LAKE DEPOSITS AS ARCHIVES OF CONTINENTAL-SCALE LANDSCAPE EVOLUTION: EXAMPLES FROM THE EOCENE GREEN RIVER FORMATION (Invited Presentation)

Lacustrine deposits have long proven invaluable for the highly detailed and geographically specific records they can provide of climatic and biologic change. Because lakes receive drainage from their surrounding drainage catchments, they also offer unique opportunities to reconstruct tectonic, magmatic, and geomorphic histories across areas vastly larger than the lakes themselves. The lakes responsible for depositing the Eocene Green River Formation for example received drainage from an area spanning ~600,000 km², which included much of the foreland and eastern hinterland of the North American Cordillera. A broad suite of field, detrital zircon geochronology, ash bed geochronology, ⁸⁷Sr/⁸⁶Sr, and stable isotopic studies have collectively demonstrated that regional drainage patterns were highly dynamic, effectively reversing polarity over a time interval of ~5 my. From ~55-50 ma the California River flowed ~800 km north from the Mohave region into a fresh to brackish lake in northeast Utah. This lake spilled across the Douglas Creek Arch into an increasingly saline lake in northwest Colorado, that eventually deposited sodium carbonate evaporite (nahcolite). The Aspen River flowed northwest from central Colorado at about the same time, into a hypersaline saline lake in southwest Wyoming that deposits world’s largest known trona accumulation. The lake in Wyoming suddenly freshened at ~50 ma in response to capture of an eastern drainage. The timing of this capture corresponds to a general westward tilting of eastern Wyoming, which is attributed to isostatic and thermal buoyancy changes resulting from removal of the Farallon flat slab. From ~50-47 ma Eocene lakes were increasingly influenced by inflow from the Idaho River, which drained the newly formed, high elevation Challis volcanic edifice ~400 km to the northwest. Inflow from this area suddenly increased at ~49 ma, resulting in southward spillage of Lake Gosiute in Wyoming, consequent expansion of Lake Uinta, and deposition of the Mahogany zone across Colorado and Utah. Challis-derived volcanic detritus filled in Lake Gosiute by ~48 ma and the eastern end of Lake Uinta by ~47 ma. From ~47-43 lacustrine deposition was restricted to a relatively small area of Utah near the Sevier fold and thrust belt.