Description
The Imperial Formation as exposed in the southeast Coyote Mountains consists of three major facies. Facies A records the initial flooding of the Salton Trough in earliest Pliocene times. The Salton Trough was a low-lying area with rugged topography. When inundated, this resulted in a highly irregular shoreline of high relief characterized by headlands, bays, islands, and submerged hills. Facies A consists of shoreline deposits associated with alluvial fans. This facies is almost entirely non-fossiliferous. Fossils, when found, are of Clypeaster sand dollar type. Facies B is highly complex in its lateral and vertical variations. This is the result of the high initial relief. The facies consists of tidally dominated units, ranging from supratidal gypsum to low-tide terraces. Sublittoral units indicate depths probably not deeper than 30 m. Facies B is highly fossiliferous with a diverse fauna which records tropical temperatures, normal-salinity marine waters, and oxygenating near-shore environments. Facies C consists of greenish siltstones and clays indicating a filling of the Salton Trough by fine clastic material from the Colorado River. The lowermost rocks of the facies are locally gypsiferous. The major portion of Facies C is non fossiliferous and may indicate rapid sedimentation, high evaporating conditions, followed by a stable salinity gradient and anoxic bottom conditions. The upper part of the unit contains sheet-like sandy coquinas which are proposed to be the result of meandering tidal channels. Overlying gradationally, is the brakish to lacustrine Palm Springs Formation. The Palm Springs Formation records the development of the Colorado River delta cone into a barrier excluding marine waters from the Salton Trough in Early Pleistocene times.
