A drumlin field at Fláajökull, Iceland: New insights from till shearing using magnetic fabrics

Abstract

The drumlins at Fláajökull, a non-surge-type glacier, Iceland provide the opportunity to investigate a modern drumlin-forming environment. Field observations indicate that the drumlins were formed sub-marginally along with an end moraine between the time period of 1966–1995. The fact that the drumlins did not extend beyond the end moraine, suggests that they were formed during this period. The Fláajökull drumlins consist of glaciofluvial outwash or till in their cores and have been streamlined with basal till. The Boulton hypothesis specifies that drumlin formation is driven by strain gradients in the glacier bed caused by hydrologic heterogeneity. This hypothesis was tested by measuring the anisotropy of magnetic susceptibility (AMS) of 457 till samples from two drumlins in the glacier forefield. In the first drumlin, two end member states of strain indicated that the basal till was sheared on the up-glacier side of the ridge (toward the overdeepened basin) and compressed on the down-glacier end with upward extrusion. These fabrics were likely formed in a recessional push-moraine on the drumlin surface. AMS fabrics in the other drumlin indicate that the tills were sheared in directions 10° and 12° northwest of the drumlin long axis; however, in one case the fabric was precisely sheared along the drumlin axis. Shear plane orientations conform to the drumlin morphology and were observed in both the drumlin core and the overlying basal till carapace in relation to the drumlin long axis. Shearing azimuths at the stoss end of the drumlin indicate some tendency for divergence suggesting higher basal resistance in the bed. New observations of hydrometer tests indicate that the Fláajökull tills are texturally similar to basal till from the Múlajökull drumlin field in central Iceland and the Horicon till, a late Wisconsin-age basal till from the Green Bay Lobe of the Laurentide Ice Sheet. In addition, ring-shear calibrations on the Horicon till provide a basis for estimating the state of strain for the tills at Fláajökull.