Facies Architecture, Depositional Environments, and Sequence Stratigraphy of the Marble Falls Formation (Morrowan-Atokan), Central Texas
Wood, Stephanie G.; Ruppel, Stephen C.; Loucks, Robert G.
The Pennsylvanian Marble Falls Formation in the southern Fort Worth Basin (central Texas) is a Morrowan-Atokan carbonate unit whose lithofacies, depositional environments, and sequence stratigraphic architecture have been investigated. Until recently, descriptions of lithofacies and interpretations of the Marble Falls Formation have only come from outcrop studies at the fringes of the Llano Uplift, making it difficult to have an understanding of the complete section and regional stratigraphy. A series of 30 cores are now available with many cores showing the lower contact with the Barnett Shale and the upper contact with the Smithwick shale. This dataset enables the complete Marble Falls section to be described and correlated to create a regional sequence stratigraphic interpretation.
On the basis of core data, the study area is interpreted to have been deposited on a shallower water platform to the south (possibly associated with the Llano Uplift) to a deeper water slope/basinal setting to the north into the Fort Worth Basin. Initial studies of cores and thin sections have identified 14 platform and basin facies. Dominant facies are (1) dark gray to mottled, argillaceous, burrowed spiculitic wackestones and packstones, (2) red algal grain-dominated packstones to grainstones, (3) skeletal foram wackestones, and (4) lithoclasts-rich debrites. Several siliciclastic mudstone facies are also present.
Because the Marble Falls section was deposited during Pennsylvanian icehouse times, the resulting depositional cycles reflect rapid changes in sea level. However, distal cores in the slope/basinal setting also have density flow deposits, so cycle stacking patterns were investigated to separate depositional units responding to sea-level changes (especially sea-level drops) from depositional units responding to local processes (e.g., gravity flows). Next, facies correlations interpreted in conjunction with chemostratigraphic data will be used to improve previous interpretations of the unit's depositional history and form a unified regional model.
Results from this study contribute to our understanding of the depositional response to glacioeustatic sea-level changes during the Pennsylvanian. These results also form the basis for a sedimentological and facies analog for middle to late Atokan shallow- to deepwater carbonates present in the Permian Basin and in the northern Fort Worth Basin.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013