Distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation, Cisco Group (Virgilian and Wolfcampian), north-central Texas

Abstract

Meter-scale transgressive-regressive cycles of the subsurface Cisco Group are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the Eastern shelf of the Midland basin during Late Pennsylvanian and Early Permian time. Five cycle types are characterized by thickness, magnitude, order, and principal lithofacies. Cycle magnitude is defined as the maximum facies shift in a cycle, indicating extent of shoreline migration. Cisco cycles belong to three orders—minor, intermediate, and major—and they are superimposed and form a stratigraphic hierarchy. Each order of cycles has a distinct range of thickness and possibly duration. A cycle is also divided into a lower sand-poor interval, during which coarse siliciclastic supply at the depositional site was diminishing, and an upper sand-rich interval, during which coarse siliciclastic supply was high. Regional thickness and lithofacies variations of sand-rich intervals indicate that progradational infilling at a depositional site lagged marine regression, suggesting a delay in sediment supply from the upland source relative to the time of base-level fall. Regional systematic variations in cycle abundance, continuity, and characteristics along depositional dip and strike record the interplay among regional topography, pattern of siliciclastic supply, and shelf subsidence, which controlled distribution of depocenters and bypass zones and, thus, stratigraphic completeness and resolution. Regional persistence of cycles suggests a eustatic control on regional, ordered transgressive-regressive events. In contrast, local variations of cycle characters suggest controls by local topography and depositional dynamics, which determined depositional loci, differential compaction, and erosion. A predominantly autocyclic Cisco record in the upper platform does not imply the absence of allogenic processes. An allocyclic Cisco record in the lower platform contains abundant autocyclic imprints, because allogenic controls on cyclic sedimentation were accomplished through local autogenic processes. Distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation is an important step in establishing a high-resolution (meter-scale) chronostratigraphy of any sedimentary record 1 .