JÖKULHLAUP-DRIVEN LANDFORM EVOLUTION DURING LATE CENOZOIC: GRAND VALLEY, CO (USA)

Grand Valley in the upper Colorado River Basin has been interpreted as primarily of fluvial origin. Multiple strath terraces along the Valley are composed of river gravels that consistently overlie the Mancos Shale. Tributary valleys between Grand Valley and Grand Mesa contain fill-terraces, which consist of boulders and cobbles transported by glacial outburst floods. The coexistence of the two end-member types of terraces in the region leads us to ask: Could they have originated contemporaneously from jökulhlaups? Evidence for the genesis of both terrace types by jökulhlaups from Grand Mesa are supported by numeric dating and subsurface imaging.

Whereas the terraces are considered to be solely fluvial in origin, this past assessment is based on very limited amount of data regarding geochronological backing, assessments of subsurface matrices, and consideration of the regional climate changes. We incorporate all three factors to provide a novel explanation for the genesis of these terraces. The terraces that flank Grand Mesa were produced by multiple glacial floods over the past 100 ka, at 85–74 ka, 67 ka and 58 ka in the northern flank and 63–61 ka in the southern flank of Grand Mesa, respectively. These dates consistently postdate MIS 6 (Bull Lake) and predate MIS 2 (Pinedale), suggesting additional local glaciations. Our subsurface investigation with electrical resistivity showed river gravels, cobbles, and even boulders constitute the Grand Valley terraces. The tomography data show the low resistivity of the highly erodible Mancos Shale and the high resistivity of the large-sized clasts, suggesting the Valley had been beveled and then covered with the clasts, with sections becoming abandoned as terraces. Our new age estimates linked to regional climatic events and the geophysical data support a new scenario of landform evolution. Thus, we suggest that glaciofluvial episodes in the Pleistocene were predominantly responsible for the evolution of these terraces.

Our novel conclusion does not stand alone in the Rocky Mountains region. Increasing amounts of age data and terraces-genesis studies elsewhere in the Rocky Mountains also suggest diluvial origins. We foresee that future research in alpine settings will drive a revised understanding of alpine-river terrace formations incorporating a diluvial framework.