EPISODIC HILLSLOPE DENUDATION DURING THE YOUNGER DRYAS IN SOUTHEAST WYOMING

Colluvial aprons emanating from Laramie River and Sybille Creek terraces are deposited on present floodplains, arroyo bottoms, and adjacent terraces. Each apron contains multiple buried colluvial deposits and intervening soils, and is sourced from eolian sand and silt that was deposited on terraces in the late Quaternary. Apron aggradation timing was established by dating buried soil C horizons with Optically Stimulated Luminescence. Episodic deposition occurred at approximately 13,000 yr B.P. 11,700 yr B.P. and 8,200 yr B.P. in nearly all aprons sampled. The two former dates correlate with the beginning and end of the Younger Dryas, and the latter with the approximate timing of the Laurentide Ice Sheet collapse.

Similar to previous Quaternary hillslope studies using the Biogeomorphic Response Model (BRM), colluvial aprons identified in this study are interpreted to result from changes in biologic-geomorphic processes dominant under transitional climatic regimes. In this setting, authors utilizing the BRM would predict increased hillslope sediment yields at the transition out of the relatively more humid Younger Dryas period because arid region hillslopes are typically sediment starved. This study, however, shows apron aggradation at both the transition into and out of the Younger Dryas. This discrepancy is most easily explained by the fact that terrace surfaces are sediment rich as a result of frequent eolian deposition.

This study supports the BRM by predicting increased hillslope sediment yields during the transitions into and out of periods of differing available moisture. Furthermore, this study establishes the impact the Younger Dryas event had on this portion of the western Great Plains. As such, increased sediment yields to footslopes and fluvial systems ultimately resulted in fluvial aggradation during the Younger Dryas. Preliminary data indicates that hillslope denudation also occurred during Heinrich Event 1, adding to the growing body of evidence indicating that terrestrial deposits in the western United States are responding to millennial-scale climate changes.