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Defining Lateral Lithostratigraphic and Chemostratigraphic Variability Within a Closely-Spaced Array of Drill Cores, Upper Cretaceous Strata, Austin, Texas

Abstract

Twelve well cores from the foundation of the AT&T Executive Education and Conference Center, University of Texas at Austin, were collected and studied for their geochemical and lithological/sedimentological content. Collectively, the cores penetrate the uppermost Del Rio and Buda formations, and formations and members of the Eagle Ford and lower Austin groups. The cores provide a unique, closely-spaced, three-dimensional sampling of geological variability over a ~350-feet by 410-feet area. The full stratigraphic interval is ~167 feet in thickness. Such a closely-spaced network of cores is not found in the deep subsurface, making the data set particularly useful in evaluating the degree of lateral variability on the scale of tens to a few hundred feet. The data set may be viewed as superior to a horizontal core taken in the deep subsurface because a horizontal core typically “porpoises” through the formation, decreasing our understanding of lateral variability of one horizon, and providing very little information in the breadth dimension. Initial evaluation of the cores reveals significant thickness variations in well-defined units, including mass flow deposits and ashes. Also revealed is the occurrence of thick mass flow deposits in some of the cores, and their absence in neighboring cores, suggesting a high degree of lateral depositional variability in some of the units. The abundance of mass flow deposits throughout the succession, coupled with abundant ripple-laminated strata within the Eagle For Group units underlines the dynamic depositional conditions at work during accumulation of the succession. A three-dimensional “cube” of stratigraphic and lateral changes in deposition will be presented, including a focus on the lateral variability in ash accumulation and diagenetic characteristics.