CHANGES IN SANDSTONE COMPOSITION ACROSS UNCONFORMITIES IN THE CLOVERLY FORMATION OF WYOMING

The Cloverly Formation of the Bighorn Basin in Wyoming contains a valuable fossil record preserving the transition between major Cretaceous floras and faunas. University of Georgia graduate student Cade Orchard's recent study of the sequence stratigraphy of the Cloverly identified two unconformities. Their presence has implications for the interpretation of the Formation’s fossil record. I conducted a tectonic provenance analysis of Cloverly sandstones to test for compositional changes across these unconformities. Thirty-seven thin sections were point counted using the Dickinson-Gazzi method. Samples were collected from the following six units in ascending order: the underlying Pryor Conglomerate (n = 1), Little Sheep Mudstone (n = 7), Himes amalgamated channel sandstone (n = 9), Greybull sandstone (n = 6), Himes upper channel sandstones (n = 8), and the overlying Sykes Mountain Formation (n = 6). The analysis supports the presence of Orchard’s postulated unconformities, one at the base and one at the top of the Himes amalgamated channel sandstone. Most samples contain over 80% quartz, while the Himes amalgamated channel contains 50–75% lithics. The quartz-rich samples plot in or near the Craton Interior provenance on Dickinson’s QFL and QmFLt diagrams, while the Himes amalgamated channel plots across the Lithic Recycled, Dissected Arc, and Transitional Recycled and Arc fields. Similarly, on the QpLvLs and QmPK diagrams, most samples plot near the quartz vertices, whereas the Himes amalgamated channel has much higher values of Lv, P, and K. The one sample of the basal Pryor Conglomerate also diverges from the quartz-rich groups, plotting in the Recycled Orogen provenance due to its Ls components, primarily chert. These abrupt stratigraphic changes in composition support the presence of two unconformities and imply substantial changes in sediment delivery. Evidence for these unconformities supports Orchard's stratigraphic analysis and raises the possibility that the anomalously long duration of the Cloverly may be largely due to two hiatuses. Accounting for these hiatuses may facilitate a reinterpretation of the fossil record of the Cloverly Formation.