Drowning of the Upper Marble Falls carbonate platform (Pennsylvanian), central Texas: A case of conflicting “signals?”

Abstract

Marble Falls and Smithwick Formation carbonates and shales were deposited on the western margin of the slowly subsiding Fort Worth Basin of central Texas during Early Pennsylvanian time. The Upper Marble Falls Member carbonate platform back-stepped towards the west–southwest as Smithwick outer shelf–slope black shales drowned platform margin areas to the east. The shelf-to-basin profile observed from subsurface data from the Fort Worth Basin and outcrops from the Llano Uplift area records platform drowning; however, the platform began to shoal upwards as it back-stepped towards the west–southwest, due to the presence of the Llano Uplift paleobathymetric high. The geometry of these facies tracts was controlled primarily by a combination of subsidence and second-order sea-level change, while the development of sequences and unconformities was controlled by third-order sea-level changes and environmental factors, and not subsidence. The termination of platform deposition in the Fort Worth Basin was caused by the interplay of third-order sea-level changes, environmental deterioration in advance of siliciclastic burial, and a lack of accommodation space. The depositional history of the Upper Marble Falls carbonate platform does not fit “classic” examples of drowned platforms described from other areas, but may be typical of platforms in slowly subsiding basins.