PETROLOGY OF VOLCANOCLASTIC ROCKS IN THE WIGGINS FORMATION, SOUTHERN ABSAROKA RANGE, WYOMING

The purpose of this undergraduate research project was to characterize volcanoclastic conglomerate layers within the southeast part of the Absaroka Mountains. The exposure described is within the Wiggins Formation that is part of the Thorofare Creek Group and is cropping out along the Wood River. The Wiggins Formation, consisting of volcaniclastic rocks and subordinate lavas of biotite-hornblende andesite, has been estimated to have a thickness of about 3,000 ft. According to previous studies, it has been recognized as the youngest formation in the southern part of the Absaroka field and dated in the Eocene (44-46 million years ago). The samples for this project were taken from a location approximately 30 miles southwest of Meeteetse, Wyoming along Road 4DT. This location has latitude of 43°55.942’ N, longitude of 109°07.847’ W, and an elevation of 7426 ft at the lower exposure along the road. Additional samples were collected from exposures upslope to the north from the road. The conglomerates are dominated by well rounded pebble (4-64mm) and cobble (64-256mm) clasts in thick beds of light-olive-gray mudflow. Based on initial thin section analyses, the clasts are predominantly igneous rocks with aphanitic to porphyritic textures. Their compositions are mafic to intermediate with plagioclase being the dominant mineral in both the phenocrysts and groundmass. Other minerals commonly occurring in the clasts are clinopyroxene and othropyroxene as well as hornblende and biotite. Some alteration is evident by the presence of chlorite, zeolite and iron oxide minerals.