A Model of Drumlin Growth at Múlajökull, Iceland

Abstract:

Observations at a rare, modern drumlin field—exposed by the recession of the Icelandic surge-type glacier, Múlajökull—allow a physically based model of drumlin formation to be formulated. These observations include the following: 1) drumlin relief resulted from basal till deposition on drumlins and erosion between them; 2) till deposition occurred during surges; 3) evidence of longitudinally compressive strain in till of the drumlins, as expected from negative flux divergence in a shearing bed, is absent; 4) during quiescent periods till was under higher effective normal stresses between drumlins than within them; 5) crevasse swarms are coincident with drumlins, and 6) water drainage at the glacier margin during the current quiescent state is through channels in low areas between drumlins.

In the new model, basal sediment transport is controlled by patterns of effective stress that are different in the quiescent and surging states. During quiescent periods, slip of ice over a sinusoidally perturbed bed, crevasse formation, and flow of subglacial water toward R-channels that lie between drumlins result in effective stresses that increase toward channels and decrease from the stoss to the lee sides of drumlins. This effective-stress pattern causes till entrainment and erosion by regelation infiltration (Rempel, 2008, JGR, 113) that peaks at drumlin heads and near R-channels and is minimized on the lee sides of drumlins, while bed shear is inhibited by effective stresses too high to allow deformation. In contrast, during surges regelation infiltration cannot occur, and high rates of bed shear—assumed to be spatially uniform due to the lack of evidence for till flux divergence—cause basal melt rates sufficient to deposit, by lodgment, till layers of up to ~2.0 m in thickness during a single surge. With multiple surge cycles, drumlins grow in height and migrate downglacier, with resultant stratigraphy in broad agreement with field observations.