Paleoenvironmental analysis of the Lower Middle Pennsylvanian deep-water sediments, Fort Worth Basin, Central Texas
Wagoner, John Charles van
Warme, John E.
Master of Arts
The Fort Worth Basin, a marginal foreland basin of the Ouachita orogenic belt was a site of sedimentation during the Pennsylvanian. Lower Middle Pennsylvanian claystones, siltstones, and sandstones were studied where exposed south of Goldthwaite, Texas on the Colorado River. Physical sedimentary structures are present such as small scale ripple laminae, flute and groove casts, convolute and horizontal laminae, and syndepositional soft sediment slumps. Repeated sequences of graded beds are commonly associated with both parallel and convolute lamination. Trace fossils abound, including Zoophycos, Neonereites, Lophoctenium, Chrondrites, and one with a rosette shape. All other faunal evidence is lacking. Such physical and organic structures may be found in many different environments. Paleogeography of the Lower Middle Pennsylvanian including proximity of the rocks at Goldthwaite to deep-water rocks of similar age exposed nearby indicate the rocks studied were deposited in deep-water. Noticable lack of micro or macro fauna, absence of medium scale cross-bedding or ripples, the presence of turbidites, and abundant claystone both slumped and unslumped further substantiate deposition below storm wave base in deep-water. On the basis of distinctive rock types, paleocurrent directions, geometries of sandstone bodies, and assemblages of sedimentary structures three facies were recognized in the rocks studied along the Colorado River. They were arbitrarily designated Facies A, Facies B, and Facies C. Collectively the three facies are a portion of a submarine fan prograding approximately westward into the Fort Worth Basin, on the western edge of the Ouachita uplift. Facies A is a sandstone filled channel in the lowerinner to mid fan region. Facies B is interchannel claystones and siltstones, and Facies C represents either overbank deposition from a channel or a fine-grained channel fill. Based on the similarity of the tectonic history and relationship to the Ouachita geosyncline between the Fort Worth Basin and the Pennsylvanian Stanley and Atoka Basins it is concluded that submarine fan deposition active in the Fort Worth Basin is a viable model to apply to the now heavily deformed rocks of the Stanley, Johns Valley, and Lower Atoka Formations in the Ouachita Mountains.