The Strawn Group (late Atokan-early Missourian) comprises a thick sequence of terrigenous clastic and carbonate facies that crop out in central Texas in two roughly triangular areas separated by Cretaceous overlap. The exposures in the Brazos River valley have been studied in detail and interpreted as fluvial, deltaic, and shallow marine deposits. The Strawn Group of the Colorado River valley has been cited in the literature, but little detailed work regarding subdivision or interpretation has been done.

The Strawn Group exposed in the Colorado River valley can be informally subdivided into two general stratigraphic units. The lower Strawn represents submarine fan and basin depositional systems that prograded southwestward. At least two but possibly several cycles of fan progradation are recognizable based on vertical succession of distal to more proximal middle-fan facies associations. In marked contrast, the upper Strawn represents three regressive and transgressive cycles of typical Pennsylvanian deltaic, perideltaic, and shallow submarine depositional systems.

Stratigraphic relationships of the Strawn Group are complex and reflect changes in the evolving Ouachita foldbelt and its associated tectonic features. The lower Strawn occurs only in the Fort Worth basin and apparently was deposited with a clinoformal relationship to the underlying basinal Smithwick Shale. It records foreland basin subsidence and regional block faulting synchronous with orogenesis. As active tectonic subsidence shifted westward, upper Strawn deltaic systems prograded to the west, onlapped the Concho platform, and truncated older units particularly on horst blocks. In the southeastern portion of the Concho platform, the overlying Canyon Group completely onlaps the Strawn Group and rests unconformably on the Marble Falls Limestone.

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