Clive Auton

Polyphase deformation at an oscillating ice margin following the Loch Lomond Readvance, central Scotland, UK

The sequence of glacitectonic disturbance of an ice-contact delta during the initial stages of deglaciation following the Loch Lomond Readvance is examined. An ice-marginal, Gilbert-type deltaic sequence exposed in Drumbeg quarry, Drymen (Scotland) displays a polyphase deformation history that is punctuated by periods of erosion and deposition. Progradation of the delta into ice-dammed Lake Blane led to a temporary stabilisation of the Loch Lomond glacier during recession from its Loch Lomond Readvance maximum position. This was followed by a NE-directed readvance into the ice-contact slope of the delta, resulting in proglacial deformation (D1) and the formation of a thrust-block ridge. In the middle to outer parts of the delta complex, sedimentation continued uninterrupted. Subsequent retreat of the ice was accompanied by erosion and deposition as the ice-contact slope was re-established. A second phase of readvance resulted in subglacial, ductile shearing (D2) and deposition of a diamicton during NE-directed over-riding of the delta sediments by the ice. The direction of ice flow subsequently changed towards the SE. This was followed by a further retreat of the ice and re-establishment of the fan-delta complex. The complexity of the glacitectonic sequence at Drumbeg records the impact of an oscillating ice margin responding to either relative deepening of the lake waters in contact with the receding snout, or climatic controls. It provides further evidence that many of the Scottish Highland glaciers were subject to active recession rather than stagnation at the end of the Loch Lomond Readvance.

Ice-proximal glaciomarine sedimentation and sea-level change in the inverness area, Scotland: A review of the deglaciation of a major ice stream of the British Late Devensian ice sheet

Abstract Evidence of both rising and falling relative sea levels and glacitectonic movements is preserved in two formations of raised glaciomarine deposits that were laid down in front of an oscillating ‘grounded’ tidewater glacier in the Inverness Firth. These changes occurred during the latter stages of the disintegration of the Moray Firth ice-stream, one of the major ice streams that drained the British main Late Devensian ice sheet. Most of the glaciomarine deposits antedate a sequence of glacio-isostatically tilted Late Devensian marine shorelines and associated littoral and estuarine deposits. The shorelines began forming at about 13,000 BP and record a progressive fall in relative sea level. A new model for the deglaciation of the Moray Firth region is proposed after a critical appraisal of published accounts of both onshore and offshore Quaternary sequences. The disintegration of the Moray Firth ice stream involved several rapid phases of retreat to pinning points, caused by iceberg calving and triggered by rising global sea level. Each retreat was followed by minor readvances or stillstands, possibly caused by short-lived accelerated periods of glacio-isostatic rebound and concomitant temporary falls in relative sea level. Two such events occurred in the Inverness Firth: the Ardersier Oscillation and the Alturlie Stillstand. Substantial differences (lower relative sea levels, later deglaciation) are apparent between the pattern of ice-retreat in the Moray Firth region and published accounts of the deglaciation of the Irish Sea basin. These differences require a reassessment of some current hypotheses concerning the disintegration of major ice streams associated with high relative sea levels. Furthermore, geological and geomorphological evidence suggesting both rising and falling sea levels in the Inverness area, prior to ca. 13,500 BP, is not fully compatible with recently published computer simulations of the dissolution of the British main Late Devensian ice sheet.

Micromorphological evidence for polyphase deformation of glaciolacustrine sediments from Strathspey, Scotland

Abstract A combination of field investigation and micromorphological analysis has been applied to polydeformed Late Devensian rhythmites and glacigenic diamicton, exposed in Strathspey, Scotland. This provided information on the geometry, kinematics and relative ages of ductile and brittle structures, and records a complex subglacial deformation history. The deformation is interpreted as resulting from a single progressive event, associated with over-riding of proglacial lake sediments by wet-based ice. The earliest deformation (‘D1’) resulted from compaction/loading (pure shear) and imposed a bedding-parallel (S1) fabric throughout the rhythmites. S1 was subsequently deformed by kink bands and minor ductile shearing during ‘D2’. A later ‘D3’ event, characterized by soft-sediment deformation and fluidization of matrix-poor sands, was accompanied by an increase in pore water pressure. This lead to hydrofracturing of the rhythmites. The most intense deformation (‘D4’), which resulted from simple shear, was partitioned into the upper part of the sequence. It produced folding, thrusting and brittle microfaulting in response to NNW-directed ice-push. These findings indicate that, in general, subglacial deformation is not homogeneous and can extend to depths of >3 m below the presumed ice-sediment interface.

Mapping groundwater vulnerability in Scotland: a new approach for the Water Framework Directive

Synopsis A new methodology for groundwater vulnerability assessment has been devised for Scotland to meet the requirements of the Water Framework Directive. Using the methodology, a new GIS-based map of groundwater vulnerability has been produced, at a working scale of 1:100 000. The map is being used by the Scottish Environment Protection Agency (SEPA) to help characterize and assess risk to groundwater bodies. The methodology assesses the vulnerability of groundwater in the uppermost aquifer to the vertical downward movement of a non-specific contaminant from the ground surface. It considers the intrinsic properties of the pathway between the ground surface and the water table. The key difference from previous vulnerability maps in Scotland and the rest of the UK is that the new method assesses vulnerability in all aquifers regardless of resource potential. This reflects the diverse environmental objectives for groundwater bodies under the Water Framework Directive. This approach provides the flexibility to combine the groundwater vulnerability map with maps of pressures, groundwater resources or other groundwater-related receptors, as required.

Ductile fault rocks and metamorphic zonation in the Dalradian of the Highland Border SW of Stonehaven, Kincardineshire

Synopsis High strain zones, characterized by shear fabrics and mylonites, have been identified within lower amphibolite-grade, coarse granular metasandstones of the Glen Lethnot Grit Formation of the Southern Highland Group (Dalradian) along the Highland Border, south-west of Stonehaven. The rocks comprise part of a polydeformed, regionally metamorphosed, Neoproterozoic sequence, bounded to the north by the post-orogenic Mount Battock Granite (Late Caledonian) and to the south-east by the Highland Boundary Fault Zone. Early regional D1 and D2 structures have been rotated by a major monoform, the Highland Border Downbend (D4), to produce a steeply dipping, downward facing succession. The mylonitic metasand-stones contain a variety of kinematic indicators, including asymmetrical pressure shadows, S-C fabrics, shear bands and an extensional crenulation cleavage, which yield a sense of shear of top-towards-the SE (present structural position). Regional syn- to post-D2 metamorphism resulted in the development of prograde pelitic mineral assemblages which overgrow earlier Dl structures. Mylonitic fabrics within the high strain zones, developed during D2, both deform and are also overgrown by syn- to post-kinematic biotite porphyroblasts. The high strain zones may represent part of a major ductile structure active during the Dalradian D2–D3 tectonothermal event, which contributed to the development of strong metamorphic gradients and narrowing of mineral assemblage zones within the Highland Border.

Switching flow patterns within the last ice sheet in northern Scotland

Synopsis A confluence zone existed within the northern sector of the last British Ice Sheet (BIS) where ice flowing from the Northern Highlands met ice streaming out of the Moray Firth and across the plain of Caithness. At Wester Clett, a complex 35m thick sequence of glacigenic sediments provides important evidence of dynamic interactions between these ice lobes that are consistent with recent computer simulations of the behaviour the last BIS. Sedimentological evidence suggests that early advance of Northern Highlands ice, probably between 33.5 and 30.5 ka, was followed by a first incursion of Moray Firth ice. Ice flowing out of Strath Halladale then deposited sands and gravels along its margin. Later, around the time of the Last Glacial Maximum, a powerful flow of Moray Firth ice diverted Northern Highlands ice to flow to the NW across the present coastline, depositing tills of mixed provenance. Cosmogenic exposure dates of c. 18 ka for summits in southern Caithness indicate that the ice sheet thinned subsequently, but flow to the NW continued. The youngest till and its associated moraine systems record a final movement of Northern Highlands ice. Subsequent ice retreat was accompanied by uncoupling of ice lobes at the Caithness–Sutherland border soon after 16 ka.

The glacial and interstadial sediments at the Burn of Benholm, Kincardineshire: evidence for onshore pre-Devensian ice movement in northeast Scotland

Pebbly clays and diamictons containing marine shell fragments and peat lenses exposed beneath subglacially deposited Late Devensian till at the Burn of Benholm provide new insights into the glacial history of Quaternary sequences in eastern Scotland. The peat yielded pollen of interstadial affinity (including Bruckenthalia spiculifolia) and non-finite radiocarbon dates. Comparisons with other pre-Late Devensian pollen records in northern Scotland suggest that the peat lenses are remnants of an Early Devensian interstadial deposit, of Oxygen Isotope Substage 5c or 5a age. Reworked faunal assemblages in the shelly sediments include Quaternary marine molluscs of low boreal aspect, as well as Mesozoic and Palaeozoic microfossils. Amino acid ratios from fragments of Arctica islandica suggest that the shells are of Oxygen Isotope Stage 9 age or older. The fabric and composition of the shelly sediments are consistent with their emplacement as deformation till during the onshore movement of glacially transported rafts of marine sediment. Folded and sheared contacts between the shelly deposits, peat lenses and the overlying Late Devensian till indicate that the fossiliferous sediments were glacitectonised during the main Late Devensian glaciation, when ice moved from Strathmore and overrode the site from the southwest. British Geological Survey.