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Geology and Petrology of
the Ute Mountains Area

fecal.
Jtuok_

Colorado

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LGEOLOGICAL SURVEY PROFESSIONAL - PAPER 481

NL

Prepared on éefiagf of the U.S. Atomic

Energ y Commission

 

V,. 418) - #4972
$4 aid. (1 A .

[iB

Geology and Petrology of
the Ute Mountains Area

Colorado

By E.. B. EKREN and F. N. HOUSER

 

GEOLOGICAXL=@ SURVEY _PROFESSIONAE PAPER 48 1

Prepared on éefia/qutfie U.S. Atomic

Energ y Commission

 

The petrography, structure, and petrology
qf a laccolithic mountain range on the

Golorado Plateau

 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON ; £965

UNITED STATES DEPARTMENT OF THE INTERIOR
STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY

Thomas B. Nolan, Director

The U.S. Geological Survey Library catalog card for this publication appears after index.

 

For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C., 20402

CONTENTS

EIL... .:.... / AJ ener alone ain
introduction.. : -L c tlc lloc
Geographic setting and accessibility ______________._
21. Te ela es <'nane a aan cok
.._:
Climate and
Purpose and scope of investigation_______________.
Previous
Fieldwork and acknowledgments__________________
Stratigraphy of the sedimentary rocks________________.
Pre-Navajo rocks..
Priassic(?) and
Glen Canyon Group.:..............l..icl..;
Navajo Sandstone. ......__.._:__:_..._.

durassic
San Rafael
Entrada
Summerville Formation-_________________
Junction Creek Sandstone. _____________.
Morrison Formation......._......._...._

Salt Wash Sandstone Member________

Recapture Shale Member___________.

Westwater Canyon Sandstone Member.

Brushy Basin Shale Member_________

Cretaceous .c.ll...
Burro Canyon Formation..................__
Dakota
Mancos Shale:
Mesaverde Group........._...._._.__..__.._
Point Lookout Nandstonc................_.._._
Quaternary
l.. Lrg Ot
General features and evidence for age of igneous
SCHVIbUY . 222220 cu arts een
Relative ages of the intrusive rocks.
Occurrence and petrography L ___
Ceneral

cli
Diorite porphyry.
Normal diorite porphyry _________________
Leucocratic diorite porphyry __-___-______
Grandiorite porphyry..:......l._.l.._._... .....
Quartz monzonite porphyry _______.__________
lll...

-O
Alteration. 2, cee cl. oe cane. as
Deuteric
Hydrothermal alteration at Mable Moun-

§MN-L .cc. v= o
Pyritization in the vicinity of igneous
contacts..
Hydrothermal (hot-spring) alteration asso-

ciated with breccia pipes______________.

rg
Fea Pa
o co co to ~1 <D ND ~T O or tr tr Gr h wa wh bo bo < B

IJ bD hJ RD bJ bQ bet bet bet feds fests pts fost pest
~ & o or t O co 00 tr Ot ia i> G b

27
27
28
28
28
28
30
30
30
31
32
33
35
35

35
35

35

 

Igneous rocks-Continued

l co eee ne sn anns
Chemical composition of the igneous rocks__.._.
Variation dingrams.:......-.:._J._......l....
Minor
Variation in the igneous rock series __________.
Variation of minor elements with rock
alteration.-_ _L: sei
Variations in igneous
Variations in sedimentary rocks adja-

cent to igneous

Origin of the igneous rocks and hornblende in-
2. AQ IEIL elan cree eae a
Structural geology and forms of the igneous bodies___.._.
General
Ute al ll oin nene ees e ae aie
McElmo s LEL rece
LLU. Lr ue «= sale
intrusive iC
Stooks.... ..... .l tot ler e Es
Black Mountain stock__________________.
"The Knees" stock....:....: .L.
Ute .no el uue
Bysmaliths......._......-_
Sentinel . r.ulcd..
Mable Mountain-.........__....0...2_.
The "'West neues
Laccoliths.: ":..". l sll .s
North Ute Peak lacoolith._...___.._......
Horse Mountain laccolith_______-_-------
East Horse laccolith:_.._.-._-._.:.__._:_..
Sundance cluster of laccoliths__---------.-
Razorback
Last Spring Isccolith:.;..............._...
Flat Iaccoolith.:.<._--.l_..;:;:........sl.e~
Three Forks
Banded
The "West Toe" cluster of intrusive bodies.
Mushroom'laccolith.......:..._._.-.._._.
Pack Trail laccolith:..-...._........}._.
Tongue
Yucca cluster of laccoliths__-_-----_-----
"'The Buttes" laccolith............._.L_.
North Black Mountain laccolith___-___---
Sills and
Precoia pipes:
Summary of geologic events_________________.L_______
Economic =s
Metallic mineral
Uranium
Fault-controlled deposits_________._--.-------

Cliff House group of claims_____.-_---
Three States Natural Gas Co. pros-

HIL

Page

35
35
37
38
38

40
40

43

CONTENTS

 

 

IV
Page Page
Economic geology-Continued | Economic geology-Continued
Metallic mineral deposits-Continued Metallic mineral deposits-Continued
Uranium deposits-Continued Uranium potential of the Ute Mountains area... 67
Bedded uranium deposits_._...__._cl._l.c._l.c. 64 Northern Ute Mountains area___-____------ 67
Karla Kay mine.... ...sa c yl 64 Southern Ute Mountains area__------------ 68
, Nonmetallic mineral deposits__________------------- 68
Coffin e nece no po Petroleum, natural gas, and carbon dioxide.... 68
Copper _ __________-__________________-- 66 Known 68
Battle Rock 66 Oil and gas possibilities of the Ute Moun-
M taing .l pe 69
Ute Creek dike prospects.. 66 dram... ln fae 69
Little Maude 66 com. l. l tal yack us 69
Relation of the metallic mineral deposits to the EAiterature cited.... . ion a cen aun 69
igneous _c 67 I' Index:: -. 71
ILLUSTRATIONS
Page
PLATE - 1. Geologic map of the Ute Mountains p In pocket
FIGURE 1. Index map of southwestern Colorado showing quadrangles in the Ute Mountains area____------------------- 3
2. Navajo Sandstone in MeElmo Canyon. 9
3. Upper unit of the Junction Creek Sandstone underlying the Salt Wash Member of the Morrison Formation --- 12
4. Salt Wash Sandstone Member, Recapture Shale Member, and Westwater Canyon Sandstone Member of the
Morrison Formation near Karla Kay 15
5. Lower part of the Dakota Sandstone and sandstone of the Burro Canyon Formation in Western McElmo Canyon. 22
6. "The Mound." The Juana Lopez Member in the Mancos 25
7. Contact of quartz monzonite porphyry with diorite 27
8. Specimens of the four main types of igneous rock from the Ute Mountaing-____----------------------------- 29
9. Specimen of hornblende granodiorite porphyry from Ute 31
10. Inclusions in intrusive igneous rocks of the Ute 34
11. Variation diagrams for igneous rocks of the Ute 37
12. Oxide weight percentages of rocks from the Ute Mountains plotted against the sum of normative quartz, albite,
and orthoclase (the differentiation 39
13. Frequency distribution histograms of semiquantitative spectrographic analyses of minor elements common to
the igneous rocks of the Ute 41
14. Diagram showing variation of five trace elements with respect to increasing silica and total pyrible content... 46
15. Graphical solution of Harker diagram to determine possible materials added or subtracted to produce quartz
monzonite from microgabbro.-._...-.L......:l: aL ance meres ue uk en cerecs 47
16. Approximate limits of composition of amphibole derived from various rocks showing plot of four samples of
hornblende from the Ute Mountaing. .- ;... Ere. dH Le» pul nh a a mie fen a a m aln iela ae uname in naal 48
17. Sketch showing inferred structural relations near the contact of the Sentinel Peak intrusive with a structural
dome thought to be the result of igneous intrusion at 55
18. Sketch showing inferred stratigraphic and structural relations of laccoliths in the Sundance cluster and mechanism
of "trapdoor" Le.. cur. -=a«-amachere -a nl uss anus ante pe mis nll ank aa an eae s eels 57
19. Two sills in Mancos Shale along Pine Creek north of Black 60
20. Boulder of brecciated lamprophyre cemented with 61
21. Fractured and brecciated sill of 61
22. Map showing location of mines and 62
23. Sketch showing stratigraphic and structural relations at the Cliff House prospect, McElmo Canyon, Montezuma
County, c sans cds saunas ssc hans theo ree 63
24. Cliff House No. 4 area, McElmo Canyon, Montezuma County, 64
25. Map and section of the Karla Kay mine, McElmo Canyon, Montezuma County, Colo_---.------------------ 65
26. 66

Sketch showing stratigraphic relations at the Karla Kay

TaBus 1.

& oue O N

~

10.
11.
12.
13.

14.

CONTENTS

TABLES

Average rainfall and temperature at Mancos: Colo: 1808-1010 :....._....:l.n. Dall Dl B L0 (___

. Sedimentary rocks exposed in the Ute !.. :.:. ___ _l._lgl 100 coil ll M _I .O.
. Chemical analyses and norms of igneous rocks from the Ute Mountaing. . lll csc nl cr re
. Semiquantitative spectrographic and chemical analyses, of igneous rocks from the Ute Mountains. __
. Semiquantitative spectrographic analyses of igneous rocks from the Ute Mountains..._...____.__________.__
. Semiquantitative spectrographic and radiometric analyses of altered and unaltered igneous and sedimentary rocks

from tho Ufo olo ol toe tilde nc Ott tc it on t

. Comparison of the chemical composition of microgabbro phenocrysts having the chemical composition of material

that might be subtracted from microgabbro to produce quartz

+ Chemical analyses of hornblende from the Ute .._. ......
. Atomic ratios of elements in hornblende on basis 24 (O, OH, Uso Leno tc Oot _

Optical properties of hornblende from the Ute Mountaing._....... ... ___ _._ [D L OTD)
Quantitative spectrographic analyses of hornblende from the Ute ;o {plc llc}.
Semiquantitative spectrographic analyses of hornblende from the Ute Mountaing_________.___________;_____
Radiometric and chemical analyses of samples from radioactive prospects, McElmo Canyon, Montezuma

County, Colo. _ .t hls (ne nooo eosin aul atte" n tot ",

Page
4

6

36
40
42

44

63

Sains area.... case ce nlc ttt ttt [Oy In pocket

 

GEOLOGY AND PETROLOGY OF THE UTE MOUNTAINS AREA, COLORADO

 

By E. B. Exrexn and F. N. Houser

ABSTRACT

The Ute Mountains area is on the Colorado Plateau in the
southwest corner of Colorado, in Montezuma County, about 20
miles northeast of the Four Corners where Colorado, Utah,
Arizona, and New Mexico join. The mountains are a laccolithic
group that rises to an altitude of nearly 10,000 feet and stands
2,500 to 4,000 feet above the surrounding area.

Sedimentary rocks exposed in the Ute Mountains area ag-
gregate about 3,800 feet in thickness and are of Triassic(?),
Jurassic, Cretaceous, and Quaternary age. The Triassic(?) and
Jurassic rocks in the area include the Navajo Sandstone, the
San Rafael Group, and the Morrison Formation. Cretaceous
rocks include the Burro Canyon Formation, the Dakota Sand-
stone, the Mancos Shale, and basal beds of the Point Lookout
Sandstone of the Mesaverde Group. Tertiary and Cretaceous
rocks younger than the lowermost beds of the Point Lookout
Sandstone have been removed by erosion.

The physical characteristics of the Navajo Sandstone (oldest
formation exposed) and formations of the San Rafael Group
are similar and indicate alternating subaerial and subaqueous
deposition. These formations are predominantly cross-stratified
sandstone that was deposited in an eolian environment, but in-
clude beds of flat-stratified sandstone, siltstone, and mudstone
that apparently were deposited in a marine or lacustrine en-
vironment. The Morrison Formation in the Ute Mountains
area is composed of fluvial and flood-plain deposits, and com-
prises four members: the Salt Wash Sandstone Member, Re-
capture Shale Member, Westwater Canyon Sandstone Member,
and Brushy Basin Shale Member. The lower two members,
Salt Wash and Recapture, and the upper two members, West-
water Canyon and Brushy Basin, intertongue and intergrade
to a considerable extent. The Burro Canyon Formation con-
sists of discontinuous fluvial conglomeratic sandstone lenses
and interbedded mudstone and claystone. In places only mud-
stone is present. The mudstone weathers hackly or fissile and
can be easily distinguished from that of the Brushy Basin Mem-
ber of the Morrison Formation, which weathers to a hard and
frothy-appearing material because of abundant bentonite. The
Dakota Sandstone consists predominantly of fluvial sandstone
and interbedded carbonaceous shale. Although the Dakota
Sandstone is separated from the Burro Canyon Formation by a
disconformity, the two formations are similar and, in places,
their distinction is difficult. The Mancos Shale in the Ute Moun-
tains area consists of approximately 1,900 feet of dark-gray
gypsiferous mudstone. The formation contains abundant ma-
rine fossils and two conspicuous lithologic marker units. The
Greenhorn Limestone Member is about 75 feet from the base,
and consists of 15 to 35 feet of dense gray limestone and inter-
bedded gray mudstone. The Juana Lopez Member is about 475
feet above the base of the Mancos ; it consists of 3 to 50 feet of
thin-bedded tan sandy fossiliferous limestone and was mapped

 

to include an overlying light-gray glauconitic coarse-grained
sandstone. Overlying the Mancos Shale on a few high places
in the Ute Mountains are thin flat-stratified persistent beds of
buff and yellowish-gray sandstone interstratified with dark-gray
mudstone similar to the Mancos. This sequence is probably the
basal part of the Point Lookout Sandstone of the Mesaverde
Group.

The igneous rocks in the Ute Mountains intrude beds of the
Point Lookout Sandstone of Late Cretaceous age. This evi-
dence is the only direct indication available of the age of the
intrusive rocks in the Utes. The lack of coarse detritus in
Upper Cretaceous rocks that crop out a few miles east of the
Ute Mountains, and that are younger than the Point Lookout
Sandstone but older than the McDermott Member of the Animas
Formation of late Cretaceous and Paleocene age indicates that
igneous activity and accompanying uplift did not occur prior
to the deposition of the McDermott. The McDermott Member
contains abundant andesite boulders probably derived from the
La Plata Mountains, a laccolithic group lying 35 miles east of
the Ute Mountains and presumably of the same age as the in-
trusive rocks of the Ute Mountains. The occurrence of andesite
boulders in the McDermott may indicate a precise age for the
laccolithic mountains on the Colorado Plateau.

The igneous rocks of the Ute Mountains form a differentiated
calc-alkaline series that ranges from microgabbro through
quartz monzonite. Field mapping indicates that the earliest
intrusive rock was microgabbro, followed by diorite, grano-
diorite, and finally quartz monzonite. All the igneous bodies
were intruded forcibly into sedimentary rock. No evidence of
wallrock replacement or assimilation was found. The textures
of the igneous rocks vary with rock type and from one intrusive
body to another. In general, the microgabbros are slightly
porphyritic, the diorites moderately porphyritic, and the grano-
diorites and quartz monzonites conspicuously porphyritic. The
groundmasses of the different types of rock make up from 40
to 60 percent of the volume and, in general, are dense; grains
range in size from less than 0.01 mm to about 0.1 mm.

Chemical analyses of 16 samples of igneous rocks from the
Ute Mountains reveal that silica ranges from about 50 percent
in the microgabbro to almost 69 percent in the quartz monzonite ;
alumina content is nearly constant (about 17 percent for all
the rocks in the series) ; total iron, magnesia, and lime decline
from high values in the microgabbro to low values in the quartz
monzonite; Na:0 and KO show distinct increases from rela-
tively low values in the microgabbro to higher values in the
quartz monzonite.

Semiquantitative spectrographic analyses indicate that sev-
eral minor elements show distinct trends. The systematic vari-
ation of the major and minor elements suggests that the igneous
rock series in the Ute Mountains resulted from differentiation
of a single-source magma.

2 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Hornblende-rich inclusions are abundant in the igneous rocks,
particularly in the diorite and granodiorite. The hornblende
in the inclusions is not distinguishable optically or chemically
from the hornblende of the phenocrysts. Chemical, optical, and
spectrographic analyses of hornblende from two inclusions and
phenocrysts from two porphyries are so similar that a common
origin is inferred. Two such origins seem most probable: the
inclusions are either fragments of early differentiates, or frag-
ments derived from a deep hornblendic substratum from which
the Ute magma itself was derived.

The largest structural features in the Ute Mountains area
are Ute and McElmo domes. Ute dome is nearly circular in
plan view and averages about 10 miles in diameter, although
the western margin is poorly defined. The relief on the dome
ranges from only about 400 feet on the north, where it is in
contact with McElmo dome, to more than 2,000 feet on the south.
The top of the dome appears to be flat, but is complicated in
detail by the occurrence of numerous laccoliths and three
stocks. Ute dome is thought to be entirely the result of forceful
injection of magma, most of the doming being caused by the
injections of three stocks. McElmo dome is also nearly cir-
cular in plan view, but the flanks of the dome pass into a series
of radiating anticlines. The total area affected by McElmo
dome and its satellitic anticlines measures about 20 miles east
to west and 10 miles north to south. The dome has about 500
feet of closure. The origin of McElmo dome and its relation
to Ute dome is uncertain, but the proximity of the two suggests
that they are related genetically. The general lack of igneous
rock at depth in McElmo dome, however, suggests that most of
the structural relief may be a result of basement uplift related
to Late Cretaceous or early Tertiary folding recognized else-
where on the Colorado Plateau.

Steeply dipping normal faults of small displacement occur
on the north, west, and south flanks of Ute dome, and on the
southwest flank and in the central part of McElmo dome. No
postintrusive faults were noted on the steeply dipping east
flank nor in the central part of Ute dome, but preintrusive frac-
ture zones appear to have controlled emplacement of some ig-
neous bodies. The possibility exists, therefore, that many of the
faults and fractures are earlier than Ute dome and reflect a
zone of weakness in the basement that localized the igneous
activity in the Ute Mountains.

Intrusive rocks in the Ute Mountains form laccoliths, bys
maliths, sills, and dikes, in addition to stocks. In general, the
least siliceous rocks occur as sills or flat-topped laccoliths; and
the more siliceous intrusive rocks occur as mushroom-shaped
laccoliths and bysmaliths. Dikes occur mainly adjacent to the
stocks. Most of the intrusive rocks (excluding dikes) probably
were fed laterally from the three stocks.

Deposits of pranium, vanadium, and copper occur in McElmo
Canyon. Although these deposits are small and currently have
no commercial value, they are of considerable geologic interest.
Uranium and vanadium occur in the Karla Kay Conglomerate
Member of the Burro Canyon Formation and in the Entrada
Sandstone. Uranium, traces of copper, and abundant barite
occur in faults northwest of the Ute Mountains. Copper, barite,
and traces of lead, zinc, and silver occur in nonradioactive
deposits in faults that lie between the intensely altered Mable
Mountain bysmalith and the uranium deposits. The Mable
Mountain bysmalith is cut by a fracture zone and has been in-
tensely mineralized with iron sulfides. 'The copper deposits and
the fault-controlled uranium deposits are spatially related to the
Mable Mountain fracture zone and may be genetically related to
the Ute igneous activity. The uranium was probably deposited

 

by ascending solutions that also carried copper. The ubiquity
of barite in both the radioactive and nonradioactive deposits
suggests that the deposits are related to the Ute igneous rocks,
which are relatively rich in barium. No evidence was found
that directly related the uranium deposits in beds of conglom-
erate and sandstone to the fault-controlled deposits. The minor
elements of the two types of uranium deposits and the nonradio-
active copper deposits are similar. The possibility exists, there
fore, that the bedded deposits of the McElmo Canyon area are
related to the Ute igneous activity.

The oil and gas possibilities of the Ute Mountains area have
been enhanced by the discovery of several major oil fields
in southeastern Utah. Both McElmo dome and Ute dome are
potentially excellent structural traps for oil. McElmo dome,
however, by 1958 had been tested by six wells drilled on or near
the crest of the structure, only one of which yielded hydro-
carbons. This well produces natural gas from the Shinarump
Member of the Chinle Formation and had an initial potential
of 500,000 cubic feet of gas per day. Carbon dioxide was found
in three other wells in Mississippian and Lower Cambrian rocks,
and two of these were completed as carbon dioxide wells. The
paucity of oil and flammable gas in McElmo dome and the abund-
ance of carbon dioxide have discouraged exploration. The
origin of carbon dioxide in the McElmo structure is unknown,
but probably is related to igneous intrusion either directly be-
neath McElmo dome or in the Ute Mountains. Carbon dioxide
probably occurs in the Ute dome and may completely fill the
potential reservoir rocks.

The Dakota Sandstone in the Ute Mountains area contains
many thin beds of coal, which, with few exceptions, are lentic-
ular and have no commercial value at present.

INTRODUCTION

This report describes the geology of the Ute lacco-
lithic mountain group on the Colorado Plateau and par-
ticularly the stratigraphy of the sedimentary rocks
exposed in the area surrounding the mountains, the pe-
trography and petrology of the igneous rocks, the struc-
ture of the sedimentary and igneous rocks, and the eco-
nomic geology.

GEOGRAPHIC SETTING AND ACCESSIBILITY

The Ute Mountains area (fig. 1) is in the southwest
corner of Colorado, in Montezuma County, about 20
miles northeast of the Four Corners where Colorado,
Utah, Arizona, and New Mexico join. The mountains
extend about 8 miles from north to south and 7 miles at
the widest part from east to west. McElmo Canyon lies
just north of the Ute Mountains. The west-trending
canyon cuts into the broad McElmo dome, which is
about 8 miles in diameter.

From the Ute Mountains area, the La Plata Moun-
tains are visible to the east, the Abajo Mountains to the
northwest, the Carrizo Mountains to the south, and
Ship Rock, the famous volcanic neck in northwestern
New Mexico, is visible to the south-southeast.

The Ute Mountains area is accessible from Cortez,
Colo., by U.S. Highway 666 to the Ute Reservation

INTRODUCTION

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FiGURE 1.-Index map of southwestern Colorado showing quadrangles mapped in the Ute Mountains area.

745-807 O-65--2

4 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Road to Towaoc, Colo., and by State Highway 32
through McElmo Canyon. A road passes south of the
mountains to Aneth, Utah. Roads have been con-
structed through parts of the eastern half of the moun-
tains by the Ute Mountain Tribal Council since the
conclusion of the field study. Many of these roads open
areas that during mapping were reached on foot.
Cortez is serviced by bus, truck, and airlines. The
nearest railroad is at Durango, Colo., 48 miles east of
Cortez, and can be reached by U.S. Highway 160.

CULTURE

The Ute Mountains are the site of the Ute Mountain
Indian Reservation, which covers most of the area
mapped except Canyon and its tributaries west
of the mountains. Most residents of the Ute Mountain
Reservation make their permanent homes at Towaoc,
Colo., which is the headquarters of the Ute Mountain
tribe and the location of the local agency of the U.S.
Bureau of Indian Affairs. A store, restaurant, filling
station, and school are available at Towaoe. During
the summer months, many Indian familie move into
the mountains where they hunt and herd sheep. In-
come from land royalties during recent years has en-
abled the Ute Indians to raise their standard of living
and to make significant capital investments in a water
system and in livestock.

Alluvium in Montezuma Valley and McElmo Can-
yon (pl. 1) supports small but productive fruit, small
grain, and cattle ranches. The alluvium occurs at low
elevations and irrigation is necessary. Dry farming
of pinto beans and wheat is profitable on higher mesas
on the north flank of McEImo dome and on the plains
north of the city of Cortez.

TOPOGRAKHY

The Ute Mountains rise 2,500 to 4,000 feet above the
surrounding area, which ranges in altitude from 4,800
feet in McElmo Canyon to 6,000 feet on mesas north of
McElmo Canyon. The altitude of Ute Peak is 9,977
feet, of Black Mountain 9,405 feet, and of Hermano
Peaks ("The Knees") in the south-central part of the
mountains 8,960 feet. Although moderately sharp
ridges and intervening valleys make up most of the
range, a few topographic benches occur on flat-topped
laccoliths.

From the east, the view of the mountain range is
striking. It has led many to visualize a man lying on
his back, with his head to the north, his feet to the south,
his arms folded upon his chest, and his face turned
toward the setting sun. This configuration has led to
the popular name, Sleeping Ute or Sleeping Ute
Mountain.

 

Various ridges and peaks of the range are known
locally by the parts of the "Sleeping Ute" that they
represent. Thus, Mable Mountain represents the head,
Ute Creek dike the headdress, Ute Peak the arms folded
over the chest, and Hermano Peaks the knees. Sentinel
Peak in the southeast part of the mountains and a topo-
graphically similar peak in the southwest are the large
toes, termed, respectively, East Toe and West Toe.

The east, south, and west sides of the mountains blend
gently with the plains or mesas of the Colorado Plateau,
at altitudes between 5,000 and 5,500 feet above sea level.
On the north side, McElmo Creek has cut a deep west-
trending canyon through the southern flank of McElmo

dome.
CLIMATE AND VEGETATION

The climate of the area is semiarid. McElmo Creek
is the only perennial stream, but springs are abundant
in the mountains, and some of them flow throughout the
year.

Rainfall and temperature recorded (table 1) at Man-
cos, Colo., 25 miles to the east, indicate approximate
rainfall and temperatures in the Ute Mountains, inas-
much as the altitude of 7,035 feet at Mancos is close to
the mean altitude of the Ute Mountains. Thunder-
storms are common over the mountains in summer and
early fall months. Prevailing winds are from the
southwest.

TABLE 1.-Average rainfall and temperature at Mancos, Colo.,
1898-1919

[After U.S. Department of Agriculture (1936, see. 22, p. 16 and 23)]

 

 

 

 

 
 
 
   

Precipitation | Temperature
Period (inches) (°F; 20 record

days)
L222 ient recu ben oe 1.36 25.5
oor ouch eli seven sb wie 1.46 20.1
March.... ts 2.02 36.8
April... LZ 1.76 44. 4
May... 1.26 51.5
June... .81 61.2
July...... 1. 91 66.2
AUGUSE 2.01 65.0
September............ 1.55 57.6
October...... 1.55 47.3
s. celle cocoa a 1.14 37.9
rel.. celine l Uc. dul ats 22 1.21 26.5
AnnUa! et dha 18. 04 45.8

 

Sagebrush, juniper, and pifion grow on the plains
north of McElmo Canyon, and sparse grass, juniper,
and pifion grow on the plains west of the mountains.
The plains south and east of the Ute Mountains have
only sparse grass, sage, and thistle. Lower slopes sur-
rounding the mountains are covered moderately with
juniper, pifion, and shrubs. Most of the middle-level
slopes in the mountains proper are covered with oak
brush, juniper, and pifion. Pine and fir grow on the
upper slopes and ridges, and small clumps of aspen
grow on the north slopes of Horse Mountain, Black
Mountain, and Ute Peak.

INTRODUCTION 5

The vegetation in the Ute Mountains is similar to
that in the Henry Mountains of Utah described in con-
siderable detail by Hunt, Averitt, and Miller (1953, p.
i gT-85).

PURPOSE AND SCOPE OF INVESTIGATION

The Ute Mountains of southwestern Colorado were
mapped by the U.S. Geological Survey as part of the
program of uranium investigations undertaken by the
Survey on behalf of the Division of Raw Materials of
the U.S. Atomic Energy Commission. The objectives
of the work were: to determine the uranium resources
and potentialities of the area; to provide detailed in-
formation on the relations of uranium ore deposits in
the sedimentary rocks of the area to the igneous rocks
of the Ute Mountains; and to fill a gap in the modern
geologic map of the Colorado Plateau.

PREVIOUS WORK

The Ute Mountains were first described by Holmes,
of the Hayden Survey (1877, p. 236-237), who referred
to them as the El Late Mountains. Cross (1894, p. 211-
214) reviewed Holmes' work and described the igneous
rocks sampled by Holmes. Two of these samples were
analyzed chemically and the analyses are reproduced
in this report. Coffin (1920) mapped the structure and
stratigraphy of McElmo dome. P. H. Metzger (written
commun., 1944) studied the geology of the McElmo
Canyon area, with emphasis on the Morrison Forma-
tion. Cadigan studied the Junction Creek Sandstone
in McElmo Canyon in 1949. In the spring of 1952,
diamond-drill exploration of radioactive areas along
faults in McElmo Canyon was undertaken by the
Atomic Energy Commission. Geologic logs of the dia-
mond-drill core are on file in the office of the Atomic
Energy Commission in Grand Junction, Colo.

The Ute Mountains and the McElmo Canyon area
were mapped in reconnaissance by E. M. Shoemaker
and W. L. Newman during a succession of brief visits
in 1951, 195%, and 1953. Shoemaker and Newman
(1953) noted approximately 30 major laccoliths, and
they concluded that the laccoliths had been fed later-
ally from two stocks-Black Mountain and Hermano
Peaks ("The Knees"). They noted that most of the
large dikes and the two stocks lie in a north-south
zone, and suggested that the orientation indicated con-
trol of intrusion by a deep-seated structure in the base-
ment complex. Geochemical studies by Shoemaker and
Newman (written commun., 1955) suggested that the
igneous rocks in the Ute Mountains may have been a

Cadigan, R. A. 1952, The correlation of the Jurassic, Bluff, and
Junction Creek sandstones in southeastern Utah and southwestern
Colorado : Pennsylvania State College master's thesis.

 

source for the vanadium and uranium found in some of
the faults and sedimentary beds of the McElmo Canyon

area.
FIELDWORK AND ACKNOWLEDGMENTS

The Ute Mountains and the adjacent McElmo
Canyon area were mapped during the spring and sum-
mer of 1955 and the summer of 1956. The geology was
mapped on aerial photographs furnished by the U.S.
Geological Survey and by the Soil Conservation Serv-
ice of the Department of Agriculture. Base maps for
the area were prepared from aerial photographs by the
use of the Kelsh plotter and a radial planimetric plotter
and slotted templates. Geology and planimetry were
transferred simultaneously from the photographs to the
base maps at a scale of 1 : 20,000. _ Horizontal and verti-
cal control was established by setting up a triangulation
net tied to the primary triangulation station "Ute" and
to Coast and Geodetic Survey leveling lines in McElmo
Canyon and Montezuma Valley. Approximately six
stations were placed in each of the five T!4-minute
quadrangles mapped. Clyde Duren, Jr., and Lloyd L.
Smith, formerly of the U.S. Geological Survey, estab-
lished many of the stations, and their assistance and
advice are gratefully acknowledged.

Geologic maps of the five 71-minute quadrangles on
planimetric bases have been published in the U.S. Geo-
logical Survey Mineral Investigations Field Studies
Map series (Ekren and Houser, 1957, 1959b, c, d ; Houser
and Ekren, 1959). Topographic maps of the Ute Moun-
tains area were published by the Geological Survey late
in 1957. The geologic map accompanying this report
was prepared by reducing the original planimetric-
geologic maps to 1: 48,000 scale and transferring the
geology to the new topographic base.

GEOLOGIC SETTING

The Ute Mountains are in the western Colorado mar-
gin of the Canyon Lands section of the Colorado
Plateau (Fenneman, 1931, p. 307). The mountains rise
above the Great Sage Plain, a broad expanse cut by
deep, steep-sided canyons that extends northward from
the Ute Mountains to the La Sal Mountains. The plain
is formed mainly on gently dipping resistant sand-
stone of the Upper Cretaceous Dakota Sandstone.
The broad Mesa Verde (Fenneman, 1931, p. 308-309)
begins 6 miles east of the Ute Mountains and extends
20 miles eastward beyond the Mancos River. West and
south of the Ute Mountains is the rugged canyon coun-
try of the San Juan River drainage.

The Ute Mountains area is on a structural terrace or
bench between Blanding basin to the west-northwest and
San Juan basin to the southeast. The laccolithic moun-
tains lie on a structural dome formed by injection of

6 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

magma and are just south of McElmo dome, a large star-
shaped structure that probably originated as a result of
basement uplift, igneous intrusion, and, possibly, move-
ment of salt. To the southwest is the Defiance uplift
and to the northeast, the San Juan uplift (Luedke and
Shoemaker, 1952; Kelley, 1955).

Within a radius of 150 miles from the Ute Mountains
are seven other laccolithic mountain groups: the La
Plata and Rico Mountains to the east, the Carrizo Moun-
tains to the south, the Henry and the Abajo Mountains
to the northwest, the La Sal Mountains to the north-
northwest, and the West Elk Mountains to the north-
east. The igneous rocks in these laccolithic mountains
are quite similar. Geologists who have studied the
Colorado Plateau generally agree that the rocks are re-
lated and that most of the laccolithic groups were formed
during the same geologic period. On the basis of geo-
morphic evidence, the laccolithic intrusive rocks and re-
lated structures are believed by Hunt (1956, p. 45) to
be of late Miocene or early Pliocene age. Shoemaker

 

(1956, p. 162), on the basis of stratigraphic evidence
discussed on page 27, concluded that the intrusions
took place near the end of the Cretaceous Period.

STRATIGRAPHY OF THE SEDIMENTARY ROCKS

The sedimentary rocks exposed in the Ute Mountains
area are listed in table 2. Except for the surficial
deposits, these rocks range in age from Triassic(?) and
Jurassic (Navajo Sandstone) to Late Cretaceous (Point
Lookout Sandstone). Tertiary and Cretaceous rocks
younger than the lowermost beds of the Point Lookout
Sandstone have been removed by erosion.

The thickness of Upper Cretaceous and Tertiary sedi-
mentary rocks that once overlay the Ute Mountains
area cannot be accurately determined. The lithology
of the thick post-Point Lookout, pre-Animas sedimen-
tary rocks of Late Cretaceous age that crop out a few
miles southeast of the Ute Mountains indicates that the
Ute Mountains area was not a topographic high during
those periods of deposition. These sediments reflect

TaBus 2.-Sedimentary rocks eaposed in the Ute Mountains area, Colorado

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

System and Group, formation, and member Character Thickness Economic value
Series (feet)
Alluvium, 3 to about 75 ft thick; composed of interbedded mud, Fanglomerate gravel in McElmo
F sand and gravel in valley fill and stream deposits; windblown Canyon and pediment gravel on
Quaternary sand and silt; talus; landslide deposits; block rubble; stream 0-75 eastern slopes of Ute Mountains
terrace gravel; pediment gravel; fanglomerate. are used in highway construction.
Unconformity
$5
$ 3 Point Lookout Sandstone Thin-bedded yellowish-gray and buff fine-grained sandstone 200 No recognized economic value in
g a interbedded with gray sandy shale. (incomplete) Ute Mountains area.
A
Mancos Shale Predominantly gray to black shaly mudstone and claystone.
Upper The Juana Lopez Member is cuesta-forming sandy fossilif- | 1,800-1,900 | Some of the claystone has ceramic
Cretaceous Juana Lopez erous limestone and shale 25 to 100 ft thick, 475 ft above base value for making brick and tile.
Member of formation.
Yellowish lenticular sandstone and conglomeratic sandstone
Dakota Sandstone interbedded with carbonaceous shale and coal. Coal beds are 110-140 Coal, ground water.
extremely lenticular and usually contain considerable amounts
of mud and silt.
er Wh d b ish d: d 1 te int Karla Kay C 1 te Member
j ite, gray, and brownish sandstone and conglomerate inter- arla Kay Conglomera ember
Lower Em bedded with nonbentonitic green and red mudstone. Includes 30-200 is uranium bearing in McElmo
Cretaceous Karla Kfi Gogglomerate the Karla Kay Conglomerate Member in McElmo Canyon. Canyon area.
ember
E Brushy Basin Shale Member Vézioololred bentonitic mudstone and a few conglomeratic sand- 150-300
4 stone lenses.
8 Westwater Canyon and Salt Wash
g Westwater Canyon Sandstone | Pale yellow-brown fine- to medium-grained sandstone inter- 75-200 Members are uranium bearing in
9 Member bedded with green bentonitic mudstone. many parts of the Colorado
a Plateau. No uranium deposits
8 Recapture Shale Member Tan and reddish-gray fine- to medium-grained sandstone inter- 0-200 are known in these members in
é bedded with red mudstone. the Ute Mountains area.
g Salt Wash Sandstone Member | Pale-brown fine- to medium-grained sandstone interbedded with 0-200
predominantly red-brown mudstone.
Predominantly pink fine- to coarse-grained, poorly sorted sand-
Upper Junction Creek Sandstone stone, cross-stratified at high angle; weathers to a "slick rim." 250-300 Ground water.
Jurassic & Correlates with the Bluff Sandstone of Utah and Arizona.
2 Flat- and thin-bedded argillaceous sandstone and siltstone, brick Upper sandstone unit is uranium
& Summerville Formation red. Includes a 20-ft fine-grained well-sorted ledge-forming 120-130 bearing in McElmo Canyon.
fig sandstone at top. Except for this unit, the formation is slope No mineral value-1958.
& forming.
a; Slick Rock Member, 70 to 80 ft thick, is sandstone that is pale
& brown and light pink at top and that grades to white and
Entrada Sandstone finally to orange red in lower part, weathers to a "slick rim." 100-115 Ground water.
Dewey Bridge Member is very fine grained brick-red argil-
laceous sandstone, 25 to 35 ft thick.
a§2
Jurassic and | 3 R3 Navajo Sandstone Predominantly orange fine-grained eolian sandstone. 300 Ground water.
Triassic(?) | 0 §5
O

 

STRATIGRAPHY OF THE SEDIMENTARY ROCKS A T

marine or marginal marine conditions, show no lateral
change in the proximity of the Ute Mountains, and un-
doubtedly were once continuous over the Ute Mountains.
The lower part of the Animas Formation (McDermott
Member), however, contains abundant andesite detritus,
and the Ute Mountains may have been uplifted first dur-
ing McDermott time (Shoemaker, 1956, p. 162). The
post-Animas (early Tertiary) sediments that crop out
a few tens of miles southeast of the Ute Mountains are
largely fluviatile in origin. Present knowledge of these
early Tertiary sediments is inadequate to determine

whether the Ute Mountains area was supplying or re- |

ceiving sediments.
PRE-NAVAJO ROCKS

In the Ute Mountains area, the sedimentary rocks
that underlie the Navajo Sandstone according to Zabel
(1955, p. 132-135) are in descending order : the Upper
Triassic(?) Kayenta Formation, Wingate Sandstone,
the Upper Triassic Chinle Formation (includes Shina-
rump Member), the Lower and Middle(?) Triassic
Moenkopi Formation, the Permian Cutler Formation,
the Pennsylvanian and Permian (?) Rico Formation, the
Middle Pennsylvanian Pinkerton Trail Formation of
Wengerd and Strikland (1954) and the Hermosa For-
mation (includes Paradox Member), the Mississippian
Leadville Limestone, the Upper Devonian Ouray Lime-
stone and Elbert Formation, and equivalents of the
Middle Cambrian Ophir Formation and Lower Cam-
brian Tintic Quartzite of northern Utah.

Several of these formations are of economic interest
in the Ute Mountains area. The Paradox Member of
the Hermosa Formation produces oil and gas in the
nearby Aneth field of Utah and is a potential producer
in the Ute Mountains area ; the Leadville in the McElmo
dome produces carbon dioxide gas, and in McElmo
Canyon the Shinarump Member produces flammable
gas.

The nearest exposures of Triassic and uppermost
Paleozoic rocks are in the Dolores River Canyon near
Dolores, Colo., in the Lia Plata Mountains, Colo., and
along the San Juan River in Utah. The rocks in the
La Plata Mountains were described in detail by Eckel
(1949, p. 8-24). Rocks older than the Hermosa For-
mation of Pennsylvanian age crop out north of Du-
rango, Colo., along the Animas River valley in the
vicinity of the Needle Mountains (Cross and Hole,
1910).

TRIASSIC(?P) AND JURASSIC
GLEN CANYON GROUP
NAVAJO SANDSTONE

The Glen Canyon Group includes the Wingate Sand-
stone, Kayenta Formation, and Navajo Sandstone.

 

The Navajo Sandstone is the only formation in the Glen
Canyon Group that is exposed in the Ute Mountain
area.

The Navajo Sandstone (Gregory, 1917, p. 57-59)
crops out along the south flank of McElmo dome in Mc-
Elmo Canyon. A thickness of about 300 feet is exposed
along Sand Creek, a tributary of McElmo Creek, in sec.
26, T. 36 N., R. 18 W. The base of the formation is not
exposed in Sand Creek, but a gamma-ray log of the
National Oil and Gas Co.'s West 1, located a few miles
east of Sand Creek, indicates a total thickness of 300
feet for the Navajo Sandstone. This thickness suggests
that the Navajo-Kayenta contact can be no more than a
few feet below the bed of Sand Creek.

Throughout the McElmo Canyon area the Navajo
Sandstone is predominantly orange and fine grained,
and is composed of subangular clear quartz grains.
"Berries" of medium-grained well-rounded quartz are
common. In general, the Navajo is thick bedded and
is alternately cross-stratified and flat stratified. The
cross-stratified beds are thickest and are separated from
flat-stratified beds by planar erosion surfaces that form
distinct horizontal partings in the weathered outcrop
(fig. 2). The sequence of cross-stratified beds horizon-
tally truncated at many intervals suggests eolian depo-
sition alternating with subaqueous deposition.

That water played a role in the deposition of the
Navajo Sandstone is also indicated by the occurrence of
a bed of dense gray limestone about 4 feet thick near
the top of the formation that crops out in the northern
part of McElmo Canyon. The limestone is nonfossilif-
erous and was probably deposited in a fresh-water lake.

The following section was measured on the east side
of Sand Creek Canyon in see. 26, T. 36 N., R. 18 W., by
V. L. Freeman and L. C. Craig of the U.S. Geological
Survey ; minor changes have been made in this section
as recorded to conform with the nomenclature used in
this report. In this report, rock color terms that are
followed by color symbols are from the National Re-
search Council "Rock-color Chart" (Goddard and
others, 1948).

1. Lower McEimo Canyon section (pl. 1)
Entrada Sandstone.
Navajo Sandstone :
Sandstone, very pale orange; some pale reddish-
brown areas; fine-grained ; large-scale cross lami-

Feet

 

nations: forms a 80. 2
Sandstone, red to yellow, very fine grained, very an-
gular; contains some disseminated coarse grains
("berries") u 1.2
Sandstone, pinkish-gray, medium fine-grained; some
larger angular grains; cross laminated __________- 7.6

Sandstone, pinkish-gray, medium fine-grained; con-
tains some larger angular grains; cross laminated - 1.9

8 GEOLOGY, PETROLOGY, UTE

1. Lower McEimo Canyon section (pl. 1)-Continued

Navajo Sandstone-Continued

Sandstone, pink and white, fine-grained, subangular,
thin-bedded, cross laminated____________________

Sandstone, silty, yellowish-gray; some dark-red
patches; very fine grained; cross laminated_____

Sandstone ; pale reddish orange to very pale orange at
top; very fine grained, subangular; massive cross-
laminated unit ase

Sandstone; pale red at base to very pale orange at
top; fine-grained, subrounded ; massive cross-lami-
nated unit.. .L... 300

Sandstone; light-brown, fine-grained, subrounded;
contains some black accessory mineral grains; cal-
careous; wedging cross lamination______________

Sandstone, pale reddish-brown (10R 6/4), fine-
grained, subangular ; contains very minor accessory
minerals; calcareous; wedging cross lamination.

Sandstone, pale reddish-brown (10R 6/4), fine to very
fine grained, subangular, calcareous_____________

Sandstone, light-brown, fine-grained, subrounded,
calcareous, cross-laminated_____________________

Sandstone, moderate reddish-orange to pale reddish-
brown, fine-grained ; subangular at base to very fine
grained at top; contains scattered black accessory
mineral grains; calcareous; massive with broad
cross lamination; base of expoSUre______________

Feet

15. T

21.9

2. 8

18. 5

38.3

8.5

43. 0

 

Total (incomplete?) Navajo Sandstone-_______ 307. 6

Baker, Dane, and Reeside (1936, p. 44) regarded the
Navajo Sandstone as a large northeastward-thinning
wedge of eolian sand. The Ute Mountains area appar-
ently is very close to the eastern edge of the wedge, for
the Navajo pinches out between McElmo Canyon and
the La Plata Mountains.

No fossils were found in the Navajo Sandstone in the
Ute Mountains area. A fossil vertebrate found in the
Navajo Sandstone of northeastern Arizona was de-
scribed by Camp and Vanderhoof (1935, p. 385) as
"a small dinosaur about the size of a turkey." Camp
(19836, p. 39) named it Segisaurus hall? and stated (1936,
p. 52) : "It represents a single member of an unknown
upland fauna and despite its primitive characters it
could be placed in either the Triassic or Jurassic."
Harshbarger and others (1957, p. 22-31) described
findings of other organic remains from the Navajo
Sandstone, but none of these are diagnostic of age. The
assignment of Triassic(?) and Jurassic age for the
Navajo Sandstone is based on a variety of evidence
summarized by Lewis and others (1961) who point out
that the age of the Navajo must be based upon: (a)
the age of the Kayenta Formation, which intertongues
with the lower part of the Navajo, and (b) the age of
the fossiliferous Carmel Formation, which intertongues
with the upper part of the Navajo.

 

MOUNTAINS AREA, COLORADO

The contact of the Navajo Sandstone with the over-
lying Entrada Sandstone is sharp in the McElmo Can-
yon area.

JURASSIC SYSTEM

SAN RAFAEL GROUP

ENTRADA SANDSTONE

The San Rafael Group in McElmo Canyon includes
the Entrada Sandstone, Summerville Formation, and
the Junction Creek Sandstone. These formations are
of Late Jurassic age and are about 1,000 feet thick.
The local nature of the exposures of the San Rafael
Group in the Ute Mountains area precludes a discussion
of sedimentary history other than the brief statements
included with the lithologic discussion. A synthesis
based on a regional study was made by Harshbarger
and others (1957, p. 23-51).

The Entrada Sandstone (Gilluly and Reeside, 1928,
p. 76) consists of two units in McElmo Canyon and its
tributaries. The two units have been named the Dewey
Bridge Member and the Slick Rock Member by Wright,
Shawe, and Lohman (1962, p. 2057). The type locality
of the Dewey Bridge Member is at Dewey Bridge,
Grand County, Utah, and the type locality of the Slick
Rock Member is at Slick Rock, San Miguel County,
Colo., about 30 miles north of the Ute Mountains area.
The Dewey Bridge Member (lower unit) is red hoodoo-
weathering sandstone that forms a bench over the re-
sistant Navajo Sandstone. The Dewey Bridge Member
formerly was thought to correlate with the Carmel
Formation. The upper unit of the Entrada, the Slick
Rock Member, is white to orange clean sandstone that
weathers to a nearly vertical rounded cliff or "slickrim."

The two sandstone units are conspicuous in the Mc-
Elmo Canyon area. (See fig. 2.) The Dewey Bridge
Member, which is 25 to 35 feet thick, is composed of
brick-red argillaceous and silty, very fine grained sand-
stone. The Slick Rock Member is 70 to 80 feet thick;
it is orange to light pink in the lower part and grades
to white and pale brown in the upper part. It is fine
grained, is composed of subangular clear quartz and
abundant well-rounded medium to medium-coarse
quartz "berries," and is cross-stratified at medium
angles. The cross-strata commonly are truncated by
planar surfaces of erosion that underlie horizontal lami-
nae. The Slick Rock Member includes a 6-foot thick
ledge at the top that may correspond to the lower part
of the Bilk Creek Sandstone Member of the Wanakah
Formation.

The following section was measured in Sand Creek
Canyon in see. 26, T. 36 N., R. 18 W., by Ekren.

STRATIGRAPHY

OF THE SEDIMENTARY ROCKS 0

 

FIGURE 2.-Navajo Sandstone (J®n) in McElmo Canyon.
stone facies of the Dewey Bridge Member of the Entrada.

2A. Sand Creek section (pl. 1)

Summerville Formation.
Entrada Sandstone:
Slick Rock Member :

Sandstone, pale-brown to light yellow-brown,
fine-grained, poorly sorted ; contains abundant
medium to medium-coarse clear quartz grains
("Entrada berries"); sandstone is limonite-
flecked and mottled; unit forms ledge ("non-
glick-rim"

Sandstone; light-pink at base grading to white
and to pale-brown at top; weathers to very
pale brown and white; very fine to fine
grained; well-rounded quartz grains or "ber-
ries" fairly common in light-pink zone, rare
in white zone, common in pale-brown zone
near top. Light-pink zone is abundantly
freckled with limonite; entire unit is "slick-
rim" weathering and is cross-stratified at
medium angle. Cross strata are bounded by
horizontal. pATHNGEE-.. -. cL

Sandstone, orange-red; weathering orange to
red; very fine grained, grains angular to sub-
angular ; bedding obscure, massive ; "slick-rim"
weathering; calcite cement__________________

Feet

48

20

 

Note horizontal partings in upper 200 feet. Bench above Navajo is formed in silty sand-
Slick Rock Member of the Entrada (Je) is above bench. The Summerville
Formation (Js), the Junction Creek Sandstone (Ji), the Morrison (Jm)}, and the Burro Canyon and Dakota (Kbd) occupy slope in the
background.

2A. Sand Creek section (pl. 1)-Continued
Entrada Sandstone-Continued
Slick Rock Member-Continued
Sandstone, light-tan, buff; weathering composed
of very fine quartz grains and a few medium-
coarse grains of well-rounded clear quartz
"berries"; the very fine grains subangular to
subrounded ; unit resistant (abundant calcar-
eous cement), does not weather "slick" as unit
above does: base 2
Dewey Bridge Member :
Sandstone, argillaceous; very fine to silt-sized
grains, brick red; hoodoo weathering. Upper
10 ft well exposed, lower part (about 15 ft)
poorly exposed and mantles bench formed on
top of resistant Navajo Sandstone__________-

Feet

30

 

Total Entrada Sandstone____..__..____.___ 106

The thickness and lithology of the Entrada Sand-

stone show little variation throughout the McElmo

Canyon area. The following section was measured in

Goodman Canyon approximately 3 miles east of Sand

Creek Canyon, by L. C. Craig, V. L. Freeman, and J. D.
Strobell, Jr.

 

 

10 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO
3. Upper McEimo Canyon section (composite) (pl. 1) 3. Upper McEimo Canyon section (composite) (pl. 1)-Con.
Entrada Sandstone-Continued
Summerville Formation. Dewey Bridge Member : Feet
Entrada Sandstone : Top contact gently undulant with sharp lithic
Slick Rock Member : Feet CHASE E _ 1) ol. 2. se eee e l a a o u te he he a he he n hn in ip e race wate nema ar hca eel
Sandstone; very pale orange (10YVR 8/2) in up- Sandstone, pale reddish-brown (10R 5/4) and
per half, pinkish cast in lower half; fine moderate reddish-orange (10R 6/6), very fine
grained; contains disseminated medium-fine, to fine-grained; composed of subangular clear
rounded to well-rounded grains or "berries"; quartz and minor accessory minerals; color
composed of subangular clear quartz and col- and weathering controlled by degree of irregu-
ored minor accessory minerals; structureless ; lar discontinuous dark clayey lamination;
lower half forms reentrant__________________ 5.6 weathers shaly to earthy and locally forms
Section A. Measured on north side of McElmo rounded biscuit-shaped cliffs_________________ 18. 1
Canyon just east of old oil rig and west of Sandstone, moderate reddish-orange (10R 6/6)
mouth of Goodman Canyon. Sec. 30, T. 36 N., to moderate orange-pink (10R 7/4), fine-
R. 17 W. grained; contains subangular clear quartz,
Sandstone, white, pale yellow weathering; pre- pink and black accessory minerals; contains
dominantly fine grained; rare medium-sized numerous layers with concentrations of
grains; composition same as unit below; unit medium-grained well-rounded frosted quartz
forms massive rounded top to "slick rim"; ffherrics"; platy bedded.. ..-.-----«---.._.__- 2.83
poorly formed irregular horizontal 8.6
Sandstone, white, predominantly fine grained; Total Entrada Sandstone-________________ 102. 8
composuu'm Same as unit belO.W; Stratification in the Dewey Bridge Member (red silty
eous; unit predominantly horizontally lami- # % es h
fated but a. few. boas. show. Ane-scale. crosk facies) in the McEIlmo Canyon area is inconspicuous
llc n {{i cma ntt... 12.1 | Most of the visible stratification is flat. The sparseness
Sandstone, white, fine- to medium coarse-grained ; of cross-stratification of either eolian or fluviatile type
composed of subangular to well-rounded clear together with fair sorting of the grains suggests
quartz and very minor black, gray, and pink deposition in a lacustrine or marine environment.
accessory minerals; cross laminated; medium The most conspicuous feature of the stratification of
Reale (o 40 4 f,“ woelso Dodd U8 -- unc >> t* 4 the Slick Rock Member of the Entrada Sandstone of
eavasione while Jo - ualegelion. yers (line the McElmo Canyon area is the large number of hori-
grained ; composition same as unit below ; hori- . 3 7
zontally laminated ; unit forms a reentrant____ 4.0 | ZObtal (planar) surfaces of erosion that are continuous
Sandstone, white to pale-yellow, fine- to medium- over long distances. These surfaces commonly separate
grained; composed of subangular to rounded wedge-stratified beds from horizontal laminae. The
clear quartz and minor green and black acces- alternating stratification suggests subaqueous and
sory minerals; highly cross laminated ; wedge subaerial deposition.
bedded at a fine to medium scale. "Pop con- Several alcoves have been weathered into the Slick
fact shhto and bovoled forms e t. Bis ** | Rock Member of the Entrada Sandstone in the McElmo
Sandstone; moderate reddish orange in lower 4
half and while to pale yellow in uprer halt . Canyon area. | Thgse alcove? were used by ancient In-
very fine to fine grained; composed of sub- dians as building sites for cliff houses. The cliff houses
angular to subrounded clear quartz and black, are most numerous and are best preserved in the Sand
gray, green, and pink accessory minerals; Creek area.
slightly wavy horizontal laminations ; weathers The contact of the Slick Rock Member with the over-
massive and rounded; color contact ranges lying siltstones and mudstones of the Summerville For-
frorn spatp do vogne and Otnee mation is sharp in McElmo Canyon, but beds of sand-
sandstone slightly calcareous, white is highly ps . A y
alcareous___.."l_. }}... afa t" 9.3 | stone similar to those in the Entrada occur in the lower
Sandstone, moderate reddish-orange (10R 6/6), part 0_f the Surpmerville Formation, and the two
fine to very fine grained ; composed of subangu- formations may intertongue.
lar clear quartz and disseminated medium-
coarse "berries"; massive and structureless in SUMMERVILLE FORMATION
lower 15 ft; discontinuous clayey laminae in The Summerville Formation of Late Jurassic age was
upper d ft pale honzonfal ~> 88 | named by Gilluly and Reeside (1928, p. 80) from ex-
Sandstone, grayish-orange-pink (10R 8/2) to F « s
Inoderare mory hoe To mealol posures on Summerville Bomt in the San Rafael Swejll,
fine-grained, poorly sorted ; contains subangular Utah. I'n the type locality the Summerville overlies
to rounded clear quartz, medium-coarse "ber- the Curtis Formation.
ries" of well-rounded frosted quartz, and white In the McElmo Canyon area the Summerville For-
to light- and dark-gray chert ; cross laminated, mation overlies the Entrada Sandstone and underlies
wedge bedded at a fine scale (6 in to 2 ft) ______ 4.5 | the Junction Creek Sandstone. The formation is domi-

STRATIGRAPHY OF THE SEDIMENTARY ROCKS 11

nantly brick red to red brown and is composed of al-
ternating and gradational beds of very fine to fine-
grained well-sorted sandstone and silty mudstone.
Stratification is generally flat. The formation is from
125 to 150 feet thick and, with the exception of a resist-
ant sandstone bed 10 feet below the top, is bench form-
ing. The lower 10 to 15 feet of the formation contains
thin beds of white to pale-brown sandstone that resem-
ble the Entrada Sandstone.

The following section was measured by Ekren in
Sand Creek Canyon, sec. 24, T. 36 N., R. 18 W.

2B. Sand Creek section (pl. 1)

Junction Creek Sandstone.

Summerville Formation : Feet
Mudstone," 5
Sandstone, pale-brown, fine-grained 1
Sandstone, red-brown, very soft, fine-grained; con-

tains abundant red-brown hematite or limonite___. 3
Mudstone.-red-brown 1
Sandstone, white to pale-green, light-green-weather-

ing, fine-grained, well-cemented; protects softer

sandstone units 4
Sandstone, red-brown to chocolate-brown, fine-

grained, soft and friable; abundant hematite____ 4
Sandstone, light-red to light-gray, soft, limonite-

flecked, flat-bedded, massive-weathering__________ 9
Siltstone, red, hard, spheroidal-weathering._________ 3

Sandstone, light-red, very fine grained ; this unit, and
the overlying sandstone and siltstone units form
RA SH Coe eed ol one l nn ana aa an aw o ware ao 10
Sandstone, light-red to white, very fine to fine-
grained, very soft; bedding obscure; bench form-
ing. te ao n o ut we oe ae an oe ai a nere io altace ara ea oe L 14
Siltstone, argillaceous, gray-green and light-red_____ 2
Sandstone, paleyellow to pink; fine to very fine
quartz grains; limonite flecked and blotched with
unknown black mineral; contains sparse black

opaque minerals; flat-bedded____________________ 8
Mudetone, red.... . 11... __ _ _edcol nemen cnd wren o 2
Sandstone, tight, hard, flat-bedded ; contains very

fine grained . 2

Sandstone and mudstone, light-red and brick-red,
alternating and gradational, flat-bedded; sand-
stone is very fine grained._______________________ 8
Mudstone, brick-red ; a few thin layers of very fine
grained sandstone usually less than 1 ft thick that
are light red to pink, dark red weathering; unit is
flat bedded wud Mods 8T
Sandstone, white buff-weathering; very fine grained
quartz "berries" ; rock is tight and hard, calcareous
cemented ; unit splotched and flecked with limonite.. 2
Siltstone and mudstone, brick-red ; forms brick-red
soil; bench forming. Layer of fine-grained sand-
stone 2.5 ft thick crops out about 6 ft above base
of unit; this sandstone is light brown to pale
brown, hard with calcareous cement, spotted with
limonite, and together with unit above probably
corresponds to the Bilk Creek Member (Goldman
and Spencer, 1041). ._ -_- CL E- _ 12

 

 

Total Summerville Formation-__________________ 127

745-807 O-65--3

 

The resistant sandstone near the top of the Summer-
ville is continuous throughout the McEImo Canyon area.
This sandstone is generally reddish brown or pink, but
near igneous intrusive rocks in the Ute Mountains and
near some faults in McElmo Canyon it is yellow brown.
It is radioactive near faults in the vicinity of Rock Creek
in sees. 22 and 23, T. 36 N., R. 18 W.

L. C. Craig, J. D. Strobell, Jr., and V. L. Freeman
measured the Summerville Formation on the west side
of Goodman Canyon about 114 miles north of the fruit
farm at the mouth of the canyon in see. 29, T. 39 N., R.
17 W.

3. Upper McEimo Canyon section (pl. 1)

Junction Creek Sandstone.
Summerville Formation :

Claystone and sandstone, grayish-red (10R 4/2).
Claystone is fine grained to medium grained, sandy,
shaly to earthy weathering. Sandstone is white,
medium fine to fine grained, noncalcareous; acces-
sory minerals as in unit below ; forms several beds
as much as 1 ft

Sandstone, grayish orange-pink (10R 8/2) to white,
medium fine- to fine-grained ; composed of rounded
clear quartz and minor amber, pink, and black ac-
cessory minerals ; forms fairly resistant

Sandstone, clayey, dark reddish-brown (10R 3/4),
very fine grained; composed of subangular clear
quartz; earthy to shaly weathering; forms small
bene. ... ..- . ss JELCLLLEILACERELALL canne a olan une a 8.1

Sandstone, grayish orange-pink (10R 8/2) to white,
medium fine- to fine-grained ; composed of rounded
clear quartz and minor amber, pink, and black ac-
cessory minerals ; predominantly wayy lamination ;
forms prominent ledge....._...__._.__________ ___

Sandstone, light grayish-red (10R 5/2) to white, non-
clayey; weathers rounded ; structureless or show-
ing wavy lamination and thin shaly partings_____ 9. 8

Sandstone, moderate orange-pink (10R 7/4) to white,
finely mottled, medium fine- to fine-grained ; some
medium-sized grains; coarser grains well-rounded,
finer grains subangular; rock slightly calcareous ;
weathers, soft, friable; indistinct bedding, 60 per-
cent cross-laminated at fine to medium scale, 40
percent horizontally laminated ; several prominent
horizontal bedding planes_______________________

Sandstone, clayey, dark-red, mottled light-green,
very fine grained, noncalcareous ; weathers rounded
to shaly and earthy.

Sandstone, white, dark-brown weathering; predomi-
nantly fine grained; clear quartz and red, pink,
green, yellow, and black accessory minerals; wayy
laminated and cross laminated on a fine scale;
forms prominent ledge; sand typical of Summer-
-__ eL L_ cL LLL LL

Sandstone and claystone interbedded. Sandstone is
pale reddish brown (10R 5/4), silt sized to medium
fine-grained sand size; weathers to shaly irregular
to rounded ledges. Claystone is dark red, very
sandy, earthy weathering; contains disseminated
sand grains. Two ledges of white brown-weather-
ing, very fine grained to fine-grained resistant non-

Feet

16. 2

4.3

15. 8

24. 4

2. 2

2. 6

12 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

3. Upper McEimo Canyon section (pl. 1)-Continued
Summerville Formation-Continued
calcareous sandstone between 29 and 34 ft above
base Of UNIC. . : -.. . .. 2 no eron eee ee baa nn en na nline as a mme
Sandstone; mottled very pale orange and pale-red ;
predominantly fine grained; composed of suban-
gular clear quartz and colored accessory minerals;
contains disseminated rounded medium fine
grained "berries"; forms prominent
Sandstone and claystone. Sandstone is pale red (5R
6/2), very fine to medium fine grained ; forms sev-
eral ledges as much as 1 ft thick. Claystone is dark
red, very sandy, nonresistant, earthy weathering..

Feet

59. 1

1.9

9. 7T

Total Summerville Formation-_______________-- 149. 1

The flat stratification and good sorting of grains in
the Summerville Formation indicate deposition in
clear water. The gradual increase in sand content up-
ward in the formation suggests deposition in a receding
sea. s

The contact of the Summerville Formation with the
overlying Junction Creek Sandstone is gradational
throughout the McElmo Canyon area. Sandstone and
mudstone of the Summerville Formation are inter-
bedded with sandstone similar to that of the overlying
Junction Creek in many places. The contact was chosen
where rocks lithologically similar to the Junction Creek
predominate above and rocks lithologically similar to
the Summerville predominate below.

JUNCTION CREEK SANDSTONE

The Junction Creek Sandstone was named by Gold-
man and Spencer (1941, p. 1750-1751) from an ex-
posure between Junction Creek and the Animas River
a few miles north of Durango, Colo. It correlates with
the Bluff Sandstone of Utah, Arizona, and New Mexico.

In the McElmo Canyon area the Junction Creek
Sandstone forms a conspicuous cliff, characteristically
a "slick rim," and its outcrops closely resemble those of
the Entrada Sandstone. Generally, however, the June-
tion Creek Sandstone is light red, reddish orange, or
pink, in contrast with the white to orange Entrada
Sandstone. In the vicinity of igneous intrusions, the
Junction Creek Sandstone is light brown. The forma-
tion averages about 280 feet in thickness and is divisible
into three gradational units based on the type of bed-
ding. The lower unit, 30 to 50 feet thick, is composed
of alternating beds of flat-stratified and cross-stratified
sandstone. The cross-strata dip at low angles and are
truncated by planar surfaces of erosion. A few of the
horizontal surfaces of erosion may be continuous across
all the exposures in McElmo Canyon. The middle unit,
150 to 250 feet thick, is composed of thick-bedded fine-
to coarse-grained sandstone that is cross-stratified at
high angles (nearly 30° in several exposures). The up-
per unit, 20 to 50 feet thick, is argillaceous fine-grained

 

reddish sandstone, commonly mottled or blotched green
and gray. This unit has obscure flat stratification and
weathers to hoodoos (fig. 3).

 

FiGur® 3.-Upper unit (Jj) of the Junction Creek Sandstone underlying
the Salt Wash Member of the Morrison Formation (Jms). Unit
weathers to hoodoos. Ute Peak, Mable Mountain, and North Black
Mountain appear in the background. Photograph courtesy of R. A.
Cadigan.

Subangular red and orange chert grains are common
in the lower and middle units. The chert commonly is
concentrated in single beds within a set of strata. The
angular chert grains contrast with the subrounded to
rounded quartz grains.

The following section was measured by Ekren on the
east side of Sand Creek in see. 24, T. 36 N., R. 18 W.

2B. Sand Creek section (pl. 1)

Salt Wash Member of the Morrison Formation.
Junction Creek Sandstone :
Upper unit :

Sandstone, argillaceous, light-red; weathers to
red with pink, green, and gray blotching;
obscure flat bedding; weathers to massive
hoodoos ; contains very fine grained quartz___ 30

Middle unit :

Sandstone, pink in variegated hues; weathers
darker pink to red brown; "slick-rim" weath-
ering; contains fine to medium fine-grained
pink-coated quartz grains, minor fine-grained
variegated chert, cross-stratified (strata near
top dip 30°) ; joints or fractures very sparse;
rock poorly cemented ; base gradational_____. 85

Sandstone, pink to orange; weathers darker pink
to reddish orange; medium fine to medium
grained, poorly sorted ; calcareous cement ; con-
tains clear subrounded quartz and sparse
medium- to coarse-grained subangular red,
gray, and orange chert; cross-stratified at
medium angles; "slick-rim" weathering; joint-
ing conspicuous in upper part ; joints "veined"
as described in unit below. A conspicuous
horizontal parting surface separates this unit
from unit

Feet

135

STRATIGRAPHY OF THE SEDIMENTARY ROCKS 13

2B. Sand Creek section (pl. 1)-Continued

Junction Creek Sandstone-Continued
Lower unit :

Sandstone, light-red to orange-pink; weathers
dark coral; fine to medium grained at base
becoming fine grained at top; alternating flat-
and cross-stratified beds; cross-strata trun-
cated by horizontal surfaces of erosion ; sand-
stone is soft and friable. Near the base of this
unit the sandstone contains more cement (cal-
careous?) and is greatly fractured ; joints are
vertical, horizontal, and at low angles, and are
filled with sandstone that is well cemented with
silica; filled joints protrude as "veins" from
weathered sandstone surfaces; joints die out
or become obscure in upper part of the unit____ 40

Feet

 

Total Junction Creek Sandstone__________ 290

The Junction Creek Sandstone of southwestern Colo-
rado has been correlated with the Bluff Sandstone of
southeastern Utah by Cadigan (in Craig and Cadigan,
1958, p. 182-185). Cadigan divided the Junction Creek
into four members on the basis of sedimentary struc-
tures. His basal member, "A," corresponds to the writ-
ers' lower unit; his "B" and "C" members correspond
to the writers' middle unit ; his "D" member to the upper
unit.

The lithology of the lower unit of the Junction Creek
Sandstone indicates alternating subaqueous and sub-
aerial deposition, which in turn suggests an oscillating
sea at the end of Summerville time.
reflects predominantly eolian deposition. Sand dunes
50 feet high were common during this period. The flat
stratification of the upper unit may indicate a return of
subaqueous conditions or deposition on a low flood plain
near sea level.

The contact with the overlying Salt Wash Member of
the Morrison Formation is locally disconformable. In
many places lenses of fluviatile sandstone of the Salt
Wash Member have been deposited in channels scoured
in the Junction Creek Sandstone by streams that re-
moved much, and in some places all, of the upper unit
of the Junction Creek Sandstone. In other places the
argillaceous sandstone of the upper unit grades into
mudstone and siltstone typical of the Salt Wash.

The irregular Junction Creek-Salt Wash contact sug-
gests that the earliest streams of Salt Wash time flowed
with sufficient velocity over a Junction Creek surface of
low relief to cause downcutting through the underlying
soft muddy sands. In places, stream deposition of sand
alternated with subaqueous (flood-plain ?) deposition of
argillaceous sand, mud, and silt.

MORRISON FORMATION

The Morrison Formation was named by Emmons,
Cross, and Eldridge (1896) from the town of Morrison,

The middle unit

 

near Denver, Colo., where the formation is about 200
feet thick and is composed of green or gray fresh-water
marls and thin beds of limestone and sandstone.

Throughout the Colorado Plateau the Morrison For-
mation is composed of stream and flood-plain deposits.
Sandstone and conglomeratic sandstone occur in ancient
stream channels, and mudstone on contiguous flood
plains. Fluviatile deposition prevailed during early
Morrison time and gradually gave way to predominant
flood-plain deposition during late Morrison time.

The Morrison of the Ute Mountains area has been
divided into four members. These are, in ascending
order: the Salt Wash Sandstone Member, Recapture
Shale Member, Westwater Canyon Sandstone Member,
and Brushy Basin Shale Member (Gilluly and Reeside,
1928; Gregory, 1938; and Lupton, 1914). A discussion
of the regional aspects of the Morrison is given by Craig
and others (1955, p. 134-159).

The lower three members-the Salt Wash, Recapture,
and Westwater Canyon-are composed predominantly
of lenticular light-tan and light-gray sandstone which
is interstratified with thin red or green siltstone and
claystone. The Brushy Basin, a bentonitic mudstone, is
mainly green, but is subordinately red, purple, and gray.

Because the upper two members, and also the lower
two, intertongue and intergrade to a considerable extent
field division is arbitrary in many places. The West-
water Canyon Member intertongues with the Brushy
Basin. The Westwater Canyon and the Recapture
Members do not intertongue, but their distinction is
extremely difficult and is based largely on the predom-
inance of red mudstone in the Recapture in contrast to
predominantly green mudstone in the Westwater Can-
yon. Where the Recapture is absent in eastern McElmo
Canyon, the Westwater Canyon possibly intertongues
with the upper lenses of the Salt Wash. The Salt Wash
Member intertongues and intergrades with the Recap-
ture to a considerable degree in most places.

The intertonguing and intergrading of members of
the Morrison Formation in the Ute Mountains area
are believed to reflect alternating deposition from two
different source areas. Sediments forming the Recap-
ture and Westwater Canyon Members were derived from
an area south of Gallup, N. Mex. (Craig and others,
1955, p. 150). Sediments forming the Salt Wash and
Brushy Basin probably were derived from a major
source area in west-central Arizona and southeastern
California. The lower three members of the Morrison
in the Ute Mountains area show considerable blending
of sediments from the two sources.

Despite the complexities of deposition during Morri-
son time and considerable variation in the thicknesses of
the individual members, the total thickness of the Mor-

14

rison is consistently between 500 and 650 feet. Where
one member is thin, generally, another is correspond-
ingly thick. The Brushy Basin Member and overlying
Burro Canyon Formation have a similar nearly con-
stant total thickness. This consistency in total thick-
ness is of value in determining structure below the
Morrison from data obtained on formations above the
Burro Canyon.

The assignment of a Late Jurassic age to ths Morri-
son in the Ute Mountains area is based on lithologic
correlation with the Morrison of other parts of the
Colorado Plateau where a Late Jurassic age seems in-
dicated by vertebrate remains (Stokes, 1944).

GEOLOGY, PETROLOGY, UTE

SALT WASH SANDSTONE MEMBER

The name Salt Wash Sandstone Member was first
proposed by Lupton (1914, p. 125-127) for a member of
the McElmo Formation. The name McElmo Formation
was later abandoned, and these rocks were divided into
the Morrison, Summerville, Entrada, and Carmel for-
mations; and the base of the Morrison was defined as
the base of the Salt Wash (Baker and others, 1927 ; Gil-
luly and Reeside, 1928).

In the Ute Mountains area, the Salt Wash Member
consists of interbedded sandstone and predominantly
red mudstone. It lies upon massive crossbedded sand-
stone of the Junction Creek and is overlain by the Re-
capture, Westwater Canyon, or Brushy Basin Members.
The sandstone lenses of Salt Wash range from white or
light gray to pale yellow or very pale brown. They are
trough cross-stratified and are composed of fine- to
medium fine-grained clear subangular to subrounded
quartz accompanied by sparse to common accessory
grains of black, white, red, or pink color. The sandstone
lenses are interbedded with dark-red, dark-purple, and
red-brown mudstone. Gray or green-gray mudstone is
extremely rare.

The Salt Wash Member is exposed continuously for a
distance of about 11 miles along the slopes of McEImo
Canyon. It is absent between the Junction Creek Sand-
stone and the Recapture Member on the structural dome
just south of Sentinel Peak in the Sentinel Peak NE
quadrangle, although some of the sandstone there
mapped as the Westwater Canyon Member is lithologic-
ally similar to the Salt Wash of McElmo Canyon and
may be a bed of the Salt Wash. The Salt Wash Mem-
ber is absent also in northeastern Arizona, a few miles
northwest of the Carrizo Mountains (L. C. Craig, oral
commun., 1956).

The thickness of the Salt Wash Member varies con-
siderably in McElmo Canyon. The member is 90 to
110 feet thick near the west end of the canyon, where
it is overlain by the Recapture Member. Farther east,

 

MOUNTAINS AREA, COLORADO

it is approximately 150 feet thick, and the Recapture
Member is absent. - Still farther east, at Trail Canyon,
the Salt Wash is 200 to 250 feet thick and the overlying
Westwater Canyon Member is thin.

The basal sandstone of the Salt Wash Member com-
monly is separated from the underlying Junction Creek
Sandstone by red-brown mudstone 3 to 50 feet thick.
Because this mudstone resembles the mudstone inter-
layered higher in the Salt Wash, it is included with the
member. The contact of the Salt Wash with the over-
lying Recapture Member, and locally with the West-
water Canyon Member, is difficult to determine. In
contrast to the Salt Wash, the sandstone beds of the
Recapture and Westwater Canyon Members contain
very sparse detrital grains of feldspar and generally
are interbedded with red and green mudstones, respec-
tively. The pink tone of the Recapture Shale Member
is distinctive and contrasts with the colors of the Salt
Wash and Westwater Canyon Members. In much of
western McElmo Canyon the contact between the Salt
Wash and the Recapture is marked by a narrow bench
formed on the top of the uppermost part of the Salt
Wash Sandstone Member. (See fig. 4.)

RECAPTURE SHALE MEMBER

The Recapture Shale Member was first named and
described by Gregory (1988, p. 58) from Recapture
Creek, southeastern Utah. In the Ute Mountains, it
is composed of interbedded sandstone and mudstone.
The sandstone beds are light gray, pinkish gray, or light
brown on fresh surfaces and weather to pinkish gray
or brownish gray. - The quartz grains are fine grained,
poorly sorted, and subangular to subrounded. Black,
orange, pink, and white accessory grains are common.
Detrital grains of feldspar are extremely sparse. The
mudstone beds are pale red, reddish green, and pale
green.

McElmo Canyon marks the approximate northern
limit of the Recapture Member in southwest Colorado
(Craig and others, 1955, p. 137-140). It is present
west of a north-south line through the junction of Sand
Creek with McElmo Creek. -In the lower McElmo Can-
yon section measured by L. C. Craig (written commun.,
1949), the Recapture is 117 feet thick. It is more than
260 feet thick south of the Ute Mountains where ex-
posed in a breached dome south of Sentinel Peak. In
this exposure the member contains more siltstone and
claystone than in McEIlmo Canyon.

The intertonguing and intergrading relations of the
Recapture with the overlying and underlying members
have been discussed. The Recapture is recognized
regionally in an oval-shaped area stretching from west
of Santa Fe, N. Mex., into southeastern Utah. In that

STRATIGRAPHY OF THE SEDIMENTARY ROCKS

15

 

FigUrE 4.-Salt Wash Sandstone Member (Jms), Recapture Shale Member (Jmr), and Westwater Canyon Sandstone Member (Jmw) of the Mor-

rison Formation viewed northward from State Highway 32 near the Karla Kay mine.

Creek Sandstone (Jj) is barely visible in lower part of photograph.
Member.

area, it has been divided into three facies (Craig and
others, 1955, p. 137-142): a conglomeratic sandstone
facies, a sandstone facies, and a claystone and sandstone
facies.. The conglomeratic sandstone facies and the
sandstone facies occupy the south-central part of the
oval area. The Ute Mountains area is on the north-
eastern edge of the claystone and sandstone facies.

WESTWATER CANYON SANDSTONE MEMBER

The Westwater Canyon Member (Gregory, 1938, p.
59) ranges from 40 to more than 200 feet in thickness
in the Ute Mountains area and averages about 100 feet.
Slightly more than 240 feet of the member was meas-
ured at the south end of the Ute Mountains. The sand-
stone beds in the Westwater Canyon are pale yellow
gray to light brown on fresh surfaces and weather to
light yellow gray or yellow. They are composed of
fine- to medium-grained subangular to subrounded
quartz and numerous white, red, or pink accessory min-
- erals, Detrital feldspar is sparse. The sandstone lenses
are cross-stratified channel-fills They form rounded
and irregular cliffs.

The Westwater Canyon Member intertongues and
intergrades with the mudstone beds of the Brushy
Basin. It is conformable with the Recapture and may
possibly intertongue with the Salt Wash. It is present

 

throughout McElmo Canyon and to the north in Trail

Upper hoodoo-weathering unit of the Junction
Note the topographic break at the top of the Salt Wash Sandstone

Canyon. The Westwater Canyon Sandstone Member
is exposed in the dome south of Sentinel Peak and in
Towaoe dome.

BRUSHY BASIN SHALE MEMBER

The uppermost member of the Morrison Formation,
the Brushy Basin Member, was first described by
Gregory (1938, p. 59) from the region around Brushy
Basin, Utah. It is composed of bentonitic varicolored
mudstone that forms slopes and becomes hard and
frothy appearing where weathered. The frothy appear-
ance is a result of swelling and subsequent drying of
bentonite in the mudstone. Swelling muds are not
known in the lower members of the Morrison nor in the
overlying Burro Canyon Formation. Moderately per-
sistent interbeds of thin resistant, very fine grained
silicified sandstone and siltstone are common in the
Brushy Basin Member. Conglomerate and conglom-
eratic sandstone are uncommon, although in many
places green friable conglomeratic sandstone about 20
feet thick occurs near the middle of the member. Local-
ly, where the Brushy Basin is thin and the Burro Can-
yon is thick, the conglomerate is near the top of the
member. The conglomerate contains many fragments
of igneous rock.

The lower part of the Brushy Basin intertongues with
the Westwater Canyon Sandstone Member in places, but

16 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

in others the contact appears to be a nearly flat surface
for long distances. The base is placed at the top of the
highest sandstone of Westwater Canyon lithology.

The upper part of the Brushy Basin intertongues in
many places with the Burro Canyon Formation (Ekren
and Houser, 19592). Localities where intertonguing is
most pronounced are generalized on the geologic map
(pl. 1).

In many areas the top of the Brushy Basin is iden-
tified by a change from bentonitic to nonbentonitic mud-
stone. Locally, the two mudstones are separated by a
ledge of silicified siltstone about 1 foot thick, partic-
ularly in exposures southwest of the Ute Mountains.
In other areas, bentonitic mudstone typical of the
Brushy Basin is overlain directly by conglomeratic
sandstone or conglomerate typical of the Burro Canyon.
In still other areas, intertonguing is extensive. Most
commonly, sandstone and conglomerate typical of the
Burro Canyon are interbedded with mudstone typical
of the Brushy Basin, but interbedding of bentonitic and
nonbentonitic mudstone has been observed in western
McElmo Canyon.

The Brushy Basin forms slopes throughout McElmo
Canyon and its tributary canyons in the Mogui SW
quadrangle west of the Ute Mountains. It ranges in
thickness from 150 to 300 feet.

The following stratigraphic sections are representa-
tive of the Morrison Formation in the Ute Mountains
area. The section in lower McElmo Canyon was meas-
ured by L. C. Craig near Tozer Gulch (pl. 1) where the
Recapture Member is present in see. 32, T. 36 N., R.
18 W. The Trail Canyon section was measured by
Houser and Ekren where the Recapture Member is ab-
sent in secs. 34 and 27, T. 36 N., R. 17 W.

4. Lower McBEimo Canyon section C of Craig (pl. 1)

Morrison Formation.
Brushy Basin Member : Feet
Top contact not well exposed.
Interval covered by debris from above except for
2 ft of grayish-red claystone at base; float in-
dicates of white to cream claystone above.
W. D. Keller reports partially devitrified ash
shards from a sample a few feet from top of
UNI 30.9
Claystone, grayish yellow-green (5GY 7/2) to
dusky yellow-green, slightly silty to very fine
grained, sandy, hard and dense; forms two
beds separated by bright-green clay parting-__ 2.0
Claystone, very pale orange, white, and very
pale green, silty to fine-grained sandy,
bentonitic __-- aam Ra Fes _ 82.4
Claystone, same as in unit below; contains sev-
eral beds of dusky yellow-green siliceous clay-
stone, as much as 1 ft thick, consisting of
dense noncalcareous matrix and disseminated
fine-grained. lc 15. 2

 

 

Claystone, very pale orange to cream, silty to
slightly sandy, bentonitic; bright orange-red
spots suggest incipient chert development ; sam-
ple of siltstone from this unit reported by W.
D. Keller to be very fine grained silica, a cherty
rock which may be a secondary product from
hydrolysis of volcanic ash________________--

Sandstone, very pale orange (10¥R 8/2) to pale-
green (5G 7/2) ; weathers very pale green to
light brown ; fine to medium fine grained ; con-
tains subangular to rounded clear quartz and
numerous pink, orange, black, green, and red
accessory minerals; forms several slabby
Tedlges~ .. . L... n cha ce n eee ole ao mele ae t ao Ie Sa e he heme ane hale

Claystone, very pale orange to grayish-orange
pink, silty to medium fine-grained, sandy,
bentonitic ; considerable slumping-____________

Claystone, very pale orange (10YR 8/2) to gray-
ish orange-pink (5YR 7/2), silty to medium
fine-grained, sandy, bentonitic. In top 2 to 3
ft of unit are slabby ledges of fine- to medium
fine-grained sandstone; contains numerous
pink and red accessory minerals and a few
chlorite .._. L .. Looe ooc eer

4. Lower McEimo Canyon section C of Craig (pl. 1)-Con.
Morrison Formation-Continued
Brushy Basin Member-Continued

Feet

52.0

6. 0

27. 0

32. 4

 

Total Brushy Basin Member-________----- 197.9

 

 

Westwater Canyon Member :

Sandstone, white to very light gray; weathers
light brown ; fine to medium fine grained ; con-
tains subangular clear quartz and minor pink
and black accessory minerals ; channeled, cross
laminated; ledge lenses out laterally from
section

Claystone, pale-green and greenish-gray, very
sandy; float indicates; bed of bright-green
hard slabby calcareous siltstone near top.
Interval poorly exposed_......_______________

Sandstone, pinkish-gray (5Y¥R 8/1) to yellowish-
gray (5Y¥ 8/1) ; weathers yellow brown to light
brown ; fine to medium fine grained ; contains
subangular to subrounded clear quartz and
numerous pink, red, and white accessory min-
erals; channeled, cross laminated____________

Claystone and sandstone; claystone, light green-
ish gray (5G 8/1) and light greenish gray
(5GY¥ 8/1) with vague mottling of pale red;
silty to sandy, grading to nonresistant very fine
to medium fine-grained clayey sandstone; this
unit is the lowest predominantly green unit__.

Sandstone, white to light greenish-gray;
weathers pale to light brown ; fine to very fine
grained; contains clear subangular quartz,
abundant black, pink, and red and green acces-
sory minerals, granules of light chert, and a
few feldspar grains; channeled, cross lami-
nated; ledge thickens 300 ft to east to form
cliff

8.5

17.9

9.1

 

Total Westwater Canyon Member_________.

79. 0

 

 

STRATIGRAPHY OF THE SEDIMENTARY ROCKS 17

4. Lower McEimo Canyon section C of Craig (pl. 1)-Con.
Morrison Formation-Continued

Recapture Member : Feet
Claystone, pale-red (5R 6/2) mottled to pale-
green, silty to medium fine-grained, sandy.

Interval very poorly exposed_________________ 28.1

Sandstone, pale-yellow; weathers light brown;
medium fine to fine grained; composed of sub-
rounded to well-rounded clear quartz and
numerous red and pink accessory minerals;
forms rounded structureless ledge____________ 2. 8

Claystone, variably sandy, pale- to grayish-red,
mottled pale-green, bentonitic. Sand is sub-
rounded and reaches medium-fine size________ 4. 4

Sandstone, very light gray to white and pale-yel-
low, very fine to medium fine-grained, poorly
sorted; contains clear subangular to sub-
rounded quartz and abundant black and minor
pink, orange, and white accessory minerals;
in irregular structureless beds with worm bur-

 

rows... 6.6
Claystone, pale-red (10R 6/2) to grayish-red

(10R 4/2), silty to somewhat sandy, slightly

bentonitic rms 8. 4

 

Sandstone, white to pale-yellow, very fine to fine-
grained ; contains clear quartz and minor pink,
black, and orange accessory minerals; chan-
neled, faintly cross laminated________________ 5.9
Claystone (70 percent) and sandstone. Clay-
stone is silty to sandy, grayish red (10R 4/2)
to pale green; sandstone weathers pale red to
greenish brown. Unit very fine to fine grained ;
forms even, continuous beds; lower two-thirds
poorly exposed
Sandstone, pinkish-gray, light-brown weathering,
fine to medium fine-grained; commonly con-
tains pink, black, and white accessory min-
erals; weathers to rounded ledge of irregular
beds_..____ a 2.8

Sandstone, clayey, pale-red to dark-red to green,
nonresistant; interval very poorly exposed___ - 3.4
Sandstone, pinkish-gray (5YR 8/1), fine-grained ;
composed of clear subangular quartz and, com-
monly, pink, black, and white accessory min-
erals; channeled, cross laminated and ripple
laminated ; some worm burrows. Unit thickens
laterally from section and fills scours that cut
through underlying unit____________________

Claystone and sandstone. Claystone grayish red
(10R 4/2), slightly silty, flaky earthy weather-
ing. Sandstone very light gray to pale yellow,
very fine to fine grained ; contains subangular
clear quartz and many colored accessory min-
erals; horizontally laminated, platy bedded ;
forms 2 ft bed in middle of unit______________ 6.7

31.6

 

 

Total Recapture Member__________________ 117.1
up
Salt Wash Member :
Sandstone, white to pale-yellow, with limonite
speckling; weathers pale brown; fine to me-
dium fine grained ; contains subangular to sub-

 

4. Lower McEimo Canyon section C of Craig (pl. 1)-Con.
Morrison Formation-Continued

Salt Wash Member-Continued
rounded clear quartz and commonly black,
white, red, and pink accessory minerals; chan-
neled, cross laminated ; contains sandy to silty
red claystone parting as much as 6 ft thick off
line of section

Interval covered except for upper 7 ft which is
interbedded claystone and sandstone; clay-
stone dark red and green, sandy to silty ; sand-
stone yellowish gray, brown-weathering very
fine to fine grained ; contains clear quartz and
pink, black, and green accessory minerals; ir-
regular but continuous beds ; float of claystone
covers lower part of unit____________________

Claystone and minor sandstone. Claystone
sandy, dark red below to dark purple above.
Sandstone yellowish gray (5Y¥ 7/2), very fine
grained ; contains clear subangular quartz and
numerous colored accessory minerals; sand-
stone forms massive, structureless ledge 1.3 ft
or more thick at top of unit_________________

Sandstone, white to pale-yellow, light-brown
weathering, very fine to fine grained; con-
tains clear subangular quartz and abundant
black, dark-gray, and orange to pink accessory
minerals; channeled, cross-laminated ; bed ap-
pears to lens out laterally from section__..____

Claystone and sandstone, interbedded. Clay-
stone sandy to silty, dark red to purplish red,
calcareous; sandstone greenish gray to light
purplish gray, very fine to medium fine grained,
finely banded. Unit forms shaly nonresistant
reentrant; bottom contact is gradational, top
contact sharp. Formation assignment de-
batable. t aan a ne on ae Eee Sede ae ie ae m Sil ale wee ot ie ae wae 3.8

Feet

87.6

29. 2

18. 1

17.2

 

 

Total Salt Wash Member_________________ 100. 9

 

 

Total Morrison Formation-_______________ 494.9

5. Trail Canyon section (pl. 1)

Morrison Formation :
Brushy Basin Member (incomplete) :
Mudstone, green. Total incomplete Brushy
Bagin Member... Le 22

 

 

Westwater Canyon(?) Member :

Sandstone, pale yellow-gray to light-yellow, fine-
grained; composed of subrounded quartz,
opaque white clay(?) grains (5 percent) ;
soft, friable; upper 2 ft is quartzite; forms
ledge; joints strike N. 40° W., dip 80° SW.___ 34

Mudstone, green, mostly covered ______________ 19

 

Total Westwater Canyon(?) Member-______ 53

 

 

Salt Wash Member :
Sandstone, - light-gray, fine-grained, friable,
quartzose; calcareous cement; abundant fine
SPOTS. _.. _. ~ coon noches .s T

18

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

5. Trail Canyon section (pl. 1)-Continued

Morrison Formation-Continued

Salt Wash Member-Continued

Sandstone, light-gray, yellow-gray weathering;
fine-grained; calcareous cement; quartzite in
basal 1.5-2.0 ft grades upward to soft, friable
sandstone; contains abundant limonite and
traces of very fine opaque white grains ; 0.2-ft
green flat-bedded mudstone at top-__________-

Sandstone; grayish tan, fine grained, and thinly
flat bedded at base; grades to light yellow
gray, fine to medium fine grained at top ; firmly
cemented at base to weakly cemented at top;
limonitic, very sparse biotite(?) fragments at
base; very sparse pale-orange grains present
throughout ; thin green mudstone at top-____-

Covered; believed - predominantly - siltstone,
clayey; contains a few thin layers of hard
gilicified sandstone._..LcL_c._c__L__LL___L_____

Sandstone, 1light-gray, limonite-spotted, pale-
brown-weathering ; contains fine-grained
quartz, opaque white clay grains, sparse, very
fine rounded pale-orange grains, interstitial
black MnO:( ?) flakes, very sparse black opaque
grains, angular opaque white to pale-green
clay grains, and green and  pale-green
graing :: thick bedded

Sandstone, light-gray to white, limonite-stained,
fine- to medium fine-grained; contains sub-
rounded clear quartz; commonly contains
white opaque fine angular clay grains, sparse
to common very fine rounded pale-orange
grains, green mud pellets and clay galls, very
sparse fine angular green grains, sparse pink
quartz grains, interstitial light-gray to white
clay, numerous green mudstone partings, lim-
onite flakes, and calcareous cement; joints are
N. 55° W.

Sandstone, light-gray to white, pink-weathering,
very fine to fine-grained ; contains sparse, very
fine orange grains; weathers to rounded nobu-
lar hoodoos; thin (1-2 ft) green mudstone
SE EOD st E Eas, 1 cen, ae ae te at see ie o eee Ne he ht ue a et uy he oe mre h frt mn te oe

Siltstone and sandstone, very fine grained,
partly ~ COVYered -_ cone cece rue eo oe erea nen a
Sandstone, light-gray, fine- to medium fine-
grained; contains subrounded quartz grains
and sparse subrounded orange and pink grains ;
calcareous cement; friable; has interstitial
mudstone ; calcite-filled joint trends N. 50° W..

Sandstone, light-gray, fine-grained; contains
subrounded to subangular quartz grains and
sparse very fine orange grains (feldspars?),
calcareous cement; current lineations N. 15°
E.; crossbedded, lenticular; fills channel in
mudstonc of unit 1.

Mudstone, red, purple, reddish-gray, sandy____-

Feet

11

12

12

48

15

16

16

16
42

 

Total Salt Wash Member___________________

Base of Morrison Formation.
Top of Junction Creek Sandstone.

201

 

CRETACEOUS SYSTEM

BURRO CANYON FORMATION

In the Ute Mountains area, the Burro Canyon Forma-
tion (Stokes and Phoenix, 1948) ranges from 30 to 200
feet in thickness and consists of green, predominantly
nonbentonitic mudstone interbedded with lenses of con-
glomerate and conglomeratic sandstone that vary con-
siderably in number and thickness. Red mudstone oc-
curs sporadically in the lower part of the formation.
The nonbentonitic mudstone weathers hackly or fissile.

The conglomerate and conglomeratic sandstone are
commonly white or light gray. They contain pebbles
and granules of colored chert, silicified limestone, and
siltstone in a matrix of quartz sand. Fragments of
petrified wood and silicified dinosaur bone are common
in the lowermost lenses. Carbonized plant remains are
extremely rare. The conglomerate lenses in the upper
part of the formation are more numerous, finer grained,
and blanket larger areas than those in the lower part.

The lowest unit in the Burro Canyon Formation, a
system of shoestring lenses of conglomerate and con-
glomeratic sandstone, has been named the Karla Kay
Conglomerate Member from an exposure at the Karla
Kay mine in McElmo Canyon (Ekren and Houser,
1959a). The Karla Kay is highly resistant to weather-
ing, and forms vertical cliffs and dark-brown knobby
outcrops. The shoestring lenses or channel fillings are
rarely more than 2,000 feet wide or more than 65 feet
thick; commonly they are 500 to 800 feet wide. The
channel fillings have been undercut in many areas, and
their former locations are marked by huge, resistant,
dark blocks-many as large as a house-that have
slumped down the slopes of soft underlying mudstone.
Where the Karla Kay Conglomerate Member is absent,
the base of the Burro Canyon is marked either by hack-
ly-weathering mudstone or by fine-pebble and granule
conglomerate or sandstone. These rocks, although ba-
sal, are stratigraphically younger than the Karla Kay.

Stratigraphic sections of the Burro Canyon follow.
Section 6 was measured by Ekren in see. 2, T. 35 N., R.
19 W., and sec. 35, T. 36 N., R. 19 W., about half a mile

_ north of Colorado Highway 32. Section 7 was measured

by Houser and Ekren in see. 30, T. 36 N., R. 18 W., on
the northwest side of Tozer Gulch, about three-quarters
of a mile west-northwest of the Karla Kay mine.

6. Wood Chuck section (pl. 1)

Dakota Sandstone.

Disconformity.
Burro Canyon Formation :
Sandstone, conglomeratic, white, weathering

white; contains granules and pebbles of white
chert, sparse red chert; grades to very fine to

STRATIGRAPHY OF THE

6. Wood Chuck section (pl. 1)-Continued

Burro Canyon Formation-Continued

medium grained sandstone at top; massive
weathering; forms a

Mudstone, green-gray ; weathers hackly and fissile
without significant swelling_________________

Sandstone, conglomeratic, palé-brown to white;
weathers buff and brown; cross-stratified (fes-
toon type) ; pebbles and granules mostly col-
ored chert as much as % in. in diameter; white
chert common

Feet

TO. 0

50. 0

20. 0

 

 

Total Burro Canyon Formation-___________ 140. 0

7. Tozer section (pl. 1., section 7)

Dakota Sandstone (not measured).
Erosional disconformity, relief of about 2 feet.
Burro Canyon Formation :

Sandstone, white, fine-grained; carbonate ce-
ment; contains subangular quartz____________

Mudstone, green; weathers hackly, without sig-
nificant swelling

Sandstone and conglomerate; sandstone light tan
gray, medium- to coarse-grained, interbedded
with conglomerate (granules to pebbles as
much as 1% in. in diameter) ; pebbles consist
mostly of chert and some quartzite and silicified
limestone; unit forms rounded, blocky, verti-
cal cliff____

Mudstone, pale-green; weathers hackly, with-
out significant

Mudstone, pale-green and red, well-silicified ;
forms resistant

Mudstone and sandstone, interbedded ; sandstone,
very light gray, fine-grained ; forms thin ledges,
predominates in lower one-third and decreases
upward, poorly exposed; mudstone, very light
gray, silty; weathers hackly, without signifi-
Cant SWElNng _ _-... ... o ce ces n e onal once eae

Karla Kay Conglomerate Member :

Conglomerate, very pale tan-gray to pale grayish-
green; weathers dark brown and black ; firmly
cemented ; subrounded pebbles range from % to
2%4 in. in diameter, averaging about 1 in., peb-
bles consist mostly of brown, gray, blue-gray,
green-gray, and red-brown chert; matrix of
coarse-grained clayey sandstone; conglomerate
and sandstone silicified somewhat differen-
tially but generally along bedding; unit cross-
stratified (festoon type), with troughs plung-
ing eastward; forms massive cliff with very
knobby surface due to pebbles; dinosaur bones
and silicified trees common. Unit occurs as
nonpersistent lenses about 600 ft wide. Total
Karla Kay Conglomerate Member-___________

2. 0

15.5

32. 0

 

46. 0

2. 0

64. 0

 

Total Burro Canyon Formation-__________ 206. 0

The amount of conglomerate and sandstone in the
Burro Canyon Formation decreases southward within
the map area, and in many exposures in the south the
Burro Canyon is entirely mudstone. Just south of

745-807 O-65--4

 

SEDIMENTARY ROCKS 19

Sentinel Peak (the "East Toe" of the Ute Mountains),
about 3 miles southwest of Towaoe, Colo., the Burro
Canyon consists of approximately 30 feet of hackly
weathering mudstone. Farther south, in the Carrizo
Mountains of New Mexico and Arizona, the upper part
of the Morrison Formation as mapped by Strobell
(1956) contains lenses of conglomeratic sandstone that
may be equivalent to those of the Burro Canyon of
southwestern Colorado, although no mudstone typical
of the Burro Canyon is present.

The top of the Burro Canyon Formation is an ero-
sional disconformity observed throughout the Ute
Mountains area. The relief on this surface ranges from
a few inches to more than 40 feet within a distance of
300 feet. White quartzite, sandstone, or less commonly,
mudstone of the Burro Canyon Formation are found at
this erosion surface in different parts of the area.

The erosional disconformity has been observed in
other areas of the Colorado Plateau (Carter, 1957), but
in places in the Slick Rock district in southwestern Colo-
rado approximately 30 miles northeast of the Ute Moun-
tains the Dakota Sandstone rests conformably upon the
Burro Canyon (Simmons, 1957).

The change in the Burro Canyon southward within
the Ute Mountains area suggests that this area may
have been near the southern or southeastern edge of the
area of deposition of coarse clastic material during
Burro Canyon time. In the Four Corners area of New
Mexico and Arizona, thin discontinuous lenses of con-
glomeratic sandstone typical of the Burro Canyon
Formation are interbedded with mudstone typical of
the Brushy Basin, and within a few feet of the Dakota
Sandstone (stratigraphic section 8). The thinness or
absence of the Burro Canyon in this region is probably
due primarily to pre-Dakota erosion but may be due,
in part, to a facies change southward from the Ute
Mountains area. Farther south the Dakota lies on suc-
cessively older formations (Craig and others, 1955, p.
161).

Brown (1950), Katich (1951), Stokes (1952), and
Simmons (1957) have presented evidence that the Burro
Canyon, or its equivalent in central Utah, the Cedar
Mountain Formation (Stokes, 1944, 1952), is of Early
Cretaceous (Aptian) age. Simmons (1957) reports that
invertebrate fossils in upper beds of the Burro Canyon
in the Slick Rock mining district were identified as
Early Cretaceous. The fossils correlate with an Early
Cretaceous fauna in Wyoming or Montana which in turn
contains fossils found in the Cedar Mountain Forma-
tion of Utah.

Little, if any, break in deposition occurred between
the Morrison and Burro Canyon Formations in the
Ute Mountains area. The complete lack of fossils

20 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

in the vicinity of the contact prevents precise deter-
mination of the boundary between the Jurassic and
Cretaceous Systems.

DAKOTA SANDSTONE

The Dakota Sandstone of Late Cretaceous age ranges
in thickness from 95 to 140 feet but averages 125 feet in
most of the Ute Mountains area. It is composed of
yellow-gray and tan carbonaceous sandstone of fluvial
origin in the lower part and yellow-brown and yellow-
gray sandstone of fluvial and beach origin in the upper
part. These are interbedded with gray to dark-gray
carbonaceous mudstone and coal that were probably de-
posited in a swamp or tidal-flat environment. The coal
beds are not persistent and range from a few inches to
3 feet in thickness. Sandstone cemented with iron
oxide is common in the upper part of the formation.

Two complete stratigraphic sections of the Dakota,
measured southwest of the mountains, follow. Section
8 was measured in sec. 6, T. 34 N., R. 18 W.; section 9 in
sec. 10, T. 331% N., R. 19 W. Both sections were meas-
ured by J. H. Irwin, Houser, and Ekren.

8. Yucca section (pl. 1)

Block rubble.
Unconformity.
Dakota Sandstone :

Mudstone and coal, gray to black, very thin bedded.
Unit forms a smooth, covered slope ; it is carbona-
ceous mudstone at base and grades to coal at top;
PASC SHATD.. .... coo ou eo loon 8.5

Sandstone, light-gray, yellowish-gray-weathering,
very fine grained ; contains subangular quartz, rare
black accessory minerals, firm calcareous cement;
grades to siltstone; flat, thin bedded; weathers
slabby, forms a ledge; contains carbonized plant
fragments, and ironstone concretion zones which
are parallel to the bedding planes; base sharp-___- 1.0

Sandstone, pale yellow-gray to yellowish-brown, fine-
to very fine-grained, well-sorted; contains sub-
rounded to subangular frosted quartz, abundant
argillaceous material (accessories obscured by iron
stains), firm calcareous and ferruginous cement;
wedge-plane crossbedding; weathers blocky, forms
a ledge; bedding contorted; contains sparse plant
fragments and a few ironstone concretions which
weather out as spheroidal nodules; base sharp-__ 5.0

Sandstone, pale brownish-gray; weathers yellowish
brown and yellowish gray ; medium to fine grained ;
contains quartz and, at intervals, abundant argil-
laceous material (accessories obscured by iron
stain) ; firmly cemented, calcareous and limonitic;
flat, thin-bedded ; weathers flaggy, forms a ledge;
contains abundant films of carbonaceous material
and epigenetic seams of calcite along horizontal
fractures: base 2. 5

Mudstone, very dark gray to black, fissile to thin-
bedded ; forms a regular slope; unit is very carbo-
naceous; contains ironstone concretions at the top
in a zone as much as 8 in. thick; base sharp-______

Feet

10. 0

 

8. Yucca section (pl. 1)-Continued
Dakota Sandstone-Continued
Sandstone, dark-gray, yellowish-gray weathering;
very fine grained; grades to siltstone; well-sorted ;
composed of subangular quartz and argillaceous
material; firm calcareous cement; very thin
bedded ; weathers rounded, forms a ledge; contains
sparse limonite and abundant plant fragments ; base
sharp hake
Mudstone, light- to medium-gray, thinly to very thinly
laminated; forms a regular slope; unit is car-
bonaceous ; upper 3 ft very impure coal containing
iimonite
Ironstone and autochthonous chert zone, dark-red,
sandy; base gradational-_______________________ 1.0
Sandstone, yellowish-gray, yellowish-brown ; weather-
ing very fine grained, fairly well sorted ; grades to
siltstone; composed of frosted quartz; poorly ce-
mented, limonitic ; thinly cross laminated (rib and
furrow); forms a ledge; contains abundant cal-
careous films and limonite stains, spots, and
streaks; base gradational__.._.___.___:_________ 3.5
Mudstone, silty, dark-gray; indistinct bedding;
weathers hackly ; forms a slope ; base gradational__ _ 3.0
Coal, impure
Mudstone, silty, dark-gray; indistinct bedding____~
Sandstone, light-gray ; weathers pale yellowish gray ;
medium- to fine-grained, fairly well sorted; con-
tains subrounded frosted quartz, some minor argil-
laceous material, firm calcareous cement; thin bed-
ded planar crossbedding; weathers smooth to
slabby, forms a ledge; contains some powdery cal-
cite and limonite flecks on fresh fracture_______-
Sandstone, light-gray, medium- to fine-grained ; con-
tains subrounded quartz, firm calcareous cement;
thin bedded ; weathers knobby, forms a slope; con-
tains common iron stains and abundant carbonace-
ous material; 1-ft-thick dark-gray carbonaceous
mudstone at top; base sharp-____________________ 6.0
Carbonaceous mudstone, dark-gray, thinly laminated ;
weathers fissile, forms irregular slope; base grada-
MONALE . 22 ne cocco o ee ean el n a ol n i neal ie malo mean med 1.5
Siltstone (contains some claystone), gray, poorly
sorted, thin-bedded; weathers blocky, forms a
slope; contains abundant carbonaceous material
and sparse limonite; base gradational____________
Mudstone (silty with minor sand grains), dark- to
medium-gray ; fissile bedding; forms an irregular
slope; unit very carbonaceous at base, becomes
sparsely carbonaceous toward top; base grada-
HORRL ECE REEL oleae ene leno anno eben be eames
Sandstone, medium-gray; weathers pale yellowish
gray ; medium- to very fine-grained ; contains some
siltstone, poorly sorted and subrounded frosted
quartz, rare black and orange accessory minerals;
argillaceous material common; flat, thin bedded;
weathers smooth, forms a ledge. This unit is dis-
continuous; base sharp.._.__.___________________ 1.0
Mudstohe, very sandy ; grades to muddy sandstone;
light gray, weathers mottled gray and yellow;
medium to very fine grained to silt size, poorly
sorted, weakly cemented, probably calcareous ; flat,
thin to very thin bedded ; forms a slope; contains
limonite stains and some clay-size matrix material

Feet

8.5

 

17.0

 

3. 0

1.0

15. 0

 

STRATIGRAPHY OF THE SEDIMENTARY ROCKS

8. Yucca section (pl. 1)-Continued

Dakota Sandstone-Continued

which resembles Brushy Basin Member of Mor-
rison. This unit grades to sandstone horizontally
in both directions of exposure; base flat_________-
Sandstone, white to very light gray, pale-yellow-
weathering; very fine grained to silt size, fairly
well sorted ; contains frosted quartz, rare to com-
mon black, orange, and green accessory minerals
and argillaceous material; flat, thin bedded;
weathers smooth, forms a ledge; contains medium-
grained quartz "berries" and a few limonite specks ;
Daso: SHATD--

Feet

13. 0

5.0

 

'Total Dakota 101. 0

Top of Burro Canyon Formation.

9. Kiva section (pl. 1)

Mancos Shale (not measured) :

Sandstone, yellow, soft, friable.

Dakota Sandstone :

Sandstone, pale yellowish-gray, tan-weathering ; fine
to very fine grained, sugary, well sorted ; contains
subrounded frosted quartz, black accessory mineral
grains, weak limonite cement; unit is a coset com-
posed mainly of flat beds, some wedge-planar-type
crossbeds, and some concave low-angle crossbeds;
weathers knobby to slabby, forms a ledge; contains
limonite stains; base sharp______________________

Mudstone and impure coal, medium-gray to black,
flatbedded; weathers hackly, forms an irregular
slope; contains limonite pockets; base sharp_____.

Sandstone, pale yellowish-gray, tan-weathering; fine
grained, sugary, well sorted; composed of sub-
rounded quartz; weakly cemented (iron) ; unit is a
coset composed of flat thin to thick beds, wedge-
planar-type crossbeds, and some concave low-angle
crossbeds; weathers pitty and slabby, forms a
ledge ; contains rib and furrow and secondary silica
overgrowths; base sharp-..___._.________________

Mudstone, dark-gray, flat-bedded; weathers fissile;
mostly a covered interval; contains abundant car-
bonaceous material and some limonite; base
SEL TD 2 + - 2 e 2 e e + 2 alee aa eee e ue n o an ue He He He a nt mee te at aloe s at tee ae te te take

Sandstone, gray, weathers light-gray ; abundant yel-
low limonite staining; carbonaceous, fine to very
fine grained, well sorted; contains subrounded to
subangular quartz; thick bedded; weathers mas-
sive and blocky, forms an irregular ledge; contains
pitted surface marks, abundant carbonized plant
remains, and, at top of the unit, a 3-ft bed of impure
coaly material and hackly-weathering carbona-
ceous mudstone; contains pale greenish-yellow
material} base ShAPD- -L. l,

Mudstone, medium-gray to black to yellowish-gray,
carbonaceous ; weathers hackly, forms a slope ; con-
tains limonite stains and beds of impure coal ; base
SRATD-_-. .... tits cease as

Ironstone, dark-red, flat-bedded, cherty; weathers
blocky to knobby, forms an irregular ledge; con-
tains abundant hematite and limonite; base
sharp... . ceeds on ene a n CC LLL...

Mudstone, same as mudstone unit above____________

4.5

4. 0

8.0

5.5

13. 0

11.0

1.0
15. 5

 

9. Kiva section (pl. 1)-Continued

Dakota Sandstone-Continued

Sandstone, light yellowish-gray, buff-weathering;
medium grained, well sorted; contains rounded,
frosted quartz, black accessory minerals; weakly
cemented by limonite(?) ; unit is a coset contain-
ing flat thin beds, and wedge-planar crossbeds, and
small-scale crossbeds; weathers smooth to blocky,
forms a ledge ; contains masses of rounded medium-
grained sandstone, firmly cemented with calcite;
base ... _._

Conglomerate, pale yellowish-gray. Matrix is fine-
grained sandstone composed of subrounded to sub-
angular frosted quartz, firmly cemented. Pebbles
are quartzite, red chert. Unit is a coset containing
flat thin beds, wedge-planar-type crossbeds and
low-angle small-scale concave crossbeds; weathers
smooth, forms a ledge; unit is short channel or
lens, thickness reaches 6 in. a short distance on
either side of the point measured ; coarser material
concentrated along the bounding surfaces of cross-
bedded sets; base irregular and gradational_____._

Sandstone, pale-yellow, medium-grained, well-sorted ;
contains subrounded frosted quartz, quartz over-
growths; weakly cemented (calcareous and limo-
nitic); wedge- and tabular-planar crossbeds and
concave low-angle small-scale crossbeds; weathers
blocky to knobby, forms an irregular ledge; thick-
ness varies considerably laterally; base grada-
LeeLee eo e o EL OCU Emas a aim ulm aera

Sandstone, gray to brownish-gray; weathers to dark
brownish-gray; medium grained, fairly well to
poorly sorted ; contains rounded quartz, red, orange,
and sparse green chert, firm cement (calcareous),
flat bedding; weathers blocky, forms an irregular
ledge ; contains calcite overgrowth ; unit grades up-
ward into overlying unit; base sharp-___________-

Mudstone (mostly covered), carbonaceous, dark-gray,
flat, thinly laminated; weathers fissile, forms a
slope; base concealed

Sandstone and rare short conglomeratic beds, white,
light-gray-weathering, medium-grained, well-
sorted; contains subangular sugary quartz, rare
black accessory minerals, weak cement ; wedge- and
tabular-planar crossbeds and low-angle small-scale
crossbeds; weathers rounded to pitty to knobby,
forms an irregular ledge; contains abundant limo-
nite at base; flashes in sunlight owing to quartz
overgrowths; base gradational__________________

Sandstone, light-gray, pale yellowish-gray-weather-
ing; medium to fine grained, well sorted ; contains
rounded frosted quartz, rare black accessory min-
erals, firm cement (calcareous?) ; flat, thin beds
(rib and furrow) ; weathers blocky, forms a ledge;
contains abundant limonite spots and stains; base
SHAD |- 25.2.2 009 20 a o se oes ce te aer te wa in e an be ment us as tt l s me oe in e an orn ie

Mudstone, carbonaceous, dark-gray, flat-bedded;
weathers fissile, forms a slope; contains a 6-in.
flaggy gray beige-weathering sandstone about 1 ft
from base; composed of angular quartz, abundant
limonite cement; contains a few iron concretions;
base sharp. Nes

21

Feet

28. 0

4. 0

7.0

3.0

4. 0

10. 0

3. 0

4.5

22

GEOLOGY, PETROLOGY, UTE

9. Kiva section (pl. 1)-Continued

Dakota Sandstone-Continued

Sandstone, very pale yellow, light grayish-yellow,
weathering, fine-grained, sugary, well-sorted ; con-
tains subangular, frosted quartz, weak cement (cal-
careous) ; unit is a coset composed of flat thick-
bedded sets, wedge-planar crossbedded sets, and
low-angle crossbeds; weathers smooth to blocky,
forms a ledge; desert varnish on resistant bedding
surfaces; contains limonite spots and streaks;
quartz flashes in sunlight owing to secondary
quarts overgrow ths...

Sandstone and short conglomeratic units, light-gray,
coarse- to medium-grained, poorly sorted ; contains
rounded to subrounded frosted quartz, pebbles of
quartzite, red chert, and clay; firmly cemented
(calcareous?) ; wedge-planar type crossbedding
(concave, low- to medium-angle, medium-scale
crossbeds) ; weathers blocky, forms a ledge; con-
tains limonite stains and some argillaceous mate-
rial; base gradational

 

Conglomerate, mottled gray and red; weathers light
gray. Matrix is coarse-grained quartz sandstone.
Pebbles in unit comprise quartzite, clay fragments,
chert, and siliceous limestone(?). Unit weathers
blocky, forms a ledge; contains a 2-in. sandstone
layer at the base which is gray, medium grained,
and well sorted and contains abundant carbon and
limonite: ba§e SRATD... . -. . ooo eo esa

Feet

6. 0

7.0

1.0

 

'Total Dakota 140. 0
Top of Burro Canyon Formation.

 

MOUNTAINS AREA, COLORADO

The fluviatile sandstone beds in the lower part of the
Dakota are much like the sandstone in the upper part
of the Burro Canyon Formation; where the discon-
formity between the two formations is not apparent,

distinction is difficult.

Several criteria were used to

distinguish the two formations, and these gave reliable

results in mapping :
Burro Canyon Formation

Carbonaceous material absent
or scarce.

Sandstone beds are commonly
conglomeratic and white.

Mudstone beds are mostly
green and red; a few are
very light gray to white.

Mudstone pebbles in conglom-
eratic material are red or
green.

Dakota Sandstone

Carbonaceous material com-
mon in all types of strata.
Sandstone contains very little
conglomeratic material ex-
cept in lowermost beds and
are yellow-brown or yellow-

gray.

Interbedded mudstone com-
monly is medium gray to
black, and, very rarely, light
gray.

Mudstone pebbles in basal
beds are mostly gray or
black, but some are green,
white, and red.

In figure 5 the contrasting colors of the Dakota Sand-
stone and the sandstone beds of the Burro Canyon

Formation are apparent.

The basal conglomerate of the Dakota has a maxi-
mum thickness of about 5 feet and is absent locally.

 

FiGurE 5.-Lower part of Dakota Sandstone (Kd) and sandstone of Burro Canyon Formation (Kb) display contrasting colors in western McElmo

Canyon.

Rounded slopes in middle ground are on Brushy Basin Member (Jmb} of Morrison Formation.
ground are in Westwater Canyon Member (mw) of Morrison Formation.

Viewed from the west.

Sandstone outcrops in fore-

 

STRATIGRAPHY OF THE SEDIMENTARY ROCKS 28

Most of it is granule conglomerate but some is pebble
conglomerate. In areas of relatively high pre-Dakota
relief, slabs, chips, and, rarely, cobbles and boulders of
quartzite are present in the conglomerate. Most of the
granules and pebbles are chert and quartzite, although
pebbles of mudstone and sandstone are common locally.

The sandstone beds in the lower part of the Dakota
are cross-stratified at high angles. These beds are
highly lenticular and have little value for structural
interpretation. The uppermost sandstone bed in the
Dakota is fairly continuous throughout the Ute Moun-
tains area and is a useful horizon marker. The top of
the Dakota was mapped at the top of this sandstone
bed. The bed contains fossil pelecypods and is prob-
ably a reworked beach deposit. The pelecypods are the
same as those occurring a few feet higher in the Mancos
Shale, identified by W. A. Cobban (written commun,
1956) as G@ryphaea n. sp.

The assignment of an age to the Dakota Sandstone
is based on its lithologic similarity to the Dakota else-
where on the Colorado Plateau where it overlies the
Burro Canyon Formation and where a Late Cretaceous
age has been established (Brown, 1950). Katich (1951,
p. 2094) mentioned, however, that faunal evidence in-
dicates that in parts of central Utah the lithologic unit
called Dakota Sandstone is Early Cretaceous in age.
Early Cretaceous fossils also have been found in a lower
sandstone of the Dakota in the Carrizo Mountain area
(J. D. Strobell, oral commun., 1957). In the latter
areas the Burro Canyon Formation has not been recog-
nized. Where the Burro Canyon Formation is recog-
nized, the Dakota Sandstone is therefore apparently of
Late Cretaceous age, and it is assigned this age in the
Ute Mountains area.

MANCOS SHALE

Cross and Purington (1899) named and described the
Mancos Shale exposed along the Mancos River in east-
ern Montezuma County, Colo., where they estimated a
thickness of 1,200 feet. The Mancos Shale in the Ute
Mountains region consists of 1,900 feet of beds com-
posed almost entirely of gray to dark-gray gypsiferous
shale. The following section was measured by Houser,
J. H. Irwin, and Ekren south of the Ute Mountains in
sees. 24 and 25, T. 3314 N., R. 19 W., on the north slope
of "The Mound" (pl. 1).

10. Mound section (pl. 1)

Top of local exposure.
Pediment deposits.

Unconformity.
Mancos Shale (incomplete) : Feet
Mudstone, pale yellow-gray, fissile-bedded ;
weathers hackly, forms a regular slope; base
$radAtIONAL - ._ l oon eee o nana me Sanit wis 18

 

10. Mound section (pl. 1)--Continued

Mancos Shale (incomplete) -Continued
Sandstone, very light gray ; weathers buff ; very
coarse to coarse grained, poorly sorted; con-
tains subrounded clear and frosted quartz,
abundant glauconite, and rounded grains of
quartzite(?) and chert(?); medium to thin
beds, small-scale low-angle crossbedding;
weathers slabby; forms an irregular ledge;
interbedded with fissile dark-gray mudstone;
contains shark teeth; base sharp-_________- 9

Juana Lopez Member :

Limestone and mudstone, interbedded, dark gray ;
fissile bedding. Limestone thin-bedded (0-2
in. thick, average %% in.), pale yellow gray,
finely crystalline, very fossiliferous; contains
pelecypods (Inoceramus perpleaus Whitfield
and Ostrea lugubris Conrad) and skate tooth
(Ptychodus whipplei Marcou). Six ft above
base of unit is fine-grained limy sandstone lens
(6 in. thick) that contains frosted quartz,
abundant black accessory minerals, and argil-
laceous material; wéathers rounded to blocky,
forms a ledge; base of sandstone lens sharp.
Limestone-mudstone unit forms an irregular
cliff; base gradational.

Total Juana Lopez Member________________ 42

Mudstone, dark-gray to black; weathers dark
gray ; very thinly laminated ; weathers hackly,
forms a regular slope; contains carbonaceous
material and limestone concretions similar to
concretionary units below ; base gradational__ 62

Limestone concretionary zone in shale unit; con-
cretions are dark gray, weather tan to buff and
rounded to blocky; a 3- to 6-in. zone at base
contains same crystalline cone-in-cone struc-
ture noted in concretionary unit below ; in some
places this structure occurs at top of limestone
concretions; limestone not composing this
crystalline structure is partly fossiliferous
(Lingula-like pelecypods) ; some of upper sur-
faces of the limestone are rounded and nodu-
lar; contains vugs lined with calcite crystals;
basal 3- to 6-in. zone contains some banding;

Feet

pase 2
Mudstone, dark-gray to black; weathers dark
gray ; lacks limestone concretions____________ 33

Limestone concretionary zone in shale unit, dark-
gray; weathers brown; concretions are 1 to 5
ft in diameter, bounded by 4- to 6-in. zone of
radiating crystalline cone-in-cone structure-. 2
Mudstone, gray to dark-gray ; fissile bedding, cal-
careous; forms a regular slope; interval is
mostly covered ; contains fossils and in places

abundant gypsum; base gradational_________ 266

Greenhorn Limestone Member :

Limestone, dense, gray; weathers light gray to
white; slightly sandy, flat, thin bedded, dis-
continuous lenticular; intertongues with mud-
stone above and below ; weathers to flat cobbles,
forms a thick ledge; has conchoidal fracture.

Total Greenhorn Limestone Member-______ 20-33

24

10. Mound section (pl. 1)-Continued
Mancos Shale (incomplete) -Continued f
Mudstone, gray, silty; contains a few black ac-
cessory minerals; weathers hackly; forms a
regular slope; zone, about 1 ft thick, 2 to 3 ft
below the top of the unit, which contains
abundant shells of Gryphaea newberryi_______ 40
Sandstone, poorly cemented except for a 1- to 2-ft
bed 14 ft below top of unit; yellow, limonite
stained, medium grained ; contains subrounded
quartz, rare green accessory minerals; base
gradatiOnfil-_... ~ 20
Sandstone, same as unit below except gray and
contains no limonite; zone 4 ft from base is
firmly cemented (calcareous) _____________---- 10
Sandstone, yellow-gray, fine-grained; contains
subangular frosted quartz, abundant green
accessory minerals; poorly cemented (calcare-
ous and ferruginous); indistinct bedding;
weathers to soil; forms irregular slope; base
.ws o.. oan mut a unlens ss 7T

Feet

Total incomplete Mancos Shale_________-- 544

Dakota Sandstone (incomplete) :

Sandstone, pale yellow-gray, weathering yellow
gray, medium- to fine-grained, well-sorted ; con-
tains subrounded frosted quartz, common black
and red accessory minerals; firmly cemented
(calcareous) thick beds; wedge-planar low-
angle small-scale crossbedding ; forms rounded
ledge; contains abundant calcareous material,
and limonite stains, streaks, and spots, and a
2-in. coal bed about 10 ft below top. Total
incomplete Dakota Sandstone-____________-- 15

The transition from swamp, tidal flat, and beach dep-
osition during late Dakota time to marine deposition
in Mancos time was gradual. In the southern and south-
western parts of the area, the Mancos sea reworked
sands of Dakota age and deposited a pale-yellow to
light-tan, weakly consolidated, structureless clayey
sandstone about 35 feet thick. Fossils from this sand-
stone were identified by W. A. Cobban (written com-
mun., 1956) as G@ryphaea n. sp.

Two conspicuous lithologic marker units are present
in the lower part of the Mancos : the Greenhorn Lime-
stone Member of the Mancos Shale (see Dane, 1957, and
Wanek, 1959, p. 681), about 75 feet above the base, and
the Juana Lopez Member of the Mancos Shale, about
475 feet above the base of the Mancos.

The Greenhorn Limestone Member is remarkably per-
sistent throughout the map area although it ranges in
thickness from 15 to about 35 feet. It consists of thin
beds of dense, locally almost lithographic, gray lime-
stone interbedded with shale. The limestone weathers
to light gray or white and forms low ledges. Three to
six feet below the base of the limestone is a persistent
bed, 3 inches to 3 feet thick, composed almost entirely of

 

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

shells of G@ryphaea newberryi Stanton (W. A. Cobban,
written commun., 1956). About a foot above this bed
is a 6- to 12-inch very light gray bed of bentonite stained
by limonite. The Greenhorn Limestone Member is a
conspicuous marker on flanks of the Ute Mountains. It
is not shown separately from the Mancos on the map
(pl. 1).

The Juana Lopez Member is composed of brown to
tan sandy fossiliferous limestone interbedded with dark-
gray shale. It was originally defined by Rankin in
1944 (p. 12, 19-20) in a section measured 6 miles north-
west of Cerillos, Santa Fe County, N. Mex., as a very
calcareous thin-bedded, very fossiliferous sandstone 10
feet thick that occurs near the top of the Carlile Shale
in northern New Mexico and that contains Prionocyclus
wyomingensis Meek and Inoceramus labiatus Schlot-
heim. Rankin, in fact, considered the top of the Juana
Lopez Sandstone Member as the top of the Carlile Shale.

In the Ute Mountains area, the Juana Lopez consists
of 42 feet (Mound section) of very fossiliferous, finely
crystalline pale yellow-gray limestone and interbedded
dark-gray fissile mudstone. The member contains /no-
ceramus perpleeus Whitfield, Ostrea lugubris Conrad,
Ptychodus whipplei Marcou, Prionocyclus wyomingen-
sis Meek, and Inoceramus dimidius White (W. A. Cob-
ban and J. B. Reeside, Jr., 1956, written commun.).
This assemblage suggests a Carlile age for the Juana
Lopez Member in the Ute Mountains.

The basal contact of the Juana Lopez Member is
gradational and is placed at the base of the lowermost
prominent limestone bed. The top of the member is
placed at the top of uppermost limestone; however, as
mapped in the mountains, and as far south as "The
Mound" (pl. 1; fig. 6), the Juana Lopez Member in-
cludes at its top a set of light-gray glauconitic cross-
stratified coarse-grained quartz sandstone beds a few
inches to as much as 9 feet thick. South of "The
Mound," however, two beds of this sandstone are sepa-
rated by about 40 feet of gray shale.

In this area the combined thicknesses of the Juana
Lopez Member and the overlying interval as defined
by these two sandstones is nearly 100 feet thick, whereas
at "The Mound" it is 50 feet thick, and northward in the
mountains proper it is less than 25 feet thick. Since the
completion of the present study, C. H. Dane (1960) re-
ported that the glauconitic sand overlying the Juana
Lopez Member at "The Mound" is of Niobrara age, con-
taining ZInoceramus deformis. According to Dane
(1960, p. 53) the glauconitic sandstone cuts sharply
downward across underlying beds northward from the
San Juan River. Dane reported (p. 53) : "At a locality
3 miles south of the Colorado State line coarse-grained
glauconitic sandstone rests directly on shales of Carlile

STRATIGRAPHY OF THE SEDIMENTARY ROCKS 20

 

FiGuURE 6.-"The Mound." The Juana Lopez Member (Kmsj) in the Mancos Shale (Km).

age within 60 feet of Rankin's Juana Lopez sandstone
member. West of Towaoc, Colo. (loc. 8), south of Ute
Mountains, coarse-grained glauconitic quartz sandstone
containing Zmnoceramus deformis of Niobrara age rests
on only 15 feet of shale above the Juana Lopez sand-
stone member. It is of interest to note that the basal
few inches of the glauconitic sand contains a few shells
of Ostrea lugubris, doubtless reworked from the under-
lying shales in which it is also present." Dane con-
cluded (p. 53) that beds of the Imoceramus deformis
zone cut down unconformably northward and that 300
feet or more of beds present south of San Juan River
are missing just south of Ute Mountains about 30 miles
to the north.

The Juana Lopez Member is sporadically exposed in
cuestas along the lower flanks of the Ute Mountains
from "The Mound" to Towaoe, Colo., and forms a low
ridge across the southern part of the Cortez SW quad-
rangle. North of the Yucca cluster of laccoliths, along
the Western slopes of the mountains, the member is hid-
den by talus and debris and its location is not certain.
The member is probably continuous throughout the Ute
Mountains, but it could not be traced in the higher parts
owing to heavy cover of vegetation and soil, the non-
resistant nature of the limestone, and general thinning
of the member, especially toward the north. The Juana

 

Lopez Member a few hundred yards east of Towaoe is
so deficient in limestone that it has no topographic
expression.

The lower part of the Mancos Shale in the vicinity of
Point Lookout, 25 miles east of the Ute Mountains, is
of Colorado age and the upper part is of early Montana
age (Pike, 1947, p. 20-24). (For a discussion of the
terms Colorado Group and Montana Group and of the
faunal evidence for these divisions, see Pike 1947, p.
20.) The Mancos in the Ute Mountains is similar to
that near Point Lookout and almost certainly correla-
tive with it.

MESAVERDE GROUP

The Mesaverde Group of Late Cretaceous age is pre-
served in many places in western Colorado. It was first
described by Holmes (1877, p. 245, 248). Later, the
major subdivisions were named by Collier (1919, p.
296) from bottom to top : the Point Lookout Sandstone,
120 feet thick, the Menefee Formation, 800 to 900 feet
thick, and the Cliff House Sandstone, 190 feet thick.
In the vicinity of the San Juan River in southwestern
Colorado and northwestern New Mexico the group
ranges from 1,200 to 1,500 feet in thickness.

Only the Point Lookout Sandstone is present in the
Ute Mountains.

26 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

POINT LOOKOUT SANDSTONE

Overlying the Mancos Shale on a few high places in
the Ute Mountains are thin flat persistent beds of buff
and yellowish-gray sandstone interstratified with dark-
gray mudstone that resembles that of the Mancos.
This sequence is believed to be the lower part of the
Point Lookout Sandstone of the Mesaverde Group be-
cause of its lithologic similarity to that sandstone at
the type locality. The maximum preserved thickness
is about 100 feet. The beds are silicated where in-
truded by diorite and granodiorite northwest of the
Black Mountain stock. In the vicinity of "The Knees"
stock, the Point Lookout rocks are slightly recrystallized
and contain abundant crystals of pyroxene, epidote,
sericite, and garnet.

QUATERNARY SYSTEM

Alluvium, landslide debris, windblown material, and
talus of Quaternary age obscure the bedrock in many
parts of the Ute Mountains area. These materials were
mapped only where they form large bodies or where
they obscure bedrock contacts. The deposits were not
studied closely, and no systematic attempt was made
to determine their relative ages ; the sequence of presen-
tation, therefore, does not reflect age.

In the valley of McElmo Creek, thick deposits of al-
luvium (pl. 1) border the creek and provide soil that
is intensively cultivated. The alluvial deposits are
mostly of silt but include beds of sand and gravel. Al-
luvium also occurs in smaller stream valleys of the Ute
Mountains area, and although it is similar to the al-
luvium along McElmo Creek, it generally contains more
gravel. In the western part of the Ute Mountains
much sheet-wash gravel was mapped as alluvium. Al-
luvium composed of reworked Mancos Shale and wind-
blown silt is found in Montezuma Valley east of the
Ute Mountains. It forms a good soil and is extensively
farmed. In places, especially in McElmo Canyon, the
alluvium is being rapidly removed by stream down-
cutting.

Stream terrace gravel (pl. 1) occurs in McEImo and
in East McElmo Canyons and vicinity. The gravel
rests on stream-leveled bedrock and consists of pebbles,
cobbles, and boulders of both igneous rock and well-
cemented sandstone. The gravel was deposited during
a period of aggradation that preceded the cycle of
downcutting and aggrading that gave rise to the valley-
fill alluvium.

Light-brown windblown silt and very fine grained
sand (pl. 1) mantle most of the mesas and pediment
surfaces in the Ute Mountains area. They form an
excellent porous soil capable of holding water for long
periods of time and are utilized in dry farming in the

 

higher parts of the area. The windblown material
was probably derived mainly from arid areas to the
southwest. The bulk of it probably accumulated dur-
ing the Pleistocene Epoch, but some is still accumulat-
ing.

Fan-shaped deposits of coarse gravel occur on the
south side of McElmo Canyon and in Montezuma Val-
ley near the base of the Point Lookout Sandstone.
These gravel deposits, classed as fanglomerate, were
spread over rather even, steeply sloping pediment sur-
faces. Those of McElmo Canyon are divided into two
groups on the basis of relative age. The older deposits
are lithified; the younger deposits are lithified only
locally and lie at the level of streams that drain the
northern parts of the mountains (pl. 1). The cobbles
and boulders in the fanglomerate of McElmo Canyon
are mostly of igneous rock; those in the deposits in
Montezuma Valley are mainly of Point Lookout Sand-
stone.

Thinner deposits of coarse gravel mantle gently slop-
ing pediment surfaces that extend radially away from
the mountains. These deposits were mapped as pedi-
ment gravel and no sharp line of distinction exists be-
tween them and the thicker fan-shaped deposits. In
places, the gravel deposits are well cemented with
caliche, but in others they are loose. Many of the
gravel-covered pediments between creek beds serve as
natural roads for the Ute Indians, and the gravels in
the larger deposits are used extensively in highway
construction.

Although, several pediment surfaces are in evidence,
they were not differentiated, and the deposits were
mapped as one unit.

Deposits of fine material, cobbles, boulders, and great
slabs of rock that have moved downward through the
combined action of gravity, water, and, in some in-
stances, snow, were mapped as landslides (pl. 1). The
largest of such deposits are on the north-facing slopes
of the Ute Mountains. A large landslide deposit that
lies between Black Mountain and Ute Peak (pl. 1) is
of interest because in places it contains abundant mate-
rial derived from the Point Lookout Sandstone, a unit
that now exists only in thin remnants in the Ute Moun-
tains. A large landslide deposit on the north slope of
Mable Mountain consists mostly of Mancos Shale and
boulders and cobbles of porphyry.

Bodies of talus (pl. 1) surround many of the igneous
intrusive bodies. The talus is thick in the vicinity of
Ute Peak, and on the west side of the peak it has
streamed downward to form a rock glacier (pl. 1). On
the north, east, and south sides of the peak, talus grades
imperceptibly into block rubble.

IGNEOUS ROCKS 27.

In places along the flanks of the mountains, blocks of
rock-principally of porphyry-have accumulated in
bodies 5 to 40 feet thick, which are classed as block
rubble. The rubble has a hummocky form, and in some
areas may include the slumped remnants of sills or
laccoliths. Water and possibly snow played major roles
in the movement of the blocks. Bodies of the rubble
commonly lie at considerable distances from possible
source rocks. They differ from landslide deposits
mainly in that they rest on gentler slopes and consist
mostly of porphyry. - Block rubble, landslide, and talus
grade from one to another, and the contacts between
them are drawn arbitrarily in many places.

IGNEOUS ROCKS

GENERAL FEATURES AND EVIDENCE FOR AGE OF
IGNEOUS ACTIVITY

The igneous rocks of the Ute Mountains are members
of a calc-alkalic series that ranges in composition from
microgabbro through quartz monzonite. Field map-
ping indicates that the succession of intrusive rocks
from earliest to latest was microgabbro, diorite, grano-
diorite, and quartz monzonite.

All the igneous bodies were intruded forcibly through
or between layers of sedimentary rock. No evidence
was found that the igneous rocks replaced or assimilated
their wall rocks during intrusion.

The igneous rocks occur in stocks, laccoliths, bysma-
liths, sills, and dikes. The dikes are radially distrib-
uted about the Black Mountain stock in the north, "The
Knees" stock in the south, and an inferred concealed
stock at Ute Peak.

The similarity of the intrusive forms, the geologic
structure, and the types of igneous rock of each of the
laccolithic mountain groups on the Colorado Plateau
strongly suggests that all the laccolithic mountains
formed at approximately the same time.

The igneous rocks in the Ute Mountains intrude sedi-
mentary rocks as young as the Point Lookout Sand-
stone of Late Cretaceous age. This intrusion is the only
direct evidence of the age. of the intrusive rocks avail-
able in the Utes. Other lines of evidence (see p. 6 to
7) indicate that igneous activity in the Ute Mountains
did not occur prior to the deposition of the McDermott
Member of the Animas Formation of latest Cretaceous
age. Shoemaker (1956, p. 162) pointed out that the
only known stratigraphic evidence that may permit pre-
cise assignment of age to the laccolithic mountains is
the occurrence of boulders of andesite in the McDermott.
He concluded that at least some of the coarse debris in
the McDermott was probably derived from the La Plata
Mountains, a laccolithic group, and that some of the in-
trusive bodies in the La Plata Mountains are probably of

745-807 O-65--5

 

latest Cretaceous age. In contrast, Hunt, Averitt, and
Miller (1953, p. 212) outlined geomorphic evidence that
suggests a middle Tertiary age for the Henry Mountains

RELATIVE AGES OF THE INTRUSIVE ROCKS

and thus, inferentially, for other laccolithic groups.
Evidence that the microgabbro was the earliest intru-
sive rock and that diorite porphyry, granodiorite por-
phyry, and quartz monzonite porphyry are successively
younger is found in both the northern and southern
parts of the mountains. In the vicinity of North Black
Mountain in the northern Ute Mountains, sills of micro-
gabbro are domed upward over the underlying North
Black Mountain laccolith of diorite porphyry (pl. 1).
On the east side of Black Mountain, quartz monzonite
porphyry has a chilled border where it is in contact
with microgabbro. In the vicinity of the Horse Moun-
tain laccolith a discordant intrusive of quartz monzonite
has a chilled border where in contact with diorite por-

phyry (fig. 7).

 

FIGURE 7.-Contact of quartz monzonite porphyry (amp) with diorite
porphyry (do). Note that flow lineation in the quartz monzonite
porphyry parallels the contact zone.

Southeast of "The Knees" stock in the southern Ute
Mountains, sills of microgabbro and diorite porphyry
are domed upward by a large underlying intrusive body
of granodiorite porphyry.

Some of the evidence for the relative ages of the
intrusive rocks, such as the doming of microgabbro by
an underlying, more silicic intrusive mass, is indirect.
The microgabbro might possibly have been intruded
into beds that had already been domed ; however, this
possibility is not considered likely because (a) the
microgabbro is generally more fractured than the un-
derlying diorite porphyry and this fact suggests doming
after solidification, and (b) there is evidence in some
exposures that the microgabbro was altered slightly by
underlying, more silicie rocks. A thin section of micro-
gabbro from an intrusive body that overlies granodiorite
porphyry southeast of "The Knees" contains signifi-
cantly more quartz and calcite than a sample of
microgabbro taken from an exposure several hundred
yards from the granodiorite. Some of the calcite and
quartz may have been introduced by 'hot ground water

28 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

when the enclosing Mancos Shale was heated by the
large underlying granodiorite laccolith.

OCCURRENCE AND PETROGRAPHY
GENERAL FEATURES

In the descriptions that follow, modal analyses of
several samples are given for each rock. Analyses that
include modes of the groundmasses are close approxi-
mations only, owing to the dense nature of the ground-
masses. The term "dense" as used in this paper implies
that the grain size of the groundmass crystals averages
less than 0.03 mm; "extremely dense" implies that the
groundmass crystals are less than about 0.01 mm in av-
erage diameter.

The texture of the rocks varies with rock type and
from one intrusive body to another. In general, the
microgabbro is slightly porphyritic ; the diorite is mod-
erately porphyritic; and the granodiorite and quartz
monzonite are conspicuously porphyritic. The ground-
masses of the various rocks make up from 40 to 60 per-
cent of the volume and, in general, are less dense in
the mafic rocks.

MICROGABBRO

The microgabbro is an aphanitic black rock closely
resembling basalt (fig. 84). It occurs as sills in the
northern part of the mountains and as small tongue-
shaped laccoliths and thin sills in the central part of
the mountains. Two varieties are recognized. One is
porphyritic and contains about 10 percent hornblende
and 3 to 10 percent augite in phenocrysts that are 2 to
10 mm in length. The texture of this rock varies from
diabasic to trachytic. Major constituents of the ground-
mass are plagioclase and chlorite. The plagioclase oc-
curs as twinned microlites that average about 0.04 by
0.1 mm and as nearly equant zoned crystals as much as
2 mm in length. Chlorite fills interstices between the
plagioclase microlites, and occurs with calcite as an
alteration product of augite, hornblende, and the larger
crystals of plagioclase. Biotite is common as tiny books
as much as 0.2 mm in length. Quartz is ubiquitous and
averages about 4 percent; magnetite and apatite are
the most common accessory minerals.

The other variety of microgabbro has seriate tex-
ture and contains very few crystals that exceed 1 mm
in length. The rock contains about 15 percent augite
and 8 percent hornblende that form the largest crystals
and range in size from about 0.1 mm to slightly more
than 3 mm. Plagioclase occurs as tabular zoned crys-
tals ranging from less than 0.1 mm to about 0.6 mm, and
as tiny microlites.

The larger crystals of plagioclase in both varieties of
microgabbro are conspicuously zoned with centers of

 

bytownite or calcic labradorite and outer zones of
andesine. These crystals are about Ang; in average
composition, and the overall plagioclase composition
is estimated to be about Ans;.

Apatite is abundant in the microgabbro, and some
crystals are as large as 1 mm in cross section.

Both varieties of microgabbro have been considerably
altered. Original augite has been altered in part or
completely to chlorite and calcite in all the microgabbro
examined in thin section. Plagioclase and hornblende
are fresh to moderately altered. The altered plagio-
clase commonly contains calcite, kaolin, and chlorite;
the altered hornblende contains calcite and chlorite.
Locally, pyrite is abundant. - Vesicles filled with quartz
and calcite are fairly common in the microgabbro.
Modes of three samples of microgabbro are listed below.

Modes of microgabbro

[Sample 56-E-77 is porphyritic; 56-E-94 and 56-E-97 have seriate texture. For
chemical analysis of 56-E-77, see table 3, sample 2]

 

 

 

   

 

Field sample
Mineral
56-E-77 | 56-E-94 | 56-E-97
c..... iki trans 3 3
Plagioclase Any. . ai 53 60
Hornblende... ... Ysa r 8 6
13 12
1 4
RT ec
7 6
y I
CHIORINe - onne reel cera adnan eaten dee 14 | *) > T| s 9
TODA: Sce cloe on on cee eden a+ ce whine aie 100 100 100

 

 

 

 

1 Relicts are now filled with chlorite, limonite, and unknown green to white opaque
mineral.
? Formed largely at the expense of pyroxene.

Locality descriptions:
56-E-77: Tongue laccoliths, northeast of "The Knees," central Ute Mountains.
56-E-94: Sill in Pine Creek, northern Ute Mountains.
56-E-97: Sill on east flank of Black Mountain, northern Ute Mountains.

DIORITE PORPHYRY

Diorite porphyry is the most abundant igneous rock
in the Ute Mountains. It occurs as laccoliths, as thin
sills ranging from a few feet to 100 feet in thickness,
and in a few dikes. Although mapped as a single unit,
the diorite porphyry is divided for purposes of descrip-
tion into two varieties: normal diorite porphyry and
leucocratic diorite porphyry. The two varieties gen-
erally are distinguishable by differences in color caused
primarily by a greater abundance of mafic minerals in
the groundmass of the normal diorite porphyry, al-
though they are petrographically gradational. Only
one variety occurs in any individual intrusive body.

The normal diorite porphyry is thought to be the
older. The two varieties of diorite were observed to
be in contact in only one place, on the northeast side of
Razorback laccolith, where leucocratic diorite porphyry
of the Razorback laccolith has intruded normal diorite
porphyry of the East Horse laccolith. Structural re-

IGNEOUS

 

FiGuRE 8.-Specimens of the four main types of igneous rock from the Ute Mountains.
porphyry (leucocratic variety), phenocrysts are hornblende and plagioclase; note the hornblendic inclusion.
southern type; phenocrysts are plagioclase and pseudomorphs after hornblende, now filled with sericite, chlorite, and calcite.

ROCKS 29

A, Microgabbro, nearly seriate texture. B, Diorite
C, Granodiorite porphyry ;
D, Quartz

monzonite porphyry ; phenocrysts are bipyramids of quartz, plagioclase, and pseudomorphs after hornblende.

lations elsewhere suggest indirectly that laccoliths of
normal diorite porphyry have influenced the location
and extent of intrusion of leucocratic diorite porphyry.
Such relationships are discussed in detail in the section
describing the structure of the individual intrusive
masses.

The shape of laccoliths bears a general relationship to
the type of diorite porphyry. The normal diorite por-

phyry occurs mainly as flat-topped laccoliths 200 to 500
feet thick. The leucocratic diorite porphyry occurs pre-
dominantly as thick (500 to 1,200 feet), steep-sided
laccoliths that have higher ratios of vertical dimension
to areal extent than laccoliths of normal diorite por-
phyry. Differences in viscosities of the two diorite por-
phyry magmas probably caused differences in the forms
of the laccoliths.

30 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

NORMAL DIORITE PORPHYRY

The fresh rock is mostly greenish gray; some is yel-
lowish gray. It weathers to a moderate yellowish
brown and medium brown, and on many outcrops is
stained medium to dark brown by iron. Some stain of
black manganese oxide coats the older exposures.

The distinction between phenocrysts and groundmass
is not microscopically sharp inasmuch as many of the
rocks have a nearly seriate texture. The phenocrysts
constitute about 50 percent of the volume of the rock.
Of these, plagioclase is the most abundant, comprising
25 to 45 percent. The plagioclase phenocrysts are
twinned and zoned and average about 0.7 by 1.0 by 2.0
mm in size. The composition ranges from calcic labra-
dorite in the cores to oligoclase in the outer zones. The
maximum difference in the anorthite content in the
zones of one crystal is about 45 percent An. The pheno-
erysts are anhedral to euhedral in outline.

Phenocrysts of hornblende and augite constitute 12
to 17 percent of the rock and average about 14 percent.
Most crystals are euhedral. The hornblende crystals
average about 0.6 by 2.5 mm but generally vary consid-
erably from these dimensions. The crystals are of the
common green variety of hornblende, and in most sam-
ples have been moderately to considerably altered to
magnetite, sericite, and chlorite. (See table 6.) The
augite phenocrysts average about 0.3 by 1.0 mm.

Accessory minerals-magnetite, apatite, and sphene-
rarely exceed 3 percent of the rock. Rare crystals of
clinozoisite near the margin of one intrusive body of
diorite porphyry are thought to have originated during
the late magmatic stage.

A medium-gray to dark greenish-gray groundmass
forms about half the volume of normal diorite porphyry
and contains grains ranging in size from less than 0.01
mm to as much as 0.1 mm. Somewhat more than 70
percent of the groundmass is sodic plagioclase, anhedral
quartz, and potassium feldspar. Some of the plagio-
clase is euhedral and a little of the potassium feldspar is
subhedral. The composition of plagioclase in the
groundmass can seldom be determined accurately.
Small amounts of hornblende or augite normally occur
in the groundmass, and chlorite is abundant. Diorite
porphyry having a composition near microgabbro con-
tains as much as 10 percent augite. Quartz is mainly in-
terstitial and formed during a late magmatic stage.
Rare rounded and embayed phenocrysts of quartz are
interpreted as having formed under conditions of fa-
vorable pressure and temperature at considerable depth,
although they may be partially assimilated crystals de-
rived from a preexisting silicic rock.

 

LEUCOCRATIC DIORITE PORPHYRY

The leucocratic diorite porphyry crystallized from a
magma slightly richer in silica and water than the
magma of the normal diorite porphyry. The rock con-
tains no pyroxene but contains more hornblende than
the normal diorite porphyry. Phenocrysts constitute
an average of 50 percent of the leucocratic diorite
porphyry. These are zoned crystals of labradorite or
calcic andesine that constitute somewhat more than 35
percent of the volume and hornblende that- constitutes
10 to 20 percent. Quartz phenocrysts are rare and aver-
age less than 3 percent. The groundmass is dense or
extremely dense and consists of sodic plagioclase,
potassium feldspar, and quartz.

The leucocratic diorite porphyry is megascopically
very similar to the granodiorite porphyry of Ute Peak
but differs from the rock at Ute Peak in its lower con-
tent of potassium feldspar and its lack of pyroxene. A
specimen of the leucocratic diorite porphyry is shown
on figure 8B.

Modes of diorite porphyry
[See table 3 for chemical analyses of 56-E-42 (sample 5), 56-E-33 (sample 6)]

 

 

 

 

   
 

 

 

 

Field sample
Constituent Normal diorite porphyry Leucocratic

diorite porphyry

55-E-47 | 56-E-42 | 56-E-37 | 56-E-33 | H-539

QUaTEREE- ecole ee occurs. Tr. 1.6 4.2 0. 4 6

Potnssinmfeldspat::.;._._...__.]..____._.. 4.8 SA |e

Plagioclase... 38.1 60. 4 68. 6 28. 2 25. 6

Hornblende. __ 4.3 7.0 1.2 17.0 19.2

Augite. ...- 8.5 1 0. 4 BM Nuse esen

-c. oo abe cn been ad (fe- neuger s (eus ee cs cae [ek ohne =n

4.0 2.2 .4 4 1.7

ADBHEG. lc ede onne due .8 Tr 78 rO
Alteration products:

Chlorite 2. luk. 12 4 12.8 8. 6 9. 4 1.0

Sericite (affer hornblendo)..L1L . 22. ice elcerece lee are anni .3

Calolbe:- 2 L ole n eee 1.8 26 1.2

Kaolinite..._.... A lowe 2

Seritib6.... sinc eons [e- ieee ganas .6

Groundmass4............_.._.._ B2. 4 44.0 47.6

.l; deco 100. 0 100. 0 100. 0 100. 0 100.0

 

 

 

 

 

1 Augite occurs as fine-grained prisms.

2 Groundmass consists mainly of sodic plagioclase, quartz, and potassium feldspar.
Although the groundmasses of 56-E-42 and of 56-E-37 were counted, the modes as
shown are considered to be approximations only.

Locality descriptions:
55-E-47: South side Yucca laccolith.
56-E-42: Urfltnamevc‘ll laccolith in Yucca cluster of laccoliths, SEX see. 33, T. 35 N.,
56-E-37: Southwest part of unnamed laccollth in the West Toe cluster, SEH
unsurveyed see. 4, T. 34 N., R.
56-E-33: West side Pack Trail laccolith.
H-539: West end Horse laccolith.
GRANODIORITE PORPHYRY

For mapping purposes, three distinctly different
rocks have been grouped together as granodiorite por-
phyry. Because the three rocks are localized geo-
graphically within the Ute Mountains, they will be
referred to as the southern, central, and northern types.

The southern type (fig. 13C) is a light-gray rock that
forms laccoliths, bysmaliths, dikes, and a stock in the

southern and south-central parts of the mountains. It

IGNEOUS ROCKS

is characterized by phenocrysts of zoned plagioclase in
equant grains as much as 10 mm across, and by pseudo-
morphic aggregates of secondary minerals that indicate
the former presence of phenocrysts of hornblende. The
plagioclase is estimated to average about An,. The
pseudomorphs after hornblende are filled with sericite,
chlorite, and calcite. Quartz phenocrysts are rare. An
extremely dense groundmass constitutes 50 percent of
the volume of this rock. Although the groundmass can-
not be effectively examined microscopically, staining by
sodium cobaltinitrite reveals the presence of abundant
potassium feldspar, and norms have as much as 16 per-
cent potassium feldspar. (See p. 36 to 37.) Sodic
plagioclase and quartz are the other principal constit-
uents of the groundmass.

The central type of grandodiorite porphyry is a light-
gray rock that occurs only in the Mushroom laccolith.
It contains abundant fine-grained crystals of quartz as
much as 0.5 mm in diameter. This rock was mapped
as tonalite porphyry in the field but was later placed
with the granodiorite on the basis of total K,0 and
Na,0. This rock is similar to the southern type of
granodiorite in that andesine (An,,) and pseudomorphs
after hornblende are the most common phenocrysts, al-
though the andesine crystals are much smaller in the
central type and rarely exceed 4 mm in diameter. The
hornblende pseudomorphs are filled with chlorite,
sericite, calcite, and iron ore.

Most of the plagioclase in the central and southern
types of granodiorite has been altered in part or com-
pletely to sericite, calcite, and chlorite. In the most in-
tensely altered rocks, pseudomorphs after plagioclase
contain fine-grained quartz or chalcedony in addition to
the minerals listed above, and the hornblende pseudo-
morphs contain partly opaque material that is prob-
ably a mixture of clay, sericite, chlorite, and calcite.
Pyrite is locally abundant. Epidote and garnet are
common in granodiorite of the southern type in the vi-
cinity of "The Knees" stock (south-central mountains).

The northern type of granodiorite porphyry, called
hornblende granodiorite porphyry, occurs in and adja-
cent to the Black Mountain stock and in the Ute Peak
bysmalith. A specimen of this rock is shown on figure
9. The rock closely resembles the diorite porphyry in
hand specimen but contains more fine-grained potas-
sium feldspar. The potassium feldspar, sanidine, is
confined to the groundmass, which is coarser than in the
other types of granodiorite porphyry and can be ana-
lyzed fairly accurately with the microscope. In addi-
tion to sanidine, the groundmass consists mainly of
sodic plagioclase and quartz. Phenocrysts in this rock
include plagioclase, hornblende, augite, magnetite, and
sparse rounded crystals of zircon. The plagioclase oc-

31

 

FIGURE 9.-Specimen of hornblende granodiorite porphyry from Ute Peak.

curs as equant crystals (0.5-2.5 mm) that are conspicu-
ously zoned from calcic labradorite in the cores to sodic
andesine and oligoclase in the outer zones. The aver-
age plagioclase (phenocrysts and groundmass) is
estimated to be about Any;.

In contrast to the generally altered character of the
granodiorite porphyry in the central and southern parts
of the mountains, the hornblende granodiorite porphyry
of Ute Peak and Black Mountain is relatively fresh.
Many of the plagioclase crystals are clear, and some
of the hornblende is fresh except for slight alteration
to pyroxene and magnetite along crystal borders.

Modes of the three types of granodiorite porphyry
are shown below.

Modes, in percent, of granodiorite porphyry
[See table 3 for chemical analysis of 56-E-82 (sample 13)]

 

 

 

 

 

   
 
  
 

 

Field sample

Northern type

Southern type | Central type (hornblende

Constituent granodiorite)

56-E-140 56-E-82 55-E-16

@UHBTLE- 2C Ie cos oo Cece e TC 1.4 7.2 4.2
Pingloclase.............s e 34. 8 37.2 65. 6
Potassium feldspar. cc c AL rede ech 12.8
HornbleRde. . . . --.. cle eas rend 18.6 1 9.8 8.2
Augite....... TF. 2.0
Magnetite .8 1.6 3.6
Apatite... 7 . 2
EMCOR L1 EE 12 -- a a o ull ae as ai ae we I aie cen a ne dak aides (ans eo ene nak ue ae Tr.
Biotite. .. .2
Chlorite .. 1.8
Calcite... 1.4
Sericite...... nee dels |duveve sur VB Leave cone ars
Groundnmiags...._._..____.___...__ 54. 4 SL
Pobal). oocli coule eu eve 100. 0 100. 0 100. 0

 

 

 

 

1 Pseudomorphs after hornblende are filled with chlorite, sericite, and calcite.

Locality descriptions:
56-E-140:
56-E-82: Mushroom laccolith.

ike north of "The Knees" stock.

55-E-16: Approximately 1,500 ft. below top of Ute Peak, east side.

QUARTZ MONZONITE PORPHYRY

The name quartz monzonite porphyry is applied to

rocks of two types in the Ute Mountains.

One type is

characterized by pyrogenic biotite phenocrysts and

32 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

occurs only in the northern mountains in the vicinity
of Black Mountain. The other type contains mica
only as secondary crystals in pseudomorphs after horn-
blende. This type occurs only in the southern part of
the mountains. The two types will be referred to as the
northern and southern types of quartz monzonite
porphyry.

The northern type of quartz monzonite porphyry is
light blue gray where unaltered and pale green to green-
ish white where altered. It contains phenocrysts of
quartz, plagioclase, and biotite that make up about 50
percent of the volume of the rock. Quartz phenocrysts
average about 9 percent occurring as rounded bipyra-
mids that range in diameter from about 0.1 to 1 mm
and average about 0.4 mm. Tabular or equant euhedral
crystals of plagioclase that reach 2 mm in length make
up from 30 to 35 percent of the rock, and biotite oc-
curring as books makes up 3 to 9 percent. The plagio-
clase crystals are conspicuously zoned with cores of
labradorite or andesine and outer zones of oligoclase
or albite. The books of biotite range from about 0.1
to 1.5 mm and average about 0.4 mm. Magnetite
usually exceeds 1 percent of the volume and occurs as
subhedral grains that average about 0.2 mm in diameter.

The groundmass of the northern type of quartz mon-
zonite is extremely dense; most of the grains average
about 0.01 mm in diameter. Although the amount of
potassium feldspar in the groundmass cannot be ac-
curately determined optically, staining of thin sections
with sodium cobaltinitrite reveals that potassium feld-
spar constitutes more than 10 percent of the rock. The
anorthite content of the fine-grained plagioclase in the
groundmass could not be determined with the micro-
scope, but chemical analyses of the rock indicate a high
content of Na,0, and the plagioclase in the groundmass
is probably albite.

The rock is generally quite altered; biotite crystals
are wholly or partly altered to chlorite and plagioclase
crystals wholly or partly altered to calcite, kaolinite,
and chlorite. Epidote is common (as much as 1 per-
cent) as subhedral grains 0.15 mm in length.

The southern type of quartz monzonite is light blue
gray where unaltered and light tan where altered. It
differs from the northern type of quartz monzonite in
grain size and texture, and is petrographically grada-
tional with the southern type of granodiorite porphyry,
which it resembles. - In the field, it is distinguished from
the southern type of granodiorite by the abundance of
quartz phenocrysts. Quartz, plagioclase, and pseudo-
morphs after hornblende are the most abundant pheno-
crysts in the quartz monzonite, and make up about 45
percent of the volume. Quartz phenocrysts average
about 4 percent, plagioclase about 35 percent, and

 

pseudomorphs after hornblende about 3 percent. Al-
though quartz phenocrysts comprise a relatively small
fraction of the rock, they are the most conspicuous
crystals owing to the fact that they are never altered
and are highly resistant to erosion. In nearly all ex-
posures the quartz crystals (as much as 5 mm in diam-
eter) protrude from the surface of the rock (fig. 8D).
Plagioclase phenocrysts are commonly 3 by 4 by 5 mm
(rarely as much as 10 mm in length) and are con-
spicuously zoned from cores of labradorite or andesine
to outer layers of oligoclase or albite. They are com-
monly altered to calcite, kaolinite, chlorite, albite, and
sericite. Pseudomorphs after hornblende are filled with
sericite, chlorite, and calcite.

Tiny phenocrysts of epidote and apatite are common
in the southern type of quartz monzonite. Strongly
zoned yellow-green, distinctly pleochroic crystals of
epidote are as much as 0.6 mm in length. Apatite crys-
tals as much as 0.5 mm in length are of euhedral tabu-
lar habit, and are commonly clear to slightly brownish
in thin section. The brown rarely penetrates the entire
crystal, but is an ubiquitous feature of the apatite in
both the quartz monzonite and granodiorite of the
southern type. Indices of refraction of this brownish
apatite range from 1.616 to 1.621. Magnetite is ex-
tremely rare in the southern type of quartz monzonite.
This scarcity is probably due to alteration because,
where magnetite is present, it is always partly altered to
limonite or hematite or both.

The groundmass of the southern type of quartz mon-
zonite, as with the northern type, is extremely dense.
Staining of thin sections with sodium cobaltinitrite re-
vealed abundant potassium feldspar in the groundmass,
and a comparison of modal analyses of the phenocrysts
with normative minerals of the rock suggests that the
groundmass is also rich in albite and quartz.

LAMPROPHYRE

A lamprophyre, spessartite, is found in the extreme
western part of the Ute Mountains in the vicinity of
Yucca laccolith, and in the northern part at Black
Mountain. The rock forms sills and short dikes that
intrude Mancos Shale in the western exposures. At
Black Mountain the spessartite appears to intrude
granodiorite porphyry, although the relations are ob-
scure because the contacts between the rocks are not
exposed. A planar structure or rock cleavage in the
spessartite dips 22° E., in contrast with the steeply
dipping and plunging granodiorite porphyry.

The rock exposed in the western part of the moun-
tains is dark brownish gray and is conspicuously
porphyritic; it contains large phenocrysts of pyroxene
and hornblende that commonly exceed 10 mm in length.

IGNEOUS ROCKS

Modes, in percent, of quartz mongzonite porphyry
[See table 3 for chemical analysis of samples 56-E-92 (sample 15), H-465 (sample 16)]

 

Field sample

 

Northern type Southern type

Constituent

 

56-E-92 56-E-13

(altered)

56-E-101 | H-465

 

   

P
Alteration products:
Chlorite (after hornblende or biotite) .
Calolbe.... . .. s
L1 .c
Sericite........ ar as
Groundmass

lr ro

 

 

 

 

 

 

 

1 Largely altered to calcite and chlorite.
2 Partly altered to chlorite.

Locality descriptions:

56-E-92: Cottonwood Creek vicinity, east side Black Mountain.

56-E-101: Black Mountain (central part).

ts Sentinel Fook dome. -."

A zeolite (thomsonite) fills cavities as much as 20 mm
in diameter and adds to the porphyritic appearance of
the rock. The pyroxene was determined to be common
augite with a 2V of 54° +=1° and N, of 1.70.

The groundmass of the rock is almost entirely plagi-
oclase and chlorite but contains sparse amounts of bio-
tite, potassium feldspar, and zeolites. The average
composition of the plagioclase is andesine which occurs
as tabular crystals that range from 0.2 to 1 mm. in
length. Biotite is common as small books as much as
1 mm in length and as tiny shreds. Potassium feldspar
occurs as small anhedral crystals that average less than
1 mm in diameter and as an alteration product of plagi-
oclase. Analcime is abundant in small cavities and as
a replacement product of plagioclase. The analcime in
the cavities shows very low birefringence and pseudo-
polysynthetic twinning. Other zeolites are heulandite,
stilbite, natrolite, and mesolite. Magnetite is abundant,
making up from 2 to 4 percent of the rock, and apatite
is a ubiquitous accessory.

The rock exposed at Black Mountain has nearly seri-
ate texture except for large hornblende crystals as much
as 30 mm in length. The rock is dark brownish gray,
and, in addition to hornblende, contains plagioclase,
augite, actinolite, and chlorite. Plagioclase occurs as
tabular subhedral crystals that range from about 0.2 to
1.5 mm across. The plagioclase is relatively unaltered,
and extinction angles indicate an average composition
of sodic labradorite for the larger crystals and calceic
andesine for the smaller crystals; the average composi-
tion is probably calcic andesine. The central parts of
a few crystals of plagioclase have been altered to potas-

 

33

sium feldspar. Augite is abundant as euhedral crystals
about 2 mm in length, most of which are rimmed with
actinolite. Chlorite with actinolite occurs in sheaves
and radiating fibrous groups 0.1 to 0.3 mm in diameter
and formed largely from fine-grained augite. Biotite
occurs as books that range in size from 0.1 mm to more
than 4 mm. A few crystals are partly altered to chlo-
rite. Magnetite and pyrite are common accessory
minerals and two zeolites, thomsonite and chabazite( ?),
coat fractures in the rock.

In the field the spessartite can easily be mistaken for
microgabbro, but the absence of quartz in the spessar-
tite and the large crystals of hornblende or augite are
distinctive. Two modes of spessartite follow.

. Modes, in percent, of spessartite
[See table 3 for chemical analysis of sample 55-E-46 (sample 1)]

 

 

 

Field sample
Mineral
55-E-46 56-E-104
ces 1 48
Wugite.:/....-l.. 17
Hornblende. ..... 3
Potassium feldsp: 1

 
 
 

Actmollte (after augite) . .
Chlorite. .

 

 

 

 

Calcite......... es +e 2 |.
. ~-... cou irene s 42 ....
...... o eus a 1

$: 21. 2006 rice ak bener bees tae 100

 

 

1 Largely altered to kaolin and analcime.
2 This value does not include analcime replacing parts of plagioclase crystals.

Localitg descriptions:
55-E-46: Yucca laccolith.
56-E-104: Black Mountain.

INCLUSIONS

Inclusions are common in the igneous rocks of the
Ute Mountains. More than 90 percent are rich in horn-
blende, although a few contain none. Xenoliths de-
rived mostly from Mesozoic sedimentary rocks are ubiq-
uitous but rare. They are mostly hornfels of Mancos
Shale and silicated sandstones derived from the Dakota
and Burro Canyon. Most of the xenoliths are less
than a foot in diameter although a few are large as a
house. Fragments of granophyric granite are fairly
common as inclusions in the northern mountains, es-
pecially in the Pack Trail laccolith (pl. 1).

The granitic inclusions superficially resemble the Pre-
cambrian granite observed in the Gulf Oil Co. Fulks 1
drilled in the NW14 see. 27, T. 37 N., R. 17 W.,
10.5 miles north-northeast of Ute Peak but differ from
it in several important ways. A characteristic feature
of the inclusions is the occurrence of large grains of
orthoclase intergrown with crystals of quartz in typical
granophyric texture. The texture of the granite in the
Gulf well, on the other hand, is hypautomorphic-granu-
lar. Most of the inclusions are nearly free of mafic

34

minerals, whereas the granite in the Gulf well is rich
in mafic minerals. A few inclusions contain biotite
and (or) hornblende, but these show a strong gneissic
fabric. According to Zabel (1955, p. 186) and Phillip
Katich (oral commun., 1958), structureless granite has
been the only Precambrian rock penetrated by the drill
in the Ute Mountains region. Thus the granitic in-
clusions in the Ute Mountains evidently were not of
local origin but were, perhaps, brought to the surface
from considerable depth. A few inclusions contain
both granite and hornblende-rich layers (fig. 104), and
the possibility exists that both the granite and the
hornblende-rich inclusions were derived from the same
substratum.

The hornblende-rich inclusions are found in all the
igneous rocks in the Ute Mountains, but are most abun-
dant in the diorite, where in some outcrops they com-
prise as much as 5 percent of the volume of the rock.
They range in diameter from less than an inch to more
than a foot. Although most of the inclusions have
sharp contacts (fig. 10) with the enclosing igneous host
rock, a few have gradational contacts-that is, the
hornblende becomes less abundant outward from the
central part of the inclusion. Some of the hornblendic
inclusions are nearly monomineralic and appear struc-
tureless; others have a distinct gneissic or schistose
texture and contain other minerals in large proportions.

Modes of three hornblende-rich inclusions and a
granite inclusion are shown below.

GEOLOGY, PETROLOGY, UTE

Modes, in percent, of inclusions
[Chemical analyses are shown in table 3]

 

Field sample

 

Mineral

EMS-64-52 | 56-E-105 | EMS-101-52

(granite)

 

 

Hornblende......._..:...;...
.IN III+.

   

Plagiociage.....cc__._.__...._
Potassium feldspar...........

 

 

 

 

 

 

2 Anio.
* Includes yellow-brown chlorite.
Locality descriptions:
56-E-223: Sentinel Peak bysmalith.
EMS-64-52:; Sentinel Peak bysmalith. Collected by E. M. Shoemaker.
56-E-105: Black Mountain stock.
EMS-101-52; Black Mountain stock. Collected by E. M. Shoemaker.

Samples 56-E-223 and EMS-64-52 are inclusions
from the Sentinel Peak bysmalith, which consists of
granodiorite whose mafic minerals are all pseudomor-

 

 

MOUNTAINS AREA, COLORADO

  

Diorite porphyry

  
 
   

Hornblende gneiss

1 INCH

 

FIGURE 10.-Inclusions in intrusive igneous rocks of the Ute Mountains.
A, Metasediment composed of argillite and hornblende gneiss in-
jected with granite. B, Hornblende-rich inclusion composed almost
entirely of hornblende. Granodiorite is the enclosing host rock.

phous after hornblende. Both samples contain biotite,
but neither they nor the host rock contains pyroxene.
The hornblende in sample 56-E-223 has been partly
altered to yellow-brown chlorite and green chlorite
near the contact with the granodiorite porphyry. This
alteration probably occurred at the same time as that
of the phenocrysts of hornblende in the host rock.

IGNEOUS ROCKS 30

Magnetite is abundant in this inclusion, and some grains
contain cores of pyrite.

Sample EMS-64-52 consists of large crystals of horn-
blende and epidote in a matrix of plagioclase, potas-
sium feldspar, biotite, and quartz. The plagioclase is
more calcic than that of the host rock, averaging about
Ans, in composition, and is not zoned. The epidote
crystals are conspicuously zoned and are commonly
engulfed by single large crystals of plagioclase.

Sample 56-E-105 has distinct gneissic structure ex-
pressed by alternating layers of light and dark minerals.
Augite is nearly as abundant as hornblende in the in-
clusion. Within the dark layers, laminae consisting
almost entirely of augite alternate with laminae com-
posed almost entirely of hornblende. Apatite is com-
mon in this inclusion and is much more abundant near
the contact with the host rock.

ALTERATION

Four types of alteration other than baking effects near
contacts have been observed in or near intrusive bodies.
They are: (a) deuteric alteration of the central parts
of laccoliths, (b) hydrothermal alteration at Mable
Mountain, (c) pyritization in the vicinity of igneous
contacts with Mancos Shale, and (d) hydrothermal or
hot-spring alteration associated with the formation of
breccia pipes in the Yucca cluster of intrusives.

DEUTERIC ALTERATION

The central parts of many laccoliths have been in-
tensely altered by deuteric action. The alteration is a
direct result of the partial sealing in of residual liquids
in the interior of an intrusive mass.

In the altered rock, phenocrysts of calcic plagioclase
have been partly replaced by more sodic plagioclase and
sericite, calcite, and minor chlorite and kaolinite.
Hornblende has been altered to sericite, chlorite, and
magnetite, and pyroxene to chlorite. The groundmass
in the altered rock generally contains more chlorite,
sericite, kaolinite, and quartz than in the fresh rock.

The Sundance laccolith, the Three Forks laccolith,
and several laccoliths in the Yucca cluster (pl. 1) have
altered central parts that are well exposed.

HYDROTHERMAL ALTERATION AT MABLE MOUNTAIN

The rocks at Mable Mountain are fractured along a
northeast-trending zone that cuts through the central
part of the mountain and appears to be an upward ex-
tension of the zone along which the Ute Creek dike was
intruded (pl. 1). Pyrite occurs in veins in the fracture
or shear zone and as discrete crystals in the rocks for
a considerable distance on either side of the fracture
zone. At the surface the pyrite is altered to red limonite

745-807 O-65-6

 

Magnetite is abundant in this inclusion, and some grains
contain cores of pyrite.

In addition to being pyritized, the rocks at Mable
Mountain have been slightly argillized and slightly
sericitized. Thin sections of altered diorite porphyry
reveal that plagioclase phenocrysts have been altered to
clay, sericite, and calcite, and phenocrysts of hornblende
to chlorite, calcite, pyrite, and magnetite.

PYRITIZATION IN THE VICINITY OF IGNEOUS CONTACTS

Exposures of contacts of diorite porphyry and Man-
cos Shale commonly reveal disseminated crystals and
irregular masses of pyrite in both the porphyry and the
shale as much as 20 feet from the contacts. Because the
shale is gypsiferous, it may have furnished most of the
sulfur for the pyrite.

HKYDROTHERMAL (HOT-SPRING) ALTERATION ASSOCIATED WITH
BRECCIA PIPES

Rocks in and above nearly circular breccia pipes in the
Mancos Shale in the extreme western part of the moun-
tains (in or adjacent to sec. 31, T. 35 N., R. 18 W.) have
been altered by hydrothermal solutions of the hot-spring
type. Above the pipes, a laccolith composed of diorite
porphyry and a sill of lamprophyre have been intensely
fractured and impregnated with travertine. The traver-
tine was apparently deposited by calcium carbonate-
bearing hot water that rose along the pipes. Pieces of
diorite porphyry fill the central parts of some of the
pipes and in places are argillized and nearly white.
Mancos Shale within the pipes is light gray green to
light tan in contrast to its normal dark-gray to black
color. The breccia pipes are discussed further on pages
59 to 60.

PETROLOGY

CHEMICAL COMPOSITION OF THE IGNEOUS ROCKS

Chemical analyses of 16 igneous rocks and of 2 in-
clusions are shown in table 3. All the analyses, with
the exception of two from Cross (1894), were made by
methods described by Shapiro and Brannock (1956).
The norms were computed by using the direct method
described by Wahlstrom (1947, p. 235-237). In this
method, diopside is not computed and excess CaO) is
allotted to wollastonite. MgO and FeQ are allotted to
enstatite, ferrosilite (iron hypersthene), and magne-
tite.

Sample 1 in table 3 is of the porphyritic spessartite
lamprophyre. This rock contains the least silica and
alumina of any rock in the Ute Mountains and is com-
pletely without quartz. The norm indicates that the
rock is undersaturated in silica. Olivine (forsterite)
is computed in the norm, although it is not actually
found in the rock. The principal mafic minerals,

36

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

3.-Chemical analyses and norms of igneous rocks from the Ute Mountains, Colo.

[Analyses, in percent, by P. L. D. Elmore, S. D. Botts, and K. E. White (samples 1, 2, 5, 6, 13, 15); Elmore, Botts, and M. D. Mack (samples 3, 4, 12); H. F. Phillips,
Elmore, and White (samples 7, 11, 14, 17, 18); W. F. Hillebrand (samples 8, 10); Elmore and Botts (samples 9, 16)?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  
 
 

Sample.......... v 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Field No...... 56-E- (56-E-77| 55-E- | 55-E- |56-E-42)56-E-33| EMS- |________ H-516 EMS- |56-E-40|56-E-82) EMS- |56-E-92) H465-1| EMS- | EMS-
46A 48A 87A 96-52 65-52 94-52 101-52 | 64-52
Laboratory No--| 148755 | 148758 | 150490 | 150489 | 148757 | 148756 | 81653 |___... 150061 |______._ 77081 | 150492 | 148759 | 81651 | 148760 | 150060 | 81654 77080
Analyses
48. 8 50.0 55.3 55.9 56. 8 58. 6 57. 4 59.42 | 61.1 62.65 | 62.6 64. 6 65.2 68. 4 68.7 68. 9 76.0 40. 8
15.5 17.3 16.7 17.2 16.7 17.4 16.7 16.79 16.9 16.68 | 17.6 16.5 16. 8 16.7 17.1 17.4 14.0 14. 4
4.8 2.9 5.0 4.8 3.1 2.6 .9 3.23 3.8 2.35 1.9 1.3 2.3 1.3 1.4 13 | .5 5.8
6.7 5.8 1.8 2.8 3.8 4.4 5.5 8. 29 2.8 2. 63 2.1 1.8 11 -78 . 64 .43 . 20 10.0
5.8 8. 4 2.5 8.1 3.1 3.0 2.2 2. 24 2.1 1.43 11 1.0 1.0 . 36 .78 . 54 .07 8.1
8.5 9.0 6.0 6.4 6.5 4.8 5.8 5. 57 4.7 4.96 4.3 8.7 3.7 1.8 17 2.3 1.3 11.3
3.8 3.3 4.0 3.9 8.7 4. 4 4.2 4.15 4.0 4. 45 5.1 3.7 4.0 6.0 4.0 5. 4 3.6 2.3
1.9 1.1 1.9 2.0 2.1 2.1 2.1 2.82 2.9 2.75 2.0 g 2.4 2.8 3.6 3.0 4.7 1.1
1.2 . 85 72 . 78 72 . 68 . 60 . 68 . 66 42 . 85 . 30 . 82 17 .20 18 . 04 1.4
. 42 . 49 .36 . 37 . 29 . 30 . 34 . 35 .31 . 28 .19 .10 16 . 10 iM . 08 .00 16
17 16 15 16 12 12 14 13 . 09 16 . 07 12 . 08 .06 . 06 . 05 .02 . 28
3.7 2.8 4.1 2.2 2.6 1.6 4.0 1.06 1. 4 . 93 1.5 1.9 1.6 1.4 2.0 1.0 . 20 2.0
.46 2.8 . 61 . 20 . 58 . 05 .20 . 44 1.3 1:5 1.2 . 34 12 .13 . 05 1.1
100.25 | 99.90 | 99.14 | 99.81 | 100.11 | 100.05 | 100.08 | 100.17 | 100.36 | 99.69 | 100.11 | 99.22 | 99.86 | 100.21 | 100.41 | 100.71 | 100.77 98. 79
Norms
0.0 7.6 11.2 9.7 11.1 9.2 8.0 11.0 15.0 14.1 17.7 26.7 27.0 19.7 26.9
11.2 6.5 11.2 11.9 12.4 12.4 12.4 16.7 17.1 16.3 11.8 16.0 14.2 16.6 21.3
82.1 27.9 33.8 33.0 31.3 37.2 35.5 35.1 33. 8 37.6 48.1 31.3 33.8 50.7 33.8
19.9 24. 4 21.9 28. 4 22. 4 20. 9 20. 4 18.7 19. 4 17.2 11.9 8.5 10.0 6.0 7.5
0 1.7 0 0 0 .2 0 0 0 0 2.6 4.3 3.9 1.5 8.9
9.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 8.5 6.3 7.8 7.8 7.5 5.5 5.5 5.3 3.6 2.8 2.5 2.5 :0 2.0
4.9 7.8 0 .2 3.5 5.1 8. 4 2.5 0 2.5 1.8 2.0 0 A 0
6.9 4.2 4.5 7.0 4.5 3.8 1.3 4.7 5.5 3. 4 2.8 1.9 3.3 1.9 2.0
0 0 «5 0 0 0 0 0 0 0 0 0 0 0 0
2.2 1.6 1.4 1.5 1.4 1.3 11 1.3 1.2 .8 £7 .6 .6 .8 4
1.0 6.4 1.4 .5 1.3 0 .5 1.0 .2 0 3.0 3. 4 2.7 .8 .38
.9 11 .8 .8 .6 £7, Ye .8 v4. .6 .4 .2 4 v2 4
9.3 0 .8 21 .6 0 2.1 2.2 .6 2.9 0 0 0 0 0
Sample material Locality Sample material Locality Sample material Locality
I. Spessartite............ Vicinity of Yucca cluster. 9. Granodiorite por- Ute Peak. 15. Quartz monzonite por- Vicinity of Black Moun-
-.. The Tongue laccolith. phyry. phyry. tain.
+ -_ Yucca cluster. 10. "'The Knees' (Cross, 1894). 16. Banded laccolith.
4. £ Do. 11. Sentinel Peak (vicinity of 17. inclusion. Vicinity of Black Moun-
5. 3 Do. "The Knees"). tain.
6. ___ Pack Trail laccolith. 12. Do (altered). ..... Mushroom laccolith. 18. Hornblendite inclu- Sentinel Peak.
e Flat laccolith. 13. Do..: cnl Do. sion.
8. Granodiorite por- Ute Peak (Cross, 1894). 14. Quartz monzonite Vicinity of Sentinel Peak.

phyry. porphyry.

chlorite, augite, and hornblende presumably contain all
the magnesium in the rock. Only about 1 percent potas-
sium feldspar was observed in thin section whereas
there is 11 percent of normative potassium feldspar.
The difference is in part due to abundant modal biotite
(not calculated in the norm), and probably in part to
KAISi;O; in solid solution in the plagioclase. The rock
is undersaturated with respect to alumina and, as a
consequence, abundant wollastonite appears in the
norm.

Sample 2 is of the porphyritic microgabbro. The
rock is moderately altered and contains about 7 percent
calcite. The microgabbro is quite low in magnesium
relative to gabbro from other parts of the United States
(Clarke, 1924, p. 465). This low value reflects the lack
of olivine and the paucity of pyroxene in the porphyritic
variety of microgabbro.

Samples 3-7 are of diorite porphyry. In general, the
analyses show more Na,0 than inferred from the mode
and indicate a considerably higher content of sodic

 

plagioclase than in the microgabbro. The K,0 content
in the five samples of diorite porphyry analyzed is
nearly constant at about 2 percent. This content is
nearly twice the K,0 in the microgabbro, and the differ-
ence is expressed in thin section by abundant small
crystals of potassium feldspar in the groundmasses of
all the diorite porphyry samples.

Six samples, 8-13, are of granodiorite porphyry and
include the southern type (10, 11), the central type (12,
13), and the northern type (8,9).

The granodiorite is richer in SiO;, K0, and Na,0
than the diorite and has slightly less MgO, CaO, and
total Fe. The K.O is fairly uniform except in sample
11, which apparently contains much albite and an ab-
normally small amount of potassium feldspar.

The central type of granodiorite porphyry is the most
silicic of the granodiorite and is the poorest in total iron
and magnesia. This type is intermediate between
granodiorite and quartz monzonite.

IGNEOUS ROCKS 37.

The northern type of granodiorite (hornblende
granodiorite of Ute Peak and Black Mountain) has the
coarsest groundmass. An approximate mode agrees well
with the norm except that there is 11 to 15 percent of
normative quartz and only 4 to 5 percent of modal
quartz. This difference is probably due entirely to
errors in identifying the minerals of the groundmass.

Samples 14-16 are of quartz monzonite porphyry.
These rocks are all characterized by conspicuous pheno-
erysts of quartz and by an extremely dense groundmass.
The extremely fine grain of the groundmass and the
high content of Na,0 in these rocks create a problem as
to rock name. The norms indicate a greater abundance
of albite than in the granodiorite but, in general, about
the same amount of orthoclase, The rocks could have
been included with the granodiorite, but their quartz
phenocrysts and low color indices make them separable
in the field, and their high silica content, total alkali
metal content, and low calcium, iron, and magnesium
content make them easily separable chemically from the
granodiorite. The name quartz monzonite is believed
justified on the basis of total alkali feldspar (albite+
potassium feldspar). The chemical analyses indicate
an average of about 68 percent silica, which is approxi-
mately 5 percent more than the average silica for gran-
odiorite. The samples of quartz monzonite porphyry
contain more Na,0 plus K0 than the granodiorite, less
calcium, and markedly less total iron and magnesium.
The K,0 content averages about 3 percent.

VARIATION DIAGRAMS

The weight percentages of six major oxides in the
analyses were plotted against the percentage of silica.
(See fig. 11.) Ferric iron was recomputed to ferrous
iron for each analysis and is shown with the reported
ferrous iron. The resulting total iron eliminates the
erratic though somewhat mutually compensating varia-
tions of the two iron oxides.

The diagrams clearly illustrate the nearly constant
alumina content of the rocks (excluding the lampro-
phyre), the steep decline of total iron, magnesia, and
lime from high values in the microgabbro to low values
in the quartz monzonite, and the gentle rise of Na:O+
K,0 with increasing silica. The curve for the sum of
the two alkalies shows a pronounced reciprocal relation-
ship with lime. This relation is a reflection of the in-
creased amount of sodic plagioclase and potassium feld-
spar in the more silicie rocks.

The alkali-lime index (Peacock, 1931) provides a
means of comparing igneous rocks throughout a region
or district. - The alkali-lime index for the igneous rocks
of the Ute Mountains is 59 (fig. 17). According to
Peacock's classification, these rocks are within the cale-

 

 

 

SAMPLE
n s
6 6
C i
m mn
o o
o o
a a
as dlls o
"S . % 1 Co
h. sgsras Size {3
1 h p tu he in o Tu gas 40
s f s¢
8 88 B8 2 5 2 8 8 I A8
Freds __ AL in" s C3
ad
SN M
< %
15 | | | |
10 - y
O
a 5
ee
0 | I |
5__ -
O o
bp
a
0 | | |

 

 

 

WEIGHT PERCENT

CaO and Na,0 plus K,0
U
I

 

 

IOT g

Na,0
on
T

 

 

 

K,0

 

 

o | I | 1
50 55 60 65 70

WEIGHT PERCENT SiO;

 

FiGURE 11.-Variation diagrams for igneous rocks of the Ute Mountains.
All analyses in weight percent recalculated (minus H;O, SO;, and
CO;) to 100 percent. Analyses are shown in table 3.

38

alkalic field. Significantly, the alkali-lime index of
the Ute Mountains rocks is similar to that of other lac-
colithic groups in the Colorado Plateau. Indices for
porphyritic rocks of the La Plata Mountains from anal-
yses published by Cross and Purington (1899, p. 6-7),
for rocks of the Carrizo Mountains from analyses made
available by E. M. Shoemaker and J. D. Strobell, and
for the Abajo Mountains from analyses furnished by
I. J. Witkind and E. M. Shoemaker, were computed as
follows:

Ta Plata abt

Carrizo a~58

Abajo 60. 5

GEOLOGY, PETROLOGY, UTE

The alkali-lime indices are not listed for the rocks in
the Henry and La Sal Mountains laccolithic groups
because they are too uncertain. Many of these rocks
are high in alkalies and low in silica (Hunt and others,
1953, p. 154; Hunt, 1958, p. 324-333). - Graphs of their
oxides show a wide dispersion of points that do not de-
fine single curves for either lime or total alkalies.
More than one igneous series may be represented in
these laccolithic groups. Analyses of only the diorite
porphyry and the monzonite porphyry in the Henry
and La Sal Mountains suggest an alkali-lime index be-
tween 59 and 62.

Thornton and Tuttle (1956) suggested a diagram to
illustrate trends from mafic to felsic rocks without re-
gard to the processes contributing to the formation of
the rocks. In this diagram, oxide weight percentages
are plotted against the sum of normative quartz, albite,
orthoclase, nepheline, leucite, and kaliophilite. This
sum is called differentiation index because of the
tendency for all magmas to change in composition
toward the system quartz-nepheline-kaliophilite. In
figure 12 the oxide weight percentages of rocks from the
Ute Mountains are plotted against the sum of normative
quartz, albite, and orthoclase, there being no normative
nepheline, leucite, or kaliophilite. Figure 12 is sub-
divided according to the scheme of Sukheswala and
Poldervaart (1958, p. 1489). Except for FeO, and
FeQ, the scattering of points is very minor, and straight
lines fit the points very well. The scattering of
and FeQ is thought to be due to mutually compensating
variations of the two individual iron oxides. SiO;
shows a gradual increase throughout the series; Al;O;
is nearly constant throughout; TiO, MgO, and CaO
show steady decreases. Na,0 and K,0 increase
throughout the series. The microgabbro 'has a differ-
entiation index of 40, and it plots in the field of late-
stage basalt. The diorite falls in the field of inter-
mediate differentiates; the granodiorite falls along the
boundary between the intermediate differentiates and
the felsic differentiates having indices ranging from

 

MOUNTAINS AREA, COLORADO

about 63 to 75. The quartz monzonite falls in the field
of felsic differentiates having indices between 82 and
87.

MINOR ELEMENTS

VARIATION IN THE IGNEOUS ROCK SERIES

The minor elements of the igneous rocks are shown
in tables 4 and 5. All amounts, except for those of
titanium and manganese, were determined by semi-
quantitative spectrographic analysis. In this proce-
dure a weighed amount of the sample mixture is burned
in a controlled d-c are and the spectrum recorded on a
photographic plate. Selected lines on the resulting
plate are visually compared with those of standard
spectra prepared in a manner similar to that for the
unknowns. The approximate visual detection limits for
the elements determined by this semiquantitative spec-
trographic method are shown in table 4A. The as-
signed group for semiquantitative results will include
the quantitative value about 60 percent of the time
(Myers, Havens, and Dunton, 1961). The chemical
analyses for titanium and manganese were by methods
described by Shapiro and Brannock (1956).

Frequency distribution of minor elements in each type
of rock is shown by histograms in figure 13. Some of
the histograms depart considerably from a normal dis-
tribution curve, as, for example, cobalt in the granodio-
rite. The spread in values is probably due partly to
variation between samples and partly to the analytical
method. Although the mean content of cobalt cannot
be determined accurately from these values, the trend
of cobalt values from about 17 ppm (parts per million)
in the diorite to a trace in the quartz monzonite is
significant.

In determining the geometric mean of an element for
which one or more values were reported as zero (amount
is below the visual detection limit), it was assumed that
because the majority of the samples showed significant
quantities, that element is present in the remaining
samples, but in amounts slightly less than the sensitiv-
ity. For example, in the diorite, 4 samples were ana-
lyzed as having less than sensitivity values for nickel
and 19 samples contained determinable amounts. In
determining a mean value for the 23 analyses, the 4
samples were assumed to contain 0.00015 percent nickel
(one-half the visual detection limit of 0.0003 percent
for nickel). Values reported as trace were considered to
be threshold amounts when computing the mean for an
element.

In comparing the amounts of trace elements in the
igneous rocks of the Ute Mountains to averages in other
parts of the world (table 4), it is apparent that the

IGNEOUS ROCKS

 

Ultramafic £5er aim Late-stage Intermediate Felsic
differentiates | ' basealstsage basalts differentiates differentiates

 

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0 20 40 60 80 100

FiGurE 12.-Oxide weight percentages of rocks from the Ute Mountains plotted against the sum of normative
quartz, albite, and orthoclase (the differentiation index). (After Thornton and Tuttle, 1956, and Sukhes-
wala and Poldervaart, 1958.)

40

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

TABLE 4.-Semiquantitative spectrographic and chemical analyses, in parts per million, of igneous rocks from the Ute Mountains

[an values, except italic, are geometric means (antilogs of mean logs) determined from semiquantitative spectrographic analyses (table 5; See Miesch and Riley, 1961). Italic
values were determined from chemical analyses (valueg for microgabbro is based on a single chemical analysis; for diorite porphyry on 5 analyses; for granodiorite on 6
analyses; for quartz monzonite on 4 analyses). Semiquantitative spectrographic determinations made by the rapid visual-comparison method. Comparisons of similar
data with those obtained by quantitative methods show that the assigned semiquantitative-class interval includes the quantitative value in about 60 percent of the de-
terminations. Tr., near threshold amount of element. - Analysts: Spectrographic, N. M. Conklin, R. G. Havens, J. C. Hamilton, P. J. Dunton; chemical, P. L. D. El-

more, 8. D. Botts, M. D. Mack, K. E. White]

 

 

 

    
  

 

 

 

 

 

 

 

Average abundance
Microgabbro Diorite Granodiorite! Quartz mon- |Lamprophyre| (world-wide distribution)
Element (3 analyses) porphyry porphyry zonite por- | (3 analyses)
(23 analyses) | (9 analyses) phyry
(6 analyses) Gabbro Diorite
(DAIM. ,s -L ear 410 740 880 60 1 230
() () Ir. Tr. () 32 +3
(2) () (0) Tr. (?) $3
68 17 11 Tr, 25 79 8 32
88 6 5 3 50 340 8 68
150 30 16 10 115 149 7 38
_______ 30 18 17 19 18 12.8 814.5
_______ ) Tr. ~30 Tr. ) 9 8 26
. _... Po. U2. L GT uis on ae na nene ar's aad Shep a 1, 240 1, 080 850 460 1, 240 1, 160 * 606
NEOIyDIONUM .. . 11.00... _. eon 000000000 ee deen <10 Tr. (] C) ) 10 3 12.5
Nickel......... 40 5 4 Tr. 15 158 8 40
Niobioum (0) (0) Tr. Tr. © 19 11 3.6
Tr. Tr. 12 15 Ts, 5 12 30
Scandium. 50 17 13 ~5 32 20 3 4.6
Strontium. 680 1, 060 1, 040 880 680 170 18 260
TT iOMIGIL 2... .. ..- Lun sech 5, 100 , 200 2,700 1, 100 7, 200 14 4, 400
315 150 74 25 245 14 150
Yiiriuim........;c.ll.l 41 23 32 ~13 30 117.9
Yiterbtlum........._... we. 5 3 3 ~1.5 4
+-. L Le. .... oul bent out oren demean n beaes bene se en 70 115 125 100 70 140 18 280

 

! Engelhardt (1936).

2 Indicates the amount is less than the sensitivity.
3 Goldschmidt (1954).

+ Sandell (1952).

5 Goldschmidt and Peters (1932¢).

8 Goldschmidt (19372).

? Sandell and Goldich (1943).

8 Sandell (1947).

Ute Mountains rocks are relatively high in barium,
strontium, scandium, and low in chromium, nickel, and
zirconium.

Barium exhibits a marked increase from about 400
ppm in the microgabbro to more than 800 ppm in the
quartz monzonite porphyry. Most of the barium is
probably contained in the potassium feldspar and bio-
tite. Strontium is most abundant in the diorite and
granodiorite, where it is probably present in both the
plagioclase and potassium feldspars. Beryllium was
detected only in the granodiorite and quartz monzonite,
and boron only in the quartz monzonite.

The amounts of cobalt, chromium, copper, manganese,
nickel, scandium, titanium, and vanadium are relatively
high in the microgabbro and low in the quartz mon-
zonite porphyry. These elements are nearly all con-
fined to the mafic minerals. In figure 14, five elements
(vanadium, copper, cobalt, chromium, and nickel) are
plotted against total silica and pyribole. In general,
the five elements decrease gradually in amount from
microgabbro to quartz monzonite. However, two
samples of diorite porphyry, 56-E-42 and 56-E-33, are
unusually high in mafic minerals for diorite in the Ute
Mountains. Sample 56-E-42 contains approximately 9
percent pyroxene and 7 percent hornblende, and al-
though sample 56-F-33 contains no pyroxene, it has 17
percent hornblende.

Gallium is markedly more abundant in the microgab-
bro than in the other rocks. Lanthanum was found in

* Otto (1936).

!! Hevesy and Hobbie (1933).

!! Rankama and Sahama (1950).

? Hevesy, Hobbie, and Holmes (1931).

18 Noll (1934).

14 Goldschmidt (1937b) (average for igneous rocks).
1s Tongeren (1938) (average for igneous rocks).

# Hevesy and Wiirstlin (1934).

appreciable quantities only in the granodiorite
porphyry. Lanthanide elements are admitted by
capture into the crystal lattices of compounds of quadri-
valent elements of similar size, such as zirconium
(Goldschmidt, 1954, p. 315). The lanthanum in the
granodiorite may be related to the greater abundance
of zircon in these rocks. Yttrium and ytterbium do not
follow the variation in lanthanum, and they show a
slight decline as silica increases in the rock series.
Traces of molybdenum were detected only in the micro-
gabbro and in the diorite porphyry. Niobium in
amounts near the visual detection limit was observed
only in the granodiorite and quartz monzonite. Lead
gradually increases from barely detectable amounts in
the microgabbro and diorite to approximately 15 ppm
in the quartz monzonite.

VARIATION OF MINOR ELEMENTS WITH ROCK ALTERATION
VARIATIONS IN IGNEOUS ROCKS

As previously mentioned in (p. 35) the intrusive rocks
of the Ute Mountains are partly or wholly altered.
Most of the alteration is due to the action of deuteric
solutions; however, the Mable Mountain bysmalith and
associated dike were altered by sulfide-bearing solutions
which were introduced after the intrusive masses had
completely solidified.

Paired samples or suites of samples of altered and
unaltered rocks were taken from several intrusive
bodies. These samples were analyzed spectrograph-

 

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42

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

TABLE 5.-Semiquantitative spectrographic analyses

 

[Sample numbers with prefix H were collected by F. N. Houser; prefix EMS, E. M. Shoemaker. Analysts: 1. N. M. Conklin; 2, R. G. Havens; 3, J. C. Hamilton. _......
looked for but not detected. - Approximate visual detection limits for the elements determined by the semiquantitative spectrographic methods at the Denver laboratory,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sample No. Elements
Rock type F4 z
Labora- | £ | si Al Fe Mg Ca Na Ti P Mn B Ba Be
Field tory (0.002)| (0.001) |(0.0008)| (0.0005) | (0.005) | (0.05) (0.7) (0.0002) | (0.2) | (0.0002) | (0.002) | (0.0002) | (0.0001)
10.0005) | (£ 0.002)
Microgabbro
porphyry.......... 56-E-76 244798 I| XX |XX XX X XX X X 0. X
56-E-77 244799 12.9%. XX XX XX X X se mell -d
Diorite 56-E-94 244713 1} XX X+ X+ X+ X X aB | Hil cever
porphyty.......... 55-E-44A 238250 2 | XX | XX X+ X- X X X VR aes eee
55-E-47B 238260 2 | XX XX X X X+ X X wKX {il
55-E-48B 238261 2 | XX | XX X X- X X X w: eileen ene
55-E-49 238262 2 | XX XX X X- X X X t. < is
55-E-50A 238263 2 | XX XX X X- X X+ X R
56-E-33 244757 I { XX XX X+ X X+ X+ X th:< R
56-E-37 244760 1 | XX XX X+ X X+ X X TK s
Altered diorite
porphyry (mate
to 56-E-87).......- 56-E-38 244761 1 | XX | XX X+ X X+ X+ X
Diorite porphyry....| 56-E-42 244764 1 | XX | XX X X X X X
56-E-78 244800 1 |XX | X+ X X X X X
56-E-175 253402 3 | XX | XX X X X X X
56-E-189-4 251988 1 | XX XX X+ X- X+ X X
H 387-2 251993 1 | XX | XX X X- X+ X X
H 308-1 251994 1 | XX | XX X X- X+ X X
H 408-1 251995 1 | XX | XX X X- X X X
H 411-2 251996 1 | XX XX X X- X X+ X
H 414-1 251997 1 | XX XX X X- X X X
H 517-1 252026 1 | XX | XX X X- X X X
583-2 252031 I | XX | XX X X- X+ X X
H 534-2 252082 1 | XX | XX X+ X- X+ X X
Altered diorite por-
phyry (mate to H
CSTR) een H 387-1 251992 1 | XX XX X X- X+ X+ X
Diorite porphyry
(mate to H 387-1)..| H 387-2 251998 I[ XX XX X X- X+ X X
Diorite porphyry....| EMS-96-52 | D-81653 |____| XX | XX X X- X X X-
Granodiorite
porphyry....._.___ 56-E-51A 244772 1 | XX | XX X X- X X X
244804 1 | XX XX X X- X X X
56-E-88 244708 1 | XX XX X+ X X+ X X
H 270-1 244834 1 | XX | XX X X- X X+ X
H 373-1 251991 1 | XX XX X X- X X X
H 515-2 252024 1 { XX XX X X- X+ X X
H 516-2 252025 1 | XX XX X+ X- X+ X X
56-E-40 244762 1 | XX XX X X- X X X
EMS-65-52 | D-77081 4 | XX X X X X X . X
Quartz monzonite }
porphyry......_... 56-E-13 244743 1 | XX XX X- . X+ X X+ X
56-E-02 244812 ape. XX X . X+ X X X
H 465-1 252004 1 |XX XX X -X X- X+ X
H 526 252030 1 | XX XX X . X X- X X+
EMS-94-52 | D-81651 | 2 | XX XX X- /K X- X X-
EMS-95-52 | D-81652 | 2 | XX | XX X- VX X- X X-
Lamporphyre... ..... 55-E-46A 253490 3 | XX XX X+ X+ X+ X X
55-E-400 238258 2 | XX XX X X X X+ X $
55-E-51 238264 2 | XX XX X+ X X X X .OX+
1 A different exposure is required for the lower detectabilities shown.
ically, and the results of the analyses are shown in | from the Mable Mountain bysmalith and con-

table 6.

Apparently, there are few consistent variations. For
example, the data from suite 7, taken from the Three
Forks intrusive mass, a laccolith with an intensely all-
tered center, show that vanadium was removed during
alteration of diorite porphyry, but samples from other
intrusive bodies show virtually no variation in va-
nadium.

Cobalt decreases in the altered rock of three suites, 6,
T, and 9, and increases in altered rock in suite 8. Suite
9 is sheared rock from Ute Creek dike which extends

 

tains copper deposits. Nickel and chromium decrease
slightly in the altered rocks of several of the suites.
Although these variations are small and generally
within a single order of magnitude, they appear to be
significant because sedimentary rocks adjacent to the
igneous intrusive rocks have corresponding increases
in these elements. Variations in equivalent uranium
are slight as determined by radiometric analyses (a
maximum of 2 ppm for one suite). Chemical deter-
minations of uranium for one suite show virtually no
variation.

IGNEOUS ROCKS 43

of igneous rocks from the Ute Mountains, Colo.

Percent
0.02 -0.05

 

( 000 X-
U.S. Geological Survey (revised August 1956,) are shown in parentheses.

Some combinations of elements affect the detectabilities. Approximate values are given. In

uxiusually favorable materials, concentrations somewhat lower than the values given may be detected. In unfavorable materials, the given detectabilities may not be

obtained for some of the elements]

 

 

 

 

 

 

 

 

 

 

 

 

Elements-Continued
Co Or Cu Ga La Mo Nb Ni Pb Sc Sr v ¥ Yb Zr
(0.0005) (0.0001) (0.0001) (0.0002) | (0.002) | (0.0005) | (0.001) (0.0003) (0.001) (0.0005) (0.0002) | (0.001) (0.001) (0.0005) (0.001)
0.00X+ | 0.00X+ 0.0X- 0. 0X 0.00X+ 0.000X+ | 0.00X+
00X+ .O0X+ .OX- . OX .O0X O00 X+ .O0OX+
O0X+ .OX- .OX- .OX .O0X O00 X .O0X+
00X- 000 X+ 00X .OX- .O0X . O00 X .OX-
00X- O00 X+ O0X .OX- .O0OX- . O00 X .O0X+
00X- . O00 X+ 00 X .OX- .O0X- . O00 X- .OOX+
00X+ . O00 X 00 X .OX- .O0X- 000 X- .O0X+
X- . O00 X+ X .OX- .O0X . 000 X .OX-
00 X .O0X O0X+ .OX .O0X . O00 X .0OX-
00 X 000 X+ O0 X .OX- .O0X . O00 X .OX-
.O0X . O00 X+ O0 X. .OX- .O0X . 000 X .0OX-
.O0X .O0X- 00X+ .OX- .O0OX- 000 X .OX-
.O0X O00 X+ 00X+ .OX- .O0X- 000 X O0X+
.O0X- 000 X+ 00 X 0X- .O0X . O00 X .OOX+
. O0X .0O0X- 00X OX- . O0X . O00 X .OX-
.O0X- 000 X+ 00X .O0OX . 000 X .OX-
.O0X- . 000 X . O0X 0X- . O0X . 000 X. .OX-
.O0X- 000 X- .O0X- 00X+ .O0X- . 000 X .0OX-
000X+ | .000X- 000 X+ .OX- .O0X . 000 X .OX-
X- . O00 X 00 X .OX- .O0X- . O00 X .OX-
00X- . O00 X 00 X- .OX- . O0X . O00 X .OX-
00X- . O00 X+ 00 X .OX- .0O0X- . O00 X .OX-
00X- .O0X 00 X .OX- .OO X- . 000 X 00 X+
.O0X- 000X+ 00 X 00 X . 000 X .OX- .OX-
.O0X- 000X+ 00 X 0X- .O0X . 000 X .OX-
. O00 X+ 000 X+ 00 X 00X+ | .000X- O00 X- 00X+
.O0X- . O00 X+ 000 X+ .O0X+ | .00X O00 X .OX
.O0X- . O00 X+ 00 X. .O0X+ | . 00X O00 X .OX-
. O0 X 000 X+ O0 X. .OX- .O0X+ 000 X+ .O0X+
. O0 X- O00 X+ 000 X+ .O0X+ .0O0X- . O00 X- .OX-
........... 000 X. 000 X+ O0 X .O0X . O00 X .OX-
.O0X- 000 X+ O0X 0X- .O0X O00 X .O0X+
.O0X- . 000 X 00X- 0X- .O0X 000 X .OX-
.O0X- . O00 X+ 00X- 00X+ .O0X 000 X OX
. O00 X 000 X 00 X 00 X .O0X 000 X 00 X
Tr . O00 X+ . O00 X+ 00X+ 00X- 000 X- .OX-
Tr . O00 X- .O0X- O0X 00 X. . 000 X. .OX-
___________ . O00 X . O00 X O0 X Tr. Tr .OX-
....................... .0O0X- 00X- | .00X- . O00 X. .OX-
. 000 X . 000 X .O0X- 00X- . Devel .O0X+
........... O00 X .O0X- NOX ({e zan p [acee ae ee we cel y OOX
00 X .O0X+ .OX- OX .O0X O00 X+ 00X+
00X- .O0X .0O0X+ 0X- .O0X 000 X O0X+
O0 X .O0X+ .OX- OX .O0X . 000 X 00 X+

 

 

 

 

 

 

 

 

VARIATIONS IN SEDIMENTARY ROCKS ADJACENT TO IGNEOUS
INTRUSIVES

The Junction Creek Sandstone exhibits color changes
wherever exposed in the vicinity of intrusive rocks in
the Ute Mountains area. Away from intrusive rock
the sandstone is predominantly orange or red. Ad-
jacent to intrusive rock, the color is light brown or yel-
low brown, probably because of the introduction of
iron oxide.

Spectrographic analyses of paired samples of red
unaltered sandstone and yellow-brown altered sandstone

 

(table 6) indicate that cobalt, chromium, gallium,
nickel, strontium, vanadium, yttrium, and ytterbium are
more abundant in the altered than in the fresh rock.
Samples of Dakota Sandstone (table 6, suite 10) yielded
generally similar results. The altered Dakota Sand-
stone was taken about 20 yards from a contact with
granodiorite porphyry, and the unaltered specimen was
collected from the same sandstone bed about a mile from
the nearest exposure of igneous rock. The altered Da-
kota Sandstone contains slightly greater amounts of
chromium, copper, gallium, nickel, and ytterbium than
the unaltered Dakota.

44 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

TaBLE 6.-Semiquantitative spectrographic analyses and radiometric analyses of
[For code see table 5. Values in italic for altered rock differ from: values for corresponding unaltered rock. Sample numbers with prefix EMS were collected by E. M.. Shoemaker.

 

 

 

 

 

  

 

 

 

 

 

 

 

 

Spectrographic analyses
Pair Sample ht!“ Rock type
or ory |
suite No. ‘é Minor elements
cl
31. Bs > |- Cs CrlOnIGalMo'Ni Po : | 'e
Igneous
1 55-E-47B 1 | 0.0X+ 0.00X- | 0.000X+ | 0.00X
"""" 1 | .OX+ .O0X- .O00X+ | .00X
2) .OX+ 00X 000X+ 00X
2 | .OX+ 00X 000X+ | .O0X
2 | .OX 00X+ 00X+ .OX-
2 | . OX 00X+ 00 X+ .OX-
yry. 2) .OX+ 00X- 000X+ O0X
3 t oe | ont) of
yry - =
2} .OX+ 00X - X 00 X-
3{mrphyr ________ 8 | .X 000X+ 000 X+ 00X
EMS-65-52 | D77081 Moggmteli tered. granodio- .OX 000X+ 00X- 00X
rite porphyr
EMS-66-52 | D77082 Altered grmodlonte porphy- | 3 | .OX . 000 X 000 X+ O0X
EMS-70-52 | D77086 Intensely altered diorite por- 3 | .OX 000 X+ Tr O0X
EMS-71-52 | D77087 Altered dlorite porphyry... 8 |. .OX .O0X- 000X O0X
EMS-72-52 | D77088 | Diorite porphyry._.________._.. 3 | .OX .O0X- 000X O0X
8 {EMS—73—52 D77089 | Altered diorite porphyry......| 3 | .OX .O0X- 000X 00X
"""" EMS-74-52 | D77090 | Diorite porphyry...___________] 3 .OX .000 X+ 000X 00 X
Hydrothermal
9 {EMS—12-53 D§8148 | Altered diorite porphyry...... 41 OX- 000 X+ | .00X+ .O0X- 00X 000 X+ |......... 000 X+
"""" EMS-14-53 | D88150 | Diorite porphyry..._______.____\ 4 00X+ .O0X- 00X- .OX+ .O0X- 00X- 00X- Tr 000 X+
Sedimentary
56-E-20 244747 Algereéi tfossiliferous Dakota | 2 | 0.0X- |...._._._..- 0.0X- | 0.00X-
andstone.
10......- 56-E-224 253493 Ugaltgnzd fossiliferous Dakota | 5 | .00X _ |-_..__.__--- .000X+ | .000X+
andstone.
56-E-15 244745 Alttered Junction Creek Sand- | 2 | .OX Tr .00 X- 000 X+
stong.
M...... 56-E-16 244746 | Unaltered Junction Creek | 2 | .OK -- |___________ .000X+ | .000X+
Sandstone.
56-E-510 244774 | Baked Mancos Shale...__..... 2 | .OX+ 0.00 X- .O0X+ .O0OX
122 H-368-1 251990 |.... O:rik\ s sos 2 | .OX .0X- .0 X+ .O0X
Unaltered _ Mancos Shale |._..| .032 000X+ | .004 .0O0X+
(average of three samples).
56-E-25 244751 Exiled sand afiimestone of the | 2 | |....____... 000 X NOX! {oll TFA See eee nees
ancos
18....... 55-E-70 253491 | Sandy I uglaestone Of the Man- | s} .ox |.._____.__. 000% A -OO0XE |-. :. 2 ccc l ero ieee eee ce lave guapa
cos

 

 

 

 

 

 

 

 

 

 

 

 

1 Chemical uranium determination.

The variations in minor metallic elements noted above
for the Junction Creek and the Dakota Sandstones are
very slight and the quantities involved are near the
limits of detection for those metals. Both sandstones
are rather poorly sorted and differences in grain size
may exist between the individual samples of a pair,
even though such differences are not apparent with the
hand lens. Other workers have found that the amount
of a minor element varries considerably with grain size
within a single stratum (V. C. Kennedy, oral commun.,
1957). This relation may explain the variation noted
in both the Junction Creek and Dakota Sandstones, but
it seems more likely that the variations are related to
the Ute instrusive rocks, because the effects of alteration
are similar in the two sandstone formations. The Man-

 

cos Shale, which shows little variation in grain size,
supplies corroborative evidence. Two samples of baked
shale contain more cobalt, chromium, nickel, strontium,
yttrium, and ytterbium than the unaltered shale (table
6, suit 12). Suite 13, baked and unbaked sandy lime-
stone from the Mancos Shale, shows little variation
except a loss of barium and a slight gain in copper,
yttrium, and ytterbium in the baked sample.

The authors conclude that the altered and baked sedi-
ments adjacent to the Ute intrusive rocks gained small
amounts of cobalt, chromium, gallium, nickel, yttrium,
ytterbium, and probably vanadium. These elements
apparently were transported by solutions from adjacent
intrusive bodies.

IGNEOUS ROCKS

45

altered and unaltered igneous and sedimentary rocks from the Ute Mountains, Colo.
Analysts-spectrographic: 1, R. G. Havens; 2, N. M. Conklin; 3, P. J. Dunton; 4, G. W. Boyes, Jr.; 5, J. C. Hamilton; radiometric: 1, C. G. Angelo; 2, J. P. Sehuch]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Spectrographic analyses-Continued Radiometric
analyses
Minor elements-Continued Major elements Equivalent | 4
bread |g
perce:
Sr ‘ v x Yb Zr Si l Al i Fe t Mg Ca ’ Na ’ K y Ti > Mn 4
rocks
0. X- 0. 0X - 0. O0X - 0. 000X 0.00X+ | XX XX X X X+ X X 0. X 0. x- | <0.001 1
.0X+ .OX- . O0X - .000X- | .00X+ | XX XX X X- X X X ~A .X- | < .00L 1
. X- .0X- . O0X . O00 X .0X- | XX XX X+ X X+ X X -% . X- . 001 1
. X- . OX - . OOX . O00X .0X- XX XX X+ X X+ X+ X o < .X- < .00L 1
.OX+ OX . O0X . 000 X+ 00X+ | XX XX XX X XX X X .X .X < .00 1
.OX+ OX .O0X+ 000 X+ 00X+ | XX XX XX X XX X X .X . X- | < .001 1
. X- .0X- . O0X . O00 X .OX- | XX XX X X- X+ X X aX .X- | < .00% 1
. X- .OX- . OOX . 000 X .0X- XX XX X X- X+ X+ X .X+ .X- | < .00L 1
. X- . OX - . O0X . 000 X .O0X+ | XX XX X X- X+ X X -% OX+ | < .00L 1
. X- .0X- . O0X . O00 X .OX- XX XX X+ X- X+ X X .X+ . X- . 003 1
.< . O0 X . O0X . 000X . O0X XX X X X X X X -& SOX ' Alis sista -
OX . O0 X . O0 X . 000 X .O0X XX X X X X X «K v. UX! {o -
OX . O0X O0X . O00 X . OOX XX X X xX X X X . X ~QX > -
OX .O0X . O0X .000X . O0 X XX X X X X X X -& C -
OX .O0X . O0 X . O00 X . O0 X XX X X X X X -£ r . X e
OX .OX . O0X . O00 X . OOX XX X X X X X .X /A s < -
OX . O0X . O0 X . O00X . O0X XX X X X X X X *. *. -
OX . O0X . O0 X . O00 X . O0X XX X X X X X .X X -X -
. OX .OX- TY. .OX- | XX XX X .OX+ .0X+ .OX+ . X+ X .OX- | 1 .0003(U) 2
. OX .O0X+ | .00X- .0O0X+ | XX XX X+ X X. - X. - X. - X . X- ! .0004(U) 2
rocks

0. 0X- 0.00X- | 0.00X- | 0.000X- | 0.00X+ | XX 0.X- | 0.X+| 0. X XX OLX 0.0X 0. X- | <0.001 1
.OX- . O0 X- 0X- | XX .X+ . X+ . X- | XX m . OX i RES ll nevis «Funds 1
00X+ . O0 X 00X- 000 X- OX XX X X+ X X+ X X- . OX . wt 4
. O00 X+ AOX- Te s Tr. | .0OX- | XX X- X- X .OX+ | X- .OX+ . OX < . 001 1
.OX+ .OX .O0X+ 000X+ | .0X- | XX XX X X XX X X . X- . X- . 002 1
.OX+ .0X- . O0X- X .OX- | XX XX X X- XX X+ X+ . X+ . OX . 002 1
. 082 .0X- »OOXK- .OX- | XX X+ X- . X+ X+ . X X ip:<€ XY M -
.OX+ .O0X- 00X- . 000 X- 00X- X+ . OX .X+ | X- XX »OX4 {sosa inces .OX- .0OX+ | < .001 1
. X- OOK: seee rai ds eee .O0X+ X . X . X+ X+ | XX SKF OX (OX ~ -

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2 Data from W. L. Newman and E. M. Shoemaker (written commun., 1952; average of three samples).

ORIGIN OF THE IGNEOUS ROCKS AND
HORNBLENDE INCLUSIONS

The igneous rocks of the Ute Mountains are a group
of intergrading hypabyssal intrusive rocks that changed
in composition from gabbro to quartz monzonite as in-
trusion proceeded. This progressive change with time
of intrusion suggests a derivation from a common an-
cester magma by some process of differentiation. The
chemical composition of the phenocrysts of the micro-
gabbro is very similar to the calculated chemical com-
position of material that might have been subtracted
from microgabbro to form quartz monzonite. In figure
15 the major oxides are plotted against silica for micro-
gabbro and quartz monzonite; the silica content of the
most mafic material that might be subtracted from the

 

microgabbro to obtain quartz monzonite is determined
by projecting the line connecting the points for K;0 to
0.5 percent, which is considered to be a realistic value
for a calc-alkalic magma. Other values are based on
the silica value thus found (points on vertical line at
left in figure 15A) ; they are compared to the chemical
composition of microgabbro phenocrysts in table 7.
Augite, hornblende, and labradorite are the earliest
mineral components of the rocks; it seems reasonable
that large quantities of these crystals were separated
from the magma by settling or by being pressed from
the liquid during movement of the magma, thus chang-
ing the composition of the rest of the magma. If this
process took place, it occurred before the magma rose

46 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

 

 

 

 

 

 

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SILICA, IN PERCENT

FIGURE 14.-Diagram showing variation of five trace elements with respect to increasing silica and total pyribole content. - The pyribole includes
pyroxene and amphibole. Pyribole in the granodiorite (excluding H-515) and quartz monzonite has been replaced by calcite, chlorite,
and sericite. Sample numbers shown at bottom of chart. Percentage of trace element determined by semiquantitative spectrographic
analysis.

IGNEOUS ROCKS 47

 

20

   

Microgabbro
56-E-77

10 |-

Quartz monzonite
H-465-1

PERCENT
Subtracted

 

 

 

SiOz, IN PERCENT

FiGURE 15.-Graphical solution of Harker diagram to determine possible
materials added or subtracted to produce quartz monzonite from
microgabbro.

Taster 7.-Comparison of the chemical composition of micro-
gabbro phenocrysts with the chemical composition of material
that might be subtracted from microgabbro to produce quartz
mongonite

[Mode of microgabbro phenocrysts was determined from thin section of field sample

56-E-77, in percent: Augite, 21.6; hornblende, 36.2; plagioclase, 40.5; magnetite,
1.7, The chemical composition of hornblende used in the calculation is an average
of four chemical analyses of hornblende from the Ute Mountains. 'The chemical
composition of augite is that given by Wahlstrom (1947, p. 238). Only plagioclase
phenocrysts larger than about 0.3 mm were counted in the mode. - These crystals
are estimated to average about 60 percent anorthite in composition]

 

 

  

Constituent Microgabbro Subtracted
phenocrysts material
HEN ALE Free ee ARTEL Ev ce aa Sok dick woe a 46.9 47.2
ATNOS. $2100 - coule 1h a baie be bei cout e baie ns 17.5 18.7
cls in nolL on beau nl beide aude cue cae een ners 10.7 1.6
CaO eect enon rs c duval a ona ae ance wee an o 13.5 12.5
MgO.. a+ 7.3 5.0
- 200... c ehe ess evan e E e Lic e 2.5 2.9
(dO perro nee neeed relo bes eee en bade se ne abe .3 .5

 

 

 

to the level of the intrusive bodies now visible, for these
bodies are petrographically uniform throughout.

The hornblendic inclusions found widely in the
igneous bodies of the Ute Mountains are also charac-
teristic of other laccolithic mountain groups on the
Colorado Plateau. The origin of these inclusions is
probably closely related to the origin of the magma or
magmas that gave rise to the laccolithic mountain
groups.

In the Henry Mountains, hornblendic inclusions com-
prise 95 percent of the total of all inclusions (Hunt and
others, 1953, p. 164). Hunt noted that the hornblende
phenocrysts in the porphyries are not sharply distin-
guishable from aggregates of hornblende in the inclu-
sions. He concluded:

It seems necessary to infer that the hornblende inclusions
were derived at great depths and have had a very different his-
tory than the obvious xenoliths. The inclusions may be altered
fragments of diverse wall rock floated from great depths, or
rock fragments from early differentiates in the magma reser-

voir, or fragments of marginal unfused layers of the substratum
from which the magma was derived.

 

A. C. Waters and C. B. Hunt (in Hunt, 1958, p. 348-
354) described hornblendic inclusions in the La Sal
Mountains and suggested that the inclusions in the
igneous rocks are either "* * * fragments of diverse
wall rocks brought to equilibrium with a magma at
great depths, or else they are the leftover unfused frag-
ments of the substratum from which the magma itself
was derived or perhaps both."

Waters and Hunt (Hunt, 1958, p. 348-355) pointed
out that the evidence in the La Sal Mountains does not
support the evolution of the igneous suite of rocks from
diorite to syenite by crystallization differentiation of a
primary basalt parent. They suggested that the igne-
ous rocks in the La Sal Mountains were derived from
a magma formed by the partial fusion of amphibolite or
related metamorphic rocks. This anatectic theory sat-
isfactorily explains: (a) the lack of basalt or its plu-
tonic equivalents in the La Sal suite of rocks, (b) the
lack of a progressive increase in silica in the later formed
rocks, and (c) the similarity of the hornblende in the
inclusions with the hornblende in the rock. The theory
does not explain the absence of the mafic alkalic rocks
which should result from fractionation of hornblende
(Bowen, 1928, p. 269-273). However, Waters and
Hunt (Hunt, 1958, p. 353) suggested that "filtration
effects" took place that decreased the amount of mafic
material carried upward by the rising magma. They
did not attempt to extend their theory to explain the
origin of all the laccolithic magmas of the Colorado
Plateau, although the theory does have plateau-wide
appeal because it could explain the hornblende inclu-
sions characteristic of all the laccolithic mountain
groups. Furthermore, if a hornblendic substratum as
broad as the plateau is assumed to exist, the theory
explains the similarity of the dioritic intrusive rocks in
the laccolithic groups.

To provide further data bearing on the origin of the
hornblendic inclusions and possibly on the magma of
the Ute Mountains, two separates of hornblende pheno-
crysts from diorite porphyry and two of hornblende
from inclusions were analyzed chemically, optically,
and spectographically. (See tables 8, 9, 10, 11, 12.)
The data derived from these analyses indicate that the
two types of hornblende are very similar. The chemi-
cal similarity is especially apparent when plotted on a
diagram designed by Hallimond (1943). In a study of
hornblende from North America, Europe, Asia, and
Africa, Hallimond found that a generalized relation
exists between parent rock and the composition of the
contained hornblende. He plotted the compositions of
hornblende from nearly 200 localities on a triangular
diagram in terms of two variables: (a) the number of
Si atoms, and (b) the number of atoms alloted to the

48

Ca, Mg; SigO;» (OH)»
9

  
    
     
  
 

 
 

Limestone
in ,

Limestone i
pargasite l

Al Al/gi Mg
700
CaNa; Mg; Al;SigO0» (OH);

 

EXPLANATION

x
Hornblende inclusion

 

o
o
w

CaMg3 A1 SisO» (OH), &

Hornblende phenocryst

FIGURE 16.-Approximate limits of composition of amphibole derived
from various rocks (Hallimond, 1943, p. 75) showing plot of four
samples of hornblende from the Ute Mountains.

vacant space in the amphibole unit cell. Figure 16 is a
reproduction of Hallimond's diagram with the com-
position points of two separates of hornblende pheno-
crysts and two separates of hornblende from inclusions
of the Ute Mountains superimposed on it. All four

TABLE 8. -Chemical analyses of hornblende from the Ute
Mountains, Colo.

[Analyses, in percent, by S. M. Berthold. Sample EMS-74-52 (collected by E. M.
Shoemaker) is from the Three Fork laccolith; 56-E-37 is from a laccolith in the
Yucca cluster (pl. 1) about 2,000 ft. southwest of Mushroom laccolith. Both samples
are of hornblende phenocrysts from diorite porphyry. Samples 56-E-105 and
56-E-223 are from hornblendic inclusions in granodiorite porphyry of Black Moun-
tain and of Sentinel Peak, respectively]

 

 

 

 

 

 

 

Field Sample...........- EMS-74-52 56-E-37 56-E-105 56-E-223
Laboratory No.........~ 151063 151064 151065 151066
43.19 43. 41 43. 84 43. 32
13. 20 12. 85 11. 48 13. 10
3. 32 3.20 4. 80 4.89
10. 88 10. 82 11. 60 10. 42
11.14 11.82 11. 34 10.90
11. 38 11. 42 11.46 10.92
2. 06 2. 04 1.70 1.98
+47 72 .81 . 99
1. 56 1.15 . 84 1.06
.10 . 20 . 05 .13
LT 1. 74 1.58 1. 62
. 58 . 36 .16 . 48
28 . 28 27 17
37 . 25 14 14
100. 55 100. 26 100. 07 100. 07
.16 .10 ¢ *.
100. 39 100. 16 100. 01 100. O1
Specific gravity. 3.13 3.12 3. 24 3. 07
FeQ:MgO...:............ . 55 . 51 . 57 . 54

 

 

 

 

 

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

fall within the diorite and amphibolite fields of
Hallimond.

TABLE 9.-Atomic ratios of elements in hornblende on basis 24
(0, OH, F)

[mmeflxodusedtomeastthechamimlnmlflsistbntoummdby Groves (1951,
306-320) who used the general formula monoclinic amphiboles: (OH, F):

Tha, Ca, K):-s (Mg, Fors, For, Ti, Mn, Als [(Si, Als Oz]]

 

 

 

 

 

 

4 A Ratio in field sample-
mi
EMS-74-52 56-E-37 56-E-105 56-E-223

Bli. bel 6.36 6. 40 6.52 6.42
Alyl- A 1.64 1.59 1.48 1.58
. 66 . 64 . 53 ys

17 .13 .09 12

.34 .36 . 54 . 55

1.34 1.34 1.44 1.20

2. 45 2. 60 2. 51 2. 41

. 04 . 03 .03 .02

. 59 . 58 . 49 . 57

1.80 1. 81 1.83 1.73

15 14 15 19

1.74 1.71 1.57 1. 60

B 12 .07 .07

5.02 5.09 5.15 5.10

. 62 . 64 . 59 . 61

1.85 1. 91 1.02 1.70

Na+Ca+K..._____.__..- 2. 54 2. 53 2. 47 2. 49

 

 

 

 

 

1 Al,, aluminum used to make up the number of metal atoms in the chain to eight;
Aly, aluminum remaining which is assumed to replace Mg in the Y group.
# ¥ group includes Ti, Fet3, Fe+?, Mg, and Mn.

TABLE 10.-Optical properties of hornblende from the Ute
Mountains

[Determinations of indices of refraction by Marie Lindberg who found a considerable
range in the indices of individual grains, especially for samples EMS-74-52 and
86-E-37. This lack of homogeneity may be due to zoning, a characteristic feature
of hornblende from the Ute Mountains. All other optical determinations were
made by E. B. Ekren. The extinction angles were measured on a four-axis univer-
sal stage using the method described by Turner (1942). Optic angles were meas-
ured directly on the universal stage. | The accuracy of these measurement is believed

 

 

 

 

 

 

 

     

to about +i°. Abbreviations used in color descriptions: gn, green; gy, gray;
It, light; med, medium; p, pale; y, yellow; 0, olive]
Field sample
Optical property
EMS-74-52 56-E-37 56-E-105 56-E-223
Min, 1.660; Min, 1.65. ...- Min, 1.659.._.| Min, 1.653.
avg, 1.664.
, 1.668; Max, 1.67- Range, 1.676- | Range, 1.665-
avg, 1.675- 1.69; avg, 1.680; avg 1.675.
1.685. 1.673-1.678. in low
range.
N sd Max, 1.71; Avg, 1.687- Max, 1.687....| Max, 1.685.
avg, 1.690- 1.692.
1.695.
y-a(calculated)...] 0.016-0.021..... 0.028....:..0.. 0.032.
y-a (Berek com- 0.019.......... 0021.......... 0.018.
pensator) « 3
ZAC... 17°41°........ 16° 16°+1°.
79°41°.......- §§¢s1°-..._... 75°1°.
pgD-y...------ py-g1n....-.--- y-gn.......-.-- pgn-y.
med gn...._..| med gn.
med to dk gn.) y-gy-gn.

 

 

 

 

 

 

TABLE 11.-Quantitative spectrographic analyses of hornblende
from the Ute Mountains

[Analyses, in percent, by K. V. Hazel]

 

 

 

 

 
  
 

Field sample......-..---- EMS-74-52 56-E-37 56-E-105 56-E-223
Laboratory No........... 151063 151064 151065 151066

0. 005 0. 006 0. 007 0. 006
Chromium. . 0007 . 002 . 002 . 002
Lithium... ® . 0007 . 0015 . 0006 . 0007
Nickel. .... . 002 . 0018 . 0021 . 0023
vansadlum......::-..._.. . 04 . 04 . 05 .06

 

 

 

 

 

IGNEOUS ROCKS 49

TABLE 12.-Semiquantitative spectrographic analyses of
hornblende from the Ute Mountains
[Analyses in percent, by K. V. Hazel. Looked for but not detected: P, Ag, As, Au,
Bi, Cd, Ce, Cs, Dy, Er, Eu, Gd, Ge, Ht, Hg, Ho, Ir, La, Lu, Nb, Nd, Os, Pd, Pr,
Pt, Rb, Re, Rh, Ru, Sb, Sm, Ta, Tb, Te, Th, Tl, Tm, U, W}

 

 

 

Field sample...._........ EMS-74-52 56-E-37 56-E-105 56-E-223
Laboratory No........... 150163 150164 150165 150166
0.003 0.003 0.003 0. 003
007 . 007 . 007 003
00007: [c. -.. couse Same coun . 00007
. 007 . 007 .007 007
. 0007 . 0015 . 0015 0015
00015 . 00007 00007 00015
. 0015 0015 . 0015 003
0007 . 0007 . 0007 0003
0007 . 0015 . 0007 0007
0007 . 0007 . 0007 0007
0015 . 0015 . 008
0015 . 0015 . 0015 0015
. 003 . 008
008 . 003 008 003
015 . 015 . 007 03
.03 . 07
003 .0015 . 0015 0015
015 . 007 . 007 007
007 .003 . 003 007

 

 

 

 

 

 

The data suggest that the phenocrysts of hornblende
and the hornblende inclusions in the Ute rocks are either
inherently similar and have a common origin, or that
physical-chemical equilibrium was nearly attained be-
tween the magma and xenolithic material that now
forms the hornblende inclusions. Hornblende from
igneous rocks of the Henry Mountains in Utah have
been analyzed by Engel (1959). Engel's chemical and
spectrographic analyses indicate that hornblende from
the Henry Mountains contains less SiO; and MgO than
hornblende from the Ute Mountains and generally con-
tains more FeO and Fe;OQ;,. Engel (1959, p. 974, 979)
pointed out that hornblende from the inclusions in the
Henry Mountains is relatively enriched in magnesium,
chromium, and nickel. The FeO : MgO ratio for an in-
clusion hornblende is much lower than in four of the
five hornblende phenocrysts. Engel (1959, p. 979) con-
cluded: "The possibility that these inclusions repre-
sent early hornblende-rich segregations is appealing be-
cause this would readily explain (1) the overlap in
composition of phenocrysts and inclusion hornblendes,
(2) the relative enrichment of Mg, Cr, and Ni in most
of the inclusion hornblendes, and (3) the constancy of
mineralogy in the inclusions, and (4) their abnormally
high hornblende content (80-95 percent in many
inclusions)."

The differences noted by Engel are not apparent
in the hornblende from the Ute Mountains. The
FeQ : MgO ratios are nearly constant (table 8) and are
very close to the ratio for the inclusion hornblende of
the Henry Mountains (Engel, 1959, p. 974, 979) ; fur-
thermore, no consistent differences in chromium and
nickel content are apparent between phenocrysts and
inclusion hornblende from the Ute Mountains (table
11). The similarities are especially significant in view

 

of the fact that the four samples were taken from four
chemically and mineralogically different rocks. The
hornblende phenocrysts (table 8, samples 1 and 2), are
from different laccoliths composed of diorite porphyry.
Sample 1 is from hornblende-rich diorite porphyry that
contains no pyroxene; sample 2, on the other hand, is
from diorite porphyry that contains about 5 percent
augite as phenocrysts, as well as about 7 percent horn-
blende as phenocrysts. Sample 3 is hornblende from
an inclusion that has a strong gneissic structure and that
contains nearly as much augite as hornblende. This
inclusion is from granodiorite porphyry of the north-
ern type, a rock that contains about 5 percent more S10;
and 1 percent more K;,0 than is average for diorite
porphyry. Sample 3 is from granodiorite of the south-
ern type; from a massive inclusion consisting of more
than 90 percent hornblende and no pyroxene. This
granodiorite (sample 11, table 3) contains more Na,0
and SiO; than do rocks classed as diorite porphyry, and
much less MgO, CaO, and total iron. Thus the horn-
blende data do not clearly indicate whether the horn-
blendic inclusions are fragments of early differentiates,
unfused fragments of a substratum that melted to give
rise to the magma, or fragments of rocks invaded by
the magma. The chemical similarity of the hornblende
in phenocrysts and that in inclusions may result from
the attainment of chemical equilibrium between inclu-
sions and magma prior to or during the rise of the
magma.

The occurrence of schistose or gneissic fabric in many
inclusions in the Ute Mountains (see, for example, fig.
10) suggests that the inclusions are unfused fragments
from a substratum that gave rise to the magma by
anatexis, as suggested by Waters and Hunt (in Hunt,
1958). The occurrence of granophyre and amphibolite
in the same inclusion (fig. 10) suggests that the same
substratum yielded both hornblende inclusions and the
relatively rare inclusions of granophyre.

That the Ute magma may have originated by anatexis,
at least in part, is suggested also by the occurrence of
anhedral zircon crystals in the intrusive masses in and
around Ute Peak. These zircons have subrounded out-
lines suggestive of mechanical abrasion and a sedimen-
tary history rather than a single-cycle igneous origin.
The differentiation toward more silicic magmas through
time as indicated in the rock series suggests, however,
that laccolithic intrusions would have been only partly
contemporaneous, if at all, with the anatectic process.
Differentiation seems to best explain the progression of
igneous rocks from gabbro to quartz monzonite, and it
seems possible that such a process could have taken
place after a magma formed by the fusion of a sub-
stratum rich in hornblendic material.

50 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS
BODIES

GENERAL FEATURES

The largest structural features in the Ute Mountains
area are Ute dome, the result of igneous intrusion, and
McElmo dome, which may be partly of igneous origin.
The central part of Ute dome is occupied by 3 stocks
and approximately 40 major igneous bodies, most of
which are laccoliths of diorite and granodiorite por-
phyry intruded into the Mancos Shale. The largest
and topographically the highest intrusive body forms
Ute Peak in the northeastern part of the Ute dome
(pl. 1):

Faults occur on the northern, western, and southern
flanks of Ute dome, and on the southwestern flank and
in the central part of McElmo dome. These faults are
described in detail in the discussions that follow.

UTE DOME

The configuration of the flanks of Ute dome is shown
on plate 1 by structure contours at the base of the
Mancos Shale. The dome is probably entirely the result
of injection of magma, and principally of three stocks
at "The Knees," Black Mountain, and Ute Peak. Be-
cause the base of the Mancos Shale is not exposed near
the stocks and because the Mancos contains few marker
beds there, the structure near the stocks is open to sev-
eral interpretations. For this reason, the central part

of the dome was not contoured on plate 1 and the in-

ferred structure is shown only by cross sections.

Ute dome is nearly circular in plan view and averages
about 10 miles in diameter. On its western side, the
dome merges with west- and southwest-plunging folds,
and its western edge is poorly defined. A broad
nose on the southwest flank of the dome may be under-
lain by a large tongue-shaped intrusive mass, and a
broad bifurcate anticline that plunges westward from
the northwest flank of the dome may also be underlain
by an igneous mass, at least in part.

Other folds along the western flank do not appear to
be closely related to igneous activity. They are closely
associated with zones of fracturing that may be tectonic
and unrelated to igneous activity.

T'wo smaller structural domes along the southern and
southeastern flanks of the Ute dome are called Sentinel
Peak dome and Towaoe dome respectively. The Sen-
tinel Peak dome lies about half a mile south of Sentinel
Peak. -It has about 900 feet of structural relief and has
been breached by erosion. Upper beds of the Junction
Creek Sandstone are exposed in the central part of the
dome, and these beds are cut by a small dike of grano-
diorite porphyry that is connected to a miniature lac-

 

colith intruded into the Recapture Shale Member of the
Morrison Formation (dike and laccolith not shown on
pl. 1). The Sentinel Peak dome almost certainly over-
lies an intrusive igneous body. This body probably was
fed from "The Knees" stock, or possibly directly from
the fissure that fed "The Knees" stock.

The top of Towaoc dome is flat and the sides slope.
gently. Configuration of the dome and its proximity
to the mountains strongly suggest that it is also due to
igneous intrusion, possibly at only moderate depth.

The relief on the flanks of Ute dome varies as a result
of regional structure. About 3 miles south of Sentinel
Peak, near the 5,300-foot structure contour (pl. 1), the
dip of the Juana Lopez Member of the Mancos Shale
steepens. If this locality is the southern extremity of
the Ute dome, there is about 2,000 feet of relief on the
south flank of the structure. East of the mountains
the regional dip is eastward and southeastward into the
San Jaun structural basin. The dip steepens about 2
miles east of the exposed igneous rock of the East
Horse laccolith, between the 5,900- and 5,800-foot struc-
ture contours. Here, there is a minimum of 1,200 feet
of relief, excluding the central part of Ute dome, which
was not contoured.

On the northeast, due to the juxtaposition of McElImo
dome, the relief is only about 400 feet in the vicinity
of Ute Peak. The relief increases westward to about
800 feet on the northwest flank of Ute dome and to more
than 1,500 feet on the western flank.

The interpretation of doming of the Dakota in the
vicinity of Black Mountain (section C-C", pl. 1) is
based on altitudes of the Point Lookout Sandstone ex-
posed on the northern flank of the mountain, and on the
altitude of the Juana Lopez Member of the Mancos
Shale on the southeast and southwest flanks. The out-
crop of the Juana Lopez Member on the southwest
flank, west of Pack Trail laccolith, indicates that the
top of the Dakota Sandstone may be as high as 7,800
feet near the contact with the Black Mountain stock.
Altitudes of the basal part of the Point Lookout Sand-
stone in the vicinity of "The Knees" stock indicate
the top of the Dakota Sandstone near the contact with
igneous rock cannot be much higher than 7,000 feet (see-
tion A-A4', pl. 1). - The structural relief at "The Knees"
could be greater than that shown in section 4-4", but
only in a very narrow strip adjacent to the walls of the
stock. ;

The structure shown in the vicinity of Ute Peak (see-
tion 3-Z3', pl. 1) is based on altitudes of the Juana
Lopez Member of the Mancos Shale exposed east of Ute
Peak on North Ute Peak laccolith and from elevations
on the base of the Point Lookout Sandstone exposed be-
tween Ute Peak and Mable Mountain.

STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES 51

The Juana Lopez Member is about 475 feet above the
base of the Mancos Shale in the Ute Mountains area.
The structure north of Ute Peak was determined by
subtracting the exposed thickness of the north Ute Peak
laccolith from elevations on the base of the Juana Lopez
Member. Inasmuch as the base of the laccolith is not
exposed, the top of the Dakota Sandstone may be lower
in the vicinity of the syncline shown on plate 1.

The configuration of the Ute dome at greater depths,
such as on the Paradox Member of the Hermosa Forma-
tion, probably differs considerably from the configura-
tion on the Dakota Sandstone. In general, at the
Paradox horizon there would probably be fewer under-
lying conformable intrusive masses and the doming
would be due almost entirely to the insertion of the
stocks. This difference could result in three domes of
small diameter but having steep sides; therefore, struc-
tural relief greater than that on the Dakota Sandstone
may occur on the Paradox Member in the vicinity of
the stocks. This conclusion is based also on observable
relations in the Henry Mountains of Utah where stocks
that are very similar to the Ute stocks are in contact
with beds as old as Permian.

Hunt (in Hunt and others, 1953, p. 139-141) proved
that the doming on each of the stocks of the Henry
Mountains is directly proportional to the diameter of
the stock. His deductions may also be applicable to the
Ute Mountains; however, several buried, probably con-
formable intrusive bodies are thought to underlie the
Dakota Sandstone at shallow depths in the Ute dome,
and an accurate analysis to determine the doming caused
only by the emplacement of the stocks is impossible.

McELMO DOME

McElmo structural dome is just north of the Ute
Mountains, and only the southern half of it lies within
the mapped area. Its structure is well exposed in
McElmo Canyon, which cuts through the southern flank
(pl:1).

The central part of the dome is nearly star-shaped
in outline and has a flat top. The dome is asymmetric,
its steepest side being on the south where the maximum
dip is about 914°. Except for the south side, the flanks
of the dome pass into a series of five anticlines. A
northwest-trending anticline that lies north of the
mapped area is 7 miles long (Coffin, 1920). An east-
trending anticline near the northern border of the
mapped area extends more than 12 miles from the east
side of McElmo dome and passes through the city of
Cortez, Colo. A moderately sharp anticline plunges
southeastward from McElmo dome in the vicinity of
Ute Peak. It is asymmetric with a steeply dipping
southwest side. Because of the uncertainties in recog-

 

nizing stratigraphic levels in the poorly exposed Man-
cos Shale, the structural saddle between this anticline
and Ute Peak may be considerably higher or lower than
shown on plate 1.

A poorly defined anticline extends southwest from
McElmo dome about 4 miles, almost parallel to a graben
that lies to the north. The fifth anticline of the series
(not shown on pl. 1) extends northeast of McEImo dome
and can be traced for approximately 6 miles.

The total area affected by McElmo dome and its satel-
litic anticlines is about 20 miles east to west and 10
miles north to south. If the configuration of the south-
east-trending anticline in the vicinity of Ute Peak is
correctly shown on plate 1, the McElmo structure has
about 500 feet of closure; the 6,600-foot contour is the
lowest closing contour.

The origin of McElmo dome and its relation to Ute
dome is uncertain. The proximity of the two domes
suggests that McElmo dome may also be underlain by
an igneous mass. - The two domes are of about the same
areal extent, and appear to have about the same struc-
tural relief. McElmo dome differs from Ute dome
mainly in the configuration of its satellitic folds. The
McElmo structure is characterized by long, relatively
narrow anticlines that plunge radially away from the
central structure, whereas Ute dome is characterized by
broad flexures that lose definition at short distances from
the central structure. Anticlines similar to those around
McElmo dome occur only on the western flank of Ute
dome and, as previously mentioned, may not be genet-
ically related to the Ute structure.

An oil well (The Three States Natural Gas Co.
(Byrd-Frost) MacIntosh 1) drilled into the central
part of the McElmo structure cuts igneous rock from
a depth of 4,600 feet to the bottom of the hole at 4,965
feet. According to Zabel (1955, p. 135), the igneous
rock from 4,600 to 4,715 feet was identified as either a
dellenite or a latite, and the rock from 4,715 to 4,965
feet as a porphyritic hornblende monzonite. Such rocks
could be related to those of the Ute Mountains. The
igneous rocks apparently were intruded into the Para-
dox Formation of Pennsylvanian age at a point 90 feet
below the highest anhydrite. In contrast, two gas wells
drilled farther north, the Schmidt 1 and the Dudley 1,
which bottomed in Devonian and Cambrian strata, re-
spectively, did not cut igneous rock. Similarly, no in-
trusive rocks were found above the granite of Precam-
brian age in the Gulf Oil Co. Fulks 1, on the northeast
flank of McElmo dome, 6 miles northeast of Goodman
Point (pl. 1). The possibility exists, therefore, that
the igneous rock in the MacIntosh 1 is part of a lacco-
lithic body extending northward from the Ute igneous
centers. Most of the structural relief of McElmo dome

52 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

and its satellitic folds may be a result of basement uplift
related to Late Cretaceous or early Tertiary folding,
such as recognized elsewhere on the Colorado Plateau
(Hunt, 1956, p. 57-58). The possibility also exists that
part of the relief is due to movement of salt. According
to Zabel (1955, p. 134), the thickness of the salt section
in the Paradox varies considerably in the McElmo area.
The salt is 1,500 feet thick in the Three States Schmidt
1; 1,350 feet thick in the Three States Dudley 1; and
only 1,155 feet thick in the Gulf Fulks 1. There is an
overall thinning of 345 feet from the Schmidt 1 on the
highest part of McElmo dome to the Fulks 1 on the
northeast flank.

FAULTS

Steeply dipping normal faults occur in the Ute Moun-
tains area on the south, southwest, and northwest flanks
of Ute dome, on the southwest flank of McEImo dome,
in the central part of McElmo dome, and along the
anticline trending eastward from McElmo dome.
Faults are conspicuously absent on the steeply dipping
east flank of Ute dome.

The greatest concentration of faults is on the north-
west flank of Ute dome. Two sets of faults appear to
have formed simultaneously in this vicinity; one set
strikes nearly west, the other set northeast. The west-
striking faults parallel west-trending folds and have
displacements that rarely exceed 30 feet.

The northeast-trending faults appear to be extensions
of a zone of faulting that cuts the southwest flank and
the central part of McElmo dome. This zone curves to
a nearly east strike in the vicinity of the Schmidt 1 gas
well on McElmo dome and continues eastward beyond
the map area toward Cortez, Colo. The faults along
this zone form a graben on the southwest flank of Mc-
Elmo dome and have displacements of as much as 180
feet, the greatest known in the Ute Mountains area.
Faults along the graben contain radioactive mineral
deposits, described on pages 62-64.

The northeast-trending faults are roughly concentric
with the northern part of the Ute Mountains and may
be related to the uplift of Ute dome. On the other
hand, they are nearly parallel to southwest-plunging
folds in the western part of the area that may predate
the doming, and to inferred fracture zones that pre-
date intrusion, which is the presumed cause of doming.
The two largest stocks, at Black Mountain and Ute
Peak, lie suggestively close to and parallel with the
southwestward extension of the fracture and fault
along which the Ute Creek dike was intruded (pl. 1).
It is conceivable, therefore, that the extensive zone of
northeast-trending faults lies en echelon to a zone of
fracturing that localized part of the igneous activity in

 

the Ute Mountains and also the uplift of McElmo dome.

"The Knees" stock appears to have been intruded
along a north-northwest zone of fracturing along which
several dikes have also been intruded. This zone of
fracturing extends south of the Sentinel Peak dome and
apparently controlled the location of the inferred in-
trusive mass of Sentinel Peak dome. Two dominant
zones of intrusion apparently were localized by pre-
existing fractures-a northeast zone and a north-north-
west zone. Black Mountain may actually lie at the
intersection of the two zones.

INTRUSIVE BODIES

Most of the intrusive bodies in the Ute Mountains
were probably injected from three stocks: Black Moun-
tain, "The Knees," and Ute Peak along conduits paral-
lel to the bedding of the sedimentary rocks. The stocks
are of small diameter and are less extensive areally
than some of the laccoliths. Except at Ute Peak, the
stocks are composite intrusive bodies containing several
rock types. The stocks appear to have discordant
sides, but they probably lifted roofs of sedimentary
rocks upward during intrusion, for doming of Mancos
strata around the stocks is very slight. 'The igneous
rock exposed on Ute Peak may be part of a bysmalith
that overlies a stock of smaller diameter.

A common feature of diorite porphyry and grano-
diorite porphyry of the northern type in the Ute Moun-
tains is the linear orientation shown by hornblende
phenocrysts. The linear orientation was not studied
in all intrusive bodies, but observations in a few lac-
coliths indicate consistent trends that are inferred to
be parallel to the flow direction of the magma. As
many as 10 readings were made on exposures which
offered observation on 3 approximately perpendicular
surfaces, and the readings were averaged to give those
plotted on plate 1. - The more conspicuous the lineation,
the fewer the readings necessary and the greater agree-
ment between them. - No fewer than four separate read-
ings were averaged at any one point. The individual
readings at each outcrop rarely spanned an arc of 20° in
either direction or plunge.

Planar orientation of minerals was observed within
a few feet of the margins of laccoliths and is inter-
preted as a primary flow structure. It is defined by
a predominance of hornblende phenocrysts oriented in
a common plane. The orientation is rare and vague
because of the lack of platy minerals such as biotite
and tabular feldspar through most of the intrusive
bodies. In a few places, linear flow structure was ob-
served in the plane of the planar flow structure.

Rock cleavage as mapped on plate 1 is defined by
closely spaced low-angle jointing that is thought to

STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES

parallel the original sides and roofs of laccoliths. In |
contrast to the planar flow structure, the rock cleavage
appears to cut mineral grains and probably formed by
contraction during cooling, parallel to the original |
sides and roofs of the laccoliths.

STOCKS
BLACK MOUNTAIN STOCK

The Black Mountain stock is in the north-central part
of Ute dome and is approximately 11/4 miles in diameter.
It is composed of granodiorite porphyry of the north-
ern type, biotite-rich quartz monzonite porphyry, and
diorite porphyry. Although no microgabbro is ex-
posed, the stock probably fed microgabbro into the sills
exposed on the flanks of the mountain. The granodio-
rite porphyry in the stock has a nearly doughnut-
shaped outcrop pattern and completely encloses a com-
posite mass of baked Mancos Shale, diorite porphyry,
and quartz monzonite porphyry. The diorite porphyry
is older than the granodiorite, which in turn is older |
than, and has been intruded by, quartz monzonite por-
phyry. The age relation of the granodiorite porphyry
and the lamprophyre exposed in the northeastern part
of the mountain is not known, for the contacts between
the rocks are not exposed.

Shale beds of the Mancos form the "hole" of the
granodiorite doughnut (pl. 1). They may be part of a
modified roof pendant that bottoms in igneous rock at
shallow depth, or they may overlie sedimentary rocks
that extend in a roughly cylindrical mass to great depth.
The northern, northwestern, and northeastern sides of
the intrusive mass are nearly vertical and appear to be
entirely discordant against gently dipping beds of Man-
cos Shale and basal sandstone beds of the Point Look-
out. The southern and southeastern sides, on the other
hand, appear to be nearly conformable with Mancos
strata. Shale exposed a half mile south-southwest of
the top of Black Mountain conformably overlies grano-
diorite porphyry, and shale near the extreme southeast
ern part of the intrusive appears to extend beneath the
intrusive as a floor. Thus the southeastern extremity
of the stock may be conformable in part, having both a |
roof and a floor.

A small neck or miniature stock composed of diorite
porphyry crops out in the west-central part of Black
Mountain, and a thin sill composed of identical rock
crops out a few hundred feet east of the neck. It seems
likely that the sill was intruded from the igneous neck.

The Black Mountain stock was probably the feeder
for nearly all the intrusive rocks exposed in the north
half of the Ute Mountains, with the exception of those
at Mable Mountain and Ute Peak, including the satel-
litic North Ute Peak and East Horse laccoliths. As

 

53

shown on plate 1, intrusive bodies surround Black Moun-
tain radially, and most of those that lie northwest, west,
and southwest of Black Mountain have, respectively,
southeast-, east-, and northeast-trending hornblende
lineation. The lineation suggests that the intrusive
masses were fed from Black Mountain along flat-lying
conduits. Several fiat-lying sills near the west side of

| Black Mountain may have been feeders for laccoliths

lying to the west. Tongue-shaped laccoliths of diorite
porphyry and a sill of microgabbro abut against the
southwest, southeast, and northeast sides of the Black
Mountain granodioritic stock, and it is inferred that
these intrusive bodies were originally connected to dio-
rite porphyry and microgabbro within the stock. The
paucity of exposed diorite porphyry and the lack of
microgabbro in the stock probably is due largely
to concealment by granodiorite that surged to a higher
level {section C-C", pl. 1) after pressing the earlier
rocks against the walls of the stock.

Metamorphism is more intense around the Black
Mountain stock than around any of the laccoliths or bys-
maliths. The Mancos Shale in and adjacent to the
stock has been intensely baked and is intruded by many
dikes, but no shattering similar to that described by
Hunt and others (1953, p. 90-151) was observed. The
width of the marginal baked zone ranges widely. On
the east side of the mountain the shale is baked for a
distance of more than one-quarter of a mile from the
stock. Although exposures are poor on the north and
west sides, baking appears to extend considerably less
than 1,000 feet from the stock. On the south side of the
mountain, baking caused by the stock cannot be distin-
guished from that caused by nearby concordant intru-
sive bodies.

The igneous rocks in the Black Mountain stock have
been neither intensely altered nor mineralized; and con-
sequently the hornblende and biotite are generally fresh
and the plagioclase is clear.

Lineation defined by hornblende crystals in the stock
generally plunges at an angle greater than 45°, a steeper
angle than those in the laccoliths.

"THE KNEES" STOCK

"The Knees" stock is exposed in the southern part
of the Ute dome, a few hundred feet southeast of two
small peaks (Hermano peaks, pl. 1) that form the
"knees" of Sleeping Ute Mountain. The stock is ap-
proximately two-thirds of a mile long and about one-
fourth of a mile wide and has nearly vertical sides that
cut beds of flat-lying Mancos Shale and Point Lookout
Sandstone.

The intrusion of the stock at "The Knees" appears
to have been controlled by a north-northwest fracture

54 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

zone along which many dikes were also intruded. The
stock is surrounded by laccoliths, bysmaliths, sills, and
dikes, including many small, thin, and discontinuous
dikes, only a few of which are shown on plate 1. The
small dikes include examples of every igneous rock
known in the Ute Mountains.

"The Knees" stock probably had the same succession
of magmas as the other stocks in the Ute Mountains,
although only granodiorite and diorite porphyry are ex-
posed in the stock proper. The occurrence of microgab-
bro (not shown in section A-4', pl. 1) and quartz mon-
zonite porphyry at depth in the stock is inferred from
exposures of these rocks in minor discordant apophyses
at the surface near the stock and from the radial ar-
rangement of larger intrusive masses around the stock.
The micogabbro was the most fluid rock intruded in the
Ute Mountains, and "The Knees" stock may have been
only a thin dike or a small neck when it was emplaced.
The microgabbro presumably makes up a very small
percentage of the total volume of rock in the stock and,
except for minor apophyses, never reached the level
of the stock that is currently exposed. Similarly the
quartz monzonite porphyry did not reach the exposed
level of the stock except as minor apophyses, probably
because earlier intrusive rocks occupied most of the
available space. The quartz monzonite apparently as-
cended along fractures that flanked the granodiorite-
and diorite-filled stock. "West Toe," for example, is
a small neck or boss of quartz monzonite porphyry that
vertically crosscuts the surrounding sedimentary rocks
and is located between "The Knees" stock and two
northeast-trending faults. Banded laccolith, composed
of quartz monzonite porphyry, has prominent vertical
ridges resulting from weathering of alternating verti-
cal zones of intensely altered and moderately altered
rock. The vertically altered zones suggest an underly-
ing feeder perhaps similar to that shown in section
A-A', plate 1.

The metamorphism adjacent to "The Knees" stock is
more intense than around Black Mountain. The Man-
cos Shale and basal beds of the Point Lookout Sand-
stone have been intensely baked for a distance of at least
a quarter of a mile from the stock; adjacent to the stock
in a zone a few tens of feet to more than 500 feet wide,
pyroxene, epidote, sphene, and sparse garnet have
crystallized in the shale. In general, the zone of meta-
morphism around "The Knees" stock is similar to the
"shatter zones" around the stocks of large diameter in
the Henry Mountains.

UTE PEAK

The Ute Peak intrusive body (pl. 1), composed of
hornblende granodiorite porphyry, has a minimum

 

thickness of 2,500 feet. It forms the highest peak in
the mountains, is very nearly circular in plan, and is
about 114 miles in diameter. Vertical or nearly vertical
lineation, defined by hornblende needles, is conspicuous
in the porphyry in the steep slopes and cliffs of the peak.

Structural relations between the granodiorite and
Mancos Shale are obscured by thick talus which almost
completely covers the contacts. Poor exposures of Man-
cos Shale and granodiorite porphyry along the north-
west flank of the peak indicate that the shale is only
slightly altered adjacent to the igneous rock, and that
it dips 20° NW to vertical.

The North Ute Peak laccolith, composed of diorite
porphyry, lies north of Ute Peak and appears to have
been fed either from the Ute Peak body or from a feeder
that underlies Ute Peak. East Horse Laccolith, also
composed of diorite porphyry, lies southeast of Ute
Peak and has rather consistent southeast-trending horn-
blende lineation that suggests that the laccolith was fed
from the direction of Ute Peak.

The vertical hornblende lineation at Ute Peak and
the steep sides of the intrusive mass indicate that Ute
Peak is either a stock or a bysmalith. (The term "bys-
malith" implies that the intrusive mass raised its sedi-
mentary roof by faulting, but that the faulting did not
penetrate below the floor of the mass.) The presence of
satellitic intrusive bodies suggests that the body is a
stock, but weak metamorphism and alteration as com-
pared with "The Knees" and Black Mountain stocks
suggest that it is not entirely a stock. It seems more
likely that the Ute Peak body is floored in part and
overlies a stock (section B-2', pl. 1).

Ute Peak is comparable to bysmaliths in the Henry
Mountains in its size, shape, and ratio of thickness to
areal extent. It is strikingly similar to the Table Moun-
tain bysmalith that was probably fed by a lateral feeder
(Hunt and others, 1953, p. 141). Particular attention
was devoted to the structure of the Mancos Shale be-
tween Black Mountain and Ute Peak in an effort to
determine if the Ute Peak mass was fed by a lateral
conduit like those described by Hunt in the Henry
Mountains. Although the exposures are poor, there is
no discernible folding of the Mancos strata to indicate
a thick feeder. This fact, together with the nearly cir-
cular outline and vertical lineation of the Ute Peak
body, suggests that the intrusive was fed by a vertical
feeder or stock near its center, rather than by a lateral
feeder from the Black Mountain stock.

The base of the Ute Peak body is probably above the
top of the Dakota Sandstone, as suggested by the lack
of sandstone in the talus along the sides of Ute Peak or
in the gravel on the benches east of the peak.

STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES 55

BYSMALITHS
SENTINEL PEAK

The Sentinel Peak intrusive mass is composed of
granodiorite porphyry of the southern type and is
approximately 1%; miles south of "The Knees" stock.
It is a bysmalith nearly 2 miles long, three-fourths of
a mile wide, and, excluding Sentinel Peak proper,
about 800 feet thick. The bysmalith may actually
comprise two separate intrusive masses that have co-
alesced. (See pl. 1.)

Almost all the north side of the bysmalith is in fault
contact with Mancos Shale; the south side probably
had a similar fault contact, but most of the shale has
been removed by erosion. On the east and west sides,
the bysmalith is in intrusive contact with older diorite
porphyry. At two places along the north side near
the top, blocks of Burro Canyon and Dakota strata
crop out. These blocks appear to be uplifted parts of
the sides and roof.

The base of the intrusive is extremely irregular,
being located stratigraphically at various horizons be-
tween the base of the Burro Canyon Formation and
the Juana Lopez Member of the Mancos Shale. At
its southern edge, the base is in the Mancos Shale,
probably at a level between the Juana Lopez Member
and the Greenhorn Limestone Member. The shale beds
are only slightly baked. The blocks of Burro Canyon
and Dakota strata that crop out along the north side
of the intrusive may mark the locations of two feeders
that extended from "The Knees" stock beneath Burro
Canyon and Dakota strata and broke upward into
Mancos Shale at the bysmalith. Except near these
conduits, the bysmalith is probably mostly in the
Mancos. e

Exposures along the southeast side of the Sentinel
Peak bysmalith suggest that the igneous rock was in-
jected in a nearly horizontal southward direction with
sufficient force to thrust beds of Mancos Shale over
beds of the Brushy Basin Member of the Morrison that
had been domed upward by an earlier intrusion. The
inferred structural relations are shown in figure 17.

N Ground surface §
TKe i etree 3

SENTINEL PEAK
BYSMALITH >

 

 
  
 

 
 

SENTINEL PEA
se
f

   

 

 

 

FIGURE 17.-Sketch showing inferred structural relations thought to be
the result of igneous intrusion at depth near the contact of the Sen-
tinel Peak intrusive with the Sentinel Peak dome. Not drawn to
scale. Tkg, granodiorite porphyry; Tkd(?), diorite porphyry ; Km,
Mancos Shale; kdb, Dakota Sandstone and Burro Canyon Formation ;
Jmb, Brushy Basin Member, and Jmwr, Westwater Canyon and Recapture
Members, all of the Morrison Formation ; Jj. Junction Creek Sandstone.

 

The most conspicuous part of the Sentinel Peak
bysmalith is Sentinel Peak itself, a vertical spine that
rises 500 feet above the rest of the bysmalith. The
spine is part of a dikelike mass that apparently in-
truded the original roof of the bysmalith, probably
along a fracture that was part of the fracture system
extending from "The Knees" stock south-southeast-
ward beyond the Sentinel Peak dome (pl. 1). The
spine has many vertical ridges formed by differential
weathering of alternating zones of moderately and in-
tensely altered rock, and probably served as a conduit
or neck that fed magma from the Sentinel Peak bysma-
lith into overlying sedimentary rocks.

MABLE MOUNTAIN

Mable Mountain, one of the largest single intrusives
in the Ute Mountains, is approximately 1 mile north-
west of Ute Peak. The mountain is composed of diorite
porphyry and has a nearly mushroom shape. It is about
1,500 feet thick in its thickest part and probably over-
lies the Ute Creek dike.

Beds of Mancos Shale and thin sills of diorite por-
phyry form the southwest side and part of the top of
Mable Mountain. The base of the intrusive body is not
exposed on the north side, where contacts with the
Mancos Shale are buried by thick talus. The base of
the body is only a few tens of feet above the top of the
Dakota near the north side of Mable Mountain, but is
several hundred feet above this level a few hundred feet
southward along Pine Creek.

On its northeast and north sides, the bysmalith is
probably in fault contact with Mancos Shale, for the
shale is greatly brecciated and the sides of the intrusive
mass are very steep. The south side is probably also
faulted, although no faults were seen during mapping.
The base of the Point Lookout Sandstone crops out at
about the 8,400-foot contour (pl. 1) near the highest
part of Mable Mountain. Only 1,500 feet to the north,
igneous rock crops out at or near the same elevation,
and a little farther north, roof rocks of the intrusive
include beds of the Greenhorn Member in the basal part
of the Mancos Shale. A fault must lie between the
Point Lookout Sandstone and the igneous mass, prob-
ably at the contact of shale and diorite porphyry as
shown on plate 1.

The Ute Creek dike probably was the feeder for the
Mable Mountain intrusive rock and may be connected
to the Black Mountain stock at depth.

Much of the diorite porphyry of Mable Mountain has
been intensely altered and mineralized with pyrite,
especially in the western part of the body. Reddish iron
stain produced by the oxidization of pyrite attracted
many prospectors to the area during the early 1900's.

56 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

THE "WEST TOE"

The "West Toe" body of quartz monzonite porphyry
is a small discordant body intruded through gently
dipping beds of the Dakota Sandstone and Mancos
Shale. It is called a bysmalith because the discordant
side contacts suggest that the intrusive lifted its roof,
but nothing is known regarding the floor. The body is
structurally very similar to that of Sentinel Peak (the
"East Toe"), and probably was a feeder for igneous rock
intrusive into overlying sedimentary beds.

LACCOLITHS
NORTH UTE PEAK LACCOLITH

The North Ute Peak laccolith is nearly continuous
with the north side of the Ute Peak bysmalith and ex-
tends half a mile north from Ute Peak. It is about 600
feet thick and a little more than a mile wide. It con-
sists of leucocratic diorite porphyry that is generally
similar to porphyry in Ute Peak bysmalith but contains
less potassium feldspar.

North Ute Peak laccolith was intruded into the Man-
cos Shale near the base of the Juana Lopez Member, or
about 475 feet stratigraphically above the base of the
Mancos. It is believed to have been intruded from the
same source as Ute Peak and to be concordant. Much
of the laccolith lies astride the southeastward-trending
syncline between the Ute Mountain mass and McElmo
dome (pl. 1).

HORSE MOUNTAIN LACCOLITH

Horse Mountain laccolith is east of Cottonwood Creek
and about 114 miles directly south of Ute Peak. It is
composed of leucocratic diorite porphyry which is
petrographically similar to the granodiorite porphyry
of Ute Peak except for a lack of abundant potassium
feldspar.

The laccolith is elongated east-southeast and is about
1 mile long and not more than half a mile wide. The
roof has been removed and only the sides of the laccolith
are preserved, but the top of Horse Mountain is thought
to be near the original top of the laccolith. The lacco-
lith is at least 1,100 feet thick and, because its horizontal
dimensions are small, it has very steep sides, parts of
which may be faulted.

Two alined vertical dikes a few hundred feet south
of Horse Mountain are probably upward offshoots from
the concealed wedge-edge of the laccolith, as probably
are dikelike bodies on the northeast side.

Linear flow structure is well displayed in most out-
crops. As in other laccoliths composed of hornblende-
bearing diorite porphyry, the flow structure is exhibited
by alined phenocrysts of hornblende. Except in places
at the ends of the laccolith, the lineation is oriented to
the east-northeast and suggests flow in that direction.

 

This lineation, together with the proximity to the Black
Mountain stock, the elongation of the laccolith toward
the east-southeast, and the structural setting at its west
end, suggests that the laccolith was fed laterally from
the Black Mountain stock.

EAST HORSE LACCOLITH

East Horse laccolith is composed of normal diorite
porphyry and is east of Horse Mountain. In plan view
it is oval, elongated N. 80° E., and is about 1%4 miles
long and 1 mile wide. It is at least 350 feet thick in
the center and tapers gradually to gently rounded sides.
It forms a low topographic dome that is dissected by
two east-trending canyons, neither of which has pene-
trated underlying sedimentary strata. The laccolith
is in Mancos Shale.

Many readings of primary flow structure as defined
by hornblende phenocrysts were taken in East Horse
laccolith. Two consistent trends were noted: in the
northern part of the laccolith, lineations plunge about
15° toward the east-southeast, and in the southern part
they plunge gently north or south, or are horizontal.
These trends suggest that two separate intrusions
occurred.

The source appears to have been under the Ute Peak
bysmalith (section 4-4", pl. 1), although the laccolith
may have been fed from the Black Mountain stock, or
from both stocks. Flow structure suggests that the
feeder was almost flat lying.

The Razorback laccolith intruded and uplifted part
of the south edge of East Horse laccolith, and thus is
the younger of the two. The straight, north side of the
Razorback laccolith clearly reflects the influence and
control of the earlier East Horse laccolith to the north.

SUNDANCE CLUSTER OF LACCOLITHS

Laccoliths of the Sundance cluster are all composed
of the normal type of diorite porphyry, and all were in-
truded into the Dakota Sandstone or the lowermost part
of the Mancos Shale at about the same time. The cluster
includes the Sundance, East Sundance, Trapdoor, and
Irwin laccoliths southeast of Black Mountain. The
Trapdoor laccolith is west of Sundance laccolith, and
Irwin laccolith (not shown on pl. 1) is east of the East
Sundance laccolith. All four laccoliths probably are
offshoots of the Black Mountain stock, through con-
cordant conduits in the strata stratigraphically lower
than the laccoliths. All members of the group have
fiat tops and, with the possible exception of the Irwin
laccolith, the base of which is not exposed, average about
350 feet in thickness.

The floors of the four laccoliths probably are at or
near the Dakota-Mancos contact. With the exception
of the Irwin laccolith, the roofs of the laccoliths are

 

STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES 57

Dakota Sandstone in some places and Mancos Shale
in others (pl. 1). Large blocks of Dakota Sandstone
apparently were tilted upward by the rising magma in
a manner analogous to the tilting of a trapdoor, and
magma then spread into the overlying Mancos Shale
(fig. 18).

RAZORBACK LACCOLITH

The Razorback laccolith is in part contiguous with
the south edge of the East Horse laccolith. It is oval
in plan view, elongated N. 80° W., and measures 1 mile
in length by a maximum of 0.4 mile in width. The
abrupt western end has a minimum thickness of 300
feet. The greatest original thickness of the laccolith
was. probably 850 feet in the central part. The roof
of Mancos Shale is missing over all the crestal part
of the laccolith but can be seen on the north side.

The southern part of the laccolith was intruded into
the Mancos Shale about 100 feet above the Greenhorn
Limestone Member. Along its north side the laccolith
is somewhat discordant, and is in contact with strati-
graphically lower beds from west to east. In its west-
ern third, the base of the laccolith lies as much as 100
feet above the Greenhorn Limestone Member of the
Mancos. On the northeast the base of the laccolith lies
below the top of the Dakota (pl. 1).

The linear flow structure defined by alined pheno-
crysts of hornblende indicates a general southeastward
movement of the magma in the western part of the
Razorback body, an eastward flow in the central part,
and an east-northeastward flow in the eastern part.
This laccolith was probably fed from the Black Moun-
tain stock by a conduit that was structurally and strati-
graphically lower than and directly under, or just to
the south of, the Horse Mountain laccolith. The con-
duit probably lies about 100 feet above the base of the

Mancos.
LAST SPRING LACCOLITHK

The Last Spring laccolith is exposed over a broad
area east of "The Knees" stock and south of the Sun-
dance cluster of laccoliths. The long dimension of this

laccolith trends northeastward. The laccolith is as

FEET
1200 -

 

much as 1 mile wide, and is about 750 feet thick in its
central part.

The roof of the laccolith is composed of Dakota
Sandstone and Mancos Shale. The fact that beds of
the Dakota are exposed at the top of the laccolith
and along the west side suggests a trapdoor effect sim-
ilar to that described for the Sundance cluster of lac-
coliths. The southwest end of the laccolith is in con-
tact with "The Knees" stock. This relation, together
with the orientation of the body, suggests that the
laccolith was fed laterally from the stock.

The Last Spring laccolith consists of granodiorite
porphyry of the southern type, but contains more
phenocrysts of quartz and crystals of epidote than most
of this rock. A petrographically similar rock crops
out a few hundred feet east of the Last Spring lacco-
lith, on slopes west of Towaoe, Colo. This rock forms
a flat-topped laccolith in mudstone beds of the Brushy
Basin Member, and probably came from the conduit
that supplied the Last Spring laccolith.

FLAT LACCOLITH

Flat laccolith is in contact with the south side of the
Last Spring laccolith, which cut and uplifted parts of
it. To the south, Banded laccolith has also cut and
uplifted parts of Flat laccolith.

Flat laccolith is 114 miles in length by a maximum of
one-third mile in width and is estimated to be 250 to
300 feet thick; its long dimension trends southeast.
The laccolith is flat-topped and has tapered sides. It
was intruded into the lower few hundred feet of the
Mancos Shale throughout its length, but is stratigraphi-
cally lower toward the southeast. Wherever exposures
are good, the contact appears concordant and the nature
of the downward crosscutting is not known.

Flat laccolith was fed from "The Knees" stock to the
northwest, as indicated by the northwest trend of linear
flow structure. (See pl. 1.)

THREE FORKS LACCOLITH

Three Forks laccolith is composed of normal diorite
porphyry and is approximately half a mile southeast

 

1000 - ft a T "oe aa ins", e e e m 2 -
+ es
er rre rie "cur. ttn...
a ae fhewe
800 re iaa a in er nl tL. e oo Mancos Shale. x ® r> Aa ~ C
rows bd - beled Clans lal N4" at" lee faw: Sec esl Jms x ~A In 2 _
sre. yous z«« L > Ig sr a.-
600 - ese n - . a= 'as wmILLI=p
oer Diorite porphyry 4 & f sum _ hoes
A00 -] - -- -- - -- - . f - : Care |-
_ Mancos Shale

 

200 -| Dakota Sandstone 2 " ancensrerns -

Burro Canyon Formation

 

Dakota Sandstone

 

 

0

1000 FEET

FiGURE 18.-Sketch showing inferred stratigraphic and structural relations of laccoliths in the Sundance cluster, and mechanism of
"trapdoor" structure.

58 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

of "The Knees" stock. It is bounded on the southwest
and southeast by younger intrusive masses of granodio-
rite and quartz monzonite. - Alined hornblende crystals
in this hornblende-rich porphyry define a general south-
east trending lineation which strongly suggests that the
magma came from "The Knees" stock.

A conspicuous feature of Three Forks laccolith is the
pronounced alteration of its central part. Pheno-
crysts of hornblende and plagioclase have been almost
completely altered to secondary minerals, whereas those
near the margins are clear and fresh. The alteration
was evidently caused by solutions and vapors that ac-
cumulated in the central part of the igneous mass,
probably immediately following consolidation. The
alteration thus is considered to be deuteric and is the
best known example of this type of alteration in the

Ute Mountains.
BANDED LACCOLITH

Banded laccolith, immediately southeast of Three
Forks laccolith, is composed of quartz monzonite por-
phyry. It is half a mile wide and originally was a
mile long, but the southeastern third has been separated
from the main body by erosion. The central part of
the laccolith is 550 feet thick; considering the lateral
dimensions, this is thicker than most laccoliths com-
posed of diorite porphyry. Most of the northwestern
part of the laccolith was intruded into a zone ranging
from 100 feet below the Dakota-Mancos contact to just
above the contact. The southeastern third intruded
the Mancos near the level of the Greenhorn Limestone
Member.

The bare, steep-faced outcrops of the quartz monzo-
nite porphyry display impressive vertical banding.
The banding is due to weathering of unevenly altered
vertical layers that are interpreted as flow structure.
In the strongly altered layers, hornblende is replaced
by chlorite and magnetite, and plagioclase by kaolinite
and abundant calcite. The alteration is probably deu-
teric. Unaltered rock occurs within short distances of
the contacts with the sedimentary rocks.

As shown on plate 1, the flow banding in the north-
western part of the laccolith strikes generally northwest
and dips slightly southwest or as much as 45° NE. In
detail, the attitude of the banding is much more diverse
than the map indicates, and is more erratic in the south-
eastern part of the laccolith than in the northwestern
part.

THE "WEST TOE" CLUSTER OF INTRUSIVE BODIES

The "West Toe" cluster includes several intrusive
bodies between "West Toe" bysmalith, "The Knees"
stock, and the western part of Mushroom laccolith. The
major intrusive bodies in this area are tongue-shaped

 

laccoliths composed of normal diorite porphyry that
were intruded into Mancos Shale a few tens of feet
above the top of the Dakota Sandstone. The laccoliths
were probably intruded laterally from "The Knees"
stock. Numerous sills crop out in the area between the
laccoliths and "The Knees" stock and are stratigraphi-
cally above the laccoliths. There are also many dikes
of granodiorite porphyry, several of which cut sills and
laccoliths of diorite porphyry.

MUSHROOM LACCOLITH

Mushroom laccolith is near the center of the Ute
Mountains midway between "The Knees" stock and
the Black Mountain stock and is composed of the cen-
tral type of granodiorite porphyry. It is 244 miles
long and almost 1 mile wide. A thickness of about 500
feet is exposed but, because the base is not exposed, the
total thickness is not known. The laccolith lies in the
Mancos Shale. It may have been fed laterally from
either Black Mountain or "The Knees," or it may have
had a nearly vertical conduit connected to a stock at
considerable depth.

Many dikes radiate outward from the approximate
center of the Mushroom laccolith. The dike rocks con-
tain crystals of andesine as much as 10 mm in length
and resemble the southern type of granodiorite por-
phyry more closely than the central type. As shown
by thin sections, the dikes contain less quartz than the
granodiorite of the laccolith.

PACK TRAIL LACCOLITH

Pack Trail laccolith, composed of diorite porphyry,
is between Mushroom laccolith and Black Mountain
in sees. 34 and 35, T. 35 N., R. 18 W. The laccolith
was intruded into Mancos Shale below the Juana
Lopez Member. Sedimentary rocks have been almost
completely removed from the flanks of the laccolith ;
only a few remnants remain. These remnants dip
steeply, but the fact that the top surface of the in-
trusive body is conformable with them suggests a
laccolith rather than a bysmalith. The base of the lac-
colith is exposed for a few feet in a deep valley cut
into it by a tributary of Cottonwood Creek in NZ
see. 35, T. 35 N., R. 18 W. The laccolith is 114 miles
long, about 1 mile wide, and at least 700 feet thick.
It was probably fed from the Black Mountain stock,
which is contiguous on the north.

TONGUE LACCOLITH

Tongue laccolith is exposed a few hundred feet east
of Mushroom laccolith and about half a mile north-
east of "The Knees" stock. It is composed of micro-
gabbro porphyry and is tongue-shaped in plan. It
probably was intruded laterally from "The Knees"

 

 

thin dimers : Hy Hes wi te

 

STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES 59

stock. It is about 200 feet thick, half a mile wide, and
of unknown length, -

The laccolith has been intruded by a younger and
smaller laccolith of diorite porphyry, and has been
domed upward along its southern extremity by grano-
diorite porphyry of the Last Spring laccolith. It is
also cut by a dike of granodiorite porphyry.

YUCCA CLUSTER OF LACCOLITHS

The Yucca cluster of laccoliths lies west and south-
west of Black Mountain. Laccoliths of this cluster are
composed of diorite porphyry of the normal type.
They have distinct northeast-trending lineation, ex-
tend almost to Black Mountain, and probably were
intruded laterally from the Black Mountain stock.

With the exception of a small laccolith in the south-
ern part of see. 83, T. 35 N., R. 18 W., the laccoliths
were intruded into Mancos Shale above the Juana
Lopez Member. The laccoliths average less than 300
feet in thickness.

"THE BUTTES" LACCOLITH

"The Buttes" laccolith is about 2 miles northwest of
the top of Black Mountain. It has the shape of a
crude equilateral triangle and is about a mile long.
It is asymmetric in cross section, and is thickest (about
500 feet) in the northwestern part.

The southern part of the laccolith was intruded some
200 feet above the base of the Mancos, the northern
part within 100 feet of the base, and the western part
at or below the top of the Dakota. Inasmuch as the
laccolith was probably fed from the south and south-
east through a sill-like conduit from Black Mountain,
the magma evidently cut downward stratigraphically
in its advance toward the northwest.

NORTH BLACK MOUNTAIN LACCOLITH

North Black Mountain laccolith, composed of nor-
mal diorite porphyry, is southwest of Mable Mountain
and northwest of Black Mountain stock. It is elongate
to the north and about 2 miles long; the north half is
about three-fourths mile wide but the south half is
narrower. The laccolith has a maximum thickness of
about 700 feet, in the south-central part.

On the southwest, the thin edge of the laccolith lies
several hundred feet above the base of the Mancos
Shale, but farther north along the west side, it lies in
the upper 100 feet of the Dakota Sandstone. Because
of poor exposures, the nature of the northward change
in stratigraphic level is not known.

The steeply dipping east side of the laccolith is well
exposed. There, Mancos strata dip 30° away from the
laccolith and can be traced upward into a nearly flat
roof (pl. 1). On the south, toward Black Mountain,

u W+ . pm

 

the fact that the Mancos dips as much as 30° away
from North Black Mountain suggests that the lacco-
lith bulges as it approaches Black Mountain stock.

SILLS AND DIKES

Near the head of Pine Creek between Black and
Mable Mountains are several sills composed of micro-
gabbro and diorite porphyry rich in mafic minerals;
two of these sills are shown on figure 19. Field rela-
tions indicate that the sills were domed locally by the
emplacement beneath them of the North Black Moun-
tain laccolith and Mable Mountain bysmalith, which
consist of more silicie rocks.

Many dikes occur in the Ute Mountains, especially
in the vicinity of "The Knees" stock and the Black
Mountain stock. Most of the dikes are composed of
granodiorite porphyry, and several of these cut sills
or laccoliths composed of diorite porphyry. The long-
est dike in the mountains is the Ute Creek dike, which
extends northeastward from the base of Mable Moun-
tain to McElmo Creek (pl. 1). This dike was probably
the feeder of the Mable Mountain bysmalith.

BRECCIA PIPES

Several breccia pipes cut the Mancos Shale in the
extreme western part of the Ute Mountains, in sees. 31
and 36, T. 35 N., Rs. 19 and 18 W. The pipes crop out
a few feet below the projected base of the westernmost
and largest laccolith in the Yucca cluster. They range
in diameter from about 10 to 150 feet and are nearly
circular, although the largest is elliptical and about 50
feet wide and 150 feet long. Relations between the
pipes, the surrounding Mancos Shale, and the base of
the laccolith are well exposed in draws leading north-
west along the northern flank of the laccolith in the
NEJ sec. 31, T. 35 N., R. 18 W. The regional dip of
the Mancos Shale in and around the northern flank of
the laccolith is about 4° W., but near the pipes the shale
dips 18° to 30° NE. (pl. 1).

The individual pipes consist of Mancos Shale and
diorite porphyry in brecciated blocks and fragments
cemented with travertine. The shale is principally in
the outer parts of the pipes. It is so altered and
hardened that it resembles mudstone, and is light gray
green to pale brown, in contrast with the dark-gray to
black fissile shale surrounding the pipes. The brec-
ciated diorite porphyry is principally in the central
parts of the pipes. In places, the porphyry fragments
have been argillized, but most of them show no altera-
tion effects other than the cementation by travertine.

The Mancos Shale below the laccolith is brecciated
into fragments that average only a few inches in diame-
ter, and the diorite porphyry for several feet above the
base of the laccolith is broken into angular fragments

60 GEOLOGY, PETROLOGY,

UTE MOUNTAINS AREA, COLORADO

 

FIGURE 19.-Two sills in Mancos Shale (Km) along Pine Creek, north of Black Mountain.
lower sill is diorite porphyry (TKd).

that range from a few inches to several feet in diame-
ter. Both the brecciated shale and porphyry are
cemented with travertine for a distance of about 15 feet
from the contact.

A sill of lamprophyre crops out in a draw south of
the locality described above, in sec. 6 of unsurveyed
T. 34 N., R. 18 W. Although no travertine-bearing
pipes are exposed in the immediate vicinity of the sill,
the sill is fractured, brecciated, and cemented with trav-
ertine. Figure 20 shows a boulder of brecciated lam-
prophyre that has slumped from the sill. The boulder
is composed entirely of angular fragments of lampro-
phyre in a matrix of travertine. Figure 21 is a photo-
graph of the sill. Some of the fractures are open and
only partly filled with travertine. - The solutions carry-
ing calcium carbonate evidently moved laterally rather
than vertically along the fractures in the sill, for the
shale beneath the sill is "tight" and unfractured.

The pipes were probably formed by the explosive re-
lease of steam and hot water. The brecciation and frac-
turing of the sill and the basal part of the laccolith may

 

Upper sill is microgabbro (TKm) about 200 feet thick ;
Viewed from the east.

have been caused by faulting and quaking prior to the
hydrothermal activity, but the possibility exists that
this brecciation was also due to the explosive action of
water which gave rise to the breccia pipes themselves.
The open nature of many of the fractures and the occur-
rence of travertine suggest that the fracturing took
place when the rocks were at shallow depth or were
exposed at the surface. The fracturing and hydrother-
mal (hot-spring) activity are therefore believed to be
much later than the igneous activity that gave rise to
the Ute Mountains.

SUMMARY OF GEOLOGIC EVENTS

The structural and stratigraphic history of the Ute
Mountains area is an integral part of the geologic his-
tory of the Colorado Plateau. Cater (1955) has out-
lined the structural history of the area of salt anticlines
on the plateau north of the Ute Mountains, and Stro-
bell (1956) has summarized the geologic history of the
Carrizo Mountains area, located approximately 25 miles
south of the Ute Mountains. The important geologic

 

ECONOMIC GEOLOGY 61

 

Ficur® 20.-Boulder of brecciated lamprophyre cemented with
travertine.

 

ee

134 Sube

FicUurE 21.-Fractured and brecciated sill of lamprophyre. Some of
the fractures are open and only partly filled with travertine.

events as recorded by the rocks exposed in the Ute
Mountains area are:

1. Predominantly subaerial deposition of the Navajo
Sandstone from sources to the west in Late Tri-
assic( ?) and Jurassic time.

2. Deposition of the Entrada Sandstone under alter-
nating subaqueous and subaerial conditions.

3. Deposition of the marine Summerville Formation.

4. Deposition of the Junction Creek Sandstone, partly
subaqueous and partly subaerial.

5. Deposition of the Morrison Formation, partly flu-
vial and partly flood plain.

6. Deposition of coarse fluvial clastics of the Burro
Canyon Formation of Early Cretaceous age from
sources west or southwest of the Ute Mountains
area.

 

7. Pronounced uplift and widespread erosion south
of the Ute Mountains area.

8. Deposition of fluvial, swamp, and littoral rocks
of the Dakota Sandstone during Late Cretaceous
time.

9. Marine invasion and deposition of the Mancos
Shale of Late Cretaceous age.

10. Deposition of marine sandstone and shale of the
Point Lookout Sandstone in an oscillating Late
Cretaceous sea.

The events that followed the deposition of the Point
Lookout Sandstone in the Ute Mountains area and the
age of the igneous rocks of the mountains can be in-
ferred only from rocks exposed southeast of the Ute
Mountains.

11. Deposition of the marine and continental shale and
sandstone of pre-Animas, post-Point Lookout
age (Menefee Formation, Cliff House Sandstone,
Lewis Shale, Pictured Cliffs Sandstone, Fruit-
land Formation, and Kirtland Shale).

12. Intrusion of igneous rocks :-

a. Microgabbro.

b. Diorite porphyry.

c. Granodiorite porphyry.

d. Quartz monzonite porphyry.

13. Erosion in the Ute and La Plata Mountains and
deposition of coarse clastics (in the McDermott
Member of the Animas Formation of Late
Cretaceous age) to the south and east.

ECONOMIC GEOLOGY
METALLIC MINERAL DEPOSITS

Small deposits of uranium, vanadium, and copper
along the northern edge of the Ute Mountains and in
the vicinity of McElmo Canyon have been extensively
prospected, but thus far have not proved to be of
much commercial value. Except for the generally
barren iron-stained pyritic alteration zones in some
of the intrusive bodies, no metallic mineral deposits
are known in the Ute Mountains proper.

The uranium deposits include (a) fault-controlled
deposits that contain abundant pyrite and sparse cop-
per minerals, and (b) flat-lying deposits containing
vanadium minerals in beds at considerable distances
from known faults. The copper deposits are all as-
sociated with faults or shear zones.

Commercial deposits of uranium and vanadium occur
in the Salt Wash Member of the Morrison Formation
in Montezuma Canyon, Utah, about 10 miles north-
west of the northwest corner of the Mogqui SW quad-
rangle, and in the Carrizo Mountains of Arizona, about
25 miles southwest of the Ute Mountains. Very large
deposits of uranium and vanadium occur in the Jack-
pile sandstone (of local usage) in the Morrison Forma-

62 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

tion far south of the Ute Mountains in the vicinity
of Grants, N. Mex. Because of the widespread occur-
rence of uranium deposits in the Salt Wash Member
on the Colorado Plateau, the Salt Wash was consid-
ered to have the greatest potential in the Ute Moun-
tains area and was studied in considerable detail dur-
ing the present work.

URANIUM DEPOSITS
FAULT-CONTROLLED DEPOSITS

CLIFF HOUSE GROUP OF CLAIMS

The Cliff House group of claims lies in sees. 22, 23,
and 27, T. 36 N., R. 18 W. All the prospects in this

 

group are about 11/4, miles north of McElmo Creek (fig.
22) and can be reached by an access road, built by the
Atomic Energy Commission, that leaves Colorado
Highway 32 just north of Battle Rock. The Cliff House
group was owned in 1957 by the Four Corners Uranium
Co., Denver, Colo. 2

Cliff House prospect.-The Cliff House prospect (fig.
22) is along a northeast-trending fault along which
beds of the Summerville Formation are displaced 180
feet down to the northwest and brought against the up-
per part of the Navajo Sandstone (fig. 23). The radio-
active area is about 100 feet long and is limited to a
zone where mudstone and sandstone of the Summerville

R. 18 W. R. 17 W.

 

 

 
 
   
 
   

 

al

§TATES NATURAL
PROSPECT

Post-Salt Wash

 

Post-Salt Wash

 

KARLA KAY
M

  
 
  
 

Pre-Salt Wash

 

 

 

1. 36 N! 31

s

(Js

T. 35 N. 6

 

   

 

 

 

 

 

 

 

Post-Salt Wash

 

 

 

 

 

|
‘Post-Salt Wash‘I
|
|

s

 

MINE

 

 

R |
LITTLE MAUDE \
|

|

|
|
|
]

 

Fisurs 22.-Map showing location of mines and prospects.

EXPLANATION
U C
Contact wa 32 ;
Prospect pit or small mine
omnes U, uranium
Fault Cu, copper

Bar and ball on downthrown side

TKd, diorite porphyry ; Jms, Salt Wash Member of the Mor-
rison Formation (stippled).

 

ECONOMIC GEOLOGY

5 N

  
  

Entrada Sandstone

\\\’ Maximum radioactivity occurs at base
of limestone

Summerville Formation

  
    
  

Limestone bed

Sandstone
Navajo Sandstone

   

Navajo Sandstone

o 200 FEET
wile Whens.

Ficur® 23.-Sketch showing stratigraphic and structural relations at
the Cliff House prospect, McElmo Canyon, Montezuma County, Colo.
Formation are in contact with a limestone bed about
4 feet thick in the Navajo Sandstone. Rocks in the pros-
pect area are intensely stained with limonite, and much
carbonate has been leached from the limestone bed.
Scintillation-counter measurements indicate that most

of the radioactive material is concentrated in sandstone .

at the base of the leached limestone. No minerals of
uranium or vanadium were identified. Sample 55-E-12
(table 13), chipped from sandstone in contact with the
leached limestone, assayed 0.038 percent eU;0;, 0.040
percent U;O;, and 0.16 percent V;0,;.

A radiometric survey was made of the fault line for
several hundred yards east and west of the prospect
area; no radioactivity above background was noted.

Cliff House No. 2 prospect.-The Cliff House No. 2
prospect is about half a mile northeast of the Cliff
House prospect (fig. 22) along the same northeast-
trending fault. In the Cliff House No. 2 area, down-
ward displacement on the northwest side of the fault is
only about 100 feet, and the Entrada Sandstone is in
contact with uppermost beds of the Summerville.
Radioactivity is only slightly above background. The
sandstone in the Summerville is strongly stained yellow-

 

63

brown by limonite near the prospect, but farther away
the sandstone changes gradually to the pink color char-
acteristic of this stratum in the McElmo Canyon area.
The sandstone is probably the same bed that is radio-
active in the Cliff House No. 4 prospect.

Clif House No. 4 prospect.-The Cliff House No. 4
workings consist of two very short drifts and several
small pits dug into or near two east-trending faults
(fig. 24). The northern fault has about 80 feet of dis-
placement with the south side down. The beds are
displaced 15 feet down on the north side of the south-
ern fault; consequently there is a small graben in the
prospect area. The southern fault dips about 80° to
the north and probably intercepts the northern fault
at depth (section 4-4", fig. 24). The highest radio-
activity occurs near the southern fault; a chip sample
(55-E-10, table 13) taken in this zone assayed 0.053
percent 0.028 percent U;O;, and less than 0.1
percent V;0O0;. The radioactive material has not been
identified; it is closely associated with limonite, hema-
tite, barite, asphaltite, and sparse copper carbonates
concentrated in a flat-lying upper sandstone bed of the
Summerville and in fault breccia adjacent to this bed.
The radioactivity diminishes southward from the
faulted area and, in general, where it diminishes the
content of limonite decreases. The upper bed of the
Summerville between the faults, and north of the north
fault, is not radioactive. In holes drilled by the U.S.
Atomic Energy Commission (AEC) along the north
and south faults and in the intervening graben there
is no increase of radioactivity at depth. Pyrite and
marcasite were identified in the AEC drill cores by
Coleman and Delevaux (1957, p. 513).

THREE STATES NATURAL GAS CO. PROSPECT

Uranium prospects in see. 24, T. 36 N., R. 18 W., are
on an oil and gas lease owned by the Three States
Natural Gas Co. and on a mineral claim located by
Ben Archibeque, Mancos, Colo. The area is about 200

TaBus 13.-Radiometric and chemical analyses, in percent, of samples from radioactive prospects, McElmo Canyon, Montezuma County

Colo.
[Analysts, 1, D. L. Schafer, H. H. Lipp, J. E. Wilson; 2, C. G. Angelo; 3, C. G. Angelo, J. P. Schuch, J. E. Wilson]

 

 

 

 

 

 
 
 

 
  
  

 

Sample No. Prospect or mine sampled Radiometric) Chemical analysis
analysis
Analysts
Labora Equivalent! Uranium | Vanadium
Field tory Name Material uranium (U;O3) pentoxide
(eU;03)
1 | Three States Natural Gas Co.__..____. 2:2. core +2202 000 oce cen 0. 049 0. 015 <0.1
1 foss} I0 reena e Seele enue evs Q eS re 3 . 028 019 <1
14 .do. . O21 006 Kul
1 .:do. O04 Io eact . 52
1 - Mo.z.: £008] . 93
1 Limonitic sandstone. . 053 . 028 xt
1] -CHA Hose. .. 211.2000... cence ees HD ESE ELECT cy ce Cee adn . 038 . 040 16
2 Leached limestone. ............. & 2002 12. . os ol ena eee nuns
3 -| Green mudstone, Brushy Basin Member... ey So s se eile el
8 Sandstone from Karla Kay Conglomerate Member. RODE ETH oon
3 Mudstone pebble conglomerate, Karla Kay . 016 . 019 82
Conglomerate Member.
3 Karla Kay Conglomerate Member.....___.__.__.. . 007 . 012 <.1

 

 

 

 

 

 

 

 

 

 

64 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

 

 

 

EXPLANATION

Contact

U
D
Fault
U, upthrown side; D, downthrown
side

 

X
Prospect pit

Radioactive material

C.)—
AEC angle diamond-drill hole

©

Four Corners Uranium Co.
Vertical solid- bit -drill hole

 

0 300 FEET
(Erron f ient oras

FIGURE 24.-Clif House No. 4 area, McElmo Canyon, Montezuma County, Colo.

Ji, Junction Creek Sandstone: Jsu, upper part, Jsr, radio-

active sandstone bed, and Jsl, lower part, all of the Summerville Formation, and Je, Entrada Sandstone.

feet south of the Schmidt 1 carbon dioxide well and
can be reached by a 2-mile access road from Colorado
Highway 32.

Two prospect pits about 60 feet apart are dug into
the Entrada Sandstone along a northeast-trending
fault that displaced beds of the Summerville Forma-
tion 10 feet down to the southeast, against the Entrada
Sandstone. The highest radioactivity is in the east
pit, which is dug into sandstone on the north side of
the fault. Of three samples taken in this pit during
the present work, the most radioactive (sample 55-E-5,
table 13) assayed 0.049 percent eU;0,;,, 0.015 percent
U;O;, and no V;0,;.

 

Veinlets of asphaltic material were noted in fault
breccia in the prospect area. The asphaltic material
has been "dried"; it is black, has a pitchy luster, is
brittle, and can be ground to a brown powder between

the fingers.
BEDDED URANIUM DEPOSITS

KARLA KAY MINE
The Karla Kay mine is near the NW cor. sec. 32,
T. 36 N., R. 18 W. The mine area is claimed by the
Four Corners Uranium Co. and can be reached by a
1%/,-mile access road that leaves Colorado Highway 32
at the house site of the Fowler Farm in McElmo
Canyon.

 

 

v

ECONOMIC GEOLOGY 65

The mine has 70 feet of workings divided among
three drifts to the north, east, and south (fig. 25). The
mine was started in radioactive talus and the workings
entered coarse clastic rocks of the lowermost part of
the Burro Canyon Formation.

Three sets of joints break the rocks in the mine-
one set strikes consistently northeast, another north-
west, and a poorly defined third set, about east. The
ground is badly broken by these fractures and by col-
lapse due to removal by underground water of mud-
stone beneath the sandstone of the Burro Canyon.

Most of the radioactive material in and around the
mine is in a lentil of mudstone conglomerate (fig. 26)
that is part of the Karla Kay Conglomerate Member
(Ekren and Houser, 19592) of the Burro Canyon
Formation. The conglomerate contains pebbles and
slabs of green mudstone in a matrix of coarse conglom-
eratic and cherty yellow-brown sandstone. It con-
tains abundant tree molds, but few carbonized tree
remnants. 'The conglomerate lentil occupies a channel
cut in bentonitic mudstone of the Brushy Basin Member
of the Morrison Formation and underlies a fine- to
medium-grained, poorly sorted quartz sandstone that
contains very sparse pebbles of variegated chert. This
sandstone bed underlies the main lens of the system of
shoestring conglomerates of the Karla Kay. Both the
mudstone conglomerate and the overlying coarse sand-
stone were eroded prior to the deposition of the main
lens of the Karla Kay east and west of the mine.

The highest radioactivity is associated with carbon-
ized plant debris in the back of the north drift at the
top of the mudstone conglomerate lentil (fig. 25). This
material is estimated to have about 0.05 percent U;O;.
Carnotite is weakly disseminated in the conglomerate
and forms a coating on pebbles of mudstone. Radio-
activity is not concentrated along any of the fractures
in the mine, many of which contain gypsum, limonite,
and dendrites of pyrolusite. Radiometric traversing
to the east, north, and west of the mine did not disclose
significant radioactivity.

The radioactive material in the Karla Kay mine has
not proved to be of commercial value (table 13), and
no ore has been shipped from the mine. Several small
deposits of uranium have been found associated with
the shoestring conglomerates of the Karla Kay west of

 

25.-Map and section of the Karla Kay mine, McElmo Canyon,
Montezuma County, Colo. Qt, Quaternary talus deposits ; Kbk,, sand-
stone, and Kbk,, mudstone, of Karla Kay Conglomerate Member of the
Burro Canyon Formation; Jmb, mudstone of Brushy Basin Shale
Member of the Morrison Formation. Level of plan view is waist
height.

 

Abundant carbon trash in back

   
   
  
  
   
 

Carbonized tree frag-

3 ments (most radio-
active material in
mine)

Qt

Joints are coated with gypsum,
lirnonite, and pyrolusite

Carnotite, weakly disseminated
in sandstone

Carnotite coating
mudstone pebbles

Drift to north

 

 

0 10 FEET

EXPLANATION

Contact
Dashed where approximately located

80
sees,

Strike and dip of joints

90
~-

Strike and dip of vertical joints

66 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

SW

NE

Burro Canyon Formation

 
 
 
 
   
   

Karla Kay Conglomeratfz Member

- KARLA KAY MINE

Quaternary talus
deposits >

0

errno Brushy Basin Shale Member of Morrison Formation _(Mudstone) ----------_-----

100 FEET

( LL LL __]

FisurE 26.-Sketch showing stratigraphic relations at the Karla Kay mine.

the Ute Mountains area, however, and some of this
material has been mined.

COFFIN'S PROSPECT

Coffin's prospect is on the John Meadows Ranch a
few feet north of Colorado Highway 32, in see. 34,
T. 36 N., R. 18 W. This prospect was described by
Coffin (1921).

The mineralized material is in flat-bedded sandstone
near the middle of the Entrada Sandstone. The pros-
pect was opened in 1913 and the pit has since been filled
with alluvium and windblown silt. The mineralized
rock is exposed in an outcrop about 130 feet long by
40 feet wide that extends south under the highway. It
is eroded away north and éast of this exposure and is
covered by alluvium to the south and west. Holes dug
for fenceposts and power poles along the road show
the mineralized zone to be about 6 inches thick. The
vanadium-uranium minerals and traces of malachite
are concentrated along bedding planes and do not seem
to be influenced or controlled by fractures. A 1-inch
seam within the mineralized zone is richer than the
rest and contains a dark-gray mineral, probably vana-
dium mica. The secondary minerals pintadoite and
carnotite occur as coatings on fresh fractures.

No ore has been shipped from this prospect. Assays
of chip samples taken along the most intensely mineral-
ized seam during the present study are shown in table 13.
A sample taken by Coffin (1921) assayed more than
1.0 percent V,0,;, and 0.09 percent U;O;. The deposit
at the prospect differs from others in the Ute Mountains
area in that vanadium is much more abundant than
uranium. The assays in table 13 indicate approxi-
mately a 100 : 1 ratio, and the assay reported by Coffin
shows a 10 : 1 ratio.

 

Collapsed beds have been restored to horizontal position.

COPPER DEPOSITS
BATTLE ROCK MINE

The Battle Rock workings are in a mineralized shear
zone that extends the full length of Battle Rock, a nat-
ural monolith in McElmo Canyon in see. 34, T. 86 N.,
R. 18 W. The shear zone trends N. 75° W. and is from
10 to 15 feet wide. The introduction of silica, barite,
limonite, and carbonates along the shear zone indurated
the normally friable Junction Creek Sandstone and
altered its color from reddish orange to brown. The
indurated rock in and adjacent to the shear zone weath-
ers in relief, and thus forms the monolith.

The workings are only partly accessible; only the
main drift was examined. Sparse copper carbonates
in a barite gangue, and abundant limonite or hematite,
or both, were found there. Rock on the mine dump
has sparse chalcopyrite. No ore has been shipped from

the mine.
UTE CREEK DIKE PROSPECTS

Several prospect pits have been dug into baked sedi-
mentary rocks and sheared diorite porphyry along the
sides of the Ute Creek dike (fig. 22). Most of these
pits are devoid of visible ore materials, although a few
show sparse copper minerals distributed along frac-
tures. A shallow shaft that exposes copper minerals
in sheared diorite was studied by E. M. Shoemaker and
W. L. Newman, who reported (written commun., 1953)
that: "The sheared diorite is intensely kaolinized lo-
cally. Copper minerals, chiefly malachite, azurite, and
copper pitch are distributed along fractures, in small
cavities, and replace altered feldspar and hornblende
phenocrysts."

LITTLE MAUDE MINE

The Little Maude mine, in see. 14, T. 35 N., R. 18 W.

(fig. 22), is a single drift approximately 50 feet long

 

ECONOMIC GEOLOGY 67

in baked and sheared limy mudstone of the Mancos
Shale. The back of the drift is the base of a thin
sill that is the featheredge of the Mable Mountain
bysmalith. The drift follows a shear zone that con-
tains much pyrite and seems to be part of the frac-
ture system along which the Ute Creek dike was
intruded.

RELATION OF THE METALLIC MINERAL DEPOSITS TO THE
IGNEOUS ROCKS

The occurrence of uranium and other metals in faults
in the Ute Mountains area is of geologic interest be-
cause of the controversy regarding the origin of the
uranium deposits on the Colorado Plateau. The fun-
damental questions regarding the uranium in the
McElmo Canyon area are these: Were the uranium
and associated metals derived from hypogene solutions ?
If so, were the solutions related to the igneous rocks
of the Ute Mountains?

The field data indicate that the copper deposits of
Battle Rock and the Ute Creek dike were almost cer-
tainly derived from hydrothermal solutions related to
the igneous activity of the Ute Mountains. These de-
posits occur in sheared or faulted zones that are radial
to the intrusive bodies (pl. 1) and, specifically, to the
Mable Mountain bysmalith that probably overlies a
fracture zone.

A large part of the Mable Mountain intrusive body
has been pyritized and altered. Intensely altered areas
partly surrounded by vein deposits of metallic minerals
(especially base metals) are a well-known feature of
mineral districts of hydrothermal origin in southwest-
ern Colorado. The pyritization at Mable Mountain
was probably hydrothermal and related to the intro-
duction of copper minerals in the veinlike deposits
of Battle Rock and along the Ute Creek dike.

The introduction of uranium and sparse copper in
the faults of the Cliff House No. 4 and Three States
Natural Gas Co. prospects probably was accomplished
during the same hydrothermal stage as the pyritization
at Mable Mountain. The abundance of pyrite and
marcasite at depth in the Cliff House No. 4 veins and
the occurrence of asphaltite in both this prospect and
that of the Three States Natural Gas Co. indicate that
the introduced material in the arcuate faults was largely
derived from ascending solutions. The asphaltite prob-
ably came from petroliferous beds that underlie the
uranium deposits, because, in the McEImo Canyon area,
petroleum is unknown in rocks younger than those that
carry the deposits.

The ubiquity of barite in fractures along McElmo
Canyon is significant because the igneous rocks of the
Ute Mountains are abnormally rich in barium. (See
table 4.)

 

Fourteen samples from the fault-controlled and the
bedded mineral deposits were analyzed spectrographi-
cally in an attempt to find clues that might indicate a
source for the metals (table 14). Analyses of barren
rock from the formations that contain these deposits
are also shown in the table for comparative purposes.
The spectrographic data indicate that iron, barium,
cobalt, chromium, copper, molybdenum, nickel, lead,
strontium, vanadium, yttrium, and ytterbium occur in
greater than background quantities in both the fault-
controlled and the bedded deposits. On the other hand,
silver, arsenic, antimony, niobium, zinc, and manganese
occur in greater than background values only in the
fault-controlled deposits. All the elements listed above,
with the exception of silver, antimony, and niobium,
also occur in greater than background values in the
nonradioactive copper deposits in Battle Rock.

Vanadium is found only in one fault-controlled radio-
active deposit, the Cliff House prospect. Mineralized
limestone in the Navajo Sandstone at this prospect con-
tains about 300 ppm of vanadium (sample EMS-9-53,
table 14), and limonitic sandstone contains 900 ppm of
vanadium (sample 55-E-12, table 13), more than 50
times the background for Navajo Sandstone. The
vanadium may be related to dried asphaltic material
that is common as veinlets in the fractures of this
vicinity.

It is concluded that most of the metals, including
uranium, were introduced into the fault-controlled de-
posits by ascending (hypogene) solutions related to the
igneous activity in the Ute Mountains. The minor
elements of the bedded uranium deposits are similar to
those in the fault-controlled deposits and they may
therefore be of the same origin. It is unlikely, however,
that the Ute magmas were the source of uranium and
other. metals in the large bedded deposits in the Morri-
son Formation of nearby areas in southwestern Colo-
rado, because the hydrothermal activity associated with
the igneous intrusions of the Ute Mountains was feeble
and therefore probably not capable of supplying the
necessary volume of metals. In contrast with the stocks
of the Henry Mountains (Hunt and others, 1953, p.
217-220) and the Abajo Mountains (I. J. Witkind,
oral commun., 1957), the stocks of the Ute Mountains
are unmineralized.

URANIUM POTENTIAL OF THE UTE MOUNTAINS AREA
NORTHERN UTE MOUNTAINS AREA

The uranium potential of the area that includes the
northern fringes of the mountains and McElmo Canyon
is believed to be extremely small. The known deposits,
described above, have thus far proved to be too poor to
be of commercial value. Intense prospecting in this

68

area of dissected and well-exposed beds of the Morrison
Formation between 1950 and 1958 failed to disclose com-
mercial deposits. Nevertheless, because the lower
members of the Morrison Formation contain uranium
deposits in adjacent areas, they were studied in consider-
able detail during geologic mapping.

The Westwater Canyon Member of the Morrison
Formation averages less than 100 feet in thickness in
McElmo Canyon. The Salt Wash Member averages
about 200 feet in thickness in the eastern part of the
canyon, where the Recapture Member is absent, but it
thins westward to about 100 feet where the Recapture
Member is present. Throughout McElmo Canyon the
Salt Wash consists of continuous lenses or rims of sand-
stone separated by lenses of reddish-brown mudstone.
In some areas, there are four or more distinct lenses of
mudstone, but in others mudstone is in extremely thin
units and the Salt Wash forms a single thick sandstone
unit. Very little green or altered mudstone is present
below or above the individual lenses of sandstone and,
where present, the altered mudstone extends for only a
few tens of feet and is only a few inches thick. Pebbles
and seams of mudstone within the sandstone lenses,
however, are commonly green. No accumulations of
"carbonaceous trash" are known, but fine particles of
carbon are fairly abundant along bedding planes.

The lack of extensive greenish or altered mudstone,
the absence of "carbonaceous trash", and the nonlenticu-
lar character of the sandstone of the Salt Wash in this
area as compared to uranium-producing districts sug-
gest that the area is generally unfavorable for uranium
deposits (Weir, 1952). The environment in which the
sandstone beds were deposited apparently did not per-
mit large accumulations of carbon, nor the development
of zones of contrasting transmissivity.

SOUTHERN UTE MOUNTAINS AREA

The lower beds of the Morrison are exposed in only
one place in the southern Ute Mountains area. This ex-
posure is approximately a quarter of a mile south of the
"East Toe" or Sentinel Peak. Here, approximately 250
feet of the Westwater Canyon Sandstone Member over-
lies 250 feet of the Recapture Shale Member of the
Morrison Formation. The Salt Wash Sandstone Mem-
ber either is absent or is in a mudstone facies mapped
as Recapture Member.

The abrupt change in lithology of the lower beds of
the Morrison may be a favorable feature. The rich
uranium deposits of the Uravan district, Montrose
County, Colo., are localized in sandstone that pinches
out completely or thins abruptly within short distances
from productive areas (Boardman and others, 1956).
It seems quite certain that zones of contrasting trans-

 

GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

missivity (Jobin, 1962) within the beds of the Salt
Wash exist between the southern Ute Mountains area
and McElmo Canyon. Uranium deposits may occur in
or near these zones. The most economical area to ex-
plore for such deposits is the western flank of the Ute
dome. Many drilling sites are readily accessible in the
canyons and on the tops of mesas and pediments lying
west of the igneous terrane. The approximate depth
to the base of the Salt Wash in such places ranges from
400 feet in areas where the Brushy Basin Member of
the Morrison is at the surface to 800 feet where the top
of the Dakota is at the surface. Although the Morri-
son Formation is present beneath many of the igneous
bodies, exploration beneath these bodies would be ex-
pensive because of the depth of drilling and the structure
and hardness of the rock.

Criteria for recognizing favorable ground in areas
containing the Westwater Canyon Sandstone Member
have never been completely outlined. Thaden and
Santos (1956) reported that ore has not been found in
the Westwater Canyon Member where the sandstone
is split into many tongues or where it is less than about
100 feet thick. The Westwater Canyon is thicker than
100 feet in the southern part of the Ute Mountains area,
but the possibility of its being a host for large deposits
is probably remote inasmuch as no deposits have been
found in this member where it is widely exposed in adja-
cent areas of Utah, Arizona, and northwestern New
Mexico.

NONMETALLIC MINERAL DEPOSITS
PETROLEUM, NATURAL GAS, AND CARBON DIOXIDE
KNOWN OCCURRENCES

Only minor amounts of oil and gas had been pro-
duced from the Ute Mountains area before 1958, and
this production was confined to McEIlmo dome. The
structure and oil and gas possibilities of McEImo dome
have been summarized by Zabel (1955), who stated that
of six wells drilled on or near the crest of the dome,
only one, the MacIntosh 1 (pl. 1), produced hydrocar-
bons. Initial potential of the well was 500,000 cu ft

flammable gas per day. The gas from this well is used

as fuel by a dry-ice plant of the Colorado Carbonics
Co. in McEImo Canyon. Two other wells also supply
this plant with carbon dioxide gas. The A. H.
Schmidt 1 yields carbon dioxide from the Leadville
limestone and had an initial potential of 20,000 Mef
(thousand cubic feet) gas per day. The Dudley 1
yields from the same rocks and had an initial potential
of 7,000 Mct gas per day.

The Fulks 1 well, drilled in the NW 14 see. 27, T. 37 N.,
R. 17 W. (not in the mapped area), yielded some car-

LITERATURE CITED 69

bon dioxide from the Mississippian and Lower Cam-
brian rocks. Granite was reached at a depth of 8,637
feet, and the hole was abandoned at 8,787 feet.

The West 1 well (pl. 1) found strong shows of oil in
the upper part of the Hermosa Formation at an approxi-
mate depth of 3,300 feet.

OIL AND GAS POSSIBILITIES OF THE UTE MOUNTAINS AREA

The oil and gas possibilities of the Ute Mountains
area have recently been enhanced by the discovery of
several major oil fields in southeastern Utah, the largest
of which is the spectacular Aneth field. Production in
these fields is from two zones in the Hermosa Forma-
tion-designated the Desert Creek and the Bluff zones
by Herman and Barkell (1957). The reservoir at
Aneth, Utah, is described as a carbonate buildup in
the Desert Creek zone and is considered by some geol-
ogists to be a reef. The reef apparently has a north-
west trend that parallels the southwest flank of the
Paradox structural basin.

UTE DOME

The possibility has been discussed by Hunt (1942)
that the domes formed by laccolithic mountains may
be oil bearing. Hunt pointed out that oil geologists
have had little interest in laccolithic mountains because
the laccoliths have been generally interpreted as mush-
room-shaped bodies overlying undisturbed strata. The
knowledge that laccoliths were injected laterally from
stocks that occupy the centers of structural domes
greatly enhances the oil and gas possibilities.

Hunt's (1942) conclusions are believed to be applica-
ble to the Ute Mountains. The Ute dome could be
an excellent structural trap, for it has large closure
and contains formations that are productive in nearby
areas.

The paucity of oil and flammable gas in McElmo
dome and the abundance of carbon dioxide may be con-
sidered negative indications as to the possibilities of oil
and gas in the Ute dome. The origin of the carbon
dioxide gas in the McElmo structure is unknown. Car-
bon dioxide in other areas is thought to have originated
from the metamorphism of hydrocarbons by contact
with hot mineralized waters, or from the metamorphic
action of magmas and hot solutions intruding limestone
(Dobbin, 1935, p. 1068-1069). Whatever the origin,
the carbon dioxide in the McElmo dome apparently is
related to igneous activity either directly below
McEImo dome or in the Ute Mountains, and it seems
probable that carbon dioxide will also be found in car-
bonate-bearing rocks in parts of the Ute dome.
Whether this gas, if it exists at all, completely fills the

 

potential reservoir rocks in the structure can be deter-
mined only by drilling.

coal

The Dakota Sandstone in the Ute Mountains area
contains abundant thin beds of coal, but with few
exceptions these beds are extremely lenticular and have
little commercial value. One bed with reportedly large
reserves is exposed a few miles east of Cortez, Colo.

M. A. Pishel (written commun., 1911) studied the
coals in the Dakota of southwestern Colorado and
stated, "The coal is low-grade bituminous in character,
containing relatively large amounts of finely dissemi-
nated siliceous matter which in the analysis shows up
as ash." One of the several samples of coal from the
Dakota that Pishel had analyzed was from the McElmo
Canyon area. This analysis follows.

Analysis of coal sample from the Dakota Sandstone, SEV, sec.
3, T. 35 N., R. 18 W., McEimo Canyon area

[Analyst unknown, sample fairly fresh]

Loss of moisture on air-drying____________ percent.. 3. 20

Analysis of air-dried sample :
do.=.= 4. 69
¥Volatile - do.... 21. 68
Fixed - carbon .___... __ Leco l_. do.... 60. 62
E ae mie ien wld in d a take a mnd do... 13. 00
Ler ries do.... . 48
Calories: ssw... . LOL 6, 232
BEL 402 lc AREA c e - an allens wld in bs aie te we 11, 218

Coal beds more than 2 feet thick were observed in
several exposures during the geologic mapping, prin-
cipally in the Moqui SW quadrangle in T. 35 N., Rs. 18
and 19 W. Local ranchers have mined small amounts
of the coal in this area for use as fuel. A coal bed ap-
proximately 8 feet thick was observed in the Sentinel
Peak NE. quadrangle, about half a mile south of Senti-
nel Peak. This bed consists of a lower seam of good
coal about 4 feet thick, impure coal about 1 foot thick,
and two upper 1-foot seams separated by impure coal
and carbonaceous shale. The exposure is limited and
the extent of the bed could not be determined.

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70

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GEOLOGY, PETROLOGY, UTE

 

 

 

MOUNTAINS AREA, COLORADO

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leum Geologists Bull., v. 43, no. 1, p. 190-201.

1959b, Preliminary geologic map of the Cortez SW quad-

rangle, Montezuma County, Colorado: U.S. Geol. Survey Min-

eral Inv. Map MF-217.

1959c, Preliminary geologic map of the Mogqui SE quad-

rangle, Montezuma County, Colorado: U.S. Geol. Survey Min-

eral Inv. Map MF-221.

19594, Preliminary geologic map of the Sentinel Peak NE
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Emmons, S. F., Cross, W. C., and Eldridge, G. H., 1896, Geology
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Engel, C. G., 1959, Igneous rocks and constituent hornblendes
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Engelhardt, Wolf yon, 1936, Die Geochemie des Barium : Chemie
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Fenneman, N. M., 1931, Physiography of western United States :
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Gilluly, James, and Reeside, J. B., Jr., 1928, Sedimentary rocks
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Goddard, E. N., chm., and others, 1948, Rock-color chart: Wash-
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Goldman, M. I., and Spencer, A. C., 1941, Correlation of Cross'
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1937b, Geochemische Verteilungsgesetze der Elemente.

IX. Die Mengenverhiltnisse der Elemente und der Atom-

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Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry :

Chicago, Univ. Chicago Press, p. 558, 594.

1954, Geochemistry : London, Oxford Univ. Press, 730 p.

Goldschmidt, V. M., and Peters, C., 1932%¢, Zur Geochemie des
Bors: Nachr. Gesell. Wiss. Gottingen, Math-phys. Klasse,
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Gregory, H. B., 1917, Geology of the Navajo country-a recon-
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Gregory, H. E., 1938, The San Juan country, a geographic and
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Groves, A. W., 1951, Silicate analysis: London, George Allen
and Unwin, Ltd., 336 p.

 

 

 

 

 

 

F ma

LITERATURE CITED T1

Hallimond, A. F., 1943, On the graphical representation of the
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Harshbarger, J. W., Repenning, C. A., and Irwin, J. H., 1957,
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T4 p.

 

1958, Stratigraphy of the uppermost Triassic and the
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Herman, George, and Barkell, C. A., 1957, Pennsylvanian stra-
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Hevesy, G., and Hobbie, R., 1983, Die Ermittlung des Molybiin
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Kalervo, and Sahama, Th. G., 1950, Geochemistry: Chicago
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Hevesy, G., Hobbie, R., and Holmes, Arthur, 1931, Lead content
of rocks: Nature, v. 128, p. 1038-1040; or see table 41.4, in
Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry :
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Hevesy, G., and Wiirstlin, K., 1934, Die HéHufigkeit des Zirko-
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see table 21.3, in Rankama, Kalervo, and Sahama, Th. G.,
1950, Geochemistry : Chicago Univ. Press, p. 566.

Holmes, W. H., 1877, Report of the San Juan district, Colorado::
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Houser, F. N., and Ekren, E. B., 1959, Preliminary geologic map
of the Mogqui SW quadrangle, Montezuma County, Colorado:
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Hunt, C. B., 1942, New interpretation of some laccolithic moun-
tains and its possible bearing on structural traps for oil and
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1956, Cenozoic geology of the Colorado Plateau: U.S.

Geol. Survey Prof. Paper 279, 99 p.

1958, Structural and igneous geology of the La Sal Moun-
tains, Utah : U.S. Geol. Survey Prof. Paper 294-I, p. 305-360.

Hunt, C. B., Averitt, Paul, and Miller, R. L., 1953, Geology and
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Jobin, D. A., 1962, Transmissive character and distribution
of uranium deposits in the exposed sedimentary rocks of the
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Katich, P. J., Jr., 1951, Recent evidence for Lower Cretaceous
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Kelley, V. C., 1955, Tectonic map of the Colorado Plateau show-
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Larsen, E. S., and Berman, Harry, 1934, The microscopic deter-
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Lewis, G. E., Irwin, J. H., and Wilson, R. F., 1961, Age of the
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Luedke, R. G., and Shoemaker, E. M., 1952, Tectonic map of
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Lupton, C. T., 1914, Oil and gas near Green River, Grand County,
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Miesch, A. T., and Riley, L. B., 1961, Basic statistical measures
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Myers, A. T., Havens, R. G., and Dunton, P. J., 1961, A spectro-
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Noll, W. yon, 1934, Geochemie des Strontiums. Mit Bemerkungen
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Otto, Helmut, 1936, Die Rolle des Mangans in den Mineralien :
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Jour. Geology, v. 39, no. 1, p. 54-67.

Pike, W. S., Jr., 1947, Intertonguing marine and nonmarine
Upper Cretaceous deposits of New Mexico, Arizona, and
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Rankama, Kalervo, and Sabhama, Th. G., 1950, Geochemistry :
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1952, The beryllium content of igneous rocks: Geochim.
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Sandell, E. B., and Goldich, S. S., 1943, The rarer metallic con-
stituents of some American igneous rocks: Jour. Geology, v.
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hama, Th. G., 1950, Geochemistry: Chicago Univ. Press, p.
626, 697.

Shapiro, Leonard, and Brannock, W. W., 1956, Rapid analysis
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Shoemaker, E. M., 1956, Structural features of the central
Colorado Plateau and their relation to uranium deposits, in
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Contributions to the geology of uranium and thorium by the
United States Geological Survey and Atomic Energy Com-
mission for the United Nations International Conference
on Peaceful Uses of Atomic Energy; Geneva, Switzerland,
1955: U.S. Geol. Survey Prof. Paper 300, p. 155-170.

Shoemaker, E. M., and Newman, W. L., 1953, Ute Mountains,
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Bull., v. 55, p. 951-992.

 

T 2 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Stokes, W. L., 1952, Lower Cretaceous in Colorado Plateau:
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Stokes, W. L., and Phoenix, D. A., 1948, Geology of the Egnar-
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Strobell, J. D., Jr., 1956, Geology of the Carrizo Mountains area
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Sukheswala, R. N., and Poldervaart, Arie, 1958, Deccan basalts
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Thaden, R. E., and Santos, E. S., 1956, Grants, New Mexico,
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Thornton, C. P., and Tuttle, O. F., 1956, Applications of the
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p. 571-583.

 

U.S. Department of Agriculture, 1936, Climatic summary of the
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Wager, L. R., and Mitchell, R. L., 1943, Preliminary observa-
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Wahlstrom, E. E., 1947, Igneous minerals and rocks: New York,
John Wiley & Sons, Inc., 367 p.

Wanek, A. A., 1959, Geology and fuel resources of the Mesa
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U.S. Geol. Survey Bull. 1072-M, p. 667-721 [1960].

Weir, D. B., 1952, Geologic guides to prospecting for carnotite
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Wengerd, S. A., and Strickland, J. W., 1954, Pennsylvanian
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Wright, J. C., Shawe, D. R., and Lohman, S. W., 1962, Defini-
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Zabel, V. H., 1955, Geology of McElmo dome, Montezuma County,
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Geology of parts of Paradox, Black Mesa, and San Juan
Basins, 1955: p. 132-136.

x3

  

 

 

 

  

 

 

A Page
Apcessibility of Srea. . 2
Acknowledgments..............____.._.__:..}0 5
Agriculture.......... 4
Alkali-lime index.... 37
Aluvium............ 26
85
variation of minor elements............... 40
AIMEIUO-2-.. -. co oon eon eg wea 4
Alumina. . 37
con- cu cece rch eras 33
69
..o ec 48
35, 40, 42
samples from radioactive prospects. ._.... 63
31, 33, 58
-_... .on ecus cc oie cement 67
ADSMOG-.- -c rence noun 28, 30, 32, 35
Arsenic... 67
elo 28, 30, 31, 33, 35, 36, 45, 49
B
Banded 58
DATING. neocon c 66, 67
40, 67
Battle Rock 66
Berthold, 9. M.. shalyst......._.............. 48
..o cee o .ll cle cenececencrerckecees 40
Bilk Creek Sandstone Member, Wanakah
..- 8
See also Entrada Sandstone.
28, 30, 31, 32, 34, 35
Black Mountain, altitude. ..................- 4
domning................ 50
Black Mountain stock........................ 58
TUDDIG. J.C.. coll 220. 27
Blitff ccc 02 0. 12
See also Junction Creek Sandstone.
BOON. os bere 40
Breccia pipes, description................__... 59
hydrothermal hot-springs alteration. . ._.. 35
Brushy Basin Shale Member, Morrison For-
.._ eon ce 13, 15
Burro Canyon Formation, comparison w1th
Dakota Sandstone................ 22
(CeSCHIDHOTM .E; LOL. 18
(Buttes 59
BySMSUENE.: 2. .Q. oll coven ee caine bens 55
C
ELA Casein mbar sass 31, 32
Camp, C. L., quoted - 8
(Carbon rss. 68
rere cect as 65
Carrizo Mountains, alkali-lime index...... 38
Chemical composition, igneous rocks.._...... 35
CHIOKIEG. ... c.. .on cloe 28, 30, 31, 32, 36
cell ccond 40, 42, 44, 49, 67
Cliff House No. 2 prospect......______________ 63
No. DFORDECL.21 2. c-. C: ln ose cece eine 63
Clift House 62
Cliff House Sandstone... 25
Cliff houses, Entrada Sandstone, Sand Creek
Arter 10

 

INDEX

[Italic page numbers indicate major references]

 

 

 

 

  
  
 
 
 
 
 
 
 
 
 

  

 
 
  
 

Page
LEL annees 4
COAL: =...... cans 69
Cobalt.:....... 38, 40, 42, 44, 67
Coffin'$ sc- ee ce 1901s 66
40, 42, 44, 66, 67
Cretaceous System.. 18
. . C_. .o lo. 4
D
Dakota Sandstone... ..o 220 20, 48
Dense, defined . ., 1.s on aril 28
Douteric AlferatIoh. : 35
Dewey Bridge Member, Entrada Sandstone... 8
.. .. .c. cone 59
Diopside . ..... 35
DiQrite LIC ec. es 28
Disconformity, top of the Burro Canyon
19
E
East Horse 56
East Sundance laccolith......._.............. 56
Economic 61
Engel, C. G., quoted.. 49
..I. 1. ie ce idence nn anes 35
Entrada Sandstone.cc..._-_c:ccll...ccc...... 8
. 12220. .on ies enensuenee 32, 35
F
FAREIOMOTREE . . L Coool ee Ee cn ags 26
1. curate bus iv ab 50, 52
.... .» cone eer sna 35
Fieldwork... 5
Flat laccolith 67
Folds....... 50
Forsterite. 35
Fossils... 8, 28, 24
G
Gallium... care 40
Geography. . 2
Geologic history. 60
Geologic setting. .... 5
Glen Canyon Group.. 7
Granodiorite porphyry .. 30
Gravel, terrace. . 26
Great Bago PIGIN. . cbc seres 5
Greenhorn Limestone Member, Mancos Shale. 24
Gryphaea........ 28, 24
Gryphaea newberryi. . 24
GyDPSUNLLL. . . c nos ou eel NLI e ne 65
II
Hazell K. V., 48
Hematite_..... 35
Henry Mountains, 51
hornblendic inclusions 47
Hermano Peaks, altitude.. 4

Hormblende......._.......- 28, 30, 31, 34, 35, 36 45, 58

 

 
 

Hornblende inclusions, origin .... 45
Horse Mountain laccolith . * 56
Hunt, C. B., quofed...:.-...-.-.-..._... : 47

Hydrothermal alteration, Mable Mountain... _ 35

 

  
   
  
 
 
 
  
   

 

 

 

 

 

 

 

 

  

Page

Hydrothermal hot-springs alteration, breccia

PIPES. .- 35
I

Igneous activity, age.....-.....--- 27

Igneous rocks, chemical composition 35
CeSCHIPMON- 21 27
inclusions... 33
occurrence 28
Origin ro +o ents 45
relation of the metallic mineral deposits“. 67

Igneous rock series, variation........._.. 38

33

Inoceramus deformis ..... -_ 24, 25
dimidius. ..... 24
111 cine ene 24
ccie ille levees 24

Intrusive bodies...... 52

Intrusive rocks, relative ages...... 27

-- 87, 67

Irwin .. ores 56

J

Juana Lopez Member, Mancos Shale...._.... 24

Junction Creek Sandstone...___...._____..--. 12, 43

Jurassic rocks.....__.... 7

Jurassic Bystem..1. .-.... ddl. 8

K

. ... 32

Karls Kay Mino... ...,... 18, 64

Kiva section .. 21

KNnCES BLOCK.. o.. ie cli 58

L

LAbIAUOFINe: : .- -cell as 45

LAcColISh$ . ore edi eer e eus 56

Lamprophyre.. . 32

Landslide CeposILS_..... .L L0... 26

\ .... cs 200000000000. i+ 40

La Plata Mountains, alkali-lime index 38

La Sal Mountains, igneous rocks...........~- 47

Last Spring 67

LeMAL 12

Leucocratic diorite porphyry...-_.._....__..- 30

LifHe. Ito ore one cennet cb d eus edaees

1.1 deena ene.

Lithology, Burro Canyon Formation......... 18
Dakots 20
Entrada Sandstone ...-- 8
Junction Creek Sandstone...........----. 12
Mancos Shale... eds 23
Morrison Formation 13
Navaio 7
Summerville Formation........._........ 10

Little Matide mine. -.-.. _.... 66

M

Mable Mountain, description. ...............- 55
hydrothermal alteration.................- 35

Mable Mountain 40

73

T4

  
 

 

   
 
  

 

 

Page
McElmo Canyon, lower section....._......_.. 7,16
Apper Sechon. ssc cooler ccc ri ici 10, 11
McElmo dome, description............_...... 50, 61
ere other einen er 69
.c. Ul ene s en eaee sevies 37
MABNOBIUMILS: -se TEO. IU ece ee s 36
Magnetite. . A 28, 30, 31, 32, 35
Shalo.2. ILl In Losa 23, 44
CY ee s ades 38, 40, 67
se cece nece: c 5
MBFORSINGI:Y 222: Eee ores eos 67
Menefee Formation....._._...._________...... 25
Mesaverde Group.. 25
Metallic mineral deposits. ............._._.... 61
Metamorphism, Black Mountain stock........ 58
The 54
Ute PORK . £ 22.0.0 Oc 54
1 22s Eo ense abed 28, 47
Minor 38
Modes, diorite porphyry......_.__._.__._____. 30
granodiorite porphyry......______._____.. 31
AnClusiOn$ s seuss dee CIL IC 34
quartz monzonite porphyry......_.______ 33
spessariIte ser .. LIL ACL. 33
IIE 67
Morrison 13
Mound Section. cc 23
Mushroom laccolith.......__..___.__.___.__._\ 68
N
ITCC IEO be enn bs 68
Navajo 7
Newman, W. L., quoted......_.:..:.__..__}.. 66
Nickels 2202000 eR 38, 40, 42, 44, 49, 67
Niobiim2. 420.000. IU ILL L.- 40, 67
Nonmetallic mineral deposits 68
Normal diorite porphyry.........___________._ 30
Norms, igneous 36
North Black Mountain Laceolith.... 59
North Ute Peak laccolith.................... 56
0
\t 30 - Coxeter aan C00 Fo care pode aie pave 1. AF AL Sve 35, 36
Origin, igneous rocks and hornblende
inclustong 2.2 00s. 45
Ostren 0000000000000. nene vien 24, 25
P
Pack Trail faccolith....._....._...0.........l 58
Foradox 51
Petrography, igneous rocks_......_......._... 28
Cen sis oo 68
PetrOIO@Y . ... nel lU acs divers s 85
Pishel, M: A.. ...l... 69
28, 30, 31, 32, 35, 58
Point Lookout Sandstone.....__....._..._... 25, 26
Fotasstum 35

 

 

 

 

 
  

   
   
 

 

  

 

INDEX
Page
7
Previous work..... :...... 2. IE- 5
Prinocyclus .. 24
Plychodue Whipplet. . . 2. (coule lud Ane ues device 24
Purpose and scope of investigation.._..._._._. 5
Pyrite... l osi. o! i 31, 35, 67
Pyritization, igneous contacts......_.___._... 35
Mable 67
65
P TOON... .. CLC... ... .! Liles poops dena 36, 49
Q
2.00. 28, 31, 32, 35
Quartz monzonite porphyry..._....___.___._. 31
Quaternary System.. 26
R
Rainfall. -. ..1 1... coo OOC bars ees belew 4
Rasorback 28, 57
Recapture Shale Member, Morrison Forma-
Afon :. 2. coco 20020 conman wea 13,14
Fock cn... ccs 22. ..I Dib eos nisi 26
8
Salt Wash Sandstone Member, Morrison For-
THAHON -.. 75s ci ns 13,14
Sand Creek, 9, 11, 12
Sandiding ... ../... . .... ooc lu Praca ees 31
San Rafael Group: :.... .;}. 2000000000 8
Seanditim. ... 22... 40
Sedimentary rocks, stratigraphy.............. 6
variations in adjacent intrusives.......... 48
halli-. .. . .- 122000. Cer ces .8
Sentinel Peak....:>...1! . 55
Sentinel Peak 50
Sericite ... {10.0000 o LUCIA OED nn 32, 35
Sills s; 2 22211 roo Hna ana oen Sea 55, 58, 59
SHIV . ccie 67
Slick Rock Member, Entrada Sandstone. .... 8
©posgartibesc. 11. 20.000 32
pene? 2:11. 0: devalue ne enas or aoe 30
LOOKS LY Neo ec evie cece nene 58
Stratigraphic section, Burro Canyon Forma-
MON: c.. -c 200 - 18
Dakota Sandstone......_...._.....__..... 20
Entrada Sandstone.......... ot 9
Junction Creek Sandstone................ 12
Mancos Shale. 23
Morrison Formation < 16
Navajo 7
Summerville Formation.................. 11
Stratigraphy, sedimentary rocks. 6
.. .oo -se ase 40, 44, 67
ci-. . oul nol Gored 50
Summerville Formation......_..._..__.._.... 10

C

 

 

 

 

 

 

 

Page
Sundance cluster of laccoliths................. 56
Sundance laccolith.........__.._.........0U.l 56
T
TABU. Vs cire eel cool ne knees 26
Temperature. ..... 4
Three Forks laccolith..........___..._...__... 67
Three States Natural Gas Co. prospect.... 63
# 33
EAU 1011 20000 COL OETA SLL IIE 38, 40
TTongne Iacoolifh.s..-crcullcl ln 2.00 58
Topography... .. # 4
'Towaboe Home. 2..- 2221000000. LET s 50
Tozer Gulch, 19
Trail Canyon, section . 17
Trapdoor 56
Trigssic Fooks. ..... LLC et onyl. 7
U
.. 0.000. oe L 42, 61, 62, 67
Uranium potential 67
Ufe Creek dike -/ .. LLL. prec Lc l? 55
Ute Creek dike prospects........_____._._.__._. 66
Ute dome, description . . 50
ced 69
relation to McElmo dome................ 51
Ute Peak, altitude........... 4
description. .. ">. I0... . 02 oer cae 54
v
00.00 40, 44, 61, 67
Variation 37
I rar ean 4
w

Wells, oil and gas . ......
West Toe, bysmalith ..

 
  

 
 
 
 

 

West Toe cluster of intrusive bodies........ .. 58
Westwater Canyon Sandstone Member,
Morrison Formation.............. 13,15
Windblown silt and sand . .... 26
Wollastonite............. - 35, 36
Wood Chuck section .. 18
X
'Xenolithis....1...._....... 33
¥
LL 40, 44, 67
0.0000 40, 44, 67
Yucca cluster of laccoliths. T. - 69
-.... L_ ECEC tes cnce 20

 

 

-i-

 

The U.S. Geological Survey Library has cataloged this publication as follows :
Ekren, Einar Bartlett, 1923- 7
Geology and petrology of the Ute Mountains area, Colo-

rado, by E. B. Ekren and F. N. Houser. Washington, U.S.

Govt. Print. Off., 1965.

v, 74 p. illus., maps (1 fold. col. in pocket) 30 em. (U.S. Geological
Survey. Professional paper 481)
Prepared on behalf of the U.S. Atomic Energy Commission.
Bibliography : p. 69-72.
(Continued on next card)

Ekren, Einar Bartlett, 1923-

Geology and petrology of the Ute Mountains area, Colo-
rado. - 1965. (Card 2)

1. Geology-Colorado-Montezuma Co. 2. Petrology-Colorado-
Montezuma Co. 3. Rocks-Analysis 4. Mines and mineral re-
sources-Colorado-Montezuma Co. I. Houser, Frederick Northrop,
1924- Joint author. II. Title. III. Title: Ute Mountains area,
Colorado. (Series)

 

> UNITED STATES DEPARTMENT OF THE INTERIOR PROFESSIONAL PAPER 481

    

  
  

   

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

Entrada Sandstone
TKqm

 

0
O
GEOLOGICAL SURVEY PLATE 1
108°37 30"
s isin 5730" (Mogul SE 1:24 000) _ R.19 W. 55" 5230" 50' ~R. 1s w. Jmb 108 a ee,
tes ~, aps p" pap -all ve | fig?” 2230 EXPLANATION
Qa
Alluvium Point Lookout Sandstone
Qt S
Talus €
{ 33 w
T. 36 N, 3 7
&,
~€ 2" £ Mancos Shale >I(-I)J
'% m : Kmsj, sandstone unit and Juana Lopez Member
eb s jL‘ = Rock glacier ff
z Lu
3 ys T
3 Bage 9! s" a* . }
fl; DV VAVDVAQA Z o s
Z : Dakota Sandstone
A Landslide deposits
2 UNCconFoRrmiTy
&. moo y %
2 “5.1, A °e°§~n x ¥ §
=, ics < F
Fo les ®, Z E
& \i S . ,
8 Block rubble u l ) a
20 20" B3 + 5 Burro Canyon Formation
é]; Ofy g § Kbk, Karla Kay Conglomerate Member
A s?
r
e:) Qfo
£
i '_Fanglomerate
_38 Qfy, younger
2 Qfo, older
0
on
bas!
§ s .
3 * « Morrison Formation
C
~ feel Jmb, Brushy Basin Shale Member.
E71) J s Jmw, Westwater Canyon Sandstone Member.
= 0 ® Jmr, Recapture Shale Member. O
& £ é Jms, Salt Wash Sandstone Member (a
- J S
g Terrace gravel 2 é
S § >
s"
Qs i o
Junction Creek Sandstone
> \ p we \ ( ext is Aag \ ) F ~ : \ Sap 7 f , Eolian sand and silt J
P $ c 777777 2% ‘ Se A ¥ \ y ) ) i( I. < \ w \ > W \ '_ ‘ } A Af < s ( f/ a VA 3 v‘ NM ¥ } 4 < {1 l A l ' 23 z wont ‘rl 2 3 4 "4 5370. - .‘ C > ' 5 7 yo P e : = c mmm 7777‘7177, UNCONFORMITY ;
* S S m C N S foo C § f > aC Wig. s ~A i | ~ UPJ X v» a ; | a ) SAN re. , ‘7 amg 2. fe > fs. # , al /. e ( Summerville Formation
~ 7 T A f ao 1; -F I A 5 oro -h y- , SO o if f x/ f , K < - ‘ Z 2%% sigs 4 /A a ; g. % 'le ' Tk 7" Tks
17'30" P $f A- 1 R t . wally ezt ite : ( { ) f ' ( _ - Ar ; C " i otes, ~ SY f “ 22 t> _ x a aste 1 1730"
| _-_) _ 5s (ks S t € WN < m, = R f vad. R fl \ R # % @ f {7 'A. > =i) & Ay A a f Cx [ |T. 35 N. Spessartite lamprophyre
$ p "S&S G U r & : ye | 4 x f « ‘ peo i -- g ( C j
Sy] -\ f 3
5 TB Al q i ( 7

 

Quartz monzonite porphyry

TKg
Dike

  

Navajo Sandstone

 

TRIASSIC (?)
AND
JURASSIC

 

 

 

 

 

 

Granodiorite porphyry Contig—
Dashed where approximately located; dotted where
concealed
TKd
-

 

Thrust fault
T, upper plate

Diorite porphyry

v
CRETACEOUS OR TERTIARY

I nant ances svn tuin 11s 5) an a's sig
4 _-y-p -__-"
p 4 f High-angle fault, showing dip
Microgabbro Dashed where approximately located; dotted where

concealed; queried where extension doubtful; U, up-
thrown side; D, downthrown side

##
rae

 

15 Shear zone
Closely spaced vertical joints or faults showing little

Geology mapped by E. B. Ekren and F. N. Houser, 1955-56 displacement of contiguous rock masses

 

Anticline
Showing trace of axial plane and direction of plunge.
Dashed where inferred

Syncline
Showing trace of axial plane and direction of plunge.
Dashed where inferred

' Z 2) R ( : g- ~ AE T h &A \ K \ Strike and dip of beds

50
-/-

Strike and dip of overturned beds

 

45
_s

Strike and dip of cleavage

 

 

60
om...

Strike and dip of planar flow structure

 

Bearing and plunge of lineation

©

Vertical lineation

 

a-

Horizontal lineation

 

 

A The {fn A' Structure contours
G Knees uln'] $000' Drawn on base of Mancos Shale. Dashed where
9000 m approximately located; short dashed where projected

above surface or projected down through a sill or

 

 

 

  
 

 

 

 

 

 

 

   

 

 

       

 

 

 

 

 

   
 

   
 

 

 

 

 

   

 

 

 

 

             

  

      
 

     
   

 

 

    

 

  
 

 
  

 
  

8000' 8000' laccolith; queries indicate very poor control. Most
f "e> hig Poco of the laccoliths were intruded at or near the
# 70004 P ; ( r te af-... 3 contact of the Dakota Sandstone and Mancos Shale,
e pos 6 as j e f g ; — " oes, Eg p and, locally, portions of upper beds of the Dakota
r j s [ ‘ im. OOC A » »
6000 ir / ml ~u~--\\jfi\\“:‘: Asli 6000 were domed upward by the intrusive rocks. - The
' ¢ ' y sms oe. —~——~~—T:__ KJbm ___ . structural datum is projected through the laccoliths
k : 5000 Jj Pre-Junction Creek Sandstone /Pre-maofifik e pmo, 5000 in order to define the underlying structure. Lack of
I 4. , wo > ra f Ay t \ 4 won NL p (p 17 5 (5 R fz f 2 R N y , \ ' stratigraphic markers within the central area
T:33v: N > e " GUP hl os .. ) } _o 4h | A 2 ¢ fl" FL-. N- Soft f y ~ ~ ' | A \ ) z | enclosed by the 7000-foot contour precludes further
§ ¢ * interpretation at the contoured horizon. Contour
interval 100 feet. Datum is mean sea level
Location of measured section
B s a
g al B Adit
10,000 4 flo Ute Peak 10,000"
= TK &
5 Horse & A
j orse| Mountain f
9000 if TKd a. 9000 Prospect
(els § ;
agoo Sord Cu, copper; U, uranium
s f \ A
C g M \ 2009. Gravel pit
6000' r | 6000 QU
Pre-Junction Creek Sandstone ‘ i
5000" | 5000' Mine
U, uranium
fos
Dry hole
@
&l Dry hole having show of oil
C 6“? C' yf
10,000 _ u | ® 10,000"
Black Mountain U | s Gas well
Horse Mountain
9000 Km ( Ot 5d EAST HORSE 9000" all
LACCOLITH Sprlng
t- 33 N 8000 ix; kms _ -_ _ ) - gone ( yt / TC 00. 8000'
7000' 7000' Unit symbols shown only on sections: >
Jom ___ BS _ _- TKqm t KJbm, Burro Canyon Formation and Brushy Basin
6000 ( % 6000' Member of Morrison Formation.
& . Jml, Westwater Canyon Sandstone, Recapture Shale,
37°0740" Xenu N ure _ Smit An an. J M Oj) ° f OOO 1T 0 O Tooak\ (_ {{f N1 ___ MF 1. \ W PN 730" 5000' Pre-Junction Creek Sandstone \ £ \ | fre junction Creek Sandstone 5000' and Salt Wash Members of Morrison Formation
5 109000! R’ZOW 57’30” a 52,30" )O' 4 j 47'30' % INTERIOR-GEOLOGICAL SURVEY, WASHINGTON, D.C.-1965-G64084
e ac o. ow

 

Base from U.S. 'Geological Survey topographic quadrangles .

#.

GEOLOGIC MAP AND SECTIONS OF THE UTE MOUNTAINS AREA, COLORADO

SCALE 1:48 000

TRUE NORTH

 

APROXIMATE MEAN
DCLINATION, 1965 1 Ya 0 1 + 2 3 4 5 MILES
- {~-

 

 

 

 

 

 

e e p e 1 6 0 1 2 3 4 5 KILOMETERS
% ; ? "* %, ECBO ORO " ree
2 ‘ CONTOUR INTERVAL 40 FEET

% DATUM IS MEAN SEA LEVEL

 

 

 

 

 

 

UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY

[See table 5 for code.

TABLE 14.-Semiquantitative spectrographic and radiometric analyses of samples from mineral deposits in the Ute Mountains area, Montezuma County, Colo
; ,

PROFESSIONAL PAPER 481
TABLE 14

hi R. G. Havens; 2 G, W. Boyes, Jr.; 3, N. M. Conklin; 4, P. R. Barnett; radiometric: 1, C. £ tes f
1, C. G. Angelo; 2, J. W. Patton: 3, D. L. Schafer, H. H Lipp, J. E. Wi S: p p y Pp » « UCs a roc e same f ation. T., trace; , not detected; (*)

 

 

 

 

 

 

  

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

s Sample No. Semiquantitative spectrographic analyses
*. Prospect Rock type Analyst Radiometric analyses
Field abora- ; ema
Tory si A1 Fe Mg Ca Na K Ti Mn Ag As Ba B B ~ K r agen 1 __
e Co Or Cu Ga La Li Mo Nb Ni Pb Se Sr v Yb Zn Zr Sb 8 Analyst | eUsOs
u
Uranium prospects, fault (percent)
controlled "%
55-E-26B 238257 Cliff House:. :... .... .--24ey Leached limestone....--.----- 1 X+ x Xx 0. X- px - o a
EMS-9-53 D-88145 |--... cc lc oot Navajo Sandstoné.._.......--- 2 | XX X X- s .K- .O0X+
EMS-8-53 D-88144 | Cliff House No. 2...-------- Sandstone in the Summerville. 2 | XX * AX+ .OX+ . OX . OOX+ 1 0.002
EMS-5-53 D-88140 | Cliff House No. 4 d 2 | XX & X+ .OX+ | OK- a "994
EMS-5A-53 | D-88141 2 | XX mat X+ .OX+ . OX .OX- 2 . 002
EMS-6-53 D-88142 2 | XX & . X+ . OX . OX . O0X 2 . 082
EMS-7-53 D-88143 2 | XX I% Xt .OX+ .OX . X- 2 .018
55-E-5 220457 3 | XX X+ .OX- .OX .O0X- 4 . 004
55-E-6 220458 3 | XX X+ .OX+ . OX .OX 000 X+ 2 . 003
55-E-7 229459 3 | XX X+ . X- .OX- .OX- 000X+ 3 . 049
3 :
Uranium prospects, bedded 3 35?
55-E-8 229460 Coffin's Prospect.... eer ioo. cian 3 | XX X- .OX+ . X- x- .OX+ | x- .OX .0X-
55-E-9 220461 o.. 1. nein Picea oe aind (671.2. . e eval count 3 | XX X . X- . X- . X+ .OX+ | x- AX+ | ~.0X-
EMS-15-53 D-88151 | Karla Kay mine..__.___.-.- Conglomerate of Early Creta- 2 | XX X+ X . X+ X+ «X X- X .OX+ 3 . 004
ceous age. 3 *
EMS-16-53 D-8§159 |: 2 onit ols .n Mier . inion. 2 | XX a+ X- X+ K K .X+ | .X- .OX+ 2 " $3)?
Copper prospects 2 . 064
EMB-10-58 |_____.____ Battle Rock mine........... Junction Creek Sandstone oxi- 2 | XX X- X PX X- JK Tr. | .OX "0x - 000 X
Vdized. | 7] € p < x *."" aii are e g 00 X- | X- === ---- x- ox-
/. gin ¢ X Tr | . OX . X- 000 X- |___... 4 ¥
251976 Barite with iron oxides.... 3 X - . X+ . 005 .OX .OX+ :OX- |~.00Xx+ LEI. X>Jé """""""""" 00 xX 000 X+ RK) o [Ae ---- x4 oX as
D-88148 Altered diorite porphyry. ._ _- 2| xx | xx x .OX+ .0x+ | | _.X+ | .X tox- "y l |a." oxy esas f aim ftl ' "ogy | *f <. 001
D-88149 Mineralized diorite porphyry .- 2 | XX XX X .X . X+ .X+ | X- .X * AB ea s aE. xe [aR ( C XZ - “x4— MX+ 0 z OX OK- 9
D-88150 Barren diorite porphyry ._.... - 2 | XX XX *+ X X- X- X- .< oke Ao toto tane | 28 -A.. © O0X- 00X 3 - |. --- :0%=- .O0X+ <. 001
251983 Vein sulfides in Mancos Shale. 3 | XX X X+ .X- | XX X+ Js.." ox- |- Hook Q.. tric. lox {ass As 000K +- oe o thp ag .OX ; . 001
Barren sandstone #4 S yas (*) #
D-80397 | Summerville Draw, Utah._.| Conglomerate of Early Creta- 4 | XX X+ . X .< ho 4. : Beds Milam . X- §0XK- A2 . 2 eac s
oils pn it f a aat aa a a ee en A te Rh Yona tot . an 2s OX 00X- |...... ©000XK- | 000K | © .00X | im rels sak. s As. |. Sis oct r els". > an X ox
244746 _ | Southern Ute Mountains...! Junction Creek Sandstone... 3 | XX X- -X 2.0 .X OX+ | X- QE |. ;O% "slc sede eles OX Tr 000%. $% <. 001
D-81690 | McElmo Canyon _.___.-..- Summerville Formation.....--|.-.----.- XX Xx -X «K+ X X- | x- . OX PORK "* : Be OL reg au. it 22 box. 2X2. I5 000% -| [00% + | \- 000 X+ TF. 'Is. 000X+ | .oox-
D-81736 | Lower McElmo Canyon..__| Entrada Sandstone... @ i | XX . X4 x x+ .X+ ox cxf 1 fox _| ax wel. Es s 'ox" | coo%=_ |:. 2 |" fhox +00X- > :OX- <. 001
D-81707 | Upper McElmo Canyon....| Navajo Sandstone........-.--- 4 [ XX X+ .OX+ .OX X L . OX DECT AL ail rege .OX (0) Ke- HL z. 000% %§i 00X+ <. 001
___________ f a 00X- . 002
. 001

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

1 Sample of the Buckhorn Conglomerate of Stokes (1944). This unit is very similar to the Karla Kay Conglomerate Member.

745-807 O-65 (In pocket)

 

 

 

Physiographic Divisions
of Alaska

By CLYDE WAHRHAFTIG

 

GEOLOGICAL -SURVEY PROFESSIONAL PAPER . 482

A classification and brief description with a
discussion of high-latitude physiographic

processes

 

 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965

UNITED STATES DEPARTMENT OF THE INTERIOR
STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY

Thomas B. Nolan, Director

Library of Congress catalog-card No. GS 65-305

 

For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402

CONTENTS

introduction. . ... ates ene -an.
Acknowledgments... ons ain west sa
The basis for the clasgification............_.....___._.._
Summary of the geologic history of Alaska_____________
Details of sculpture-physiographic evolution in the last
million Years. awe n
Glaciated highlands and mountains. _____________.
Glaciated
Ungslaciated
Unglaciated lowlands.._............_...........
Description of the physiographic divisions. __._____.____._
Interior uce cae aos nn
Arctic Coastal

Rocky Mountain
Arctic

Arctic Mountains province._.__.___._________

De Long Mountaing............._....._..

Noatak Lowlands............t.._L.....L

Daird

Central and eastern Brooks
Ambler-Chandalar Ridge and Lowland sec-

tION..: ctl obs

intermontane
Northern Plateaus province.
Porcupine

Ogiivie

Tinting Valley...... ..... .c gere.
Yukon-Tanana
Northway-Tanacross Lowland___________.

Yukon Flats

Rampart Trough........s..............
Kokrine-Hodzana Highlands

Western Alaska province.

Kanuti Flats..
Tozitna-Melozitna Lowland_____________.

Indian River Upland....__......._:__°.._

rah River section......-..........2..cc...

Koyukuk
Kobuk-Selawik Lowland________________._

Selawik l:...

Buckland River

Nulato Hills....
Tanana-Kuskokwim Lowland._..---.-..._.

Nowitna Lowland..................l....

 

Description of the physiographic divisions-Continued
Intermontane Plateaus-Continued
Western Alaska province-Continued
Kuskokwim
Innoko
Nushagak-Big River Hills.
Holitna Lowland....s...................
Nushagak-Bristol Bay Lowland.-.-..-..----
Pering Shelf. LL .._
Yukon-Kuskokwim Coastal Lowland...
Bering
Pacific Mountain
Alaska-Aleutian province._...._..___..._.....
Aleutian Islandsg..-..--....~._-...._..._._.
AMeutian
Alaska Range (southern part) ______---__-
Alaska Range (central and eastern part)...
Northern Foothills of the Alaska Range...
Coastal Trough province..................._.
Cook Inlet-Susitna
Broad Pass Depression.
Talkeetna Mountains. ....___._.._____._._
Upper Matanuska Valley.-_.____________.
Clearwater Mountaing....._..._.._...._.._.
CGulkana Upland..........:_..........-.
Copper River Lowland._........._.._._.
Wrangell! Mountains.. _....._......._.._..
Chatham Trough.......................
Xupreanof Lowland......................
Pacific Border Ranges province.______.___.._..
Kodiak Mountaing......................
Kenai-Chugach Mountains._...____..___..
St. Elias Moulntains.............._......
Fairweather Range. ........_...__.
Gulf of Alaska coastal
Chilkat-Baranof Mountains--._________..
Prince of Wales Mountains.
Coast
Boundary Ranges............_.._.._....
Coastal Foothills............._.........
Referehtes CWE. s 2-1. e eZ Leen cele cena ne can

HI

Page

IV CONTENTS

ILLUSTRATIONS

[Plates are in pocket]

Prate 1. Maps of physiographic divisions of Alaska.

2. Photographs illustrating minor physiographic features and microrelief in Alaska.

3. Aerial photographs illustrating the physiography of the Rocky Mountain System and Intermontane Plateaus in
Alaska.

4. Topographic maps and aerial photographs illustrating the physiography of the Interior Plains, Rocky Mountain
System, and Intermontane Plateaus in Alaska.

5. Topographic maps illustrating the physiography of the Pacific Mountain System in Alaska.

6. Aerial photographs illustrating the physiography of the Pacific Mountain System in Alaska.

FiaurEs 1-6. Map of Alaska showing- Page
1; Location of 1: 250,000 topographic GID .L 4
2./ Mean daily minimum temperatures in snene 9
$. Mean daily maximum temperatures in 200. 10
4. Average annual ssn ns bgs ~ ws 11
5. Extent of existing glaciers, Pleistocene glaciers, and permafrost regions______L____________________ 12
6. Drainage basing of major cn oes 19
TABLE
Page

TaBL® 1. The geologic Hime Scale...... -.. an amass 6

PHYSIOGRAPHIC DIVISIONS OF ALASKA

By Waurnarttc

ABSTRACT

Alaska occupies the great northwestern peninsula of North
America, which slopes and. drains westward to the Bering and
Chukchi Seas. Most of the State is mountainous or hilly al-
though plains 20-100 miles wide abound. The central part,
which slopes westward, consists of interspersed plains, plateaus,
and rounded mountains, extending from beyond the Canadian
border to the west coast; this part is bordered on the north
and south by high rugged ranges which effectively cut off the
bulk of the peninsula from the Arctic and Pacific Oceans. The
northern range is the Arctic Mountains province of the Rocky
Mountain System, dominated by the Brooks Range whose sum-
mit altitudes are 6,000-8,000 feet. North of this province are
the Arctic Foothills, also part of the Rocky Mountain System,
and the Arctic Coastal Plain, the northwestern extension of
the Interior Plains. The southern mountain barrier, part of the
Pacific Mountain System, is a pair of ridges separated by a
line of discontinuous depressions, the Coastal Trough province.
The northern ridge of the pair is the Alaska-Aleutian province;
and the southern, the Pacific Border Ranges province. The
highest peaks of North America, rising to an altitude of more
than 20,000 feet, are here; and mountains 8,000-12,000 feet in
altitude are common.

The part between the Arctic Mountains and the Pacific
Mountain System is a disordered assemblage of flat plains and
rolling uplands, surmounted here and there by groups of low
mountains; the whole region declines in relief and altitude
westward. It is divided from east to west into five provinces:
the Northern Plateaus province, having uplands at altitudes of
3,000-5,000 feet, formed on Paleozoic and crystalline rocks;
the Western Alaska province, having uplands at altitudes of
1,500-3,000 feet, formed on late Mesozoic sedimentary and vol-
canic rocks; Seward Peninsula and the Abklun Mountains,
two distinctive relatively mountainous provinces; and the Ber-
ing Shelf, a nearly featureless plain, which is mostly sub-
merged but has mountainous islands. Southeastern Alaska
includes parts of the Coast Mountains, Coastal Trough, and
Pacific Border Ranges; all are provinces of the Pacific Moun-
tain System.

By far the largest river is the Yukon, which, together with
the Kuskokwim and other rivers that flow to the Bering and
Chukchi Seas, drains all the Intermontane Plateaus and parts
of the two mountain systems. The Bering-Chukchi-Arctic
Ocean Divide is in the Arctic Mountains province; and the
Bering Sea-Pacific Ocean Divide is partly on the crest of the
Alaska-Aleutian province but for long stretches is in the low-
lands of the Coastal Trough where in places it follows eskers.

Each of the provinces is divided for purposes of description
into sections, 60 in all; and some of the sections are broken
into subsections. The general topography, drainage, lakes,

glaciers, permafrost conditions, and geology of each section are
briefly described.

The Rocky Mountain System, Intermontane Plateaus, and
Pacific Mountain System together constitute the North Ameri-
can Cordillera, one of the major physiographic features of the
continent. Throughout most of its history the North American
Cordillera has been the site of geosynclinal sedimentation,
dominantly miogeosynclinal with interbedded carbonate and
well-sorted clastic rocks in the Rocky Mountain System and
northern part of the Intermontane Plateaus and dominantly
eugeosynclinal with interbedded volcanic and poorly sorted
clastic rocks in the Pacific Mountain System and southern part
of the Intermontane Plateaus. Orogenic activity, accompanied
by the invasion of immense granitic batholiths in the Pacific
Mountain System and Intermontane Plateaus, has affected the
North American Cordillera almost continuously since early
Jurassic time. Near the end of Cretaceous time, orogenic ac-
tivity reached its climax; and most of Alaska was converted to
dry land, which has remained ever since. In Cenozoic time
Alaska has been subjected to faulting, warping, and local fold-
ing. These processes formed highlands, whose erosion pro-
duced large quantities of poorly consolidated sediments, and
basins, in which these sediments were deposited and are now
preserved. Deformation continues and is particularly strong
along the Pacific Coast. Active volcanoes are in the Aleutian
Islands, Alaska Peninsula, and Wrangell Mountains.

Alaska has bedrock structure of great variety and complex-
ity, which is reflected in the number and types of its moun-
tainlands, uplands, and lowlands. Structural trends are pre-
dominantly parallel to the Pacific Coast; they swing from
northeastward in the southwestern part to eastward in central
and northern Alaska and southeastward in southeastern
Alaska.

For the last 2 or 3 million years, frost climates have pre-
vailed in Alaska, and the geomorphic processes have been pre-
dominantly either glacial or periglacial. Most of Alaska north
of the Pacific coastal belt is underlain by permafrost. The
firn line today ranges from 3,000 feet on the south coast to
6,000 feet on the north coast and is 8,000 feet in the eastern
interior; during the glacial stages of the Pleistocene, it prob-
ably averaged 1,000-2,000 feet lower; thus, the Pacific Moun-
tain System, which even today has extensive glaciers and
mountain icecaps, was covered almost completely by the vast
cordilleran ice sheet. The Arctic Mountains province, Ahklun
Mountains, and southern Seward Peninsula were also intensely
glaciated, whereas, in contrast, most of the Intermontane
Plateaus, Arctic Foothills, and Arctic Coastal Plain were never
glaciated.

Glaciated uplands that were buried by icecaps were eroded
into blocklike groups of mountains having rounded hummocky

1

2 PHYSIOGRAPHIC DIVISIONS OF ALASKA

summits, isolated by networks of broad steep-sided U-shaped
valleys and low passes. Ridges and peaks that rose above the
level of the icecaps are jagged and knifelike. Ranges dominated
by such ridges have extreme relief, and their valley§$ head in
steep-walled glacier-filled cirques.

Glaciated lowlands are irregular and consist of end and
ground moraines, drumlings, chaotic assemblages of irregular
hills and hollows (stagnant ice topography), kames, eskers,
and glacial-lake plains. Rock-basin and moraine-dammed lakes
- of great size, depth, and beauty are common around the mar-
gins of the glaciated lowlands.

Unglaciated uplands have been sculptured largely by creep
and solifluction under an arctic permafrost climate, as the
abundance of patterned ground, solifluction sheets and lobes,
and altiplanation terraces attests. Ridges of the unglaciated
uplands have broad rounded summits and gentle convex sides
which are commonly mantled in their lower parts by wind-
borne silt. The uplands are cut by narrow flat- floored valleys
which have V-shaped tributary gulches.

Unglaciated lowlands are generally broad silt plains which
have meander belts along the wildly meandering axial streams.
Near the meander belts are flat plains dotted with thaw lakes
(in places making up more than 50 percent of the area) and
sporadic pingos. Toward the surrounding uplands are rolling
silt-covered benches pocked by thaw sinks. Near the margins
of the Pleistocene ice sheets, the lowlands have extensive out-
wash fans and aprons, commonly trenched by shallow ter-
raced valleys; the streams are commonly braided. Fields of
stabilized and active sand dunes are present locally.

INTRODUCTION

Alaska, the 49th State of the United States of
America, occupies the great peninsula at the northwest
corner of the North American continent and is sep-
arated from the conterminous United States by part of
western Canada. It is one of the last regions of North
America to be explored, and maps adequate for de-
limiting its physiographic divisions did not exist before
the last 20 years. In this paper the State is divided
into 12 physiographic provinces and 60 smaller di-
visions (pl. 1), all but two of which are described
briefly.

Almost the entire State of Alaska is included in the
North American Cordillera, the great mountainous
backbone of western North America. Mountain-build-
ing activity has been recurrent throughout the geo-
logic history of Alaska and has continued to the pres-
ent time. The great variety of structures produced by
the mountain-building activity and the differential
movements of the recent geologic past have combined
to give the State its extreme topographic diversity.
Alaska's position at the northwestern corner of North
America, close to the Eurasian landmass, has caused it
to play a major part in the biologic, as well as geologic,
history of the earth. At various times in the geologic
past Alaska has been connected with Siberia and has
served as a migration route for plants, animals, and

men between the Eastern and Western Hemispheres
(Hopkins, 19592).

Although Alaska had long been inhabited by peoples
of American Indian and Eskimo stock, the first Euro-
peans to see it were Russian explorers under the cap-
tainship of Vitus Bering in 1741. The Russians con-
quered it and explored its southern coastal areas but
did not penetrate deeply into the interior. It was part
of the Russian Empire until 1867, when it was sold to
the United States for $7,200,000. Systematic explora-
tion of Alaska by expeditions of the United States
Government began in 1883 (Brooks, 1906, p. 121-123).
The first expeditions were sent by the Army and the
Revenue-Marine Service; they generally had with them
a geologist who prepared a report on and map of the
region traversed. About 1898 the task of exploring the
geology and geography of Alaska, except for the coast-
line, was taken over almost exclusively by the U.S.
Geological Survey.

The early exploring parties were searching for routes
of travel and commerce-navigable rivers and passes
over the high mountain ranges that border the Pacific
Ocean. They were in Alaska for the summer only,
when the swampy lowlands are largely impassable.
They therefore traveled by boat along the rivers, and
later by horse across plateaus and low mountains. They
mapped the country topographically as far as it could
be seen on either side of their route, in part by instru-
mental observation on the spot and in part by pano-
ramic photographs from which the topography could
be determined by careful measurements in the office the
following winter (Bagley, 1917). The narratives of
their explorations-for example, Spurr's (1900) ac-
count of the exploration of southwestern Alaska-are
stories of high adventure, full of danger and hardship.
Their discoveries were summarized in two reports of
the Geological Survey : that by Brooks in 1906 on the
geography and geology of Alaska and that by P. S.
Smith in 1939 on the areal geology of Alaska.

Brooks recognized that Alaska has two great moun-
tain systems and an intermontane plateau region be-
tween them, and he saw that these are the northern
continuations of the mountain ranges of the United
States and Canada. He also recognized most of the
subdivisions of the southern mountain system that we
use today, but the Intermontane Plateaus seemed to be
a chaotic jumble of hills and lowlands, which he d1d not
attempt to classify.

As late as 1940 most of Alaska was still unmapped,
topographically and geologically. The application of
aerial photography to topographic mapping, and par-
ticularly the invention of the metrogon lens which
permitted horizon-to-horizon photography from a

INTRODUCTION 3

single airplane flight, greatly accelerated mapping in
Alaska. By 1946, in response to wartime needs, a crude
map of the then territory had been prepared on a scale
of 1 :1,000,000, with a contour interval of 1,000 feet.
Since that time the topographic mapping of Alaska has
been progressing rapidly under the programs of the
U.S. Geological Survey, the U.S. Coast and Geodetic
Survey, and the U.S. Army Map Service. By 1970
nearly all the State will be covered by topographic
maps of excellent quality having a scale of 1 :250,000
and a contour interval of 200 feet (see fig. 1) ; and the
greater part of it will be covered also by maps having
a scale of 1 : 63,360 and a contour interval of 50 or 100
feet; all maps will be prepared by photogrammetric
methods from vertical or near-vertical aerial photo-
graphs.

Geologic mapping, which is not amenable to accurate
surveys from the air, has necessarily proceeded much
more slowly. Nevertheless, in the last 15 years geologic
maps on a scale of 1 :250,000 have been made for all
northern Alaska and much of western Alaska, the
Brooks Range and southwestern Alaska.

ACKNOWLEDGMENTS

This wealth of new geographic and geologic infor-
mation has made desirable a new classification of the
State into physiographic divisions. The classification
in this report was prepared intermittently between
1949 and 1959; most of the work was done between
1956 and 1959. The physiographic units and their
boundaries were determined with the advice and as-
sistance of the following geologists of the U.S. Geo-
logical Survey : Robert S. Bickel, Earl Brabb, William
P. Brosgé, Robert L. Detterman, Arthur Grantz, J. M.
Hoare, David M. Hopkins, E. H. Lathram, Marvin D.
Mangus, Don J. Miller, William W. Patton, C. LL.
Sainsbury, and John R. Williams. It is hoped that
the publication of this classification will bring about
a clearer understanding of the geography of Alaska
and will stimulate research into the history of the
formation of the Alaskan landscape.

In addition to advice and assistance, the geologists
named above also contributed unpublished information
on the geology and physiography of Alaska. Arthur H.
Lachenbruch provided information on the formation
of ice-wedge polygons. A. R. Tagg and G. W. Holmes
provided photographs of microrelief features.

Especial thanks are due Bradford Washburn, Direc-
tor of the Boston Museum of Science, for permission
to use his excellent aerial photographs of Alaska. Other
aerial photographs were provided by the U.S. Navy
and U.S. Air Force.

The work was done under the supervision of George
O. Gates and G. D. Eberlein, and their encouragement
and advice are gratefully acknowledged.

THE BASIS FOR THE CLASSIFICATION

The purpose of a physiographic classification of a
region as large and diversified as Alaska is to divide it
into areas that are so homogeneous topographically and
distinct from the areas around them that the physical -
appearance of the region can be easily apprehended
and described. The boundaries of the physiographic
divisions are therefore drawn where the topography
changes in character. The physical divisions must be
such that one can describe them accurately in short,
general statements. If the units are too large, they
cannot be described in general terms without doing
violence to the facts about their parts; and if they are
too small, major relations of topography, geology, and
drainage cannot be described because the units do not
include them.

Although the basis for selecting the units is largely
topographical, a major use of the classification is to

. deduce the history of the topography in order to under-

stand why there are mountains in one place and valleys
in another. Such a history is necessarily geologic, and
the geologic structure must be considered in determin-
ing which areas shall be designated physiographic
units. For example, the Upper Matanuska Valley and
the Broad Pass Depression are shown as physiographic
units because they are structurally controlled troughs,
although many valleys that are just as wide are not
considered physiographic units because they bear no
relation to the structure.

The terminology follows the scheme used for the
physical divisions of the conterminous United States
by Fenneman and others (1946), in which the great
physiographic features of North America were broken
into major divisions, each major division, into prov-
inces, and each province into sections. In addition, in
the classification of Alaska, some division of sections
into subsections has been necessary. As far as possible,
the boundary lines 'were drawn to correspond with
those of Bostock (1948) for the Canadian Cordillera so
that the physiographic units would match across the
international boundary. Bostock's names were used for
the units unless Alaskan names had already appeared
in the literature or seemed more appropriate. Bostock's
grouping into units corresponding to provinces and
major divisions could not be adhered to in all details.

The major divisions and their boundaries are shown
on plate 1. These in turn have been divided into 12
provinces, which are shown 'on plate 1. The provinces
have been divided into 60 sections, whose descriptions
begin on page 18. The number in parentheses follow-

OF ALASKA

PHYSIOGRAPHIC DIVISIONS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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SUMMARY OF THE GEOLOGIC HISTORY OF ALASKA 5

ing the title of each province or section or the first men-
tion in the text of each subsection corresponds to the
numbering of that area on plate 1.

The descriptions of the individual provinces and see-
tions include: a brief sketch of the topography; some
salient features of the drainage; statements on the

lakes, glaciers, and permafrost; and a condensed ac-

count of the geology as it affects physiographic devel-
opment. These statements are based on the literature
dealing with the geology and geography of Alaska,
chiefly publications of the U.S. Geological Survey ; on
topographic maps and aerial photographs; and to
a very large extent, especially for the geology, on un-
published information given freely by my colleagues
of the Geological Survey and listed on plate 1.

T'wo sections shown on plate 1, the Old Crow Plain
(8) and the Duke Depression (50), are not described
in text. These lie largely in Canada and have very
small prongs that extend into Alaska. Descriptions for
these areas were given by Bostock (1948, p. 76, 96-97).

SUMMARY OF THE GEOLOGIC HISTORY OF ALASKA

Alaska has already been indicated to be a region of
intense orogenic (mountain-building) activity. Any
understanding of its present topography depends,
therefore, on some knowledge of its geologic history.
A brief account of the geologic history is given in this
section and summarized in table 1.

Most of Alaska has been studied geologically only in
an exploratory manner. In his cross-country travels
the geological explorer attempted to interpret, on the
basis of a single traverse, the geology for many miles
on either side. Only in recent years has there been
much geologic mapping that attempted to cover uni-
formly the geology of a large area. Despite the speed
at which the early Alaskan geologists worked, the
extrapolations they had to make from rock outcrops
near at hand to mountains miles away, and the lack of
accurate maps on which to plot their observations, few
of their interpretations of the geology have been found
to be in serious error. Nevertheless, our knowledge of
the geologic history of the State is still fragmentary
and hypothetical. The most recent general summary
of the geology of Alaska, on which this brief account
is based, is that of Miller and others (1959). The
geology of Alaska, at the same scale as plate 1, is shown
on the map by Dutro and Payne (195%).

The oldest rock unit in Alaska is the schist of the
Yukon-Tanana Upland and the Alaska Range-called
the Birch Creek Schist from outcrops along Birch
Creek. This schist was originally sand and mud de-
posited in ancient seas and is thought to have been
tightly folded and metamorphosed in early Precam-
brian time. P

173-592 O-66-2

Apparently most, if not all, of Alaska was under
water during much of the Paleozoic Era, for thick de-
posits of this age are found throughout the State. The
Paleozoic rocks of the Brooks Range consist largely
of limestone, sandstone, and shale, or of their meta-
morphic equivalents. In the central part of Alaska,
the rocks also consist largely of limestone, sandstone,
and shale; chert, volcanic rock, and graywacke as old
as Ordovician in age are interbedded with the other
rocks. In southern and southeastern Alaska, graywacke
and volcanic rock-the rocks of orogenic belts-are
common throughout the Paleozoic sequence, inter-
bedded with great thicknesses of limestone, slate, schist,
and nonmarine red beds.

Even in the early Paleozoic rocks there are frag-
mentary records of mountain-building activity, for
angular unconformities exist between rocks of Ordovi-
cian, Silurian, and Devonian age in southeastern
Alaska and similar unconformities in central and
northern Alaska record a period of mountain building
at the beginning of Devonian time. Whatever the
nature of these orogenies, they have had practically no
effect on the present topography of Alaska.

In Mississippian time extensive submarine volcanic
eruptions occurred in various parts of Alaska, and
during part of the Permian and Triassic most of
Alaska south of the Yukon River was a great sub-
marine volcanic field. The ancient basalt flows of this
age constitute the greenstone formations common in
Alaska ; the most famous of these is the Nikolai Green-
stone of the Kennicott Copper district. After the erup-
tion of the greenstone, more limestone and shale were
deposited over the sea floor.

A new period of orogenic activity began in Jurassic
time. It was heralded by the eruption, in southern
Alaska, of andesite flows and tuffs. These were in-
truded in mid-Jurassic time by an enormous granitic
batholith that stretched from the Talkeetna Mountains
through the southern Alaska Range to the Aleutian
Range. From this time on, parts of Alaska were
rapidly uplifted to form mountains, from which sedi-
ments were eroded and deposited in adjacent basins
that were as rapidly subsiding. These sediments became
the great thicknesses of graywacke, argillite, and con-
glomerate that make up great parts of the mountain
ranges of southern and central Alaska. Not only were
the blocks of land uplifted and depressed, but they
were also squeezed together and slid over and past each
other ; therefore, throughout large parts of Alaska the
consolidated bedrock is tightly folded, broken by
numerous faults, and slightly metamorphosed. The
deformation was not continuous but was probably of a
pulsating nature. Periods of rapid change alternated

PHYSIOGRAPHIC DIVISIONS OF ALASKA

TaBu® 1.-Geologic time scale

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Time of beginning, in Era Period Typical fossils Events in Alaska and elsewhere
years before present !
8,700 years Recent Civilized man. Retreat of the glaciers; rise of sea to
P Quater- present level.
nary z (L
ris leisto- ammoth, giant beaver. | Growth of vast ice sheets and glaciers;
About 2 million cene ’ cold climates; growth of mountains 'to
their present height.

Cenozoic Various kinds of horses, Most of Alaska probably 'dry land. Ac-
camels, saber-toothed cumulation of gravels in many low
tigers, aInd other mam- areas. ficumulafin of coal in svgagps

ans M mals. In sea, mol- to form Nenana atanuska, an er-
About 63 million Tertiary lusks (clams and ing River coal fields. A ,period of
snails) and echino- waning mountain-building activity ex-
derms (sea urchins and cept in the Aleutians and the Gulf of
sand dollars). Alaska coastal belt.

Age of dinosaurs on Period of culmination of great mountain-
land; in sea, age of building period in Alaska. Intrusion
complicated nautilus- of Coast Range batholith and other
like animals called batholiths. Accumulation of vast thick-
ammonites. nesses of sand, shale and impure

About 135 million Cretaceous sandy muds in basins where Kenai-
Chugach - Mountains, __ Kuskokwim

Mountains, and Nulato Hills now stand,

firs well as dnorth of Brooks Ilt'ssngef

F olding and thrusting of roc 0

Mesozoic Brooks Range and elsewhere.

Period of growing mountain-building ac-
tivity in Alaska. Volcanic deposits in
Jurassic the § alrliz‘eetna Mountains find Si111;1('iusi01cti
ali of the Talkeetna batholith. Sand an
About 230 million shale in Cook Inlet.
"Pringsio Small dinosaurs, End of period of outflow of submarine
amphibians. lavas over much of southern Alaska.
Amphibians, clubmoss Early part of period of outflow of sub-
Permian and fern forests on marine lavas over much of southern
land. Trilobites, Alaska.
early corals, and -
Tk 3 primitive fishes in Submarine lavas and dark sedimentary
About 345 million 2 | Pennsylvanian sea. rocks in southern Alaska.
O
SE
$- Chert in central Alaska. Limestone in
5 Mississippian northern Alaska.
O

Most primitive fishes. Sandstone and shale in northern Alaska;

Paleozoic First forests on land. limestone and other sedimentary rocks

deposited in central and southern
Devonian Alaska. Apparently a period of moun-
tain-building in Alaska, in which the
mountains may have trended northeast
or north.
About 600 million Chiefly trilobites and In southeastern Alaska accumulation of
Silurian brachiopods (lIamp- pure limestone in reefs associated with
shells). volcanic rocks.
fas Little record in Alaska. Limestone in
Ordovician Seward Peninsula.
Cambrian Earliest trilobites. Little record in Alaska.
Oldest age deter- No fossil record. Formation of Birch Creek Schist of central
mined from rocks Alaska (Fairbanks area, Yukon-Tanana
by radioactivity: Precambrian Precambrian Upland, and Alaska Range).

3300 million

 

 

 

 

 

! Date of Pleistocene-Recent boundary is based on Charlesworth (1957, v. 2, p.
1525-1526); date of boundary between Tertiary and Quaternary is based on Leakey
and others (1961) and on J. F. Evernden and G. H. Curtis, quoted in Hay (1963);

other dates from Kulp (1961).

Glaciation.

2 It has been determined that ice advanced and then melted away at least 4 times
in Central United States and probably also in Alaska, 'The last such advance and
the one that left the clearest mark on the topography is called the Wisconsin

SUMMARY OF THE GEOLOGIC HISTORY OF ALASKA T

with periods of relative stability, and a block that was
uplifted at one time might be depressed at another.

Orogenic activity reached its climax in late Early
and Late Cretaceous time. At that time, the rocks were
tightly folded and had a cleavage ; and great batholiths
were intruded in the Alaska Range, in the Interior
Plateaus, in the Kodiak and Chugach Mountains, and
throughout southeastern Alaska and adjacent British
Columbia. The Paleozoic rocks of the Brooks Range
were thrust northward and folded into great flat folds
overturned to the north. By the end of Cretaceous
time, most of Alaska was dry land-a segment of the
great mountain belt that borders the Pacific Ocean on
the east, north, and west. The part of that belt in
North America is called the North American Cor-
dillera. In Alaska the structures produced by this
orogeny are great arcuate belts roughly parallel to the
shore of the Gulf of Alaska, and structural trends are
generally northwestward in southeastern Alaska,
roughly due west throughout most of central and
northern Alaska, and southwestward in western and
southwestern Alaska.

The structures characteristic of the North American
Cordillera in Alaska have their counterparts farther
south in Canada and the conterminous United States.
Thus the bedded rocks of the Pacific Mountain System
consist largely of great thicknesses of tightly folded
graywacke, argillite, conglomerate, and basalt and
andesite flows and tuffs. Rocks of the Rocky Mountain
System are mainly limestone, quartzite, slate, and shale
that were folded and thrust northward or eastward,
away from the Pacific. The greatest intrusive and
volcanic activity has been in the Pacific Mountain
System, which contains an almost continuous belt of
granitic batholiths from Mexico to the Aleutian Islands
as well as a chain of active and recently active vol-
canoes. The evidence of igneous activity decreases
across the cordillera away from the Pacific, and rela-
tively little igneous activity occurred in the north front
of the Brooks Range.

.. Orogenic activity continued in the North American
"Cordillera throughout the Tertiary. The belt immedi-
ately adjacent to the Pacific Ocean has been the most
active. Tertiary rocks along the coast of the Gulf of
Alaska have been tightly folded, thrust, and raised to
great heights. The great chain of volcanoes in the
Aleutians suggests the presence of magma at depth.
Both the Aleutians and the Gulf of Alaska coast have
experienced many strong earthquakes in recent years.

In the remainder of Alaska, the orogenic activity of
Cenozoic time was much less intense than that of the
Cretaceous. It has consisted mainly of block faulting
and, in late Cenozoic time, of broad gentle warping

and uplift. In the Alaska Range and elsewhere, bodies
of coarse poorly consolidated conglomerate and sand-
stone which are folded and faulted indicate at least one
period of intensive mountain building in central
Alaska in Tertiary time.

Cenozoic time (the last 60 million years) has prob-
ably been largely an era of erosion in Alaska, inter-
spersed at times by local depression of the ground
surface which caused continental sediments to accumu-
late in basinlike areas of relatively small extent. It is
the period of the formation of the physiography of
Alaska. It is much more difficult to reconstruct the his-
tory of the formation of this landscape than it is to
unravel the history of the rocks on which it is formed,
even though the landscape is younger than the rocks.
The advantage to a geologist of a sequence of sedi-
mentary rocks is that the rocks themselves contain the
record of the history of their deposition, stacked in
order like the pages of a book. The landscape, how-
ever, does not generally retain the record of its forma-
tion. The record of erosion is like messages on a black-
board, for each stage of the erosional process destroys
most of the product of the stage before it, and to work
out the successive stages we must rely on undestroyed
relics of the old topographies, which are hard to find
and even harder to interpret.

Many hypotheses have been proposed for the evolu-
tion of the drainage of Alaska and for the distribution
of its mountains and lowlands. Information on the
later geologic history of Alaska is as yet insufficient to
eliminate any but a few of these hypotheses or to give
a history of the development of drainage that explains
all known features. Therefore, no account is given
here of the development of the major physiographic
features or of the development of the drainage.

Certain facts of the physiography of Alaska, how-
ever, must be taken into account in formulating any
history of its physiographic development. Although
the mountain systems broadly parallel the major belts
of internal structure of the cordillera, there are sig-
nificant local differences, particularly in southwestern
Alaska, where the north front of the Pacific Mountain
System trends diagonally across several structural
belts. The Western Alaska province of the Inter-
montane Plateaus is underlain largely by folded and
faulted marine and continental sedimentary rocks of
Cretaceous age. The Northern Plateaus province, on
the other hand, is underlain chiefly by metamorphic
rocks of Paleozoic and Precambrian age, including the
oldest rocks in Alaska, and by large granitoid batho-
liths. There is no break in the structure of the under-

8 PHYSIOGRAPHIC DIVISIONS

lying rocks at the boundaries of this province with the
mountain system to the north and south. Many of the
lowlands are underlain, beneath a thin cover of soft
Tertiary or Quaternary sedimentary deposits, by the
same kinds of rock-erystalline or partly metamor-
phosed-that make up the mountains that surround
them. It seems, therefore, that the present topography
neither can be explained as being an immediate product
of the main period of rock deformation and igneous
activity in the North American Cordillera, nor can it
be explained entirely by differential erosion of the
rocks deformed in that period. Folding and faulting
of more recent age, probably during late Tertiary or
Quaternary time, were at least partly responsible for
the topography.

The drainage similarly shows many anomalies. The
major drainage divides do not generally coincide with
major mountain axes but lie for long distances in low-
lands to the north or south of them; and a large num-
ber of rivers cross mountain ranges through deep
canyons from one lowland to another, even though the
lowlands join around the ends of these ranges.

DETAILS OF SCULPTURE-PHYSIOGRAPHIC EVOLU-

TION IN THE LAST MILLION YEARS

Although the drainage and the larger physiographic
features of Alaska may have originated far back in the
Tertiary and possibly even earlier, the erosion and de-
position that was responsible for the detailed form of
mountainsides and valley floors has probably taken
place throughout Quaternary time, when the present
climate in Alaska was being established. Quaternary
climatic patterns are unusual in the history of the earth
in that extreme cold prevails in the polar regions. Much
work has been done in Alaska and elsewhere on the
erosional processes prevailing in these cold climates,
and we have some idea about how the details of
topography were formed.

The Quaternary has been a period when vast sheets
of ice, compacted from snow, covered large parts of
North America, Europe, Asia, Patagonia, and Antare-
tica and pack ice covered the polar seas. Several periods
of growth of these ice sheets, accompanied by cooler
and wetter conditions around the world, have alter-
nated with periods having climates similar to, or even
warmer and drier than, those of today. We know that
much, though not all, of Alaska was covered with ice
during the Pleistocene glacial advances; large parts
are still ice covered. Furthermore, north of the south-
ern coastal areas, Alaska is a region of permafrost
that is, a region where large areas of the ground a few
feet below the surface are frozen the year around. A

OF ALASKA

climate in which glaciers or permafrost can form is
called a frost climate and is generally characterized by
an average annual temperature that is below freezing.
Figures 2, 3, and 4 indicate in a general way the
climate of Alaska by showing the average January
minimum and July maximum temperatures and the
annual precipitation.

The layer of permafrost has profoundly affected the
weathering and erosion of the rocks and produced a
characteristic topography over much of interior and
northern Alaska. Figure 5 shows the extent of the
permafrost areas and the extent of Pleistocene and
modern glaciation in Alaska.

In addition to its direct effect in the formation of
glaciers and permafrost, the arctic and subarctic cli-
mate of Alaska has an indirect effect on the processes
of erosion by its control on vegetation. Much of Alaska
is above or beyond timberline and is covered by a dense
mat of low bushes, herbs, grasses, and moss; parts are
so high and cold that they are barren rock deserts. The
distribution of the major types of vegetation in Alaska
is given by Sigafoos (1958), and some of the climatic
factors controlling the distribution of vegetation are
discussed by Hopkins (1959c¢).

The physiographic sections of Alaska that are shown
on plate 1 can be conveniently divided into four groups,
in each of which a fairly uniform topography prevails :
(a) glaciated highlands and mountains, (b) glaciated
lowlands, (c) unglaciated highlands, and (d) unglaci-
ated lowlands. The physiographic processes that pre-
vail in each of these groups are discussed in the follow-
ing four sections. The boundaries between the glaciated
and unglaciated regions are not everywhere sharp, for
in places where the older glacial advances were much
more extensive than the younger glacial advances the
glaciated topography produced by the older advances
has been greatly modified by nonglacial erosional
processes.

The material in the following four sections is based
in large part on the following books and articles:
Black (1951, 1954); Bostock (1948); Coats (1950) ;
Eakin (1916, p. 76-92); Eardley (1988b); Eckel
(1958) ; Flint (1957); Hopkins (1949); Hopkins and
Sigafoos (1951); Hopkins, Karlstrom, and others
(1955) ; Lachenbruch (19602, 1960b, 1962) ; Leffingwell
(1919, p. 179-214) ; Livingston (1954) ; Muller (1947) ;
Péwé (1954, 1955a) ; Péwé and others (1953) ; Plafker
and Miller (1958); Porsild (1938); Sharp (1947,
1958); Taber (1943); Tarr and Martin (1914) ;
Trainer (1953); Twenhofel (1952); Wahrhaftig
(1949, 1958a); Wahrhaftig and Cox (1959); Wal-
lace (1948) ; Washburn (1956) ; and Williams (1958).

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DETAILS OF SCULPTURE-PHYSIOGRAPHIC EVOLUTION IN THE LAST MILLION YEARS 13

GLACIATED HIGHLANDS AND MOUNTAINS

During the glacial expansions of the Pleistocene,
snow accumulated on the mountains and avalanched
into the heads of valleys, where it compacted into ice
and flowed down the valleys as glaciers, just as it does
on a smaller scale in the higher mountains of Alaska
today. Eventually, most parts of the Pacific Mountain
System and the Brooks Range were buried beneath a
great network of glaciers and icefields; and smaller ice
sheets existed on the Ahklun Mountains, on many
mountains in the Intermontane Plateaus, and on the
Seward Peninsula.

The grinding and quarrying action of the glaciers
enlarged the heads of the valleys into steep-walled
flat-floored theaterlike basins or cirques and steepened
or undermined the surrounding mountainsides (pl. 5,
figs. 9 and 10; pl. 6, figs. 6 and 7). Freezing of water
in joint cracks in the mountains that stood above the
ice split out blocks that fell to the glaciers below to be
carried away by the ice. As a result these mountains
were left as jagged knife-edge ridges (arétes) and
mountain spires (horns) with steep craggy cliffs that
are commonly accessible only to experienced moun-
taineers (pl. 6, figs. 1, 4, and 6). In the glaciated re-
gions, the upper limit that was reached by the most re-
cent ice sheet can be recognized as the lowest level at
which these jagged cliffs occur.

The cliffs and pinnacles are higher and steeper in
areas of widely jointed rocks such as granitic intrusions
and are less steep in closely jointed or slabby rocks
such as schist and shale. Mountains formed of gently
dipping sedimentary rocks have a cliff-and-bench to-
pography (pl. 3, fig. 2) ; the cliffs are made of resis-
tant widely jointed rocks such as limestone and quartz-
ite and the benches are formed on soft marls and shales.

The ice generally flowed in the direction that its up-
per surface sloped whether or not this coincided with
the slope of the buried landscape. As it flowed, the
ice-using as tools the rock fragments embedded in it
-quarried, ground, and polished the buried land and
carried away all weathered and broken material. Hill-
tops overridden by the ice were rounded and flattened.
Layers of soft rock and cracks and planes of weakness
such as joints and faults were etched out by the ice,
and the intervening masses were left as rounded knobs.
Such glaciated country, free of all soil and most loose
material, presents magnificent fresh outcrops for geo-
logic study (pl. 6, figs. 7 and 11; pl. 5, fig. 11).

Glacial erosion widened valleys so that they now
have U-shaped cross profiles with steep bounding walls
and broad gentle floors; it straightened them by plan-
ing off projecting spurs; and it gave them irregular
longitudinal profiles, eroding parts into deep rock
basins (now filled by lakes) and in other parts forming

773-592 O-66-3

giant steps by quarrying action. In mountainous areas
almost completely buried by ice, the movement of the
glaciers tended to lower the passes between the former
drainage systems; and such areas eventually came to
consist of blocklike mountains separated by a network
of valleys and low passes (pl. 5, fig. 4).

In most of their erosion the ice sheets merely modi-
fied a pattern of valleys that already existed; they did
not entirely destroy old drainage systems, nor did they
create new valleys. The material removed was carried
to the lower margins-where melting and evaporation
balanced the inflow of ice-and was deposited there as
heaped-up lines of ridges or end moraines. Melt water
flowing along the sides of the glaciers carved notches
and valleys that are now one-sided (the former other
side was ice) and deposited plains of sand and gravel
that now stand as benches (pl. 2, fig. 10). Beyond these
lines of marginal deposits the topography of the un-
glaciated parts of the mountains is usually markedly
different (pl. 5, fig. 7) ; however, this distinct margin
is visible only around the later great glacial advances.
Where ancient glaciation was more extensive than later
advances, its forms can be recognized in subdued sharp-
crested mountains and open cirquelike valleys without
steep headwalls or lakes (pl. 5, fig. 6), but in most
places the older morainal deposits have largely been re-
moved by erosion (pl. 3, fig. 7).

GLACIATED LO WLANDS

Glaciated lowlands are largely regions in which ma-
terial was deposited by ice. The material deposited
directly from ice (till) is generally a very poorly sorted
mixture of boulders, sand, and clay. The few basins
eroded by the glaciers in the lowlands, generally where
the glaciers flowed out of the bordering mountains, are
now filled by lakes many miles across.

As a glacier advances it may push up a ridge of
debris at its front. This ridge, or moraine-generally
an are enclosing the mouth of the valley from which
the glacier flowed-remains when the glacier melts.
There may be many parallel ridges, each pushed up by
a short advance that punctuated a general period of
glacier retreat. Few of the arcuate ridges enclosing the
mouths of glacial valleys are of such simple ice-push
origin, however. The debris scraped up by the glacier
tended to be concentrated near the glacier front. When
the glacier melted away, this debris was left as a belt
of hilly country somewhat higher than the ground it
encloses (pl. 3, fig. 10; pl. 4, fig. 18).

Some till was plastered directly on the ground from
the lower part of the moving ice. If the resulting sheet
of till is thin, it merely subdues the topography that
existed before it was deposited. If it is thick, the pre-
glacial topography may be obliterated, and a new to-

14 PHYSIOGRAPHIC DIVISIONS OF ALASKA

pography imposed, which generally has smooth stream-
lined hills (drumlins) and hollows elongated in the
direction of ice flow. Such sheets of till, whether thick
or thin, are known as ground moraine.

As the glaciers melted away, the ice that was pro-
tected from sunlight by a covering of moraine did not
melt as rapidly as the bare ice on either side of it.
Rapid lowering of the bare ice surface left the moraine-
covered ice standing as steep-sided ridges. The moraine
slid off these ridges onto the bare ice, and the formerly
protected ice melted down in turn. The repeated in-
version of topography that this process produced
caused the debris to slide back and forth and resulted
eventually in accumulation of the debris as great piles
of open bouldery till (pl. 2, fig. 10). In the last stages
of this process the glacier was too thin to flow, and the
ice became stagnant. The last blocks of ice to melt-
half buried by accumulations of till around them-
left hollows that are now commonly filled by irregular
ponds and lakes. Large areas of the glaciated lowlands
are marked by this chaotic stagnant ice topography
(pl. 3, fig. 10; pl. 4, fig. 13).

Melt water from the glaciers circulated through and
between the blocks of stagnant ice, sorting and rework-
ing much of the morainal material. Gravel deposited
by water flowing in tubes within and beneath the ice
stands as narrow sinuous ridges (eskers) now that the
ice has melted away (pl. 2, fig. 13). Elsewhere, the
water deposited the gravel to form plains bounded by
walls of stagnant ice. The ice later melted, leaving
these plains as flat-topped ridges and hills (kames).
Some of these flat ridges coalesce downstream to form
outwash plains-plains made of the sand and gravel
washed out of the ice by glacial melt water (pl. 5, fig.
8; pl. 6, fig. 5).

Much glacial melt water and water trapped from
surrounding uplands flowed along the margin of the
ice sheet; in many places its channel would have one
wall of ice; some segments of its channel would be en-
tirely in ice; and other segments would be cut across
low spurs of the surrounding uplands. The remains of
such channelways form benches and notches on the
gently sloping valley walls or are narrow flat-floored
steep-sided winding valleys that end abruptly down-
stream and upstream where the water formerly passed
over ice. The streams, if any, that flow in these valleys
today are too small to have cut them; at many places
their direction of flow is the reverse of that of the melt
water (pl. 5, fig. 2; pl. 6, fig. 9).

Large parts of some lowlands, the Copper River
Lowland, for example, were flooded by lakes made
when the rivers draining them were dammed by gla-
ciers from the surrounding mountains. Finely ground
rock debris that was washed into these lakes from the

surrounding glaciers accumulated as layers of clay on
the lake floors; today these floors are flat plains (pl.
6, fig. 3).

The result of this variety of processes of deposition
and erosion accompanying the advance and retreat of
the glaciers is that the glaciated lowlands are much
more irregular and more diversified topographically
than are the lowlands of unglaciated regions.

UNGLACIATED UPLANDS

The slopes of the unglaciated uplands of Alaska are
modeled through the freezing and thawing of the thin
surface layer above the permafrost. This thin layer
of debris is formed by the freezing of water in joints
and cracks that shatters the bedrock into small frag-
ments. During freezing the ground heaves outward
perpendicular to the slope. This heaving is caused
partly by the expansion that water undergoes when it
turns to ice and partly by the growth of ice crystals as
water is drawn to them through capillary openings in
the soil. When the ground thaws it tends to settle ver-
tically downward under the influence of gravity. The
small displacements that result shift the surface layer
downslope, and over the years their cumulative effect
is considerable. This slow, almost imperceptible motion
is called creep.

The layer of repeatedly frozen and thawed material
is kept close to saturation by ice melting in the frozen
soil beneath it. In its saturated condition during the
spring thaw it flows downslope as a thick sludge, a type
of motion known as solifluction. Both creep and soli-
fluction can take place on very slight slopes.

Freezing and thawing not only transport the sur-
face layer, they also rearrange the soil and sort the
soil constituents. The manner in which this rearrange-
ment and sorting are accomplished is not yet com-
pletely understood and what is known is too complex
to be told in this short review. However, the products
are striking and are easily recognized. On nearly level
surfaces the coarse fragments in the soil are sorted
from the fine constituents and are arranged into poly-
gonal networks of stones about centers of silt or clay
(pl. 2, fig. 12). The silt and clay centers of the poly-
gons are commonly raised into low mounds. This is
one type of patterned ground (pl. 2, fig. 11). On some
sloping surfaces the polygons are elongated downslope
through creep and solifluction and pass into stripes of
stones and fine earth extending down the slope (pl. 2,
fig. 6). On other slopes the soil is arranged into small
benches or terraces having steep banks of stones or turf.
In some parts of Alaska, notably the Alaska Range,
the stripes occur on south-facing slopes and the terraces
on north-facing slopes.

 

DETAILS OF SCULPTURE-PHYSIOGRAPHIC

The rate at which hills are eroded by creep and soli-
fluction and the forms of the hills that are produced
depend to a very large extent upon the rate at which
the material is moved down the slope by these processes.
The rate of movement, in turn, depends upon many
factors-among which are the steepness of the slope,
the depth of thaw, the amount of water in the soil, the
size of the soil fragments, and the extent of vegetation.
These factors vary not only from place to place but
from year to year. In general, one may assume that,
other things being equal, the rate of movement in-
creases with steepness of slope and with increase in
water content of the soil. Large blocks move more
slowly than fine soil, and vegetation inhibits movement
by decreasing the depth of penetration of the thaw and
by holding the soil fast with its roots.

If the rate of movement on the upper part of the
slope is greater than that on the lower part, material
will pile up at midslope. Lobelike masses (solifluction
lobes), particularly common near the upper limit of
vegetation, show that this piling up of material is com-
mon (pl. 2, fig. 5). If the rate of transport is constant
throughout the slope, erosion can take place only near
the top of the slope, for the layer of material moving
down the slope protects the lower parts from frost
penetration and erosion. The tops of the hills will be
flattened, whereas the lower parts of the slopes will re-
tain their steepness and will be eroded only where the
rate of transport increases at least in proportion to the
distance from the top of the slope. Hence, where the
rate of transport increases with the steepness of the
slope, erosion of the lower part of the slope takes place
only where the steepness increases downslope. The ten-
dency of erosion by creep and solifluction, therefore,
is to produce hills with broad gently rounded or flat
summits and convex side slopes. Such hills are charac-
teristic of the uplands throughout the unglaciated parts
of Alaska (pl. 3, figs. 5, 6, and 9; pl. 4, figs. 5, 10,
and 11).

If the downslope increase in the rate of transport is
greater than the increase necessary to erode the slope
evenly, the lower part of the slope is degraded more
rapidly than the upper part. The part of the slope
where degradation increases downslope must be grow-
ing steeper, because its base is being lowered more
rapidly than its top. This part of the slope may grow
into a craggy outcrop or a cliff (pl. 3, fig. 4). Common
manifestations of this downslope increase in rate of
degradation are tors, or bosses; these are resistant
masses of unjointed bedrock a few tens of feet across
and as high as a hundred feet, left standing on hill-
tops as the material around them moves downslope by
creep and solifluction (pl. 2, fig. 3).

15

An extreme example of downslope mass movement is
a landslide. Where saturated with water, poorly con-
solidated or badly shattered rock may suddenly or
slowly move downhill over surfaces of slipping. Con-
trary to popular impression, the water does not act as
a lubricant; its effect is much more complicated. In
part it breaks down the soil structures, such as clods,
that hold the mineral and rock fragments together or
dissolves salts or other chemicals binding or cementing
the rock; thus the rock is weakened for sliding. Its
greatest influence, however, is its buoyancy. The rock
is buoyed up by the weight of water filling its pores
much as a stone is easier to lift when it is submerged in
water, because the weight of the water it displaces acts
against the weight of the stone. This buoyant weight
counteracts the component of weight directed against
the slope, which is the component that maintains the
frictional resistance against sliding. At the same time
the weight of the water is added to the component
directed along the slope-the component that causes the
sliding. Permafrost provides abundant ground water
that keeps the ground saturated during the summer ; be-
cause the permafrost is impervious (its pores are
clogged with ice) this water cannot drain away. Hence,
permafrost areas have frequent landslides (pl. 2, fig. 7).

A major factor in the landslide hazard is the steep-
ness of the slope. Saturated silt and clay will slide or
flow on slopes of 10° or less, but more massive material
will stand on much steeper slopes, even in vertical
cliffs, although it may contain much water. Anything
that undercuts or steepens a slope is likely to cause
landslides. The outer banks of meanders and the walls
of canyons that are being deepened by streams are
likely places for landslides.

Landslides are characterized by hummocky topogra-
phy and irregular hillocks and depressions. At the
upper end is a bowl-shaped hollow, bounded by steep
scarps that mark the limits of the area of landslide
subsidence. The lower end of some landslides is a bulg-
ing prominence, which may dam a stream to form a
lake. The rocks of a landslide area are displaced,
broken, and disturbed; characteristically, blocks are
rotated backward toward the mountain as they move
downslope.

Poorly consolidated rocks are generally protected
from the erosional activity of rainwash by a blanket of
vegetation. Areas laid bare by landslides lack this
protection, and rainwash quickly carves exposures of
sand and clay and of poorly consolidated sandstone
and claystone into intricate patterns of steep-walled
gullies. In Alaska the activity of rainwash alternates
with freezing and thawing, and this alternation pro-
duces in the poorly consolidated rocks erosional forms
of great beauty and regularity. The finest of these are

EVOLUTION IN THE LAST MILLION YEARS

16 PHYSIOGRAPHIC DIVISIONS OF ALASKA

the exposures of coal-bearing rocks in the Nenana coal
field, in the Northern Foothills of the Alaska Range
(pl. 2, fig. 8). On exposures of conglomerate, pebbles
and cobbles are loosened by freezing and thawing and
bounce down the slopes, loosening other pebbles in their
paths; the faint hollows formed by the first-falling
pebbles channel other pebbles to fall their way so that
an initially smooth cliff is gradually transformed into
many closely spaced ravines which have rounded floors
and are separated by knifelike ridges (pl. 2, fig. 9).
The higher ridge crests and mountain tops in the un-
glaciated parts of Alaska are notched into giant steps
or terraces several hundred to a thousand feet wide,
whose banks of coarse rubble are 50-200 feet high (pl.
2, fig. 1). The material on the upper surfaces of these
terraces is progressively finer and more saturated with
water toward the back of the terrace. The origin of
these altiplanation terraces is not clearly understood,
for they have not been studied in detail. They are
probably an example of erosion produced by soliflue-
tion and creep acting with the frost-sorting process.
The lower slopes of the upland hills are mantled
with loess (wind-deposited silt) blown from the flood
plains of the glacial streams. Part of this loess mantle
is moved to the adjacent valley bottoms by torrents,
mudflows, or landslides; so, the lower slopes of the
hills are marked by a fine tracery of shallow gullies
(pl. 3, fig. 9). In its transport to the valley bottoms,
the reworked loess carries down and buries great quan-
tities of vegetable material and even animal remains.
Subsequently the whole mass freezes. The placer
gravels of interior Alaska are commonly covered with
several tens to a few hundred feet of this frozen muck.
Transitional in character between the forms of gla-
ciated uplands and those of unglaciated uplands are
rock glaciers; these are tonguelike or lobelike masses
of coarse blocky rubble 50-250 feet high, resembling
glaciers or lava flows, that move through the flow of
their interstitial ice. They move outward at a rate of a
few feet per year from the lower ends of talus cones,
and hence are found only at the bases of steep cliffs
in areas of frequent rockfalls. The interstitial ice is
necessary for the movement of rock glaciers, and the
accumulation of boulders is necessary for the protec-
tion of the ice, which would otherwise melt. They are
common in cirques and along the walls of glaciated
valleys a few hundred to 2,000 feet below the firn line.
The many cycles of glaciation in the glaciated parts
of Alaska were matched by alterations of intense and
mild frost action, creep, and solifluction in the ungla-
ciated parts. In general, during the cold and wet gla-
cial periods the level at which the more alpine forms
of erosion were active was lowered, and the accumula-
tion of loess was more rapid. During the warmer in-

terglacial periods the silt tended to slide into the val-
leys to form muck, and the hillsides were moderately
gullied. From a study of these deposits one can work
out a succession of alternately severe and mild climates
in interior Alaska that corresponds with at least the
later stages of glacial advance and retreat in the ad-
joining glaciated areas.

UNGLACIATED LOWLANDS

The topography of the unglaciated lowlands of
Alaska has been formed by the deposition of material
brought in from adjacent highlands and glaciated areas
by rivers and wind and by the action on this material
of processes involving freezing and thawing that take
place in permafrost areas. Most of the great rivers that
flow through these lowlands had their sources in ice
sheets during the Pleistocene Epoch, and many of them
still head in glaciers. They brought and are still bring-
ing great quantities of sand and gravel and finely
ground rock flour from the glaciers. Where the rivers
enter the lowlands, this coarse material was deposited
in the form of broad, low fan-shaped deposits (out-
wash fans) that are crossed by the bare flood plains of
the braided rivers (pl. 3, fig. 8). The gravel deposits
raise the beds of the rivers and cause them to overspill
their banks and change their courses to flow through
adjacent lower areas. Such a shift of the Nenana River
in 1921 destroyed a segment of the newly built Alaska
Railroad. This shifting of the streams builds the out-
wash fans. Most of the fans were built during the
Pleistocene ice advances, when the rivers were much
more heavily laden than they are now. Because they
have less material to carry from the glaciers, the rivers
have removed some of the gravel they formerly de-
posited and now flow through the upper parts of their
outwash fans in terraced valleys a few feet to a few
hundred feet deep and 14-4 miles wide. The outwash
terraces can be traced for miles upstream into the sur-
rounding mountains, where they generally end at the
end moraines of the ice advances of the time they were
deposited (pl. 4, fig. 13; pl. 6, fig. 2).

Downstream from the fans the rivers carry silt and
clay. Banks of these compact sticky materials are not
as easily eroded as are the loose banks of gravel and
sand. Consequently, the rivers cannot spread freely
over their flood plains in networks of small channels
but instead concentrate in one or two large channels,
which meander across the silt-covered plains (pl. 4,
figs. 6 and 7).

Many of the streams rise in unglaciated regions.
Some of these are provided with abundant debris by
frost action in highland source areas and behave much
as glacial streams do. Others carry very little inorganic
debris and meander sluggishly over flat marshy lake-

DETAILS OF SCULPTURE-PHYSIOGRAPHIC

dotted plains. Peat accumulates in the lowland flats
along these sluggish streams and extensive logjams
occur in their courses.

The glacial rivers are subjected to daily and annual
floods. The major annual flood occurs during the
spring ice breakup, which progresses from the head-
waters downstream. The ice floes form great jams and
back up the rivers, which then flood the lowlands and
deposit silt and clay in the vegetation of their flood
plains. Freezing of this mixture of silt, clay, and
slowly decaying vegetation keeps pace with accumula-
tion. Generally the percentage of ice in this type of
permafrost is as great as or greater than the combined
percent of inorganic silt and organic matter.

During periods of low water the wind blows great
clouds of dried mud and sand from the beds of the gla-
cial streams. The fine material settles out of the air to
mantle the plains and lower slopes of the surrounding
hills as loess, which has accumulated to great thick-
ness. The great dust storms that blow across the Alaska
and Richardson Highways from the beds of the Delta,
Donjek, and White Rivers are examples of the loess-
forming process in action. Such storms must have been
much more common and extensive during the glacial
advances of the Pleistocene.

The sand that was blown from the river flats accu-
mulates as dunes. Some dunes form long low ridges
parallel to the direction of the wind. Others are great
wavelike ridges at right angles to the wind and have
gentle windward slopes and steep lee slopes (pl. 4, fig.
12). Most of these are now stable and overgrown with
vegetation, but active dune deserts, barren as any in the
Sahara, cover parts of the Koyukuk Flats and Kobuk
Valley.

Ice accumulates in the silt in the form of interstitial
cement, thin horizontal sheets, and thick vertical
wedges that form polygonal networks, another type of
polygonal ground (pl. 2, fig. 2). The ice wedges start
from tension cracks that open during winter periods
of rapid and profound drop in temperature. Later,
water trickles into these cracks and freezes, adding to
the permafrost. The cracks open during cold spells of
following winters, and the ico wedges gradually
thicken. The ice wedges are marked at the surface by
shallow troughs bordered by low ridges of material
thrust aside when the ground warmed and expanded
in spring and summer after the contraction crack had
filled with ice. Generally, the vegetation growing in
these troughs is different from that growing in the in-
tertrough areas. When the climate warms or the vege-
tation cover is removed, the ice wedges melt and the
ground above them settles. Fields cleared in such ice-
wedge terrain have to be abandoned after a few years.

EVOLUTION

17

The meandering rivers rapidly erode the frozen silty
outer banks of their meanders at the same time that
silt and sand bars are built in the slack water at the
insides of the meanders. The rate of migration of the
channels by this process has been estimated to be as
much as 75 feet per year. The pattern of delicate al-
ternation of bars and swales (known as meander-scroll
pattern) left behind as the rivers migrate laterally is
slowly buried by silt and vegetation, which is added to
the body of permafrost (pl. 4, figs. 6 and 7).

The permafrost beneath the great silt-covered flats,
50 percent or more ice by volume, is in delicate equilib-
rium with the prevailing temperature and is generally
protected from thawing by an insulating layer of moss,
grass, and other low plants. If this vegetation layer
is broken, the soil beneath it thaws and collapses, form-
ing a pit which is filled with water. This water, in
turn, thaws the banks of the pit, which is thus enlarged
to a pond or lake called a thaw or thermokarst lake;
the lake grows by collapse of the walls of the pit until
the water has an opportunity to drain off. Silt and
vegetation again cover the hollow and ice forms in the
soil once more. The cycle may then repeat itself (pl. 4,
fg. 8).

Loess and muck on hillsides, also frozen, can thaw
and collapse by this process. Generally, much of the
water drains out of the pits so formed, and an irregu-
lar topography of steep-walled flat-bottomed thaw
sinks, which are a few feet to a few tens of feet deep
and some of which contain lakes, forms on rolling
silt-covered lowlands and terraces (pl. 4, fig. 7). Such
topography is easily mistaken for morainal topog-.
raphy.

Pingos, rounded or conical ice-covered turf mounds
a few feet to 100 feet high, are characteristic features
of the lowlands of the northern parts of Alaska and in
valleys in central Alaska (pl. 2, fig. 4). Pingos form
when water trapped under great pressure in layers
within the permafrost (between the permafrost and
newly frozen ground) or beneath permafrost breaks
through the confining frozen ground and forms a great
blister of ice just beneath the turf-for water kept
liquid at subfreezing temperatures only by high con-
fining pressure freezes instantly when the pressure
is released.

The processes that affect the unglaciated lowlands
are for the most part depositional processes that re-
duce or erase irregularities on the lowland surface.
The permafrost prevents downward percolation of
ground water, and summer melting of ice at the top of
the permafrost keeps the vegetation on these relatively
smooth plains saturated. Consequently, large parts of
the unglaciated lowlands are marshy flats, impassable

IN THE LAST MILLION YEARS

18 PHYSIOGRAPHIC DIVISIONS OF ALASKA

to overland travel except in winter (pl. 4, figs. 1, 3,
and 7).

DESCRIPTION OF THE PHYSIOGRAPHIC DIVISIONS

Two of the major physiographic features of North
America extend into Alaska-the Interior Plains and
the North America Cordillera. The Arctic Coastal
Plain is the only continuation of the Interior Plains in
Alaska. The North American Cordillera, which in-
cludes most of Alaska, consists of three of the major
divisions of Fenneman's (1946) classification-the
Rocky Mountain System, the Intermontane Plateaus,
and the Pacific Mountain System, which form three
parallel belts from the conterminous United States
through Canada to Alaska (pl. 1). In Bostock's (1948)
terminology, the Rocky Mountain System is the East-
ern System and the Pacific Mountain System, the
Western System. The names used by Fenneman had
been applied to Alaska by Brooks in 1906, and there
seems to be no reason for abandoning them here in
favor of Bostock's terms.

The Rocky Mountain System in Alaska is represented
by the Brooks Range-a mountainland 80 miles wide
and 600 miles long that rises to altitudes of 4,000-9,000
feet-and by the Arctic Foothills-a rolling upland
about 80-100 miles wide on the north side of the Brooks
Range. The Brooks Range faces the Arctic Foothills
with an abrupt scarp, and similarly rises abruptly
above lowlands and low plateaus on the south (pl. 1).

The Intermontane Plateaus system in Alaska consists
of a heterogeneous assemblage of low mountain ranges,
rolling uplands, and alluvium-floored lowlands that de-
cline in average altitude and relief westward from the
Canadian border to the Bering and Chukchi Seas. Alti-
tudes of uplands and mountains rarely exceed 6,000
feet in the east and 4,000 feet in the west, and are
generally below 3,000 feet.

The Pacific Mountain System is an arcuate belt of
high mountains that borders the Pacific: Ocean. Gen-
erally, the system consists of two ranges of mountains
and a belt of intervening lowlands. The northern range
is represented in Alaska by the Aleutian Range, the
Alaska Range, and the Coast Mountains; and the
southern range, by the Kodiak, Kenai-Chugach,
Baranof, and Prince of Wales Mountains. Intervening
lowlands include the Cook Inlet-Susitna Lowland, the
Copper River Lowland, and the Kupreanof Lowland.
The northern and southern ranges come together and
culminate in the St. Elias Mountains. Relief in the
Pacific Mountain System is extreme: the lowlands lie
near or even below sea level and the mountains rise to
altitudes of 10,000-20,000 feet.

The Intermontane Plateaus are drained by the
Yukon River, the largest river in Alaska, and by other

streams flowing to the Bering and Chukchi Seas. The
divide between the Bering and Pacific drainages fol-
lows in part the northern ridge of the Pacific Mountain
System and is in part in the depressions south of that
ridge. The divide between the Bering-Chukchi and
Arctic Ocean drainage lies mostly in the Brooks Range,
the main part of the Rocky Mountain System in
Alaska. The major rivers and their drainage basins are
shown in figure 6.

INTERIOR PLAINS
ARCTIC COASTAL PLAIN (1)

General topography.-The Arctic Coastal Plain is a
smooth plain rising imperceptibly from the Arctic
Ocean to a maximum altitude of 600 feet at its southern
margin. The coastline makes little break in the profile
of the coastal plain and shelf, and the shore is generally
only 1-10 feet above the ocean; the highest coastal
cliffs are only 50 feet high. The Arctic Coastal Plain
province is divided into the Teshekpuk (1a) and White
Hills (1b) sections. Scattered groups of low hills rise
above the plain in the White Hills section; the
Teshekpuk section is flat. Locally, an abrupt scarp
50-200 feet high separates the coastal plain from the
Arctic Foothills. Locally pingos are sufficiently abun-
dant to give an undulatory skyline. The part of the
coastal plain between the Kuk and Colville Rivers has
scattered longitudinal sand dunes 10-20 feet high
trending N. 55°%-75° E.

Drainage.-The Arctic Coastal Plain is very poorly
drained and consequently is very marshy in summer.
It is crossed by rivers which head in highlands to the
south. Rivers west of the Colville River meander slug-
gishly in valleys incised 50-300 feet; those east of the
Colville cross the plain in braided channels and are
building deltas into the Arctic Ocean.

Lakes.-The Teshekpuk Lake section of the Arctic
Coastal Plain province is covered by elongated thaw
lakes oriented N. 15° W.; these range from a few feet
to 9 miles long, are from 2 to 20 feet deep, and are oval
or rectangular in shape (pl. 4, fig. 3). The lakes expand
about 1 meter per year in places, and several genera-
tions of drained lake basins may be seen.

Glaciers and permafrost.-There are no glaciers. The
entire land area is underlain by permafrost at least
1,000 feet thick. The permafrost table (base of zone of
summer thaw) is 4-4 feet below the surface. A net-
work of ice-wedge polygons covers the coastal plain
(pl. 2, fig. 2). These are oriented parallel and perpen-
dicular to receding shorelines because of stress differ-
ences set up by horizontal temperature gradients.
Random polygons form in areas of more uniform
stress.

19

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Geology.-The Teshekpuk Lake section is underlain
by 10-150 feet of unconsolidated Quaternary marine
sediments resting on nearly flat Cretaceous sedimentary
rocks containing coal. The White Hills section con-
tains, in addition, lower Tertiary sedimentary deposits.

References.-Black (1952, 1955); Black and Barks-
dale (1949); W. P. Brosgé (oral commun., 1959) ;
Chapman and Sable (1960); Coulter and others
(1960); Lachenbruch (1960a, 1960b, 1962); A. H.
Lachenbruch (oral commun., 1962) ; Leffingwell (1919,
p. 150-155, 211); W. W. Patton, Jr. (oral commun.,
1959) ; Porsild (1938) ; Smith and Mertie (1930) ; I. L.
Tailleur (oral commun., 1959).

The Arctic Coastal Plain is covered by the following
1:250,000 topographic maps: Barrow, Barter Island,
Flaxman Island, Beechy Point, Harrison Bay, Teshek-
puk, Meade River, Wainwright, Demarcation Point,
Mount Michelson, Sagavanirktok, Umiat, Ikpikpuk
River, Lookout Ridge, Utukok River, and Point Lay.

ROCKY MOUNTAIN SYSTEM
ARCTIC FOOTHILLS (2)

General topography.-The Arctic Foothills consist
of rolling plateaus and low linear mountains; they are
divided into two sections. The northern section (2a)
rises from an altitude of 600 feet on the north to 1,200
feet on the south and has broad east-trending ridges,
dominated locally by mesalike mountains. The southern
section (2b) is 1,200-3,500 feet in altitude, has local
relief of as much as 2,500 feet, and is characterized by
irregular buttes, knobs, mesas, east-trending ridges
and intervening gently undulating tundra plains (pl. 3,
figs. 3 and 4).

Drainage.-The Arctic Foothills are crossed by
north-flowing rivers from sources in the Brooks Range.
The Colville River, the largest stream, has an anomal-
ous east-trending course for more than 220 miles along
the boundary between the northern and southern sec-
tions. Most streams have swift, braided courses across
broad gravel flats that are locally covered in winter
with extensive sheets of aufeis, or anchor ice, that
freezes to the riverbeds; this filling of the channels
causes the streams to flood their gravel flats.

Lakes.-A few thaw lakes are present in the river
valleys and on some divides. The upper valleys of
major rivers from the Brooks Range contain many
morainal lakes.

Glaciers and permafrost.-There are no glaciers. The
entire province is underlain by permafrost. Ice wedges,
stone stripes, polygonal ground, and other features
of a frost climate are common.

Geology.-The northern section is underlain by
Cretaceous sedimentary rocks deformed into long

PHYSIOGRAPHIC DIVISIONS OF ALASKA

linear folds of the Appalachian type. Unequal erosion
of layers of rock that differ in hardness has produced
the linear-ridge topography. The southern section is
underlain by diverse sedimentary rocks of Devonian to
Cretaceous age together with mafic intrusions, all
tightly folded and overthrust to the north. A pre-
glacial gravel-covered pediment surface is preserved on
some divides between north-flowing rivers. Hummocky
morainal ridges border most valleys issuing from the
central Brooks Range.

References.-Chapman and Sable (1960); J. T.
Dutro (oral commun., 1959) ; Keller and others (1961) ;
W. W. Patton, Jr. (written commun., 1959); Payne
and others (1951) ; Schrader (1904, p. 45-46) ; Smith
and Mertie (1930); I. L. Tailleur (written commun,
1959). f

The Arctic Foothills are covered by the following
1:250,000 topographic maps: Meade River, Wain-
wright, Demarcation Point, Mount Michelson, Saga-
vanirktok, Umiat, Ikpikpuk River, Lookout Ridge,
Utukok River, Point Lay, Philip Smith Mountains,
Chandler Lake, Killik River, Howard Pass, Misheguk
Mountain, De Long Mountains, Point Hope, and
Noatak.

ARCTIC MOUNTAINS PROVINCE

The Arctic Mountains province consists of moun-
tains and hills carved chiefly from folded and over-
thrust Paleozoic and Mesozoic sedimentary rocks. It is
divided into the following sections: De Long Moun-
tains (3), Noatak Lowlands (4), Baird Mountains (5),
central and eastern Brooks Range (6), and Ambler-
Chandalar Ridge and Lowland section (7).

DE LONG MOUNTAINS (3)

General topography.-The central part of the De
Long Mountains consists of rugged glaciated ridges
that are 4,000-4,900 feet in altitude and have a local
relief of 1,500-3,000 feet. Narrow even-crested ridges
in the lower eastern and western parts rise to 3,000-
4,000 feet. Many passes about 3,500 feet in altitude
cross the range. The north boundary with the Arctic
Foothills is irregular and indistinct, but the south
front is abrupt. ~

Drainage.-Streams from the De Long Mountains
flow south and west to the Noatak River and the
Chukchi Sea and north to the Arctic Ocean. The
drainage divide is at the north edge of the mountains.
Asymmetry of passes, barbed drainage, wind gaps,
perched tributaries, and abandoned valley systems sug-
gest that the divide has moved northward by stream
capture.

Lakes, glaciers, and permafrost.-There are no lakes
or glaciers in the De Long Mountains. The entire sec-
tion is underlain by permafrost.

ARCTIC MOUNTAINS

Geology.-The De Long Mountains consist of folded
! and faulted sedimentary rocks of Devonian to Creta-
ceous age, intruded by massive diabase sills that are the
chief cliff-forming units; structural trends are west-
ward in the eastern and northern mountains and
change to southwestward in the southwestern part. The
eastern and northern De Long Mountains are a great
sheet thrust north over the rocks of the Artic Foothills.

References.-Mangus and others (1950) ; Smith and
Mertie (1930) ; I. L. Tailleur (written commun., 1959).

The De Long Mountains are covered by the follow-
ing 1: 250,000 topographic maps: Howard Pass, Mish-
eguk Mountain, and De Long Mountains.

NOATAK LOWLANDS (4)

General topography.-Two broad lowlands sur-
rounded by hills and separated by a rolling upland lie
along the Noatak River. The Mission Lowland (4a) is
a broad tundra flat, containing thaw lakes and pingos
25-300 feet high and crossed by the forested flood plain
of the Noatak River; it merges with the surrounding
foothills by silt uplands intricately dissected by thaw
sinks. The Aniuk Lowland (4b) is an irregular rolling
plain that slopes gradually upward on the south to
merge with a subsummit upland in the Baird Moun-
tains. The intervening upland is the Cutler River
Upland (4c).

Drainage.-The two lowlands and the Cutler River
Upland are drained entirely by the Noatak River,
which rises in the western part of the Schwatka Moun-
tains. The Noatak crosses the Cutler River Upland
and the Igichuk Hills south of the Mission Lowland
by narrow cliffed gorges a few hundred feet deep.

Lakes.-The Mission Lowland has numerous thaw
lakes. There are scattered morainal and thaw lakes
in the Aniuk Lowland.

Glaciers and permafrost.-There are no glaciers. The
entire section is underlain by permafrost, and pingos
abound in the Mission Lowland.

Geology.-Bedrock geology beneath the lowlands is
probably similar to that of surrounding uplands and
mountains. The entire valley of the Noatak was prob-
ably glaciated in pre-Wisconsin time, but glaciers of
Wisconsin time occupied only part of the Aniuk Low-
land and reached only the north edge of the Mission
Lowland. The depth of alluvial fill in the lowlands is
unknown. Rounded gravel is reported 850 feet above
the Noatak in the Cutler River Upland, and the course
of the Noatak across the upland may be superposed.

References.-Smith (1913, p. 27-33, 92-93); Smith
and Mertie (1930); I. L. Tailleur (written commun.,
1949).

The Noatak Lowlands are covered by the following
1: 250,000 topographic maps: Howard Pass, Misheguk

PROVINCE 21

Mountain, De Long Mountains, Ambler River, Baird
Mountains, and Noatak.

BAIRD MOUNTAINS (5)

General topography.-Moderately rugged mountains
having rounded to sharp summits 2,500-3,000 feet in
altitude rise abruptly from lowlands on the south and
west to a subsummit upland along the crest of the
Baird Mountains. This subsummit upland slopes gently
northward and merges with the Aniuk Lowland and
Cutler River Upland. Scattered groups of higher
mountains (3,500-4,500 feet in altitude) rise above
the subsummit upland ; they were centers of glaciation
in Pleistocene time. The indistinct boundary with the
Schwatka Mountains on the east is drawn where the
relief increases abruptly eastward.

Drainage.-The Baird Mountains are drained by
streams that flow north to the Noatak River and south
to the Kobuk River. The south-flowing streams head
in narrow ravines having steep headwalls, several
hundred feet high, incised in broad flat passes that are
the beheaded parts of north-draining valleys. This re-
lationship indicates that the divide is migrating to the
north by headward erosion.

Lakes and glaciers.-There are no lakes or glaciers in
the Baird Mountains.

Geology.-Schist, quartzite, and limestone of Paleo-
zoic age make up most of the Baird Mountains. Struc-
tural trends are eastward and the internal structure is
probably anticlinorial. Differential erosion involving
limestone and volcanic rocks of a northeast-trending
anticline along the northwest border of the mountains
has produced prominent northeast-trending ridges.

References.-Smith and Mertie (1930) ; I. L. Tail-
leur (written commun., 1959).

The Baird Mountains are covered by the following
1:250,000 topographic maps: Ambler River, Baird
Mountains, and Noatak.

CENTRAL AND EASTERN BROOKS RANGE (6)

General topography.-The central and eastern
Brooks Range is a wilderness of rugged glaciated east-
trending ridges that rise to generally accordant sum-
mits 7,000-8,000 feet in altitude in the northern part
and 4,000-6,000 feet in altitude in the southern part.
The easterly grain of the topography is due to belts of
hard and soft sedimentary and volcanic rocks. The
mountains have cliff-and-bench slopes characteristic of
glacially eroded bedded rocks. Abrupt mountain fronts
face foothills and lowlands on the north (pl. 3, figs. 1,
2, and 3, pl. 4, fig. 4).

Drainage.-The drainage divide between the Bering
Sea and Arctic Ocean drainages is near the north edge
of the range west of long. 149° W. and in the center
of the range east of long. 149° W. The major rivers

22 PHYSIOGRAPHIC DIVISIONS OF ALASKA

flow north to the Arctic Ocean and south to the Yukon,
Koyukuk, and Kobuk Rivers in flat-floored glaciated
valleys 4-2 miles wide; they have a broad dendritic
pattern. Minor tributaries flow east and west parallel
to the structure, superposing a trellised pattern on the
dendritic pattern of the major drainage.

Lakes.-Large rock-basin lakes lie at the mouths of
several large glaciated valleys on the north and south
sides of the range. The Brooks Range in general is
characterized by a paucity of lakes for a glaciated area.

Glaciers.-Small cirque glaciers are common in the
higher parts of the range, in the Schwatka Mountains
(6a), and in mountains around Mount Doonerak. The
firn line is at an altitude of about 6,000 feet in north-
facing cirques and about 8,000 feet in south-facing
cirques. Valley glaciers 6 miles long are fed from
cirques and small icecaps in the Romanzof Mountains
(6b).

Geology.-The central and eastern Brooks Range is
composed chiefly of Paleozoic limestone, shale, quart-
zite, slate, and schist. Northeast of the Sagavanirktok
River the Paleozoic rocks are in faulted folds over-
turned to the north. Elsewhere, they are in giant plates
or nappes thrust to the north. The deformation is of
Laramide age. The north front of the range is made
of light-colored cliff-forming Mississippian limestone
(see pl. 3, figs. 2 and 3). Rocks south of lat. 68° N.
are metamorphosed and generally equivalent in age to
those farther north. Granitic intrusions underlie the
higher parts of the Schwatka Mountains (62) and
Romanzof Mountains (6b), both of which rise to
8,500-9,000 feet in altitude.

References.-Brosgé (1960; oral commun., 1959) ;
Brosgé and others (1952); Chapman and Eberlein
(1951); R. L. Detterman (oral commun., 1959) ;
Detterman and others (1958) ; Keller and Detterman
(1951) ; Keller and Morris (1952) ; Keller and others
(1961); Mangus (1953); Mangus and others (1950) ;
W. W. Patton, Jr. (oral commun., 1959) ; Patton and
others (1951); Payne and others (1951); Schrader
(1904); Smith (1913, p. 32-35); Smith and Mertie
(1930) ; I. L. Tailleur (oral commun., 1959).

The central and eastern Brooks Range is covered by
the following 1: 250,000 topographic maps: Demarca-
tion Point, Mount Michelson, Sagavanirktok, Table
Mountain, Arctic, Philip Smith Mountains, Chandler
Lake, Killik River, Howard Pass, Christian, Chanda-
lar, Wiseman, Survey Pass, and Ambler River.

AMBLER-CHANDALAR RIDGE AND LOWLAND SECTION (7)
General topography.-This section consists of one

or two east-trending lines of lowlands and low passes

3-10 miles wide and 200-2,000 feet above sea level,

bordered on the north by the abrupt front of the

Brooks Range. Along the south side is a discontinuous
line of rolling to rugged ridges, 25-75 miles long and
5-10 miles wide, rising to 3,000-4,500 feet in altitude.
Some of these ridges were intensely glaciated. Within
the lowlands are east-trending ridges 5-10 miles long.

Drainage.-The western part of the section is
drained by tributaries of the Kobuk River; the central
part, by the Koyukuk River and its tributaries; and the
eastern part, by the Chandalar River. Most streams
flow south out of the Brooks Range across both the low-
lands and the ridges to lowlands farther south. The
drainage was probably superposed but may have been
disoriented later by glaciers. The Chandalar River
flows east along the eastern part of the trough.

Lakes.-Several large lakes fill ice-carved rock basins
in deep narrow canyons across the southern ridge.
Areas of ground and end moraines contain many
ponds. The flood plains of the major streams have thaw
lakes and oxbow lakes.

Glaciers and permafrost.-The section contains no

. glaciers but is underlain by continuous permafrost.

Geology.-The ridges are composed in part of re-
sistant massive greenstone (metamorphosed basalt)
of Mesozoic(?) age. The lowlands are underlain
largely by Cretaceous sedimentary rocks, folded into
synclines. Pleistocene glaciers from the Brooks Range
extended across the lowland and through passes in the
line of ridges.

References.-Brosgé (1960; oral commun., 1959) ;
W. W. Patton, Jr., (oral commun., 1959) ; I. L. Tail-
leur, (oral commun., 1959) ; Smith and Mertie, (1930).

The Ambler-Chandalar Ridge and Lowland section
is covered by the following 1:250,000 topographic
maps: Chandalar, Wiseman, Survey Pass, Ambler
River, Bettles, Hughes, and Shungnak.

INTERMONTANE PLATEAUS
NORTHERN PLATEAUS PROVINCE

The Northern Plateaus province consists of uplands
and lowlands carved chiefly in Paleozoic and Precam-
brian(?) rocks. It is divided into the following see-
tions: Porcupine Plateau (8), Old Crow Plain (9),
(not described), Ogilvie Mountains (10), Tintina
Valley (Eagle Trough) (11), Yukon-Tanana Upland
(12), Northway-Tanacross Lowland (13), Yukon Flats
section (14), Rampart Trough (15), and Kokrine-
Hodzana Highlands (16).

PORCUPINE PLATEAU (8)
General topography.-Low ridges having gentle
slopes and rounded to flat summits 1,500-2,500 feet in
altitude dominate the topography of the Porcupine
Plateau; a few domes and mountains rise to 3,500 feet.
Valley floors are broad; and valley patterns, irregular,
having many imperceptible divides. Thazzik Mountain

NORTHERN PLATEAUS PROVINCE 23

(8a) in the extreme west, a rugged glaciated mountain
group, rises to 5,800 feet.

Drainage.-The entire plateau, except the extreme
northeastern part, is drained by tributaries of the
Yukon River. The Chandalar, Sheenjek, and Coleen
Rivers rise in the Brooks Range and flow south across
the plateau in broad valleys floored with moraines and
outwash terraces. The Porcupine River crosses the
plateau in a narrow cliff-lined canyon 50-500 feet deep.
The Black and Little Black Rivers, which drain the
southeastern part of the area, meander through broad
irregular flats.

Lakes.-A few moraine-dammed lakes lie in gla-
ciated passes and valleys along the north margin of the
plateau. The largest of these is Old John Lake, 5 miles
long and 2 miles wide. Scattered thaw lakes occur in
lowlands and low passes.

Glaciers and permafrost.-There are no glaciers.
The entire section is underlain by ¢ontinuous perma-
frost.

Geology.-The northern part is underlain by crystal-
line schist, granite, quartzite, slate, and mafic rocks,
probably mostly Paleozoic in age ; the southeastern part
is underlain by moderately deformed Paleozoic and
Mesozoic sedimentary rocks. Basinlike areas of Ter-
tiary rocks and flat-lying Cenozoic flows occur along
the Porcupine River.

References.-Bostock (1948); W. P. Brosgé (oral
commun., 1959) ; FitzGerald (1944); Kindle (1908) ;
Mertie (193802).

The Porcupine Plateau is covered by the following
1: 250,000 topographic maps: Table Mountain, Arctic,
Coleen, Christian, Chandalar, Black River, and Char-
ley River.

OGILVIE MOUNTAINS (10)

General Topography.-The Ogilvie Mountains have
sharp crestlines, precipitous slopes, and deep narrow
valleys; they rise to 5,000 feet in altitude, and local
relief is as much as 4,000 feet. The ridges are inter-
connected and passes are few. The narrow valleys are
interrupted by gorges where rivers cross cliff-forming
layers of rock.

Drainage.-The Ogilvie Mountains are drained by
the Kandik, Nation, and Tatonduk Rivers, all tribu-
taries of the Yukon River.

Lakes.-Small thaw lakes and oxbow lakes are along
the Yukon River in the southern part of the moun-
tains.

Glaciers and permafrost.-There are no glaciers.
Most of the section is underlain by permafrost, and
pingos are common.

Geology.-Moderately folded and faulted sedimen-
tary and volcanic rocks ranging in age from late Pre-
cambrian to Cretaceous make up the mountains. Some

formations of limestone, dolomite, quartzite, and green-
stone are in massive cliff-forming beds. Continental
sandstone and conglomerate of Late Cretaceous and
early Tertiary (?) age underlie a lowland on the upper

Nation River.

References.-Bostock (1948, p. 59); Brabb (1962,
oral commun., 1962) ; Mertie (1932).

The Ogilvie Mountains are covered by the following
1: 250,000 topographic maps : Charley River and Eagle.

TINTINA VALLEY (11)
General topography.-The Tintina Valley is a nar-
row belt of low country consisting of low rounded
ridges and open valleys, which pass northwest into
loess-covered terraces and a lake-dotted plain that
connects with the Yukon Flats section. Relief in the
southeastern part (southeast of Woodchopper Creek)
is 1,000-1,500 feet, and ridges rise to 2,000-2,500 feet
in altitude. Discontinuous low hills on the north sepa-
rate this part from the Yukon River, and the moun-
tains of the Yukon-Tanana Upland rise gradually
above it on the southwest.

The northwestern part, around Circle Hot Springs
and Birch Creek, is a lake-dotted plain that slopes
gradually upward to the base of surrounding hills, and
that is separated from the Yukon River by a flat-
topped gravel ridge about 400 feet high and 3-4 miles
wide.

Drainage. -The southeastern part in Alaska is
drained chiefly by small north-flowing streams that
rise in the upland to the south and have superposed
courses to the Yukon River in narrow valleys across
hills of resistant rocks on the north. The northwestern
part is drained largely by Birch Creek, which flows
parallel to the Yukon into the Yukon Flats.

Lakes.-There are no large lakes in the southeastern
part. The plain of the northwestern part has several
thaw lakes; the largest, Medicine Lake, is nearly 2
miles across.

Glaciers and permafrost.-The section contains no
glaciers but is generally underlain by permafrost.

Geology.-The Tintina Valley is underlain by a syn-
clinal belt of highly deformed, easily eroded conti-
nental sedimentary rocks of Cretaceous and Tertiary
age that are probably in sedimentary contact with the
metamorphic and granitic rocks of the Yukon-Tanana
Upland on the south and with the well-consolidated
Paleozoic and Mesozoic sedimentary rocks of the Ogil-
vie Mountains on the north. Northwest of Woodchop-
per Creek, loess mantles flat-topped ridges and hides
most bedrock outcrops. The geology beneath the plain
near Circle Hot Springs is unknown.

References.-Brabb (1962; oral commun., 1962) ;
Mertie (1930b, 1937, and 1942).

24 PHYSIOGRAPHIC DIVISIONS OF ALASKA

The Tintina Valley is covered by the following
1:250,000 topographic maps: Charley River, Eagle,
and Circle.

YUKON-TANANA UPLAND (12)

General topography.-The Yukon-Tanana Upland
is the Alaskan equivalent of the Klondike Plateau in
Yukon Territory. Rounded even-topped ridges with
gentle side slopes characterize this section of broad un-
dulating divides and flat-topped spurs (pl. 3, figs. 6
and 9). In the western part (12a) these rounded ridges
trend northeast to east; they have ridge-crest altitudes

of 1,500-3,000 feet and rise 500-1,500 feet above adja- .

cent valley floors. The ridges are surmounted by com-
pact rugged mountains 4,000-5,000 feet in altitude.
Ridges in the eastern part (12b) have no preferred di-
rection, are 3,000-5,000 feet in altitude but have some
domes as high as 6,800 feet, and rise 1,500-3,000 feet
above adjacent valleys (pl. 3, fig. 6). In the extreme
northeast the ridges rival the Ogilvie Mountains in
ruggedness. Valleys in the western part are generally
flat, alluvium floored, and 4-1 mile wide to within a
few miles of headwaters. Streams in the eastern part
that drain to the Yukon flow in narrow V-shaped ter-
raced canyons, but the headwaters of the Fortymile
and Ladue Rivers are broad alluvium-floored basins.

Drainage.-The entire section is in the Yukon drain-
age basin. Streams flow south to the Tanana River and
north to the Yukon River. Most streams in the western
part follow courses parallel to the structural trends of
bedrock, and several streams have sharp bends involv-
ing reversal of direction around the ends of ridges of
hard rock. Drainage divides are very irregular. Small
streams tend to migrate laterally southward (pl. 4,
fg. 5).

Lakes.-The few lakes in this section are mainly
thaw lakes in valley floors and low passes.

Glaciers and permafrost.-There are no glaciers.
The entire section is underlain by discontinuous perma-
frost. Periglacial mass-wasting is active at high alti-
tudes, and ice wedges lace the frozen muck of valley
bottoms.

Geology.-A belt of highly deformed Paleozoic
sedimentary and volcanic rocks containing conspicuous
limestone units, overthrust and overturned to the
north, extends along the north side of the upland. The
rest of the upland is chiefly Precambrian (?) schist and
gneiss but has scattered small elliptical granitic in-
trusions in the northwestern part; large irregular
batholiths make up much of the southeastern part. In
the western part a thick mantle of windborne silt lies
on the lower slopes of hills, and thick accumulations
of muck overlie deep stream gravels in the valleys.
Alluvial deposits of gold and other metals abound

throughout the upland. Pingos are common in valleys
and on lower hill slopes (pl. 2, fig. 4).

References.-Bostock (1948, p. 59-73) ; Holmes and
others (1965); D. M. Hopkins (oral commun., 1959) ;
Mertie (1937); Péwé (1955a, 1955b, 1958) ; Prindle
(1905, p. 17); Williams (1959); Williams and others
(1959).

The Yukon-Tanana Upland is covered by the fol-
lowing 1:250,000 topographic maps: Charley River,
Circle, Livengood, Tanana, Eagle, Big Delta, Fair-
banks, Kantishna River, Tanacross, and Mount Hayes.

NORTEWAY-TANACROSS LOWLAND (13)

General - topography.-The - Northway-Tanacross
Lowland consists of three small basins, separated by
screens of low rolling hills The two basins along the
north side of the lowland are nearly level plains,
broadly oval in plan. Scattered longitudinal dunes
mark the floor of the eastern one of these two basins.
The third basin, on the southeast, is a gently rolling
moraine-covered plain.

Drainage.-The entire lowland is drained by the
Tanana River, which may have captured the lowland
in early Pleistocene time, for the drainage divide with
the Yukon River is only 2-5 miles north of the Tanana
and the north tributaries of the Tanana are steep
barbed streams. The headwaters of the Yukon drain-
age north of the divide are underfit streams in broad
valleys that head in wind gaps.

The main tributaries of the Tanana rise in glaciers
in mountains to the south, and their deposits of out-
wash have pushed the Tanana against the north side
of the lowland. The upper courses of these streams
are swift and braided; their lower courses and the
course of the Tanana are sluggish and meandering.

Lakes.-Large lakes in reentrants in the surround-
ing hills may be caused by alluviation of the lowland.
Thaw lakes abound in areas of fine alluvium, which are
as much as 70 percent lake surface. Oxbow lakes and
morainal ponds are also present.

Glaciers and permafrost.-The lowland has no gla-
ciers; it is in the area of discontinuous permafrost.

Geology.-The basins are mantled with outwash
gravel, silt, and morainal deposits. The two northern
basins were probably occupied by a lake dammed by
a glacier at Cathedral Rapids. Tertiary rocks have
been reported on the north side and may extend be-
neath the Quaternary deposits, Bedrock hills are Pre-
cambrian(?) schist and Mesozoic granitic intrusions.
The Taylor Highway immediately north of Tetlin
Junction passes through a field of stabilized sand dunes.

References.-D. F. Barnes (oral commun., 1956) ;
Black (1951, p. 98-100); Mertie (1937, pl. 1) ; Moffit
(1954a, pl. 6) ; Wallace (1946).

NORTHERN PLATEAUS PROVINCE 25

The Northway-Tanacross Lowland is covered by the
following 1:250,000 topographic maps: Tanacross,
Mount Hayes, and Nabesna.

YUKON FLATS SECTION (14)

General topography.-The central part of the Yukon
Flats section consists of marshy lake-dotted flats rising
from 300 feet in altitude on the west to 600-900 feet
on the north and east. The northern part of the flats is
made up of gently sloping outwash fans of the Chan-
dalar, Christian, and Sheenjek Rivers; the southeastern
part of the flats is the broad gentle outwash fan of the
Yukon River. Other areas are nearly flat flood plains.
Rolling silt- and gravel-covered marginal terraces hav-
ing sharp escarpments 150-600 feet high rise above the
flats and slope gradually upward to altitudes of about
1,500 feet at the base of surrounding uplands and
mountains. Their boundaries with surrounding up-
lands and mountains are gradational.

Drainage.-The Yukon Flats section is drained by
the Yukon River, which has a braided course south-
east of the bend at Fort Yukon and a meandering
course, containing many sloughs, southwest of the bend
at Fort Yukon. Most tributaries rise in surrounding
uplands and mountains and have meandering courses
through the flats.

Lakes.-Thaw lakes are abundant throughout the

flats. Thaw lakes and thaw sinks are common on the.

marginal terraces.

Glaciers and permafrost.-There are no glaciers.
Permafrost probably underlies most of the section ex-
cept rivers, recently abandoned meander belts, and
large thaw lakes.

Geology.-Escarpments bounding the Yukon Flats
expose well-consolidated or crystalline rocks of Paleo-
zoic and possibly Mesozoic age. The marginal terraces
are capped with gravel on which rests a layer of wind-
borne silt. A well drilled at Fort Yukon in 1954 dis-
closed 48 feet of aeolian sand of late Pleistocene or
Recent age, underlain by 100 feet of sandy gravel of
Pleistocene age, underlain in turn by at least 292 feet
of fine lake sediments of late Pliocene or early Pleisto-
cene age. On the basis of this well it is thought that
the Yukon Flats are the site of a late Tertiary lake
that occupied a downwarped basin.

References.-Williams (1955, p. 124-125;
written communs., 1955-1959).

The Yukon Flats section is covered by the following
1: 250,000 topographic maps: Coleen, Christian, Black
River, Fort Yukon, Beaver, Charley River, Circle, and
Livengood.

1960;

RAMPART TROUGKH (15)
General topography.-The Rampart Trough is a
structurally controlled depression having gently roll-

ing topography 500-1,500 feet in altitude; it is incised
500-2,500 feet below highlands on either side. Ter-
races on tributaries of the Yukon River near Rampart
are 20 feet, 100 feet, 150 feet, 250 feet, and 500 feet
above stream level.

Drainage.-The Yukon River enters the east end of
the trough through a narrow rocky gorge and swings
in broad bends from one side of the trough to the other
within a narrow flood plain. Near the southwest end,
a ridge of hard rock separates the Yukon River from
the trough. Short tributaries rise in hills to the south,
and flow across the trough and through the bedrock
ridge on its north side to the Yukon. The Yukon and
its tributaries appear to be superposed from a surface
at least 1,500 feet in altitude.

Lakes.-Scattered thaw lakes lie on the Yukon flood
plain and elsewhere in the trough.

Glaciers and permafrost.-The Rampart Trough
contains no glaciers. Permafrost underlies all the low-
land except the Yukon flood plain.

Geology.-The Rampart Trough was eroded along
a tightly folded belt of soft continental coal-bearing
rocks of Tertiary age. Hard rock hills and the sur-
rounding uplands are partly metamorphosed sedimen-
tary and volcanic rocks of Mississippian age that strike
about N. 60° E. and are cut by granitic intrusions.

References.-D. M. Hopkins (oral commun., 1959) ;
Mertie (1937, p. 174-185) ; R. A. Paige (written com-
mun., 1958).

The Rampart Trough is covered by the following
1: 250,000 topographic maps: Livengood and Tanana.
KOKRINE-HODZANA HIGHLANDS (16)

General topography.-The Kokrine-Hodzana High-
lands consist of even-topped rounded ridges rising to
2,000-4,000 feet in altitude surmounted by isolated
areas of more rugged mountains. A rugged compact
highland in the northeastern part has many peaks be-
tween 4,500 and 5,700 feet in altitude. The Ray Moun-
tains (16a), rising to 5,500 feet, have cirques and gla-
ciated valleys and craggy cliffed tors that rise abruptly
from broad granite ridgetops. Valleys have alluviated
floors to within a few miles of their heads.

Drainage.-The irregular drainage divide between
the Yukon River and its large tributary, the Koyukuk
River, passes through these highlands. Drainage to
the Yukon is by way of the Hodzana, Tozitna, Melo-
zitna and Dall Rivers and many shorter streams.
Drainage to the Koyukuk is by the Kanuti River and
the South Fork of the Koyukuk.

Lakes. There are a few thaw lakes in the lowland
areas and a few lakes in north-facing cirques in the
Kokrine Hills (16h) and the Ray Mountains.

26 PHYSIOGRAPHIC DIVISIONS OF ALASKA

Glaciers and permafrost.-There are no glaciers.
The entire section is probably underlain by permafrost.
This section contains classic examples of altiplanation
terraces, stone polygons, and other periglacial phe-
nomena.

Geology.-The highlands are underlain chiefly by
Paleozoic and schist and gneiss hav-
ing a northeast-trending structural grain, cut by sev-
eral granitic intrusions, the largest of which is the
granite batholith that upholds the Ray Mountains.
Small placers of tin and gold occur in the southern
part of the highlands.

References.-Eakin (1916); Maddren (1913); J.
R. Williams (written commun., 1959).

The Kokrine-Hodzana Highlands are covered by the
following 1:250,000 topographic maps: Chandalar,
Wiseman, Beaver, Bettles, Livengood, Tanana, Melo-
zitna and Ruby.

WESTERN ALASKA PROVINCE

The Western Alaska province consists of uplands and
lowlands underlain chiefly by folded and faulted Cre-
taceous rocks. It is divided into the following sections :
Kanuti Flats (17), Tozitna-Melozitna Lowland (18),
Indian River Upland (19), Pah River section (20),
Koyukuk Flats (21), Kobuk-Selawik Lowland (22),
Selawik Hills (23), Buckland River Lowland (24),
Nulato Hills (25), Tanana-Kuskokwim Lowland (26),
Nowitna Lowland (27), Kuskokwim Mountains (28),
Innoko Lowlands (29), Nushagak-Big River Hills
(30), Holitna Lowland (31), and Nushagak-Bristol
Bay Lowland (32).

KANUTI FLATS (17)

General topography.-The Kanuti Flats form an ir-
regular shaped lake-dotted plain 400-1,000 feet in alti-
tude that merges with low surrounding hills. Scattered
low irregular hills rise in the central part of the plain,
which is crossed by the forest-covered meander belts
of the Koyukuk and Kanuti Rivers.

Drainage.-The Kanuti Flats are drained by the
Koyukuk River and its tributaries. The Kanuti River,
which drains the southern part of the plain, flows
through a narrow canyon in the Indian River Upland
before joining the Koyukuk River.

Lakes.-There are numerous thaw lakes, some as
large as 2 miles across. Some parts of the flats are more
than 50 percent lake surface.

Glaciers and permafrost.-The flats contain no gla-
ciers. The section is underlain by permafrost except
beneath large lakes, rivers, and recently formed flood
plains.

Geology.-The geology of the Kanuti Flats is un-
known.

References.-None.

The Kanuti Flats are shown on Bettles 1:250,000
topographic map.

TOZITNA-MELOZITNA LOWLAND (18)

General topography.- The Tozitna-Melozitna Low-
land is a long, narrow rolling plain, 5-10 miles wide,
at the heads of the Tozitna and Melozitna Rivers. The
pass between these streams is less than 1,000 feet in al-
titude.

Drainage.-The lowland is drained by the Tozitna
and Melozitna Rivers, which flow south from the low-
land in narrow gorges across the Kokrine-Hodzana
Highlands to the Yukon River.

Lakes.-The lowland contains numerous thaw lakes.
Oxbow lakes are common along the Melozitna River.

Glaciers and permafrost.-The section has no gla-
ciers; it is in the area of discontinuous permafrost.

Geology.-Nothing is known of the geology of this
lowland.

References.-None.

The Tozitna-Melozitna Lowland is covered by the
following 1:250,000 topographic maps: Tanana and
Melozitna.

INDIAN RIVER UPLAND (19)

General topography.-Groups of low gentle ridges
having rounded accordant summits at 1,500-2,000 feet
altitude are interspersed with irregular lowlands and
broad flat divides. The ridges in the southeastern part
are generally parallel and trend northeastward ; ridges
in the northwestern part have irregular trends. A few
mountains rise to 4,000 feet in altitude. The Koyukuk
and Kanuti Rivers cross the upland in narrow canyons
a few hundred feet deep.

Drainage.-Most of the Indian River Upland is
drained by the Koyukuk River and its tributaries. The
northwest corner drains to the Kobuk River and the
southeastern part drains by the Melozitna River to
the Yukon. Many of the streams have extremely ir-
regular courses.

Lakes.-Numerous thaw lakes, the largest 2%%4 miles
across, are in the lowlands, valleys, and broad passes.

Glaciers and permafrost.-There are no glaciers.
The entire land area, except recent flood plains, is
underlain by permafrost, and periglacial processes pre-
dominate. Altiplanation terraces are common at high
altitudes.

Geology.-The Indian River Upland is underlain
chiefly by folded sedimentary and volcanic rocks of
Mesozoic age, in which sandstone, shale, and conglom-
erate predominate. These rocks are intruded by small
granitic stocks, and are overlain by remnants of flat-
lying lavas of Tertiary or Quaternary age. Structural
trends are northeastward in the southeastern part but
are poorly defined in the northern part.

WESTERN ALASKA PROVINCE 27

References.-Cass (1959e) ; W. W. Patton, Jr. (oral
commun., 1959).

The Indian River Upland is covered by the follow-
ing 1:250,000 topographic maps: Bettles, Hughes,
Tanana, Melozitna, Kateel River, Ruby, and Nulato.

PAH RIVER SECTION (20)

General topography.-The Pah River section is an
area of diversified topography. Compact groups of
hills and low mountains 20-40 miles long and rising
to 4,000 feet in altitude are surrounded by rolling
plateaus 500-1,500 feet in altitude and broad lowland
flats 5-10 miles across. The lower parts of the moun-
tain groups consist of gently rounded ridges; their
higher glaciated parts contain broad, shallow cirques
having flaring walls (pl. 3, fig. 7).

Drainage.-The northern and western parts of the
Pah River section drain to the Selawik and Kobuk
Rivers. The southern and eastern parts drain via the
Huslia and Hogatza Rivers to the Koyukuk River.
The major streams meander sluggishly through the
broad lowlands. The Pah River, which drains the
Pah River flats (20b), flows north to the Kobuk
through a narrow canyon across the Lockwood Hills
(20a).

Lakes.-Numerous thaw lakes lie in the lowland flats.
The central part of the Pah River flats is probably 50
percent lake surface. (See pl. 3, fig. 7.) A few small
cirque lakes occur in the higher glaciated parts of the
Lockwood Hills and the Zane Hills (20c¢c).

Glaciers and permafrost.-There are no glaciers. The
entire section is underlain by permafrost, and peri-
glacial erosional processes predominate. Altiplanation
terraces are common below the level of glaciation in the
Zane Hills and the Purcell Mountains (20d).

Geology.-The Pah River section is underlain by
Mesozoic volcanic and sedimentary rocks that are
intensely deformed and locally contact metamorphosed,
without strong persistent structural grain, and by
Mesozoic granitic stocks and batholiths.

References.-W. W. Patton, Jr. (oral commun.
1959) ; I. L. Tailleur (oral commun., 1959).

The Pah River section is covered by the following
1: 250,000 topographic maps: Hughes and Shungnak.

KOYUKUK FLATS (21)

General topography.-The Koyukuk Flats form an
extensive lowland of irregular outline at the junction
of the Yukon and Koyukuk Rivers. The central parts
of the Koyukuk Flats are flat plains 5-20 miles wide,
along the major rivers. The parts immediately adja-
cent to the rivers are meander belts 5-10 miles wide;
the parts farther away are dotted by thaw lakes. Broad
rolling silt plains, in part mantled by dunes and in part
pocked by thaw sinks, stand 100-200 feet above these

central plains and merge imperceptibly with the sur-
rounding uplands. Several low bedrock hills rise from
the center of the lowland (pl. 4, fig. 7).

Drainage.-The Koyukuk Flats are drained by the
Yukon River and its tributaries. Streams meander
wildly across the lowland and have numerous meander-
ing side sloughs. Lateral migration of meanders is as
much as 75 feet per year, and elaborate patterns of
bars and swales (meander-scroll pattern) are left be-
hind (pl. 4, figs. 6 and 7).

Lakes.-The meander belt has innumerable narrow
meander-scroll lakes and some oxbow lakes; these are
generally silted by floods, and the newly formed ground
freezes perennially. Subsequently, thaw lakes form in
the frozen ground and pass through a complicated
cycle of enlargement, coalescence, and destruction by
infilling or drainage. These thaw lakes are abundant
away from the rivers (pl. 4, fig. 7).

Glaciers and permafrost.-No glaciers exist in the
flats. All the land area except recently formed flood
plains is underlain by permafrost.

Geology.-The bedrock hills and surrounding up-
lands are chiefly Cretaceous sedimentary rocks, older
Mesozoic volcanic rocks, and some intrusions. Low
basalt hills rise from the central part of the lowland.
The plains are underlain by water-laid and windborne
silt. Sand dunes are common; a large barren area of
active sand dunes lies in the northwestern part. North-
east-trending scarplets and low rises that cross the low-
land presumably mark active faults.

References.-Cass (1957, 1959e, 1959f); Eardley
(1938a, 1938b) ; Elias and Vosburgh (1946); W. W.
Patton, Jr. (oral commun., 1959) ; Péwé (1948).

The Koyukuk Flats are covered by the following
1: 250,000 topographic maps: Hughes, Shungnak, Me-
lozitna, Kateel River, Ruby, and Nulato.

KOBUK-SELAWIK LOWLAND (22)

General topography.-The Kobuk-Selawik Lowland
consists chiefly of broad river flood plains and lake-
dotted lowlands that pass at their seaward margins
into deltas. The Baldwin Peninsula, which separates
Hotham Inlet from Kotzebue Sound, is a rolling lake-
dotted lowland containing hills as high as 350 feet in
altitude, bordered by bluffs. The Waring Mountains
(22a) are an east-trending group of low rounded hills
less than 2,000 feet in altitude. The upper valley of
the Kobuk River is bordered by gravel and sand
terraces 100-200 feet above river level that are dotted
with thaw lakes and thaw sinks and, on the south side
of the river, have large areas of both stabilized and
active sand dunes.

Drainage.-The lowland is drained mainly by the
Kobuk and Selawik Rivers. Most streams are sluggish,

28 PHYSIOGRAPHIC DIVISIONS

meandering, of low gradient, and have numerous side
sloughs.

Lakes.-The area around the Selawik River, in par-
ticular, has numerous large thaw lakes. Hotham Inlet
and Selawik Lake are large bodies of water at sea level
that are kept nearly fresh by the great outflow of the
Selawik, Kobuk, and Noatak Rivers.

Glaciers and permafrost.-(Glaciers are absent. Most
land area is underlain by permafrost. Pingos are abun-
dant in the lowland around the Selawik River.

Geology.-Most of the lowland areas are underlain
by morainal deposits and by stream and lake deposits
of unknown thickness. Baldwin Peninsula is probably
the end moraine of a pre-Wisconsin glacial advance.
In Wisconsin time glaciers from the Brooks Range
sent tongues into the upper valley of the Kobuk but did
not advance farther. The Waring Hills are underlain
by Cretaceous sedimentary rocks.

References.-A. L. Fernald (written commun.,
1958) ; D. M. Hopkins (oral commun., 1959); W. W.
Patton, Jr. (oral commun., 1959); Smith (1913);
Smith and Mertie (1930); I. L. Tailleur (oral
commun., 1959).

The Kobuk-Selawik Lowland is covered by the fol-
lowing 1:250,000 topographic maps: Ambler River,
Baird Mountains, Noatak, Shungnak, Selawik, and
Kotzebue.

SELAWIK HILLS (23)

General topography.-The Selawik Hills are gentle
hills having rounded to flat summits as much as 3,300
feet in altitude. The hills rise fairly abruptly, with a
relatively straight scarp, from the Kobuk-Selawik
Lowland on the north and decline gently to the Buck-
land River Lowland on the south. Altiplanation ter-
races are common on the higher summits (pl. 2, fig. 1).

Drainage.-The hills are drained by short streams
that flow south and west to the Buckland River or
north to the Kauk River, Selawik River, and Selawik
Lake.

Lakes, glaciers, and permafrost.-There are no lakes
or glaciers. The entire section is underlain by perma-
frost.

Geology.-The Selawik Hills are underlain chiefly
by Paleozoic and Mesozoic metamorphosed volcanic
rocks and granitic intrusive rocks. Quaternary volcanic
rocks lie on the flanks.

References.-W . W. Patton (oral commun., 1959).

The Selawik Hills are covered by the following
1: 250,000 topographic maps: Selawik and Candle.

BUCKLAND RIVER LOWLAND (24)

General topography.-The Buckland River Lowland

is a rolling lowland having slopes of a few feet to a

OF ALASKA

few hundred feet per mile and consisting largely of the
original surfaces of lava flows.

Drainage.-The lowland is drained mostly by the
Buckland River. The Tagagawik River drains the ex-
treme eastern part, and the Koyuk River the southern
prong.

Lakes.-Small thaw and oxbow lakes are common
along the Buckland River and in other flat valleys.
A few thaw lakes lie on some flat interfluves.

Glaciers and permafrost.-There are no glaciers. The
entire section is probably underlain by permafrost.

Geology.-The lowland is underlain chiefly by flat-
lying lava flows of Quaternary age, mantled by a thick
layer of windborne silt.

References.-D. M. Hopkins (oral commun., 1959) ;
W. W. Patton, Jr. (oral commun., 1959).

The Buckland River Lowland is shown on the
1 : 250,000 topographic map of Candle quadrangle.

NULATO HILLS (2)

General topography.-The Nulato Hills consist, in
general, of northeast-trending even-crested ridges,
1,000-2,000 feet in altitude, having rounded summits
and gentle slopes. Valleys are narrow and have flat
floors that are generally trenched in their upstream
parts to depths of about 30 feet. Local relief is 500-
1,500 feet. The topography is relatively fine textured;
gullies are spaced 500-1,500 feet apart and second-
order tributaries are %%-1 mile apart (pl. 4, fig. 10).
Three highland areas of steeper ridges rise to about
4,000 feet in altitude.

Drainage.-Streams on the east side of the section
flow to the Yukon River and those on the west side to
Norton Sound. Major streams are markedly parallel,
flowing either northeast or southwest, and their courses
are eroded along northeast-trending fault zones. Valley
heads are generally connected by low passes along the
faults.

Lakes.-There are a few thaw lakes in the valleys.

Glaciers and permafrost.-There are no glaciers. The
entire section is probably underlain by permafrost.

Geology.-Almost all the hills are composed of
tightly folded sandstone, conglomerate, and shale of
Cretaceous age. The folds trend about N. 45° E. but
bend around to northward in the northern part. The
rocks are cut by northeast- and north-trending faults.
A few mountains are underlain by post-Cretaceous in-
trusive and volcanic rocks. Older rocks, chiefly of
volcanic origin, make up the hills in the extreme
northern part and extreme southern part.

References.-R. S. Bickel (oral commun., 1958) ;
Cass (1957, 1959a, 1959b, 1959c, 1959f) ; Harrington
(1918); Hoare and Coonrad (1959b); Patton and
Bickel (1956a, 1956b) ; Smith and Eakin (1911).

WESTERN ALASKA PROVINCE 20

The Nulato Hills are covered by the following
1 : 250,000 topographic maps: Shungnak, Kateel River,
Candle, Nulato, Norton Bay, Unalakleet, St. Michael,
Holy Cross, Kwiguk, Russian Mission, and Marshall.

TANANA-KUSKOKWIM LOWLAND (26)

General - topography.-The - Tanana-Kuskokwim
Lowland is a broad depression bordering the Alaska
Range on the north; its surfaces are of diversified
origin. Coalescing outwash fans from the Alaska
Range slope 20-50 feet per mile northward to flood
plains along the axial streams of the lowland. Rivers
from the range flow for a few miles at the heads of the
fans in broad terraced valleys 50-200 feet deep. Semi-
circular belts of morainal topography lie on the upper
ends of some fans. (See pl. 3, fig. 10 and pl. 4, fig. 13.)
The flood plains of the Kuskokwim and Kantishna
Rivers and of the Tanana River west of Tolovana are
incised 50-200 feet below the level of the lowland.
Several nearly level projections of the lowland extend
into uplands on the north. Large fields of stabilized
dunes cover the northern part of the lowland and lower
slopes of adjacent hills between Nenana and McGrath
(pl. 4, fig. 12).

Drainage.-The central and eastern parts of the low-
land are drained by the Tanana River, and the south-
western part is drained by the Kuskokwim River.
Braided glacial streams rising in the Alaska Range
(pl. 3, fig. 8) flow north across the lowland at intervals
of 5-20 miles. Outwash has pushed the axial streams-
the Tanana, Kuskokwim, and Kantishna Rivers-
against the base of hills on the north side. Tightly
meandering tributaries of low gradient flow into the
section from the north.

Lakes.-Thaw lakes abound in areas of fine alluvium.
Thaw sinks are abundant in areas of thick loess cover.

Glaciers and permafrost.-The lowland contains no
glaciers. The entire section is an area of permafrost.
Porous gravel at the heads of the outwash fans, how-
ever, has a deep water table and dry permafrost
(ground perennially at temperatures below freezing
but having no ice). ;

Geology.-The outwash fans grade from coarse
gravel near the Alaska Range to sand and silt along the
axial streams. Areas north of the axial streams are
underlain by thick deposits of "muck," a mixture of
frozen organic matter and silt. Parts of the south-
western part of the lowland have thick loess cover, but
the central and eastern parts are free of loess south of
the Tanana River. Scattered low hills of granite, ultra-
mafic rocks, and Precambrian(?) schist rise above the
outwash. Tertiary conglomerate in the foothills of the
Alaska Range plunges beneath the lowland in a mono-
cline, and the heads of the outwash fans may rest on a

pediment cut across this conglomerate. The base of the
alluvial fill near Fairbanks is at or below sea level.

References.-D. F. Barnes (oral commun., 1956) ;
Barnes and McCarthy (unpub. data, 1954); Drury
(1956) ; Fernald (1955, 1960); Holmes and Benning-
hoff (1957) ; Kachadoorian (1956) ; Péwé (1954, 1955b,
1958); Péwé and others (1953) ; Reed (1961); Wahr-
haftig (19582) ; Williams and others (1959) ; Andrea-
sen and others (1964).

The Tanana-Kuskokwim Lowland is covered by the
following 1 : 250,000 topographic maps : Livengood, Big
Delta, Fairbanks, Kantishna River, Mount McKinley,
Medfra, Talkeetna, and McGrath.

NOWITNA LOWLAND (27)
General topography.-The Nowitna Lowland is a
rolling silt-covered tableland ranging from 250 to 900
feet in altitude and having a local relief of 50-250 feet
and slopes of 100-150 feet per mile into which the flat
flood plains of the major rivers (valleys 114-10 miles
wide) have been incised 150-300 feet. A line of gentle
bedrock hills in the center rises to 1,500 feet. The table-
land is pocked with thaw sinks. The part of the table-
land south of the line of hills is covered with longi-
tudinal and sigmoid dunes and has been dissected by
steep-walled gullied canyons (pl. 4, fig. 8).

Drainage.-The entire lowland is drained by the
Yukon River, which follows the north boundary. The
confluence of the Yukon with the Tanana River is in
the eastern part of the lowland. The southern part of
the lowland is drained by the Nowitna River, a tribu-
tary of the Yukon, and its tributaries. Parallel drain-
age of small tributaries of the Chitinana River and
other streams in silt uplands of the eastern part may be
consequent upon the flanks of a recent upwarp (pl. 4,
fig: 9).

Lakes.-Oxbow lakes are common in the central
parts of the meander belts. Thaw lakes abound in the
marginal areas and throughout the silt and dune-
covered uplands.

Glaciers and permafrost.-The lowland contains no
glaciers; it is underlain by permafrost, except in re-
cently abandoned flood plains.

Geology.-Bedrock in the hills is similar to that of
surrounding highlands-schist and gneiss on the west
and Cretaceous sedimentary rocks on the east, all cut
by granitic intrusions. Tilted and faulted Tertiary and
possibly Quaternary sedimentary deposits are exposed
on the south bank of the Yukon. Most of the lowland
is covered by windborne silt and sand of unknown
thickness. Depth of alluvium is at least 180 feet.

References.-Cass (1959d, 1959e); Eakin (1914,
1918) ; Eardley (1938, 1988b).

30 PHYSIOGRAPHIC DIVISIONS OF ALASKA

The Nowitna Lowland is covered by the following
1: 250,000 topographic maps: Tanana, Melozitna, Kan-
tishna River, and Ruby.

KUSKOKWIM MOUNTAINS (2)

General topography.-The Kuskokwim Mountains
are a monotonous succession of northeast-trending
ridges having rounded to flat summits 1,500-2,000 feet
in altitude and broad gentle slopes (pl. 4, fig. 11).
Ridge crests north of the Kuskokwim River are ac-
cordant at about 2,000 feet and are surmounted at
intervals of 10-30 miles by isolated circular groups of
rugged glaciated mountains 3,000-4,400 feet in altitude.
Valleys have flat floors 1-5 miles wide.

Drainage.-The Kuskokwim Mountains are drained
by tributaries of the Yukon and Kuskokwim Rivers.
Major streams generally flow northeast to southwest
along valleys that are probably controlled by faults;
streams are fast and meandering and generally lie near
the northwest walls of their valleys. The Kuskokwim
River crosses the mountains in a gorge 100-400 feet
deep incised in an older valley about 1,000 feet deep
and 2-8 miles wide.

Lakes.-Liakes are few. There are oxbow and thaw
lakes in the valleys and a few cirque lakes in the
glaciated mountains.

Glaciers and permafrost.-There are no glaciers.
Permafrost underlies most of the section, and peri-
glacial erosional processes predominate.

Geology.-Most of the Kuskokwim Mountains are
made of tightly folded Cretaceous rocks that strike
northeast. Graywacke upholds the ridges, and argillite
underlies the valleys. The northeastern and northwest-
ern parts are underlain by Paleozoic sedimentary
rocks and Precambrian(?) schist. The isolated circu-
lar groups of high mountains are underlain by
monzonitic intrusions and their surrounding hornfels
aureoles. Flat-lying basalt caps the remnants of a mid-
Tertiary erosion surface. Pleistocene and Recent block
faulting has occurred south of the Kuskokwim River.

References.-Brown (1926a, 1926b); Cady and
others (1955); Eakin (1918); Hoare and Coonrad
(1959a, and 1959b); Mertie and Harrington (1924).

The Kuskokwim Mountains are covered by the fol-
lowing 1: 250,000 topographic maps: Kantishna River,
Ruby, Nulato, Mount McKinley, Medfra, Ophir, Una-
lakleet, McGrath, Iditarod, Holy Cross, Sleetmute,
Russian Mission, Taylor Mountains, and Bethel.

INNOKO LO WLANDS (29)

General topography.-The Innoko Lowlands are a
group of flat river flood plains, dendritic in pattern,

whose bounding slopes are generally steep banks cut

into the surrounding hills; in places, however, gentle
silt-covered slopes merge with the surrounding hills.

Drainage.-The Yukon River and a large tributary,
the Innoko River, cross the lowlands. The main part
of the lowlands has a complex intersecting network of
meandering sloughs of these two streams.

Lakes.-Oxbow and meander-scroll lakes are abund-
ant in recently abandoned flood plains and partly
silted sloughs. Thaw lakes abound in old flood plains
and on gentle silt-covered slopes. The lower parts of
many tributaries from surrounding hills are dammed
by alluvium from the Yukon and form narrow dendritic
lakes.

Glaciers and permafrost.-No glaciers exist in the
lowlands. Much of the section is underlain by perma-
frost.

Geology.-Bedrock geology is probably the same as
that of the surrounding hills. The plains are mantled
by river-flood-plain deposits and by windborne silt,
which also extends up the slopes of the surrounding
hills.

References.-None.

The Innoko Lowlands are covered by the following
1:250,000 topographic maps: Ophir, Unalakleet, Idi-
tarod, Holy Cross, and Russian Mission.

NUSHAGAK-BIG RIVER HILLS (30)

General topography.-The Nushagak-Big River
Hills are largely rounded, flat-topped ridges rising to
an altitude of 1,500 feet on the west and 2,500 feet on
the east; the hills have broad gentle slopes and broad
flat or gently sloping valleys. Local relief is 1,000-
2,500 feet. Mountains in the northeastern part rise to
an altitude of 4,200 feet. Ridges trend northeastward
in the eastern part but have no preferred trend in the
southwestern part.

Drainage.-The northern part of the hills drains to
the Kuskokwim River via the Big, Stony, Swift, and
Holitna Rivers; the southern part is drained by the
Mulchatna and Nushagak Rivers. The rivers that rise
from glaciers in the Alaska Range and flow across the
hills, like the Stony and Swift, are braided muddy
streams. Others, like the Holitna, are clear and mean-
dering.

Lakes.-A few thaw lakes are in some valleys. Ponds
are abundant in the moraine-mantled eastern part of
the hills.

Glaciers and permafrost.-There are no glaciers.
Most of the section is underlain by permafrost, and
periglacial erosional processes predominate.

Geology.-Most of the hills consist of tightly folded
Mesozoic graywacke, argillite, conglomerate, and
greenstone flows. There is a central northeast-trending
belt of Paleozoic rocks, including steep isolated ridges
of limestone. Early Tertiary intrusions and their meta-

SEWARD PENINSULA 31

morphic aureoles uphold two small cireular groups of
high mountains in the southwestern part.

References.-Capps (1935); J. M. Hoare (written
commun., 1958); C. L. Sainsbury (oral commun,
1959) ; Smith (1917).

The Nushagak-Big River Hills are covered by the
following 1 : 250,000 topographic maps: McGrath, Idi-
tarod, Lime Hills, Sleetmute, Lake Clark, Taylor
Mountains, Iliamna, and Dillingham.

HOLITNA LOWLAND (31)

General topography.-The Holitna Lowland is
largely a moraine-covered plain 300-800 feet in altitude
and is crossed by several low arcuate hummocky
ridges marking the end moraines of glacial advances
and by broad outwash and meander plains along rivers.
The Lime Hills, conspicuous isolated steep-sided ridges
in the southern part of the lowland, rise to an alti-
tude of 1,000-2,300 feet.

Drainage.-The Holitna Lowland is drained by the
Kuskokwim River and three of its tributaries, the
Stony and Swift Rivers, which are glacial streams
from the Alaska Range that have braided gravelly
courses, and the Holitna River, a clear meandering
stream that rises in uplands to the south.

Lakes.-There are numerous morainal and thaw
lakes throughout the lowland.

Glaciers and permafrost.-There are no glaciers.
This section is probably one of discontinuous perma-
frost.

Geology.-The bedrock hills are of Mesozoic gray-
wacke, argillite, and conglomerate and early Paleozoic
limestone. Most of the lowland is underlain by moraine
and outwash together with thick accumulations of
windborne silt.

References.-J. M. Hoare (oral commun., 1959);
J. N. Platt (written commun., 1956) ; Smith (1917).

The Holitna Lowland is covered by the following
1 : 250,000 topographic maps: McGrath, Iditarod, Lime
Hills, Sleetmute, and Taylor Mountains.

NUSHAGAK-BRISTOL BAY LOWLAND (32)

General topography.-The Nushagak-Bristol Bay
Lowland is a moraine- and outwash-mantled lowland
having local relief of 50-250 feet and rising from sea
level to an altitude of 300-500 feet at its inner mar-
gins. High steep-sided outliers of the Ahklun Moun-
tains rise from the western part. Arcuate belts of
morainal topography, 100-300 feet high and 1-5 miles
wide, enclose large deep glacial lakes on the southeast
margin and cross parts of the lowland west of the
Nushagak River.

Drainage.-The lowland is drained by the Nushagak
and other large rivers that flow into Bristol Bay. Most
streams rise in large lakes in ice-carved basins border-

ing the surrounding mountains and flow into tidal
estuaries that appear to be drowned river mouths.

Lakes.-The lowland is dotted with morainal and
thaw lakes. Large lakes occupy ice-scoured basins
along the margins of the lowland. The largest of these,
Lake Iliamna, is 80 miles long and 20 miles wide.

Glaciers and permafrost.-There are no glaciers in
this section, and permafrost is sporadic or absent.

Geology.-The lowland is underlain by several hun-
dred feet of outwash and morainal deposits that are
mantled in part by silt and peat. Outwash deposits
are coarse near the mountains and grade to fine sand
along the coast. Quarternary deposits thin to a feather-
edge along the base of surrounding mountains. A
small area of low stabilized and active dunes lies east
of the Nushagak River.

References.-Muller (1952, 1953, 1955, 1956).

The Nushagak-Bristol Bay Lowland is covered by
the following 1:250,000 topographic maps: Taylor
Mountains, Iliamna, Dillingham, Naknek, Nushagak
Bay, Ugashik, Bristol Bay, Chignik, Port Moller, Fort
Randall, False Pass, and Unimak.

SEWARD PENINSULA (33)

General topography.-The Seward Peninsula con-
tains: extensive uplands of broad convex hills and flat
divides that are 500-2,000 feet in altitude and are in-
dented by sharp V-shaped valleys (pl. 3, fig. 5); iso-
lated groups of rugged glaciated mountains, 20-60
miles long and 10 miles wide, having peaks 2,500-4,700
feet in altitude (pl. 4, fig. 2) ; and coastal lowland and
interior basins.

Drainage.-Many small rivers, whose lower courses
are sluggish and meandering, drain the peninsula.
Some of these build deltas into the heads of protected
lagoons and bays. The interior basins are drained
through narrow canyons across intervening uplands.

Lakes.-The lowlands have numerous thaw lakes.
There are several rock-basin and morainal lakes in the
glaciated Bendeleben (332) and Kigluaik (33b)
Mountains. Lakes fill several large shallow volcanic
craters in the northern part of the peninsula and sev-
eral depressions between lava flows in the central up-
land; some of the depressions were accentuated by
faulting and warping.

Glaciers and permafrost.-The Seward Peninsula
has no glaciers. The entire peninsula is underlain by
permafrost; periglacial erosional processes predomi-
nate (pl. 2, fig. 5) and ice-wedge polygons are common.

Geology.-The bedrock of the peninsula is chiefly
Paleozoic schist, gneiss, marble, and metamorphosed
volcanic rocks, all of which are cut by granitic intrusive
masses. Structural trends in the metamorphic rocks
are chiefly northward. The York Mountains (38¢) are

32 PHYSIOGRAPHIC DIVISIONS OF ALASKA

carved in a mass of resistant marble (pl. 4, fig. 2).
The Kigluaik, Bendeleben, and Darby Mountains have
recent scarplets along their bases and may be Cenozoic
uplifts. A Quaternary lava plateau lies in the north-
central part. The southern and western mountains are
extensively glaciated. In exposures of;beach placer de-
posits along the south coast, layers of till are inter-
bedded with beach and shore deposits that are both
above and below sea level; it is therefore possible to
correlate glacial advances in the Seward Peninsula
with the history of rise and fall of sea level in late
Cenozoic time.

References.-Collier (1902); Collier and others
(1908) ; Hopkins (1949, 1955, 19592, 1959b, 1963; writ-
ten communs, 1956, 1959); Hopkins and Hopkins
(1958); Hopkins and others (1960); Moffit (1913, pl.
1); Smith (1910); A. R. Tagg (oral commun., 1962).

Seward Peninsula is covered by the following 1 : 250,-
000 topographic maps : Selawik, Kotzebue, Shishmaref,
Candle, Bendeleben, Teller, Norton Bay, Solomon, and
Nome.

BERING SHELF

The Bering Shelf is the largely submerged, nearly
level plain close to sea level that joins Alaska with
the Chukotsk Peninsula of Siberia. It contains two
sections, the Yukon-Kuskokwim Coastal Lowland, a
largely emerged area, and the Bering Platform, a
largely submerged area.

YUKON-KUSKOKWIM COASTAL LOWLAND (34)

General - topography.-The - Yukon-Kuskokwim
Coastal Lowland is a triangular lake-dotted marshy
plain rising from sea level on its west margin to
100-300 feet at its east end. Many low hills of
basalt surmounted by cinder cones and broad shallow
volcanic craters and a few craggy mountains of older
rocks 2,300-2,450 feet high, rise from the western part
of the plain. Low beach ridges, marked by lines of
thaw lakes, lie along part of the west coast.

The Norton Bay Lowland ($4a) is a lake-dotted
coastal plain on the east side of Norton Bay, similar to
the northern end of the Yukon-Kuskokwim Coastal
Lowland. At its western extremity is an isolated range
of hills, the Denbigh Hills.

Drainage.-The lowland is crossed by meandering
streams of extremely low gradiant, many of them dis-
tributaries or former channels of the Yukon River;
these flow to the Bering Sea. The Yukon River flows
along the base of hills on the north side of the lowland
and is building a delta into the Bering Sea. The Kus-
kokwim River on the southeast side ends in a marine
estuary that appears to be a drowned river mouth.

Lakes.-The lowland is dotted with innumerable
thaw lakes, many of them 10 or more miles long. Some

have scalloped shorelines and probably formed through
the coalescence of several smaller lakes. Probably 30-
50 percent of the lowland is lake surface.

Glaciers and permafrost.-The area contains no gla-
ciers; it is underlain by discontinuous permafrost.

Geology.-The lowland is underlain by Quarternary
sand and silt to unknown depth. Basalt flows and
cinder cones are of Tertiary and Quarternary age.
Other bedrock hills consist of Cretaceous sedimentary
rocks, cut by early Tertiary intrusions, and of crystal-
line rocks of unknown age.

References.-Coonrad (1957).

The Yukon-Kuskokwim Coastal Lowland is covered
by the following 1: 250,000 topographic maps: Unala-
kleet, Saint Michael, Holy Cross, Kwiguk, Black, Rus-
sian Misson, Marshall, Hooper Bay, Bethel, Baird In-
let, Nunivak Island, Goodnews, and Kuskokwim Bay.

BERING PLATFORM (35)

General topography.-The Bering Platform is a
montonously smooth submarine plain 100-500 feet
deep bordered on the southwest by a submarine scarp
several thousand feet deep. A coastal lowland at the
head of Norton Sound is included in the platform.
Several islands rise abruptly from the plain. Most of
the islands are rolling uplands a few hundred to 1,000
feet high bordered by wave-cut cliffs. St. Lawrence
Island (35a), the largest, is about 100 miles long and
20 miles wide. It is chiefly a lake-dotted bedrock plain
less than 100 feet in altitude above which isolated moun-
tain groups bordered by old sea cliffs rise to altitudes
of 1,000-1,500 feet (pl. 4, fig. 1). A large shield vol-
cano with many vents is on the north coast of St. Law-
rence Island. St. Paul and Nunivak Islands consist
largely of undissected volcanic topography.

Drainage.-Many small rivers drain St. Lawrence
Island and Nunivak Island; most small islands have
no permanent streams.

Lakes.-Thaw lakes abound on the lowlands of St.
Lawrence Island and the lower parts of Nunivak
Island; there are small crater lakes on Nunivak and
the Pribilof Islands.

Glaciers and permafrost.-There are no glaciers.
Part of St. Lawrence Island and possibly Nunivak
Island may be underlain by permafrost.

Geology.-The Pribilof Islands (35b), St. Matthew
Island (385¢), Nunivak Island (35d), and north-central
St. Lawrence Island are made of Cenozoic basalt flows
and pyroclastic debris interbedded with some sedi-
ments. Cinder cones are present on the Pribilofs and
Nunivak Island. St. Lawrence, Diomede, and King
Islands are underlain largely by intensely deformed
Paleozoic and Mesozoic sedimentary and volcanic
rocks and granitic intrusions.

ALASKA-ALEUTIAN PROVINCE 33

References.-Barth (1956); Dutro and Payne
(1957) ; Flint (1958); Hopkins (19592).

Land areas of the Bering Platform are covered by
the following 1:250,000 topographic maps: Norton
Bay, St. Lawrence, St. Matthew, Nunivak Island, Cape
Mendenhall, and Pribilof.

AHKLUN MOUNTAINS (36)

General topography.-Groups of rugged steep-
walled mountains, having sharp summits 2,000-5,000
feet in altitude, separated by broad flat valleys and
lowlands, rise abruptly above the lowlands and low
hills on the north and east. Mountains in the south-
western part have rounded summits 1,500-2,500 feet
in altitude.

Drainage.-The Ahklun Mountains are drained by
shallow, clear streams that flow directly to the Bering
Sea on the south and west, to the Nushagak River via
the Nuyakuk River on the northeast, and to the Kus-
kokwim River on the northwest. Most rivers are incised
in bedrock gorges 20-50 feet deep in the downstream
parts of their valleys. Drainage is roughly radial, and
several streams in the northwestern part flow through
canyons that cut directly across structurally controlled
ridges.

Lakes.-This province is outstanding for the num-
ber and beauty of its glacial lakes, which are long
narrow bodies of water in U-shaped canyons. The larg-
est, Lake Nerka, is 29 miles long, and at least 40 lakes
are more than 2 miles long. Lake depths as great as
900 feet have been reported.

Glaciers and permafrost.-A few small cirque gla-
ciers are found in the highest parts of the mountains
from Mount Waskey northward. Permafrost occurs
sporadically.

Geology.-The mountains are made of strongly de-
formed sedimentary and volcanic rocks of late Paleo-
zoic and Mesozoic age together with some bodies of
older schist. These rocks are cut by great northeast-
trending faults along which many of the valleys have
been eroded. Structural trends control many ridges.
Small granitic masses surrounded by more resistant
hornfels have formed many ringlike mountain groups.
Late Cenozoic basalts lie on the floor of Togiak valley.
The entire province was intensely glaciated.

References.-Harrington (1921) ; J. M. Hoare (writ-
ten commun., 1958) ; Hoare and Coonrad (19592, 1959b,
1961a, 1961b) ; Mertie (1938, 1940).

The Ahklun Mountains are covered by the following
1:250,000 topographic maps: Taylor - Mountains,
Bethel, Dillingham, Goodnews, Nushagak Bay, and
Hagemeister Island.

PACIFIC MOUNTAIN SYSTEM
ALASKA-ALEUTIAN PROVINCE

The Alaska-Aleutian province consists of an arcuate
belt of mountain ranges along the north side of the
Pacific Mountain System in Alaska. It is divided into
the following sections: Aleutian Islands (37), Aleu-
tian Range (88), Alaska Range (southern part) (39),
Alaska Range (central and eastern parts) (40), and
Northern Foothills of the Alaska Range (41).

ALEUTIAN ISLANDS (37)

General topography.-The Aleutian Islands are a
chain of islands surmounting the crest of a submarine
ridge 1,400 miles long, 20-60 miles wide, and 12,000
feet high above the sea floor on either side. An arcuate
line of 57 volcanoes of Quaternary age, 27 reported
active, rise 2,000-9,000 feet above sea level along the
north side of the Aleutian Islands. Other topography
in the Aleutian Islands is of two types: (a) wave-cut
platforms less than 600 feet above sea level, bordered by
low sea cliffs, and (b) intensely glaciated mountainous
islands 600-3,000 feet above sea level, indented with
fiords and bordered by cliffs as high as 2,000 feet (pl.
5, fig. 12). Broad level intertidal platforms border
some islands; they were probably produced by frost
weathering. f

Drainage-Streams in the Aleutian Islands are
short and swift. Many plunge into the sea over water-
falls Volcanoes of porous rock have widely spaced
stream courses that are filled with water only during
exceptionally heavy rains.

Lakes.-Many small lakes occupy irregular ice-
carved basins in rolling topography on the glaciated
islands. Numerous ponds were enlarged when ice, ex-
panding by freezing, shoved the banks aside to form
ramparts of soil and turf. Lakes fill a few volcanic
craters and calderas.

Glaciers and permafrost.-The firn line is at an alti-
tude of about 3,000 feet east of Unimak Pass and about
4,500 feet west of it. Most high volcanoes bear icecaps
or small glaciers, and there are a few cirque glaciers on
the mountainous islands. There is probably no perma-
frost in the Aleutian Islands, but periglacial erosional
processes are active because of the cold, wet climate.

Geology.-The linear chain of volcanoes on the north
side of the islands is of constructional origin and late
Cenozoic age; it includes many calderas. The remain-
ing islands appear to be emerged parts of tilted fault
blocks consisting chiefly of faulted and folded Ceno-
zoic volcanic rocks, locally mildly metamorphosed;
granitic intrusions of Cenozoic age are present on
Sedanka, Unalaska, Ilak, and other islands. Submarine
topography of the Aleutian ridge shows it to be com-
plexly blockfaulted along its crest.

3A PHYSIOGRAPHIC DIVISIONS OF ALASKA

References.-Bradley (1948); Byers (1959) ; Coats
(1950, 1953, 1956a, 1956b, 1956c, 19592, 1959b) ; Coats
and others (1961) ; Drewes and others (1961) ; Fraser
and Barnett (1959) ; Fraser and Snyder (1959) ; Gates
and Gibson (1956) ; J. P. Schafer (unpub. data, 1956) ;
Gibson and Nichols (1953); Kennedy and Waldron
(1955); Murray (1945); Nelson (1959); Powers and
others (1960); P. C. Scruton (unpub. data, 1953) ;
Sharp (1946) ; Simons and Mathewson (1955) ; Snyder
(1957, 1959).

The Aleutian Islands are covered by the following
1: 250,000 topographic maps: Port Moller, Fort Ran-
dall, False Pass, Unimak, Unalaska, Umnak, Samalga
Island, Amukta, Seguam, Atka, Adak, Gareloi Island,
Rat Islands, Kiska, and Attu.

ALEUTIAN RANGE (38)

General topography.-The Aleutian Range consists
of rounded east-trending ridges 1,000-4,000 feet in
altitude, surmounted at intervals of 5-85 miles by vol-
canoes 4,500-8,500 feet in altitude (pl. 6, fig. 8). It
merges northward with the Bristol Bay-Nushagak
Lowland and has an abrupt and rugged south coast.
The range is extensively glaciated as shown by the U-
shaped valleys, cirques, and other features of glacial
erosion. Most of the volcanoes reached their final
growth after the extensive glaciation of the range.

Drainage.-The drainage divide between the Bering
Sea and the Pacific Ocean is generally along the high-
est ridges, within 10 miles of the south coast. Streams
to the Pacific are short and steep; those flowing to
Bering Sea are longer and have braided channels.

Lakes.-Along the north side of the range are many
large lakes, partly held in by end moraines. Most of
them extend well below sea level. The largest is Lake
Tliamna.

Glaciers and permafrost.-The firn line is at an
altitude of about 3,000-3,500 feet along the axis of the
range and rises northward across the range to 4,000-
5,000 feet in the northwestern part. Most volcanoes
have glaciers on all sides and some have summit ice-
fields. There is probably no permafrost, but periglacial
erosional processes are active in the cold, wet climate.

Geology.-Most of the range is composed of mildly
deformed folded and faulted Mesozoic and Cenozoic
sedimentary rocks, locally intruded by granitic stocks
and surmounted at intervals by volcanic piles of late
Tertiary to Recent age. Many volcanoes are calderas
(pl. 5, fig. 5). A major fault extends along the north
side of the eastern part of the range, separating the
sedimentary rocks from a large Mesozoic granitic
batholith on the north.

References.-Atwood (1911) ; Coats (1950) ; Curtis
and others (1954) ; Griggs (1922) ; Keller and Reiser

(1959) ; Knappen (1929) ; Mather (1925) ; Muller and
others (1954); Smith (1925a); Smith and Baker-
(1924) ; Wilcox (1959).

The Aleutian Range is covered by the following
1:250,000 topographic maps: Iliamna, Afognak,
Mount Katmai, Naknek, Karluk, Ugashik, Bristol
Bay, Sutwik Island, Chignik, Stepovak Bay, Port
Moller, and Semeonof Island.

ALASKA RANGE (SOUTHERN PART) (39)

General topography.-Between Rainy Pass and Lake
Chakachamna the Alaska Range consists of many
parallel rugged glaciated north-trending ridges 7,000-
12,000 feet in altitude; south of Lake Chakachamna
the ridges trend northeast and are 4,000-6,000 feet
in altitude. Between the ridges lie broad glaciated val-
leys which have floors less than 3,000 feet in altitude.
Local relief is between 4,000 and 9,000 feet. Many
spirelike mountains rise in the central part of the
range.

Drainage-Large braided glacial streams follow
the north- and northeast-trending valleys; they flow
north or south to the Kuskokwim River, southwest
to the Nushagak or Kvichak Rivers, and east to the
Susitna River and Cook Inlet.

Lakes.-Many large lakes occupy glaciated valleys
within and on the margins of the range; the largest of
these is Lake Clark, 49 miles long and 1-4 miles wide.

Glaciers and permafrost.-Extensive systems of val-
ley glaciers radiate from the higher mountains. The
firn line is lower and the glaciers are larger on the
southeast side of the range than on the northwest and
west side of the range. The extent of permafrost is
unknown. $

Geology.-Most of the range is underlain by large
granitic batholiths, intrusive into moderately meta-
morphosed and highly deformed Paleozoic and Meso-
zoic volcanic and sedimentary rocks, which form scat-
tered areas of lower mountains. Structural trends are
generally northerly, but change abruptly to northeast-
erly and easterly northward across Rainy Pass. Mount
Spurr, Mount Iliamna, and Mount Redoubt are large
active volcanoes. Well-bedded Jurassic sedimentary
rocks form prominent hogbacks and cuestas dipping
southward off the south flank of the range toward
Cook Inlet (pl. 2, fig. 10).

References.-Capps (1935, 1940); Juhle (1955) ;
Juhle and Coulter (1955); Moffit (1927); Spurr
(1900).

The southern part of the Alaska Range is covered
by the following 1:250,000 topographic maps: Tal-
keetna, McGrath, Tyonek, Lime Hills, Kenai, Lake
Clark, Seldovia, and Iliamna.

ALASKA-ALEUTIAN PROVINCE

ALASKA RANGE (CENTRAL AND EASTERN PART) (40)

General topography.-The central and eastern part
of the Alaska Range consists of two or three parallel
rugged glaciated ridges, 6,000-9,000 feet in altitude,
surmounted by groups of extremely rugged snow-
capped mountains more than 9,500 feet in altitude (pl.
6, fig. 1). The Mentasta-Nutzotin Mountain segment
(40a) at the east end of the Alaska Range has a single
axial ridge. The ridges are broken at intervals of
10-50 miles by cross-drainage or low passes; most of
the drainage appears superposed (pl. 5, fig. 1 and pl. 6,
fig. 2). The range rises abruptly from lower country
on either side, and its longitudinal profile, seen from a
distance, is irregular. Mount McKinley, 20,269 feet
high and the highest mountain in North America, is in
this part of the Alaska Range. (pl. 6, fig. 2).

Drainage.-The central and eastern part of the
Alaska Range is crossed at places 25-100 miles apart
by north-flowing tributaries of the Tanana and Yukon
Rivers. Most of the range drains to the Tanana. The
western part drains to the Kuskokwim River and parts
of the south flank drain to the Susitna and Copper
Rivers. Streams are swift and braided, and most rivers
head in glaciers.

Lakes.-There are a few rock-basin lakes and many
small ponds in areas of ground moraine. Lakes are
rare for a glaciated area.

Glaciers and permafrost.-The firn line on the south
side of the range is 5,000-7,000 feet in altitude and on
the north side is 6,000-8,000 feet in altitude; this
change reflects the northward decrease in cloudiness
and precipitation as one passes from the Gulf of Alaska
coast to the interior. The high mountains are sheathed
in ice, and valley glaciers as much as 40 miles long and
5 miles wide radiate from them. For some glaciers (for
example, Black Rapids Glacier and Muldrow Glacier)
short periods of rapid advance have alternated with
long periods of stagnation. Short valley glaciers lie in
north-facing valleys in the lower parts of the range.
Rock glaciers are common. Permafrost is extensive and
solifluction features are well developed.

Geology.-The internal structure of the Alaska
Range is a complex synclinorium having Cretaceous
rocks in the center and Paleozoic and Precambrian (?)
rocks on the flanks. This synclinorium is cut by great
longitudinal faults that trend approximately parallel
to the length of the range and are marked by lines of
valleys and low passes. The synclinorium was probably
formed near the close of the Mesozoic Era. Many
roughly oval granitic stocks and batholiths support
groups of high mountains that have cliffs as high as
5,000 feet (pl. 6, fig. 1).

35

Synclinal areas of Tertiary rocks underlie lowlands
that trend parallel to the length of the range. Much of
the major topography of the range was probably pro-
duced from mid-Tertiary structures by removal of
easily eroded Tertiary rocks to form lowlands. Re-
cently formed scarplets as high as 30 feet can be seen
on several longitudinal faults. At least four periods of
glaciation have been recognized; the earliest is indi-
cated only by scattered giant granite erratiecs on up-
lands in the foothills to the north.

References.-Brooks (1911); Capps (1912, 1913,
1916a, 1932, 1940); Maddren (1917); Moffit (1912,
1914, 1954a) ; Wahrhaftig (1958a, 1958b) ; Wahrhaftig
and Cox (1959).

The central and eastern Alaska Range is covered by
the following 1 : 250,000 topographic maps; Tanacross,
Mount Hayes, Healy, Mount McKinley, Nabesna,
Gulkana, Talkeetna Mountains, Talkeetna, McGrath,
and Tyonek.

NORTHERN FOOTHILLS OF THE ALASKA RANGE (41)

General topography.-The Northern Foothills of the
Alaska Range are flat-topped east-trending ridges
2,000-4,500 feet in altitude, 3-7 miles wide, and 5-20
miles long that are separated by rolling lowlands 700-
1,500 feet in altitude and 2-10 miles wide. The foot-
hills are largely unglaciated, but some valleys were
widened during the Pleistocene Epoch by glaciers from
the Alaska Range. Colorful badlands abound in areas
of rapid erosion in soft Tertiary rocks (pl. 2, figs. 8
and 9).

Drainage.-The major streams of the foothills are
superimposed across the topography. Most streams are
nearly parallel, rise for the most part in the Alaska
Range, and flow north to N. 20° W. across the ridges
in rugged impassable V-shaped canyons and across the
lowlands in broad terraced valleys. The entire section
drains to the Tanana River.

Lakes.-A few small lakes of thaw. origin lie in the
lowland passes, and morainal areas have shallow ir-
regular ponds.

Glaciers and permafrost.-The entire section is below
the firn line, and there are no local glaciers, although a
few glaciers from the Alaska Range terminate in the
foothills Permafrost is extensive, and polygonal
ground and solifluction features are well developed
(pl. 2, figs. 6 and 11).

Geology.-Crystalline schist. and granitic intrusive
rocks make up most of the ridges, which are anticlinal.
Poorly consolidated Tertiary rocks underlie the low-
lands; thick coarse conglomerate near the top of the
Tertiary section forms cuestas and ridges where it dips
20°-60°, and broad dissected plateaus where it is flat
lying. The topography reflects closely the structure of

36 PHYSIOGRAPHIC DIVISIONS OF ALASKA

monoclines and short, broad flat-topped anticlines hav-
ing steep north flanks. Flights of tilted terraces on
north-flowing streams indicate Quaternary tilting and
uplift of the Alaska Range. The Tertiary rocks con-
tain thick beds of subbituminous coal.

References.-Capps (1912, 1919, 1940) ; Holmes and
Benninghoff (1957); Maddren (1917); Wahrhaftig
(1951, 1958a, 1958b); Wahrhaftig and Hickcox
(1955) ; Wahrhaftig and others (1951).

The Northern Foothills of the Alaska Range are
covered by the following 1 : 250,000 topographic maps :
Big Delta, Fairbanks, Kantishna River, Mount Hayes,
Healy, and Mount McKinley.

COASTAL TROUGKH PROVINCE E

The Coastal Trough province is a belt of lowlands
extending the length of the Pacific Mountain System,
interrupted at intervals by oval mountain groups. It is
divided into the following sections: Cook Inlet-Susitna
Lowland (42), Broad Pass Depression (43), Talkeetna
Mountains (44), Upper Matanuska Valley (45), Clear-
water Mountains (46), Gulkana Upland (47), Copper
River Lowland (48), Wrangell Mountains (49), Duke
Depression (50) (not described), Chatham Trough
(51), and Kupreanof Lowland (52).

COOK INLET-SUSITNA LOWLAND (42)

General topography.-The Cook Inlet-Susitna Low-
land is a glaciated lowland containing areas of ground
moraine and stagnant ice topography, drumlin fields,
eskers, and outwash plains (pl. 6, figs. 5 and 9). Most
of the lowland is less than 500 feet above sea level and
has a local relief of 50-250 feet. Rolling upland areas
near the bordering mountain ranges rise to about
3,000 feet in altitude, and isolated mountains as high
as 4,800 feet rise from the central part of the lowland.
The Cook Inlet-Susitna Lowland is the major popula-
tion center of Alaska and contains most of the devel-
oped agricultural land.

Drainage.-The lowland is drained by the Susitna
River and other streams that flow into Cook Inlet.
Most of these streams head in glaciers in the surround-
ing mountains. The shores of Cook Inlet are for the
most part gently curving steep bluffs 50-250 feet high.

Lakes.-Three large lakes-Tustumena, Skilak, and
Beluga-fill ice-carved basins at the margins of sur-
rounding mountains. Lake Tustumena, the largest, is
23 miles long and 7 miles wide. Hundreds of small
irregular lakes and ponds occur in areas of stagnant
ice topography and on ground moraines. Stringe-
moore ponds are common in marshes.

Glaciers and permafrost.-The section is almost ice-
free, although one glacier reaches the lowland from
the Alaska Range on the west, and sporadic permafrost
is present in the northern part.

Geology.-Bedrock beneath the lowland consists
mainly of poorly consolidated coal-bearing rocks of
Tertiary age, generally mildly deformed or flat lying;
this rock is mantled by glacial moraine and out wash and
marine and lake deposits. Sequences of moraines record
successive glacial advances. The boundaries of the low-
lands are of two kinds: (a) abrupt straight mountain
fronts that are probably faultline scarps, and (b) up-
lands of hard pre-Tertiary rocks that slope gently
toward the lowland. The uplands are probably up-
lifted parts of the surface on which the Tertiary rocks
were deposited; the edge of the lowland generally
marks the edge of the Tertiary cover, which dips
gently away from the mountains. The isolated moun-
tains in the center of the lowland generally consist of
metamorphic and granitic rocks of Mesozoic age.

References.-Barnes and Cobb (1959); Barnes and
Payne (1956); Barnes and Sokol (1959); Brooks
(1911); Capps (1940); T. N. V. Karlstrom (written
commun., 1957) ; Karlstrom (1955, 1960) ; Martin and
others (1915) ; Miller and Dobrovolny (1959) ; Trainer
(1953, 1960).

The Cook Inlet-Susitna Lowland is covered by the
following 1:250,000 topographic maps: Talkeetna
Mountains, Talkeetna, Anchorage, Tyonek, Kenai, and

Seldovia.
BROAD PASS DEPRESSION (43)

General topography.-The Broad Pass Depression,
1,000-2,500 feet in altitude and 5 miles wide, is a trough
having a glaciated floor; it opens on the east to a broad
glaciated lowland with rolling morainal topography
and central outwash flats. The bounding mountain
walls of the trough are several thousand feet high.
Long, narrow drumlinlike hills on the floor of the
trough trend parallel to its axis, and the main streams
in the trough are incised in rock-walled gorges a few
hundred feet deep. The trough opens on its south end
to the Cook Inlet-Susitna Lowland.

Drainage.-The divide between the Bering Sea and
Pacific Ocean drainages crosses this depression in two
places and is marked by nearly imperceptible passes.
The southwestern part drains by the Chulitna River
to the Susitna River; the central part, by the Nenana
River north to the Yukon River; and the eastern part,
by the headwaters of the Susitna. Most streams head
in glaciers in the surrounding mountains and are swift,
turbid, and braided.

Lakes.-Many long, narrow lakes lie in morainal de-
pressions in the central part of the trough. Morainal
and thaw lakes are common in the eastern part.

Glaciers and permafrost.-There are no glaciers.
Most of the depression is underlain by permafrost.

Geology.-Patches of poorly consolidated Tertiary
coal-bearing rocks, in fault contact with older rocks

COASTAL TROUGH PROVINCE

of the surrounding mountains, show that this depres-
sion marks a graben of Tertiary age. Most of the
bedrock consists of highly deformed slightly metamor-
phosed Paleozoic and Mesozoic rocks that are also ex-
posed in the surrounding mountains. Ground moraine
mantles the lowland.

References.-Capps (1940) ; Hopkins (1951) ; Wahr-
haftig (1944, 19582).

The Broad Pass Depression is covered by the fol-
lowing 1: 250,000 topographic maps: Healy, Talkeetna
Mountains, and Talkeetna.

TALKEETNA MOUNTAINS (44)

General topography.-The Talkeetna Mountains
are an oval highland of diversified topography that
interrupts the belt of lowlands of the Coastal Trough
province. The central Talkeetna Mountains (44c) are
a compact group of extremely rugged radial ridges
6,000-8,800 feet in altitude, having only few low
passes, that isolate steep-walled U-shaped valleys.
Accordant flat ridge crests in the western and eastern
parts of the central Talkeetna Mountains suggest a
warped peneplain that plunges beneath Tertiary rocks
in the adjacent lowlands (pl. 6, fig. 5). The glaciated
Chulitna Mountains (442), a compact group of moun-
tain blocks separated by low passes, are isolated from
the central Talkeetna Mountains by the Fog Lakes
Upland (44h), a northeast-trending area of broad roll-
ing summits, 3,000-4,500 feet in altitude, which has a
glacially sculptured mammillated surface in its south-
western part but is unglaciated in the northeastern
part. A similar upland (the Clarence Lake Upland,
44d) borders the mountains on the east.

Drainage.-The central Talkeetna Mountains have a
radial drainage of large braided glacial streams that
are tributary to the Susitna, Matanuska, and Copper
Rivers. The extreme northern part drains to the Yukon
River via the Nenana River. The Susitna River flows
west across the Talkeetna Mountains in a narrow steep-
walled gorge that in places is more than 1,000 feet deep.
West-flowing streams in the southwestern Talkeetna
Mountains have many long southern tributaries and
few or no northern tributaries; low slanting solar rays
from the south, favoring the growth of glaciers in
shaded north-facing valley heads and inhibiting their
growth on sunny south-facing slopes, probably caused
this asymmetry.

Lakes.-There are few lakes in the southern part of
the Talkeetna Mountains. Many lakes, some 5 miles
long, occupy ice-carved and moraine-dammed basins
in the northern part.

Glaciers and permafrost.-The firn line is between
altitudes of 6,500 and 7,000 feet. Glaciers 5-15 miles
long lie at the heads of most valleys in the central

37

Talkeetna Mountains. A few cirque-glaciers occupy
north-facing valley heads in the northeastern Talkeetna
Mountains and Chulitna Mountains. Rock glaciers are
common in the southeastern Talkeetna Mountains and
in the Chulitna Mountains. Permafrost probably
underlies most of the section; altiplanation terraces
are present in unglaciated parts of the northeastern
Talkeetna Mountains.

Geology.-A large mid-Jurassic batholith in the
central and western Talkeetna Mountains intrudes
Jurassic volcanic rocks and older rocks and is eroded
into cliffs and spires. The southeastern Talkeetna
foothills (44e) are composed of soft sandstone and
shale of Jurassic and Cretaceous age, capped by flat-
lying cliff-forming Tertiary basalt flows aggregating
several thousand feet in thickness. The northern part
of the Talkeetna Mountains consists of Paleozoic and
Mesozoic greenstone, gray wacke, and argillite in north-
east-trending belts. The greenstone forms rugged
mountains.

References.-Barnes (1962); Capps (1927, 1940) ;
Chapin (1918); Grantz (1953, 1960a, 1960b, 19612,
1961b, oral commun., 1957) ; Moffit (1915); Paige and
Knopf (1907) ; J. R. Williams, oral commun. 1957).

The Talkeetna Mountains are covered by the follow-
ing 1:250,000 topographic maps: Healy, Talkeetna
Mountains, Talkeetna, and Anchorage.

UPPER MATANUSKA VALLEY (45)

General topography.-The Upper Matanuska Valley
is a glaciated trough 2-5 miles wide containing longi-
tudinal bedrock hills 500-1,000 feet high and having
steep bounding walls several thousand feet high. Alti-
tude of its floor ranges from 800 feet on the west to
2,000 feet on the east.

Drainage.-The Upper Matanuska Valley is drained
entirely by the Matanuska River, which flows westward
along the trough.

Lakes.-Many small narrow lakes occupy ice-carved
bedrock basins, and ponds are common in morainal
areas.

Glaciers and permafrost.-The terminus of the
Matanuska Glacier reaches the east end of the trough.
Permafrost is present in the eastern part of the trough,
but its extent is unknown.

Geology.-The Upper Matanuska Valley is a struc-
turally controlled trough bounded on the north by a
major fault, the Castle Mountain fault, and on the
south by a steep unconformity and faults. It is under-
lain by easily eroded rocks of Cretaceous and Tertiary
age, which are highly deformed and were intruded by
gabbro sills and stocks. It contains several coal fields.
The bordering mountains are of older and more re-
sistant rocks.

38 PHYSIOGRAPHIC DIVISIONS OF ALASKA

References.-Barnes (1962); Barnes and Payne
(1956); Capps (1927); Grantz (1953, 1960b, 1961a;
oral commun., 1959); Grantz and Jones (1960) ;
Martin and Katz (1912).

The Upper Matanuska Valley is shown on the
1: 250,000 topographic map of Anchorage quadrangle.

CLEARWATER MOUNTAINS (46)

General topography.-The Clearwater Mountains
consist of two or three steep, rugged east-trending
ridges rising to altitudes of 5,500-6,500 feet, separated
by U-shaped valleys 3,000-3,500 feet in altitude. They
are intensely glaciated. The ridges are asymmetrical;
long spurs on their north sides separate large com-
pound cirques; their south sides are relatively smooth
mountain walls grooved by short steep canyons.

Drainage.-The entire section is tributary to the
Susitna River.

Lakes.-There are a few rock-basin lakes in cirques
and passes. The largest lake is less than 1 mile long.

Glaciers.-The north slopes of the highest peaks
have a few cirque-glaciers.

Geology.-The Clearwater Mountains are underlain
chiefly by Triassic greenstone and Mesozoic argillite
and graywacke. The rocks are highly deformed, strike
generally east, and dip steeply.

References. Kachadoorian and others (1954) ; Moffit
(1912) ; Ross (1933).

The Clearwater Mountains are covered by the follow-
ing 1:250,000 topographic maps: Mount Hayes and
Healy.

GULKANA UPLAND (47)

General topography.-The Gulkana Upland consists
of rounded east-trending ridges separated by lowlands
2-10 miles wide. The ridge crests, 3,500-5,500 feet in
altitude, are 4-15 miles apart and are cut at intervals of
5-15 miles by notches and gaps that were eroded by
glaciers or glacial melt water (pl. 5, fig. 2). The low-
lands are floored by glacial deposits showing morainal
and stagnant-ice topography and contain large esker
systems (pl. 5, fig. 3).

Drainage.-The southeastern and eastern part drains
south to the Copper River; the western part drains
southwest to the Susitna River; and the north-central
part drains north via the Delta River to the Tanana
and Yukon. The drainage divide between the Pacific
Ocean and Bering Sea has an irregular course through
this section and is in part along eskers (pl. 5, fig. 3).

Lakes.-Many long, narrow lakes occupy rock-cut
basins in notches through the ridges. Irregular lakes
abound in some areas of morainal topography.

Glaciers and permafrost.-A few cirque glaciers lie
on the north sides of the highest ridges. The lower
ends of a few glaciers from the Alaska Range are in

this section. The upland is underlain by permafrost
and contains ice-wedges, pingos, and altiplanation
terraces.

Geology.-Bedrock is chiefly greenstone and of late
Paleozoic and Mesozoic age; structure trends eastward.
Areas of relatively low relief in the northern part are
underlain by poorly consolidated Tertiary sedimentary
rocks.

References.-Kachadoorian and others (1954) ;
Kachadoorian and Péwé (1955) ; Moffit (1912, 19542).

The Gulkana Upland is covered by the following
1:250,000 topographic maps: Mount Hayes, Healy,
Gulkana, and Talkeetna Mountains.

COPPER RIVER LOWLAND (48)

General topography.-The eastern part of the Cop-
per River Lowland (482) is a relatively smooth plain,
1,000-2,000 feet in altitude trenched by the valleys of
the Copper River and its tributaries, which have steep
walls 100-500 feet high (pl. 6, fig. 3). The Copper and
Chitina valleys, eastward prongs of this lowland, con-
tain longitudinal morainal and ice-scoured bedrock
ridges that rise above axial outwash plains. The
western part of the Copper River Lowland, the Lake
Louise Plateau (48b), is a rolling upland, 2,200-3,500
feet in altitude, and has morainal and stagnant-ice
topography; the broad valley of the Nelchina and
Tazlina Rivers separates this upland from the Chugach
Mountains.

Drainage.-The eastern and southern parts of the
Copper River Lowland are drained by the Copper
River and its tributaries. The northwestern part is
drained by the Susitna River. Low passes lead to the
heads of the Delta, Tok, and Matanuska Rivers. Most
rivers head in glaciers in surrounding mountains and
have braided upper courses. Salty ground water has
formed salt springs and mud volcanoes.

Lakes.-Large lakes occupy deep basins in the moun-
tain fronts. Thaw lakes are abundant in the eastern
plain. Lakes occupy abandoned melt-water channels;
those in morainal depressions in the western upland are
as much as 6 miles across. Beaches and wave-cut cliffs
border lakes more than 2 miles wide whereas irregular
muskeg marshes encroach on smaller lakes.

Glaciers and permafrost.-There are no glaciers. The
entire lowland is underlain by permafrost. The perma-
frost table is within 5 feet of the surface and perma-
frost is at least 100 feet thick.

Geology.-Bedrock beneath the southern part of the
lowland is chiefly easily eroded sandstone and shale of
Mesozoic age; bedrock beneath the northern part is
chiefly resistant late Paleozoic and Mesozoic metamor-
phosed volcanic rocks. Tertiary gravels cap some hills.
Ground and end moraine and stagnant ice deposits

PACIFIC BORDER RANGES PROVINCE 39

mantle much of the lowland. The eastern plain is
underlain by glaciolacustrine and glaciofluvial deposits
at least 500 feet thick.

References.-Andreasen and others (1958) ; Grantz
(1961b; oral commun., 1959); Grantz and others
(1962); Mendenhall (1905); Moffit , (1938, 1954a) ;
D. R. Nichols (written commun., 1960) ; J. R. Williams
(written commun., 1957).

The Copper River Lowland is covered by the follow-
ing 1:250,000 topographic maps: Nabesna, Gulkana,
Talkeetna Mountains, McCarthy, Valdez, and Anchor-
age.

WRANGELL MOUNTAINS (49)

General topography.-The Wrangell Mountains are
an oval group of great shield and composite volcanoes
(Mount Wrangell, 14,005 feet in altitude, is still ac-
tive) that rises above a low plain on the north and west
and above heavily glaciated clified and castellated
ridges on the south and east (pl. 6, fig. 3). Six vol-
canoes at altitudes higher than 12,000 feet-the highest
is Mount Blackburn, 16,523 feet-make up the greater
part of the mountains.

Drainage.-Seventy-five percent of the section drains
to the Copper River, which encircles the mountains on
the west. The remainder drains to the Tanana River
via the Nabesna and Chisana Rivers and to the Yukon
River via the White River.

Lakes.-There are a few rock-basin lakes in the ex-
treme northern part. Several ice-marginal lakes lie in
Skolai Pass at the east end of the mountains.

Glaciers and permafrost.-The firn line is at an alti-
tude of about 7,000 feet. A large icecap covers most of
' the high mountains and feeds large valley glaciers.
Rock glaciers are common in the southeastern Wrangell
Mountains. Permafrost is probably present in the
glacier-free areas, but its extent is unknown.

Geology.-The Wrangell Mountains are a great pile
of Cenozoic volcanic rocks that rests on deformed
Paleozoic and Mesozoic sedimentary and volcanic
rocks, among which are cliff-forming units of limestone
and greenstone. Some granitic masses intrude the
Mesozoic rocks. An important belt of copper deposits,
including the Kennicott Mine, lies on the south side of
the Wrangell Mountains.

References.-Capps (1915,
(1905) ; (1938, 19542).

The Wrangell Mountains are covered by the follow-
ing 1:250,000 topographic maps: Nabesna, Gulkana,
McCarthy, and Valdez.

CHATHAM TROUGE (51)

General topography.-The Chatham Trough is a
deep, straight trench, 4-15 miles wide, which is entirely
below the sea except for its north end. Average depth

19162); Mendenhall

of water in the trough is more than 1,900 feet, and its
maximum depth is 2,900 feet. Mountains on either side
rise to 2,500-5,000 feet above sea level.

Geology.-The Chatham Trough probably marks a
major fault line. Rocks on opposite sides of the trough
do not match across the trough, either in their structure
or in their age. It probably owes its greater depth to
glacial erosion of relatively soft rocks.

References.-Lathram and others (1959).

The Chatham Trough is covered by the following
1: 250,000 topographic maps: Skagway, Juneau, Sitka,
and Port Alexander.

KUPREANOF LOWLAND (59)

General topography.-The Kupreanof Lowland con-
sists of islands and channels. Islands of rolling heavily
glaciated terrain having a local relief of 300-500 feet
and a maximum relief of 1,000-1,500 feet are separated
by an intricate network of waterways. (See northeast
part of fig. 11, pl. 5.) Scattered blocklike mountains
having rounded hummocky summits 2,000-3,000 feet in
altitude rise above the general level of the lowland.
Parts of some islands are plains which are a few feet
above sea level and are cut across rocks of varying
hardness.

Drainage.-The islands of the lowland are drained
by many short clear streams that generally follow
linear depressions etched by the Pleistocene ice sheets
along joints, faults, bedding, and schistosity.

Lakes.-There are abundant lakes in glacially
scoured basins. Parts of some islands are almost 50
percent lake surface.

Glaciers and permafrost.-There are no glaciers or
permafrost.

Geology.-The lowland is underlain mainly by well-
consolidated faulted and folded Paleozoic and Mesozoic
sedimentary rocks, locally metamorphosed. Small ellip-
tical granitic and ultramafic masses underlie most of
the high mountains. The northern part of the lowland
has an extensive Cenozoic basalt field. Small patches
of Tertiary sedimentary rocks have been found.

References.-Buddington and Chapin (1929) ; G. D.
Eberlein (oral commun., 1957); Sainsbury (1961) ;
Sainsbury and Twenhofel (1954).

The Kupreanof Lowland is covered by the following
1: 250,000 topographic maps: Sumdum, Sitka, Peters-
burg, Port Alexander, Ketchikan, Craig, and Prince
Rupert.

PACIFIC BORDER RANGES PROVINCE

The Pacific Border Ranges province consists of
several mountain ranges bordering the Pacific Coast
and a coastal shelf that is present in places between
them and the ocean. It is divided into the following
sections: Kodiak Mountains (53), Kenai-Chugach

40 PHYSIOGRAPHIC DIVISIONS OF ALASKA

Mountains (54), St. Elias Mountains (55), Fair-
weather Range (55a), Gulf of Alaska coastal section
(56), Chilkat-Baranof Mountains (57), and Prince of
Wales Mountains (58).

KODIAK MOUNTAINS (53)

General topography.-The Kodiak Mountains in-
clude a group of mountainous islands that are the
structural continuation of the Kenai-Chugach Moun-
tains (54) but whose topography is more finely tex-
tured and on a smaller scale than that of the Kenai-
Chugach Mountains. The Kodiak Mountains section is
mostly glaciated, but the glaciation of western Kodiak
Island was very early. Summit altitudes are between
2,000 and 4,000 feet. Kodiak Island has a rugged north-
east-trending divide having horns and arétes from
which broad smooth ridges extend northwestward. The
topography southeast of the divide has a strong north-
easterly grain normal to the drainage (pl. 5, fig. 7).
The coastline is extremely irregular, having many
fords and islands. The northern part of Afognak
Island is a hilly lowland, and the western part of
Kodiak Island has many broad valleys.

Drainage.-The islands of the Kodiak Mountains are
drained mostly by swift, clear streams that are less
than 10 miles long. Two rivers, each about 25 miles
long, drain much of southwestern Kodiak Island.

Lakes.-There are several lakes more than a mile
long in the southwestern part of Kodiak Island and
on Afognak Island. Small ponds are scattered over the
glacially sculptured topography. The glaciated valleys
heading in the main divide have chains of paternoster
lakes.

Glaciers and permafrost.-The firn line is between
altitudes of 3,000 and 3,500 feet along the main divide
of Kodiak Island, which has 40 cirque glaciers, all less
than 2 miles long ; the firn line rises to much more than
4,000 feet in the northwestern part of Kodiak Island.
Permafrost is probably absent.

Geology.-The Kodiak Mountains are underlain
mostly by Mesozoic argillite and graywacke. Older
rocks, chiefly greenstone and schist, lie along the north-
west coast. The main divide of Kodiak Island is under-
lain by a granitic batholith. Northeast-trending belts
of downfaulted and easily eroded Tertiary rocks lie
on the southeast side of Kodiak Island and also make
up the Trinity Islands. Lateral moraines, ice-marginal
drainage channels through the ends of ridges, and old
greatly modified cirques half buried in alluvium (pl.
5, fig. 6) indicate that western Kodiak Island was not
covered by ice of the last glaciation and that ice from
the Aleutian Range banked against its western shore.

References.-Capps (1937); Maddren (1919).

The Kodiak Mountains are covered by the following
1: 250,000 topographic maps: Afognak, Kodiak, Kar-
luk, Kaguyak, and Trinity Islands. §

KENAI-CHEUGACK MOUNTAINS (54)

General topography.-The Kenai-Chugach Moun-
tains form a rugged barrier along the north coast of
the Gulf of Alaska. High segments of the mountains
are dominated by extremely rugged east-trending
ridges 7,000-13,000 feet in altitude. Low segments con-
sist of discrete massive mountains 5-10 miles across and
3,000-6,000 feet in altitude, separated by a reticulate
system of through valleys and passes 4-1 mile wide
that are eroded along joints and cleavage (pl. 5, fig. 4).
The entire range has been heavily glaciated, and the
topography is characterized by horns, arétes, cirques,
U-shaped valleys and passes, rock-basin lakes, and
grooved and mammillated topography. The south coast
is deeply indented by fiords and sounds, and ridges ex-
tend southward as chains of islands (pl. 5, fig. 8). The
north front is an abrupt mountain wall.

Drainage.-The drainage divide, generally an ice
divide, is along the highest ridges, and is commonly
only a few miles from the Pacific Ocean. Streams are
short and swift; most head in glaciers. The Copper
River crosses the eastern part of the Chugach Moun-
tains in a canyon 6,000-7,000 feet deep.

Lakes.-Large lakes fill many ice-carved basins along
the north margin of the Chugach Mountains and
throughout the northern Kenai Mountains. Lake
George is an ice-margin lake dammed by the Knik
Glacier; it empties in an annual flood.

Glaciers and permafrost.-The firn line rises from an
altitude of 2,500-3,500 feet on the south side of the
mountains to 7,000-8,000 feet on the north side of the
central Chugach Mountains. All higher parts of the
range are buried in great icefields, from which valley
and piedmont glaciers radiate. Many of the glaciers
on the south side of the mountains end in tidewater.
The extent of permafrost is unknown.

Geology.-The Kenai-Chugach Mountains are com-
posed chiefly of dark-gray argillite and graywacke of
Mesozoic age that are mildly metamorphosed and have
a pronounced vertical cleavage that strikes parallel to
the trend of the range. In the Prince William Sound
area large bodies of greenstone are associated with the
argillite and gray wacke. A belt of Paleozoic and Meso-
zoic schist, greenstone, chert, and limestone lies along
the north edge of the Kenai and Chugach Mountains.
All these rocks are cut by granitic intrusions.

References.-Barnes (1943); Capps (1916b, 1940) ;
Capps and Johnson (1915) ; Grant and Higgins (1910,
1913); Grantz (1961@a, 1961b; oral commun., 1957) ;
Landes (1927); Martin and others (1915); Miller

PACIFIC BORDER RANGES PROVINCE 41

(1958; oral commun., 1959) ; Moffit (1914, 1935, 1954b) ;
Park (19833) ; Plafker (1955) ; Tarr and Martin (1914,
p. 232-450) ; Tuck (1933) ; J. R. Williams (oral com-
mun., 1957).

The Kenai-Chugach Mountains are covered by the
following 1:250,000 topographic maps: McCarthy,
Valdez, Anchorage, Bering Glacier, Cordova, Seward,
Kenai, Blying Sound, and Seldovia.

ST. ELIAS MOUNTAINS (55)

General topography.-The St. Elias Mountains are
probably the most spectacular mountains of North
America. Massive isolated blocklike mountains 14,000-
19,000 feet in altitude rise at intervals of 5-30 miles
from a myriad of narrow ridges and sharp peaks 8,000-
10,000 feet in altitude that, seen from a distance, gives
the impression of a broad ice dome. The average alti-
tude of icefields in the interconnected valley system is
3,000-7,000 feet. Local relief is extreme and jagged
cliffs abound (pl. 6, fig. 4).

Drainage.-Drainage is almost entirely by glaciers.
The ice divides between drainages of the Yukon, Cop-
per, and Alsek Rivers and the Pacific Ocean meet in
this range. The Alsek River flows west to the Pacific
across this range from lowlands on the northeast side
and separates the Fairweather Range subsection from
the rest of the mountains.

Lakes.-There are no lakes.

Glaciers and permafrost.-All parts of the range
gentle enough to hold snow are sheathed in glacial ice.
A continuous network of icefields and glaciers 4-15
miles wide and as much as 80 miles long penetrates the
range and feeds piedmont glaciers to the south. The
extent of permafrost is unknown.

Geology.-The high mountains are probably under-
lain by crystalline schist and granitic intrusive masses.
A belt of Permian and Triassic volcanic and sediment-
ary rocks extend along the north side of the range.
Lower Cretaceous sedimentary rocks lie in down-
faulted basins in the center of the range and probably
underlie ice-filled valleys. The entire sequence is thrust
southward against Cretaceous and Cenozoic rocks;
thrusting may be active today. Cenozoic volcanoes are
present in the northern part of the range; some of these
may still be active.

References.-Blackwelder (1907a, 1907b) ; Bostock
(1948, p. 92-101, 1952, p. 6-8) ; Capps (19612) ; Filippi
(1900) ; Kindle (1953) ; Miller (1958) ; Moffit (1938) ;
Muller (1954); Odell (1950); Plafker and Miller
(1957, 1958); Russell (1893); Sharp (1943, 1947);
Sharp and Rigsby (1956); Tarr and Butler (1909) ;
Tarr and Martin (1912, 1914, p. 23-231); Washburn
(1936).

The St. Elias Mountains are covered by the follow-
ing 1:250,000 topographic maps: McCarthy, Bering
Glacier, Mount St. Elias, and Yakutat.

FAIRWEATHER RANGE (554)
(Subsection of the St. Elias Mountains)

General topography.-The Fairweather Range is an
exceedingly steep and high unbroken barrier between
the Pacific Ocean and Glacier Bay; mountains rise to
12,000-15,000 feet in altitude only 15 miles from tide-
water (pl. 6, fig. 6). Peaks are high ice-clad pyramids
having steep-cliffed walls, sharp ridges, and spirelike
summits. There are a few subsummit ice plateaus but
no passes across the range.

Drainage.-The Fairweather Range is drained en-
tirely by glaciers; most of these discharge into the
Pacific Ocean or Glacier Bay.

Lakes.-There are no lakes.

Glaciers and permafrost.-Most of the range is above
firn line (4,000 ft) and supports vigorous glaciers that
descend to tidewater. Glaciers on the west side have
not advanced or retreated in recent years; those on the
east side have retreated in the last 60 years and expose
fiords having walls nearly 6,000 feet high. Permafrost
is probably absent.

Geology.-The Fairweather Range is underlain
mainly by crystalline schist that has northwesterly
structural trends parallel to the length of the range.
Many large granitic stocks and three large elliptical
layered mafic bodies have intruded the schist. The
range is bounded on its southwest side by a major
fault, the Fairweather fault, on which a lateral dis-
placement of 21 feet took place in July 1958.

References.-Mertie (1931); Miller (1953b, 1960) ;
Rossman (1963a, 1963b) ; Tocher and Miller (1959).

The Fairweather Range is covered by the following
1:250,000 topographic maps: Skagway and Mount
Fairweather.

GULF OF ALASKA COASTAL SECTION (56)

General topography.-The Gulf of Alaska coastal
section has a diversified topography carved in Terti-
ary rocks. A coastal plain marked by longitudinal
beach and dune ridges, crossed in places by outwash
plains and by belts of morainal topography, is backed
by marine terraces as high as 800 feet in altitude and
by rugged intricately gullied mountain ridges as high
as 12,000 feet. The straight exposed coastline is broken
at intervals of 50-100 miles by large fiords (pl. 6,
fig. 10).

Drainage.-Short melt-water streams of large vol-
ume cross the lowland. Bars built by coastal currents
cause the river mouths to go through cycles of west-

42 PHYSIOGRAPHIC DIVISIONS OF ALASKA

ward migration followed by breakthrough at their
original sites during periods of high runoff.

Lakes.-There are many ephemeral lakes along the
margins of the piedmont glaciers. A few large lakes
occupy ice-carved basins.

Glaciers and permafrost.-The firn line is at an alti-
tude of 2,000-4,000 feet. Icefields on higher mountains
and valley glaciers in most of the valleys coming from
the St. Elias and Chugach Mountains feed enormous
piedmont glaciers, of which the Malaspina Glacier (pl.
6, fig. 4) is the largest. Glacial advances within the last
thousand years are greater than any advance recorded
in the Pleistocene. Permafrost is absent.

Geology.-The Cenozoic rocks are intensely de-
formed yet easily eroded claystone, sandstone, and con-
glomeratic sandy mudstone, all tightly folded and
thrust to the south. Large thrust faults separate this
section from mountains to the north and northeast.
Marine terraces show that the area has been uplifted
rapidly. The conglomeratic sandy mudstone inter-
bedded in the Cenozoic section is interpreted to be
marine tillite; it indicates recurrent tidewater glacia-
tion on this coast as far back as Pliocene time or earlier.
An earthquake-induced landslide on July 9, 1958,
created a flood wave on Lituya Bay that splashed to
a height of 1,700 feet on the side of a mountain, sweep-
ing the forest in its path into the bay.

References.-Mertie (1931); Miller (1958a, 1958b,
1957, 1958, 1960; written commun., 1957) ; Plafker and
Miller (1957, 1958) ; Twenhofel (1952).

The Gulf of Alaska coastal section is covered by -the
following 1:250,000 topographic maps: Mount St.
Elias, Bering Glacier, Cordova, Yakutat, Icy Bay,
Middleton Island, and Mount Fairweather.

CHILKAT-BARANOF MOUNTAINS (57)
General topography.-The Chilkat-Baranof Moun-
tains are a highland of diversified topography, which
is divided into four subsections: the Alsek Ranges
(57a), a subsection of rugged glaciated mountains
4,000-7,500 feet altitude, containing horns and arétes
(pl. 6, fig. 6) ; the Glacier Bay subsection (57b), a low-
land, largely drowned, that contains isolated rounded
mountains; the Chichagof Highland (57c¢), consisting
mainly of northwest-trending ridges whose summits
are accordant, rounded, and 3,000-3,500 feet in altitude
and of long fiords and through valleys; and the Bara-
nof Mountains (57d), a rugged asymmetric chain
3,000-5,300 feet in altitude, having a steep eastern slope
(pl. 5, fig. 10) and a more gentle southwest slope
deeply indented by fiords. The southern two-thirds
of the Chilkat-Baranof Mountains consists of islands.
A narrow strandflat lies on the west coast of Chichagof
and Baranof Islands.

Drainage.-The Chilkat-Baranof Mountains are
drained by short, swift streams that flow directly to
the ocean. Chains of cascades are common on the east
side of Baranof Island (pl. 5, fig. 10).

Lakes.-Liakes abound in ice-carved basins in Bara-
nof and southwestern Chichagof Islands (pl. 5, fig. 10).
Elsewhere lakes are few.

Glaciers and permafrost.-The Alsek Ranges have
large icefields containing tidal glaciers; Glacier Bay
was filled with ice at least 2,000 feet thick as late as
1750, and glaciers have retreated more than 50 miles
since then to expose the bay. Mountains on Baranof
Island higher than 4,500 feet support cirque glaciers
and small icefields (pl. 5, fig. 10). Permafrost is prob-
ably absent.

Geology.-Northwest-trending belts of Paleozoic
and Mesozoic sedimentary and volcanic rocks underlie
the Alsek Ranges and Glacier Bay subsections; north-
west-trending belts of crystalline schist and gneiss and
large areas of migmatitic and granitic rocks underlie
the Chichagof Highland and the northeastern part of
the Baranof Mountains and are bordered on the west
by a belt of Mesozoic graywacke and greenstone. The
rocks are cut by many northwest-trending faults. Quat-
ernary volcanoes make up southern Kruzof Island.

References.-Buddington and Chapin (1929) ; Guild
and Balsley (1942); Knopf (1912); Lathram and
others (1959); Reed and Coats (1942); Rossman
(1963b; oral commun., 1956) ; Sainsbury and T'wen-
hofel (1954) ; Seitz (1959).

The Chilkat-Baranof Mountains are covered by the
following 1:250,000 topographic maps: Skagway,
Juneau, Mount Fairweather, Sitka, and Port Alex-

ander.
PRINCE OF WALES MOUNTAINS (58)

General topography.-The Prince of Wales Moun-
tains are moderately rugged glaciated mountains hav-
ing rounded hummocky summits 2,000-3,500 feet in
altitude and some spirelike arétes as much as 3,800
feet in altitude. They are dissected by steep-walled U-
shaped valleys and by fiords 600-1,000 feet deep. (See
pl. 5, fig. 11.) Several passes less than 500 feet in
altitude cross the range. The northeast front is
abrupt, and the northwest boundary is indistinet. Karst
topography is found in areas of marble on Dall and
Long Islands in the southwest Prince of Wales Moun-
tains.

Drainage.-Short, swift streams, having many lakes
and waterfalls, drain the mountains and generally
follow trenches eroded by Pleistocene glaciers along
joints, faults, and bedding.

Lakes.-There are many rock-basin and cirque lakes,
a few as much as 2,000 feet above sea level. The larg-
est lake is 7 miles long and 1 mile wide.

REFERENCES CITED 43

Glaciers and permafrost.-One or two very small
glaciers lie on the protected north sides of the highest
peaks. There is no permafrost.

Geology.-The Prince of Wales Mountains are un-
derlain in part by well-consolidated slightly metamor-
phosed Paleozoic sedimentary and volcanic rocks and
in part by crystalline schist and marble. Several small
granitic stocks cut these rocks. The mountains were
entirely covered by the Pleistocene cordilleran ice sheet,
which was fed partly from local centers but mainly
from the Coast Mountains to the east.

References.-Buddington and Chapin (1929) ; Con-
don (1961) ; G. D. Eberlein (oral commun., 1959).

The Prince of Wales Mountains are covered by the
following 1:250,000 topographic maps: Ketchikan,
Craig, Prince Rupert, and Dixon Entrance.

COAST MOUNTAINS

The Coast Mountains form a massive mountain

barrier underlain by the Coast Range batholith. The
province can be divided into the Boundary Ranges
(59) and the Coastal Foothills (60).

BOUNDARY RANGES (50)

General topography.-The Boundary Ranges are a
glacier-covered upland 5,000-7,000 feet in altitude
dissected by a dendritic pattern of deep steep-walled
U-shaped valleys. The ridges have rounded accordant
summits and are surmounted by arétes and horns ris-
ing to 8,000-10,000 feet. Many of the valleys are
drowned and form fiords. Passes are scarce, and valley
heads are isolated. The mountains give an impression
of great bulk and are bordered largely by cliffs that
plunge several thousand feet to tidewater. (See pl. 5,
fig. 9; pl. 6, fig. 7.)

Drainage.-The summit of the Coast Mountains co-
incides approximately with the international boundary ;
most of the range in Alaska is drained by glacial
streams less than 20 miles long. Large braided rivers
flow southwestward across the range at intervals of
30-120 miles from low-lying areas in northwestern
British Columbia.

'Lakes.-A few small lakes lie in rock basins on valley
floors and in mountainside hollows in the western,
glacier-free part of the range.

Glaciers and permafrost.-The firn line is about
4,500-5,000 feet in altitude. Extensive mountain ice-
caps, the largest 90 miles long, feed many valley
glaciers, some of which descend to tidewater. Extent
of permafrost is unknown.

Geology.-The Boundary Ranges are underlain _

mostly by the massive granitic rocks of the Coast Range
batholith ; a belt of schist and phyllite along its western
margin and migmatized roof pendants within the

batholith give a strong northwesterly grain to the
topography.

References.-Buddington (1929); Buddington and
Chapin (1929) ; Lathram and others (1959) ; Sainsbury
and Twenhofel (1954) ; Twenhofel (1952).

The Boundary Ranges are covered by the following
1:250,000 topographic maps: Atlin, Skagway, Taku
River, Juneau, Sumdum, Bradfield Canal, Petersburg,
Ketchikan, and Prince Rupert.

COASTAL FOOTHILLS (60)

General topography.-The Coastal Foothills consist
of blocks of high mountains 3-30 miles across separated
by flat-floored valleys and straits 4-10 miles wide;
they include closely spaced mountainous islands and
peninsulas 1,000-4,500 feet in altitude. Mountains
less than 3,500 feet in altitude were glacially over-
ridden and have rounded hummocky summits (pl. 6,
fig. 11). Higher mountains are generally sharp crested.
The boundaries with bordering sections are indistinct.

Drainage.-Few streams are more than 10 miles long.
The lower parts of most valleys are drowned, forming
inlets and harbors.

Lakes.-There are many rock-basin lakes, the largest
8 miles long and 1 mile wide.

Glaciers and permafrost.-The Coastal Foothills are
almost entirely ice free. A few small glaciers lie on the
north sides of the high peaks on Admiralty Island.
There is no permafrost.

Geology.-Northwest-trending belts of metamorphic
rocks, cut by many faults parallel to the northwesterly
trend of the rocks, give the topography a pronounced
northwest grain. Small granitic and ultramafic bodies
and westerly projections of the Coast Range batholith
cut the metamorphic rocks. Southwest Admiralty
Island is a high Tertiary basalt plateau.

References.-Buddington and Chapin (1929) ; Lath-
ram and others (1959); Sainsbury and Twenhofel
(1954).

The Coastal Foothills section is covered by the fol-
lowing 1:250,000 topographic maps: Taku River,
Juneau, Sumdum, Sitka, Bradfield Canal, Petersburg,
Ketchikan, Craig, and Prince Rupert.

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44

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1911, The Mount McKinley region, Alaska, with de-
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1915, An ancient volcanic eruption in the Upper
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1916a, The Chisana-White River district, Alaska :
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1916b, The Turnagain-Knik region: U.S. Geol. Sur-
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1919, The Kantishna region, Alaska: U.S. Geol. Sur-
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1927, Geology of the upper Matanuska Valley, with a
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1932, The eastern portion of Mount McKinley Na-
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1935, The Southern Alaska Range: U.S. Geol. Survey
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1937, Kodiak and adjacent islands, Alaska:
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1940, Geology of the Alaska Railroad region: U.S.
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Capps, S. R., and Johnson, B. L., 1915, The Ellamar district,
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Cass, J. T., 1957, Reconnaissance geologic map of the Kateel
River quadrangle, Alaska: U.S. Geol. Survey Misc. Geol.
Inv. Map 1-243, scale 1: 250,000.

1959a, Reconnaissance geologic map of the Norton
Bay quadrangle, Alaska: U.S. Geol. Survey Misc. Geol.
Inv. Map I-286, scale 1: 250,000. ‘

1959b, Reconnaissance geologic map of the Candle
quadrangle, Alaska: U.S. Geol. Survey Misc. Geol. Inv.
Map I-287, scale 1: 250,000.

1959c¢, Reconnaissance geologic map of the Unalakleet
quadrangle, Alaska: U.S. Geol. Survey Misc. Geol. Inv.
Map I-288, scale 1 : 250,000.

1959d, Reconnaissance geologic map of the Ruby
quadrangle, Alaska: U.S. Geol. Survey Misc. Geol. Inv.
Map I-289, scale 1: 250,000.

U.S.

REFERENCES CITED

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1959f, Reconnaissance geologic map of the Nulato
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Chapin, Theodore, 1918, The Nelchina-Susitna region, Alaska :
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Chapman, R. M., and Sable, E. G., 1960, Geology of the Utukok-
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1953, Geology of Buldir Island, Aleutian Islands,
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--- 1956a, Geology of northern Adak Island, Alaska:

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1956b, Geology of northern Kanaga Island, [Alaska] :
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1956c, Reconnaissance geology of some western
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1959a, Geologic reconnaissance of Gareloi Island,
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1959b, Geologic reconnaissance of Semisopochnoi
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45

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U.S.

46

Grant U. S., and Higgins, D. F., 1913, Coastal glaciers of Prince
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19602, Generalized geologic map of the Nelchina area,
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1960b, Geologic map of Talkeetna Mountains (A-2)
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1960c, Geologic map of Talkeetna Mountains (A-1)
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1961a, Geologic map and cross sections of the An-
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Grantz, Arthur, White, D. E., Whitehead, H. C., and Tagg,
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Alaska: Am. Assoc. Petroleum Geologists Bull., v. 46, p.
1990-2002.

Griggs, R. F., 1922, The Valley of Ten Thousand Smokes
[Alaska] : Washington, Natl. Geog. Soc., 340 p.

Guild, P. W., and Balsley, J. R., Jr., 1942, Chromite deposits
of Red Bluff Bay and vicinity, Baranof Island, Alaska :
U.S. Geol. Survey Bull. 936-G, p. 171-187.

Harrington, G. L., 1918, The Anvik-Andreafski region, Alaska
(including the Marshall district) : U.S. Geol. Survey Bull.
683, 70 p.

1921, Mineral resources of the Goodnews Bay region :
U.S. Geol. Survey Bull. 714-E, p. 207-228.

Hay, R. L., 1963, Stratigraphy of beds I through IV, Olduvai
Gorge, Tanganyika: Science, v. 189, p. 829-833.

Hoare, J. M., and Coonrad, W. L., 1959a, Geology of the Bethel
quadrangle, Alaska: U.S. Geol. Survey Misc. Geol. Inv.
Map I-285, scale 1 : 250,000.

1959b, Geology of the Russian Mission quadrangle,
Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map I-292,
scale 1: 250,000.

1961a, Geologic map of Goodnews quadrangle, Alaska :
U.S. Geol. Survey Misc. Geol. Inv. Map I-339, scale
1 : 250,000.

1961b, Geologic map of Hagemeister Island quad-
rangle, Alaska: U.S. Geol. Survey Misc. Inv. Map I-321,
scale 1: 250,000.

Holmes, G. W., and Benninghoff, W. S., 1957, Terrain study of
the Army Test Area, Fort Greely, Alaska: U.S. Geol.
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Holmes, G. W., Hopkins, D. M., and Foster, H. L., 1965, Dis-
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PHYSIOGRAPHIC DIVISIONS OF ALASKA

Hopkins, D. M., 1949, Thaw lakes and thaw sinks in the
Imuruk Lake area, Seward Peninsula, Alaska: Jour.
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1951, Lignite deposits near Broad Pass Station,
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963-E, p. 187-191.

1955, Northern Seward Peninsula, in Hopkins, D. M.,
Karlstrom, T. N. V., and others, 1955, Permafrost and
ground water in Alaska: U.S. Geol. Survey Prof. Paper
264-F, p. 120-124.

1959a, Cenozoic history of the Bering Sea land
bridge [Alaska]: Science, v. 129, p. 1519-1528.

1959b, History of Imuruk Lake, Seward Peninsula,
Alaska: Geol. Soc. America Bull., v. 70, p. 1033-1046.

1959c, Some characteristics of the climate in forest
and tundra regions in Alaska: Arctic, v. 12, p. 215-220.

1963, Geology of the Imuruk Lake area, Seward
Peninsula, Alaska: U.S. Geol. Survey Bull. 1141-C, 101 p.

Hopkins, J. P., and Hopkins, D. M., 1958, Seward Peninsula,
in Williams, Howel, ed., Landscapes of Alaska-their
geologic evolution: Berkeley, California Univ. Press, p.
104-110.

Hopkins, D. M., MacNeil, F. S., and Leopold, E. B., 1960, The
coastal plain at Nome, Alaska-a late Cenozoic type sec-
tion for the Bering Strait region in Chronology and Clima-
tology of the Quaternary: Internat. Geol. Cong., 21st,
Copenhagen 1960, Proc., pt. 4, p. 46-57.

Hopkins, D. M., and Sigafoos, R. S., 1951, Frost action and
vegetation patterns on Seward Peninsula, Alaska: U.S.
Geol. Survey Bull. 974-C, p. 51-101.

Hopkins, D. M., Karlstrom, T. N. V., and others, 1955, Perma-
frost and ground water in Alaska: U.S. Geol. Survey Prof.
Paper 264-F, p. 113-146.

Juhle, R. W., 1955, Iliamna Volcano and its basement: U.S.
Geol. Survey open-file rept., Apr. 27, 1955, 74 p., 38 pls.

Juhle, R. W., and Coulter, H. W., 1955, The Mount Spurr
eruption, July 9, 1953: Am. Geophys. Union Trans., v. 36,
p. 199-202.

Kachadoorian, Reuben, 1956, Engineering geology of the Nen-
ana-Rex area, Alaska: U.S. Geol. Survey open-file rept.,
Apr. 24, 1956, 20 p., 2 figs.

Kachadoorian, Reuben, Hopkins, D. M., and Nichols, D. R.,
1954, A preliminary report of geologic factors affecting
highway construction in the area between the Susitna and
Maclaren Rivers, Alaska: U.S. Geol. Survey open-file rept.,
June 21, 1954, 74 p., 8 figs.

Kachadoorian, Reuben, and Péwé, Troy L., 1955, Engineering
geology of the southern half of the Mount Hayes A-5
quadrangle, Alaska: U.S. Geol. Survey open-file rept.,
May 17, 1955, 27 p., 5 figs.

Karlstrom, T. N. V., 1955, Kenai lowland, in Hopkins, D. M.,
Karlstrom, T. N. V., and others, Permafrost and ground
water in Alaska: U.S. Geol. Survey Prof, Paper 264-F,
p. 133-134.

-__- 1960, The Cook Inlet, Alaska, glacial record and
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Keller, A. S., and Detterman, R. L., 1951, Preliminary report
on the stratigraphy and structure of the Shaviovik and
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1919, The beach placers of the west coast of Kodiak
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Mangus, M. D., 1953, Regional interpretation of the geology of
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47

Martin, G. C., and Katz, F. J., 1912, Geology and coal fields of
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Martin, G. C., Johnson, B. L., and Grant, U. S., 1915, Geology
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Mather, K. F., 1925, Mineral resources of the Kamishak Bay
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Mendenhall, W. C., 1905, Geology of the central Copper River
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1930b, Geology of the Eagle-Circle district, Alaska :
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1931, Notes on the geography and geology of Lituya
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1932, The Tatonduk-Nation district [Alaska]: U.S.
Geol. Survey Bull. 836-E, p. 347-443.

1937, The Yukon-Tanana region, Alaska: U.S. Geol.
Survey Bull. 872, 276 p.

1938, The Nushagak district, Alaska:
Survey Bull. 903, 96 p.

1940, The Goodnews platinum deposits, Alaska:
U.S. Geol. Survey Bull. 918, 97 p.

1942, Tertiary deposits of the Eagle-Circle district,
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Mertie, J. B., Jr., and Harrington, G. L., 1924, The Ruby-
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Miller, D. J., 1958a, Late Cenozoic marine glacial sediments and
marine terraces of Middleton Island, Alaska: Jour. Geol-
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1953b, Preliminary geologic map of Tertiary rocks in
the southeastern part of the Lituya district, Alaska, and
correlated columnar sections of Tertiary rocks in the
Lituya district, Alaska: U.S. Geol. Survey open-file map,
May 26, 1953, 1 map, 1 sec.

1957, Geology of the southeastern part of the Robinson
Mountains, Yakataga district, Alaska: U.S. Geol. Survey
Oil and Gas Inv. Map OM-187, scale 1: 68,360.

1958, Gulf of Alaska area, in Williams, Howel, ed.,
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1960, Giant waves in Lituya Bay, Alaska:
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Miller, D. J., Payne, T. G., and Gryc, George, 1959, Geology of
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131 p.

Miller, R. D., and Dobrovolny, Ernest, 1959, Surficial geology
of Anchorage and vicinity, Alaska : U.S. Geol. Survey Bull.
1093, 128 p.

Moffit, F. H., 1912, Headwater regions of Gulkana and Susitna
Rivers, Alaska, with accounts of the Valdez Creek and
Chistochina placer districts: U.S. Geol. Survey Bull.
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1913, Geology of the Nome and Grand Central quad-
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1914, Geology of the Hanagita-Bremner region,
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U.8.

48 PHYSIOGRAPHIC DIVISIONS OF ALASKA

Moffit, F. H., 1915, The Broad Pass region, Alaska, with sections
on Quaternary deposits, igneous rocks, and glaciation, by
J. E. Pogue: U.S. Geol. Survey Bull. 608, 80 p.

1927, The Iniskin-Chinitna Peninsula and the Snug
Harbor district, Alaska : U.S. Geol. Survey Bull. 789, 71 p.

1935, Geology of the Tonsina district, Alaska: U.S.
Geol. Survey Bull. 866, 38 p.

1938, Geology of the Chitina Valley and adjacent
area, Alaska: U.S. Geol. Survey Bull. 894, 137 p.

1954a, Geology of the eastern part of the Alaska
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D, p. 63-218.

1954b, Geology of the Prince William Sound region,
Alaska: U.S. Geol. Survey Bull. 989-E, p. 225-310.

Muller, E. H., 1952, The glacial geology of the Naknek dis-
trict, the Bristol Bay region, Alaska: Ann Arbor, Mich.,
University Microfilms, 98 p.

1953, Northern Alaska Peninsula and eastern Kil-
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tiple glaciation in Alaska, a progress report: U.S. Geol.
Survey Circ. 289, p. 2-8.

1955, Bristol Bay region, in Hopkins, D. M., Karl-
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in Alaska: U.S. Geol. Survey Prof. Paper 264-F, p. 131-
133.

1956, Quaternary glaciation in the Bristol Bay region,
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Muller, E. H., Jubhle, Werners, and Coulter, H. W., 1954, Cur-
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Muller, J. E., 1954, Preliminary map, Kluane Lake [west half]
Yukon Territory: Canada Geol. Survey Paper 53-20, 9 p.

Muller, S. W., 1947, Permafrost or permanently frozen ground
and related engineering problems: Ann Arbor, Mich.,
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Murray, H. W., 1945, Profiles of the Aleutian Trench: Geol.
Soc. America Bull., v. 56, p. 757-781.

Nelson, W. H., 1959, Geology of Segula, Davidof, and Khvostok
Islands, Alaska: U.S. Geol. Survey Bull. 1028-K, p. 257-
266.

Odell, N. E., 1950, Notes on the geology of the St. Elias Range,
Alaska-Yukon Territory, North America: Geol. Soc. Lon-
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Paige, Sidney, and Knopf, Adolph, 1907, Geologic reconnais-
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U.S. Geol. Survey Bull. 327, 71 p.

Park, C. F., Jr., 1933, The Girdwood district, Alaska :
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Patton, W. W., Jr., and Bickel, R. S., 1956a, Geologic map and
structure sections along part of the lower Yukon River,
Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map I-197,
scale 1 : 200,000.

1956b, Geologic map and structure sections of the
Shaktolik River area, Alaska: U.S. Geol. Survey Misc.
Geol. Inv. Map I-226, scale 1 : 80,000.

Patton, W. W., Jr.. Magnus, M. D., and Brosgé, W. P., 1951,
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U.S.

Payne, T. G., and others, 1951, Geology of the Arctic slope of
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Péwé, T. L., 1948, Terrain and permafrost of the Galena Air
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1954, Effect of permafrost on cultivated fields, Fair-
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1955a, Origin of the upland silt near Fairbanks,
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1955b, Middle Tanana Valley in Hopkins, D. M.,
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1958, Geology of the Fairbanks (D-2) quadrangle,
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Péwé, T. L., and others, 1953, Multiple glaciation in Alaska, a
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Plafker, George, 1955, Geologic investigations of proposed
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Plafker, George, and Miller, D. J., 1957, Reconnaissance geol-
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1958, Glacial features and surficial deposits of the
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Porsild, A. E., 1938, Earth mounds in unglaciated arctic north-
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Powers, H. A., Coats, R. R., and Nelson, W. H., 1960, Geology
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Prindle, L. M., 1905, The gold placers of the Fortymile, Birch
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Ross, C. P., 1933, The Valdez Creek mining district, Alaska :
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Rossman, D. L., 1963a, Geology and petrology of two stocks of
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Sainsbury, C. L., 1961, Geology of part of the Craig C-2 quad-
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1949, The frost-moved rubbles of Jumbo Dome and
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Wahrhaftig, Clyde, and Cox, Allan, 1959, Rock glaciers in the
Alaska Range: Geol. Soc. America Bull., v. 70, p. 383-486.

Wahrhaftig, Clyde, and Hickceox, C. A., 1955, Geology and coal
deposits, Jarvis Creek coal field, Alaska: U.S. Geol. Survey
Bull. 989-G, p. 353-367.

Wahrhaftig, Clyde, Hickcox, C. A., and Freedman, Jacob, 1951,
Coal deposits on Healy and Lignite Creeks, Nenana coal
field, Alaska, with a section on clay deposits on Healy
Creek, by E. H. Cobb, in Barnes, F. F., and others, Coal
investigation in south-central Alaska, 1944-46: U.S. Geol.
Survey Bull. 963-E, p. 141-165.

Wallace, R. E., 1946, Terrain analysis in the vicinity of North-
way, Alaska, with special reference to permafrost: U.S.
Geol. Survey Permafrost Program Prog. Rept. 3, 34 p.

1948, Cave-in lakes in the Nabesna, Chisana, and
Tenana River valleys, eastern Alaska: Jour. Geology, v.
56, p. 171-181.

Washburn, A. L., 1956, Classification of patterned ground and
review of suggested origins: Geol. Soc. America Bull., v.
67, p. 823-865. t

Washburn, Bradford, 1936, Exploring Yukon's glacial strong-
hold: Natl. Geog. Mag., v. 69, p. 715-748.

Watson, C. E., 1959, Climate of Alaska in Climates of the
States: U.S. Weather Bureau, Climatography U.S. no.
60-49, 24 p.

50 p PHYSIOGRAPHIC DIVISIONS OF ALASKA

Wilcox, R. E., 1959, Some effects of recent volcanic ash falls, ___ 1959, Geology of the western part of the Big Delta
with especial reference to Alaska: U.S. Geol. Survey (D-6) quadrangle, Alaska: U.S. Geol. Survey Misc. Geol.
Bull. 1028-N, p. 409-476. Inv. Map I-297, scale 1: 63,360.

Williams, Howel, ed., 1958, Landscapes of Alaska-their geo- - ______- 1960, Cenozoic sediments beneath the central Yukon
logic evolution: Berkeley, California Univ. Press, 148 p. Flats, Alaska, in Short papers in the geological sciences:

Williams, J. R., 1955, Yukon Flats, in Hopkins, D. M., Karl- U.S. Geol. Survey Prof. Paper 400-B, p. B329.
strom, T. N. V., and others, 1955, Permafrost and ground - Williams, J. R., Péwé, T. L., and Paige, R. A., 1959, Geology of
water in Alaska: U.S. Geol. Survey Prof. Paper 264-F, the Fairbanks (D-1) quadrangle, Alaska: U.S. Geol.

p. 124-126. Survey Geol. Quad. Map GQ-124, scale 1: 63,360.

A Page
Acknowledgments........:..................} 3

 
  

Alaska-Aleutian province........_..... aus
Alaska 0... 5, 85,1, 18
Aleutian Islands

Aleutian Range.
Alsek Ranges.........

   
    

 
 
 
  
  
 

 

 
  

 

Altiplanation terraces.... ._...._. 16, 26, 27, 28, 37, 38
Ambler-Chandalar Ridge and Lowland sec-

rer eve- Col- EJ
20
AniokLowland....................\ 21
Arctic Coastal 18
Arctic 20
Arctic Mountains province..............__... 20
CO. cre eaces 13, 40, 42, 43
AUSA .n bandh au 20

B

35
fDaird #1
Baranof Mountains.
Batholith...
Bering Platform.
Bering Shelf......
Birch Creck
nh reese
Boundary Ranges....
Broad Pass Depression. .

Brooks Range...........................< 5, 18, 21, 7
Buckland River Lowland....._._.___________ 28
C
0-2... .u cols 33, 34
Canadian Cordillera. 3
Cenozole 7
Chatham 89

Chichagof Highland...._.__._____.
Chilkat-Baranof Mountains........
Chugach
Chulitna Mountains...............
Cinder cones.... ese?
se cL ole cn ience cen 13, 38, 40

Clarence Lake Upland..................._...
Classification, physiographic, basis
Clearwater Mountains........._...
Cliff-and-bench topography.......
Climate, Alasks...................

Quaternary patterns. =
Coastal Foothills:
Coastal Trough province. ..._________________ 86
Coast Mountains........_._.___.
Cook Inlet-Susitna Lowland...
Copper River Lowland...._.....___...
Cordillera, North American
Craters, volcanic.........__.....

   

  
 
  

Cutler River Upland...._.................... 21

INDEX

D Page
'De Long Mountalnis.:.t-._....ll....._._....ls. 20
Devonian history....

 
 
 
  
 
 
  

 

  

   

 

 
 
 

 

End moraines.
Eskers........
Exploration..
F

Fairweather 41
Fog Lakes 37
ol 40, 41, 42, 43
Firn line... 333537394042“
L2. -.. cone cles aln a

G
Geologic history, summary........_._______.. 6
Geologic time scale_..._.......
Glacial lakes......

2 .s. c 20 ne 400 ee etbe ave beaut s shows
Glaciated areas, physiographic evolution... ... 18
-... 10:0 III LLL. 8, 13, 35, 42
'Dlacier 42
Glaciers............~. 22, 33, 34, 35, 37, 39, 40, 41, 42, 43
(GroOUnI 00. 14, 36
Gulf of Alaska Coastal section........_...._.. 41
Gulkan§® Upland: .... ...s

H
Holiffis lls
f
TCB -.-. co... ool 17, 20, 24, 38
polygonal ground..........
Indian River Upland.........
Innoko Lowlands...
Inferior Plaing........_........

 

 

 
  

Intermontane Plateaus..........._.....__....
J
T ..: 1... XL e eevee cense 5
K
T L 12s oul eo . ene NCEA canvass 14
Kanuti sat k 6
(Karst 42
Kenai-Chugach Mountains...........___..__.. 18, 40
| Kobuk River valley...l._...__....... A...... 17, 27
Kobuk-Selawik Lowland.......__._________._ 27
Kodiak Mountains........
Kokrine-Hodzana Highlands..
Koyukuk Flats..._.__..._.
Kupreanof Lowland............
Kuskokwim Mountains....................... 80

L Page

Lake Louise Plateau......................_... 38
Lakes, glacial...._....... 14, 22, 31, 33, 34, 36, 38, 39, 42
16-mIAMEINAL .: C2 39, 40, 41

  

thaw or thermokarst............ cee 17

 

  

 

 
 

   

 

 

   

MeAHnOORR 21 .co 000 ATL cest 15, 16, 17, 27
Meander-scroll pattern_..........._...._..._.. 17, 27
-Mentasta-Nutzotin Mountain segment (of the
Alaska ..... 35
5
Metamorphism. ...... 5
Mission Lowland...... 21
Mississippian history..............._....__... 5
22 een ce 13, 14, 31
Mountain building... 2,5
NMC. .:. ...o col.. ccc cE. cc uel e. 35
-o ice enn 16, 24, 29
Mudflows....

MUG NOICBROES. 220: 2.0. ...ll cues 38

N
Nikolai Greenstone........................_.. 5
Noatak #1
North American Cordillera.............._... 2,7, 18
Northern Plateaus province.... 7, 22
Northway-Tanacross Lowland. #4
Nowitna Lowland..._... 29
NMulato Hills............. 28
Nunivak Island........... 32
Nushagak-Big River Hills............_....... 80
Nushagak-Bristol Bay Lowland....._._...... 81

0
28
Ordovician history...... 5
Orogenic activity........ 5
Outwash, TARS :. 2000 12A. cedes 16, 20
PIMING .. 42.0.2 000000. ALLE eases 14, 31, 36

P
Pacific Border Ranges province........._..... 89

Pacific Mountain System......

sh River 27
Pan 27
PAIGOROIC ...i 5
Patterned cs.. 0A 14,17
llc Pon An reine dedi erences 17
Peneplain. . . 2.0000 00. oor eed eee 87
Permafrost........ 8, 14, 15, 17, 18
Ys: aids ann 20
Permafrost 18
Formian 5

51

 

 

 

52
Page
Physiography, evolution of......_....._.___._. 7,8
PINGO8..... .u or nce cee 17, 18, 21, 23, 24, 28, 38
Pleistocene history.._._........... 6
Polygonal ground.. - 14, 17, 20, 35
Polygons, c.. 02.20. ece 14
Porcupine lloc 22
Precambrian history. * 5
PribiofAslanids. .. ..o 00.000. 2 bel dO ave 32
Prince of Wales Mountains..........__._____.. 18, 42
Purcel! Mountaing.. ...... 200. 27
Q
Quaternary history,... 8
R
iBAINWASh ..-... ... 000. ... ler avea sare 15
Rampart 25
Ray. cel. 25
'Rock 16, 35, 37, 39
Rocky Mountain System...._________.___.. 7, 18, 20 -
Romanzof Mountaing......................... 22
8
S6. Elise 18, 41
St. Lawrence 82
St. Matthew 32
Schwatka Mountaing................_........ 22
Selawik Hills... .. rebels 28

INDEX

Page

Seward ...... 81
Silnrlan RigtOrys.12. 2. . .o 5.2202 IAI cc 5
Solffluction. ...s . 22. . 220.0000 Ie 14, 15, 16, 35
Solifluetion IObeg. . 1 . 2020 core ouly once 15
Stagnant ice topography.......__.___.___.. 14, 36, 38
Stone gifipe . . . .. osu, nev co as 14, 20
. }; :. . 1 creel o en 42
StreAnt CApHITGL : 2.2292. .. chy Aer eins he 20, 24
Superposed drainage................. 21, 22, 23, 25, 35
Bw alos. .s 2 0202. 00. : A V. uveal re sabe nude by 17

TP

Talkeetna Mountaing......................... 5,87
Tanana-Kuskokwim Lowland..........._.... 29
'permindlogy...... 0002 ise 3
TTeFPACES.L .... .. co. 12 . bo .o ous oue eae 16, 20
OHEWAShLL . 02.4 aus. scan eni 16
'Pertlary history.... ... L0. eel e 7,8
Teshekpuk Lake section..........____.__.___.. 18
Thaw lake............ 17, 18, 20, 23, 24, 25, 26, 27, 30, 32
TRHAW SIM. .L . 12. .. ...o... s oven ae e 17, 25
Thazsik Mountain.... ...... 0290.0 _ 22
17
THOPY L ILI IIL: ved vos a 13, 14
Tinting Valley .. . ..... edes 28
TPorrent#...s. 0020.0 200000 III vue deans 16
POMS LL. %. ens 15

 

Page

Tozitna-Melozitna Lowland.......__...__._.. 26

highOry . 20.02. 000 ce. cen ale 5
U

U-shaped valleys......._...___.._._. 13, 33, 40, 42, 43

Unglaciated areas, physiographic evolution... 14, 16

Upper Matanuska Valley. ..__...___________.. 87
v

-. Leduc sod 8,15, 17

Volcanic rocks, Quaternary......_.___.. 32, 33, 34, 39

Voleshlem../1: ..... ..... ...i... Jo.. 5

Volcanoes, active... 33, 34, 30, 41
w

Western Alaska province. 7, 26

White Hills section. 18

Mrangell Mountaing. L..... 89
¥

Yukon Flats Sechion... ... .\ 25

Yukon-Kuskokwim Coastal Lowland. 82

Yukon River.. 001... 18, 19

¥Yukon-Tanana Upland........._._...__..l_... 5, 24
Z

Tome BHlls.. c_ lt vt oin #7

U.S. GOVERNMENT PRINTING OFFICE : 1966 0-773-592
i < i%
FA »

*

 

 

UNITED STATES DEPARTMENT. OF THE INTERIOR f Was . : PROFESSIONAL PAPER 482
GEOLOGICAL SURVEY ‘ ' PLATE 3

 

  

 

+

 

: f *%," "& FIGURE 3. NORTH FRONT OF THE BROOKS RANGE AND THE PLAINS AND PEDIMENTS OF THE SOUTHERN
FIGURE 1. MOUNTAINS CARVED IN MILDLY METAMORPHOSED SEpIMENTARY ROCKS OF PALEOZOIC AGE,

: SECTION OF THE ARCTIC FOOTHILLS: (ISOLATED MOUNTAINS OF MISSISSIPPIAN LIMESTONE RISE ERO M
t C SOUTHERN BROOKS RANGE. LOOKING WEST ACROSS THE SOUTHERI‘Q JOHN RIVER. - OFFICAL U.S. NAVY PHGO- FIGURE 2. GLACIATED MOUNTAINS CARVED IN RECUMBENT FOLDS AND OVERTHRUST PLATES - OF PALEOZO!IC ROLLING TUNDRA PLAINS CARVED ON HIGHLY DEFORMED YET SOFT LATE PALEOZOIC AND MESsoZoIC ROCKS.
TOGRAPH $ - ' ; + a ROCKS, CHIEFLY LIMESTONE OF MISSISSIPPIAN AGE, NORTHERN BROOKS RANGE. LOOKING WEST FROM

MORAINE DEPOSITED BY THE ANAKTUVUK GLACIER IN LATE PLEISTOCENE TIME IN RIGHT FOREGROUND.

f OVEN THE TRIELEIN RIVER _ OFFICIAL-U s. NAVYPHOTOGRAFPE LOOKING WEST FROM OVER THE ANAKTUVUK RIVER NEAR ITS HEAD. OFFICIAL U.S. NAVY PHOTOGRAPH

spe-

 

                 

0 : FIGURE 4. RIDGES OF RESISTANT ROCK RISE ABOVE TUNDRA PLAINS CUT oN soFT ROCKS IN THE TIGHTLY
C N FOLDED SOUTHERN SECTION OF THE ARCTIC FOOTHILLS. LOOKING WEST FROM OVER THE ETIVILUK RIVER.
OFFICIAL U.S. NAVY PHOTOGRAPH

      

EXPLANATION :

yt/ ¢:

Location of point from which ob-

lique aerial photograph was

an" ; taken. Arrow indicates direction
(a mo' 1 of camera

   

 
  

Foothills

colt
'e

POINT HC

 
   
 

   

    

%
~.

FIGURE 4 _ FIGURE 3
<3 BROOKS

     
  
  
 

   
   

 

 

{FIGURE 2

s sw fe - % wert mad aas erage --as -. .o peso e shins l walla stds... om

 

FIGURE 5 ._
<3

  
   
 
 

Seward
Peninsula

          
 
 
 
 

y? TANANA

%.
/ ;
S

me

 

2 A/s \
{Gs EMMQQ’

AIRBAGNES 's

 

1
U

\
J:\(‘X\

ST

   

 
  

   

  
 

p

   

   

 

    
  

 
  

       

 

 

   

  
  

 

 

 

    

BETHEL
‘\..fl/\\ (o) I
; } i
NUNIVAK ISLAND
Lm/M‘Wg
nmn Sows, & SEAC
”7mm,/ K.
2YAKEUTAT® 1 4
\p #
HTLINGHAM \V
ALASKA
&
heads
®
AK
I
M m- I C G : fs > A
p A f If ( 433?“
FIGURE 5. GENTLY UNDULATING UPLAND SURFACES AND SHARP-V-SHAPED CANYONS TYPICAL OF THE Up- FORT RANDALL ¢ 178 .

LANDS OF NORTHERN SEWARD PENINSULA. ESCARPMENTS ALONG VALLEY IN FOREGROUND ARE EDGEs OF cpno® ~>GC 5 e

DISSECTED INTRAVALLEY LAVA FLOW. FINELY ETCHED GULLIES ON NEARBY SLOPES ARE CUT on LOESS; sl" % A. .. s 3 az

WHICH BLANKETS MOST OF THE UPLAND SURFACE. PLACER MINES ALONG RIVER. LOOKING WEST ACROSS wal ~ ~" AN **

ADAK %

> a ars 6°", Bp

 

VALLEY OF INMACHUK RIVER, ABOUT 20 MILES SOUTH OF DEERING. GOODHOPE BAY IN UPPER RIGHT
CORNER. PHOTOGRAPHY BY U.S. AIR FORCE

 

      

 

 

     

 

 

 

 

* FIGURE 6. ROUNDED MOUNTAINS WITH GENTIY SLOPING SIDES, NaARRowW ALLUVIUM-FLOORED VALLEYS, AND
FIGURE 11. MAP OF ALASKA, WITH BOUNDARIES OF PHYSIOGRAPHIC PROVINCES, SHOWING LOCATIONS OF PHOTOGRAPHS V-SHAPED TRIBUTARY GULCHES, FEATURESOF UPLAND AREAS THROUGHOUT INTERIOR ALASKA. SOUTHERN
PART OF THE YUKON-TANANA UPLAND. RELEF IS ABOUT 1500-2000 FT. BEDROCK IS NORTHEAST-TRENDING

PRECAMBRIAN SCHIST. VIEW EAST UP THE 2HENA RIVER. PHOTOGRAPH BY U.S. AIR FORCE

 

FIGURE 7. A COMPACT HIGHLAND ABOUT 4000 FT HIGH IN THE PAH RIVER SECTION. THE SHARP-CRESTED FIGURE 8. BRAIDED STREAM ON AN OUTWASH FAN IN THE SOUTHERN PART OF THE TANANA-KUSKOKWIM FIGURE 9. GOLD DREDGING IN A VALLEY IN THE YUKON-TANANA UPLAND NEAR FAIRBANKS. NOTICE THE FIGURE 10. END MORAINE ON THE LITTLE DELTA RIVER , SOUTHEAST PART OF THE TANANA-KUSKOKwWIM Low-
HIGHER RIDGES, GLACIATED IN PRE-WISCONSIN TIME, HAVE BEEN MODIFIED BY PERIGLACIAL MASS WASTING LOWLAND, NORTHWEST OF MOUNT McKINLEY. PHOTOGRAPH BY BRADFORD WASHBURN ROUNDED CRESTS AND SMOOTH GENTLE SLOPES OF THE BEDROCK HILLS. THEIR LOWER sLOPES ARE LAND, SHOWING IRREGULAR TOPOGRAPHY OF MOUNDS AND WATER-FILLED HOLLOWs. VIEW NORTHWEST
IN LATER COLD PERIODS. MORAINAL TOPOGRAPHY HAS BEEN DESTROYED. THE SMOOTH ROUNDED RIDGES t THICKLY MANTLED WITH LOESS,.IN WHICH SHALLOW GULLIES WERE CUT (MARKED BY LINES OF ASPENS) ACROSS MORAINE. PHOTOGRAPH BY BRADFORD WASHBURN

AND V-SHAPED CANYONS OF THE LOWER HILLS WERE SCULPTURED LARGELY BY MASS WASTING. THE LAKE-
DOTTED PAH RIVER FLATS ARE BEYOND THE HILLS. VIEW NORTH ALONG THE ZANE HILLS. PHOTOGRAPH BY
U.S. AIR FORCE

THE BANKS OF THE DREDGE AREA ARE FORMED OF FROZEN SILT AND MUCK, WHICH MUST BE REMOVED BY
HYDRAULICKING BEFORE THE GOLD- BEARING GRAVELS CAN BE THAWED AND DREDED. PHOTOGRAPH BY
| BRADFORD WASHBURN

 

 

 

INTERIOR-GEOLOGICAL SURVEY. WASHINGTON. D. C.-1965-G64166

 

AERIAL PHOTOGRAPHS ILLUSTRATINg} THE PHYSIOGRAPHY OF THE ROCKY MOUNTAIN SYSTEM AND INTERMONTANE PLATEAUS IN ALASKA

4
|

 

 

UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY

 

PROFESSIONAL PAPER 482
PLATE 6

 

 

FIGURE 1. MOUNTAINS OF THE ALASKA RANGE. CLIFFS IN FOREGROU_I\?D.,<WHICH DROP 5000 FT, ARE IN GRANITE, AND
ARE CAPPED BY OLDER METAMORPHIC ROCKS, VISIBLE ON. MOUNTAINTOP AT RIGHT. MOUNT HUNTER, a 14,580 FT

GRANITE MOUNTAIN@IS IN THE DISTANCE. (VIEW .WEST-EFROM..OVER'*THE-RUTH GLACIER. PHOTOGRAPH BY BRADFORD
WASHBURN &

 

 

 

FIGURE 5. VIEW NORTH ACROSS THE LOWER MATANUSKA VALLEY, COOK INLET-SUSITNA LOWLAND. THE MATANUSKA RIVER
AND THE TOWN OF PALMER ARE ON THE RIGHT. DIRECTLY ABOVE THE RIVER,. AT THE BASE OF THE MOUNTAINS, WISH-
BONE HILL, A SYNCLINAL MOUNTAIN OF CONGLOMERATE,IS THE CENTER OF A MaAjoR COAL FIELD. A COMPLEX SYS-
TEM OF ESKERS AND CREVASSE FILLINGS IN CENTER FOREGROUND PASSES INTO AN OUTWASH PLAIN ON THE
LEFT. THE FARMS ARE ON A RICH LOESS SOIL THINNING WESTWARD FROM THE MATANUSKA RIVER. THE FRONT OF
THE TALKEETNA MOUNTAINS IN THE BACKGROUND IS A LARGE FAULT-LINE SCARP. A DISSECTED SURFACE OF Low
RELIEF CAPS RIDGES ON THE LEFT AND SLOPES WESTWARD BENEATH TERTIARY sEDIMENTS IN THE SUSITNA VALLEY.
MOUNT McKINLEY, 120 MILES AWAY AND 20,269 FT HIGH, AT LEFT HORIZON. PHOTOGRAPH BY U.S. AIR FORCE

 

FIGURE 8. GLACIALLY SCULPTURED TOPOGRAPHY ON GENTLY DIPPING JURASSIC SANDSTONE IN THE EASTERN
ALEUTIAN MOUNTAINS. GULLIES FOLLOW FAULTS AND JOINTS. MOUNT DOUGLAS ANDFOURPEAKED MOUNTAIN,

TWO ACTIVE VOLCANOES, IN THE DISTANCE. KAMISHAK BAY, ANARM OF COOK INLET, ON THE LEFT. PHOTO-
CRAPH BY US. AIR FORCE

 

 

 

FIGURE 2. THE ALASKA RANGE NORTHEAST oF MOUNT MCKINLEY (20,269 FT) SEVEN NORTH-FLOwWING RIVERS
WERE SUPERIMPOSED ON EAST-TRENDING BELTS OF HARD ROCKS (RIGHT) AND SOFT TERTIARY ROCKS THAT UNDERLIE
THE LINE -OF. PLAINS AND PASSES EXTENDING FROM THE LEFT OF THE PICTURE TOWARD THE BASE OF MOUNT
McKINLEY. THE NORTHERN LIMIT OF THE PLEISTOCENE CORDILLERAN ICE SHEET Is IN THE PICTURE - GLACIERS FROM
MOUNTAINS ON THE LEFT EXTENDED NORTH DOWN THE VALLEYS, BUT THE MOUNTAINS ON THE RIGHT WERE NOT GLA-

CIATED. THE PLAIN IN THE RIGHT DISTANCE is LESS THAN 2000 FT ABOVE SEA LEVEL.

VIEW SOUTHWEST TO MOUNT
McKINLEY. PHOTOGRAPH BY BRADFORD WASHBURN.

 

FIGURE 3. PLEISTOCENE VOLCANOES OF THE WESTERN WRANGELL MOUNTAINS. MOUNT DRUM (12,002 FT) ON LEFT,
MOUNT SANFORp (16,208 FT) IN CENTER, AND MOUNT WRANGELL (14,005 FT), STILL ACTIVE, ON RIGHT.. 'DEFEORMED
PALEOZOIC AND MESOZOIC SEDIMENTARY AND VOLCANIC ROCKS ARE EXPOSED IN THE LOW MOUNTAINS AT THEIR
BASE. THE BLUFFS OF THE COPPER RIVER: IN THE MIDDLE DISTANCE: ARE OF PLEISTOCENE GLACIAL AND LAKE
DEPOSITS AND INTERBEDDED VOLCANIC MUDFLOWS OF THE COPPER RIVER LOWLAND.

RICHARDSON HIGHWAY AND
TONSINA RIVER IN FOREGROUND. PHOTOGRAPH BY BRADFORD WASHBURN

 

 

 

 

 

\We Seward
Peninsula

 

  

 

 

   

 

   
 

     
 

   

EXPLANATION

£

Location of point from which ob-
lique aerial photograph was
taken. Arrow indicates direction
of camera

wm

1
\Figure 4
FIGURE 10W~§ 1

  
   
     

 

C,
(aweu Re 7
«.

 

 

 

  

 

FIGURE 12. MAP

FIGURE 9. ABANDONED MARGINAL pRAINAGEWAY ON THE NORTH SIDE OF KENAI PENINSULA, cooOK INLET-SUSITN A
LOWLAND. TINY STREAMS MEanpER THROUGH THESE NARROW WINDING MARSH-COVERED PLAINS, WHICH
WERE PRESUMABLY CARVED BY MELTWATER DURING THE WANING OF A LATE PLEISTOCENE ICE SHEET WHEN

ICE STILL FILLED COOK INLET. MOUNT REpOoUBT(10,197FT) AND MOUNT ILIAMNA (10,016 FT),

VOLCANOES OF THE
sOoOUTHERN ALASKA RANGE, RISE ON THE HORIZON BEYOND COOK INLET.

PHOTOGRAPH BY U.S. AIR FORCE

 

 

 

AERIAL

OF ALASKA, WITH BOUNDARIES OF PHYSIOGRAPHIC

 

PHOTOGRAPHS ILLUSTRATING THE PHYSIOGRAPHY OF

 

 

PROVINCES, SHOWING LOCATIONS OF PHOTOGRAPHS

 

FIGURE 10. SOUTH FACE OF THE ROBINSON MOUNTAINS, 5500-6500 FT HIGH, WITH COASTAL PLAIN AND MARINE
TERRACES, IN THE GULF OF ALASKA COASTAL BELT. SOFT TERTIARY ROCKS ON THE SOUTH LIMmB OF A BROAD
sYNCLINE ARE EXPOSED IN THE MOUNTAIN FRONT. SOME OF THE DARK BANDS ARE MARINE TILLITE. BERING
GLACIER IS IN THE LEFT DISTANCE, BEYONDTHE END OF THE RANGE, AND THE HIGH MOUNTAINS OF THE CHUGACH
RANGE ARE IN CENTER DISTANCE. PHOTOGRAPH BY BRADFORD WASHBURN

 

 

 

 

 

FIGURE 4. THE MALASPINA GLACIER AND THE SOUTH FRONT OF THE ST. ELIAS MOUNTAINS. MOUNT ST. ELIAs (18.008
FT), AT LEFT,. RISES 16,000-17,000 FT ABOVE THE GLACIER SURFACE IN THE FOREGROUND. THE PATTERN oF

INTENSELY DEFORMED SUPERGLACIAL MORAINES Is CHARACTERISTIC OF GLACIERS THAT EMERGE ONTO
BROAD LOWLANDS. PHOTOGRAPH BY BRADFORD WASHBURN

 

FIGURE 6. FAIRWEATHER RANGE, IN THE BACKGROUND, TOWERS
OvER THE. ALSEK RANGES IN THE FOREGROUND. MmoUNT LA
PEROUSE (10.728 FT) - LEFT OF.; CENTER AND MOUNT CRILLGN
(12,726 FT) NEAR RIGHT, ON SKYLINE. GEIKIE GLACIER, IN CENTER
FOREGROUND, .IS SURROGUNDED BY MOUNTAINS 4500 FT HiGH.

VIEW WEST FROM OVER GLACIER BAY. PHOTOGRAPH BY R. H,.
SARGENT, 1929

 

FIGURE 7: COAST MOUNTAINS. THE SUMMIT UPLAND, OF GRANITIC
ROCKS, IS ABOUT 6000 FT HIGH, AND THE SHARP PEAKS ON THE
SKYLINE RISE TO ABOUT 8000-8400FT. VIEW soUTHEAST FROM THE

HEAD OF ENDICOTT ARM UP THE DAWES GLACIER. PHOTOGRAPH BY
R. H. SARGENT, 1929

 

 

FIGURE 11. ROUNDED MOUNTAINS WITH HUMMOCKY SUMMITS AND
ROCK-BASIN LAKES IN THE COASTAL FOOTHILLS, ONCE BURIED BY
PLEISTOCENE ICE SHEETS. VIEW WEST OVER EASTERN REVILLAGI-

GEDO ISLAND, SOUTHEASTERN ALASKA. PHOTOGRAPH BY R. H.
SARGENT, 1929.

 

 

THE PACIFIC MOUNTAIN SYSTEM IN ALASKA

 

 

 

INTERIOR-GEOLOGICAL sUryEy, WASHINGTON. D. C.-1965 - G64166