The Composition, Texture, Structure, and Probable Origin of the "Gatchell" Sand

Citation

Regan, Louis John (1941) The Composition, Texture, Structure, and Probable Origin of the "Gatchell" Sand. Master's thesis, California Institute of Technology. doi:10.7907/N2K8-7D80. https://resolver.caltech.edu/CaltechTHESIS:03292010-131419884

Abstract

The purpose of the research has been to gather data on the structural and stratigraphic relations, composition, and texture of the "Gatchell" sand, and to attempt to interpret the conditions of deposition of the sand body in the light of this data. The "Gatchell" is the producing sand of the East Coalinea Extension Oil Field. Several structure contour maps, an isopach map and two structure sections are presented to show the general structural relations of the "Gatchell". The sand body is thicker on the north than on the south; moreover the rate of eastward thickening is much more rapid in the northern area. The zero isopach, or line of pinch-out of the sane body, is remarkably straight and trends about north-south. The sand is arkosic, containing between 20-30% feldspar. The feldspar is practically all potash feldspar, orthoclase and microcline being present in subequal amounts. Accessory or heavy minerals are scanty both in number of species and in total amount present. The assemblage is a very stable one, relatively rich in zircon, garnet, and tourmaline. The presence of glaucophane points to a Coast Range origin for at least some of the material. Whether the bulk of the sand was derived from the east or from the west cannot yet be shown. The sand is very angular and coarse, but very well sorted, It is suggested that the agent transporting and depositing the sand was marine currents. Since the northern sand is older,* it is suggested that the sand body grew from north to south, and that the immediate source of the material may have been on the north. The internal structure of the sand cannot be shown by bedding, which is lacking, nor by mineral zones, but a generalized picture can be obtained from a consideration of charts showing the median grain size of the sand against depth. A number of these charts are presented in the appendix; it is suggested that in the southern area, at least, "Gatchell" sands transgressed from east to west over Hondo silts. The sand in wells on the west, near the line of pinch-out, is represented in wells on the east by the upper part of the sand body only. The environment of deposition of the "Gatchell" cannot yet be demonstrated. However, four hypotheses are suggested and discussed, which fit some of the evidence. These hypotheses are (l) the baymouth bar-spit hypothesis (2) the offshore bar hypothesis (3) the shoreface terrace hypothesis and (4) the interfingering hypothesis.

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