Origin of the Eocene sands of the Coalinga district, California. The lower and middle Eocene Foraminifera of the Coalinga district, California

Abstract

Origin of the Eocene sands of the Coalinga district, California: The purpose of this paper is to attempt to explain the origin and the conditions of deposition of the Gatchell and related Eocene sands of the Coalinga district, California. The method of study has been to gather data bearing on the position of the Eocene basin of deposition, location of positive areas, and sources and direction of movement of the sediments; such data is grouped into three types: 1. The age and correlation of Eocene formations are of prime importance in the problem. The correlations used in this study are based largely on Foraminifera; in some instances, where paleontological data is lacking, correlations are based on heavy minerals. 2. Lithologic data presented in this report deals in part with the distribution of sands and silts in the Coalinga district. However, the main body of this data comprizes a considerable number of heavy mineral analyses, including analyses of Eocene sands and also of possible Eocene source rocks. Four distinct assemblages of heavy are present in the Eocene sands studied, each assemblage indicating a change in source rocks. These analyses can be orrelated with possible source rock analyses, which include granitic, Franciscan, and Cretaceous rocks. In addition to heavy mineral data, a smaller amount of data on the light minerals of the various Eocene sands is presented. 3. Evidence bearing on the position of positive areas includes structural data dealing with the main unconformities in the Eocene section, and stratigraphic data dealing with the thicknesses of Eocene formations in various parts of the area. It is concluded that during the Eocene, there were three contemporaneously existing positive areas bordering the area of deposition. The final part of the report deals with the interpretation of the data outlined above. A number of maps have been constructed showing probable progressive stages during the deposition of the Eocene sediments. The Cantua sandstone is believed to have been derived from the granitic rocks now exposed west of the San Andreas fault in the approximate latitude of Coalinga. The Gatchell sand is believed to have been derived largely from Cretaceous rocks in a positive area west of the present Reef Ridge; it is pictured as having been deposited as a spit built northward from this positive area, across the southeastern end of the Vallecitos channel. The Yokut is regarded as having a Coast Range source. The Domengine was very clearly derived from the Franciscan rocks of the Coast Ranges. The lower and middle Eocene Foraminifera of the Coalinga district, California: During the last three years, the author has been gathering data on the texture, composition, and distribution of the Gatchell sand and other Eocene sands in the vicinity of Coalinga, California, with the hope that detailed knowledge of this type would lead to an explanation of the origin of the Gatchell sand body, and to an understanding of the factors controlling the distribution of sand and silt in the Eocene sediments of this area. Because of the extreme lenticularity of the Eocene formations, a uniform basis for correlations is required for the correct interpretation of the data described above. The Eocene shales contain abundant Foraminifera, and these organisms provide such a basis for detailed correlation. This paper deals with the Foraminiferal assemblages occuring in the interval between the base of the Kreyenhagen, and the Cretaceous; this interval was chosen, since it contains the sands related to the general problem outlined above. One outcropping section, located at Oil City, a few miles north of the town of Coalinga, was studied, and the data from this section was corroborated and amplified by the study of three well sections; the location of these four sections is shown on the index map, page 4. A check list of species identified is presented, together with a chart showing the ranges of these species in each of the sections. The faunal changes agree very closely with the Eocene B, C, and D zones described by Laiming, and these divisions have been indicated on the Range Chart. The ecologic significance of the assemblages is considered, and it is concluded that there was a shallowing and a warming of the sea from deep, cold water conditions in D time to warm, shallow conditions in B3 time, followed by shallow warm water conditions in B2 and B1 time.