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Quaternary Sedimentary Environments in the Long Beach Area, Los Angeles Basin, California

C. Powell II1, E. Clifton1, B. Edwards1, K. McDougall2, D. J. Ponti1, J. Tinsley1, and J. Walker1
1U.S. Geol Survey, Menlo Park, CA
2U.S. Geol Survey, Flagstaff, AZ

Fossils and sedimentary features from two wells in Long Beach,Webster School (LWEB) and Port of Long Beach Pier C (LBPC), located only five kilometers apart, reflect environmental differences and cannot be correlated on the basis of lithologies. Instead, the environmental differences are a function of lateral facies changes across the region, effects of deformational structures active throughout much of the Quaternary, and/or the inherent geometries of constituent depositional components. Overall, sediments encountered at LWEB reflects fluvial, coastal, and shallow marine environments, whereas at LBPC offshore marine deposits pervail. The uppermost Holocene package of sediments at LWEB reflect largely fluvial and lacustrine environments. At LBPC, however, shallow bay lithofacies are encountered. These differences reflect facies changes across the modern coastal zone. Beneath the upper Holocene package at LWEB, several shallow marine to coastal bay successions appear to be only partly represented at LBPC. These deposits are likely absent at LBPC due to growth of the Wilmington anticline during middle to late Pleistocene time. At LWEB, the type Silverado aquifer is a 100-m thick succession of fluvial channel sands and gravels situated from 180 to 280 m subsurface. This system of river deposits is not encountered at LBPC, presumably reflecting the rather restricted geographic extent of this fluvial system. Overlying both the Silverado aquifer and a shallow marine sand at LWEB is a 10-m thick silty clay unit suggesting mid-shelf or deeper marine deposition. This unit is also not present at LBPC, probably owing to its being pinched out and/or not deposited at the fold crest. At LBPC, similar lithofacies are much thicker, and only occur beneath a major unconformity at a depth of 252 m. These strata are probably considerably older than strata from equivalent depths at LWEB.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California