PALYNOLOGY OF THE "PECTEN CLAY" IN THE PLIOCENE SAN JOAQUIN FORMATION, KETTLEMAN HILLS, CENTRAL CALIFORNIA, USA

The Pliocene San Joaquin Formation in the southern Central Valley of California is composed of interbedded marine and freshwater sandstones, siltstones, and claystones. An anomalous, organic-rich clay unit, informally named the "Pecten Clay", occurs in the lower portion of this stratigraphic sequence. Samples were collected above, below, and through the organic-rich clay sequence and processed to recover acid-insoluble palynomorphs to help determine the depositional environment of this unit.

Samples both above and below the "Pecten Clay" are similar in palynomorph content and suggest that the depositional environment was fluvial-lacustrine and the local vegetation source for palynomorphs was a warm temperate and relatively dry, oak-pine-juniper woodland with a sparse groundcover of wild flowers, herbs, and grasses. The modern vegetation most similar to that represented by pollen and spores in these samples lives today in the northern Central Valley of California near Redding and Red Bluff.

In sharp contrast, samples from within the "Pecten Clay" contain a palynomorph assemblage dominated by conifers (including pines, firs, spruce, hemlock, Douglas fir, redwood, and cedars), alders, oaks, basswood, elm, walnut, hickory nut, wingnut, hazel nut, and spice bush, with an abundance of algal colonies (Pediastrum and Botryococcus), the floating water fern Azolla, terrestrial ferns, clubmosses, and marine dinoflagellate cysts. Judging from the representation of taxa in this palynoflora, the terrestrial vegetation contributing pollen and spores to the depositional site was a cool temperate, conifer-dominated forest, with an understory of ferns and clubmosses, not unlike the moist, coastal forest found today along the northern California coast.

The palynomorph assemblage from the "Pecten Clay" records a major change in both climate and depositional environment, which we interpret the result of a marine incursion due to world-wide sea level rise or tectonic lowering of the Coast Ranges, allowing cold Pacific Ocean waters to enter the San Joaquin Valley. Cooler water in the inland sea/embayment probably altered the local environment to more coastal-like conditions with cool and moist on-shore winds. In response, vegetation surrounding the inland sea changed to be more like that of coastal northern California.