GEOLOGIC MAPPING IN AREAS SURROUNDING THE SOUTH PLATTE RIVER CORRIDOR IN EASTERN COLORADO REVEALS EVIDENCE OF RAPID INCISION WITHIN TRIBUTARY BOX CANYONS DURING HISTORICAL TIMES

Geologic mapping of surficial deposits in areas surrounding the South Platte River corridor in eastern Colorado documents a period during historical times of rapid incision within ephemeral, headward-eroding box canyons north of the town of Fort Morgan. The box canyons reflect a long geologic history, with older paleovalleys incised into Pierre Shale bedrock and filled with at least 4 m of middle Pleistocene sediment. The deposits consist of interstratified sand, pebbly sand, silt, and poorly sorted sandy pebble gravel, overlain by mostly finer grained, weakly stratified sandy silt with lenses and stringers of sand and fine pebbles, and massive silty very fine sand. The sediments are interpreted as stream deposits overlain by loess or eolian sand deposits that were largely reworked by sheetwash processes. A middle Pleistocene age for this section is indicated by the presence of an eroded pedogenic carbonate horizon with stage III morphology at the contact between it and the sediment that overlies it. The overlying deposits, about 1.5 m thick, consist of weakly stratified silt with scattered, poorly sorted sand stringers, and minimal soil development. This uppermost section is interpreted as latest Holocene sheetwash deposits derived primarily from the reworking of late Pleistocene and Holocene loess that blankets the low drainage divides. A bison tooth, collected (along with bison bones) at a depth of about 92 cm from young sheetwash deposits in one of the box canyons, has a ¹⁴C age estimate that calibrates to the early 1820's. Based on this age estimate, the box canyon floors aggraded about a meter and then were entrenched by as much as 5–6 meters to create steep-walled gullies exposing the paleovalley sediment fill, all within the last approximately 190 years.