Geology and paleoenvironment of the Bunker gas field, Solano County, California

Abstract

Bunker Gas Field is in Solano County, T. 6 N., R. 1 and 2 E., Mount Diablo base and meridian, in the Sacramento Valley, the northern part of the Great Valley of California. Subsurface structure contour maps of the field indicate a north-west- trending, faulted, domal to anticlinal fold which plunges to the southeast. Paleotopographic maps of the Bunker Gas Field demonstrate that: 1. The direction of the paleofluvial valleys and paleoslope during Late Cretaceous time was generally in a south-south- westerly direction and the sediment source was in the north-northeast. 2. The direction of the paleofluvial valleys and paleoslope during late Paleocene time was generally to the west and the sediment source was to the east of the field. This paleotopography is inferred to have persisted during early Eocene time. Lithologic characteristics (such as grain size, sorting, etc.) and stratigraphic sequence illustrate that subsurface deposystems in the Bunker Gas Field were predominantly of the shelf-facies type. Both progradational and retrogradational suites of strata occur within the stratigraphic sequence. The occurrence of a shallow regressive and transgressive sea was the prominent paleogeographic feature within the study area. In Late Cretaceous the study area was covered by a shallow regressive sea and was characterized by deltaic conditions during the period that the Mokelumne sand/shale lithesome was deposited. In early late Paleocene the study area was covered by a shallow transgressive sea and a mid-neritic environment prevailed during the period that the McCormick sandstone was deposited. In late late Paleocene the area was covered by a shallow regressive sea and exhibited a littoral environment at the time when the Martinez shale was deposited. In early Eocene the study area was covered by a shallow transgressive sea and was characterized by a nearshore environment during the period that the Hamilton sandstone was deposited.