MIDDLE EOCENE DEPOSITIONAL SYSTEMS OF WESTERN WYOMING

Tephrostratigraphic investigations of the Bridger Formation are used to construct paleogeographic and depositional models for western Wyoming during the middle Eocene. Electron microprobe analyses of vitric separates have resulted in preliminary cross-basin correlations and potential correlations to a source area in the Absaroka Volcanic Province (AVP). The Church Butte (Bridger B) and Henrys Fork (Bridger C) tuffs are ashfall tuffs preserved in fluvial and lacustrine depositional environments in the southern Green River Basin. Highly pumiceous sheet sands are stratigraphically higher (Bridger D), and are also found 150 km away in the northern Green River Basin. These blue sheet sands are interpreted as fluvially reworked ashflow tuffs from the AVP. Phenocryst phases present include biotite, hornblende, and sanidine rimmed plagioclase. Chemical analyses of whole rock samples identify them as high-K dacites. A correlation of these sheet sands to a stratigraphic section in the southeastern AVP is suggested.

More than 300 km south of the inferred source (AVP) these fluvial deposits contain occasional cobble-size pumice clasts, and highly pumiceous lateral accretion elements indicative of a meandering system. Soft sediment deformation, plane bedding, and laterally extensive alluvial terraces suggest choking of fluvial channels and rapid deposition of pyroclastic material. A change in alluvial architecture is coincident with the introduction of pyroclastic sediments, and fluvial response to pyroclastic sediment loads may have been quite rapid. Such episodic sediment loads may have passed downstream in a wavelike fashion, altering channel dynamics and valley-floor landforms as it passed. At least three stratigraphically and chemically distinct horizons within the sheet sands are interpreted to represent reworked ashflow tuffs preserved in distal deposits more than 300 km from the source area. These have the potential to serve as high resolution chronostratigraphic markers. Such a depositional model requires the development of thoroughgoing fluvial systems in the Wind River Basin in addition to the breaching of the Wind River Uplift by the same fluvial systems.