DISTINGUISHING THE ROLES OF AUTOGENIC VERSUS ALLOGENIC PROCESSES IN CYCLIC SEDIMENTATION, CISCO GROUP (VIRGILIAN AND WOLFCAMPIAN), NORTH-CENTRAL TEXAS

Meter-scale transgressive-regressive cycles of the subsurface Cisco Gr oup are composed of marine and nonmarine carbonate and siliciclastic r ocks deposited on the Eastern shelf of the Midland basin during Late P ennsylvanian and Early Permian time. Five cycle types are characterize d by thickness, magnitude, order, and principal lithofacies, Cycle mag nitude is defined as the maximum facies shift in a cycle, indicating e xtent of shoreline migration. Cisco cycles belong to three orders-mino r, intermediate, and major-and they are superimposed and form a strati graphic hierarchy. Each order of cycles has a distinct range of thickn ess and possibly duration. A cycle is also divided into a lower sand-p oor interval, during which coarse siliciclastic supply at the depositi onal site was diminishing, and an upper sand-rich interval, during whi ch coarse siliciclastic supply was high. Regional thickness and lithof acies variations of sand-rich intervals indicate that progradational i nfilling 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 rec ord the interplay among regional topography, pattern of siliciclastic supply, and shelf subsidence, which controlled distribution of depocen ters and bypass zones and, thus, stratigraphic completeness and resolu tion. Regional persistence of cycles suggests a eustatic control on re gional, ordered transgressive-regressive events. In contrast, local va riations of cycle characters suggest controls by local topography and depositional dynamics, which determined depositional loci, differentia l 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 aut ocyclic imprints, because allogenic controls on cyclic sedimentation w ere 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).