Imperial Valley is the southern part of a large northwesterly trending valley in southeastern California. This valley is a part of a large basin of deposition which existed during parts of Tertiary and Quaternary time. The stratigraphy discussed in this paper is based on a study of the exposed Tertiary and Quaternary sediments in the region bounded by the Santa Rosa Mountains on the north, Salton Sea on the east, Mexico on the south, and the crystalline rocks of the Coast Range on the west.

The rocks exposed in this region may be divided as follows:

Basement complex. Granite and metamorphic rocks
Split Mountain formation--0 to 2,700 feet. Non-marine fanglomerates and sandstones intercalated with marine sandstones and shales unconformably overlying basement complex. Miocene?
Alverson Canyon formation--0 to 700 feet. Non-marine unassorted sediments and associated basic igneous flows and tuffaceous sediments unconformably overlying all older rocks. Unfossiliferous

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Imperial formation--0 to 3,600 feet. Marine mudstones, siltstones, and sandstones unconformably overlying all older rocks. Upper Miocene
Palm Spring formation--0 to 6,100 feet. Non-marine mudstones, siltstones, and sandstones conformably overlying Imperial formation
Borrego formation--0 to 7,600 feet. Non-marine mudstones, siltstones, sandstones, and conglomerates probably unconformably overlying all older rocks
Terrace deposits--0 to 200 feet
Lake Coahuila deposits--0 to 100 feet. Thin veneer of lake marls covering most of surface below ancient beach line
Salton Sea deposits
Recent alluvium

The geology of the region surveyed indicates that the sediments are probably less than 14,000 feet thick due to the various unconformities within the sedimentary section. No data are available concerning the depth of the sediments in the central part of Imperial Valley. If the present topographic basin represents the central part of the basin of deposition, the sediments may extend to a depth of 22,000 feet or more.

The principal microfossils represented in the succession are Foraminifera and Ostracoda. Sixty species of the small Foraminifera were found in the Imperial formation, and most of these are confined to the lower few hundred feet of strata.

A gradual change to brackish-water conditions is indicated with the passage of Imperial time. This was followed by a comparatively abrupt change to fresh-water and slightly saline environments of the Palm Spring and Borrego formations. These locally contain Ostracoda, Chara, Rotalia beccarii, and a few species of Elphidium. The latter group of microfossils is represented in the deposits of Lake Coahuila, which covered the Salton Sink in relatively recent time. Some of the species are living in the Salton Sea, which now partially occupies the Salton Sink.

The foraminiferal fauna of the Imperial formation is not found elsewhere in California. It is clearly related to Miocene faunas of the Gulf of Mexico and Caribbean borderlands and is considered to be upper Miocene in age. A meager fauna from the Split Mountain formation suggests Miocene age. The microfossils of the Palm Spring and Borrego formations have no apparent value at this time in determining geologic age, but they can be used in local correlation.

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