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The AAPG/Datapages Combined Publications Database
Terrigenous clastic- and carbonate-rock units forming the middle third of the Strawn Group were deposited in the Fort Worth basin and on the adjacent Concho platform of north-central Texas. Four significant transgressive-regressive cycles comprising the interval between the top of the Brannon Bridge Limestone and the top of the Brazos River Formation have been evaluated employing data gathered from 4,000 well logs and 35 measured sections. Subsurface maps indicate that four discrete, vertically persistent, deltaic depocenters, two carbonate banks, and an embayment strand-plain complex are present within the area. The average thickness for the total vertical stratigraphic interval is 1,000 ft (300 m), or approximately 250 ft (75 m) per cycle of deltaic progradation and aba donment.
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 for all four cycles involve thin, usually less than 140 ft (42 m) thick, multilateral, high-constructive elongate and lobate systems. For the lower 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 (320 km) downdip from the source area. Carbonate-bank deposition subsequently was established on the distal ends of these oldest delta sands and westward deltaic progradation was less extensive with the upper two cycles. A strand-pl in--embayment system composed of mudflats, chenier sandstone bodies, and thin bay-head deltas developed between the two principal deltaic depocenters on the platform. The Midland basin on the west was a poorly defined, gradually deepening, depression; no true Desmoinesian shelf-edge or slope systems have been discovered.
High-constructive delta systems attributed to the Dobbs Valley (cycle II) and Brazos River (cycle IV) units have sandstone accumulations in excess of 200 ft (60 m) at one depocenter along the northwestern margin of the platform and at a second one on the northwestern rim of the Fort Worth basin. These thicker deltaic complexes contain linear, multistoried sandstone bodies whose geometries resemble bar-finger sands of the modern Mississippi delta. Valley-fill fluvial deposits incise the high-conservative deltaic facies. The Arbuckle and Wichita Mountains were the major sources for the more arkosic, northern delta systems. The Ouachita foldbelt furnished the chert-rich detritus for the fluvial-deltaic facies on the Concho platform.
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