COMBINED BIOTIC AND SEDIMENTOLOGIC EVIDENCE FOR MILLENNIAL-SCALE (6TH-ORDER) SEA LEVEL FLUCTUATIONS WITHIN THE PORTERSVILLE SHALE (LATE PENNSYLVANIAN), CONEMAUGH GROUP, SOUTHEASTERN OHIO

The Portersville Shale (Missourian/Kasimovian) is a thin (<1.1 m) unit of fossiliferous, dark gray shale previously interpreted to be the transgressive record of a 4th order glacio-eustatic rise in relative sea level. Earlier macrofaunal analyses recorded changes in community structure through the Portersville shale that were interpreted to represent at least one fluctuation in sea level during deposition. To further test the hypothesis of associated biotic and relative sea level changes and determine whether parasequences are resolvable within the Portersville Shale, bulk sediment samples were collected at 10-cm intervals from three locations. Samples were disaggregated, and the weight percent sand fraction, amount of glaucony mineral grains present, and the relative abundance of each prominent microfossil taxon recovered were plotted versus stratigraphic depth. Microfossils included pelagic forms, such as conodonts, and benthic organisms, including diminutive brachiopods, bryozoans, crinoids, and ostracodes. Taxa were grouped into communities using cluster analysis, and environmental gradients were examined using detrended correspondence analysis.

The sedimentary and biotic data sets show congruent variations in grain size and faunal communities within the three sections which indicate the presence of 5th order fluctuations in water depth. In addition, higher order variations in grain size and changes in faunal communities occur that are not correlative between all the sections. We interpret these 6th order fluctuations to be the result of changes in local environment and sediment input related to channel avulsion of a nearby delta. Since the complete 4th order transgressive event, which likely lasted on the order of 10 kyr, is not present within the study area, we interpret the 6th order fluctuations as millennial-scale shifts in sediment source and concomitant changes in environmental quality resulting from variation in local sedimentation rates.