Bed Deformation beneath the Baltic Ice Stream, Southern Sweden: Implications for the evolution of the subglacial system beneath short-lived, episodic ice streams
Bed Deformation beneath the Baltic Ice Stream, Southern Sweden: Implications for the evolution of the subglacial system beneath short-lived, episodic ice streams
Previously Published Material: Preliminary results of the local ice flow history near Dalby, Sweden, were presented by myself at the 2014 Geological Society of America Annual Meeting (October 19-22) in Vancouver, Canada. Implications for the evolution of the subglacial system were not included at this meeting.
Abstract ID#: 35836
English Abstract:
Temporal variability of the subglacial system is of critical importance for the dynamics of ice sheets, particularly in the case of ice streams. Herein, we explore the dynamics of the Baltic Ice Stream (BIS) through the application of Anisotropy of Magnetic Susceptibility (AMS) till fabric analysis. AMS provides a precise, unbiased, and robust assessment of the preferred orientations of silt-sized and smaller magnetic grains within the till matrix. AMS faithfully records shear strength and direction in laboratory and field situations where shear direction or ice flow is known (i.e., drumlins and flutes). In this study systematic AMS fabric analysis within an 8-meter section exposed in a quarry near Dalby, Skȧne County, Sweden, reveals the dynamic nature of the BIS. This section contains over 6 meters of a dark grey, silt-clay BIS till overlain by a 1.5 m of the regional surface till – a weakly compacted, sand-rich diamicton likely deposited by the Scandinavian Ice Sheet (SIS) immediately prior to deglaciation. AMS fabric orientations in the BIS till display an apparent rotation of ice flow from northeasterly to the southwesterly, beginning approximately 0.5 m below and terminating coincident with a boulder line and discontinuous sand lenses located 3 meters from the base of the section. Above the boulder line, ice flow is from the SW. Additionally, systematic, non-linear trends in fabric strength (S1 eigenvalue) and magnitude of anisotropy (P’) punctuated by excursions coincident with the boulder line illustrate the temporal variability of bed deformation beneath the BIS during the late-stage Weichselian Young Baltic Advance (19-17ka). Each of these parameters display marked changes across the boulder line, indicating a pronounced change in the dynamics of the subglacial system during and following its formation. The surface till displays relatively high P’ and susceptibilities; however, the five sample sites possessed weak fabrics (mean S1 = 0.612) consistent with observations (e.g., lack of striated clasts) at the exposure indicating small amounts of shear. Fabric orientations from this unit indicate ice flow from the NNE indicating a SIS origin. These results have implications for the nature and evolution of bed deformation during the advance and retreat of short-lived ice streams.