Abstract

Age and uranium content of detrital zircon in the Cretaceous to Eocene strata of the Powder River basin, Buffalo, Wyoming. Department of Geology, Union College, Schenectady, New York, June 2010.

This study addresses changes in sedimentary provenance revealed through the characteristics of detrital zircon during the unroofing of the Bighorn Mountains and the synchronous formation of the Powder River Basin (PRB) in northeastern Wyoming. Detrital zircons from three Mesozoic-Cenozoic sedimentary formations in the PRB and the Cambrian Flathead Sandstone from Alcova Reservoir were evaluated for U/Pb age, uranium content, and grain roundedness. Basement unroofing and uplift of the Bighorns resulted from the Laramide Orogeny that caused differential uplift of Precambrian basement rocks and their overlying strata to form the backbone of the Bighorn Mountains. Detrital zircon from the Cambrian Flathead Formation, Maastrichtian Lance Formation, Paleocene Fort Union Formation, and Eocene Wasatch Formation show up-section changes in U/Pb ages, uranium content, and roundedness. Zircons from the Lance and Fort Union formations are dominated by Cretaceous grains and a wide range of Precambrian grains with ages centered around 1000, 1400, and 1800 Ma. Zircons in the Eocene Wasatch Formation have a very different pattern: approximately 93% are Precambrian with nearly 60% in a cluster around 1800 Ma. Uranium content from the three formations show a decrease up section from the Lance Formation containing the highest mean U concentration (385 ppm), to the Fort Union Formation with (359 ppm), and the Wasatch Formation (267 ppm). These data suggest uplift and erosion of a source rock with abundant Precambrian grains. The high degree of rounding suggests these detrital zircons in the Wasatch Formation are predominantly recycled and not first cycle from the Archean basement. Furthermore, the rounded and recycled Wasatch grains may have significance in the formation of uranium in the PRB due to their susceptibility of dissolution causing roll-front deposits.