Abstract: Panch Rico Formation, Salinas Valley, CA: Integrative Stratigrpahic and Paleoecologic Methods
WHITTLESEY, KAREN E., and DAVID
J. BOTTJER, University
of Southern California, Los Angeles, CA 90089-0740
The latest Miocene Pancho Rico Formation, containing fossiliferous, diatomaceous mudstones, represents the final incursion of marine conditions in the Salinas Valley of California. Unlike the Monterey Formation, these diatomaceous rocks are highly bioturbated and record deposition below 100 meters of water, in thoroughly oxygenated conditions.
The strata contain abundant fossil barnacles in shell beds and small reef-like buildups, including the giant (up to 30 cm long) barnacle, Tamiosomagregaria. Unlike any other barnacle, extant or fossil, Tamiosoma lived and flourished in a muddy, diatomdominated depositional regime. In response to high rates of sediment deposition, the barnacles grew upward with narrow, elongate bases.
In the type area for Tamiosoma (Wildhorse Canyon), hundreds of meters of Pancho Rico strata are exposed, which are relatively undeformed by folding or faulting and are unconformably overlain by the nonmarine Paso Robles Formation. Techniques used to study the geology of the area include ichnofabric analysis, x-ray diffraction, paleoecology, thin sections, and bioerosion analysis.
Although isolated smaller barnacles are present throughout the formation,
the vast majority of Tamiosoma fossils are found in one stratigraphic
interval, which is approximately 2 m in thickness. This interval extends
laterally for as much as 500 meters, although fossil density varies on
a meter to decimeter scale. This layer can be subdivided into separate
layers of shell bed concentrations (approximately 20-30 cm thick) and undisturbed
diatomaceous layers, in which complete barnacles, bivalves, gastropods,
and bryozoans are present.
AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California