Date of Award

Summer 6-19-1968

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology and Geophysics

First Advisor

Moore, C. H.

Second Advisor

Perkins, B.F.

Abstract

During Fredericksburg (Early Cretaceous) time the entire Callahan Divide area of west-central Texas was the site of clayey, lime-mud accumulation in a normal-marine, shallow-shelf environment. Lateral homogeneity of the quiet-water environment was soon interrupted locally by the deposition of bioclastic lime-sands in a linear, highly agitated shoal-zone coinciding with a portion of the axis of a regional, positive, Paleozoic structural feature known as the Concho Arch. Localized shoal conditions persisted throughout most of Fredericksburg time and resulted in the deposition of a thick sequence of grainstones and packstones collectively referred to in this paper as the Callahan Complex. Numerous diastems within the Callahan Complex resulted from intermittent subaerial exposure, lithification and subsequent submergence of the area and attest to a complex depositional history of the sand body. Diastems in the lime-sand sequence resulted from the combined effect of (1) rapid accumulation of sediment on topographically high areas of the sea bottom and (2) fluctuations of sea level resulting from regional differential subsidence of

the structurally high Callahan Divide area with respect to flanking basins. Relief on the sea bottom was due primarily to ero- sional relief produced during the periods of subaerial exposure and provided sites for the shallow shoal-areas in which well-sorted, fine-grained, bioclastic material was concentrated. Passageways for marine currents through the shoalarea formed as channels normal to the linear trend. Well- sorted ooliths and coarse, poorly sorted, bioclastic grains characterize the two types of channel lithologies. During its deposition the Callahan Complex separated a marly, lime-mud depositional area to the northeast and a less-marly, tidal-flat-dolomite depositional area to the southwest. The sand body (1) acted as a barrier to the passage of terrigenous clay being received from a distant source-to the northeast and (2) locally inhibited normal circulation of marine waters.

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