COMPARISON OF HEAVY MINERAL SUITES FROM LARAMIDE BASEMENT ROCKS AND PALEOCENE STRATA IN THE POWDER RIVER BASIN, EASTERN WYOMING

Lithologies present in the source area of a sedimentary basin exert a fundamental first-order control on the sediment supply rate and types of depositional environments that will develop in the basin. Paleocene-Early Eocene exposure of Precambrian basement rocks is inferred for Laramide uplifts in the western U.S. based on clast compositions in proximal alluvial strata and extensive lakes that developed in response to exposure of erosion-resistant lithologies within the uplift cores. Less studied, more distal fluvial systems may contain an integrated record of basement exposure which will help elucidate sediment dispersal patterns during this critical time. We sampled sand from modern rivers draining Precambrian rocks in the Beartooth, Bighorn, Medicine Bow, and Laramie ranges, as well as Paleocene-Eocene strata in the Powder River basin, to explore for connections among heavy mineral suites present in the basin strata and those from discrete basement blocks. In the Powder River basin, the Paleocene Fort Union Formation is subdivided into the Tullock, Lebo, and Tongue River Members. Limited early work supports early Paleocene exposure of basement rocks that supplied a moderate amount of heavy mineral grains to the Tullock Member in the northern part of the basin. To the south, abundant heavy mineral grains and igneous fragments suggest more extensive exposure of basement rocks in sediment source areas. However, early workers were unable to link specific basement blocks to basin heavy mineral suites. Using rutile, garnet and apatite trace elements, recent workers developed discrimination diagrams to make more direct connections between source and sink and illuminate the thermal history of sediment source regions. We will fingerprint each sampled basement uplifts’ rutile, garnet, and apatite trace element profiles from modern river sand and compare the results with trace element compositions of the same minerals present in the basin strata. Assessment of pilot datasets will allow us to develop a more nuanced provenance interpretation for the Paleocene-Eocene strata in the Powder River basin, and if the basement block signature is significant, pinpoint specific spatial and temporal sources of sediment, potentially clarifying the pattern and timing of basement daylighting in the northern Laramide province.