Effects of glacial-interglacial climate change on mass wasting, southeastern Minnesota.

摘要

Glacial-interglacial changes in mass wasting in the lower Root River basin, southeastern Minnesota, were investigated using a combination of field stratigraphy and sedimentology, analyses of mass movement mechanics, and numerical modelling of soil heat and water flow. Most deposits in the study area are interpreted as products of mass wasting. Some are specifically attributed to solifluction; the mass wasting processes responsible for other depositional units cannot be determined from the field evidence. Radiocarbon dates, inclusion of silt reworked from loess units of known age, stratigraphic relationships with late Wisconsinan fluvial sediment, and geomorphic relationships with the late Wisconsinan Savanna Terrace, all indicate that most mass wasting deposits in the study area are Wisconsinan. Mass wasting began some time before 25,000 14C yr B.P., and continued until at least 13,580 O 60 14C yr B.P. (Beta-82839). There is little evidence of Holocene mass wasting on major valley-wall slopes, except possible slow soil creep.; Slope stability analyses indicate that clast-supported diamictons mantling many steep slopes in the study area should not be susceptible to rapid sliding failure even if saturated to the surface. This provides an explanation for the Holocene stability of these slopes. This result also supports interpretation of these diamictons as solifluction deposits, rather than the product of rapid flows, although deposition by rapid flows is suggested by clast fabrics.; Numerical modelling simulations of soil moisture and ground frost regime were carried out using paleoclimatic scenarios for 21 ka, 16 ka, and 14 ka, based on general circulation model output. Permafrost develops under the 21 ka paleoclimatic scenario, and this leads to a major increase in soil wetness and frost heave potential. This supports the hypothesis that permafrost development was the specific factor responsible in large part for the late Wisconsinan mass wasting episode.