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Evaporitic conditions in the Michigan Basin commenced in Late Ordovician time and continued into the Early Devonian. Five major evaporite (anhydrite/halite) cycles in the Silurian Salina Group (termed “A-1,” “A-2,” “B,” “D,” and “F”) can be recognized. They are further subdivided into two or more cycles of evaporite deposition by intercalations of less soluble members. Only the lowermost Salina evaporite produced a sylvinite deposit. The water surface was enlarged in subsequent evaporite cycles, which did not go beyond halite saturation, suggesting an enlargement of the water supply. Additional seawater came from the Kokomo Sea in Indiana; continental waters likely also entered at times from the Moose River Basin. The Chatham sag eventually fed Michigan Basin brines into Ohio and Pennsylvania. A progressive overstepping of one evaporite unit over the one below indicates either subsidence affecting a wider area of rising sea level, the latter suggested by the inundation of northern Ohio beginning with the “B” unit of salt deposition. In no case does brine depth appear to have exceeded some tens of meters. The present morphology of reefs is dictated as much by compaction of surrounding micrites as by renewed growth on older reef mounds where rates of subsidence remained moderate. The amount of clay influx in the upper part of the Salina suggests that humid periods became more frequent at the expense of the duration of dryer periods. Post-Salina rocks initially contain numerous short-lived evaporite cycles, and then grade to open-marine sediments, deposited before the Michigan Basin became emergent land in the late Paleozoic

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