Abstract

Terrigeneous clastic and carbonate depositional systems comprising the upper two thirds of the Strawn Group (Desmoinesian and lower Missourian Series) were deposited in the Fort Worth basin and on the adjacent Concho platform of north-central Texas. Eight major transgressive-regressive cycles have been evaluated using data gathered from 4,000 well logs and 35 measured sections. Subsurface isolith maps indicate the presence of four discrete, vertically persistent, deltaic depocenters, two carbonate banks, one carbonate platform and an enbayment-strandplain complex. Variations in the rate of subsidence for the foreland basin and platform, as well as the beginning of subsidence to form the Midland basin, controlled the lateral distribution and overall lilthofacies geometry for these systems.

When active subsidence ended in the central region of the Fort Worth basin during the Middle Desmoinesian, Strawn deltas prograded across the filled foreland basin and out onto the stable, gradually subsiding Concho platform. Major deltaic facies laid down on the platform are thin, ususally less than 140 feet (43 m) thick, multilateral, lobate and elongate high-constructive systems. For the lowest two cycles of deltation, progradation extended to the western margin of the platform. Carbonate bank deposition associated with the Anson system subsequently became established on the distal ends of these oldest deltaic sandstone units, as westward deltaic progradation was less extensive for later cycles. A strandplan-embayment complex composed of mudflats, chenier sandstone bodies and thin, usually less than 50 feet (15 m) thick, bayhead deltas accumulated between the principal deltaic depocenters on the platform.

During the Desmoinesian Epoch the Midland basin was a porly defined, gradually deepening depression beyond the western margin of the Concho platform. No true Desmoinesian shelf-edge or slope systems have been identified in this area. With the beginning of the Missourian, however, evidence from the upper two Strawn depositional cycles indicates the inception of a Knox slope system on the northwestern border of the platform. Further to the south, accelerated subsidence in the Midland basin brought about the development of a distinct hinge line along which the vertical accretion of the outer Palo Pinto-Winchell carbonate bank occurred.

High-constructive delta systems attributable to the Dobbs Valley (Cycle II) and Brazos River (Cycle IV) intervals involve net sandstone accumulations in excess of 200 feet (61 m) per unit at one depocenter along the northwestern margin of the Concho platform (Knox-Baylor depression) and at a second on the northwestern rim of the Fort Worth basin. These thicker deltaic complexes are linear, multistoried sandstone bodies whose geometries, particularly with the more westerly Haskell delta, resemble barfinger sands of the modern Mississippi delta. Valley-fill fluvial deposits incise the high-constructive deltaic facies (Bowie) that were deposited in the Fort Worth basin. Distal deltaic facies of the Henrietta fan delta first appear in northern Clay County near the end of the Desmoinesian, but extensive southwesterly progradation did not occur until the Middle Missourian. The Arbuckle and Wichita mountains were the principle sources for the more arkosic Bowie, Haskell and Henrietta delta systems. The Ouachita fold belt supplied the chert-rich detritus for the fluvial-deltaic facies laid down on the Concho platform.