Evaporite facies of the Michigan Basin

ABSTRACT The Michigan Basin is one of the world’s important sedimentary basins that contains significant quantities of evaporites. Here, evaporites are found in deposits of Ordovician through Mississippian age rocks; however, most of the thick evaporite accumulations occur in Silurian and Devonian intervals. Halite is most significant in the Silurian Salina Group, with a maximum aggregate thickness of halite exceeding 650 m (2150 ft). During the earliest evaporite deposition in the Salina Group (A-1 Evaporite), sylvite was widely deposited in the north-central portion of the basin within the upper 91.4 m (300 ft) of the formation. Devonian salt is also present in the north-central portion of the basin in the Horner Member of the Lucas Formation, where maximum aggregate net thickness of halite reaches 125 m (410 ft). Recrystallization of much of the halite obscures the primary depositional crystal geometry; however, some well-preserved beds do show crystal growth that is interpreted as bottom-growth chevrons, which likely suggest shallow-water deposition. Throughout the rest of the Michigan Basin, in both space and time, the evaporite phase deposited is CaSO4. In the shallowest portions of the Mississippian Michigan Formation, the sulfate mineral phase is gypsum; everywhere else in the basin, all the evaporitic sulfate deposits are anhydrite. Although the dehydration of the gypsum to anhydrite has slightly altered the original depositional morphology, some primary growth geometry is still evident. Subtidal and sabkha morphologies can be documented in all the anhydrite/gypsum deposits of the Michigan Basin. Based on historic production, evaporite minerals have added an estimated $15.5 billion (in 2013 dollars) to the industrial mineral economy of Michigan since the first commercial development in the 1860s.