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Depositional Environments of Organic-Rich Calcareous Shale in the Western Anticosti Basin: the Upper Ordovician Macasty Formation, Quebec, Canada

Kulkarni, Atul R.; Hattori, Keiko; Desrochers, Andre

The Macasty Formation, equivalent to the Utica shale in the US and southern Quebec, consists of organic-rich black calcareous shale deposited on top of the shallow water Mingan carbonate platform in a foreland basin during the Taconic orogeny. The sharp boundary between the two units suggests an abrupt change in the basin configuration. The Macasty shale is composed of very fine-grained quartz, feldspar and clay minerals cemented by calcite (up to 45%). We examined the entire section of the Macasty shale (~ 80 m in thickness) in drill hole LGPL on Anticosti Island. The compositions including the mineralogy and major and minor element abundance did not show any significant changes during the deposition of the shale. The siliciclastic components were primarily supplied from granitic rocks of the Grenville province after extensive weathering. These components were deposited in anoxic environments that allowed the accumulation of redox-sensitive elements and organic matter (up to 8 % TOC), and the reduction of marine sulfate to sulfide (0.7 to 1.6 wt% S). The organic matter has δ13C values ranging from -29 to -23 ‰, which are similar to those of Ordovician sedimentary rocks in other areas. Sulphur isotope compositions for disseminated fine-grained pyrite range from -11 to +1 ‰, which suggest the deposition in a basin with a slightly restricted supply of open ocean water. The sulfide concentrated chalcophile elements, such as As (up to 13 ppm), Cu (up to 61 ppm), Mo (up to 179 ppm) and Pb (up to 39 ppm). The organic matter was responsible for the reduction of soluble U6+ to U4+ to be fixed in the sediments, leading to high U/Th ratios (up to 3.5) in organic-rich shale. The abundant calcite in the shale shows δ13C values from -2.8 -to +1.0 ‰ and δ18OSMOW from -8.6 to -2 ‰. Combined with the finely crystalline texture of calcite, it is interpreted to have been precipitated at temperatures from 40 to 55 C after the deposition of the siliciclastic material and organics, but before any significant compaction. The introduction of this slightly oxidizing diagenetic solution was accompanied by the crystallization of coarse-grained pyrite with variably high δ34S values from - 3 to +28 ‰.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013