Detrital U-Pb geochronology provenance analyses: case studies in the Greater Green River Basin, Wyoming, and the Book Cliffs, Utah

Abstract

Heavy mineral modal abundances and U-Pb geochronology are used to determine sediment provenance, provenance changes through time, and timing of crustal exhumation. Optimal preparation of datable detrital minerals (zircon, rutile, monazite, etc.) for provenance research using U-Pb geochronology has been a subject of debate and concern. Potential biases that are a concern consist of preferentially including or excluding datable detrital grains during mineral separation and data processing techniques. Exclusion of grains can lead to under- representation of source areas for a given sedimentary unit and alter the U-Pb age signature. The purpose of this study is to provide insight into this potential biasing by performing Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) U-Pb analyses of rutile and zircon with an optimized mineral separation and grain selection procedure. The Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN®) was thus used to provide insight into potential biasing by automated quantification of modal abundances of minerals. A reliable technique for sample preparation was developed based on the LA-ICP-MS U-Pb analyses and QEMSCAN® mineralogical data. This technique includes minimal separation steps without preferential loss of mineral grains, representative sample splitting, and random and representative selection of grains to be dated. The result is a more comprehensive dataset for provenance analysis. The Late Cretaceous sedimentary rocks of the Pine Ridge Sandstone and Almond Formation of the Mesaverde Group, Lewis Shale and Fox Hills Sandstone in the Greater Green River Basin, Rawlins, Wyoming, were investigated to test and develop mineral separation techniques. The methods developed here were also used to test whether there are variations in U- Pb provenance signal and modal mineralogy due to changes in depositional facies using samples from the Upper Cretaceous Blackhawk Formation and Castlegate Sandstone from the Book Cliffs, Utah. These stratal units were selected because while the age of the potential source areas are well known and the sequence stratigraphy, and sedimentology of the strata is well characterized, provenance has not been determined. The results from LA-ICP-MS and QEMCAN analyses of Greater Green River Basin sedimentary rocks show that standard mineral separation procedures are not ideal for detrital provenance investigations. The standard mineral separation procedures introduce bias, resulting in a misrepresentation of the modal mineralogy and provenance age signal. LA-ICP-MS and QEMSCAN analyses of sedimentary samples from the Book Cliffs, Utah show that there are variations in the U-Pb provenance signal and modal mineralogy between samples from different depositional facies.