Estuarine Sedimentation, Sediment Character, and Foraminiferal Distribution in Central San Francisco Bay, California

Summary

Central San Francisco Bay is the deepest subembayment in the San Francisco Bay estuary and hence has the largest water volume of any of the subembayments. It also has the strongest tidal currents and the coarsest sediment within the estuary. Tidal currents are strongest over the west-central part of central bay and, correspondingly, this area is dominated by sand-size sediment. Much of the area east of a line from Angel Island to Alcatraz Island is characterized by muddy sand to sandy mud, and the area to the west of this line is sandy. The sand-size sediment over west-central bay furthermore is molded by the energetic tidal currents into bedforms of varying sizes and wavelengths. Bedforms typically occur in water depths of 15-25 m.

High resolution bathymetry (multibeam) from 1997 and 2008 allow for subdivision of the west-central bayfloor into four basic types based on morphologic expression: featureless, sand waves, disrupted/man-altered, and bedrock knobs. Featureless and sand-wave morphologies dominate the bayfloor of west-central bay. Disrupted bayfloor has a direct association with areas that are undergoing alteration due to human activities, such as sand-mining lease areas, dredging, and disposal of dredge spoils.

Change detection analysis, comparing the 1997 and 2008 multibeam data sets, shows that significant change has occurred in west-central bay during the roughly 10 years between surveys. The surveyed area lost about 5.45 million m³ of sediment during the decade. Sand-mining lease areas within west-central bay lost 6.77 million m³ as the bayfloor deepened. Nonlease areas gained 1.32 million m³ of sediment as the bayfloor shallowed slightly outside of sand-mining lease areas. Furthermore, bedform asymmetry did not change significantly, but some bedforms did migrate some tens of meters.

Gravity cores show that the area east of Angel and Alcatraz Islands is floored by clayey silts or silty sand whereas the area to the west of the islands is floored dominantly by sand- to coarse sand-sized sediment. Sandy areas also include Raccoon Strait, off Point Tiburon, and on the subtidal Alcatraz, Point Knox, and Presidio Shoals. Drab-colored silty clays are the dominant sediment observed in gravity cores from central bay. Their dominance along the length of the core suggests that silty clays have been deposited consistently over much of this subembayment for the time period covered by the recovered sediments (Woodrow and others, this report). Stratification types include weakly-defined laminae, 1-3 mm thick. Few examples of horizontal lamination in very fine sand or silt were observed. Cross lamination, including ripples, was observed in seven cores. Erosional surfaces were evident in almost every core where x-radiographs were available (they are very difficult to observe visually). Minor cut-and-fill structures also were noted in three cores and inclined strata were observed in three cores.

Textural patterns in central bay indicate that silts and clays dominate the shallow water areas and margins of the bay. Sand dominates the tidal channel just east of Angel and Alcatraz Islands and to the west of the islands to the Golden Gate. The pattern of sand-sized sediment, as determined by particle-size analysis, suggests that sand movement is easterly from the west-central part of the bay. A second pattern of sand movement is to the south from the southwestern extremity of San Pablo Bay (boundary approximated by the location of the Richmond-San Rafael Bridge).

Age dates for central bay sediment samples were obtained by carbon-14 radiometric age dating. Age dates were determined from shell material that was interpreted to be largely in-place (not transported). Age dates subsequently were reservoir corrected and then converted to calendar years. Sediments sampled from central bay cores range in age from 330 to 4,155 years before present.

Foraminiferal distribution in the San Francisco Bay estuary is fairly well documented except for central bay. This study fills the data gap for both natural and introduced species of benthic foraminifera. Thirty-five species of arenaceous and calcareous benthic foraminiferal fauna were identified in 55 grab samples obtained in central bay in 1998. This includes the invasive Japanese species Trochamina hadai, thought to have been first introduced into San Francisco Bay in the early 1980s. Four assemblages of foraminifers were recognized based on cluster analysis: shallow east bay, intermediate east bay, deep western bay estuarine, and deep western bay marine. The foraminiferal distributions verify that west central bay is the most oceanically influenced region of the estuary. Foraminifers also document sediment transport by the relocation of species from their natural habitat. For example, T. hadai dominated (68-98 percent) the benthic fauna of shallow-water depths (shallow east bay assemblage) of central bay. The occurrence of this invasive species is similar to that in its home waters in Japan and along the East Pacific seaboard.

The three chapters in this report consist of:
Chapter I. Estuarine Sedimentation in Central San Francisco Bay, California By John L. Chin, Donald L. Woodrow, Mary McGann, Florence L. Wong, and Bruce E. Jaffe
Chapter II. Gravity Cores, Radiocarbon Dates, and Grain-size of Surficial Sediments, Central San Francisco Bay, California By Donald L. Woodrow, John C. Chin, Florence Wong, Theresa Fregoso, and Bruce Jaffe
Chapter III. 1998 Central San Francisco Bay Foraminiferal Study By Mary McGann

First posted August 16, 2010