LOESS PSEUDOKARST IN WEST-CENTRAL NEBRASKA: PIPING, COLLAPSE, CATASTROPHIC DRAINAGE, GULLYING, AND CANYON DEVELOPMENT

The south side of the Platte River Valley in Lincoln County, Nebraska is characterized by ~60 m of Wisconsinan to Late Holocene loess (Gilman Canyon Formation, Peoria Loess, and Bignell Loess) underlying uplands consisting of nearly flat tables surrounded by deeply-eroded canyons. Local relief is ~100 m, but the gradients of table surfaces are low enough to pond water in shallow closed depressions. Cracks extending several meters downward from table surfaces through the Bignell and Peoria Loesses are generated by desiccation and propagated by gravity as headward erosion and mass wasting proceed at table margins and in advancing drainages. These cracks can be enlarged by subsurface flow to create subsoil pipes, a form of loess pseudokarst. Accounts of piping and linked geomorphic processes in the study area are mostly anecdotal, but accelerated gully erosion over a period of decades has already been documented in the loess tablelands of central Nebraska.

Serial observations (since 1988, and through archival aerial photographs, since 1938) of an area on the western flank of a 3.8 km-wide table along Moran Canyon provide some of the best direct evidence for loess pseudokarst and associated geomorphic processes in Nebraska. Rainwater collecting in a closed depression (probably enhanced by engineering) atop the table was able to percolate slowly through the Bignell and Peoria Loesses, flow horizontally above or near the base of the Peoria Loess, and erode subsurface pipes to diameters that reached or exceeded 120 cm. The collapse of these subsurface pipes and the concomitant erosion of a very steep-walled, “V”-shaped gully exceeding 30 m in depth took place during 1988-1992, but mostly during a period of mere months, if not less. That period included a single, large (200+ mm) rainfall event occurring over a 30-hour period in the fall of 1991 and the melting of heavy snow from the subsequent winter of 1991-1992. Pipe collapse, rapid runoff, and gullying together appear to have combined in at least one locally catastrophic event that produced debris-flow like deposits containing 30+ cm blocks of intact loess. Piping and related processes are likely to have contributed to the long-term development of canyons across the enclosing region, perhaps in a combination of events similar to the documented account.