Sedimentologic evidence for structural and topographic evolution following the onset of strike slip, E San Francisco Bay area, CA
- CSUH, Hayward, CA (United States)
Mid- to Upper Miocene continental (Orinda and Mulholland Fms.) and shallow marine (Neroly Fm.) strata in the Upper San Leandro Reservoir watershed (SLR) area east of San Francisco Bay preserve important information on structural and landscape evolution during the early phases of strike slip along the Pacific-North American plate boundary. The SLR area lies between the Hayward and Calaveras Faults, major strands of the San Andreas Fault system, and is bisected by the NW-striking Cull Creek Fault (CCF). Geologic mapping delineates five completely intercalated lithofacies in the Mulholland Fm. at SLR. The conglomerate-dominated, sandstone-dominated, and interbedded conglomerate, sandstone, and siltstone facies represent fluvial channel and floodplain deposits; the sandstone + mudstone facies represent lacustrine-deltaic and shallow lacustrine deposits; the shale facies records open lacustrine deposition. Sparse unidirectional paleocurrent indicators show southerly and easterly transport west of the CCF and both westerly and easterly transport east of the CCF. Conglomerate-rich and sand-rich facies tracts are juxtaposed along the CCF. Clast assemblages in Mulholland conglomerates include abundant chart, graywacke, blueschist, and vein quartz, suggesting derivation from a Franciscan-dominated source terrane. Clast assemblages in the gradationally underlying and interfingering Neroly Fm. suggest that it shared the same source terrane; this is atypical for the primarily andesitic (Sierra-derived) Neroly. Fluvial deposits are volumetrically dominant in the Mulholland Fm. at SLR; open-lacustrine shales occur in stratigraphically isolated lenses ranging from > 1 km to < 100 m along strike. This suggests numerous small lakes on a broad drainage plain rather than the single large lake envisioned by previous workers.
- OSTI ID:
- 5589543
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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