HEAVY MINERAL ANALYSIS OF THE BASAL WAPITI FORMATION IN THE SOUTH FORK SHOSHONE RIVER VALLEY, WYOMING

More than 1500 meters of Eocene rocks of the Absaroka Volcanic Supergroup are exposed in the upper South Fork Shoshone River Valley. As much as 300 meters of well-stratified, light-colored distal facies volcaniclastic sandstone, conglomerate, mudstone, and tuff occurs at the base of the volcanic succession. These rocks unconformably overlie the syn-Laramide Willwood Formation, and are conformably overlain by the proximal and vent facies volcanic rocks. Some of the conglomeratic units are quartzite bearing. Our goal is to characterize the depositional environment of these conglomeratic sandstones, including the heavy mineral provenance and detrital zircon ages. This work will provide a better understanding of the paleogeography of the region during the early episodes of Absaroka Volcanism and at the onset of the Heart Mountain faulting.

The samples of conglomeratic sandstone were collected from the basal volcanic succession. One was from Deer Creek, the second from the Ishawooa Hills and the third from the mouth of Boulder Creek. Each of the samples were rich in garnet, magnetite, and ilmentite, with lesser. Zircon occurred in small quantities in each sample. The preponderance of garnet and the presence of quartzite granules indicate a provenance rich in metasedimentary rocks. The most likely source area for the basal Wapiti Formation is from the now-eroded ancestral Targhee uplift to the northwest. Alternatively, the lower Wapiti Formation can be recycled from older Pinyon or Harebell Formations, each of which were derived from the ancestral Targhee uplift.