INSIGHTS INTO DRUMLIN DEVELOPMENT AT MULAJÖKULL, ICELAND: A GEOPHYSICAL APPROACH

Drumlins form at the ice-bed interface as the result of subglacial slip, but the mechanisms responsible for their formation remain enigmatic. Resolution of drumlin internal stratigraphy can help to provide insight into how drumlins form and into associated basal slip mechanics. Stratigraphic logging techniques illuminate the internal stratigraphy of drumlins but are inherently limited to areas of natural exposure. Utilizing ground penetrating radar (GPR), we imaged the internal stratigraphy of seven drumlins from a recently exposed active drumlin field located in the forefield of Múlajökull, Iceland. Data were collected with 100 and 200 MHz antennas that had maximum penetration depths of 8 m and 4 m. In general, echograms demonstrated distinct layering of diamictons: longitudinal profiles contained down-ice plunging reflectors of pseudo planer morphology and transverse profiles contained horizontal bedding (plunge is used instead of dip in this case due the beds’ pseudo planar morphology.) The plunges of the longitudinal beds were steeper than the surface directly above the bed at the drumlin head but became shallower than the surface above the bed at the lee side. Transverse to drumlin long axes, profiles exhibited unconformities along the drumlin flanks and head. In the transverse profiles, horizontal bedding at depth correlated with the down-ice-dipping beds observed in the longitudinal profiles. These geophysical observations are supported by stratigraphic logs recorded on the same drumlins. The stratigraphic pattern observed indicates that the drumlins of the Múlajökull forefield formed by a combination of depositional and erosional processes. These observations agree with a new model of drumlin formation that invokes deposition during glacial surges and erosion during normal ice flow.