The Oldest Known Eocene Loess in the Western United States: Evidence From a Fluvial‐Eolian Transition Zone

Abstract

The timing, stratigraphic development, and mechanisms of late Paleogene loess deposition remain poorly understood, yet they are critical to understanding how tectonics, fluvial reworking, and eolian transport interacted during a key interval of western North American landscape evolution. Here we combine lithofacies analysis with high‐resolution grain‐size analysis, quartz surface morphology, and detrital zircon U–Pb geochronology to refine the onset and depositional context of lower Eocene loess in western Wyoming and northeastern Colorado. In Wyoming, our data documents a transition from a fluvial to eolian dominant deposition. Loess‐like bimodal grain‐size distributions with characteristic eolian quartz surface features first emerged at 39.7 ± 0.4 Ma, ∼4 Myr earlier than previously recognized. This is the oldest and coarsest reported Paleogene loess in the western U.S.A. In Colorado, the earliest loess appeared by at least 35.1 ± 0.3 Ma and lasted at least into 31.1 ± 0.4 Ma. This study establishes a previously unrecognized fluvial–eolian transition zone in western Wyoming and supports an eastward younging and fining trend for the loess system. Comparison with coeval global loess records shows that loess systems consistently formed adjacent to major mountains during global cooling, suggesting that rivers originating at high topography supplied the necessary sediment, while concurrent global cooling promoted aridification for wind transport. Furthermore, a comparison to Asian loess shows that similar boundary conditions can produce comparable loess records via different atmospheric processes, implicating that synoptic‐scale climate variability may have played a major role in western U.S.A. loess formation.