ABSTRACT

Terrigenous clastic and carbonate rock units making up the middle third of the Strawn Group were laid down in the Fort Worth basin and on the adjacent Concho platform of north-central Texas. Four major transgressive-regressive cycles involving the interval between the top of the Brannon Bridge Limestone and the top of the Brazos River Formation have been evaluated utilizing data gathered from 4,000 well logs and 35 measured sections. Subsurface isolith maps indicate that four discrete vertically persistent, deltaic depocenters, two carbonate banks, one carbonate platform, and an embayment-strandplain complex are present within the area. Average thickness for the total vertical stratigraphic interval is 1,000 ft, or approximately 250 ft per cycle of deltaic progradation and abandonment.

When active tectonic downwarping diminished in the central part of the Fort Worth basin, middle Strawn cratonic deltas prograded across the filled foreland basin and out onto the stable, gradually subsiding Concho platform. Deltaic facies present on the platform with each of the four cycles involve thin, usually less than 140 ft thick, multilateral, high-constructive elongate and lobate systems. For the lowest two cycles, the Buck Creek and Dobbs Valley sandstones of outcrop, deltaic progradation extended to the western margin of the platform, more than 200 mi downdip from the source area. Carbonate bank deposition subsequently established itself on the distal ends of these oldest delta sandstone units, as westward progradation was less extensive with the upper two cycles. A strandplain-embayment system composed of mudflats, chenier sandstone bodies, and thin, bayhead deltas developed between the two principal deltaic depocenters on the platform. The Midland basin to the west was a poorly defined, gradually deepening, depression; no true Desmoinesian shelf-edge or slope systems have been identified along the basin's eastern margin.

High-constructive delta systems attributable to the Dobbs Valley (Cycle II) and Brazos River (Cycle IV) intervals have net sandstone accumulations in excess of 200 ft per unit at one depocenter along the northwestern margin of the Concho platform and at a second one on the northwestern rim of the Fort Worth basin. These thicker deltaic complexes are linear, multistoried sandstone bodies whose geometries resemble barfinger sands of the modern Mississippi delta. Valley-fill fluvial deposits incise the high-constructive deltaic facies. The Arbuckle and Wichita Mountains were the principal sources for the more arkosic, northern delta systems. The Ouachita fold belt supplied the chert-rich detritus for the fluvial-deltaic facies laid down on the Concho platform.