QUATERNARY STRATIGRAPHY OF THE LA BREA PLAIN, NORTHERN SHELF OF THE LOS ANGELES BASIN

Lithologic, paleomagnetic, strontium-isotope (⁸⁷Sr/⁸⁶Sr), tephra, amino-acid, and faunal data obtained from cored boreholes across the La Brea Plain define five unconformable Quaternary chronostratigraphic units. Resting unconformably on folded Miocene marine strata, the oldest Quaternary unit is fine-grained, normally magnetized, and contains a Repettian and overlying Venturian benthic foraminiferal fauna indicative of upper middle bathyal depths (1500 to 500 m). Strontium-isotope (⁸⁷Sr/⁸⁶Sr) and paleomagnetic data imply that this sequence was deposited during the early Pleistocene Jaramillo geomagnetic chron (~0.986-1.053 Ma). The top of the tilted Jaramillo sequence was truncated, and an onlapping sequence of chiefly massive (bioturbated) silt and silt loam was deposited that contains the Brunhes-Matuyama magnetic reversal boundary (BM, ~780 ka), and overlying Bishop ash (~760 ka). A Wheelerian Stage benthic foraminiferal fauna (upper bathyal depths, 150 to 500 m) and marine mollusks of shelfal aspect (water depths ~50 to 100 m) indicate deposition of this unit near the shelf-edge. The overlying unit encompasses a heterogeneous stratigraphic sequence deposited, at least locally, on a high-relief erosional surface. The unit’s basal sequence is a coarse-grained non-marine paleochannel fill. Interbedded silt and sand (tidal-flat and channel deposits and shallow-water marine strata) that contains wood and charcoal characterize the upper sequence. The Lava Creek ash (~665 ka) is present in the lower part of the upper sequence. A middle-Pleistocene erosional episode truncates the older deposits. Overlying this abrasion platform is a sand that contains shallow water and intertidal marine mollusks with amino-acid ratios comparable to mollusks in the type Upper Bent Spring Chronozone (oxygen-isotope stage 9 highstand, ~320 ka). The marine sequence transitions into a broadly upward-fining fluvial sequence locally capped by a paleosol that is, in turn, buried by late Pleistocene, and locally Holocene, fluvial sediment.