DECIPHERING DEBLOIS' DEGLACIAL DEPOSITIONAL DEVELOPMENT

The most recent fieldwork of our ongoing study focused on the large, multi-lobate Deblois sand plain, one of a group of flat-topped landforms in eastern Maine deposited during and after the last Pleistocene glaciation of North America. Three separate segments of the Deblois sand plain, each at ~ 61 m elevation and composed of sediments deposited in a coarsening-upward sequence of glaciomarine mud overlain by sand and gravel, are identified as: (1) the eastern plain, sloping south 3 m/km; (2) the West Barrens, sloping south 2.5 m/km; and (3) the southwest plain, sloping <2 m/km, located southwest of the West Barrens and separated from it by the Narraguagus River.

Numerous meltwater channels interrupt the flat surfaces of both the east plain and the West Barrens. The southwest plain's morphology indicates subsequent coastal modification, including possible marine scarps and terraces. With one exception, all plain surfaces terminate abruptly in steeply sloping sides.

GPR profiles from the eastern plain show multidirectional dipping reflectors and some weakly U-shaped reflectors at depth. Radar profiles from the West Barrens and southwest plain show similar features, with some channel cut-and-fill reflectors and numerous small-scale packages of clinoforms. These clinoform sets are 2 to 5 m thick and <100 m wide. The inclined reflector sets represent slip face surfaces whose orientations shift both laterally and at different depths, with bearings from 45° to 315°. Taken together, the multidirectional orientation of these clinoform packages suggests a rapidly shifting prograding deposit.

Many questions remain before an accurate depositional history for the Deblois region may be developed: Are all three plain segments depositionally alike? Could the segments represent different episodes in the region's deglacial depositional history? Were the southwest plain and the West Barrens originally continuous, and then incised by the Narraguagus River, or were they deposited separately, as some clinoform orientations in the southwest plain suggest? Are the steep-sloped margins of these features depositional, indicating a deltaic origin, or erosional, suggesting later stream modification or groundwater sapping?