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.
Description:
Includes bibliographical references (pages 86-92)
California State University, Northridge. Department of Geological Sciences.