Pleasants Sandstone Member, Williams Formation, Santa Ana Mountains: A Southern California Upper Cretaceous Shelf Sandstone: ABSTRACT

Abstract

The Williams Formation consists of two distinct members: a medium to coarse-grained sandstone and conglomeratic unit (Schulz Ranch Sandstone Member), and a fine to medium-grained sandstone unit (Pleasants Sandstone member). This study of the Pleasants Sandstone Member focused on strata in a linear outcrop belt from Modjeska Grade (southeast) to Baker Canyon to Black Star Canyon (northwest). The dominantly fine-grained clayey sandstones of the Pleasants are interbedded with numerous medium to coarse-grained sandstone beds. The lack of clay minerals, the medium to coarse-grained texture, rarity of microfossils, primary sedimentary structures, transported shell materials, numerous escape burrows, and the thickness of individual units suggest that these strata were deposited rapidly, as tempestites on a narrow shelf. The intensity and duration of storm activity determined the thickness of units, as well as their preservability. Most of this sediment influx came from the vicinity of Modjeska Grade, the study section interpreted as having been in the shallowest part of the Pleasants paleoenvironment. The fine-grained clayey sandstones become progressively more dominant toward Black Star Canyon. These strata represent sediments deposited by waning storm activity, as well as hemipelagic sedimentation. These rocks exhibit a complete range of biologically to physically dominated sedimentologic fabrics, production ofmore » which depended on sedimentation rates, infaunal organism densities, and dissolved oxygen levels. Diagenetic carbonate concretions are abundant and are useful in determining the initial sediment fabric of the deposits. Carbonate concretions formed through the decomposition of abundant organic matter (plant and animal) in porous water-saturated sediments.« less