Microstructures in diamictons—evidence of subglacial bed conditions

Abstract

Examination of microstructures within diamictons from southern Ontario, Canada, and southern Bavaria, and Baden-Württemburg, Germany reveal evidence illustrative of certain subglacial bed conditions. The microstructures from Ontario were obtained from a complex glacial mélange unit that forms the internal structure of an isolated drumlin/megaflute on the north shore of Lake Erie. The evidence from Germany was taken from individual drumlins within drumlin fields in the Inn-Chiemsee, Iller, Rhein and Salzach glacier areas. The structures within the diamicton from Ontario reveal clear evidence of massive shear associated with high externally applied strain. There are kink-band arrays, high angle shear zones, crenulation and sedimentary “dyke” and water escapes structures. In the German diamictons there is a lack of evidence of high levels of bulk deformation. There is, however, a very high carbonate content. Some evidence points to the possibility that these diamictons may contain intraclasts of paleosols. Interpretation of both sets of diamictons suggests that the Ontario sediments were subject to conditions at the bed of an ice mass best typified as a deformable bed. The German sediments may be interpreted as: (1) being so saturated that any deformation must have been by inter-grain slip (pervasive deformation) and thus no structures are recorded; or (2) being already cemented by the carbonates and sufficiently rigid to cause strain to be taken up elsewhere; or (3) were frozen prior to transposition and were again too rigid to suffer deformation; or (4) suffered no deformation whatsoever. The analyses of these two sets of sediments have important implications for subglacial conditions before and during drumlin and other subglacial bedform development.