Abstract: Stratigraphic Revelations Regarding the Rincon Shale (Lower Miocene) in the Santa Barbara Coastal Area, California
Richard G. Stanley, Mary Lou Cotton, David Bukry, Mark V. Filewicz, Zenon C. Valin, David R. Vork
The Rincon Shale is superbly exposed in excavations at the Tajiguas landfill, about 40 km west of Santa Barbara. Here, the Rincon is 449 m thick and consists of bathyal marine mudstone, shale, and dolomite. The Rincon abruptly overlies shallow marine sandstone of the Vaqueros Formation, and is conformably overlain by a 70-cm-thick tuff at the base of the Monterey Formation.
New, detailed biostratigraphic analysis demonstrates that the Rincon at the Tajiguas landfill is entirely of early Miocene age, rather than Oligocene and Miocene as previously reported. Samples from 0.38 and 0.84 m above the base of the Rincon yield planktic foraminifers of the lower Miocene N4-N5 (undifferentiated) zones of Bolli and Saunders (1985). Benthic foraminifers indicate that the entire Rincon at this locality is within the Saucesian benthic foraminiferal stage of Kleinpell (1938, 1980). Calcareous nannofossils from the lowest 43 m of the Rincon are poorly preserved but indicative of the lower part of early Miocene zone CN1. "Blooms" (abundant, low-diversity assemblages) of
small Reticulofienestra sp. and Braarudosphaera bigelowii occur at several levels within the lower 30 m of the Rincon.
At the Tajiguas landfill, the Rincon/Vaqueros contact records rapid bathymetric deepening from neritic to bathyal depths, primarily in response to tectonic subsidence but possibly also in concert with eustatic sea level rise. The Rincon/Vaqueros contact can be no older than the Miocene/Oligocene boundary, dated at 23.8 Ma by Cande and Kent (1992). The tuff at the Monterey/Rincon contact probably is about the same age as welded tuff dated isotopically at 17.79 + 0.10 Ma on Tranquillon Mountain, about 42 km west of the landfill. Thus, the period of time represented by the Rincon at the Tajiguas landfill is 6.1 million yr or less.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994