MICROBIAL MATS AS INDICATORS OF CONDENSATION IN BLACK SHALES - EXAMPLES FROM THE PENNSYLVANIAN CYCLOTHEMS OF THE MID-CONTINENT, USA

Organic-rich 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 from Iowa, Kansas, and Oklahoma. The shales contain microbial mats, mudstone clasts, Planolites burrows, and phosphate concretions; however, all of these components are not seen in every core.

In two of the cores, the number of microbial mats decreases regionally, from as much as several hundred per meter to being devoid of mats. The intervals that are devoid of microbial mats generally are instead enriched in either phosphate concretions, clay clasts, or both, and have more clay-rich matrix than is associated with microbial mats. Also, where microbial mats are abundant, the mat-rich facies is ‘sandwiched’ between mat-poor or mat-free facies with a gradational contact.

Both the microbial mats as well as the phosphate concretions reflect the overall condensed environment. However, the lack of siliciclastic fine-grained detritus with microbial mats indicates even stronger condensation than where phosphate concretions occur. The gradation of phosphate concretions with clays into microbial mat-rich settings in two of the cyclothems therefore seems to reflect increasing condensation within this basinal system. The basin itself, however, cannot have been very deep, inasmuch as it is likely that the microbes that formed the mats were, at least in part, oxygenic photosynthetic organisms and therefore needed light to flourish. In summary, 1) deposition of these black shales occurred under at least partly dysoxic conditions indicated by Planolites burrows, 2) condensation was strongest away from basin margins, and 3) the organic carbon content of these shales was likely enhanced by in situ microbial mat growth.