In the San Francisco Bay area, estuarine, lacustrine, and fluvial depositional environments existed contemporaneously within the Mulholland Formation, which was deposited in the Contra Costa basin from 7.7 to 6.5 Ma. The estuarine facies is differentiated from the nonmarine facies on the bases of ostracodes and sedimentology. Presence of the estuarine facies in the Mulholland Formation demonstrates that the Contra Costa basin was at sea level, the western part of the basin being a marine embayment for about 1 m.y. Absence of fresh Sierra Nevada-derived material in the lower Mulholland Formation indicates that a topographically elevated area existed east of the Contra Costa basin, effectively deflecting the Sierran debris, which is represented by the coeval Neroly Formation found in the eastern central Coast Ranges. To the west of the Contra Costa basin, some gaps in the Coast Ranges allowed connection with the Pacific Ocean.

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