TOC, C/N, δ13C, AND δ15N OF SHALES FROM THE MISSISSIPPIAN MICHIGAN FORMATION, WESTERN MICHIGAN

The Mississippian Michigan Formation, exposed in an abandoned gypsum mine in Wyoming, Michigan, is composed of very thick beds of gypsum interbedded with much thinner beds of siliciclastic-rich dolomite and shale. Samples of shale (15 total) from three cyclic units (unit 3, oldest; unit 1, youngest) exposed in the mine were collected. Each of the three beds of shale has a distinct appearance, especially that in unit 1, which is much thicker (mean, 1.0m), more brown in color, and more silt-rich than the gray shales in units 2 (mean thickness, 0.3m) and 3. Proxies [total organic carbon (TOC), carbon/total nitrogen (C/N) ratios, organic δ¹³C, and total δ¹⁵N] were used to help determine the depositional environment of the shales. To remove inorganic carbon for TOC and organic δ¹³C analyses, samples were treated with HCL. As others have also observed, acid-treatment consistently decreased the percent nitrogen in the samples. However, we did not see a consistent change in δ¹⁵N for acid-treated samples. Nevertheless, results of analyses for only untreated samples were used for all nitrogen proxies. Low percentages of TOC (0.20 - 0.51%) in our samples indicate low productivity of organic matter. C/N (3.98 – 8.50) and δ¹³C (-22.77 to -25.39‰ PDB) imply carbon input from mixed marine-derived water and freshwater sources, which supports a margined marine depositional environment as suggested by previous studies. On C/N versus δ¹³C plots, the data for units 1, 2, and 3 fall in three different fields. Unit 1 is most nearly characteristic of a freshwater source, and unit 2 is most typical of a marine source, which is consistent with field observations. δ¹⁵N (1.78 – 3.77‰) remains enigmatic; however, it may also indicate a mix of sources. The lack of terrestrial plant indicators suggests an arid environment with little vegetation. An arid environment would promote the evaporation of seawater, producing gypsum, the most common rock type in the mine. Freshwater influx into the brine would halt gypsum precipitation and allow clays to be deposited.