Matilija Sandstone; a transition from deep-water turbidite to shallow-marine deposition in the Eocene of California

Abstract

ABSTRACT The Matilija Sandstone (middle-upper Eocene), exposed in the Santa Ynez Mountains, California, records a major regressive event in the Eocene Santa Ynez Basin in which turbidites were deposited in the basin and subsequently covered by shallow-marine complexes. The Matilija Sandstone is underlain by the Juncal Formation and overlain by the Cozy Dell Shale. The Juncal Formation--a distal turbidite sequence--grades upward into a proximal turbidite sequence in the lower part of the Matilija Sandstone. This proximal turbidite sequence is overlain by a shallow-marine complex which is divisible into coastal and restricted-coastal lithofacies. Recognition and interpretation of these facies is based on fossil occurrences, petrography, biogenic and sedimentary features. Deposition of the Matilija Sandstone documents shoaling from outer neritic and/or bathyal depths in the Juncal Formation and basal part of the Matilija Sandstone to neritic and littoral depths in the upper part of the Matilija. It closed in a rapid transgression which culminated in the deposition of the Cozy Dell Shale (neritic to bathyal depths). The vertical sequence of sedimentary features, facies, and aerial distribution of the Matilija Sandstone suggest that it was deposited as a progradational deltaic complex. Progradation resulted in deposition by traction currents on the shelf, high-concentration sediment gravity flows initiated by slumping and sliding of material on the slope to upper basin, and deposition by lower concentration sediment gravity flows in the basin. As the delta prograded and filled the basin, it covered the deeper-water sediments derived from it.