Abstract: A Miocene Age for the "Pico" Formation in the Northern Los Angeles Basin?
Daniel J. Ponti, John A. Barron, Andrei M. Sarna-Wojcicki, Mary Lou Cotton
Five Los Angeles MetroRail borings drilled along a 4.3-km-long section of Wilshire Boulevard near Hancock Park encounter dipping siltstones between depths of ~30 to at least 60 m. We assign these sediments to the "Pico" Formation based on core samples that contain inferred late Pliocene to Pleistocene Wheelerian and Venturian (~0.7-2.5 Ma) benthic foraminiferal assemblages. Both the lithology and the foram assemblages appear to change systematically from east to west along Wilshire Boulevard and suggest a westward (up-section) shoaling, typical for the Pico. Three ashes within the section, however, indicate a late Miocene age for this biofacies (~5.4-6.0 Ma). Diatom biostratigraphy corroborates the ash data and suggests that at least part of the section dates between 5.6 and 5.8 Ma. T is age is clearly older than the foraminifera indicate. To explain this discrepancy, we speculate that the Wilshire Boulevard section may have been deposited on a bank or near the margin of the Miocene basin in an environment that did not permit the occurrence or preservation of key Miocene taxa.
While it is understood that benthic foraminiferal stage boundaries are time transgressive, the Wilshire Boulevard data illustrate that the problem is potentially quite severe. Without independent age control, basin-wide structural interpretations could be subject to significant error. Lacking other corroborating data, inferred faults and unconformities suggested by missing faunal zones and abbreviated stratigraphic sections may have unknown tectonic and stratigraphic significance, particularly in basin-margin settings.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994