SEDIMENT SUPPLY AND TECTONIC CONTROLS ON THE STRATIGRAPHIC ARCHITECTURE OF THE CISCO GROUP, SOUTHERN EASTERN SHELF OF THE PERMIAN BASIN

Several cyclothems of the Cisco Group in the Eastern Shelf record the influence of sediment supply and tectonism on the growth of a mixed siliciclastic-carbonate shelf system. These mechanisms acted together with glacio-eustatism to create considerable horizontal and vertical facies changes. I investigate the stratigraphic architecture of seven fourth-order sequences of the Virgilian interval of the Cisco Group with the aim of unraveling the role of each mechanism in the evolution of the southern Eastern Shelf.

The seven sequences span about 3.7 Myr and together create a 200- to 450-m succession of shallow-marine clastic and carbonate ramp systems. Correlation of >5,000 wireline logs were used to map depositional systems and delineate the 3D trajectory of the shelf margin. Moreover, aeromagnetic datasets provided insights about the basement morphology and structural framework.

Results suggest that shelf-edge deltas were primary controls on shelf-margin growth. In contrast, areas beyond the influence of these deltas show little or no progradation but contain carbonate ramps or patch reefs. Four sediment fairways concentrated fluvial channels that fed these delta systems. Sediment flux coming out of these channel systems varied in each sequence, controlling the amount of progradational area (200 to 3,900 km²) and direction of predominant sediment transport in the shelf (NE-SW to SE-NW).

The basement presents magnetic anomalies that commonly match with those related to contacts and fault systems observed in outcrops of the Llano Uplift. Changes of thickness and depositional style occur over these anomalies suggesting structural controls on the sedimentation. Concurrence of incised-valleys and patch reefs follows the pattern of magnetic anomalies related to structural highs. Likewise, the shelf-margin geometry in each sequence present similar direction to other anomalies related with faults.

The mechanisms observed herein shaped the geometry and architecture of the southern Eastern Shelf. Moreover, they controlled the delivery of siliciclastic sediments across the shelf to the basin. Further investigation is needed to understand the regional controls, specifically in the context of the Ouachita-Marathon orogeny.