From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, November 16, 2007
Characterization, Variations, and
Department of Geological Sciences, Jackson School of Geosciences
Abstract: T.E. Playton, C. Kerans, X. Janson
Allochthonous, seaward-dipping deposits that flank reefal carbonate platforms (reefal carbonate foreslopes) display high degrees of variability along strike and dip due to diverse sediment sources and resedimentation processes. This variability has made the development of predictive models that link carbonate foreslope composition to stratal architecture challenging. Aside from being a subject with continued academic interest, carbonate slope and basin systems are increasingly important as hydrocarbon plays (Tengiz Field, Kazakhstan; Poza Rica Field, Mexico; Midland Basin fields, West Texas), where predictive capability is critical. This study examines detailed outcrop data and well-constrained examples from literature of carbonate slope and basin systems, and addresses the 1.) component depositional elements and fundamental stratal geometries of reefal carbonate foreslopes, 2.) compositional and geometrical variations that exist, and the 3.) parameters that control these variations.
Collapse of early-lithified reefal material, sand and pebble shedding from shallow water environments, and fine-grained background sediment accumulation are fundamental processes of reefal carbonate slopes, and result in different sediment gravity flow types with distinct compositions, textures, angles of repose, and bedding architecture. The primary elements that construct reefal carbonate foreslopes can be categorized as 1.) collapse debris elements (breccias and blocks) from brittle reef failure, 2.) grain-shedding elements (grainstones and gravels) from offbank transport of grains and fragments, and 3.) hemipelagic elements (mud-dominated fabrics) that record slope quiescence. In terms of seismic-scale stratal geometries, clinoformal, accretionary margin systems are distinguished from escarpment margin systems that are characterized by onlap. Characterization of foreslope examples in terms of their elements and stratal patterns shows a spectrum of gross sediment composition, margin configuration, stratal evolution, internal deposit architecture, depositional profiles, and scales. Groups of foreslope systems that possess similar combinations of compositional and geometrical characteristics delineate major foreslope types that share common depositional processes and controls.
The differences described above for reefal carbonate foreslope systems are linked to the interplay of multiple controls, including accommodation, reef faunal assemblage and downslope extent, platform type, slope height, tectonic setting, and siliciclastic input. This study provides 1.) an approach for characterizing reefal carbonate foreslopes, 2.) models that outline the compositional and architectural variations observed, and 3.) a discussion on the interplay of parameters that control these variations.