SEDIMENTOLOGICAL AND GEOCHEMICAL CHARACTERIZATIONS REVEAL A MAJOR SHIFT IN DEPOSITIONAL ENVIRONMENTS OF THE DEVONIAN CHATTANOOGA SHALE IN NORTHEASTERN ALABAMA

The Devonian Chattanooga Shale contains primarily hydrocarbon-rich, black shales spread widely in southeastern North America. Although Devonian strata along the Atlantic Seaboard are thought to be associated with marine sediments, the depositional environments of the Chattanooga Shale remain controversial. While some researchers have proposed that these black shales were deposited in deep offshore environments, some recent studies show that the shales might be formed in shallower, more near-shore waters. In the present study, two Chattanooga Shale outcrops near Fort Payne and Big Ridge in northeastern Alabama were analyzed to reconstruct the depositional environments, combining sedimentological and geochemical approaches. The two outcrops share similar sedimentological features, i.e., thinly laminated fissile grayish shale strata in the lower part and nearly homogeneous black blocky shale in the upper part, which indicates a transition of depositional environments. Our 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: 0.4±0.21 mg CO₂/g) suggest a Type II to Type III kerogen of mixed aquatic and terrestrial origins. The δ¹³C values of TOC varied from -30.7‰ to -27.7‰, exhibiting negative excursions in the upper part of the formation that may be attributed to greater terrigenous input and/or increased degrees of organic matter. The predominance of short-chain n-alkanes over long-chain n-alkanes and low values of terrigenous/aquatic ratio (TAR) indicates that the organic matter was dominated by algal materials. Low values of Pr/Ph (1.7±0.1) and the presence of hydrocarbon potential (0.5±0.2) indicate that the shales were deposited in anoxic environments, while fluctuations of these values suggest the presence of episodic oxygenation. TAR and Pr/Ph showed a similar trend—the values increased rapidly from the lower to upper part, coinciding with a rapid rise in the values of quartz to clay mineral ratios. This shift suggests that the depositional environment became shallower and less reducing from the lower part to the upper part of the formation.