HEAVY MINERAL ANALYSIS AND CORRELATION OF LATE EOCENE SANDSTONES OF THE WILLISTON BASIN

Late Eocene sandstone exposed at the Medicine Pole Hills (MPH) in southwestern North Dakota has been correlated with the Chalky Buttes Member (CBM) sandstone of the Chadron Formation, which is exposed at numerous buttes in the Williston Basin. The MPH sandstone was sampled from an excavation pit. The CBM sandstone samples were taken from Rattlesnake Butte (RSB), Square Butte (SQB), and Little Badlands (LB). Heavy minerals separated from fine to medium-grained sand were identified using optical microscopy and microanalysis with a scanning electron microscope-energy dispersive spectrometer system. The abundance of heavy minerals in the separated grain size(s) varied considerably (8.5% in MPH, < 1% in CBM samples) as did the percentages of heavy minerals that were opaque (0.4% in MPH, 7-30% in CBM samples). Non-opaque heavy mineral abundances were very different in the four samples. The MPH sample was dominated by diopside and hornblende; RSB by epidote, garnet, and biotite; SQB by rutile, epidote, staurolite, and apatite; and LB by rutile, staurolite, epidote, and aluminosilicates. Published heavy mineral analyses, mostly from other Chadron Formation (CF) localities, were generally dominated by zircon, staurolite, aluminosilicates, epidote, and tourmaline. The distinct heavy mineral assemblages are interpreted to represent rather different sources. MPH was dominated by a Tertiary volcanic (Tv) source, with some Precambrian metamorphic-plutonic (PCmp) source input. RSB was dominated by a PCmp source, with some input from Tv. SQB and LB were dominated by ultrastable grains from recycled sediment (RS) and/or a PCmp source, and other grains from a PCmp source. The MPH and RSB samples had similar PCmp sources, but very different Tv sources. The SQB and LB sources (RS and/or PCmp) were very different from MPH and RSB sources. Most published CF analyses were similar to the SQB and LB samples in that they were dominated by ultrastable grains, but differed in that zircon > tourmaline dominated rather than rutile. The distinct heavy mineral assemblage in the MPH sandstone suggests it should not be correlated with the CBM sandstone. Additionally, results from CBM samples (and CF data) suggest the CBM may represent a complex depositional setting that consisted of multiple depositional systems receiving sediment from multiple, changing sources over a significant interval of time.