Paleoceanography of Silurian Seaways in the Midwestern Basins and Arches Region

Abstract

Prima facie isotopic, salinity, and other geochemical data of the kind normally cited as evidence for a set of stated oceanographic parameters scarcely exist for the mostly highly dolomitized rocks of the Silurian System in the midwestern basins and arches region. This lack requires that Silurian paleoceanographic reconstructions be highly generalized as projections from a recently published integrative stratigraphy and from prior proposals of regional cyclicity in sedimentation, bathymetry, and salinity. We propose that midwestern Silurian sea level fluctuated only in modest amounts from late Early to middle Late Silurian time. Further, bathymetric differentiation among some of the basins and arches was modest. These bathymetric generalities for three selected time slices are representative of a complex history of Silurian cyclicity: mid‐Wenlockian time, 50 to more than 75 m in the basins, about 30 m on the intervening platforms, and less than 15 m in parts of major carbonate bank and reef areas; latest Wenlockian, about 100 m in the Illinois Basin, about 20 m to supratidal on the carbonate shelves, and 10 m and, at times, less in the Appalachian and Michigan evaporite basins; and mid‐Ludlovian, only a modest 10–15 m deeper than that of latest Wenlockian time. The mid‐Ludlovian deepening was sufficient, however, to restore circulation and salinity to normal on the interbasin platforms and to near normal in the evaporite basins. Near the evaporite basins, net current flow was toward and into the basins through belts of increasing evaporation. Such currents became increasingly enriched in Mg ions and effected early dolomitization of Silurian carbonate, including reef sediments. Biotas ranged from normal marine, high‐diversity communities, which were most widespread during mid‐Wenlockian time, to variably restricted, algae‐dominated communities and were even nonexistent in the evaporite basins during latest Wenlockian time. Generally, shallow depth may have been more important in effecting all these gradients than were latitudinally related climatic differences. As an explanation for the pattern of cyclicity proposed here, global controls of sea level are invoked, but these are not well understood, and localized structural and biologic controls within the North American craton accounted for some details of the paleoceanographic gradients.