Chemostratigraphy And Paleoenvironment Of The Smithwick Formation, Fort Worth Basin, San Saba County, Texas

Abstract

The Early Pennsylvanian-Age Smithwick Formation was deposited in the tectonically active Fort Worth Basin in North Central Texas during a time of well-documented global climatic instability during the Late Paleozoic ice age. Geochemical and stable isotopic results from two cores recovered from the southwestern portion of the FWB provide paleoenvironmental insight into the conditions under which the Smithwick was deposited. Specifically, major element geochemistry and mineralogical results suggest a depositional setting grading from a carbonate-rich facies (Marble Falls and Big Saline) into a transgressive siliciclastic-dominated mudrock (Smithwick) that coarsens upward into a progradational mudrock and silt/sand sequence (Atoka and Strawn). While robust geochronological constraints are lacking for Middle Carboniferous strata, subtle oscillations in the detrital geochemical proxies (Si/Al, Ti/Al) may provide a temporal framework assuming they are a result of orbital forcing. Furthermore, thin siderite-rich intervals may indicate discrete periods of amplified eolian deposition due to the aridification associated with glaciation on Gondwana. Alternatively, the siderites may reflect a more localized signature of changes in a tropical paleoclimate. Integration of trace metal, Fe-S-TOC relationships, and the overall lack of preserved fauna suggest largely suboxic bottom water conditions in the basin. Average TOC for the Smithwick is 1.5%, and the organic matter is dominantly marine algal based on C/N molar ratios of 6.6 to 17.5 and ä13C values of -23.1 to -25.7 /. Integration and interpretation of the results suggest that the Smithwick represents a marginally productive environment that existed during the early stages of subsidence of the Fort Worth Basin.