STABLE ISOTOPE ANALYSIS OF THE BIGHORN BASIN DOLOMITES AND INTERPRETATION OF DOLOMITIZATION

The Bighorn Basin is a structural basin filled with over 6000 meters of sedimentary deposits that is Cambrian to Miocene in age. The Bighorn Basin contains Katian stage dolomites that were analyzed for δ¹⁸O and δ¹³C values for lateral correlation and dolomitization interpretation. We conducted a stable isotope analysis using an IRMS on a total of 34 dolomite samples from three different core depth intervals of the Bighorn Basin Dolomites. Two core depths taken at Riley Ridge, Wyoming at a depth between 5190-5200 and 4590-4600 meters. As well as a core from an outcrop in Wyoming, at a depth between 1-7 meters, a surface sample of the Madison limestone formation, and at the Darby Unconformity. Dolomite samples are composed of dolo-wackestone/packstone texture with type 1 and/or type 4 dolomite texture matrix and porosity ranging between 8 to 22%. Stylolites, echinoderm fossils, vugs, bioturbation, late diagenetic calcite, and microfractures filled with dolomite and/or calcite are common throughout the samples. Results of δ¹⁸O, and δ¹³C dolomite values range from -6.472 to -3.277‰ for δ¹⁸O and -2.163 to 3.688‰ for δ¹³C. The Bighorn Dolomite δ¹⁸O and δ¹³C values form into separate distinct clusters associated with depth interval. Riley Ridge interval forms a unique cluster, Wyoming outcrop interval forms a positive linear relationship, and surface samples yield no distinctive pattern. Interpretation of dolomitization from δ¹⁸O and δ¹³C results suggest a two-stage event: mixing zone dolomitization and burial dolomitization. Results of dolomite δ¹⁸O values, the presence of microcrystalline dolomite spar crystals, and the linear relationship between δ¹⁸O and δ¹³C values within Riley Ridge are consistent with the mixing zone dolomitization model. Possible later burial dolomitization occurred as evidenced by the presence of microfractures, stylolites, vugs, replacement dolomite, and late diagenetic calcite in the dolomite samples. Future work should include testing the timing of dolomitization, XRD analysis for clay composition, and trace element analysis of Sr, Fe, Mn, and Na on the Bighorn dolomite samples to provide information on the nature of the dolomitizing fluids and supply a precise interpretation of dolomitization.