HEAVY MINERAL ANALYSIS OF LATE EOCENE SANDSTONE FROM THE MEDICINE POLE HILLS, SOUTHWESTERN NORTH DAKOTA

Heavy mineral analysis of latest Eocene sandstone exposed at the Medicine Pole Hills (MPH) in southwestern North Dakota was used to assess its possible correlation with the Chalky Buttes Member (CBM) sandstone of the Chadron Formation, which is exposed at numerous buttes in the Williston Basin. Heavy minerals (0.25-0.30 mm grains) were identified using optical microscopy and microanalysis with an SEM-EDS system. Analyses of nine samples from seven units exposed in a trench cut through 5.7 meters of MPH sandstone show considerable variability in heavy mineral abundances. Total heavy minerals varied from 0.4 to 8.5%. Among heavy minerals, opaque minerals varied from 0.8 to 7.9%. Non-opaque heavy minerals were dominated by calcic amphibole (19-47%) and epidote (13-48%); four samples also had abundant diopside (19-51%), and one had abundant biotite (36%). Amphibole compositions ranged from actinolite to various types of hornblende. Epidote grains varied from clinozoisite (Cz) to epidote (Ep), with Cz₃₅Ep₆₅-Cz₀Ep₁₀₀ being most abundant. Relatively constant amphibole/epidote ratios suggest a common Precambrian metamorphic-plutonic (PCmp) source. Trace element analysis of diopside separates suggests an igneous or meta-igneous source. The Ti and Al concentrations in biotite grains support an igneous source. The percentage of heavy minerals and the abundances of diopside relative to amphiboles and epidote generally increase with increasing overall grain size of the samples, suggesting the diopside abundances were in part controlled by grain size. Variations in source contributions and/or post-depositional dissolution (indicated by sawtooth terminations on diopside) may have also been important. Overall, heavy mineral data suggest the MPH sandstone received sediment from three distinct sources: PCmp source(s), a diopside-bearing mafic Tertiary volcanic (Tv) or PCmp source, and a biotite-bearing Tv source. Despite the variability among the seven units, cluster analysis shows that the MPH sandstone is distinct from heavy mineral assemblages of CBM sandstone samples, which are generally dominated by PCmp and/or ultrastable grains from recycled sediment and/or PCmp sources. Based on the apparent source differences, the MPH sandstone should not be correlated with CBM sandstones as has been suggested.