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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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In the Colorado River Valley of central Texas about 1,200 ft (366 m) of Pennsylvanian (Desmoinesian) terrigenous clastic and carbonate facies were deposited in the Fort Worth basin. Paleocurrent and petrographic data indicate that the clastic provenance, to the west and northwest, was the rising Ouachita orogenic foldbelt.
Surface study suggests that the Strawn Group includes two main stratigraphic divisions: (1) a lower sequence representing basin and submarine fan sediments deposited as active tectonic subsidence substantially increased water depths in the Fort Worth basin; and, (2) an upper progradational sequence of fluvial-deltaic and carbonate platform sediments deposited as passive isostatic subsidence or regional uplift to the east accompanied progressive shoaling of the sediment surface. The lower Strawn is characterized by sediment gravity flows suggestive of submarine fan deposition. It is subdivided into four facies (from proximal to distal): (1) massive channeled sandstone; (2) massive amalgamated sandstone; (3) turbidite; and (4) shale facies. The upper Strawn constitutes a delta-platform ssemblage that includes: (1) delta facies including channel-mouth-bar sands, delta-front sands, delta-slope shales, and interdistributary fine clastics; and (2) carbonate platform facies that includes phylloid algal mounds and perideltaic bioclastic limestones.
The lower Strawn basinal shale and submarine fan sequence is gradational with the underlying basinal Smithwick Shale (Atokan) and records foreland basin subsidence synchronous with orogenesis in the Ouachita foldbelt. In contrast, the basal contact of the upper Strawn is variable. Generally, the upper Strawn is gradational with the lower Strawn but locally it rests disconformably on the Marble Falls Formation (Morrowan-Atokan). This stratigraphic relationship suggests that regional compressive forces from the Ouachitas were depositionally more important than previously supposed.
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