Depositional environments of the Eocene Domengine Formation near Coalinga, Fresno County, California

Abstract

The upper lower through lower middle Eocene Domengine Sandstone crops out in the southern Diablo Range in the Alcalde and Big Blue Hills. The formation reaches a maximum thickness of 65 m and is overlain by the deep marine shales of the Kreyenhagen Formation. The Domengine unconformably overlies the Cretaceous Panoche Formation in the Alcalde Hills and the lower Eocene Yokut Sandstone in the Big Blue Hills. The Domengine in the Alcalde Hills was deposited on a fluvial-dominated delta. The formation contains interdistributary bay, swamp, distributary channel and levee, and overbank flood deposits. Distributary channel deposits consist of pebble conglomerate and cross-bedded sandstone. The associated levee deposits consist of sandy mudstone and fine-grained sandstone and contain mudstone pods and carbonaceous material. Overbank flooding between the distributaries formed claystone deposits. The beach deposits of the interdistributary bay are represented by fine-grained sandstone which contains cross bedding, and parallel and wavy laminations lined with carbonaceous material. Lenses of Ostrea idriaensis and other shallow water fauna are abundant. Coarse, fossiliferous sandstone deposits were formed by storms within the inter distributary bay. North of Los Gatos Creek, in the Big Blue Hills, the base of the Domengine is marked by a basal pebble bed. The bed represents a transgressive lag and contains abundant black chert and fossil fragments. The majority of the Domengine consists of shoreface and transition zone deposits. The lower shoreface deposits consist of structureless (90% to 100% bioturbated) muddy sandstone and contain a wide variety of foraminifera and molluscs. Wood fragments are abundant. The middle shoreface deposits consist of predominantly structureless (70% to 90% bioturbated) fine sandstone which contains some parallel laminations. Structureless mudstone and sandy mudstone deposits represent the transition zone environment and contain foraminifera. Storm deposits, represented by fine sandstone beds and conglomeratic sandstone, occur within the shoreface and transition zone deposits. The deposits of the Domengine Formation were derived from a sedimentary source terrain, possibly the Paleocene and Mesozoic rocks to the west. The deposits are the result of an Eocene westward transgression across an eastward-sloping shoreline. A fluvial-dominated delta formed south of Los Gatos Creek and there was deposition of shoreface and transition zone deposits along a rocky coastline to the north. The lack of significant interdistibutary bay and transition zone deposits in the northern and southernmost portions of the study area indicate that there was erosion followed by quick subsidence prior to the deposition of the deep-marine shale of the Kreyenhagen Formation.