COLLUVIAL APRONS AS INDICATORS OF LATE PLEISTOCENE AND HOLOCENE CLIMATIC CHANGE IN SOUTHEASTERN WYOMING

Colluvial aprons emanating from Laramie River and Sybille Creek terraces are deposited on present floodplains, adjacent terraces, and arroyo bottoms. Estimated numerical ages were determined for aprons and terraces by Optically Stimulated Luminescence (OSL). Aprons ranging from 1,500-15,000 yr B.P. are present along all mapped and dated terraces (ranging from 16,000-100,000 yr B.P.) in the 300 km² study area. Each apron contains multiple buried colluvial deposits and intervening soils. Pedogenic alteration of colluvial apron deposits shows a relatively complex sequence of cumulic, welded, and buried soils. Overprinting by carbonates is common throughout buried soils, but is most dramatic in buried A horizons. Apron deposits in mid-slope positions, regardless of age or position relative to potential eolian sources, are texturally similar, being dominated by fine sand and silt with minor quantities of gravel. Deposits consistently exhibit coarsening upward sequences with significant increases in gravel, often concentrated in A horizon stone lines.

Similar to previous Quaternary hillslope studies, these deposits are interpreted to result from changes in geomorphic processes dominant in relatively arid vs. relatively humid climate regimes. Under arid regimes, eolian deflation of contemporaneous floodplains and/or terrace deposits results in eolian deposition of fine sand and silt on terrace surfaces. The transition to wetter periods, typified by increased and/or greater intensity rainfall coupled with low vegetation, results in erosion by overland flow and apron aggradation. Apron gravel concentrations increase through time as terrace and eolian sand are depleted from shoulder slopes, exposing terrace gravels to erosion. Estimated numerical ages determined for C horizon sediments by OSL identify transitions between arid and humid time periods.