Geological Sciences

摘要

Determining the age of the Michigan Formation is difficult due to the general lack of biostratigraphic indicators. The literature shows that historically the formation has been assigned a chronostratigraphic position of Osagean- Meramecian (equivalent to Salem and St. Louis Limestones of the Illinois Basin) based mainly on the fauna of the conformably overlying Bayport Limestone. However, recent studies using pollen and spores suggest the age of the Michigan and Bayport Formations is Chesterian (uppermost Mississippian). Gypsum samples (Kent and Iosco Counties) were analyzed for strontium ( ~(87)Sr/~(86)Sr; range 0.70792 to 0.70806; mean 0.70799±0.00006; n = 8) and sulfur (δ~(34)S; range +15.2 to +17.0; mean +16.3±0.4; n = 40) isotopes. Consistent isotopic values, both stratigraphically and laterally, indicate a marine origin for the gypsum, and that the isotopic signature was not significantly affected by nonmarine waters or diagenesis. As a result, the ~(87)Sr/~(86)Sr values can be compared to the global secular curve for Mississippian marine precipitates to help determine the formation's age. These data indicate the formation is either Kinderhookian or Chesterian. Because none of the data collected by prior researchers, including the recent pollen and spore data, suggest a Kinderhookian age, we suggest the isotopic data support a Chesterian age.