LATE QUATERNARY DEPOSITIONAL HISTORY, HOLOCENE SEA-LEVEL CHANGES, AND VERTICAL CRUSTAL MOVEMENT, sOUTHERN SAN FRANCISCO BAY, CALIFORNIA THE GOLDEN GATE Entrance to San Francisco Bay, 1933. This is the way the entrance to the bay probably looked before settlement by man. View is northeast from above the Pacific Ocean. Today, the nearest points of land are spanned by the Golden Gate Bridge. The strait framed by the Golden Gate separates the Tiburon Peninsula (left) from Angel Island (right). The combined San Joaquin and Sacramento Rivers flowed through canyons now occupied by the distant strait and the Golden Gate 15,000 years ago. Photograph by Ansel Adams. Late Quaternary Depositional History, Holocene Sea-Level Changes, and Vertical Crustal Movement, Southern San Francisco Bay, California By BRIAN F. ATWATER, CHARLES W. HEDEL, and EDWARD J. HELLEY 6 EO LOG LICA L SURVEY PROFESSIONAL PAPEE 101i UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Atwater, Brian F. Late Quaternary depositional history, Holocene sea-level changes, and vertical crust movement, southern San Francisco Bay, California. (Geological.Survey Professional Paper 1014) Bibliography: p. 14-15. Supt. of Docs. No.: 119.16:1014 1. Geology, Stratigraphic-Quaternary. 2. Sea level-Pacific coast. 3. Earth movements-California- San Francisco Bay. I. Hedel, Charles W., joint author. II. Helley, Edward J., joint author. | III. ”11>“itlezLate quaternary depositional history.... IV. Series: United States Geological Survey Professional aper 1014. QE696.A89 551.7°93°091641 16-608265 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock Number 024-001-02966-9 CONTENTS Page Page Abstract =.. :: 2. :s tran niiwa's at rie Pac's RA vh sin an a n aien a 1 Holocene sea levels-Continued introduction.. ;...; ..... +e dizih arn ravh ana shan gal's 1 Numerical uncertainties in sea-level data-Continued Acknowledgments .s 1 Postdepositional subsidence ..................... 9 Methods ;> :.... no nist hns bin a be wan naira aind 2 The Holocene transgression .........;............... 9 Late Quaternary depositional history ..................... 3 Vertical movement of the Earth's crust .................. 12 Estuarine deposits sans 3 Evidence and rates of subsidence .................... 12 Terrestrial deposits .:. .;... 34. c avis nig 4 Quaternary sediments at least 200 m below present Folocene:sea levels.. sass 6 sea level ........ Ararat n t hike aa a n vae ae 18 Relation of dated samples to former sea levels ........ 6 Wisconsin thalwegs higher than the deepest Salt-marsludeposits -.. r. 6 Sangamon estuarine deposits .................. 13 Intertidal or uppermost subtidal deposits ......... 7 Relative sea-level rise greater than that expected Freshwater marsh deposits ....................... 7 from eustatic sea-level changes alone .......... 13 Numerical uncertainties in sea-level data ............. 8 Age of the bedrock depression containing southern Modern sample elevation ......................... 8 San Francisco Bay 002%} r. cea nass 14 Sample elevation at time of deposition ............ 8 Summary and conclusions ..................... rat iyens, 14 References Cited 1.0.00... rv inh sin sla s rana k ae ad saa Pa a's 14 ILLUSTRATIONS Page PratTE 1. Cross sections showing late Quaternary sediments beneath southern San Francisco Bay, California ......._.._._____. In pocket FigurE 1. Map showing the modern San Francisco Bay estuary and adjacent continental-shelf area 2 2. Chart showing chronology for sediments under southern San Francisco Bay ...... 4 3. Photographs of representative pelecypods from post-Wisconsin estuarine sediments under southern San Francisco Bay :.... .... ... .;... .. 3.0.2 Amine inin rily List bese Bok die ronnie nesta digee Ie 1 Alge c » iin arine 7 4. Scanning electron micrographs of representative foraminifers, an ostracode, diatoms, and marsh-plant seeds from post- Wisconsin estuarine sediments under southern San Francisco Bay .... 8 5. Chart showing comparison of stratigraphic nomenclature for sediments under southern San Francisco Bay .................... 9 6. Graph showing Holocene sea-level changes in the vicinity of southern San Francisco Bay ...... iA 7. Graph showing longitudinal profiles of buried land surfaces under southern San Francisco Bay ...... 12 TABLES Page TABLE 1. Description and interpretation of sediments and bedrock under southern San Francisco Bay 5 C on Holocene SEAICVELR 3. oven reer ch ele Avani nants ana ide (die cd Hi Leia oh aden eid ne ne dean dobar cein d tity any 10 3. Evidence and inferred rates of crustal down-warping in the vicinity of southern San Francisco Bay ...... 13 v LATE QUATERNARY DEPOSITIONAL HISTORY, HOLOCENE SEA-LEVEL CHANGES, AND VERTICAL CRUSTAL MOVEMENT, SOUTHERN SAN FRANCISCO BAY, CALIFORNIA By BrIAN F. ATwATER, CHARLES W. HEDEL, and EDwARD J. HELLEY ABSTRACT Sediments collected for bridge foundation studies at southern San Francisco Bay, Calif., record estuaries that formed during Sangamon (100,000 years ago) and post-Wisconsin (less than 10,000 years ago) high stands of sea level. The estuarine deposits of Sangamon and post-Wisconsin ages are separated by alluvial and eolian deposits and by erosional unconformities and surfaces of nondeposition, features that indicate lowered base levels and ocean ward migrations of the shoreline accompanying low stands of the sea. Estuarine deposits of mid-Wisconsin age appear to be absent, suggesting that sea level was not near its present height 30,000-40,000 years ago in central California. Holocene sea-level changes are measured from the elevations and apparent ''C ages of plant remains from 13 core samples. Uncertainties of +2 to +4 m in the elevations of the dated sea levels represent the sum of errors in determination of (1) sample elevation relative to present sea level, (2) sample elevation relative to sea level at the time of accumulation of the dated material, and (3) postdepositional subsidence of the sample due to compaction of underlying sediments. Sea level in the vicinity of southern San Francisco Bay rose about 2 cm/yr from 9,500 to 8,000 years ago. The rate of relative sea-level rise then declined about tenfold from 8,000 to 6,000 years ago, and it has averaged 0.1-0.2 cm/yr from 6,000 years ago to the present. This submergence history indicates that the rising sea entered the Golden Gate 10,000-11,000 years ago and spread across land areas as rapidly as 30 m/yr until 8,000 years ago. Subsequent shoreline changes were more gradual because of the decrease in rate of sea-level rise. Some of the sediments under southern San Francisco Bay appear to be below the level at which they initially accumulated. The vertical crustal movement suggested by these sediments may be summarized as follows: (1) Some Quaternary(?) sediments have sustained at least 100 m of tectonic subsidence in less than 1.5 million years ( < 0.07 mm/yr) relative to the likely elevation of the lowest Pleistocene land surface; (2) the deepest Sangamon estu- arine deposits subsided tectonically about 20-40 m in about 0.1 million years mm/yr) relative to the assumed initial elevations of the thalwegs buried by these sediments; and (3) Holocene salt-marsh deposits have undergone about 5 m of tectonic and possibly isostatic subsidence in about 6,000 years (0.810.7 mm/yr) relative to elevations which might be expected from eustatic sea-level changes alone. INTRODUCTION Geologists long ago recognized geomorphic evi- dence of late Quaternary sea-level changes in the San Francisco Bay area. Lawson (1894, p. 266) interpreted the numerous islands, peninsulas, and small embayments near San Francisco (fig. 1) as former hills, ridges, and stream valleys recently submerged by the sea. He suggested that San Fran- cisco Bay did not exist prior to this submergence and that the ancestral drainage of the San Joaquin, Sacramento Rivers flowed through the Golden Gate to a coastline some distance west of the present one. Gilbert (1917, p. 16-24) cited partial submergence of aboriginal shell mounds and historic retreat of the bayward edges of salt marshes as evidence of an ongoing relative sea-level rise. Both Lawson and Gilbert initially attributed these sea-level changes to tectonic subsidence of the land. Subsequently, Louderback (1941; 1951, p. 84, 86) suggested that San Francisco Bay was created by a worldwide sea-level rise of about 100 m caused by melting of late Pleistocene (Wisconsin) glaciers. The purpose of this study is to elucidate the late Quaternary history of the site of southern San Fran- cisco Bay by dating post-Wisconsin sea levels, docu- menting earlier sea-level fluctuations, and measur- ing vertical crustal movement. This history con- tributes to the understanding of the recent sedi- mentary and tectonic history of the densely popu- lated area around southern San Francisco Bay. In addition, it accounts for the foundation character- istics of late Quaternary sediments beneath and peripheral to the bay. Finally, it provides a time perspective on the principal geographic features of the San Francisco Bay area. ACKNOWLEDGMENTS Most of the core samples on which this study is based were collected and logged by the California Division of Bay Toll Crossings. State engineers and geologists who participated in bridge foundation 123° DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA 122° ! /A 38° 10 15 20 MILES 0. 156 +10. 15-20 (25 30 KILOMETERS 1 < al San Joaquin *y Golden & C Gate Kl Valley CZ % (25 hohe" j ~ Farallon C+ San FranClchtxt 4 “$3 Island ' sanwys «! a + > > § % % t & u A s a?” a" y 4 C REV <% R I aas % #". C3 j Sacramento Valley > $ iN ¢ P 1 $ a ¢ *% {, o <# 0 juz Q? © JA {y/ e- ** / 18 , A V+ 4 fie? sean) -/ a % ~» 7 " & " mP SF" wi) , AS & f a & 3 3 4, 4 v R it}? A”? 2‘fo CS??? Jése iaxlwfi P m sigan, al tte alt FIGURE 1.-Modern San Francisco Bay and adjacent continental-shelf area. As used in this report, southern San Francisco Bay refers to the part of the bay that lies south of the latitude of the Golden Gate. The 100-m isobath approximates the shoreline position 15,000 years ago. studies include: S. K. Atkinson, T. Baker, R. H. Barnwell, C. C. Bohannon, F. R. Carraro, T. Daukas, J. W. Driskill, G. G. Emerson, T. Fox, S. Hernon, H. S. Hunt, S. Kelsey, M. C. Knickelbein, B. McCreary, J. Marzotto, M. Matsumoto, D. Morgan, J. M. Oxen- dine, J. W. Rolston, R. S. Samuelson, W. R. Schott, W. M. Schwartz, P. D. Trask, C. Venskus, and J. E. Wright. Fossil identifications for this report were made by W. O. Addicott (mollusks and barnacles), J. A. Barron and C. W;: Hedel (diatoms), R. E. Arnal and B. F. Atwater (foraminifers), and P. C. Valentine (ostracodes). Photographs of pelecypods were taken by Kenji Sakamoto, and scanning electron micro- graphs of microfossils were taken by Robert L. Oscarson. METHODS The stratigraphy developed in this paper is based on samples collected for bridge foundation studies. Since 1946 the California Division of Bay Toll Crossings has contracted the drilling of approx- imately 600 boreholes into sediments under southern San Francisco Bay. All of the boreholes penetrated firm terrestrial sediments beneath Holocene estu- arine deposits, and some extended more than 100 m below sea level. Borehole information assembled by Bay Toll Crossings engineers consists of samples, hand speci- men descriptions, stratigraphic cross sections, and the results of soil-mechanics tests. The samples, representing 20-50 percent of the drilled sediments, were collected in metal tubes 5-8 cm in diameter. Some samples were sealed in the tubes, whereas others were extruded and placed in glass jars. Many of the sealed tube samples were subsequently destroyed during laboratory tests. About 5,000 remaining tube and jar samples have been donated to the U.S. Geological Survey. Most Bay Toll Crossings samples have changed during storage. Holocene estuarine deposits have undergone the most obvious alteration, including precipitation of gypsum and jarosite (Andrei Sarna- Wojcicki and Julius Schlocker, oral commun., 1974), LATE QUATERNARY DEPOSITIONAL HISTORY 3 replacement of framboidal pyrite by limonite, and color changes from gray to olive and brown. In addition, calcareous fossils may have dissolved in these sediments because of oxidation of pyrite or other reactions that increase the concentration of hydrogen ions. However, the common absence or decomposition of calcareous fossils in desiccated samples of Holocene estuarine deposits (such as in borehole 11, section A-A', pl. 1) is probably due to dissolution prior to coring. This natural dissolution is evidenced by the absence of calcareous fossils in many of the fresh core samples we have collected from Holocene estuarine mud. Sediments from 18 Bay Toll Crossings boreholes were examined for this study. Paleoecological inter- pretations are based in part on color, texture, authi- genic mineralogy, bulk density or moisture content, and penetration resistance of sediments, and on megascopic plant and animal fossils. In addition, splits of about 175 samples from these boreholes were washed through sieves with a mesh opening of 0.063 mm for separation of foraminifers, ostracodes, dia- toms, seeds, and microscopic authigenic minerals. Nearly all of these washed samples come from sediments less than 60 m below modern sea level, and most come from Holocene estuarine deposits. Finally, paleoecological interpretations were extrap- olated to additional Bay Toll Crossings boreholes using the hand-specimen descriptions and physical properties recorded by the soils engineers. Carbon-14 dates (pl. 1) were prepared with pro- portional gas counting techniques by Isotopes, Inc. Because of the small size of samples and the ten- dency of plant fragments to disintegrate when washed, carbon was obtained by burning entire sample splits rather than by concentrating plant fragments. LATE QUATERNARY DEPOSITIONAL HISTORY Numerous worldwide fluctuations of sea level during late Quaternary time are known from marine- terrace and paleoclimatic records, but to date we have discerned only a few corresponding changes in depositional environment at the site of southern San Francisco Bay (fig. 2). Evidence of some fluctuations that are not presently recognized in sediments under the bay may have been removed by erosion. Evidence of others is probably contained in sedi- ments located more than 60 m below sea level and not examined during this study. ESTUARINE DEPOSITS Deposits of the earliest known estuary situated in the vicinity of the present southern San Fran- cisco Bay contain a tuff approximately 1 m.y. (mil- lion years) old (table 1; Sarna-Wojcicki, 1976) and are consequently of early Pleistocene age. This earliest estuary probably connected with the Pacific Ocean via a shallow marine embayment at the site south of San Francisco in which sediments of the Merced Formation were deposited. These sediments were later deformed and uplifted. Freshwater drained to this estuary from the Great Valley, as evidenced by the presence of Great Valley detritus in a coeval part of the Merced Formation (Hall, 1966). The youngest known estuarine deposits of Pleis- tocene age under southern San Francisco Bay were probably produced by Sangamon high stands of the sea. Although no unconformities have been recog- nized within the Sangamon estuarine deposits, the record of sea levels at New Guinea (fig. 2; Chappell, 1974, p. 568) suggests that the Sangamon estuary may have expanded and contracted several times in response to sea-level changes 70,000-130,000 years ago. The maximum areal extent of the Sangamon estuary was probably at least as great as that of the post- Wisconsin estuary because, despite postdeposi- tional erosion, Sangamon estuarine deposits under- lie most of the area of the post- Wisconsin estuarine deposits (pl. 1). Data from the Atlantic continental shelf of the United States suggest that sea level approached its present height 30,000-40,000 years ago (middle Wisconsin) (Milliman and Emery, 1968). Estuarine sediments of this age appear to be absent under southern San Francisco Bay. Presumably, if the volume of estuarine deposits that would have accu- mulated at the site of southern San Francisco Bay during this high stand of sea level would be com- parable to the volume that has accumulated during post-Wisconsin time, a considerable part of these middle Wisconsin estuarine sediments should have survived late Wisconsin erosion, as suggested by the preservation of large volumes of Sangamon estu- arine deposits (pl. 1). Consequently, the apparent absence of middle Wisconsin estuarine deposits under southern San Francisco Bay indicates that local sea level did not approach its present height 30,000-40,000 years ago. The improbability of a middle Wisconsin high stand of the sea in California has also been argued by Birkeland (1972, p. 446), who cited the general absence of post-Sangamon marine terraces in central and southern California. Estuarine sediments have accumulated at the site of southern San Francisco Bay during the past 10,000 years in response to the post- Wisconsin sea- level rise. Representative fossils from these sedi- ments are illustrated in figures 3 and 4. 4 DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA Northern Southeastern San Francisco San Francisco peninsula peninsula (Bonilla, (LaJoie and ; d 1959, p. 31; others, 1974) an??? Southern San Francisco Bay Schlocker and 9% others, 1958) Alluvial Alluvial Estuarine fan deposits deposits deposits (Holocene) (Holocene) Dune (Holocene) sand (late |--} / \/A\J -- : Pleisto- Eolian sehe and Alluvial deposits Holocene) deposits (lete (late Pleisto- Pleisto- cene and cene and Z Holocene) Holo - g cene) M 2 Alluvial ¥ f a c ls 3 deposits 3 (late Alluvial Pleisto- i Colm.a o che) deposits Estuarine Formation I2 (late deposits (late Plei (late 3 isistocens) Pleistocene) Pleisto- W &C cene) C 0 o pails M Terrestrial and Merced Santa Clara estuarine? qugs'ts’ Formation Formation undiy! 7a . a (Pliocene? and (Pliocene (Pliocene ; Pleistocene) and and Fleisto_ (TULF | Pigigco: [Tuff Tuff H _ E NS I NL As A. INL CALI Bedrock (1323;°°!() (Mesozoic Bedrock I eol and (Mesozoic and Tertiary) Tertiary) Glaciation and inter - glaciation Sea levels in New Guinea (Chappell, 1974, p. 568) terminations California Elevation, in meters -25 0 l 1 marine terrace in -50 Lowest widespread Age, in m.y. Mid-continent, North America Sierra Nevada Epoch !! Paleoclimate cycles: +- 3 5 € S - | Tahoe Holocene Illinoian Glaciation <0.7 m.y.) Late Pleistocene Mono Basin Glaciation (>0.1 m.y., | Yarmouthian | [Interglaciation| Pleistocene Kansan Glaciation Sherwin Glaciation (>0.7 m.y.) Aftonian Interglaciation . | Early Pleistocene McGee Glaciation (<2.6 m.y.) FiGur® 2.-Chronology for sediments under southern San Francisco Bay and their time-stratigraphic correlation with sediments of adjacent land areas, with late Quaternary glacial, climatic, and sea-level events, and with the lowest widespread marine terrace in California. Correlations are approximate where lines are dashed. Vertical scale of right side of main stratigraphic column is approximately proportional to thickness of units near borehole 2 (map, pl. 1). Wavy lines indicate unconformities, zig-zag lines indicate interfingering of time-equivalent units. Glacial chronology for mid-continental North America and epoch boundaries after Berggren and VanCouvering (1974, p. 144); glacial chronology for Sierra Nevada after Wahrhaftig and Birman (1965, p. 308, 331); paleoclimatic cycles from Broecker and VanDonk (1970, p. 181); age range of marine terrace from compilation by Birkeland (1972, p. 443).Correlation of tuff in type Merced Formation after Sarna-Wojcicki (1976). Radiometric ages of this tuff range from 0.4 to 2.6 m.y., but the most probable age is around 1.0 m.y. (A. Sarna-Wojcicki, written commun., 1975; see table 2). TERRESTRIAL DEPOSITS Estuarine deposits of different ages under south- ern San Francisco Bay are separated by alluvial and eolian sediments and by erosional unconformities and surfaces of nondeposition. These features indi- cate lowered base levels and oceanward migrations of the shoreline accompanying low stands of the sea. Alluvial deposits of Wisconsin age under south- ern San Francisco Bay were not recognized by Louderback (1951, p. 87-88) and Treasher (1963, p. 24). These authors apparently mistook alluvial sedi- ments for desiccated estuarine deposits. Presum- ably, the Sangamon estuarine deposits exposed at the Wisconsin land surface sustained oxidation, desiccation, leaching of calcareous fossils, and pene- tration by roots to a depth of several meters. Sedi- ments between the Sangamon and post-Wisconsin estuarine deposits, however, average 15-25 m in thickness, contain much more sand and gravel than the adjacent estuarine deposits, and lack fora- minifers, diatoms, and estuarine molluscs (pl. 1). Consequently, we interpret most of the Posey Forma- tion of Trask and Rolston (1951) and much of the older bay mud of Treasher (1963) as alluvial deposits rather than desiccated estuarine sediments (fig. 5). Eolian sands of Wisconsin age under southern San Francisco Bay were probably derived from flood plains and beaches on the broad continental shelf that was exposed during low sea-level stands. These sands were transported across low-lying parts of San Unit LATE QUATERNARY DEPOSITIONAL HISTORY Lithology Stratigraphy Color notations given according to the Munsell system Age Environment of deposition TABLE 1.-Description and interpretation of sediments and bedrock under southern San Francisco Bay Correlations Estuarine deposits (Holocene; post- Wisconsin). Alluvial deposits (late Pleisto- cene and Holocene; Wisconsin in part). Eolian deposits (late Pleistocene and Holocene; Wisconsin in part). Estuarine deposits (late Pleistocene; Sangamon). Clay and silty clay with rare sand and gravel. Dark gray (5Y 3/0.5 to 5Y 4/1) fresh, dark grayish brown (2.5Y 4/2) and olive gray (5Y 4/2) in oxidized samples. Commonly contain framboidal pyrite. Slight to strong H2S odor. Gypsum, limonite, and jarosite present in desiccated samples. Soft and normally consolidated; bulk density 1.45-1.75 g/cm, higher in sand and lower in peaty mud; moisture content 40-90 percent, lower in sand and higher in peaty mud. Typically contain diatoms, pelecy- pods, and foraminifers; rarely contain ostracodes. Locally include peaty muds containing roots and sparse seeds of estuarine marsh plants. Clay, silt, sand, gravel, and rare peat. Graded sequences common beneath contact with Holocene estuarine deposits. Inorganic sediments gray, brown, and yellowish brown; peat black; clay and silt commonly mottled. Fine-grained sediments firm to stiff, overconsolidated: bulk density 1.9-2.2 g/cm? (120-135 lb/ft?), moisture content 15-25 percent; softer gray clay and silt locally present immediately beneath Holo- cene estuarine deposits. Bulk density of peat 1.3-1.5 g/cm? (80-95 1b/ft?). Fine-grained deposits contain sparse roots and root molds; peats contain at least 50 percent plant remains, commonly including Scirpus(?) seeds. Fine- and medium-grained sand with subordinate silt. Sand grains angular to rounded, mainly sub- rounded and rounded. Yellowish brown, dark grayish brown (2.5Y 4/2), and olive gray (5Y 4/2.5). Bulk density 1.9-2.2 g/cm*, moisture content 15-25 percent. Remains of burrowing estuarine invertebrates locally present at contact with Holocene estuarine deposits. Clay and silty clay with rare sand and gravel. Dark greenish gray (5GY 4/1) fresh, commonly altering to olive gray and dark grayish brown during storage; locally mot- tled immediately beneath unconformable contact with fluvial deposits. Gypsum commonly precipitated during storage. Typically firm and slightly overcon- solidated: bulk density 1.7-1.85 g/cm*, moisture content 30-50 per- cent; locally stiffer near uncon- formable contact with Wisconsin sediments. Cover stream valleys, alluvial fans, and sand dunes of latest Pleistocene (Wisconsin) and early Holocene age. Locally interfinger with late Holocene allu- vium at margins of present estuary. Maximum thick- ness about 40 m over drowned valleys near San Francisco. Disconformably overlie Sangamon(?) estuarine deposits. Thickness typically increases southeastward from less than 15 m near San Francisco to more than 25 m between Menlo Park and Fre- mont (pl. 1, C-C"). Overlie and locally may interfinger with Wisconsin alluvial deposits. Probably extend southeastward from Golden Gate to the vicinity of San Mateo (pl. 1, map). Maximum thick- ness, near Oak- land, at least 15 m (Trask and Rolston, 1951, p. 1104). Overlie pre- Sangamon terres- trial deposits. Maximum thickness approximately 30 m. Presently forming. Oldest dated deposits approxi- mately 9,300 years ago; oldest deposits near Golden Gate, if pre- served, probably 10,000- 11,000 years ago. Oldest deposits over 40,000 years old but probably less than 100,000 years old. Include late Holo- cene deposits under mar- gins of southern San Francisco Bay. Overlie peaty sediments older than 40,000 years in borehole 7 (pl. 1, A-A'); therefore, oldest eolian deposits may be over 40,000 years old. Deposi- tion may have persisted into Holocene time, but most eolian deposits prob- ably accumulated during latest Pleistocene (Wiscon- sin) time. Disconformably overlain by terrestrial deposits over 40,000 years old in bore- hole 7 (pl. 1 A-A'); therefore older than 40,000 years. Separated from underlying tuff of Merced Formation (1.1+0.4 m.y. old) by at least 20 m of alluvial sediments and at least one major unconformity in borehole 2 (location of this borehole shown on map, pl. 1). Ca. 9,500 years ago: small brackish estuary extending no further south than Hunters Point. Ca. 9,500-7,000 years ago: rapidly enlarging estuary, progressively less diluted by fresh water and locally fringed by tidal marshes Ca. 7,000 years ago to 1850 A.D.: slowly enlarging estuary with fringing salt marshes that have prograded during the past several thousand years. Alluvial fans and trunk stream of valley now occupied by southern San Francisco Bay estuary. Trunk stream was a tributary to ancestral San Joaquin-Sacramento Rivers. Peats immediately beneath Holocene estuarine deposits near San Francisco record freshwater-marsh conditions in valleys that were probably ponded as base levels rose rapidly in response to early Holocene sea-level changes. Dunes locally separated by stream valleys and bedrock hills. Sand derived from flood plains and beaches on broad continental shelf exposed dur- ing lower sea-level stands and transported from the west across low-lying parts of San Francisco and through Golden Gate (Cooper, 1967, p. 48-52). Extensive primary beach sands of Holocene age in southern San Francisco Bay are unlikely because of the predominance of clay-size sediments in known Holocene shoreline deposits. A fluvial environment for eolian deposits is excluded by their height above presumably coeval fluvial deposits and by their uniform grain size. Estuary probably similar in size and shape to the Holocene San Francisco Bay. Water depths of at least 20-40 m near mouth of estuary indi- cated by molluscan assem- blage east of Yerba Buena Island (W. O. Addicott, written comm., 1973; sample eleva- tion 45 m below present sea level). Equivalent to bay mud of Trask and Rolston (1951) and to normally consolidated member of younger bay mud of Treasher (1963, p. 15). Coeval with Holocene and late Pleistocene alluvial fan deposits of Helley, Lajoie, and Burke (1972). Include Posey For- mation of Trask and Rolston (1951), some basal peat in bay mud of Trask and Rolston (1951), preconsolidated member of younger bay mud of Treasher (1963, p. 15), and part of sand deposits and bay mud of Treasher (1963, p. 15). Lithologically correlative and probably coeval with some of extensive dune sands in San Francisco (Cooper, 1967; Schlocker and others, 1958; Bonilla, 1965) and with the Merritt Sand in Oakland (Lawson, 1914, p. 15). Equivalent to most of the Merritt Sand of Trask and Rolston (1951) and to some of the sand deposits of Treasher (1963, p. 15). Probably coeval with the lowest widespread marine terrace of coastal California (fig. 2) and with some of the late Pleistocene alluvial fan deposits of Helley, Lajoie, and Burke (1972). Include most of the San Antonio Formation. DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA Table 1.-Description and interpretation of sediments and bedrock under San Francisco Bay-Continued Environment Unit Lithology Stratigraphy Age of deposition Correlations Estuarine Commonly contain estuarine dia- Represent the youngest deposits (late ' toms, pelecypods, and foraminifers major Pleistocene high Pleistocene; similar to those in Holocene sea-level stand, as shown Sangamon)- deposits. Locally include peaty by their great extent and Continued clays near contact with older terres- thickness and by the Terrestrial and estuarine deposits, un- divided (Plio- cene? to late trial deposits. Clay, silt, sand, and gravel. Estuarine deposits contain sparse mollusks, foraminifers, ostra- codes, and diatoms and include a tuff 87 m below present sea level in Unconformably overlie Mesozoic and Tertiary(?) rocks. Extend at least 130 m below apparent absence of over- lying pre-Holocene estu- arine deposits. Therefore correlated with the Sangamon Interglaciation and regarded as 75,000- 125,000 years old. Oldest deposits probably Pli- ocene, youngest deposits coeval with Sangamon estuary. Potassium-argon dates on tuff in type Merced Forma- tion are 0.44%0.18 m.y., 0.74 0.5 m.y., 1.110.5 m.y., 2.1+0.3 m.y. (G. B. Dalrym- ple and M. Lanphere, writ- ten commun., 1975), and 1.510.8 m.y.(Hall, 1966). A fission-track date on this tuff is 1.140.4 m.y. (C. Nae- ser, written commun., 1974). Basins subjected to oscillating sea levels and modified by tec- tonic processes. Basins of about 1 m.y. ago disrupted by later folding and faulting (Lajoie and others, 1974). Pliocene(?) and early Pleistocene sediments coeval with Merced and Santa Clara Form- ations. Younger sediments possibly coeval with some of late Pleistocene allu- vial fan deposits of Helley, Lajoie, and Burke (1972). Equivalent to parts of Alameda Forma- tion (Trask and Pleistocene). borehole 2 (location of this borehole sea level east of shown on map, pl. 1). This tuff cor- Yerba Buena relates with a tuff in the type Island (Trask Merced Formation (Sarna- and Rolston, 1951, Woijcicki, 1976). p. 1081) and at least 200 m below sea level in B-B' and C-C' (pl. 1) (Finlayson and others, 1967, pl. 5; Meade, 1967, p. 40-41). Bedrock Predominantly 'clastic sedimentary Late Mesozoic Francis can rocks and serpentinite form the known stratigra- phic and structural base- ment. Although directly overlain by Quaternary deposits near San Fran- cisco, Franciscan rocks may be mantled by other Mesozoic or Tertiary rocks rocks and igneous rocks, in part altered by low-temperature metamorphism. (Mesozoic and Tertiary). under other parts of southern San Francisco Bay. Francisco by prevailing westerly winds (Cooper, 1967, p. 48-52). - 'The lowest base level at the Golden Gate during Wisconsin time was probably about 70 m below present sea level. This base level is suggested by bedrock sills located about 5 km east and northeast of the Golden Gate, the lowest of which is 65 m below present sea level (Carlson and McCulloch, 1970). Consequently, assuming that sea level during the late Wisconsin glacial maximum stood about 100 m below present sea level (Flint, 1971, p. 322), the late Wisconsin shoreline must have been located west of the Golden Gate on what is now the continental shelf (fig. 1). Plant fossils dated at 23,600 years old and found 10 km southeast of Menlo Park (map, pl. 1) about 8 m below present sea level show a relative abundance of cedar and Douglas fir and a relative scarcity of oak and redwood, thus suggesting a cooler, wetter cli- mate than today (David P. Adam, oral commun, 1973). Associated animal fossils indicate that Rancholabrean vertebrates, including camels, bison, horses, sloths, and mammoths, roamed the late Wisconsin valley now occupied by southern San Francisco Bay (Edward J. Helley and Kenneth R. Lajoie, unpub. data). Rolston, 1951) and older bay mud of Treasher (1963, p. 15). These rocks formed in various ancient marine, continental, and plutonic environments that were subsequently destroyed by crustal movement. HOLOCENE SEA LEVELS RELATION OF DATED SAMPLES TO FORMER SEA LEVELS Holocene sea levels are typically dated by the radiocarbon ages of plant or animal remains that initially accumulated near sea level. The roots of salt-marsh plants are the principal materials dated for this study and represent upper intertidal condi- tions. Additional control on the age of former sea levels is given by dated plant remains from intertidal or uppermost subtidal environments and from pre- dominantly freshwater marsh environments at or above the highest tides (table 2). SALT-MARSH DEPOSITS With the possible exception of eelgrass (Zostera), an aquatic plant which is very rare or absent in San Francisco Bay today, plants which inhabit salt marshes are the only ones that can produce peaty estuarine muds composed largely of roots in growth positions. Therefore the principal feature of salt- marsh deposits that differentiates them from other estuarine sediments is the abundance of roots in growth positions. Additional diagnostic character- istics of salt-marsh deposits include a typical abun- dance of arenaceous foraminifers, a scarcity or HOLOCENE SEA LEVELS Ficur® 3.-Representative pelecypods from post-Wisconsin estuarine sediments under southern San Francisco Bay. Scale bar represents 1 cm. A, Ostrea lurida Carpenter; B, Balanus sp. (barnacle) attached to a fragment of Clinocardium cf. C. nuttalli (Conrad); C, Clinocardium nuttalli (Conrad); D, Cryptomya californica (Conrad); E, Macoma nasuta (Conrad); F, Macoma balthica (Linné); G, Mytilus edulis (Linné), fragment. c absence of calcareous fossils (Phleger, 1970, p. 532- 533), and a local abundance of framboidal pyrite and insect remains (table 2). The salt-marsh deposits used to date sea-level changes were collected at or slightly above the contact between Holocene estuarine deposits and underlying terrestrial sediments, thereby minimiz- ing uncertainties due to postdepositional compac- tion and settlement. By analogy with the historic salt marshes of southern San Francisco Bay (Hinde, 1951, p. 219; Nichols and Wright, 1971; map, pl. 1), these basal salt-marsh deposits accumulated in the upper intertidal zone at the fringes of the estuary, but at lower sea levels than today. Most of these marshes were eventually submerged by the rising sea as evidenced by the common overlapping of basal salt- marsh deposits by lower intertidal and subtidal sediments. Salt-marsh deposits were not sampled near the base of Holocene estuarine deposits in some Bay Toll Crossings boreholes, such as numbers 6, 7, 18, 24, 29, and 39 (pl. 1). The apparent absence of basal salt- marsh deposits in these boreholes may be explained in part by gaps between core samples. No basal marsh deposits were found, however, in boreholes 13 and 26-29 (pl. 1), in which basal estuarine sediments were carefully sampled. Collectively, this evidence suggests that salt-marsh plants may have been unable to colonize stretches of Holocene shorelines because the rate of relative sea-level rise exceeded local rates of sedimentation (Bloom, 1967a, p. 36), or that salt-marsh deposits submerged by rising Holo- cene sea levels may have been locally removed by wave erosion. INTERTIDAL AND UPPERMOST SUBTIDAL DEPOSITS Estuarine sediments lacking roots in growth positions, containing shallow-water calcareous fos- sils, and resting on alluvial or eolian deposits prob- ably accumulated in intertidal or uppermost subtidal environments. The plant remains in these sediments are regarded as detrital. FRESHWATER MARSH DEPOSITS Sediments composed dominantly of plant re- mains, containing Scirpus? seeds, and lacking fora- minifers and diatoms larger than 0.063 mm are found immediately beneath Holocene estuarine de- posits in buried Wisconsin valleys near San Fran- cisco at elevations between -25 and -45 m (A-A', pl. 1; Calif. Div. Bay Toll Crossings, 1966, and unpub. cross sections, 1970). These sediments probably ac- cumulated in marshes located at or above the highest tides. Freshwater marsh deposits do not appear to underlie Holocene estuarine sediments between San 8 DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA s L; Q L1 FraurE 4.-Scanning electron micrographs of foraminifers (A-E), an ostracode (F), diatoms (G-O), and marsh-plant seeds (P, Q) from post-Wisconsin estuarine sediments under southern San Francisco Bay. Scale bar represents 0.1 mm. A, Elphidium incertum obscurum (Williamson) var. obscurum Voloshinova; B, Elphidium gunteri Cole; C, Ammonia beccarii (Linné); D, Elphidiella hannai (Cushman and Grant); £, Trochammina inflata (Montagu); F, Cytheromorpha sp.; G, Actinoptychus splendens (Shadbolt) Ralfs; H, Coscinodiscus sp. (note framboidal pyrite in frustule); I, Aulacodiscus kittoni Arnott; J. Biddulphia sp.; K, Isthmia nervosa Kutzing; L, Campylodiscus sp.; M, Cymbella cf. C. aspera (Ehrenberg) Cleve; N, Surirella sp.; O, Surirella? sp.; P, Salicornia sp.; Q, Scirpus? sp. Mateo and Hayward and between Menlo Park and Fremont (sections B-B' and C-C", pl. 1). NUMERICAL UNCERTAINTIES IN SEA-LEVEL DATA The 100-320-year uncertainties in the **C ages of Holocene sea levels (table 2) incorporate laboratory errors only. Other possible errors are difficult to estimate. The possibility of contamination of salt- marsh deposits by roots of younger marsh plants (Kaye and Barghoorn, 1964, p. 72) is ignored because the resultant decrease in sample age is probably no greater than a few centuries under conditions of rising sea level. The error caused by the time gap between death of plant material and its final accu- mulation as detritus is also ignored except in bore- hole 17 (section B-B', pl. 1), where detrital wood must be regarded as reworked fossil material because it is several thousand years older than the associated estuarine sediments. The elevations of Holocene sea levels are deter- mined by the equation s, where His the elevation of a former sea level, h is the modern sample elevation, ho is the sample elevation at the time of deposition, and s is the postdeposi- tional subsidence of the sample. Measured and esti- mated errors contributed by these factors typically total +2-+4 m in this study. MODERN SAMPLE ELEVATION (A) The uncertainty in modern sample elevation (table 2) is the approximate sum of errors in (1) the elevation of the top of the borehole, (2) the depth of the sample interval within the borehole, and (3) the position of the dated sample within the sample interval. This uncertainty averages about +0.5 m and does not exceed +1.0 m. SAMPLE ELEVATION AT TIME OF DEPOSITION (A,,) The elevation ranges of modern salt-marsh plants approximate the elevations at which Holocene salt-marsh deposits accumulated. The lowest natural surfaces of tidal marshes in the San Francisco Bay estuary are located about 0.5 m below mean sea level and are colonized by cordgrass HOLOCENE SEA LEVELS 9 This report Trask and Treasher Rolston (1951) (1963, p. 24) Adjacent land Southern San Francisco Bay areas Younger : r bay mud, Alluvtal EstuarEne normally deposits deposits Bay mud consol- (Holocene) (Holocene) dated member Eolian 2 ig Pa # Alluvial deposits tiefifst 3 2 Nif, (late Pleistocene & '_ Ste. Merritt $ {4 52 and Holocene) alsatggene Sand i 5,82 Holo- 5 3g cene) ® *+ A Posey AIIuVEaI Formation deposits Fie: tiate ) Estuarine see ayy eistocens deposits (late SanA Pleistocene) Antonio Formation - -LA__~ Older bay mud 5 ; 5 Alameda Terrestrial and estuarine deposits, Formation undivided (Pliocene? and Pleistocene) m‘ i $5 & N Tuff Tuff FiGur® 5.-Comparison of stratigraphic nomenclature for sedi- ments under southern San Francisco Bay. Wavy lines indi- cate unconformities; zig-zag lines indicate interfingering of time-equivalent units. Lines between columns correlate rock- stratigraphic units and are dashed where correlations are approximate. (Spartina foliosa) or tules (predominantly Scirpus californicus). These large plants are replaced by smaller plants, predominantly pickleweed (Salicor- nia pacifica) and salt grass (Distichlis spicata), at elevations greater than 0.5-1.0 m above mean sea level (Hinde, 1954; Atwater and Hedel, 1976). The highest elevation of abundant Salicornia and Distichlis approximately equals that of the highest tides, varies geographically with local tidal amplification, and ranges from about 1.5 m above mean sea level near San Francisco to about 2.0 m above mean sea level near Menlo Park. Although plant remains have not been positively identified, most of the dated tidal-marsh deposits appear to contain the small roots and rhizomes of high-marsh plants (Salicornia and Distichlis) rather than the robust rhizomes of low- marsh plants (Spartina and Scirpus). These dated roots and rhizomes probably penetrated a few decimeters below the surfaces of the marshes. Consequently, most of the dated salt-marsh plant remains probably accumulated about 0-1.5 to 0-2.0 m above former mean sea levels, the upper limit varying with local tidal range (table 2). The paleobathymetry of the dated intertidal or uppermost subtidal deposits has an uncertainty of +1.5 m. Freshwater marsh deposits provide only upper limits on the elevation of sea level at the time of their accumulation (table 3). POSTDEPOSITIONAL SUBSIDENCE (s) Postdepositional subsidence (table 2) is likely where sediment overburden has placed a load on compressible sediments beneath the dated samples. Errors in thickness and consolidation of the com- pressible sediments lead to uncertainties as large as +2.5 m in the subsidence estimates. Uncertainty about the thickness of compressible sediments beneath dated estuarine deposits and freshwater peats results from 1- to 3-m discontinu- ities in sampling between dated horizons and the underlying noncompressible terrestrial deposits. Typically, the location of the contact within the discontinuity is unknown and cannot be determined without very costly additional drilling. Settlement corrections assume the following ranges of consolidation, expressed in percent of initial thickness: freshwater-marsh peat (organic matter content probably greater than 50 percent), 15- 40; salt-marsh mud (organic matter content probably less than 25 percent), 33-67; nonmarsh estuarine mud, 40-80; and soft alluvial clay within 5 m of the dated sample, 80-100. These estimates are based on the following data: (1) freshwater peat buried by 3-10 m of Holocene estuarine mud for 1,000-6,000 years in Connecticut has been compressed to 13-44 percent of its initial thickness (Bloom, 1964, p. 603) and (2) a column of mud with an initial porosity of 80 percent, an initial thickness of 3 m, and an overburden of 30 m will theoretically compress to 50 percent of its initial thickness (Weller, 1959, p. 290). The settlement corrections generally neglect compaction of Pleistocene estuarine deposits and stiff or coarse-grained terrestrial sediments. Between Menlo Park and Fremont, however, they include about 0.5 m of compaction of Pleistocene clay due to ground water withdrawal (Poland, 1971). Repeated leveling around the margins of southern San Francisco Bay by the U.S. Geological Survey and the U.S. Coast and Geodetic Survey suggests that this historic land subsidence dwindles north- west of Menlo Park and Fremont. THE HOLOCENE TRANSGRESSION The small quantity and large uncertainties of sea-level data prevent resolution of low- 10 DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA TABLE 2.-Data on Holocene sea levels [All linear measurements in meters; see plate 1 for location of boreholes] Bore- hole No. Sample elevation relative to modern Principal mean sea - dated level (A) material Postdepositional subsidence (s) of dated sample caused by compaction of underlying sediments T = thickness of compressible sediments S = settlement Elevation of mean sea Sample elevation at time of deposition relative to former mean sea level (ho)2 Environmental indicators! level at time of ac- cumulation of dated material relative to modern mean sea Pleistocene deposits, groundwater withdrawal (Poland, 1971) Late Pleistocene and Holocene terrestrial deposits Holocene estuarine deposits S T S S Total s level (H) Age and analytical uncer- tainty in "C years before 1950 A.D. (half life =5,568 years) (see pl. 1) 25 36° 32 33 21 11 10° 16 33 -7.9£0.5 Roots of salt- marsh plants. -6.610.3 Fragments of salt- marsh plants. Roots of salt- marsh plants. -9.010.3 -11.420.5 -10.8:0.5 ~11.810.5 _ -- do----- -24.6+0.7 eigen ~21.0£1.0 . -18.710.3 -do- Remains of fresh- or brackish- water marsh plants Chunks of wood. -32.5¢1.0 -39.510.5 --------- -do---------- Abundant roots, prob- Abundant roots, prob- 1.0+1.0 ably in growth positions; Trocham- mina inflata (Fa); Campylodiscus (Dp). Abundant plant frag- ments, probably detrital, and Sali- cornia seeds (A, s); Elphidium spp. (Fc); Ostrea lurida (P,8); shells broken and probably detrital -0.5¢1.5 Abundant roots, prob- 1.01.0 ably in growth positions; Trocham- mina inflata (Fa); framboidal pyrite. 1.0£1.0 ably in growth positions; Trocham- mina inflata (Fa); Campylodiscus (Dp). e-dde f Abundant roots in growth positions; Trochammina inflata (Fa); Cymbella (Dp). Abundant roots, prob- ably in growth positions; Campylodiscus (Dp). 1.0£1.0 0.740.7 0.910.9 Abundant roots, prob- 0.7+0.7 ably in growth positions; Campylodiscus (Dp); framboidal pyrite. Abundant roots, prob- 0.7+0.7 ably in growth positions; Trocham- mina sp. (Fa); Campylodiscus (Dp), Hyalodiscus (Dc). Abundant roots, prob- 0.910.9 ably in growth positions; Campylodiscus (Dp). Abundant roots in growth positions, above and abundant highest Scirpus? seeds (A, f); tides. rare limonitized framboidal pyrite. Abundant detrital wood; Ammonia beccarii, Elphidium sp. (Fc); Cythero- morpha sp. (0); Cocconeis, Surirella, Cymbella (Dp); Valvata (G, £, Psidium (G, £); Thoecamoebina (fresh- and brackish- water protozoans); mosquito larvae (R. E. Arnal, written commun., 1974). Probably 0.01.5 2.0 0.3-1.0 0.0-2.0 0.0-1.0 0.0-1.0 0.5-2.5 1.5-2.5 0.0-2.0 0.0-1.0 0 0.5-3.0 0.0-1.5 0.0-0.4 0.3-0.7 2411.6 _ -6.543.1 0.1-1.5 0 0 0 0.8:0.171 _ -5.3¢2.5 0.0-3.0 2.0-3.0 0.0-0.8 0.3-0.7 2412.1 - -7.6%3.4 0.0-2.0 0.0-1.0 0.0-0.3 0.3-0.7 - -10.8:2.8 0.0-2.0 2.0-3.0 0.0-0.8 0.3-0.7 1.941.6 -9.918.1 -12.5¢1.2 0.2-5.0 0 0 0 2.6124 -7.513.6 0.6-5.0 2.0-3.0 0.0-0.8 0 3.2126 -22.114.0 0.0-4.0 0.0-2.0 0.0-0.5 0 2.2122 -19.513.9 0.0-2.0 2.0-3.0 0.0-0.8 0 L4t1.4 -18.212.6 Below -31.5+2.0 0-2 0 -39.512.0 3,360£105 5,7451185 5,845£100 6,200+320 6,4851110 6,855¢z115 8,230£135 8,295£135 8,3651135 8,8851145 9,255:310 HOLOCENE SEA LEVELS 11 TABLE 2.-Data on Holocene sea levels-Continued Postdepositional subsidence (s) of dated sample caused by compaction of underlying sediments T = thickness of compressible sediments S = settlement Elevation of _ Age and mean sea analytical level at uncer- Sample time of ac- tainty in elevation Pleistocene cumulation "C years Sample at time of Late Pleistocene deposits, of dated before elevation deposition Holocene and Holocene groundwater material 1950 A.D. relative relative estuarine terrestrial withdrawal relative to (half life to modern Principal to former deposits deposits (Poland, 1971) modern =5,568 Bore- mean sea ated Environmental mean sea mean sea years) hole No. level (k) _ material indicators! level (h o)2 T S T S S Total s level (H) (see pl. 1) 2 -37.5:0.5 - Remains of - Predominance of Above 0 0 2-6 0-4 0 212 - Below 9,280:120 fresh-water - plant remains over - highest -35.512.5 marsh plants inorganic detritus; _ tides. abundant Scirpus? seeds (A, f). 'Explanation of symbols: A=marsh angiosperm Fa = foraminifer, arenaceous; typically lives in salt marshes Fc = foraminifer, calcareous; typically lives in lower intertidal and subtidal environments O = ostracode Dp = diatom, pennate Dc = diatom, centric P = pelecypod G = gastropod s = salinity range predominantly saltwater and brackish water f= salinity range predominantly brackish water and freshwater See figures 3 and 4 for photographs of some of these organisms, and plate 1 for additional ecological information. *Estimates based on analogy with vertical distribution of seed plants, diatoms, and foraminifers (Hinde, 1954; Arnal and others, 1977; Phleger, 1970, p. 532-533) in modern environments. *Borehole not shown on cross sections (pl. 1). amplitude sea-level oscillations, so a smoothed curve was visually fitted to the data points (fig. 6). This curve displays a more than tenfold decrease in slope between 8,000 and 6,000 years ago. On the average, sea level in the vicinity of southern San Francisco Bay rose about 2 ecm /yr from 9,500 to 8,000 years ago, but it has risen only 0.1-0.2 ecm/yr from 6,000 years ago to the present. A similar change in rate appears in sea-level records from other parts of the world (for example, Milliman and Emery, 1968) and probably coincides with the disappearance of the large Wis- consin ice sheets (Bloom, 1971, p. 368). Extrapolation of the sea-level curve to the pre- sumed late Wisconsin base level, located about 65-70° m below modern sea level at the Golden Gate (A, fig. 7), indicates that the rising sea entered the Golden Gate 10,000-11,000 years ago. Driven by the sea-level rise of several centimeters per year, an estuary then spread rapidly, advancing about 30 m/yr along the course of the trunk stream draining the site of southern San Francisco Bay and reaching the vicin- ity of Menlo Park by 8,000 (G, fig. 7) years ago (fig. 7). Subsequent shoreline changes have been more grad- ual because of the decrease in rate of sea-level rise. A change from predominantly fresh- and brackish-water diatoms in estuarine sediments about 8,500-9,300 years old to predominantly marine diatom assemblages in younger sediments (bore- holes 4, 5, 6, section A-A', pl. 1) indicates increasing salinity. This increase was probably caused by the rapid early growth of the estuary. Assuming a nearly constant long term inflow of freshwater, the large increases in water volume accompanying this growth should have resulted in substantially greater Mean sea T T T T T T T T T level -10 |- is A |_ -l C w |_ w- '-20 I- a 3 z EXPLANATION > (> Southern San Francisco Bay 3 0 x Probable range of error in age g: -30 |- and elevation of former sea level U A t (table 2). Arrow indicates former ~! sea level below some point w |- 2 ¥1. (4 in rectangle ~~~~~~~~~~ Micronesia -40 |- a -50 1 1 1 1 1 1 | 1 1 0 1 2 3 4 5 6 %. 8 9 10 AGE, IN THOUSANDS OF YEARS BEFORE PRESENT Figure 6.-Holocene sea-level changes in the vicinity of southern San Francisco Bay. The curve for Micronesia, shown for comparison, approximates Holocene eustatic sea-level changes according to Bloom (1970, p. 1901). average water salinities by permitting larger propor- tions of saltwater to enter the estuary. Changing rates of sea-level rise largely control the stratigraphic distribution of salt-marsh deposits. The sea-level rise of several centimeters per year probably exceeded most local sedimentation rates, so tidal marshes before 8,000 years ago were scarce and small and, moreover, were ultimately submerged by 12 DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA 121° Northwest Southeast T T T T T T T T T Mean 21 Mean sea |- 3° ,- - sea level 54 level San Pablo Bay 3 "l a>. G# +20 |- w @ w 3 2 g Oakland - [C Land surface buried it w a ln by Sangamon N estuarine deposits -| - 250 K4 -80 |- + 2. = * a % & <. Hayward - C -| -300 , A B D oE F G _ % -100 {3 | (ee tar st 4 n 1 - San Mateo = 10 0 10 20 30 40 50 60 70 so & g be DISTANCE FROM GOLDEN GATE, IN KILOMETERS ta Line of profiles G Fremont > a \ EXPLANATION & Menlo Park Buried thalwegs Buried bedrock sills (Carlson and O Covered with post-Wisconsin estu- McCullock. 1970) 1 arine deposits (pl. 1) aA _ Across line of profile O - Covered with sediments, probably of post-Wisconsin age, recogniz- ed in seismic reflection profiles (Carlson and others, 1970, p. 105) A - Not across line of profile 8 - Radiocarbon age of Holocene sea level in 10° years before pres- ent (fig. 6) O Covered with Sangamon estuarine deposits; elevations estimated from unpublished borehole logs and cross sections by Calif. Div. of Bay Toll Crossings, 1948-71 FiGurE 7.-Longitudinal profiles of buried land surfaces under southern San Francisco Bay. Profiles are based primarily on the elevations of buried thalwegs (lines of maximum valley depth) at points A, E, F, and G. Point E is located between boreholes 4 and 5 (A-A', pl. 1), point F, approximately at borehole 17 (B-B', pl. 1), and point G, between boreholes 32 and 34 (C-C ', pl. 1). The upper profile is also based on the elevations of bedrock sills (points A, B, and D), which indicate the deepest possible base levels during late Wisconsin time. All vertical bars show approximate uncertainties in elevations of thalwegs and sills. We speculate that the late Wisconsin trunk stream draining the site of southern San Francisco Bay met the drainage of the San Joaquin-Sacramento Rivers north and east of Angel Island because the bedrock sill between Angel Island and Tiburon Peninsula (B) is about 15 m lower than the sill between San Francisco and Angel Island (C); therefore the profile of the land surface buried by post-Wisconsin estuarine deposits loops north of Angel Island. The intersection of Holocene sea levels with this profile shows the rapid longitudinal growth of the post- Wisconsin estuary from 10,000 to 8,000 years ago and the subsequent decline in the rate of longitudinal growth. the rising sea. Later, the declining rate of sea-level rise was approached and finally surpassed by the rate of sediment accumulation in much of the estu- ary, permitting progradation of mudflats and salt marshes. Most of the bayward growth of marshes has occurred during the past several thousand years, as evidenced by the lower intertidal and subtidal deposits which typically lie no more than a few meters below mean sea level beneath the historic salt marshes (boreholes 13, 26, 27, 29, 30, 32, 38, 39, sections B-B' and C-C", pl. 1). VERTICAL MOVEMENT OF THE EARTH'S CRUST EVIDENCE AND RATES OF SUBSIDENCE Evidence of vertical crustal movement at the site of southern San Francisco Bay was first reported by Lawson (1914, p. 3), who observed that late Cenozoic structural features bound the valley that is partly flooded by the bay. Subsequently, Louderback (1951, p. 83) proposed that the bedrock surface under this valley had been downwarped during Quaternary time. Both men asserted that the valley containing southern San Francisco Bay had been shaped not only by running water but also by vertical tectonic movement of the Earth's crust. Several additional lines of evidence suggest that subsidence of the valley floor has taken place and permit rough estimates of the rates of subsidence (table 3): (1) Quaternary(?) sediments are situated at least 200 m below present sea level, (2) Wisconsin VERTICAL MOVEMENT OF THE EARTH'S CRUST 13 TABLE 3.-Evidence and inferred rates of crustal downwarping in the vicinity of southern San Francisco Bay Rate Evidence (mm/yr) How measured Time period Origin Quaternary(?) sedi- '>0.07 - Elevation difference 1.5 m.y. Tectonic ments situated at between deepest (Quaternary) least 200 m below Quaternary sedi- present sea level. ments and level at which they were probably deposited. Wisconsin thalweg 20.2+0.1 Differences in eleva- 0.1 m.y. Do. situated higher 20.4%0.1 tion between the (late Pleisto- than the deepest +0.420.1 thalwegs covered cene; Sangamon Sangamon estu- with Sangamon and Wisconsin). arine deposits. and post-Wisconsin estuarine deposits (fig. 6). Relative sea-level '0.80.7 - Comparison of post- 6,000 years Tectonic rise is greater Wisconsin sea (Holocene). and than that expected levels at south- possibly ern San Francisco isostatic. Bay with those at Micronesia, an area of presumed crustal stability (fig. 5). from eustatic sea- level changes alone. 'Applies to most of site of southern San Francisco Bay below latitude of Hayward. *Applies to site between boreholes 4 and 5 (A-A', pl. 1). 'Applies to site near borehole 17 (B-B', pl. 1). 'Applies to site between boreholes 32 and 34 (C-C", pl. 1). trunk streams were unable to cut through the deepest Sangamon estuarine deposits, and (3) the late Holo- cene rise in sea level exceeds what might be expected from eustatic sea-leyel changes alone. QUATERNARY SEDIMENTS AT LEAST 200 M BELOW PRESENT SEA LEVEL Sediments of Quaternary age may extend at least 200 m below present sea level under much of southern San Francisco Bay, particularly between San Mateo and Hayward and between Menlo Park and Fremont (Finlayson and others, 1967, pl. 5; Meade, 1967, p. 40-41). Assuming that they were deposited during the past 1.5 million years and within 100 m of present sea level, these sediments probably have been lowered by tectonic subsidence of at least 0.07 mm/yr. Conceivably, an extremely low stand of sea level could have allowed streams draining the bay basin to grade to a base level 200 m below present sea level, thereby allowing Quaternary sediments to accumu- late at lower elevations than we assume. However, such a stand of the sea during Pleistocene time is unlikely because it is lower than what might be expected from ice volumes during Pleistocene glacial maxima (Flint, 1971, p. 84) or from the overall rise of sea level independent of glacioeustatic fluctuations (Vasil ' kovskiy, 1973, p. 857). WISCONSIN THALWEGS HIGHER THAN THE DEEPEST SANGAMON ESTUARINE DEPOSITS The volume of Sangamon estuarine deposits remaining after Wisconsin erosion appears to be close to the volume of post-Wisconsin estuarine sediments under most of southern San Francisco Bay (pl. 1). The Sangamon deposits persisted in such quantity because Wisconsin trunk streams were unable to cut through them: the thalwegs (lines of maximum valley depth) of Wisconsin trunk stream are located at least 15 m above the deepest San- gamon estuarine deposits. Assuming that the trunk stream draining the bay basin during the low stand of sea level before the first major marine transgression of Sangamon age was similar in location, base level, and gradient to the trunk streams during the major low stands of sea level of Wisconsin age, the early Sangamon estuarine deposits must be tectonically depressed below the Wisconsin thalwegs. The differences in elevation between the ancient land surfaces covered with Sangamon and post- Wisconsin estuarine deposits should approximate the magnitude of this tectonic subsidence during Sangamon and Wisconsin time, a period of about 100,000 years. Assuming that the initial longitudinal profiles of these land surfaces were similar in gradi- ent and elevation, the present differences in eleva- tion between the profiles (fig. 7) indicate the follow- ing average rates of tectonic subsidence : 0.2+0.1 mm/yr, between San Francisco and Oakland; 0.4+0.1 mm/yr, between San Mateo and Hayward and between Menlo Park and Fremont. RELATIVE SEA-LEVEL RISE GREATER THAN THAT EXPECTED FROM EUSTATIC SEA-LEVEL CHANGES ALONE The record of post-Wisconsin sea levels at south- ern San Francisco Bay can be used as a datum to determine vertical crustal movement by comparison with sea-level data from areas of presumed crustal stability. Sea level has risen more rapidly in the vicinity of southern San Francisco Bay than in Micronesia by 0.8+0.7 mm/yr during the past 6,000 years (fig. 6). Assuming that the relative sea-level rise in Micronesia approximates purely eustatic changes (Bloom, 1970, p. 1901), the more rapid submergence at the site of southern San Francisco Bay represents downward crustal movement. Tectonic subsidence is probably the principal component of this crustal movement. Given the range of our uncertainties, the rates of tectonic sub- sidence during the last 100,000 years of Pleistocene time may be considered equivalent to the rate of late Holocene crustal downwarping (table 3). If the actual rate of Holocene downwarping is as high as 1 mm/yr, however, then tectonic subsidence has accel- erated or has been augmented by isostatic down- warping of the continental margin. The likelihood of isostatic adjustment to post- Wisconsin seawater loads has been established for the Atlantic coast of the United States by Bloom (1967 b), but at present we cannot differentiate its effects from those of tec- 14 DEPOSITIONAL HISTORY, SEA-LEVEL CHANGES, AND CRUSTAL MOVEMENT, CALIFORNIA tonic subsidence in the vicinity of southern San Francisco Bay. AGE OF THE BEDROCK DEPRESSION CONTAINING SOUTHERN SAN FRANCISCO BAY Rates of tectonic subsidence can be extrapolated to estimate the age of the structural trough that contains southern San Francisco Bay. For example, the top of Mesozoic bedrock under southern San Francisco Bay is located as much as 200 m below present sea level near Menlo Park (R. M. Hazelwood, oral commun., 1975). Assuming that streams flowing in this area cut no deeper than 100 m below present sea level during a Pleistocene glaciation, this bed- rock surface has undergone at least 100 m of tectonic subsidence. Extrapolation of local tectonic subsid- ence rates, which averaged 0.3-0.5 mm/yr during Sangamon and Wisconsin time, indicates that this subsidence began as recently as 0.2-0.3 m.y. ago. The structural trough appears to be older in some other areas. The depth to bedrock along the east shore of southern San Francisco Bay near Hayward, for instance, probably exceeds 300 m (R. M. Hazel- wood, oral commun., 1975). At subsidence rates of 0.3-0.5 mm/yr, this depth suggests an age of at least 0.4-0.7 m.y. Furthermore, a tuff about 1 m.y. old is found in estuarine sediments 87 m below present sea level in borehole 2 (table 1). This tuff indicates that a structural trough may have been located just east of the site of San Francisco during early Pleistocene. SUMMARY AND CONCLUSIONS 1. Two units of estuarine deposits are recognized in sediments less than 60 m below present sea level at the site of southern San Francisco Bay (table 1; pl. 1). These deposits were produced by Sangamon and post- Wisconsin high stands of sea level (fig. 2). Estuarine sediments of middle Wisconsin age appear to be absent, suggesting that local sea level was not near its present height 30,000-40,000 years ago. 2. The estuarine deposits of Sangamon and post- Wisconsin age are separated by alluvial and eolian deposits and by erosional unconformities and sur- faces of nondeposition (pl. 1). These features indicate lowered base levels and ocean ward migrations of the shoreline accompanying low stands of the sea. 3. The late Wisconsin shoreline was situated some distance west of the site of San Francisco (fig. 1). The rising post- Wisconsin sea entered the Golden Gate 10,000-11,000 years ago. As sea level rose about 2 cm/yr, an estuary spread across adjacent land areas as rapidly as 30 m/yr until about 8,000 years ago. Subsequent inundation has been more gradual because the rate of sea-level rise has declined at least tenfold since 8,000 years ago, averaging only 0.1-0.2 mm/yr from 6,000 years ago to the present (figs. 6, 7). 4. Some of the sediments under southern San Francisco Bay appear to be situated below the level at which they initially accumulated. The vertical crustal movement suggested by, these sediments (table 3) may be summarized as follows: (a) Some Quaternary(?) sediments have tectonically subsided at least 0.07 mm/yr relative to the lowest likely Pleistocene land surface; (b) the deepest Sangamon estuarine deposits have subsided tectonically 0.2-0.4+0.1 mm/yr relative to the assumed initial elevations of the thalwegs buried by these sediments; and (c) Holocene salt-marsh deposits have under- gone about 0.8+0.7 mm/yr of tectonic and possibly isostatic subsidence relative to elevations that might be expected from eustatic sea-level changes alone. 5. Rates of tectonic subsidence and the depth to Mesozoic bedrock indicate that the structural depres- sion containing southern San Francisco Bay may be as young as 0.2-0.3 m.y. in some areas. REFERENCES CITED Arnal, R. E., Quinterno, P. J., Conomos, T. J., and Gram, K., 1977, Trends in the distribution of recent Foraminifera in San Fran- cisco Bay: Springer-Verlack, Bandy memorial volume in micropaleontology (in press). Atwater, B. F., and Hedel, C. W., 1976, Distribution of tidal-marsh plants with respect to elevation and water salinity in the natural tidal marshes of the northern San Francisco Bay estuary, California: U.S. Geol. Survey, open-file report 76- 389, 41 p. Bastin, E. S., and Davis, C. A., 1909, Peat deposits of Maine: U.S. Geol. Survey Bull. 376, 127 p. Berggren, W. A., and VanCouvering, J. A., 1975, The Late Neo- gene: Palaeogeography, Palaeoclimatology, Palaececology, v. 16, p. 1-216. Birkeland, P. W., 1972, Late Quaternary eustatic sea-level changes along the Malibu Coast, Los Angeles County, California: Jour. Geology, v. 80, p. 432-448. Bloom, A. L., 1964, Peat accumulation and compaction in a Con- necticut coastal marsh: Jour. Sed. Petrology, v. 34, no. 3, p. 599-603. -1967a, Coastal geomorphology of Connecticut; Final Rept., Geography Br., Office Naval Research, contract NORN-401 (45), 72 p. 1967b, Pleistocene shorelines, a new test of isostasy: Geol. Soc. America Bull., v. 78, p. 1477-1494. 1970, Paludal stratigraphy of Truk, Ponape, and Kusaie, Eastern Caroline Islands: Geol. Soc. America Bull., v. 81, p. 1895-1904. 1971, Glacial-eustatic and isostatic controls of sea level since the last glaciation, in Turekian, K. K., ed., The late Cenozoic glacial ages: Yale Univ. Press, p. 355-379. Bonilla, M. G., 1959, Geological observations in the epicentral area of the San Francisco earthquake of March, 1957: Cali- fornia Div. Mines Spec. Rept. 57, p. 25-37. 1965, Geologic map of the San Francisco south quad- rangle, California: U.S. Geol. Survey open-file map, scale 1:20,000. Broecker, W. S., and VanDonk, J., 1970, Insolation changes, ice REFERENCES CITED 15 volumes and the '°O record in deep-sea cores: Rev. Geophysics and Space Physics, v. 8, p. 169-198. California Division of Bay Toll Crossings, 1966, Final report of preliminary soil exploration for a southern crossing of San Francisco Bay: California State Dept. Public Works, 72 p. Carlson, P. R., Alpha, T. R., and McCulloch, D. S., 1970, The floor of San Francisco Bay: California Geology, v. 23, p. 97- 107. Carlson, P. R., and McCulloch, D. S., 1970, Bedrock-surface map of - central San Francisco Bay, California: U.S. Geol. Survey open -file map, scale 1:27,200. Chappell, John, 1974, Geology of coral terraces, Huon Peninsula, New Guinea-A study of Quaternary tectonic movements and sea-level changes: Geol. Soc. America Bull., v. 85, p. 553-570. Cooper, W. S., 1967, Coastal dunes of California: Geol. Soc. America Mem. 104, 131 p. Finlayson, D. J., Hansen, W. R., Ford, R. S., Scott, R. G., and Vantine, J. V., 1967, Evaluation of ground water resources, south Bay-App. A, geology: California Dept. Water Re- sources Bull. 118-1, 153 p. Flint, R. F., 1971, Glacial and Quaternary geology: John Wiley and Sons, New York, 892 p. Gilbert, G. K., 1917, Hydraulicmining debris in the Sierra Nevada: U.S. Geol. Survey Prof. Paper 105, 154 p. Hall, N. T., 1966, Late Cenozoic stratigraphy between Mussel Rock and Fleischhacker Zoo, San Francisco Peninsula, in A walker's guide to the geology of San Francisco: California Div. Mines and Geology, Spec. Supp. Mineral Inf. Service, v. 19, no. 11, p. 22-25. Hanna, G. D., 1944, Diatomaceae, in Shimer, H. W., and Shrock, R. R., editors, Index fossils of North America: New York, John Wiley and Sons, Inc., 837 p. Helley, E. J., Lajoie, K. R., and Burke, D. B., 1972, Geologic map of Late Cenozoic deposits, Alameda County, California: U.S. Geol. Survey Misc. Field Studies Map MF-429, scale 1:62,500. Hinde, H. P., 1954, Vertical distribution of salt marsh phanero- gams in relation to tide levels: Ecol. Mon., v. 24, p. 209-225. Kaye, C. A., and Barghoorn, E. S., 1964, Late Quateruary sea- level change and crustal rise at Boston, Massachusetts, with notes on the autocompaction of peat: Geol. Soc. America Bull., v. 75, p. 63-80. Lajoie, K. R., Helley, E. J., Nichols, D. R., and Burke, D. B., 1974, Geologic map of unconsolidated and moderately consolidated deposits of San Mateo County, California: U.S. Geol. Survey Misc. Field Studies Map MF-575, scale 1:62,500. Lawson, A. C., 1894, The geomorphogeny of the coast of Northern California: California Univ. Dept. Geology Bull., v. 1, p. 241- 271. 1914, Geology of the San Francisco District: U.S. Geol. Survey United States, Geol. Atlas, 24 p. Louderback, G. D., 1941, Development of San Francisco Bay abs. : Geol. Soc. America Bull., v. 52, p. 1952. 1951, Geologic history of San Francisco Bay, in Jenkins, O. P., ed., Geologic guidebook to the San Francisco Bay counties: California Div. Mines Bull. 154, p. 75-94. Meade, R. H., 1967, Petrology of sediments underlying areas of land subsidence in central California: U.S. Geol. Survey Prof. Paper 497-C, 83 p. Milliman, J. D., and Emery, K. O., 1968, Sea levels during the past 35,000 years: Science, v. 162, p. 1121-1123. Nichols, D. R., and Wright, N. A., 1971, Preliminary map of his- toric margins of marshland, San Francisco Bay, California: U.S. Geol. Survey open-file map, scale 1:125,000. Phleger, F. B., 1970, Foraminiferal populations and marine marsh processes: Limnology and Oceanography, v. 15, p. 522-534. Poland, J. F., 1971, Land subsidence in the Santa Clara Valley, Alameda, San Mateo, and Santa Clara Counties, California: U.S. Geol. Survey Misc. Field Studies Map MF-336, scale 1:125,000. Radbruch, D.H., and Schlocker, Julius, 1958, Engineering geol- ogy of the Islais Creek Basin, San Francisco, California: U.S. Geol. Survey Misc. Geol. Inv. Map I-264, scale 1:12,000. Sarna-Wojcicki, Andrei, 1976, Correlation of late Cenozoic tuffs in the central Coast Ranges of California by means of trace- and minor-element chemistry: U.S. Geol. Survey Prof. Paper 972, 30 p. Schlocker, Julius, Bonilla, M. G., and Radbruch, Dorothy, 1958, Geology of the San Francisco North quadrangle, California: U.S. Geol. Survey Misc. Geol. Inv. Map I-272, scale 1:24,000. Trask, P. D., and Rolston, J. W., 1951, Engineering geology of San Francisco Bay, California: Geol. Soc. America Bull., v. 62, p. 1079-1110. Treasher, R. C., 1963, Geology of the sedimentary deposits in San Francisco Bay, California: California Div. Mines and Geol. Spec. Rept. 82, p. 11-24. Vasil'kovskiy, N. P., 1973, Sea level changes in the geological past: P. P. Shirshov Inst. Oceanology, U.S.S:R. Acad. Sci- ences, p. 847-859. Wahrhaftig, Clyde, and Birman, J. H., 1965, The Quaternary of the Pacific mountain system in California, in Wright, H. E., Jr., and Frey, D. G., eds., The Quaternary of the United States: Princeton Univ. Press, p. 299-340. Weller, J. M., 1959, Compaction of sediments: Am. Assoc. Petrol- eum Geologists Bull., v. 43, no. 2, 273-310. 2 Few - 6. m £ & Earth Selences & Map Library Ple ~ §0 MeCone Hall v. 1014 UNITED STATES DEPARTMENT OF THE INTERIOR University of California, Berkeley PROFESSIONAL PAPER 1014 J y) X“? GEOLOGICAL SURVEY PLATE 1 PPY o- Ene I 7 & A A 122° 80 pOssHOt* in ccr EXPLANATION 100 -%, *a (+8 Conact Location s, ~f 22 is ] - 30 ontac (California coordi- auf £ * EL 2 | $2 i Fault Map 3 Field 3 ailes gone 2) 3 5" |22| yes 2V4_ High-water shoreline No. Agency Contract Date No. Sampler N. E. so -< 3 IZEI 3 $f #7 A 14) MI ® 2C ----~- Circa 1850. Shown where 1 BTC 82-032 1969 " 34 Shelby _ 456,440 _ 1,460,540 =t. - * f J22F cL , different from 1970 shore- 2° BTC 101 1948 42 Calif. 461,200 _ 1,461,340 8 *+ * 4 2&1 > PT line by more than 200 m 33 BTC 2011 1965 _ XI __ Dand M 439,000 _ 1,464,465 60 -| .- TRY: £2 9). 4 BTC 82-032 1969 37. Shelby 457,380 1,465,450 ii n f~| zl»: 28 37°45 5 BTC 101 Jods - s3 - calif, - d46t.190 1464.660 x <+] San Francisco } 12 p+ € e !5 Borehole 6 BTC 101 1948 _ 84 Calif. 461,100 _ 1,468,620 e 28 # 12 s Th vI 7 BTC 101 1948 85 Calif. 461,100 _ 1,472,720 os It, Oakland * / 34 2h d 8 BTC 101 1948 77 Calif. 460,920 _ 1,475,580 aaa 1:1 TM. +2 f - Hz I3; "E 10 9 BTC 101 1948 78 Calif. 461,110 1,479,960 & ' te" 2! 2h), ys, o BTC 2001 1955 12 Calif. 453,100 _ 1,480,380 zo -[ fl Alameda 16.12 ¢ ps 11 BTC 101 1948 49 Calif. 463,420 - 1,482,740 fi 2 s F322“: Z:— 12 BTC 2001 1955 18 Calif. 461,540 _ 1,493,870 =-} i: San Francisco Bay ZLHAYWARD FAULT *" 13 USGS 1975 SM-1 Davis 391,400 1,480,500 y <5, Qpa s:: 2 l: 4 *> 14 CDH 04-1 34644 1966 8 Rotary 392,900 _ 1,486,700 0 —f¢)’ > Cg. % *I 2% -* J- 0 15 BTC 5007 1963 1 Calif. 395,400 - 1,489,500 i: s 3 £912 FZ. v, 16 BTC 5002 1959 1 Calif. 396,800 - 1,490,700 ='. - . 3,12 | 2 . x 17 BTC 5002 1959 3 Calif. 399,400 1,492,900 & E seléyl + sy 7 18 BTC 5002 1959 5 Calif. 402,100 _ 1,497,800 5 :" asp. ,R {Q *s , ast 19 BTC 5002 1959 6 Calif. 403,000 _ 1,500,300 D $ v4 d 2%. I zl 37 #s w 20 BTC 5002 1959 7 Calif. 404,800 _ 1,505,000 uz "11; x 1912 le' TL ig = 21 BTC 5002 1959 8 Calif. 405,800 _ 1,507,400 c I cap ] 4. $ 2 L+ < 23 BTC 5002 1959 9 Calif. 406,100 _ 1,509,800 8 -~ ~> v5 ff} §. "irl a. 23 BTC 5002 1959 10 Calif. 407,400 _ 1,512,200 is: «af 34. f 42 w ae c. 0 24 BTC 5002 1959 11 Calif. 408,400 _ 1,514,600 a &. § iol. $ QA $o z #4] BTC 5002 1959 12 Calif. 410,100 _ 1,518,900 I >60 weal> <> 2 4 12 (* ae e 26 UsGs 1974 _ K-4 _ Davis 414,800 _ 1,522,300 * x fixflzzlxfi Foals |- 59 © 27 USGS 1974 K-3 Davis 415,700 _ 1,525,500 wal */ j 2h, es" 28 USGS 1974 K-2 Davis 415,100 _ 1,527,000 v 042 ff; $A 29 USGS 1975 3-18-1 - Davis 360,260 - 1,520,130 -80 -]. 3 gt] | <> las. 304 BTC 86-041 1971 44 Shelby 364,780 _ 1,524,900 a ga ~ 5131 ¢ 31 BTC 86-041 1971 54 Shelby 366,640 1,526,850 -E) % 324 BTC 86-041 1971 60 Shelby 367,150 . 1,527,680 7" ig .s: 3p 33 BTC 86-040 1970 24 Shelby 369,210 1,529,460 +100 1 {; ’:" : : 2 34 BTC 86-040 1970 15 Shelby 210,650 _- 1,531,220 L3” 215] el 37° 30° 385 BTC 86-041 1971 65 Shelby 312,370 _ 1,533,250 ~IT% fl. 36° BTC 86-041 1971 67 Shelby 373,100 _ 1,533,810 tog =[. 4474 j f 37 BTC 86-041 1971 74 Shelby 374,880 1,534,880 Fi: a af Qpha 38° BTC 86-041 1971 110 Shelby 376,340 1,535,820 J“; Jey, 39 BTC 86-041 1971 93 Shelby 379,200 1,538,500 7% *, "{. e 49 40 BTC 86-041 1971 104 Shelby 380,190 1,541,880 -140 _‘ 5s we 9:- e Geholonglggfataralldized from; Brilnilla((1965)). Helley and # B ig: others , Lajoie and others (1974), Nichol d 41 BTC 86-041 1971 127 Shelby 381,070 1,544,600 Y 2 a S: Wright and Sehlocker and others (1195205 an 42 BTC 86-041 1971 126 Shelby 382,020 1,545,700 woul > £ a a a“ o r pA *+ "p 7+ . 8%" f BTC, State of California Division of Bay Toll Crossings; CDH, State of California Division of - 160 & h 3 75>}; £ SCALE 1:250 000 Highways; USGS, U.S. Geological Survey. Ti 5 3}?th k- -50 9 | 5 10 MILES Afan oen, 2 a a ; : wfe *5'% 1 1 i | 1 | 1 1 | e etas oun Calif., California sampler (5 cm diameter); D and M, Dames and Moore Type U sampler (5 cm Ps s., Tr T rrr gr Co don s pot eg te diameter); Davis, Davis sampler (2 cm diameter) (Bastin and Davis, 1909, p. 61); Rotary, rotary bit $x . 05°, j°.° 0 5 10 15 KILOMETERS al t : (5 cm diameter); Shelby, Shelby sampler (8 cm diameter). -180 - r: ig" 'e lee 122°00 Ask a mikes *, q* N p yue Six ¥ BP a ain) tu ® 5 : # F as me fiftlg * Borehole nof shown on cfoss section. nr Nea mo s eta Naf g a £ v nl, Lie's1*>a0.000| 9. :s 'C gif c® nel Sa to eso oll g g t AiR g eno pret MAP OF THE SOUTHERN SAN FRANCISCO BAY AREA, SHOWING GENERALIZED GEOLOGY ANID LOCATION OF BOREHOLES x Upper part of BTC boring resampled by USGS with Davis sampler. B B' DESCRIPTION OF UNITS S IE? fate ark "s *** *d 45 fats “v— Estuarine deposits (Holocene). On geologic map, pattern represents dlis- =I IY. >* b e tribution of salt marshes about 1850 IL $ Z hi v4 § & ASH r p. Alluvial d ts (Hol RADIOCARBON DATES OF SEDIMENTS UNDER 199 =1 p/ « s ".- so oo oa SOUTHERN SAN FRANCISCO BAY axl S lit © r: a M Alluvial deposits (late Pleistocene and Holocene; Wisconsin in part) 6 a ¥ <4 U.S Age. i 14p 20 g l‘:r w. i: -j<_ Eolian deposits (late Pleistocene and Holocene; Wisconsin in part) 9; , In. hm l“ a ‘Ar E > j (V) Borehole Geological Isotopes, Elevation, Dated years before g L4} *a o°l| 6 g Terrestrial deposits, undivided (Pleistocene and Holocene) No. - Survey No. Inc. No. in meters material present -[) Jp : * "~ "ar / a slff‘ 4“, Hayward o. f [. 2. |- 20 Estuarine deposits (late Pleistocene; Sangamon) 60 == v l% 'c. v ¥ 38 4P I-6475 -7.9 Peaty mud 3,360+105 t TM s 'a Alluvial deposits (late Pleist > 24 p a Af "m_ __ 0, 4 posits (late Pleistocene) 6 28PCPI 1-6976 -34.4 Peat 9.3304 150 a S o * f Abanidenedisireant Isven / . roo § 30PCP2 1-6978 ~39.9 wood 9,670+155 LA: $ We. San Mateo ":d t ug Colma Formation (late Pleistocene). Eolian, alluvial, and estuarine(?) 6 . 1-6979 <§6.7 Peat _ >4.0,000 ao |( i sae, (s e Cu" 4 deposits (bonllin. 1959). Not shown in cross cection 1 EQ 34 I-7730 -34.4 -do- 9,620+155 Iii 3 Z“ £ {fl hes Syst Merced Formation (Pliocene and early Pleistocene) Chiefly neritic and 4 EQ 35 I-7731 -37.0 Peaty mud 8,160+250 awe g ‘Z {SAN ANDREAS §: * a_ L: ~ 10 littoral deposits (Hall, 1966). Not shown in cross section 16 EQ 36 I-7732 -18.7 -do- 8,365+135 3 *" 4 % ”Isl b ThE .! i i i é & n ct s DF" ia ¢ A 17 EQ 37 1-7733 -29.3 Wood 10,700+155 5p. SE, <. ~f i'fll I; i> s Sagtglsggzatliggrmatlon (Pliocene and early Pleistocene). Not shown in 21 EQ 38 1-7734 29.2 Peaty mud - 6,855+115 fan 4 Jin R 2d .> SDF? "f S4 > 35 EQ 39 11-7742 - 6.6 -do- 3,930+105 «- 5 rfh‘ ys GX d cel £: ha 3755345) 5 Bedrock (Mesozoic and Tertiary) 33 EQ 40 I-7743 -10.8 -do- 6,200+320 If; 's$ : 'A p mm s: i mm -: ee . 2 sli A5 32 EQ 41 I-7746 -11.4 -do- 5,8451100 0 —$|:4 1:16, a X f j fs : HAYWARD FAULT—air»: — 0 36 EQ 42 17747 >9.0 do- 5,745+185 1"; 'rge: "gy. bss °o a arts al . °|$| w 4 EQ 55 I-7814 -39.5 Wood 9,255%4310 5 ~I(F. 5" 2 -& S T< $ 53 i a *+ *C edd. i SYMBOL MPLE DESCRIPTION 3 EQ 58 I-7817 -32.5 Peat 8,885+145 & 5L 3" {2 fo-JSI v, V. r CC s e 10 EQ 61 I-7820 -21.0 Peaty mud 8,295+135 = 720 =I 5h: ¥ o* {5.04 [Q vl < Sample intervals. Vertical lines show boreholes 12 EQ 62 I-7824 =11.8 -do- 6,485+110 " 3 l“? $ f es Sh Wol < I] Environment of deposition interpreted from color, texture, megascop- 11 EQ $2 I-7934 -24.6 -do- §,2304135 6 Shh; 3:1 li ,‘:4— -10 z ic organic constituents, and bulk density or moisture content 2 £Q 85 11-1939 =37.$ Peat 9.2804155 § 0 - ll: : r Z] {fr' s 8 F Environment of deposition interpreted from microscopic constituents 7 EQ 86 I-7940 -27.5 -do- _ >40,000 > S If: "L1 a a: < in addition to above properties 6 EQ 87 1-7941 -28.0 Wood 8,735+*270 = =[ ps3 4S" |- rr § { 35 EQ 92 I-8088 =18.3 8,905+275 > 5 2} I+, r Sediment size f [s. 5. P : e # Mainly silt or clay [Locations of boreholes shown on map; boreholes 3, 10, and 36 not shown on cross sections. -60 j :\ i dll 5 y 6 Mainly sand Typical uncertainty in sample elevation is +0.5 m. Datum is mean sea level. Dates prepared Sf: A hos f. f: b= -20 6 Y by James Buckley of Isotopes, Inc., 50 Van Buren Ave., Westwood, N. J. ** C half-ife=5,568 =[ f y! vay *oAl gebbly sagd or gravel years] <1 2 IP" *% x eaty mud or peat -80 =- S’LL 6 as? 4 t ¥ Silt or clay with sparse roots in growth positions s ett °C and with sparse roots in th positi SI; & --°°|$|L s led f Sand with spa in growth positions 8:11 5:24 I: a 5: & # Calcium carbonate or iron concretion Hrc h7v [gh Ir ® f= -30 : ; -100 I“) an: If Diatoms larger than 0.063 mm' : Centric genera, typically marine, in- 5:13: ding-Z9- clude Biddulphia, Coscinodiscus, Actinoptychus, Arachnoidiscus, wi Tsi. hmia, Melosira, : iscus: 1 SOURCES OF ADDITIONAL SUBSURFACE INFORMA TION po age! hus Ist zmza,P elosira, and Hiyallocfzsicus, also Aulacodiscus (borehole-s .1’ 3% 120 =I SIR; 3 2s" 4, 11). Pennate genera, indicative of freshwater to marine conditions, § Be a aka include Campylodiscus and Surirella; also Cymbella (boreholes 1, 2, Section A-A'-Contacts between boreholes located in part by using logs sud Sh: al 4, 11), Pleurosigma? (boreholes 2, 11), and Pinnularia? (boreholes 1, of additional borings by the California Division of Bay ‘gi "39 2; 6, 1D Toll Crossings under contracts 101 (1948) and 82-032 -{40 ==] SF’V' Environmental significance of diatom symmetry after Hanna (1944, (1969). Distances between boreholes range from 0.1 to [:: p. 712) 1.0 km. Contacts west of borehole 1 after Radbruch mem SI}: _- 0 Number of centric genera exceeds number of pennate genera and Schlocker (1958). Microfossils in holes 5 and 6 e £ 0 Number of pennate genera exceeds number of centric genera. Under- identified by Gregory Frantz, 1974; foraminifers in holes -160 - Slvr § line indicates number of pennate individuals exceeds 1,000 per graim 9 and 10 identified by Sarah Pierce, 1973 r”! = -50 of dry sediment Section B-B'-Contacts between boreholes located in part by using logs ZJ ”a: e s" fess . F \% Foraminifers of additional borings by the California Division of Bay oin maine Sp Tenens ia iin an en noen ine hay 0 Salt-marsh assemblage Toll Crossings under contracts 5002 (1959) and 5007 Trochammina inflata predominant, calcareous species rare or abserint (1963). Typical distance between boreholes is 0.5- 6 Shallow-water estuarine assemblage 1 1.0 km. Foraminifers in hole 22 identified by Lucy K. C , Approximate paleobathymetry 0-10 m (Arnal and others, 1977). Am- Sidoric and James C. Ingle, Jr., 1973 C monia beccarii, Elphidium incertum, E. incertum obscurum, E. gurnteri, section C-C _C § betehales tot % C Af',’ r* Elphidiella hannai, and Trochammina inflata ection C-C - oitagtds‘fetwlein lore b0 esh 024??? mIpart. by usmg logs —S|AA’ < \:‘ h it: pe ® Deep-water estuarine assemblage of additional borings by the Ca ifornia Division of Bay vr '> a us yI Approximate paleobathymetry 5-30 m (Arnal and others, 1977). Boli- Toll Crossings under contracts 86-040 (1970) and 86- F;* $? 'J 041 (1971). Typical distanc ‘b boret F 100 —S|;Lf 3sz £ vina striatula and Buliminella elegantissima in addition to shallow es- o . Typical distance between boreholes is 0.1 a 1D f'b'. l? H A} - 30 tuarine species —S| 3: 1): 0° Als 0 Estuarine ostracodes' ap =, Ir: 5, te % % Coyote Hills TL?) & Cytheromorpha sp.; also there sp. (borehole 17), and Cyprideis Sn? % :> Ifi°.Qha tye If“) beaconensis, Megacy there sp., Hemicythere sp., and Perissocytheridea _Sl;§ 2p. 1 2 9% fag? sp. (borehole 33) 2 "Mo: \.eX 31,4 luv 9 » EFSAN ANet aU kl 34 2 "soe: Menlo Park S $41: a a Estuarine pelecypods! S va tito: £ o m ae i 1g [rv \ o - kx 0@# DOJ F TM E s SCirpuS" sp uw. ot TM *A") px,3360 s'y oe - 20 “Stfl ele : I4 z 105 9 Siffit B 2 F#: (vite * 912: pss i S Detrital wood % Jo {* ®mIZ € y 8 p - z *I(l >,; +y 6 INL CV 3 4 © 8365 Radiocarbon age of wood or marsh-plant remains in '*C years O | ie Sat z: Ara I‘LAL: -= -10 z +135 I: 2) ”*:,b- Q'._ ees £, 6 a -40 4,1 SWE r* * & > S'vLJ T 005i: Tid A I; 1Assemblage is based on data from all samples except where otherwise noted. i #1 IAI-V s $ §f°~| | oes . a Individual samples contain only some of the listed members id SV Tek. ~el(lf 4% 4 jar Sh; "TG.. 's 1&5 wx d i _60 _1 | «(a b ‘ool lVArAVJV“ If; <3 $2, 6:13:72“: 2 (s al 3|1fj ! 170 3 OOII It: as n PLANAR FEATURES SHOWN ON CROSS SECTIONS Av S4. 3 #. yig $3va +l, JS: v:< CV) O0 5.2515 yess Conformable or slightly disconformable contact. Hiatus 0-10" years Sir; > AZ g a SKI: ¥ 3. S\N; Major disconformity. Hiatus 10°-10° years mul 7% J: PE'llAiV-‘r Ly: 4 fro. 5.02] |fi * 3% 5p w-w--r Angular unconformity. Hiatus 10° years -100 TSLVM eye "5°18!“ ¥2iL. - a 3 d. vor v v we we man cam Li. vas, CHL } - mC ta, Fault zones sft> PAQ C £; UE :* -120 =f (x Xs. Jé‘T-AISI: 3 '\ . . =E: * "oan 9511 Iffrflv B #88, f A r s < [w= -40 va ¢ byl 5 ”ills" 3 ~140 =[ f.: Sige: oue ~[L: '¢ . 18. i y A vs 1.2 l > J 3s » '6 -160 = SI C h sz< co sh? ft. fe- 50 m r; x (Av } Z [23A4cnvr i eet dren reno a e r es arenas wha alea a a f usgs gie (l>7"44A4rV‘AI—v Lr424J4<4rVTLJLNSq 4a ts & 4 8-290 . 'a gl els 9 o Goof-037° $9: Cfinobaod x Interior-Geological Survey, Menlo Park, CA.-1977-G76361 1 0 1 2 3 4 5 MILES 1 0 1 2 3 4 5 6 7 8 KILOMETERS . 1 ’ I t I VERTICAL EXAGGERATION X 200 DATUM IS MEAN SEA LEVEL CROSS SECTIONS SHOWING LATE QUATERNARY SEDIMENTS BENEATH SOUTHERN SAN FRANCISCO BAY, CALIFORNIA