Laurentide and Cordilleran fast ice flow: some sedimentological evidence from Wisconsinan subglacial till and its substrate

Abstract

Dreimanis, A. 2003 (March): Laurentide and Cordilleran fast ice flow: some sedimentological evidence from Wisconsinan subglacial till and its substrate. Boreas, Vol. 32, With the aid of three case studies we summarize sedimentological and structural evidence in Wisconsinan subglacial till and its substrate that can be used to infer the presence of former fast-moving ice. We compare sedimentological characteristics of till with its stone fabric, stone surface erosional features and orientations of fractures, faults and folds in till and substrate, in order to interpret the processes that formed the till. In the cases we studied, stones in tills that experienced ductile deformation (inferred to be associated with fast-moving ice) show variable alignment of latest surface striae. Moreover, striae are inconsistent with the orientations of lee ends and chattermarks on the same stones, and with the dip directions of shallow shear planes and (or) steeper tension fractures in the till matrix. These observations indicate that stones were rotating in a flowing till matrix prior to their final emplacement. Ductile shear of fine-textured, water-saturated till and Jeffrey-like rotation of stones are invoked where most of the following occur together: (i) delicate striae follow the curvature of stone surfaces; (ii) a-axis fabrics are multimodal to girdle-like; (iii) stone a axis eigenvectors, striae orientation and stone lee end alignments fall between the modes of stone fabrics; (iv) stone pavements are uneven with vertically oriented and inverted bullet-shaped stones; (v) soft sediment clasts and (or) fragile fossils are preserved.

A polyphase glacitectonic model for ice-marginal retreat and terminal moraine development: the Middle Pleistocene British Ice Sheet, northern Norfolk, UK

“…The simplest explanation is that the traction zone at the base of the A6 ice was effectively decoupled from the underlying cohesive bed (A5 deformation structure), either by a thin deformable bed, potentially represented by early soft sediment deformation within the Weybourne Town Till, or a veneer of pressurised water (Kjaer et al, 2006;Fischer et al, 2011). Basal conditions, such as those inferred here, often accompany fast or surging ice flow behaviour (Ó Cofaigh and Evans, 2001;Lian et al, 2003). Generic recognition of surging behaviour is provided by a range of sedimentological and landform features, however, few features on their own are unequivocally diagnostic of this type of flow mechanism (Evans et al, 1999;Evans and Rea, 1999;Evans and Rea, 2005;Evans et al, 2008).…”

Sedimentological Observations from the Tiskilwa Till, Illinois, and Sky Pilot Till, Manitoba

“…Classical fabric interpretation (e.g. Dowdeswell and Sharp, 1986;Hicock et al, 1996;Lian et al, 2003) would suggest that till genesis can be inferred from fabric strength and shape. However, laboratory results using a ring-shear device suggest that fabric strength is not unique to a specific genesis (Hooyer and Iverson, 2000b).…”

Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK

“…Elsewhere on Anglesey, this unit has been interpreted as glacitectonite produced by subglacial shearing and reworking of localised pre-existing bedrock and regolith . This subglacial interpretation is endorsed because of the high-consolidation of the diamicton, presence of striated and keel-shaped limestone clasts and a minor far-travelled lithological component (Boulton & Paul, 1976;Lian et al, 2003;Benn & Evans, 2010) observed in exposures at Beaumaris and Trwyn y Penrhyn (Section 3; Greenly, 1919). Scouring of the upper-surface of the basal diamicton to produce the deep sediment-filled scours is considered to have occurred by focussed meltwater incision of the subglacial bed.…”

“…Instead, the high consolidation of the diamicton, the presence of clasts from local and far-travelled sources plus the striated and keel-shaped limestone clasts are suggestive of subglacial till accretion (cf. Boulton & Paul, 1976;Lian et al, 2003;Benn & Evans, 2010). Flame-like margins along the base of the diamicton unit (Y1-Y2 scour) imply rapid loading and dewatering of the underlying basal lag.…”