ABSTRACT: Influence of Sea Level Fluctuation on Cretaceous D Sandstone

James Puckette, George Bolling, Zuhair Al-Shaieb

Integrated core and wire-line log data indicate that the D sandstone reservoir in the Wallbanger field, Elbert County, Colorado, is a valleyfill sequence complex. This sequence is represented by fluvial and marine lithofacies. The lower part of this sequence consists of fining-upward channel sandstone facies in sharp basal contact with the underlying shale. Rip-up clasts, medium- to large-scale tabular cross bedding, and ripple marks are the major sedimentary structures. The upper part of the sequence consists of intensely burrowed sandstones, siltstones, and shales with distinct marine trace fossils. Shale to sandstone ratios increase upward with a noticeable decrease in trace fossil content.

D Sandstone deposition represents the regressive episode between the Huntsman and Graneros transgressions. Sea-level drop approximately 95 Ma caused incising of the D sandstone valley system into existing D and Huntsman sequences. Deposition within this valley was dominated by fluvial processes. A subsequent rise in sea level resulted in the deposition of the burrowed marine sands, silts, and shales of the upper interval. Less-burrowed sands, silts, and darker deeper-marine shales become the dominant lithologies with increasing water depth. The D valley-fill sandstone is very fine- to medium-grained plagioclase-rich subarkose. Silica cement in the form of syntaxial quartz overgrowths has significantly reduced primary intergranular porosity. Secondary porosity is predominant and is cha acterized by oversized pores formed by dissolution of feldspars and lithic fragments.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990