Abstract: Sedimentology and Diagenesis of Lower Salina Group (Upper Silurian) Evaporites in Michigan Basin

R. D. Nurmi, G. M. Friedman

Sedimentologic analysis of the lower Salina Group and the distribution of its evaporite and carbonate lithofacies suggest that the Michigan basin periodically was desiccated during the early Cayugan. The deposition of the Salina Group began with sea-level lowering at the end of the Niagaran. The pinnacles and marginal platform were subaerially exposed during the first major Cayugan sea-level lowering as evidenced by erosional features, weathering surfaces (siliceous crusts, clay seams), and diagenetic features of vadose origin directly below the Salina Group. In the basin-center area, Cayugan sedimentation began with the subaqueous interstitial precipitation of lenticular gypsum crystals. Sulfate reducing bacteria may have restricted or prevented the accumulation of gypsu at the sediment water interface. The lenticular gypsum crystals have been replaced by finely crystalline anhydrite. Anhydrite laminae formed by coalescence in zones where gypsum crystals were abundant. This interlaminated limestone and anhydrite, the basal lithofacies of the Salina Group, is transitional with the Middle Silurian limestones and in the basin-center area is interbedded with the A-1 salt. On the northwestern interpinnacle area, the carbonates and evaporites coeval with the limestone-anhydrite laminites are shallower water deposits that have undergone extensive halite replacement. These rocks include nonplanar and planar stromatolites, flat-pebble halite conglomerates, and vertically orientated, prismatic gypsum crystals replaced by halite. These crystal forms are suggestive of precipitation at the sediment-water interface. Marine-playa stages, or periodic desiccations, are marked by sylvanite and calcium-borate precipitation, erosional solution surfaces, gypsum and halite sands, and halite crystals having chevron-shaped zones of brine inclusions.

After the deposition of the basinal, subtidal A-1 carbonate, the Cayugan sea again contracted to the basin-center area. Evidence for this sea-level lowering is found at the top of the A-1 carbonate basinward of the pinnacles and includes erosional surfaces, channels, LLH-type stromatolites, nodular anhydrite, and disrupted crusts. Within the carbonates above and below the A-1 salt in the center of the basin, sedimentologic features, such as ooids, peloids, planar and nonplanar stromatolites, and nodular anhydrite, indicate shallow-water deposition. Extensive replacement by halite has masked or obliterated many sedimentologic features but has preserved algal growth forms within stromatolitic zones.

AAPG Search and Discovery Article #90978©1975 GCAGS-GC Section SEPM Annual Meeting, Jackson, Mississippi