ABSTRACT

"Facies analysis of the transgressive and regressive sequences of late Pennsylvanian age on the Eastern Shelf of the Midland Basin indicates that carbonate deposition occurred preferentially at moderate water depths and on mid-shelf positions. Deepest water areas on the shelf were mostly sites of shale deposition, with phosphate nodule-bearing shales being characteristic. During transgression, depositional trends started with sand and mud deposition in shoal waters, then changed to carbonate deposition in moderate water depths, and finally changed to phosphatic shale if deep water conditions occurred. During regression, the sequence was reversed, although local deltaic input often prevented the accumulation of carbonates in the regressive sequence. Carbonate deposition on the shelf occurred within a restricted zone that shifted back and forth in response to rising and falling water levels. Widespread carbonate sheets are the result of lateral migration of the carbonate generating zone, and are time transgressive; not the result of synchronous carbonate deposition across the shelf.

The restriction of major carbonate deposition to mid-shelf locations is the result of large amounts of fine-grained terrigenous mud being delivered to the shelf. Carbonates are precluded from the shore zone by sand and mud deposition. Fine-grained sediment passed through the carbonate depositional zone by suspension in nepheloid layers, and supplied enough mud to the deeper water areas to preclude carbonate deposition on the deep outer shelf. Sedimentation patterns of this type (with mixed carbonate-siliciclastic deposition) occurs on ramp-topped shelves, having a gradient of increasing water depth out to the shelf edge.