Virgil and lower Wolfcamp repetitive environments and the depositional model, North-central Texas

Virgil and lower Wolfcamp rocks on the Eastern Shelf in North-central Texas are composed of several intergradational depositional systems comprising 1,200 to 1,500 feet of off-lapping, predominantly terrigenous sediments. At least a dozen major and numerous minor repetitive sequences consist of superposed deposittional systems, composed of more or less homotaxial component facies. Rapidly shifting fluvial-delta sites and associated interdeltaic and open shelf environments on the slowly subsiding shelf were subjected to marine destruction, mud compaction subsidence, and marine transgression. Variations of the basic sequence in time and space resulted from shifting depositional systems. Pluvial variants are downslopes, and deltaic and interdeltaic variants are concentrated in intermediate areas. These facies tracts shifted irregularly southwestward during Virgil and Wolfcamp deposition as the average strandline migrated with westward shelf progradation. Westward pointing deltas locally extended subaerial environments far downslope. Delta sequences between bases of successive delta systems are diachronous and aperiodic as deltation irregularly reoccupied former delta sites. Sequences between bases of successive transgressive limestone facies are also interpreted to be aperiodic and diachronous, but bounding limestones display regional continuity. Delta and fluvial constructional facies represent relatively brief, discrete time intervals, while destructional, interdeltaic, and transgressive facies involved greater time resulting in complex chronology within sequences. The fluvial-deltaic model for Virgil and Lower Wolfcamp rocks make it unnecessary to invoke external cyclic control to explain these North-central Texas deposits. The self-regulating model can operate under continuous sediment supply and continuous but slow shelf subsidence. The model is based on fades relationships and processes rather than absolute scale and geometrical comparison with Recent models. The diachronous nature of fades required by the model and supported by stratigraphic evidence indicates that repetitive deposition was primarily governed by sedimentary processes active within the local basin.