Late Devonian Earliest Carboniferous Environments Of Deposition And Hydrocarbon Potential

Author : Man Lu
Publisher :
ISBN 13 :
Total Pages : 86 pages
Book Rating : 4.:/5 (922 download)

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Download or read book Sedimentological and Geochemical Records of Depositional Environments of the Late Devonian Chattanooga Shale written by Man Lu and published by . This book was released on 2015 with total page 86 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Late Devonian is characterized by substantial changes in both land and marine biodiversity. The organic matter-rich Chattanooga Shale was deposited in shelfal waters, thereby serving as an invaluable archive recording changes in land and marine biota as well as terrestrial-marine biogeochemical linkages in the Late Devonian. In the present study, we identified carbon sources and reconstructed depositional environments of the Chattanooga Shale using two outcrops in northeastern Alabama, combining sedimentological and geochemical approaches. The two outcrops share similar sedimentological features (i.e., thinly laminated, fissile, grayish shale strata in the lower part of the outcrop and nearly homogeneous, black, blocky shale in the upper part). The lower part may be assigned to the low to middle units of the Gassaway Member and contains the top of the Dowelltown Member deposited around the Frasnian-Famennian boundary. The upper part is likely equivalent to the upper Gassaway Member. Rock-Eval parameters and the carbon number distribution of normal alkanes show that the organic matter in the Chattanooga Shale is in thermogenic gas window and contains Type II to III kerogen. Low Pristane/Phytane values show that the depositional environment was reducing with interruptions of oxic periods. In the upper part, the values of (Pristane/nC17) / (Phytane/nC18) increase, showing that water may become more oxic. Furthermore, terrestrial organic matter input increased toward the top of the outcrops, as evidenced by increased proportions of long-chain n-alkanes, higher terrigenous/aquatic ratios, lower nC17/nC27 values, and elevated concentration of polycyclic aromatic hydrocarbons derived from terrestrial plants. These data indicate that terrestrial plants became an increasingly important carbon source during the deposition. The presence of pyrosynthetic polycyclic aromatic hydrocarbon compounds can be attributed to prevalent forest fire, in accompanying with the diversification of early land plants in the Eastern Laurasia. These data demonstrate that enhanced inputs of terrestrial organic matter to shelfal waters may have played a vital role in the formation of black shales in the Late Devonian.