PROVENANCE OF THE PALEOCENE TURLOCK MEMBER USING DETRITAL GARNET AND APATITE, FORT UNION FORMATION, POWDER RIVER BASIN

The Laramide Belt of the western United States includes the Powder River Basin in Wyoming and Montana. One question that remains unsettled is the relative timing of uplift of the Laramide ranges that surround the basin. The Paleocene Tullock Member of the Fort Union Formation is characterized by sandy siltstone, limestone, and sandstone deposited in alluvial and fluvial systems. Previous methods of establishing provenance for the region included sandstone petrography and bulk heavy mineral analysis. These methods produced very different results, with separate studies suggesting evidence for the start of mountain building events ranging from the beginning of the Paleocene epoch to the end. Prior research also determined the sediments’ origins to be the Black Hills uplift east of the basin; other research disagrees with this and says the Black Hills had not yet been elevated by the time of deposition. My research aims to re-examine the provenance of the Tullock Member through analysis of detrital garnet and apatite found in sandstone. Following mineral separation, I will be using electron microprobe analysis of garnet for major elements and laser ablation inductively coupled plasma mass spectrometry of apatite for trace elements. I will be comparing my findings with data from modern rivers draining the surrounding Laramide uplifts. This work is important both for further understanding the timing of the Laramide orogeny, but also for the economical structures supported by the Powder River Basin, as the basin is the largest supplier of coal in the U.S. but is due to run out in the next few decades.