Glaciotectonic structural analysis of Devils Lake, North Dakota.

Abstract

The ice-shoved terrain south of Devils Lake was formed by the last advance of the Late Wisconsinan glaciers approximately 12,000 years ago. Two series of ice-shoved ridges and associated source depressions (lake basins) formed during this event. Many of these features formed as a result of proglacial and subglacial thrusting processes. Icethrusting was facilitated by high pore-water pressures in the underlying Spiritwood Aquifer. Permafrost conditions may have contributed to over pressurizing the aquifer. Two study sites in the Devils Lake area revealed a kame deposit at the head of the Big Coulee Spillway Valley and an ice-shoved hill that was overridden (Devils Lake Mountain). This study has demonstrated that till micromorphology can be utilized to discriminate between subglacial and gravity-driven deformation processes. Field and microstructural evidence of the Devils Lake Mountain suggest that the sediments were subjected to high finite shear strains within a subglacial environment. Microstructural observations in this study also indicate that subglacial till can be identified by pressure shadow artifacts and highly crushed quartz grains. In addition, a consistent southeastward directional trend was revealed in both macro and microscopic deformation structures. The flow tills in a kame at Sullys Hill suggest sediment deformation under low confining pressures. This was documented by the lack of pressure shadows and crushed quartz grains. Similar deformation microstructures were identified in both glacigenic deposits indicating formational processes under disparate glacial environments. Microstructural evidence in the flow till suggests that the sediment behaved in fluid-like manner during till redeposition. Characteristic microstructures in both glacigenic deposits included: discrete shears, till pebble fabrics, galaxy or turbate structures, clast haloes, pressure shadows, crushed quartz grains, clay and iron translocations, boudins, and plasma fabrics.