Reconstruction of the Depositional Environments of the Devonian Chattanooga Shale in northeastern Alabama

Abstract:

The Chattanooga Shale is a hydrocarbon-rich, Devonian age, black shale well-exposed in several outcrops in Alabama, Tennessee, Georgia, and Kentucky. Although Devonian strata along the Atlantic Seaboard are thought to be deposited in marine environments, the depositional environments of the Chattanooga Shale remain controversial. In particular, it remains unknown why much fewer fossils have been found in outcrops located in northeastern Alabama relative to the outcrops elsewhere. We hypothesized that the fossil scarcity may be due to more depleted oxygen conditions during the shale deposition in Alabama than at other locations. We collected samples from two outcrops of the Chattanooga Shale which are respectively located near Fort Payne and Collinsville in northeastern Alabama, where nearly complete stratigraphic successions from the underlying Silurian Red Mountain Formation to the overlying Mississippian Maury Formation were found. These two outcrops share similar sedimentological features (e.g., thinly laminated fissile grayish and black shale layers in the lower part and nearly homogeneous black blocky shale in the upper part) which indicate a transition from shallow to deep water environments. Based on the TOC concentrations (2.8-12.0 wt%) and Rock-Eval pyrolysis data (HI: 19.5±13.5; OI: 6.5±2.5; S1: 0.24±0.06 mg HC/g; S2: 2.8±1.1 mg HC/g; S3: 2.1±0.4 mg CO₂/g), kerogen in these outcrops is classified as type I-II, which may be found in lacustrine and marine settings. The δ¹³C values of total organic carbon varied from -30.7‰ to -27.7‰, a range indicative of lacustrine environments. We are presently examining a series of parameters derived from the distribution of normal and branched alkanes, steranes, and hopanes, which will offer important information about the sources of shale organic matter and the redox conditions of the depositional environments.