EXPLAINING THE "HOLE" IN THE COLORADO HIGH PLAINS

Although the crystalline core of the Rockies has been the focus of Bud Wobus’ career, he inspired at least a couple of his proteges to look eastward from those high surfaces. In that direction, the High Plains of Colorado are thinly mantled with the Ogallala Formation sands and gravels, which were shed from the Rockies between about 12 and 6 million years ago. Since then, the fluvial systems that distributed these gravels shifted from being depositional to erosional, leaving large “holes” through the Ogallala and the sedimentary rocks beneath it abutting the Colorado Rockies. This shift from deposition to incision signals a geologically recent change in either climate or tectonics. We developed a simple fluvial sediment transport model that allows us to contrast patterns of fluvial incision driven by changes in hydrology and sediment flux (“climate”) with those driven by changes in rock uplift patterns relative to sea level (“tectonics”). Our modeling suggests that there may be diagnostic differences between the spatial and temporal patterns of incision caused by these different processes. In particular, incision driven by climate change is most commonly accompanied by downstream migrating waves of incision and decreases in channel gradient, while under most circumstances incision driven by tectonics will be accompanied by upstream migrating incision and increases in channel gradient. We apply our modeling to the case of the North American High Plains, where the Ogallala surface has been incised up to 500 m by the fluvial systems draining the core of the Rockies. Although this incision has been interpreted as reflecting a tectonic rejuvenation of the High Plains, our analysis suggests that climate change is at least as plausible an explanation to explain the incision of this landscape.