Abstract

The Tertiary stratigraphy of the southeastern San Joaquin Basin includes sediments of Eocene through Pliocene age. The Tertiary rests unconformably on Sierran basement rocks, which include pre-Upper Cretaceous granitic, mafic and metamorphic rocks.

The Walker Formation is a nonmarine unit ranging in age from Eocene to early Miocene. Depositional settings for the Walker vary from lahar and debris flows in older outcrops of Adobe Canyon, to braided stream deposits in the lower Miocene outcrops near Caliente Creek. The Walker is conformably overlain by the Vedder Sand, a Zemorrian age marine sandstone representing shelf and slope environments. Predominantly a subsurface unit, outcrops of the Vedder are confined to the Adobe Canyon area. The upper Oligocene-lower Miocene Bealville Fanglomerate represents debris flows, talus and colluvium derived from the Edison fault in the area of Caliente Creek, where it unconformably overlies basement and interfingers to the northwest with the Walker Formation.

The Zemorrian (lower Saucesian?) age Pyramid Hill Sand Member of the Jewett Sand represents a transgressive shelf sequence. Along the east side of the basin, the Pyramid Hill Sand lies unconformably on both the Vedder and the Walker. The upper Jewett Sand and overlying Freeman Silt indicate an increase in water depth during the Saucesian. Foraminifera from outcrops of the Jewett Sand indicate a lower bathyal water depth (2000m+). The Olcese interfingers with the underlying Freeman Silt and the overlying Round Mountain Silt. The Saucesian-Relizian age marine Olcese Sand contains evidence of a range of paleoenvironments including nonmarine, estuarine and outer shelf depositional settings.

The lower-upper Miocene Bena Gravel unconformably overlies the Bealville Fanglomerate. The Bena contains both an alluvial fan and a nearshore facies which form a fan-delta system with proximal parts of the Edison Shale (=Round Mountain Silt and Fruitvale Shale). The Upper Relizian (?) to Luisian Round Mountain Silt contains inner shelf faunas at the base, an outer shelf-upper bathyal diatomaceous interval assignable to the Denticulopsis lauta zone, and a tooth and bone bed near the top which is correlative with strata containing upper middle bathyal foraminifera (1000-1500m). The Round Mountain Silt is conformably overlain by the Mohnian Fruitvale Shale, representing a shelf-slope marine environment. Conformably and unconformably overlying the Fruitvale Shale, the upper Miocene “Santa Margarita” Formation is indicative of a regressive, littoral, upper Miocene sea.

Upper Miocene, nonmarine sands, clays and muds of the Chanac Formation interfinger with and conformably overlie the “Santa Margarita” Formation. The latest Miocene to Pleistocene(?) Kern River Formation unconformably overlies the nonmarine Chanac Formation and older units and interfingers with the marine Etchegoin Formation. The Kern River, a generally coarse grained fluvial unit, includes bar deposits, channel fill deposits and finer grained facies reflecting gradual channel abandonment. Cross-cutting channels with low depth-to-width ratios such as those represented by the Kern River are a result of variable flow and unstable banks.