FORMATION CONDITIONS OF SUBGLACIAL MICROSCALE EROSIONAL FORMS

Formation conditions of Subglacial Microscale Erosional Forms



Formation conditions of Sub glacial Microscale Eros ional Forms

Introduction

The sub glacial processes at Briksdalsbreen, Norway, are examined by a combination of sedimentology, thin section and scanning electron microscope (SEM) analysis of till samples from an exposed subglacial surface and from beneath the glacier(Paterson, 2004).

Discussion

Studies of a fluted surface indicate that subglacial deformation is occurring on a field scale with flutes forming behind most clasts 0·6 m high. At the thin section scale (0·014-2·0 mm) it is seen that deformation is by rotation and attenuation and is dependent on till texture (Hallet, 2006). At the SEM scale (0·1-0·4 mm) it is seen that erosion is controlled by abrasion and percussion which produces distinct grain 'styles' as part of an Erosional continuum(Hallet, 2006). Overall it is shown that rotation and attenuation is a dominant process at all scales and that the clast interactions associated with different scale perturbations within the shear zone control erosion and deposition, as well as landform and fabric production.

The Athabasca Glacier, resting on a rigid bed, provides an excellent example of subglacial ice and till erosion. The presence of a thin mobile till layer is shown by the presence of flutes, saturated till layer, push moraines and ploughed boulders (Hallet, 2006). Cross-cutting striations, v-shaped striations and reversed stoss-and-lee clasts are indicative of clasts rotating within this layer. As the till moves it erodes the bedrock and clasts within it (Paterson, 2004). A combination of erosion by ice and till produces stoss-and-lee-clasts and generates striations on flutes and embedded clasts, as well as eroding the bedrock into a continuum of smoothed, rounded and streamlined forms.

The existing subglacial mega flood explanation of glacial sediment deposition and landform production in south-central Alberta relies upon four sets of observations and associated interpretations: (1) a subglacial genesis for all glacilacustrine sediments lying stratigraphically below a local melt-out till or “ice stagnation facies”; (2) a melt-out genesis for the “ice stagnation facies”; (3) a syn-depositional status for the glacitectonic disturbance visible in most exposures due to periodic ice-bed re-coupling; and (4) a subglacial mega flood origin for a “truncation surface” and clast lag that occurs at the top of the sedimentary exposures and the carving of hummocky terrain and flutes in the area(Paterson, 2004). It provide alternative interpretations of the sediment-landform assemblages of the area and demonstrate that alternative, simpler interpretations of the evidence can be proposed. Specifically, it is proposed: (1) a proglacial rather than subglacial origin of glacilacustrine sediments; (2) a glacilacustrine origin for the “melt-out till” and its subsequent glacitectonic disturbance by minor glacier readvances; (3) post depositional rather than syn-depositional glacitectonic disturbance due to a significant advance by glacier ice after the accumulation of a thick sequence of glacilacustrine sediments; (4) that some linear chains of hummocks are the morphological expression of glacitectonic folding and thrusting; (5) that the use of the truncation surfaces to propose a subglacial fluvial origin for hummocky moraine is flawed logic, because ...