Sedimentology and paleoecology of Pliocene shallow marine conglomerates, Salton Trough region, California

Abstract

The basal portion of the Pliocene Imperial Formation, Salton Trough region, California, includes conglomerates which were deposited in a rift-related setting at the head of the Gulf of California. Conglomerates and associated pebbly sandstones contain a fully marine fauna and form a transgressive sequence which unconformably overlies a basement of Miocene volcanics. Sequences deposited adjacent to rock cliffs include stratified conglomerate of beach origin and clast-supported boulder conglomerate which formed as talus. These types of sediment were also transported downslope as sediment gravity flows and accumulated in topographically irregular, nearshore areas as a variety of boulder to pebble conglomerates. The conglomerates contain two types of displaced faunal assemblages which reflect different types of rocky shore to shallow subtidal communities. An assemblage dominated by small oysters and barnacles represents a low diversity community which lived on and among volcanic clasts in muddy, protected coves. Another assemblage represents a diverse community of corals, bryozoans, gastropods, bivalves, annelids, barnacles, decapods and echinoids which lived on and among boulders along coastlines. Both assemblages have analogues in the modern Gulf of California. Local depletion of volcanic clasts resulted in deposition of nearshore sandstone which prograded over conglomerate deposits. This sandstone is composed of volcanic rock fragments and abundant skeletal fragments which represent taxa from several coastal communities.