CHEMOSTRATIGRAPHY AND PALEODEPOSITIONAL CONDITIONS OF THE PENNSYLVANIAN SMITHWICK FORMATION, FORT WORTH BASIN, TEXAS

The Pennsylvanian-age Smithwick Formation was deposited in the Fort Worth Basin of north-central Texas. It is predominantly composed of organic-rich (TOC: 0.4-3.7%) mudrock, but represents a gradational lithologic package. The base of the formation is partially Ca-diluted, transitioning out of the underlying carbonate Marble Falls Formation, and grades upward into a coarser siliciclastic facies ultimately into the overlying clastics of the Atokan and Strawn formations. In addition, the mudrock strata are punctuated with layered siderite, potentially of microbial origin. An integrated geochemical and petrographic study was undertaken on two Smithwick cores from the southwestern portion of the Fort Worth Basin, from San Saba and Brown counties, Texas. Relationships in the major element geochemistry distinguish the various sub-facies found throughout the section and can be used to define the formation boundaries. The Smithwick is marginally enriched in redox-sensitive trace metals (V, Ni, Zn, U, and Mo) relative to an average shale, indicating a sub-oxic to anoxic depositional environment. The preservation of organic matter within the strata is further evidence for oxygen-depleted paleodepositional conditions. Microfossils, namely radiolarians and sponge spicules, preserved in the siderite layers indicate at least intermittent high levels of productivity. The presence of the siderite layers along with chemostratigraphic fluctuations can be used to infer hydrographic or water chemistry variability during deposition. The stratigraphic evaluation of the Smithwick provides an important analog for other transitional mudrock systems.