PROVENANCE OF GREATER GREEN RIVER BASIN SEDIMENTS, WYOMING: DETRITAL MINERAL MODES COMBINED WITH RUTILE AND ZIRCON CHRONOMETRY

Detrital provenance analysis is currently dominated by zircon U-Pb geochronology, a powerful tool, but biased towards providing source age information only for those rock types that can grow zircon. We present quantitative detrital mineral mode data from sandstones in the Greater Green River Basin using automated scanning electron microscope analyses (QEMSCAN^®) combined with zircon and rutile chronometry for a more comprehensive provenance characterization.

The Greater Green River Basin in Wyoming was part of the Cretaceous Western Interior Seaway, a broad foreland basin formed during the Sevier orogeny. The basin was dissected during the Laramide Orogeny, with paleocurrent measurements indicating a provenance change from south-derived axially transported sediments within the Western Interior Seaway to north-derived sediments eroded off the Uinta Uplift (Pyles and Slatt, 2007). We investigate Campanian to Maastrichtian sandstones from near the Rawlins uplift, WY, for further evidence for this provenance change.

The modes of detrital minerals obtained by QEMSCAN^® show shifts occurring above and below the lowstand system tracts of the Dad Member of the Lewis Shale at c. 70 Ma. In particular, barite, pyrite, ilmenite, illite/muscovite, chlorite and the TiO₂ polymorphs show significant trends that may be attributed to local or far-field source changes.

Preliminary zircon U-Pb data show three significant age populations but no significant change in age provenance through the section investigated. A ~1600-1800 Ma Paleoproterozoic group is interpreted to derive from the Yavapai-Mazatzal terrane (YMT) suggesting with prominent exposures in the Sevier orogen. A ~400-700 Ma population is proposed to be derived from the Appalachian orogen in the southeast, and the pre- to syn-depositional age population of ~70-120 Ma is interpreted as first cycle detrital input from magmatic rocks of the Sevier orogen.

Most U-Pb rutile data are very discordant or dominated by common Pb, the latter interpreted as a signature of mafic source rocks. Intergrowths and alterations also complicate interpretation. Near concordant grains show major age populations mimicking the zircon data at ~1700 Ma (sourced from YMT), ~500-350 Ma (Appalachian provenance) and 85-120 Ma (Sevier exhumation and cooling).