QUATERNARY STRATIGRAPHY AND HYDROSTRATIGRAPHY OF THE LONG BEACH AREA, SOUTHWEST LOS ANGELES BASIN, CA

Two 1400 foot-deep, continuously cored boreholes were drilled in Long Beach by the USGS in partnership with Los Angeles County Department of Public Works and the Water Replenishment District of Southern California to evaluate seawater intrusion pathways into fresh-water aquifers. The cores provide critical stratigraphic and lithofacies data that, coupled with new, integrated geophysical studies, enable construction of a sequence-stratigraphic model to help resolve the complex hydrostratigraphy in this area of the L.A. basin. The northernmost well (LWEB), was drilled in the type area of the Silverado aquifer, the primary fresh-water aquifer in the basin, and is located on the northeast limb of the Wilmington anticline. The second well (LBPC), located 5 km to the south at the Port of Long Beach, was drilled near the anticline axis.

In borehole LWEB, at least six unconformity-bounded sequences, controlled by eustatic fluctuations in sea level, contain the Gaspur, Gage, and Lynwood aquifers. The Gaspur aquifer is of fluvial origin and occurs within an ancestral L. A. River channel cut during the previous sea-level lowstand (~18 ky b.p.), whereas the underlying Gage and Lynwood aquifers are a complex mix of bay, tidal, and upper shoreface lithofacies apparently associated with sea-level highstands. Stratigraphically below the Lynwood is the type Silverado aquifer, which was deposited in a series of fluvial channels likely associated with marine lowstands. In LWEB, all sequences below the Gaspur dip gently to the northeast, and preliminary age determinations indicate that the entire section is of mid-Pleistocene and younger age.

In borehole LBPC, most of the Lynwood and Gage aquifer sequences appear to be absent or thinned, suggestive of ongoing uplift of the Wilmington anticline. The Silverado gravels are not present in LBPC. Instead, probable time-correlative sequences consist of marine silt of dominantly lower shoreface and offshore origin. Mid-late Pleistocene activity of the Wilmington anticline therefore appears to have significantly influenced the geometry and distribution of aquifer facies in the region.