Sedimentology and Depositional Environments of Basin-Center Evaporites, Lower Salina Group (Upper Silurian), Michigan Basin1

Abstract

Sedimentologic analysis of the lower Salina Group and distribution of the evaporite and carbonate lithofacies suggest that the Michigan basin was desiccated periodically. Deposition of the Salina Group began with lowering of sea-level at the end of the Niagaran Epoch. The marginal platform and pinnacle reefs were exposed subaerially during this first major Cayugan lowering of sea level, as shown by erosional features, weathering surfaces (siliceous crusts, clay seams), and diagenetic features of vadose origin in rocks directly below the Saline Group. In the basin-center area, Cayugan sedimentation began with subaqueous interstitial precipitation of lenticular gypsum crystals. Sedimentologic evidence within the basic-center evaporites suggests that deposition of the first Cayugan evaporite unit began in the deepest water in the Michigan basin; the basin became progressively shallower by desiccation and infilling. The interpretation is supported by depositional analyses of Cayugan carbonate units.