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Microbial Mats as Indicators of Condensation in Black Shales – Examples From the Pennsylvanian Cyclothems of the Mid-Continent U.S.


Black shales in Pennsylvanian cyclothems occur in many of the eustatically-controlled cyclic successions throughout the North American mid-continent. This study presents the sedimentology of three representative black shales from three stratigraphic levels in five locations, the lower Desoinesian Excello Shale from south-central Iowa and northeast Kansas, the lower Missourian Hushpuckney Shale from south-eastern Kansas, and the lower Virgillian Heebner Shale from east-central Kansas and north-central Oklahoma. The studied black shales contain microbial mats, mudstone clasts, Planolites burrows, and phosphate concretions; however, not every core shows all of these components. In the Excello, the amount of microbial mats decreases from south-central Iowa to northeast Kansas, and for the Heebner, a significant decrease is observed from east-central Kansas to north-central Oklahoma. The cores that are devoid of microbial mats generally contain high amounts of either phosphate concretions and/or clay clasts; both of these facies show more clay-rich matrix than in sediments with microbial mats. Also, each core with abundant microbial mats grades vertically into facies with minor or no microbial mat occurrence towards the upper and lower boundaries of the black shales. Both the microbial mats as well as the phosphate concretions reflect deposition in a condensed environment. However, the lack of siliciclastic fine-grained input suggests that the microbial mats indicate even stronger condensation than the phosphate concretions. The gradation of clay-bearing sediments with phosphate concretions to clay-poor sediments with microbial mats in two cyclothems therefore seems to reflect increasing condensation within this basinal system. The basin itself, however, cannot have been very deep. Not only does the close relationship of in part shallow-marine carbonates with the black shales indicate an overall shallow basin, but also the presence of microbial mats that need light to flourish reflects clear water and moderate water depth. Further, abundant Planolites burrows with the microbial mats corroborates oxic to dysoxic conditions. Therefore, condensation was strongest away from basin margins; however, the high organic content of these shales was likely a consequence of microbial mat growth in situ.