Description
The Brown Mountain Sandstone, Maestrichtian in age, is comprised of large, lenticular sandstones stratigraphically bounded by shale. Based on outcrop and subsurface analyses, the Brown Mountain Sandstone is inferred to have been deposited in sand-rich submarine fan systems with source areas to the east/northeast. Overall subsurface sandstone geometry reflects a proximal-to-source direction to the northeast. Paleocurrent orientations have a dominant southwesterly component, paleontological data infer a water depth of 600-1,500 feet, and petrographic data indicate a "granitic" source terrane. In addition, two distinctive lithofacies are recognized based on differences in grain size and sedimentary structures. These lithofacies are interpreted to be the result of sand deposition in at least two separately fed fan systems that include slope-fed fan systems through delta front gullies and canyon-fed fan systems. The analyses and inferences made for the Brown Mountain Sandstone fit well into the tectonic regime of a composite forearc basin. Based on depositional character of the Brown Mountain Sandstone and stratigraphically contiguous units, the depositional model includes an overall progradational slope system for the Maestrichtian part of the sequence. This is corroborated by postulated eustatic sea-level curves and the resultant time-equivalent facies patterns present throughout the Upper Cretaceous section. The Brown Mountain Sandstone has been correlated to time-equivalent units to the north and east. Correlative biostratigraphic units, based on D-2 foraminiferal fauna in the Sacramento basin, include the upper Tracy submarine fan sandstones. Petrofacies correlation, based on sandstone composition and stratigraphic position, places the Brown Mountain Sandstone in the upper part of the “Rumsey" petrofacies. This "Rumsey" petrofacies is continuous into the northern San Joaquin and Sacramento basins.
