PALEOCEANOGRAPHY OF THE DECORAH FORMATION (ORDOVICIAN, MOHAWKIAN), NORTHERN MISSISSIPPI VALLEY

Recent stratigraphic studies of the Decorah Formation, focusing on its sedimentology, brachiopod zonation, and bentonite geochemistry and correlation, have led to a new depositional interpretation for the unit. The Decorah was deposited in a tectonic downwarp, the Hollandale Embayment, which was bounded on the northwest by the Transcontinental Arch and on the east by the Wisconsin Arch. A detailed chronostratigraphic cross-section has been developed, along a transect extending northwest from the Wisconsin Arch into the Hollandale Embayment. The Decorah consists of two major lithofacies, a lower shale and an upper carbonate, which form reciprocal wedges, the shale thinning southeastward and the carbonate thinning northwestward. The shale facies was deposited in sporadically dysoxic bottom water, as shown by patterns of bioturbation, faunal diversity, and taphonomy. We propose that the Transcontinental Arch was emergent and supplied clastic sediment, forming a shale facies that is thickest near the source area. Freshwater runoff from the land helped to stratify the water column, leading to dysoxia at depth. The overlying carbonate facies was deposited in more oxic conditions, as shown by abundant bioturbation, diverse benthic faunas, and fossil grainstone beds. The carbonate strata are thinner and less grainy northwestward (downramp) into the Hollandale basin. The smaller amount of clastic influx indicates that the Transcontinental Arch was more submerged, leading to less runoff but a deeper marine water column. However, it is the basal carbonate facies, not the underlying dysoxic shales, that show high TOC (max. > 50%) and a positive carbon isotope excursion of 3 per mil. These facts argue for an interval of high phytoplankton productivity in the epeiric sea. We suggest that the sealevel rise produced a thicker photic zone, with a chlorophyll maximum capable of tapping nutrient-rich water at depth. The Transcontinental Arch continued to supply nutrients until it was inundated in post-Decorah time. The influence of cratonic topography as interpreted here supports the “aquafacies” concept of semi-isolation within regions of the North American epeiric sea during the Ordovician.