Three-Dimensional Geometry of Coastal Deposits in the Pleistocene Merced Formation, Northern San Francisco Peninsula, California

Grove, Karen and McGuire, Terry
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The Pleistocene Merced Formation, located in coastal cliffs and the subsurface in San Francisco and San Mateo Counties, provides an opportunity to explore the detailed 3D geometry of reservoir/aquifer units and adjacent trapping/aquiclude units in shallow-marine deposits. About 2 km of sediments were deposited in a transtensional basin within the San Andreas fault system. The period of rapid subsidence, along with the frequent, high-amplitude sea-level changes associated with glacial/interglacial cycles, produced a series of unconformity-bounded sequences (i.e., parasequences) that preserve regressions formed when the coastline migrated seaward during high stand to falling parts of the sea-level cycle. The unconformities between sequences are commonly marked by transgressive lags; few sediments are preserved during the low stand and rising parts of the sea-level cycle because of wave erosion in the landward-migrating nearshore zone.

Subsequent transpression has inverted the basin—uplift along the San Andreas and Serra faults has exposed the northeastward-dipping sediments in coastal cliffs along the northwestern edge of the San Francisco Peninsula, with oldest beds at the southern end of the outcrop belt and younger beds at the north end. Northeast of the faults, the sediments are flat lying and mostly covered by the latest Pleistocene Colma Formation. These sedIments have been explored via drilling for monitoring and production wells that supply part or most of the water for communities in the northern part of the peninsula.

Our focus is on the upper 600 m of the formation (past ~0.5 m.y.), where we correlated sequences among sea cliff exposures and subsurface wells. These sequences have thin estuarine or offshore mud at the base and coarsen upward into nearshore to backshore sand and gravel. Our paleogeographic reconstructions show the limited lateral extent of coastal facies, with aquifer/reservoir units in the nearshore to backshore sand sandwiched between estuarine (aquiclude/trapping) mud. Older sequences are dominantly offshore mud with little reservoir capacity; youngest (northeasternmost) sequences are dominantly backshore sand that lacks the mud interbeds.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013