Description
The Imperial Formation crops out extensively in the Salton Sea trough in the western Colorado Desert. The formation is divided into the lower Latrania Member and the upper Burrobend Member. In its type section the Latrania Member consists of green micaceous sandy siltstone and shale which are laterally correlative with commercial quantities of gypsum and anhydrite. Higher in the section the member contains fine-grained calcareous sandstone and shale interbeds. The Burrobend Member consists of massive siltstone and claystone. Islands and basement ridges were present in at least two areas in the Coyote Mountains and one locality near the Fish Creek Mountains at the time of Latrania deposition. The Latrania Member contain s planktonic foraminifera of the N 19 zone, or early Pliocene, and is correlative with the abundantly megafossiliferous localities in the Coyote Mountains and Alverson and Barrett Canyons. The Burrobend Member belongs to the N 20 and N 21 zones or middle and late Pliocene and is transitional with the Blancan-aged (Plio-Pleistocene) Palm Spring Formation. The Imperial Formation and Panamic molluscan faunas are products of interoceanic interchange between the Caribbean and East Pacific. The areas of oceanic communication were across Central America and northern South America. Oceanic interchange between three areas seemingly ceased during or slightly before the early Pliocene. Two hundred species of molluscs, corals, barnacles, bryozoa, and echinoids are listed from the Imperial Formation and Caribbean cognates, or closely related species, are suggested. Faunal changes, in the form of almost complete extinctions of genera and species, occurred in the Pliocene coral and echinoid faunas as well as in certain groups of the molluscs at the Plio -Pleistocene transition. These extinctions could be related to changes in salinity or sedimentation or to the deterioration of the thermal regime at this time in the Gulf of California. Normally saline waters prevailed during the deposition of all the Imperial Formation with the exceptions of the hyper-saline evaporite of the lowest Latrania Member and the hyposaline claystone of the uppermost Burrobend Member. The distributional data and character of the Pliocene corals indicates that the water temperatures declined from a high during the Pliocene. Temperatures were lowest during the Pleistocene and rose slightly during the Recent but were still cooler than during the Pliocene. Distributional and relative abundance data for selected Latrania Member molluscs suggests that water depths were shallowest in the central Coyote Mountains and progressively deepened southeast and northeast. The Latrania member shows well developed lateral biotic zonation which is seemingly depth controlled. The echinoids are the shallowest-water faunal elements. There was offshore development of Pinna and Atrina reefs and Ostrea californica bioherms. The Latrania coral reefs were the most basinward biotic accumulation.
