COASTAL SEDIMENTARY RESPONSE TO SEA-LEVEL CHANGES: PLEISTOCENE MERCED AND COLMA FORMATIONS, SAN FRANCISCO, CALIFORNIA

A nearly 2-km-thick sequence of coastal sediments provides one of the most complete records of Pleistocene sea-level changes. The basin has since been inverted and sediments have been uplifted; they are exposed along the Pacific coast just south of San Francisco. Clifton and Hunter divided these sediments (Merced and Colma Formations) into ~40 sequences of generally shallowing-upward facies controlled largely by eustatic sea-level changes. The older sequences are dominated by continental shelf deposits and the younger sequences are dominated by nearshore, backshore and embayment deposits. The younger sequences, deposited since ~500 ka, continue to the east, where they form layered aquifers that the San Francisco Public Utilities Commission is using to augment public water supplies. We used subsurface data from well borings—electric logs, well cuttings, and drillers' lithologic descriptions—to correlate sequences between coastal exposures and inland wells, and to clarify environments of deposition and their lateral variations. Correlations are impeded by a blind thrust fault that has offset older units and strongly folded younger units. In this tectonically-active coastal zone, facies change abruptly and are difficult to correlate, but sea-level low stands have created erosional unconformities that form easier-to-correlate sequence boundaries. In the coastal outcrops, open-ocean sediments are common, whereas in the subsurface to the east embayment deposits, formed behind barrier dunes, predominate. Coastline position varies among sea-level cycles, but in general, the observed lateral and vertical variations suggest the following scenario: (1) erosional surfaces created during sea-level low stands and during rising sea level as the nearshore wave zone grinds its way landward; (2) embayment mud deposited as sea level reaches its high-stand level, followed by deposition of sand as the embayment fills and the barrier dune sand blows eastward; (3) simultaneous deposition of nearshore sediments to the west, followed by deposition of backshore sand, as sea level lowers and sand dunes move back westward. Embayments tend to better preserve the late transgressional parts of sea-level cycles, whereas the open-ocean coast tends to better preserve the early regressional parts of these cycles.