ABSTRACT

Terrigenous clastic depositional systems of the Upper Mississippian Chester Group and the overlying Lower Pennsylvanian Pottsville Group in the Black Warrior basin of Alabama and Mississippi were deposited in distinctly different tectonic settings. The predominantly deltaic Chester sandstone units accumulated on the stable northern shelf of the basin and had a cratonic source to the north or northwest. Detailed subsurface mapping of these cratonic delta systems indicates that the northern shelf can be subdivided into a terrigenous clastic western element (Parkwood and Floyd Formations) and a largely carbonate eastern element (Bangor, Hartselle, Monteagle, and Pride Mountain Formations). Total thickness of the Chester interval on the shelf averages 1,200 ft. Pottsville sediments, in contrast, had a principal source to the southwest of the Black Warrior basin. They represent the thick clastic wedge shed from the Ouachita orogenic belt. Pottsville deposition occurred in a rapidly subsiding foreland basin and involved a maximum sediment accumulation exceeding 12,000 ft in the basinal core.

With the Chester group four cycles of deltaic progradation have been identified through data gathered from 600 well logs. Individual genetic intervals between the Tuscumbia Limestone and the base of the Pennsylvanian System are bounded by thin marine transgressive carbonate units. Two deltaic depocenters, a carbonate shelf and ramp, and a shallow basin carbonaceous shale unit comprise the principal depositional systems along the northern margin of the basin. The lowest interval (Lewis) involves a high-constructive lobate delta system whose maximum sandstone axes extend southeastward from Lee and Itawamba Counties, Mississippi, as far as Tuscaloosa County, Alabama. Maximum net sandstone thickness for each of the four lobes is approximately 60 ft. The second interval includes two distinct subdelta complexes of roughly the same age: the Evans High-Destructive Wave-Dominated Delta System on the western side of the northern shelf and the more easterly Hartselle High-Destructive Wave-Dominated Delta system in northwestern Alabama. The Hartselle net sandstone isolith pattern evidences a pronounced northwest-southeast elongation parallel to the shelf edge. Based on outcrop work, some researchers have interpreted the Hartselle as being a barrier bar system. Overlying the Hartselle-Evans genetic unit are the laterally equivalent Muldon Delta complex (Mississippi) and Bangor carbonate shelf (Alabama). The Muldon system is a thick (up to 450 ft of net sandstone), multistoried, high-constructive elongate group of delta lobes that takes in the subsurface Rea, Abernathy, Sanders, and Carter Sandstones. Further to the east, Bangor Limestone thicknesses in excess of 600 ft occur over interior portions of the shelf. Despite this build up, no true shelf edge or slope systems developed. In the interval between the Millerella Limestone and the top of the Mississipian System, there is a fourth Chesterian delta cycle. The distal edge of an extensive Pennington Delta, having an Appalachian source, prograded into northeastern Alabama. Further to the west, the Gilmer high-constructive delta system marks the last Paleozoic cratonic delta system laid down on the northern shelf.

With the surface and shallow subsurface Pottsville of the Black Warrior basin in Alabama, the 2,000 ft stratigraphic interval can be subdivided into a minimum of seven vertical genetic components. In contrast with the Chester units, however, laterally extensive coal seams rather than marine transgressive limestone tongues form the bounding elements. On the surface, the lowest Pottsville unit has no productive coal seams and is dominated by massive, quartz-arenite sandstone bodies interbedded with dark gray shale. These rock units record the presence of a basal Pennsylvanian microtidal barrier bar system and associated lagoon. Higher, in genetic intervals 2 through 4, the leading edge of the northward-prograding Ouachita clastic wedge reached north Alabama. It is preserved as prodelta mudstone, bar finger sands, crevasse splay units, destructional sheet sand, and delta plain facies of high-constructive deltas. Intervals 5, 6, and 7 contain a greater quantity of conglomeratic debris. Coals are less numerous, but commonly thicker in this proximal deltaic setting than with the distal delta. Petrographically, the upper Pottsville sandstone bodies are phyllarenites and polylitharenites (metamorphic, volcanic, and sedimentary rock fragments) and contain significant quantities of vein quartz and chert. Careful petrochemical study of the Pennington Delta, subsurface Lewis, Evans, and Muldon systems, and the Pottsville detaic facies should be able to document the respective Appalachian orogenic, cratonic, and Ouachita orogenic source areas for these three distinct Carboniferous terrigenous clastic elements of basinal fill in the Black Warrior basin.