Brice R. Rea

Geomorphology and sedimentology of surging glaciers: a land-systems approach

The identification of surging glaciers and ice streams in glaciated landscapes is of major importance to the understanding of ice-sheet dynamics and for reconstructing ice sheets and climate. No single landform or diagnostic criterion has yet been found with which to identify surging glaciers. A surging-glacier land-system model is constructed using observations and measurements from contemporary surging-glacier snouts in Iceland, Svalbard, U.S.A. and Canada for differentiating ancient surging margins from other non-surging palaeoglaciers. This integrates the suite of landforms, sediments and stratigraphy produced at surging-glacier margins. Landforms produced during surging include thrust moraines, concertina eskers and subglacial crevasse-squeeze ridges. Sedimentary sequences are usually characterized by multiple stacked diamictons and stratified interbeds, which display severe glaciotectonic contortion and faulting. Hummocky moraine, comprising interbedded stratified sediments and mass-flow diamictons, has also been associated with surge margins where large quantities of supraglacial and englacial debris entrained during the surge event have melted out in situ. An example of the application of the land-system model is presented for east-central Alberta, Canada. A surging palaeo-ice stream is identified within this part of the southwestern Laurentide ice sheet, where thrust-block moraines, crevasse-squeeze ridges, flutings, hummocky moraine and glaciotectonized sediments are juxtaposed.

Surficial geology and geomorphology of the Brúarjökull surging glacier landsystem

Abstract Please click here to download the map associated with this article. A 1:30,000 scale map poster of the snout and proglacial landscape of the surging Icelandic glacier Brúarjökull, provides a spatial and temporal assessment of the geomorphic impacts of surging in glaciated terrains. Based upon aerial photography from 1998, 1999 and 2000, the map identifies the major landforms that are regarded as diagnostic of glacier surging when viewed in a landsystem framework; specifically, thrust block and push moraines, overridden thrust block moraines, zig-zag eskers, crevasse squeeze ridges, long utings, hummocky moraine and ice-cored, pitted outwash. This landscape imprint, when identi_ed in ancient glaciated terrains, can therefore be used as indicative of surge activity in palaeoglaciological reconstructions.

Ice stream motion facilitated by a shallow-deforming and accreting bed

Ice streams drain large portions of ice sheets and play a fundamental role in governing their response to atmospheric and oceanic forcing, with implications for sea-level change. The mechanisms that generate ice stream flow remain elusive. Basal sliding and/or bed deformation have been hypothesized, but ice stream beds are largely inaccessible. Here we present a comprehensive, multi-scale study of the internal structure of mega-scale glacial lineations (MSGLs) formed at the bed of a palaeo ice stream. Analyses were undertaken at macro- and microscales, using multiple techniques including X-ray tomography, thin sections and ground penetrating radar (GPR) acquisitions. Results reveal homogeneity in stratigraphy, kinematics, granulometry and petrography. The consistency of the physical and geological properties demonstrates a continuously accreting, shallow-deforming, bed and invariant basal conditions. This implies that ice stream basal motion on soft sediment beds during MSGL formation is accommodated by plastic deformation, facilitated by continuous sediment supply and an inefficient drainage system.

Surging glacier landsystem of Tungnaárjökull, Iceland

Abstract Please click here to download the map associated with this article. A1:30,000 scale map of the snout and proglacial landscape of the surging Icelandic glacier Tungnaárjökull, based upon aerial photography from 1995, immediately after a surge, allows an assessment of the spatial variability in landform-sediment imprints of catastrophic glacier advance across upland bedrock ridges. The ice-margin parallel alignment of the bedrock ridges locally strongly directs proglacial meltwater drainage and initiates strong compression in the ice during surging, resulting in the development of prominent ice-cored hummocky moraine composed of glacifluvial sediment. Diagnostic surge landforms elsewhere on the foreland include thrust block and push moraines, overridden ice-cored thrust block moraines, crevasse squeeze ridges, long flutings, hummocky moraine and ice-cored, pitted outwash.

Rock weathering in blockfields: some preliminary data from mountain plateaus in North Norway

Abstract The formation of blockfields is a process usually attributed to weathering. In mountain areas this is generally assumed to be mechanical weathering (frost shattering). Evidence from two high plateaus [900 and 1350 m above sea level (a.s.l.)] in North Norway (c. 70°N) suggests that chemical action is at least as important as mechanical activity in blockfield formation. The bedrock in both areas consists of complex banded gabbros. Blockfields circumscribe ice masses and are generally > 1 m thick. They contain high percentages of material in the silt and clay sized fractions, including a variety of clay minerals: gibbsite, chlorite, vermiculite and kaolinite, as well as magnetite/maghemite. The blockfield thickness and presence of these weathering products suggests both a considerable (pre-Pleistocene) length of time required for development as well as warmer conditions than are found now (mean annual air temperature c. 0°C) or in the period since deglaciation. It is suggested that these blockfields represent a preglacial palaeosurface which formed initially under warmer conditions and has survived, largely intact, beneath all the Pleistocene ice sheets.

GlaRe, a GIS tool to reconstruct the 3D surface of palaeoglaciers

Glacier reconstructions are widely used in palaeoclimatic studies and this paper presents a new semi-automated method for generating glacier reconstructions: GlaRe, is a toolbox coded in Python and operating in ArcGIS. This toolbox provides tools to generate the ice thickness from the bed topography along a palaeoglacier flowline applying the standard flow law for ice, and generates the 3D surface of the palaeoglacier using multiple interpolation methods. The toolbox performance has been evaluated using two extant glaciers, an icefield and a cirque/valley glacier from which the subglacial topography is known, using the basic reconstruction routine in GlaRe. Results in terms of ice surface, ice extent and equilibrium line altitude show excellent agreement that confirms the robustness of this procedure in the reconstruction of palaeoglaciers from glacial landforms such as frontal moraines. GlaRe is a tool for palaeoglacier 3D surface reconstruction from bed topography.GlaRe is coded in Python and runs in ArcGIS as a toolbox.The theory of perfect plasticity equilibrium glacier profile is considered.Interpolation methods for glacier 3D surface creation are described and discussed.GlaRe is tested with two extant glaciers, showing a small, acceptable error.

Contemporaneous, localized, basal ice-flow variations : implications for bedrock erosion and the origin of p-forms

A detailed study of a proglacial bedrock site and a subglacial cavity of an outlet of Oksfjordjokelen, Norway, is presented together with observations from the foreland of Konowbreen, Spitsbergen. Striation directions and subglacial observations indicate that local ice-flow paths were highly variable, deviating at angles of approximately 90° from the main ice-flow direction. Stepped bedrock topography appears conducive to the production of highly variable ice-flow paths, because the high bed roughness creates a locally variable stress regime within the ice, including low-pressure, lee-side areas into which ice can flow. If ice flow is sustained along a specific path and the ice contains debris, then abrasion should produce an erosional bedform. Models are proposed whereby locally variable ice-flow patterns could produce erosional bedforms, which would be described as p-forms, purely through mechanical abrasion.

Using UAV acquired photography and structure from motion techniques for studying glacier landforms: application to the glacial flutes at Isfallsglaciären

© 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Glacier and ice sheet retreat exposes freshly deglaciated terrain which often contains small-scale fragile geomorphological features which could provide insight into subglacial or submarginal processes. Subaerial exposure results in potentially rapid landscape modification or even disappearance of the minor-relief landforms as wind, weather, water and vegetation impact on the newly exposed surface. Ongoing retreat of many ice masses means there is a growing opportunity to obtain high resolution geospatial data from glacier forelands to aid in the understanding of recent subglacial and submarginal processes. Here we used an unmanned aerial vehicle to capture close-range aerial photography of the foreland of Isfallsglaciaren, a small polythermal glacier situated in Swedish Lapland. An orthophoto and a digital elevation model with ~2 cm horizontal resolution were created from this photography using structure from motion software. These geospatial data was used to create a geomorphological map of the foreland, documenting moraines, fans, channels and flutes. The unprecedented resolution of the data enabled us to derive morphological metrics (length, width and relief) of the smallest flutes, which is not possible with other data products normally used for glacial landform metrics mapping. The map and flute metrics compare well with previous studies, highlighting the potential of this technique for rapidly documenting glacier foreland geomorphology at an unprecedented scale and resolution. The vast majority of flutes were found to have an associated stoss-side boulder, with the remainder having a likely explanation for boulder absence (burial or erosion). Furthermore, the size of this boulder was found to strongly correlate with the width and relief of the lee-side flute. This is consistent with the lee-side cavity infill model of flute formation. Whether this model is applicable to all flutes, or multiple mechanisms are required, awaits further study.

Glacial geomorphology of the Tweedsmuir Hills, Central Southern Uplands, Scotland

The Quaternary glacial history of the Tweedsmuir Hills, Central Southern Uplands, Scotland, has received little attention since the 1980s, with earlier studies focussing on single lines of geomorphic evidence in isolated valleys. This study presents the first systematic glacial geomorphological assessment of the region, covering approximately 300 km2 with the map designed to be presented at A0. Mapping from remotely sensed imagery and field investigation reveal a large number of moraines and meltwater channels, both within valleys and occasionally extending to the plateau, alongside a range of peri- and para-glacial features, including solifluction lobes, alluvial fans, debris cones, river terraces and rock slope failures. Aspects of the mapped geomorphology are consistent with plateau icefield landsystems mapped elsewhere in Britain and this will hopefully form the basis for palaeoglaciological reconstructions which will improve our understanding of the extent and dynamics of former ice masses in the region.

Late Weichselian deglaciation and sea level history of St Jonsfjorden, Spitsbergen: A contribution to ice sheet reconstruction

Abstract The elevation and age of surveyed shorelines in St Jonsfjorden, western Spitsbergen, provide a chronology for Late Weichselian deglaciation and a proxy for former ice thickness. Support is provided for previous reports of two sets of shorelines on NW Spitsbergen, the oldest (at 60 m in St Jonsfjorden) dating to a pre‐late Weichselian glaciation and the youngest documenting postglacial rebound. The remarkable survival of the older shorelines suggests that a large area of NW Spitsbergen was either covered by protective, cold‐based ice or was subject to only local glaciation during the late Weichselian even though glacier ice extended to the shelf edge in the Isfjorden Trough. A marine limit of 46 m at the mouth of St Jonsfjorden is compatible with the regional late Weichselian marine limit (LWML) dated to ∼13 ka BP. A partial relative sea level curve and equidistant shoreline diagram are produced for St Jonsfjorden and used in conjunction with relative sea level curves based on data from previous studies to produce an isobase map for 9 ka BP. The curves indicate that western sites were subject to initial slow emergence followed by rapid emergence between 10 ka and 8.2 ka BP, trends characteristic of areas affected by a full glacial sea or at least subject to early break up of a thin or discontinuous glacier ice cover. A step in the 9 ka BP shoreline long profile over the western end of the peninsula separating Isfjorden and van Mijenfjorden suggests that Holocene neotectonics may have produced anomalously high rebound rates in that area.