Approximately 1,900 ft (580 m) of continental rocks of latest Virgilian, Wolfcampian, and early Leonardian age are exposed in an area of about 3,800 mi2 (9,900 km2) between the Brazos and Red Rivers of north-central Texas. The stratigraphic complexity of these strata has impeded internal correlation and mapping since the rocks were first described by W. F. Cummins in the late 19th century. Precise correlation of this discontinuously stratified fluvial sequence with well-defined, limestone-bounded, fluvial, deltaic, and marine formations to the south has been hampered because of (1) the pronounced change in lithology that accompanies this marked facies transition, (2) a shift in strike of approximately 65° that coincides with a change in facies thus amplifying the stratigraphic complexity of the region, and (3) generally poor exposure of the gently inclined strata.

The continental sequence is composed of approximately 25 major and numerous minor, upward-fining, principally fluviogenic cycles. Sandstone units (5-60 ft or 1.5-18 m thick) mark the bases of these cycles and occur as regionally persistent zones with multistory and multilateral geometry. These generally resistant, cuesta-forming units interfinger with limestone-bearing strata or are separated from them along strike by intervening zones of red mudstone. The fine-grained upper portion of the cycles (10 to > 100 ft or 3 to > 30 m thick) is predominantly concretionary red mudstone, although gray and variegated claystone lenses, thin siltstone and sandstone beds, and lenticular and channel-fill conglomerates are characteristic.

Precise mapping of sandstone units and correlation with prominent limestone pinch-outs have permitted a stratigraphic tie with the marine section of the Colorado and Brazos River valleys. Continental rocks are divided into the Bowie and Wichita groups; equivalent marine strata are divided into the Cisco and Albany groups. Formations have been defined in each group to allow a maximum degree of intergroup correlation.

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