ABSTRACT

That part of the middle Eocene Domengine Formation which was deposited in the deeper parts of the Sacramento Valley basin near the present confluence of the Sacramento and San Joaquin Rivers, and along the valley's shelf immediately to the west, accumulated under predominantly transgressive marine conditions. Major textural varieties of this feldspathic subgraywacke unit in the Potrero Hills-Rio Vista area include clayey sandstone, sandy siltstone, and silty shale.

Source of sediment making up the Domengine sandstone was the Eocene Sierra Nevada batholithic complex and its western foothill belt of high and low-rank metasedimentary and metavolcanic rock. Detrital contribution from a Coast Range source is possible, but remains unsupported by conclusive evidence. Pyroclastic debris from either the Sierra Nevada foothills or a western island chain comprises a third and minor type of source material.

Intensive, but heterogeneous, kaolinitic alteration of all feldspar varieties observed in Domengine samples indicates weathering in a source area of moderate relief under warm and humid climatic conditions. Greater alteration of potassium relative to sodium feldspar suggests high groundwater levels, moderate to low source area relief, high rainfall, and subtropical temperatures.

Five lithologic subunits are recognizable in the Domengine Formation on the basis of outcrop evidence, core description, and electric and radioactivity logs. Basal unit A is a sublittoral, prodelta accumulation of silty fine sandstone which was deposited on the lower Eocene Capay Formation erosion surface. Overlying unit B represents part of a deltaic complex formed when deposition temporarily exceeded the increasing capacity of the sub represiding basin. Unit C is a tidal shelf deposit that is correlative with coastal marsh lignite accumulations to the south, and that forms a transition between the deltaic deposits of unit B and the protected bay environment characteristic of unit D. Terminal unit E was deposited at shallow, sublittoral depths under open marine conditions.