Late Pennsylvanian Shelf in North-Central Texas: ABSTRACT

Abstract

Upper Pennsylvanian rocks were mapped on the surface and into the subsurface across 30,000 sq mi of north-central Texas. The study area includes most of the late Paleozoic eastern shelf of the West Texas basin. Strata in 2,500 wells were correlated using sample and mechanical logs. For surface mapping, formal stratigraphic nomenclature was used; in subsurface correlation, the strata were subdivided into 22 nearly isochronous stratigraphic intervals. Structural and isopach maps, and percentage isolith, ratio, three-component, and trend-surface lithofacies maps were constructed of various stratigraphic intervals. Upper Pennsylvanian rocks dip less than 1° west. The average strike rotates from N45°E to N20°E to nearly north-south at the tops of the Desmoinesian, Missourian, and Virgilian rocks. Three major facies are present: marine mudstone, marine limestone banks, and a paralic facies of sandstone and mudstone intercalated within beds of limestone. The average composition for Upper Pennsylvanian sediments in 2,500 wells is 68% mudstone, 15% limestone, and 17% sandstone. In Missourian rocks the marine mudstone (67%) and limestone banks (21%) dominate, whereas in Virgilian rocks, mudstone (69%) and paralic sandstone (18%) are the major lithofacies. Little is known of the marine mudstone facies other than its distribution. The limestone banks are skeletal deposits that were wave and current resistant. Composited, the banks are elongate biostromal trends as thick as 200 ft, up to 180 mi long, and 40 mi wide. Biohermal deposits are as thick as 1,200 ft, 40 mi wide at the base, and 5 mi wide at the top. The primary bank builders, which behaved as sediment baffles, were phylloid algae and bryozoans mostly on calcareous muds (micrites). Related facies include osagiid algae and crinoidal grainstones (sparites). On the eastern shelf three elongate banks persisted throughout Missourian time; two parallel northeast-striking biostromes and one east-striking biostrome along the Red River uplift. Following an epeirogenic warp of the Ouachita source a north-trending Virgilian bank persisted along the eastern edge of the West Texas basin. The most extensive bank growth occurred during two Missourian and one Virgilian transgressions. The distribution of the sandstone, and the interpretations of the textures and structures in the sandstones, indicate that braided and anastomosing streams, deltas, and nearshore bars were major types of depositions in Missourian and Virgilian paralic facies. Principal source areas were both east and north. Paralic facies are most significant in Virgilian outcrops, because the shorelines had migrated far west and south of Missourian strandline positions. A logical model for Late Pennsylvanian eastern shelf position is an ameboidlike front of terrigenous sandstone-mudstone facies advancing locally and episodically into a marine mudstone-limestone bank facies. Modern counterparts of these facies are known. End_of_Article - Last_Page 749------------