John Menzies

Micromorphological analyses of microfabrics and microstructures indicative of deformation processes in glacial sediments

Abstract Various forms of sediment deformation can be detected within glacial sediments at the microscopic scale. Analyses of these forms leads to a preliminary classification of microfabrics and microstructures of brittle, ductile and polyphase modes of deformation in glacial sediments. With the development of a taxonomy of different microfabrics and microstructures these processes, once differentiated, permit insights into glacial sediments to a scale and level of detail hitherto unknown. Examples are presented that illustrate some of these different forms of deformation often within a single glacial sediment sample. This research suggests many of the past ideas with regard to details of glacial depositional processes, especially in terms of diamicton deposition and subsequent classifications, need to be re-evaluated.

Microstructures in diamictons: evidence of subglacial bed conditions

Abstract Examination of microstructures within diamictons from southern Ontario, Canada, and southern Bavaria, and Baden-Wurttemburg, Germany reveal evidence illustrative of certain subglacial bed conditions. The microstructures from Ontario were obtained from a complex glacial melange 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.

8 – Subglacial environments

Publisher Summary This chapter provides information on subglacial environments. The subglacial environment is a large group of inter-related factors that are indirectly affected by external factors. The end product of these diverse influences reduces to a series of subglacial interface bed types that can be distinguished by thermal and rheological variations that are subject to an enormous dynamic range of temporal and spatial variations. Subglacial interfaces and related bed types are functions of the prevailing basal ice and bed conditions. These conditions are the results of complex relationships among basal ice dynamics, subglacial sediments and bedrocks, subglacial hydraulics, and the ambient thermal state for any given area of glacier bed. The importance of subglacial sediments in terrigenous glaciated areas of the world cannot be undervalued. Volumetrically, other than glaciomarine sediments, these sediments cover the largest areas of the Earths glaciated surface. Sediments and landforms of subglacial environments can be subdivided into those developed under active ice flow and passive or “dying” ice flow.

Microstructures within a modern debris flow deposit derived from Quaternary glacial diamicton—a comparative micromorphological study

Abstract Examination of a modern debris flow in comparison with a Quaternary diamicton from which it has been derived reveals significant differentiating sedimentological characteristics. The two sediments were examined using micromorphological analyses. Thin sections of the debris flow show distinct primary microstructure sets that appear indicative of debris flow processes of turbate micro-deformation during flow and subsequent flow deceleration. Comparison of the micromorphological character of both sediments reveal many similarities but the differences seem indicative of flow processes within the active debris flow. Large turbate structures are common within the debris flow sediment and although many more turbate structures occur within the in situ diamicton, they are smaller in size possibly indicative that those structures within the debris flow sediment are not inherited but are neo-formed. Perhaps, the most distinctive and individual characteristic separating the debris flow sediment from the older diamicton is the presence of “tiled” structures. These structures result from downslope flow deceleration and dewatering resulting in transverse bands of clays developing in rhythmic-like patterns. Likewise, the debris flow sediment exhibits a “marbled” appearance, presumably indicative of intense deformation, a characteristic not found in the in situ Quaternary diamicton.

Subglacial hydraulic conditions and their possible impact upon subglacial bed formation

Abstract In reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions. A speculative classification of subglacial temperate beds is presented. Subglacial beds are subdivided into “H”, “Q”, and “M” beds. “H” beds are typically hard beds in which Upper interface meltwater activity dominates. “M” beds are those in whichd soft mobile subglacial debris conditions prevail and in which advective pervasive f flow of saturated debris occurs. In “Q” beds both of the other bed types occur but are of a spatially and temporally transitory nature. Thus this latter bed type may be the most common beneath ice masses. It is under an “M”-bed state that many drumlin subglacial bedforms and associated forms are thought to develop. Discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed. It is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a “feedback effect”. Such conditions are thought to be found most likely at the Lower interface of a fast moving ice mass possibly typical of an ice stream. In using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates, basal ice velocity, debris porosity and bed width are established with reference to mobile debris thicknesses. Using this relationship, a set of possible parameter values that would allow bed formation to be achieved are presented. The impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated. The wider effect of “M”-bedd conditionsd on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised.

Evidence, from microstructures, of deformable bed conditions within drumlins, Chimney Bluffs, New York State

Abstract Discussion on the process of drumlin formation has been ongoing for over a century. At present, two dominant schools of thought prevail, viz. formation by (a) subglacial hydraulic mechanisms, and (b) due to subglacial sediment deformation. The latter mechanism depends on the existence of a subglacial deformable bed. Evidence from part of the large central New York drumlin field reveals evidence of syn-depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in Chimney Bluffs State Park. These diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange-style deposition under subglacial deformable bed conditions. The evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement. It is likely that formation of the drumlins occurred under these same subglacial bed conditions.

Microstructures in Diamictites of the Lower Gowganda Formation (Huronian), Near Elliot Lake, Ontario: Evidence for Deforming-Bed Conditions at the Grounding Line?

ABSTRACT Diamictites examined in the lower Gowganda Formation (2.5-2.2 Ga) to the north of Elliot Lake reveal a large number of macrostructures and microstructures. From micromorphological analyses the microstructures show a group of structures indicative of both ductile and brittle deformation likely developed during glacial sedimentation. The diamictites possibly were deposited in a proximal subglacial or subaqueous environment as a product of soft deforming-bed sediments being extruded beneath an active ice sheet at its grounding-line margin as it entered a marine basin. This is the first known example of glacial deforming-bed conditions being shown to have possibly developed beneath Precambrian ice sheets.

Drumlins — Products of controlled or uncontrolled glaciodynamic response?

Abstract Two schools of thought on the process of drumlin formation have recently developed that have created a controversy within this ara of glacial geology. One hypothesis contends that drumlins are, in the main, developed within a soft mobile subglacial debris zone. The other school argues that for those drumlins that contain primary deposited stratified sediment, formation during catastrophic subglacial flood events is necessary. A classification of subglacial bed types is presented that allows for the accommodation of both schools. It is proposed that a new paradigm for subglacial research may be needed in order that a closer relationship between glaciodynamics and sedimentological processes be established in future attempts at explaining glacial processes and froms.

Subglacial bedforms—an introduction

Abstract This introduction provides of a brief review of a set of papers based on a Special Symposium held at the XII INQUA Congress, Ottawa, 1987. Attention is also drawn to the major concepts and controversies behind the formation of subglacial bedforms and to major issues for future research.

Sedimentological and micromorphological examination of a Late Devensian multiple diamicton sequence near Moneydie, Perthshire, east-central Scotland.

Synopsis Re-investigation of a site at Shochie Burn near Moneydie in east-central Scotland suggests that earlier interpretations need to be modified. At this site, in the past, two tills were recognized as indicative of two separate ice advances. Previously, this site was known as a stratotype for the Perth Readvance in eastern Scotland. The evidence for a regional readvance is no longer accepted but the two-till problem remains unexplained. Present investigations suggest that the two tills are part of a large package of sediments deposited as stacked layers of a subglacial deforming bed that have been tectonized by a very short and local ice readvance perhaps of the type that occurs as a winter readvance. Sedimentological and micromorphological evidence points to the tills as not being separate but of the same depositional phase undergoing subglacial deformation and/or local marginal tectonization following subglacial transport in a deforming bed environment resulting in the incorporation of proximal outwash sediments. The till at Shochie Burn exhibits evidence, both at the macro- and microscale, of intense deformation under high porewater pressures that are attributable to the strong plasmic microstructures, fold structures, shear zones, Riedel shears, and of possible submarginal freezing conditions resulting in localized brecciation and foliation structure development. This is the first site in this area to indicate the presence of subglacial deforming bed conditions followed by marginal glaciotectonism, therefore continued research is necessary to establish this modified view of subglacial conditions in this part of Scotland.