David J. Graham

Graphical representation of particle shape using triangular diagrams: an excel spreadsheet method.

Analysis of the shape of sedimentary particles can provide information about their transport history and aid facies differentiation and the characterization of depositional environments. Triangular (Sneed and Folk) diagrams, employing ratios of the three orthogonal particle axes, have been advocated as the most appropriate method for unbiased presentation of primary particle shape data. A spreadsheet method for the production of these diagrams is described. Clast data-sets from a range of environments are presented using this method. An alternative use of the spreadsheet for the presentation of sedimentary fabric shape is suggested. Copyright

Moraine-mound formation by englacial thrusting: the Younger Dryas moraines of Cwm Idwal, North Wales

Abstract The Younger Dryas (c. 11-10 ka bp) moraine-mound complex (‘hummocky moraine’) in the historically important site of Cwm Idwal, North Wales, has previously been interpreted using periglacial, subglacial, ice-marginal and englacial models. In this paper the morphology and sedimentology of these landforms is described and the competing hypotheses tested against this evidence. It is demonstrated that an englacial thrusting model, developed for polythermal glaciers in Svalbard, best fits the available evidence. Thrusting probably resulted from longitudinal compression against a reverse bedrock slope, although a frozen snout, downglacier of sliding ice, may also have been a trigger. It is suggested that the role of ice-deformation, especially thrusting, in landform development has been underestimated, and that the englacial thrusting model may find application in the interpretation of other sites in the palaeo-landform record.

Conversions of Surface Grain-Size Samples Collected and Recorded Using Different Procedures

AbstractInformation about the grain-size distribution of the surface layer of sediment exposed on riverbeds is often critical in studies of fluvial hydraulics, geomorphology, and ecology. A variety of sampling and analysis techniques are in common usage that produce grain-size distributions that are not directly comparable. This paper seeks to explore the appropriate conversions between different types of surface grain-size sampling methods. This is particularly timely in the light of increasingly widespread use of automatic and semiautomatic image-based measurement methods, the comparability of which with conventional measurement methods is relatively poorly constrained. For conversions between area-by-number (paint-and-pick) and grid-by-number (pebble-count) samples, the empirically derived conversion factor (±2.2) was found to be greater than that predicted by the Kellerhals and Bray model (±2), but the errors associated with using the value predicted by the model were small (3.8% in mm). For conversio...

Comment: Photographic Techniques for Characterizing Streambed Particle Sizes

This article was published in the journal, Transactions of the American Fisheries Society [© American Fisheries Society] and is also available at: http://afs.allenpress.com/perlserv/?request=get-toc&issn=1548-8659&volume=134&issue=6

Origin and significance of dispersed facies basal ice: Svínafellsjökull, Iceland

Dispersed facies basal ice - massive (i.e. structureless) ice with dispersed debris aggregates - is present at the margins of many glaciers and, as a product of internal glacial processes, has the potential to provide important information about the mechanisms of glacier flow and the nature of the subglacial environment. The origin of dispersed facies is poorly understood, with several hypotheses having been advanced for its formation, and there is disagreement as to whether it is largely a sedimentary or a tectonic feature. We test these established hypotheses at the temperate glacier Svinafellsjokull, Iceland, and find that none fully account for dispersed facies characteristics at this location. Instead, dispersed facies physical, sedimentological and stable-isotope (d 18 O, dD) character- istics favour a predominantly tectonic origin that we suggest comprises the regelation and strain- induced metamorphism of debris-rich basal ice that has been entrained into an englacial position by tectonic processes operating at the base of an icefall. Further thickening of the resultant dispersed facies may also occur tectonically as a result of ice flow against the reverse bed slope of a terminal overdeepening. Lack of efficient subglacial drainage in the region of the overdeepening may limit basal melting and thus favour basal ice preservation, including the preservation of dispersed facies. Despite the relatively low sediment content of dispersed facies (� 1.6% by volume), its thickness (up to 25 m) and ubiquity at Svinafellsjokull results in a significant contribution to annual sediment discharge (1635-3270 m 3 a -1 ) that is � 6.5 times that contributed by debris-rich stratified facies basal ice.

The search for seismic signatures of movement at the glacier bed in a polythermal valley glacier

Abstract A passive seismology experiment was conducted across the main overdeepening of Storglaciären in the Tarfala valley, northern Sweden, to investigate the spatial and temporal distribution of basal microseismic waveforms in relation to known dynamics of this small polythermal sub-arctic glacier. The high ablation rate made it difficult to keep geophones buried and well coupled to the glacier during the experiment and reduced the number of days of good-quality data collection. The characterization of typical and atypical waveforms showed that the dominant waveforms were from near-surface events such as crevassing. Waveforms resembling basal microseismic signals were very rare, and seldom observed on more than two seismic stations simultaneously. The analysis of waveforms, amplitudes and particle motions suggested a near-field origin for most events. Even though basal sliding is known to occur in the overdeepening, no convincing examples of basal waveforms were detected, suggesting basal microseismic signals are rare or difficult to detect beneath polythermal glaciers like Storglaciären. We discuss the reasons for failing to locate basal signals, consider the origin of common waveforms and make recommendations for setting up passive seismology experiments on glaciers with high ablation rates.

Book Review: Image analysis, sediments and paleoenvironments

Book review of: Image analysis, sediments and paleoenvironments (Developments in Paleoenvironmental Research, Volume 7). Edited by Pierre Francus, Dortrecht: Springer, 2004, hardback ISBN 1-4020-2061-9.