Subglacial bedforms of the Zemgale Ice Lobe, south-eastern Baltic

Abstract

Presented in this paper are the results of the mapping of ∼6600 subglacial bedforms of the Zemgale Ice Lobe (ZIL), their composition and internal structure in the south-eastern Baltic region. Topographic maps at scale 1:10,000 were mainly used to map bedforms in Latvia, while a digital elevation model with cell size of 5 m was used for North Central Lithuania. The ZIL operated during the deglaciation of the Late Weichselian Scandinavian Ice Sheet (SIS), at least in the Middle Lithuanian and North Lithuanian glacial phases, and created a subglacial landform assemblage consisting of glacial lineations such as drumlins, mega-flutings, Mega Scale Glacial Lineations (MSGLs) and transversal morphologies, such as ribbed moraines. It is observed that lineations of different scales co-exist, suggesting subglacial bedform transition and possibly a continuum of bedforms. Highly elongated lineations with a length of up to 24 km and an elongation ratio of up to 1:50, interpreted as MSGLs, are observed in the NE part of the Middle Lithuanian Plain. In the so-called Zemgale drumlin field, 20% of lineations have elongation ratios >10:1, indicating the fastest ice flow in the central part of the main body of the lobe. Based on test surveys and investigated outcrops, the drumlin cores consist of sorted sediments with different levels and depths of glaciotectonic deformation overlain by subglacial till. The main drumlin-forming ice stress was mostly parallel to the crests of drumlins, while some ice stress from inter-drumlin depressions acted during the final episodes of their formation. The observed sediment structures indicate multiple episodes of the basal ice/bed coupling and decoupling, which can best be explained by the mosaic ice-bed deformation model. Ribbed moraines are characterized by a complicated structure of multiple till units interbedded with sorted sediments that have been formed by repeated subglacial overthrusting.