ANALYSIS OF ENIGMATIC PORPHYRY COBBLES IN THE CA. 90 MA FRONTIER FORMATION, EASTERN BIGHORN BASIN, WYOMING

Intermediate to felsic porphyry and granite cobbles from the ca. 90 Ma Frontier Formation in eastern Bighorn Basin (Wyoming) bear a striking resemblance to in-situ outcrops in the southeastern Beartooth Mountains (Montana). Two porphyry cobbles from the Frontier Formation were dated at 96 Ma, while in-situ porphyry outcrops in the Beartooth Mountains were similarly dated to 98–96 Ma (both by independent researchers). We are testing the hypothesis that the Beartooth Mountains area is the provenance for the Frontier Formation igneous cobbles with detailed field and hand-sample observations alongside petrographic, scanning electron microscope, geochronological, and trace element analyses. To date, observations of the specimens include in-depth inspection with a hand lens on both weathered and freshly cut faces. Frontier Formation cobbles are 7–9 cm across on average, rounded to subrounded, and are heavily weathered. The aphanitic groundmass in the exterior of several porphyry cobbles is dark grey compared to a pinkish red on the interior. The phenocrysts of the porphyries are predominantly plagioclase and range from 1-10 mm across. Several rhombohedral cavities on the exterior of the porphyries suggest preferential weathering of specific minerals. Granite cobbles are composed mainly of quartz and orthoclase, with very little mica present. There is a similarity in the pattern of fracturing between the porphyry and granite cobbles that is perpendicular to the long axis of the cobbles. Given the preliminary findings, the porphyry and granite cobbles appear similar to in-situ outcrops exposed in the Beartooth Mountains. Comparative analysis of specimens collected from the Beartooth Mountains will also be conducted; these samples will be collected from target locations and analyzed in the same manner to determine if the Beartooth Mountains is the provenance of the Frontier Formation cobbles. Comparison may yield discrepancies in the presence of certain minerals that can be explained by the weathered nature of Frontier Formation cobbles and the rhombohedral cavities on porphyry cobble exteriors. Determining the provenance of igneous cobbles in the Frontier Formation in Bighorn Basin may provide temporal constraints on the tectonic and magmatic development of the northern Laramide foreland region.