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Stratigraphic and Structural Relationships of Strawn Group, Brown, Coleman, and Runnels Counties, Texas

Gordon D. Ware

The Strawn Group (Atokan-Missourian) is exposed in two approximately triangular areas in north-central Texas: the Brazos River valley and the Colorado River valley. The exposures in the Brazos River valley are relatively well understood and are represented by cycles of fluvial-deltaic and transgressive shallow marine facies. These exposures have been traced into the subsurface, and grade laterally into a mostly carbonate shelf succession.

The Strawn Group in the Colorado River valley has not been extensively studied, but recent work is increasing our knowledge of this significant area. In the Colorado River valley, the Strawn Group is subdivided into two lithogenetic units which accumulated in at least two depositional settings. The lower Strawn Group constitutes basin-fill sediments, and represents outer shelf, slope, and submarine-fan depositional environments. The upper Strawn, although exhibiting significant differences, is essentially correlatable to the Strawn Group known in the Brazos River valley and westward into the subsurface.

Subsurface mapping westward from the Colorado River valley demonstrates the general applicability of published cyclic interpretations based on surface and subsurface work in north-central Texas. However, in the Colorado River valley and westward into the subsurface, mapping reveals a system of faults presumed to be related to the horst and graben structural development in the Llano uplift area. In addition, the Concho arch, a feature closely related to the Llano uplift, was a positive area which resisted subsidence and acted as a buttress to terrigenous clastic deposition. It thus became a core for a persistent algal buildup.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.