Simon J. Carr

Till fabric patterns and significance: particle response to subglacial stress

Abstract The analysis of particle arrangements within glacial sediments (fabric) has long been considered to reflect the behaviour of the ice that transported and deposited, or deformed the sediment. Particles can develop specific fabric arrangements as a response to bulk sediment strain driven by effective stress at the bed of a glacier. As such, fabric analysis has been used to try and understand the complex relationship between glacier and ice sheet dynamics and the geological evidence of former glaciation. Developments have focused on the use of stereonet patterns and statistical measures (vector and eigenvector analysis) of clast fabric to infer the process of sediment deposition/deformation, and consequent ice flow directions and dynamics. However, as this study identifies, fundamental processes of particle orientation (March, Jeffery and Taylor rotation mechanisms) would suggest more complex explanations than have been previously reported. This study reports the results of a detailed field and laboratory investigation of macro-scale (clast) and microfabrics from a till sequence in Central Scotland. The results suggest a systematic behaviour of different particle size fractions, which may be interpreted in terms of the strain response of sediment. Three fabric patterns are identified that illustrate consistent, progressively shifting and more complex particle behaviour. It is suggested these orientation patterns indicate the degree to which subglacial sediment has undergone strain, and the nature of the deforming bed (erosional or accretionary). It is concluded that this approach may yield fundamental information about the strain conditions underlying different ice masses, providing critical data linking glaciological modelling with the sedimentary evidence of glaciation.

Micromorphological criteria for discriminating subglacial and glacimarine sediments: evidence from a contemporary tidewater glacier, Spitsbergen

Abstract Sedimentological and micromorphological evidence is presented from four sites within St. Jonsfjorden, western Spitsbergen. Sedimentological and geomorphological evidence is used to distinguish sites comprising undisturbed glacimarine sediments from glacially over-ridden glacimarine sediments and subglacial diamicts. Impregnated thin sections are used to define associations of micromorphological characteristics that allow diamicts formed under these different conditions to be discriminated. In situ glacimarine deposits display winnowed textures, dropstone structures and distinct vertical orientations of sand grains, whilst subglacial conditions result in structures indicative of deformation under moderate to high stress regimes. These criteria may help to determine the origin of ancient and Quaternary diamicts of questionable subglacial or glacimarine origin.

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.

A glaciological approach for the discrimination of Loch Lomond Stadial glacial landforms in the Brecon Beacons, South Wales

The interpretation of small ridge systems observed within cirques is often ambiguous and glacial, periglacial and mass-movement origins have been proposed. Much of this ambiguity centres around the interpretation of purely geomorphological data, with little consideration of the glaciological constraints upon the dimensions of small snowpatches and niche glaciers. Application of a glaciological approach to reconstruction, using an estimation of former glacier ablation gradient and the temperature at the reconstructed equilibrium line allows the calculation of glacier mass-balance, total velocity and basal slippage, which may be compared with observed data from contemporary glaciers. The approach outlined is applied to five sites in the Brecon Beacons, South Wales and suggests that four sites reflect upland glaciation during the Loch Lomond Stadial. A fifth site is rejected as being of glacial origin at this time and is suggested to reflect earlier glaciation during different environmental conditions, probably during final wastage of the Last Glacial Maximum Welsh ice cap.

The North Sea basin

There has been a substantial volume of research examining glaciation of the North Sea basin, and considerable controversy regarding specific glacial episodes or ice-sheet extents. This chapter summarizes the key themes relating to the glaciation of the North Sea and identifies key areas that need further analysis. There is scattered evidence for extensive glaciation in Europe including the North Sea during the Menapian stage of the early Quaternary. The dominant river system feeding the North Sea delta flowed westward across Poland and northern Germany, draining the entire Baltic region and Eastern Europe. The absence of sediments of east European provenance in North Sea sediments younger than the Menapian led Bijlsma to suggest that the depositional system of Europe had been comprehensively altered by regional glaciation, scouring a “proto-Baltic” basin. Until the late 1940s, all models suggesting extensive glaciation of the North sea were based on inference from tills identified in coastal sections along the margins of the basin or from rare seabed samples trawled up in fishing nets. Two critical developments, in seismic analysis, and the recovery of shallow sediment cores, revolutionized the understanding of the glacial history of the North Sea.

LATE QUATERNARY MORAINES ALONG THE SEKHOKONG RANGE, EASTERN LESOTHO: CONTRASTING THE GEOMORPHIC HISTORY OF NORTH- AND SOUTH-FACING SLOPES

Abstract. Considerable Quaternary environmental reconstruction for the high Drakensberg is based on geomorphological and sedimentological work undertaken along the northern aspects of the Sekhokong mountain range of eastern Lesotho. Given that no previous investigations have focused on the southern aspects, this paper documents the observed geomorphology and provides a more complete palaeo‐environmental picture for this range. Data on the morphology, sedimentology and micromorphology for two linear debris ridges are presented. It is demonstrated that the two ridges are most likely moraines originating from a small niche glacier. The combined use of macro‐ and micro‐scale sedimentology is proven to be an essential tool in ascribing a glacial process origin for the landforms, given the complex depositional history they have undergone. AMS ages obtained from the deposits (14 700 cal. yrs bp and 19 350 cal. yrs bp) places these in the time‐scale of the Last Glacial Maximum. The study demonstrates rather contrasting aspect‐controlled palaegeomorphological environments along the Sekhokong range, which is also reflected in the dissimilar contemporary biophysical micro‐environments. It is suggested that the south‐facing slopes were dominated by glacial processes during the Last Glacial Maximum (LGM), as is evident from the moraines, while the proposition for previously described north‐facing glacial cirques is rejected based on the absence of erosional/depositional evidence and greater insolation received on these warmerequator‐facing slopes. Rather, we propose that the observed north‐facing hollows are a product of a multitude of geomorphic processes spanning several tens to hundreds of thousands of years.

Role of particle size in till-fabric characteristics: systematic variation in till fabric from Vestari-Hagafellsjökull, Iceland

Till-fabric analysis has often been used to interpret glacier flow directions and subglacial dynamics using vector-based statistics, but recent data suggest that such analysis may also effectively indicate former glacier dynamics. The results of a fabric investigation of subglacially strained till deposited during a surge of Vestari-Hagafellsjokull, Iceland, are presented. Till fabric was collected at four sites within a limited area where ice-flow direction during deposition was known from subglacial bedforms at the site. Analysis was carried out on elongate grains (axial ratio <1.5:1) at seven size fractions, with the a-axis length ranging from 0.25 to 32 mm. The largest grains tend to be parallel to ice flow, whereas smaller grains reflect a mix of parallel and transverse orientations. The implications of these data for the role and validity of till-fabric analysis are discussed, with reference to vector analyses and the compilation of fabric shape envelopes. It is noted that, in contrast to laboratory experimental data, neither March nor Jeffery mechanisms explain the fabric configurations reported. It is concluded that a standardized approach is necessary for collecting fabric data, and in many situations analysis of data populations at a range of particle sizes is desirable.

Complex relationships between younger dryas glacial, periglacial and paraglacial landforms, Brecon Beacons, South Wales

The Brecon Beacons represent the most southerly area subject to marginal glaciation during the Younger Dryas/Loch Lomond Stadial in the British Isles and the interpretation of upland landforms in this area therefore has particular palaeo-environmental significance. This study reports geomorphological evidence within two trough-heads in the central Brecon Beacons previously thought to reflect Younger Dryas glaciation. A complex suite of landforms is identified at both sites, reflecting a range of environmental processes dating from throughout the Lateglacial period. Possible Younger Dryas terminal and recessional moraines are identified at each site, consisting of passively and actively transported debris, which have subsequently undergone considerable paraglacial modification. A number of other non-glacial landforms and sediments are identified, including a protalus/pronival rampart. The initial interpretations are tested using a glaciological approach of reconstructing former glacier dynamics, and suggests that some of the terminal moraines observed reflect mountain glaciation prior to the Younger Dryas, and may demonstrate final stages of retreat from the Last Glacial Maximum (LGM). A morphostratigraphic sequence for both sites is reconstructed, reflecting landscape evolution in a range of environments from final LGM deglaciation through to Holocene landscape stability. It is concluded that associations of such landforms may be common in marginal areas of former glaciation, and that interpretations of former niche/cirque glaciation should be supported by the reconstruction of probable mass-balance, glacier dynamics and palaeoclimatic implications to improve on geomorphological speculation.

10 Mýrdalsjökull's Forefields Under the Microscope. The Micromorphology of Meltout and Subglacial Tills

Abstract Samples for micromorphological analyses were collected in the forefields of three Mýrdalsjokull outlet glaciers and lobes. Samples represent supraglacial meltout deposits, subglacial tills and related water escape structures (WES) and lacustrine/pond deposits. Micromorphology revealed that the meltout tills possess a distinct suite of microstructures, most notably coatings around grains. Together with the particular porosity, meltout deposits can be clearly distinguished from subglacial tills, even from the strongly dilated Mýrdalsjokull tills. WES and lacustrine deposits possess overlapping structures, but porosity and especially plasmic fabric help in distinguishing the groups and separate them from meltout and subglacial sediments.

The impact of pre-restoration land-use and disturbance on sediment structure, hydrology and the sediment geochemical environment in restored saltmarshes.

Saltmarshes are being lost or degraded as a result of human activity resulting in loss of critical ecosystem services including the provision of wild species diversity, water quality regulation and flood regulation. To compensate, saltmarshes are being restored or re-created, usually driven by legislative requirements for increased habitat diversity, flood regulation and sustainable coastal defense. Yet, there is increasing evidence that restoration may not deliver anticipated ecosystem services; this is frequently attributed to poor drainage and sediment anoxia. However, physical sediment characteristics, hydrology and the sediment geochemical environment are rarely examined in restoration schemes, despite such factors being critical for plant succession. This study presents the novel integration of 3D-computed X-ray microtomography to quantify sediment structure and porosity, with water level and geochemical data to understand the impact of pre-restoration land use and disturbance on the structure and functioning of restored saltmarshes. The study combines a broad-scale investigation of physical sediment characteristics in nine de-embanked saltmarshes across SE England, with an intensive study at one site examining water levels, sediment structure and the sediment geochemical environment. De-embankment does not restore the hydrological regime, or the physical/chemical framework in the saltmarshes and evidence of disturbance includes a reduction in microporosity, pore connectivity and water storage capacity, a lack of connectivity between the sub-surface environment and overlying floodwaters, and impeded sub-surface water flow and drainage. This has significant consequences for the sediment geochemical environment. This disturbance is evident for at least two decades following restoration and is likely to be irreversible. It has important implications for plant establishment in particular, ecosystem services including flood regulation, nutrient cycling and wild species diversity and for future restoration design.