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Master of Science (MS)
The Bighorn Basin (Wyoming, U.S.A.) contains some of the best exposed and studied nonmarine early Paleogene strata. Over a century of research has produced a highly resolved record of early Paleogene terrestrial climatic and biotic change as well as extensive documentation of spatiotemporal variability in basin-scale stratigraphy. The basin also offers the opportunity to integrate these data with the uplift and erosional history of the Laramide uplifts that surround the Bighorn Basin. Herein we provide a comprehensive provenance analysis of the early Paleogene Fort Union and Willwood formations in the Bighorn Basin from paleocurrent measurements (n = 510 measurements), detrital zircon U-Pb geochronology (n = 2,258 age determinations), and sandstone compositions (n = 76 thin sections) obtained from fluvial sand bodies distributed widely across the basin. We aim to address the following questions; why there are lithologic changes between the Fort Union and the Willlwood formations, why is there spatial variation of grain size within the Willwood formation, and does the boundary sandstone represent a greater efficiency of sediment transport or a change in provenance.
From these new data, and data compiled from previous studies (May et al, 2013) we are able to present a comprehensive paleodrainage and unroofing history for the Bighorn, Owl Creek, and Beartooth Mountains as well as identify hinterland sediment sources in the Sevier fold-and-thrust belt. Broadly, we observe data consistent with (1) erosion of latest Cretaceous shales from the Bighorn Mountains and westward transport into the basin; (2) erosion of Late Cretaceous shales as well as lower Mesozoic and upper Paleozoic siliciclastics from the Owl Creek Mountains and its transport north and northwest into the basin; and (3) erosion of lower Mesozoic sedimentary cover, Paleozoic sedimentary cover, and crystalline basement from the Beartooth Mountains eastward into the northern Bighorn Basin. Similar to previous studies, we find evidence for a system of transverse rivers contributing water and sediment to an axial river system that drained north into southern Montana during both the Paleocene and Eocene. Additionally, the data indicate asymmetric unroofing histories on either side of the Bighorn and Owl Creek mountains, implying a drainage divide that we attribute to the vergence direction of the underlying basement reverse faults and exacerbated by the prevailing paleoclimate. In the southwestern and northern portions of the basin we find evidence for (1) sediment sourced from the diverse Phanerozoic cover and the Neoproterozoic Brigham Group quartzites in the Paris thrust sheet of southeastern Idaho; (2) erosion of sediment from the Idaho Batholith; and (3) sediment likely generated from the crystalline basement of the Tobacco Root Mountains and Madison Range in Montana transported southward and eastward to the Bighorn Basin. We suggest this sediment was funneled through the hypothesized Monida transverse structural zone situated between the Helena and Wyoming salients of the Sevier fold-and-thrust belt, then into the Absaroka Basin, and across the Cody Arch into the Bighorn Basin. This provenance pattern appears to have largely continued until ~50 Ma at which point more proximal source areas were available in the Absaroka volcanic province. Basin-scale patterns in the stratigraphy of the Fort Union and Willwood formations were a product of catchment size and the lithologies eroded from the associated highlands. Mudrock-dominated strata in the eastern and southeastern Bighorn Basin are inferred to be caused by comparably smaller catchment areas and the finer-grained Mesozoic strata eroded. The conglomeratic and sand-dominated strata of the southwestern area of the Bighorn Basin are inferred to be caused by large, braided fluvial systems with catchments that extended for hundreds of kilometers into the Sevier fold-and-thrust belt and erosion of more resistant source lithologies. The northernmost early Paleogene strata represents the coalescing of these fluvial systems as well as rivers whose catchments extended into southwestern Montana that contained more resistant, crystalline lithologies. These factors generated the thick, laterally extensive fluvial sand bodies common in that area of the basin.
Western Washington University
Subjects – Names (LCNAF)
Bighorn Basin (Mont. and Wyo.)
Subject – LCSH
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Welch, Jessica L., "Provenance of early Paleogene strata in the Bighorn Basin (Wyoming, U.S.A.): Implications for Laramide tectonism and basin-scale stratigraphic patterns" (2021). WWU Graduate School Collection. 1040.