Seasonal changes in basal conditions at Briksdalsbreen, Norway: the winter–spring transition

The winter–spring transition is a dynamic time within the glacier system, because it marks a period of instability as the glacier undergoes a change in state from winter to summer. This period is normally associated with sudden pressure fluctuations resulting in hydrological instabilities within the subglacial drainage system. New data are presented from wireless multi-sensor subglacial probes incorporated within the till at Briksdalsbreen, Norway. Water pressure readings recorded a two-phase winter–spring transition. Event 1 occurred early in the year (December–January) and marked the start of activity within the subglacial environment following the winter. However, this did not result in any permanent changes in subglacial activity and was followed by a period of quiescence. Event 2 occurred later in the year in accordance with changing external weather conditions and the retreat of the snow pack. It was characterized by high-magnitude pressure peaks and diurnal oscillations in connected regions. The variations in sensor trends that followed this event suggested that a transition in the morphology of the subglacial drainage system had occurred in response to these pressure fluctuations. Event 2 also showed some similarities with spring events recorded at valley glaciers in the Alps. A conceptual model is presented associating the form of the winter–spring transition with respect to the location of the probes within connected and unconnected regions of the subglacial drainage system. These data provide further evidence for temporal and spatial heterogeneous subglacial drainage systems and processes. The identification and analysis of subglacial activity during the winter–spring transition can contribute to the interpretation of hydro-mechanical processes occurring within the subglacial environment and their effect on glacier dynamics.     

Proglacial glaciotectonic deformation at Melabakkar-Ásbakkar, west Iceland

A study of proglacial deformation associated with a Late Weichselian glaciomarine sequence was carried out at Melabakkar-Ásbakkar, west Iceland. At this site, coarse-grained sediments have been deformed into compressive structures with no associated push moraine morphology. Two large structures were examined, Structure A which consists of large-scale reverse (and normal) faulting and overturned bedding; and Structure B, which is more complex, with open folding, high-angle reverse faulting, nappe structures and normal faulting. The structures were interpreted as the result of increasing compressive proglacial deformation, followed by subglacial deformation, which destroyed the surface morphology of the push moraine and incorporated some of the sediments into a subglacial diamicton. The results from this study were compared with other examples of proglacial deformation, and it is suggested that at sites where deformation was restricted to the margin, longitudinal strain was lower than at sites where deformation extended out into the foreland. It is also suggested that if deformation increases downglacier, this is indicative of an overall glacial advance, whilst if the deformation decreases downglacier, this is indicative of a glacial retreat.     

Exotic marbles on Cheshire cheese: High-street geology in Bath

Most shoppers in Bath ignore the cladding on the front of the'Co-op'. Those pausing to look probably consider it marble. In fact, it is Larvikite, a type of syenite commonly used to clad high-street stores throughout Britain. A survey of shop fronts, banks and other services in this city reveals a wide variety of exotic imported marbles and granites attached to local Bath Stone at street level. Modern stone cladding is also found to extend beyond the post-war shopping area. Stromatolitic limestones and porphyritic granites peep out beneath Victorian shop windows in Milsom Street. Moreover, not all the cladding in this street is found to be modern. Scottish and Shap Granites, introduced into Bath in the nineteenth century, clad former banks and a Victorian department store.     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

Proglacial glaciotectonic deformation associated with glaciolacustrine sedimentation,Lake Pukaki,New Zealand

The Quaternary glaciogenic sediments exposed on the southwest shore of Lake Pukaki were investigated. The sections consisted of the Pukaki Diamicton, which is composed of four lithofacies: (i) homogeneous facies (PDH)—a grey matrix-supported homogeneous subglacial diamicton; (ii) coarse facies (PDC)—a very coarse matrix-supported diamicton, which was interpreted as a proximal glaciolacustrine sediment; (iii) laminated facies (PDL)—a cream coloured, fine-grained, matrix-supported diamicton, with grade laminations of silt, sand and gravel, interpreted as a more distal glaciolacustrine facies; and (iv) fine facies (PDF)—a cream coloured fine-grained, silt-rich matrix-supported diamicton, with lenses of sand and gravels, which was interpreted as the most distal glaciolacustrine facies. It is suggested that these sediments were produced by two small ice advances during a period of general retreat. Furthermore, the sections showed a combination of three types of glaciotectonic deformation; gravity tectonics, proglacial glaciotectonics and subglacial glaciotectonics. Two of the moraines showed an unusual style of glaciotectonic deformation, i.e. proglacial deformation on the proximal face and gravitational slumping on the distal face. It is suggested that this style of deformation is diagnostic of proglacial deformation into a waterbody associated with a retreating margin.     

A new geobarometer for the assemblage biotite-muscovite-chlorite-quartz

The equilibrium thermodynamics of the reaction:
And the equilibrium constant is composed of activities formulated using ideal mixing on sites. Consideration is given to the evaluation of uncertainties in pressures calculated using the geobarometer. Preliminary testing suggests that the geobarometer has considerable potential. Much wider testing is now required.     

Recent Coastal Change in Canterbury- the case of Lake Forsyth/Wairewa

Documentary and anecdotal evidence is presented to supplement geomorphological evidence for the conversion of Lake Forsyth (Wairewa) from marine inlet to brackish to freshwater lake within near historic time. Growth of the barrier across the lake outlet is likely to continue in the future.     

Proglacial glaciotectonic deformation and the origin of the Cromer Ridge push moraine complex, North Norfolk, England

The landscape of northeast Norfolk is dominated by a high (>50 m) ridge which has been interpreted as an end moraine (Cromer Ridge). This feature is truncated by coastal erosion at Trimingham. Evidence of large- and small-scale compressive styles of deformation is found throughout the sequence, except at the very top, where late Anglian/early Hoxnian lake sediments are found within an undeformed kettle hole. The deformation consists of open folds (including chevron folds) and listric thrust faults. It is suggested that these are the result of a single compressive event, which was caused by proglacial glaciotectonic deformation. It is inferred that this deformation is due to a combination of frontal pushing and compressive stresses transmitted through a subglacial deforming wedge. It is also shown that strain increases towards the ice sheet margin, as reflected by the deformational styles (from open folding up-glacier to listric thrust faulting down-glacier). The Cromer Ridge is shown to be a push moraine complex related to an actively retreating ice margin.