- Email: [email protected]
Anglesey - a landscape carved by ice: a new web-based, interactive landform map for Anglesey North Wales from the British Geological Survey
Abstracts / Quaternary International 279-280 (2012) 346–461
of N2O during abrupt climate change and contribute to an explanation of the observed concentration changes in polar ice. BEDROCK CONTROLS ON SUBGLACIAL LANDFORM DISTRIBUTION AND GEOMORPHOLOGICAL PROCESSES BENEATH A PALAEO ICE STREAM Emrys Phillips. British Geological Survey, United Kingdom E-mail address: [email protected]
Ice streams play an important role as regulators of the behaviour of modern ice sheets, taking the form of corridors of fast flowing ice. Although the geomorphological and sedimentary signatures of palaeo ice streams have received significant attention, the influence of bedrock geology on the processes occurring beneath former ice streams is less well known. The role played by bedrock geology on landform distribution has recently been recognised from the ‘footprint’ left by a Late Weichselian Irish Sea Ice Stream. This major ice stream helped regulate the British and Irish Ice Sheet, feeding ice from its onset zone in southern Scotland, down through the Irish Sea, across Anglesey as far south as the Isles of Scilly at its maximum extent. The spatial relationships displayed between the landform assemblage zones and bedrock geology on Anglesey (NW Wales) and the Rhins of Galloway (SW Scotland), both sites occurring close to the eastern margin of the ice stream, provides clear evidence for a link between landform morphology and distribution, and the disposition of the main tectonostratigraphical units within the bedrock. Changes in landform morphology are shown to have been locally controlled by largescale faults (e.g. the Berw Fault, Anglesey) and/or major lithological boundaries (e.g. base Permian unconformity, Rhins of Galloway), with less durable bedrock lithologies influencing the location and lateral extent of relatively faster flowing portions of the ice stream. In contrast, bedrock highs acted as ‘sticky spots’ within the bed, with the orientation of penetrative, regional bedrock structures also potentially impeding forward motion. The variation in landform elongation ratio across both study areas has been used as a proxy for relative changes in the velocity of the overriding ice, enabling the construction of ice flow velocity and trajectory maps across these exposed parts of bed of the Irish Sea Ice Stream. SPLIT, SQUIRT AND SEAL: THE ANATOMY OF A HYDROFRACTURE SYSTEM Emrys Phillips. British Geological Survey, United Kingdom E-mail address: [email protected]
Hydrofracture systems within subglacial to ice marginal settings represent a visible expression of the passage of pressurised meltwater through these glacial environments and provide a clear record of fluctuating hydrostatic pressure. A detailed study of a hydrofracture system cutting Devonian sandstone bedrock exposed at the Meads of St. John, near Inverness (NE Scotland) has revealed that this complex, multiphase system was active over a prolonged period and accommodated a several phases of fluid flow. The main bedding-parallel, feeder conduits are linked by steeply inclined to subvertical, transgressive dykes developed along contemporaneous fractures and normal faults. Micromorphological analysis has revealed that the individual hydrofractures comprise three main stages of fill:
Stage 1 - Initial fracture propagation, followed by the introduction of a thinly laminated clay, silt and fine sand fill. The clay lined the hydrofracture walls, sealing the adjacent wallrock reducing its permeability; Stage 2 - Repeated reactivation of the hydrofracture and deposition of a thinly laminated sequence of coarse silt to fine sand. Sedimentary structures provide a record of palaeoflow and fluctuating porewater pressure during deposition; and Stage 3 - Late-stage liquefaction of earlier sand and silt fills and injection of cross-cutting veinlets of remobilised sediment, possibly in response to stress release during subsequent brittle failure.
379
A comparison of the results of this study with published engineering ‘hydrofrac’ data indicates that: (1) changes in the style of sedimentation can be directly related to the fluctuation in overpressure during hydrofracing; and (2) the overpressures required to reactivate the hydrofracture decreases over time. The development and repeated reactivation of subglacial hydrofracture systems has the potential dramatically increase the permeability of the bed, facilitating the migration of meltwater beneath glaciers and ice sheets.
ANGLESEY - A LANDSCAPE CARVED BY ICE: A NEW WEB-BASED, INTERACTIVE LANDFORM MAP FOR ANGLESEY NORTH WALES FROM THE BRITISH GEOLOGICAL SURVEY Emrys Phillips. British Geological Survey, United Kingdom E-mail address: [email protected]
During the last ice age, the island of Anglesey (NW Wales) occupied a unique position close to the eastern margin of a the Irish Sea Ice Stream; a fast moving corridor of ice which fed ice from central and southwest Scotland, through the Irish Sea basin, eventually reaching as far south as the Isles of Scilly. This ice stream, one of the largest identified within the Late Devensian British and Irish Ice Sheet, left a distinctive ‘footprint’ on Anglesey in the form of a distinct assemblage of subglacial landforms and sediments. A new web-based educational product describing the glacial history of Anglesey has been launched by the British Geological Survey (BGS). At its heart is a new landform map of the island draped over a colour shaded NEXTMap TM digital elevation model. Tabbed and dropdown menus, as well as embedded controls allow the user to navigate around this new interactive product, revealing additional information on the bedrock geology and landform assemblage zones, browse through a photo gallery containing images of the glacial landforms and sediments, access informative summaries of how the main glacial features (including drumlins, meltwater channels) were formed, as well as zoom into, and switch on and off the various layers contained within the interactive map. The target audience of the Anglesey i-map includes school students, geography and earth science teachers, undergraduates and academic researchers, as well as anyone interested in the landscape around them evolved. Free-todownload (A0 size, pdf format) maps and photo gallery wall posters are available as part of this new BGS product.
CRONUS-EARTH PROJECT: RESULTS Fred Phillips. New Mexico Tech, United States E-mail address: [email protected]
The CRONUS-Earth Project was funded by the U.S. National Science Foundation in 2005, with the objective of obtaining a definitive set of geological calibration samples to quantify the production rates of terrestrial cosmogenic nuclides and to resolve discrepancies in cosmogenic nuclide systematics and scaling. Samples have been obtained and analyzed from five primary calibration sites, and numerous secondary sites, yielding a data set that is much more consistent than those from previous studies. One important finding is that scaling methods based on neutron-monitor counting data over-predict nuclide production at high elevation. This at least partly results from response multiplicity in the neutron monitors. Low-energy neutron production of 36Cl has been calibrated using depthprofile data. Calculations indicate that scaling may vary among different nuclides, in particular for 3He. We have developed an on-line application to facilitate the calculation of internally consistent and globally applicable cosmogenic chronologies. 40
AR/39AR GEOCHRONOLOGY OF FELDSPAR MEGACRYSTS FROM LATE PLEISTOCENE VOLCANOES, NEWER VOLCANIC PROVINCE, SOUTHEAST AUSTRALIA David Phillips. The University of Melbourne, Australia
E-mail address: [email protected]
of N2O during abrupt climate change and contribute to an explanation of the observed concentration changes in polar ice. BEDROCK CONTROLS ON SUBGLACIAL LANDFORM DISTRIBUTION AND GEOMORPHOLOGICAL PROCESSES BENEATH A PALAEO ICE STREAM Emrys Phillips. British Geological Survey, United Kingdom E-mail address: [email protected]
Ice streams play an important role as regulators of the behaviour of modern ice sheets, taking the form of corridors of fast flowing ice. Although the geomorphological and sedimentary signatures of palaeo ice streams have received significant attention, the influence of bedrock geology on the processes occurring beneath former ice streams is less well known. The role played by bedrock geology on landform distribution has recently been recognised from the ‘footprint’ left by a Late Weichselian Irish Sea Ice Stream. This major ice stream helped regulate the British and Irish Ice Sheet, feeding ice from its onset zone in southern Scotland, down through the Irish Sea, across Anglesey as far south as the Isles of Scilly at its maximum extent. The spatial relationships displayed between the landform assemblage zones and bedrock geology on Anglesey (NW Wales) and the Rhins of Galloway (SW Scotland), both sites occurring close to the eastern margin of the ice stream, provides clear evidence for a link between landform morphology and distribution, and the disposition of the main tectonostratigraphical units within the bedrock. Changes in landform morphology are shown to have been locally controlled by largescale faults (e.g. the Berw Fault, Anglesey) and/or major lithological boundaries (e.g. base Permian unconformity, Rhins of Galloway), with less durable bedrock lithologies influencing the location and lateral extent of relatively faster flowing portions of the ice stream. In contrast, bedrock highs acted as ‘sticky spots’ within the bed, with the orientation of penetrative, regional bedrock structures also potentially impeding forward motion. The variation in landform elongation ratio across both study areas has been used as a proxy for relative changes in the velocity of the overriding ice, enabling the construction of ice flow velocity and trajectory maps across these exposed parts of bed of the Irish Sea Ice Stream. SPLIT, SQUIRT AND SEAL: THE ANATOMY OF A HYDROFRACTURE SYSTEM Emrys Phillips. British Geological Survey, United Kingdom E-mail address: [email protected]
Hydrofracture systems within subglacial to ice marginal settings represent a visible expression of the passage of pressurised meltwater through these glacial environments and provide a clear record of fluctuating hydrostatic pressure. A detailed study of a hydrofracture system cutting Devonian sandstone bedrock exposed at the Meads of St. John, near Inverness (NE Scotland) has revealed that this complex, multiphase system was active over a prolonged period and accommodated a several phases of fluid flow. The main bedding-parallel, feeder conduits are linked by steeply inclined to subvertical, transgressive dykes developed along contemporaneous fractures and normal faults. Micromorphological analysis has revealed that the individual hydrofractures comprise three main stages of fill:
Stage 1 - Initial fracture propagation, followed by the introduction of a thinly laminated clay, silt and fine sand fill. The clay lined the hydrofracture walls, sealing the adjacent wallrock reducing its permeability; Stage 2 - Repeated reactivation of the hydrofracture and deposition of a thinly laminated sequence of coarse silt to fine sand. Sedimentary structures provide a record of palaeoflow and fluctuating porewater pressure during deposition; and Stage 3 - Late-stage liquefaction of earlier sand and silt fills and injection of cross-cutting veinlets of remobilised sediment, possibly in response to stress release during subsequent brittle failure.
379
A comparison of the results of this study with published engineering ‘hydrofrac’ data indicates that: (1) changes in the style of sedimentation can be directly related to the fluctuation in overpressure during hydrofracing; and (2) the overpressures required to reactivate the hydrofracture decreases over time. The development and repeated reactivation of subglacial hydrofracture systems has the potential dramatically increase the permeability of the bed, facilitating the migration of meltwater beneath glaciers and ice sheets.
ANGLESEY - A LANDSCAPE CARVED BY ICE: A NEW WEB-BASED, INTERACTIVE LANDFORM MAP FOR ANGLESEY NORTH WALES FROM THE BRITISH GEOLOGICAL SURVEY Emrys Phillips. British Geological Survey, United Kingdom E-mail address: [email protected]
During the last ice age, the island of Anglesey (NW Wales) occupied a unique position close to the eastern margin of a the Irish Sea Ice Stream; a fast moving corridor of ice which fed ice from central and southwest Scotland, through the Irish Sea basin, eventually reaching as far south as the Isles of Scilly. This ice stream, one of the largest identified within the Late Devensian British and Irish Ice Sheet, left a distinctive ‘footprint’ on Anglesey in the form of a distinct assemblage of subglacial landforms and sediments. A new web-based educational product describing the glacial history of Anglesey has been launched by the British Geological Survey (BGS). At its heart is a new landform map of the island draped over a colour shaded NEXTMap TM digital elevation model. Tabbed and dropdown menus, as well as embedded controls allow the user to navigate around this new interactive product, revealing additional information on the bedrock geology and landform assemblage zones, browse through a photo gallery containing images of the glacial landforms and sediments, access informative summaries of how the main glacial features (including drumlins, meltwater channels) were formed, as well as zoom into, and switch on and off the various layers contained within the interactive map. The target audience of the Anglesey i-map includes school students, geography and earth science teachers, undergraduates and academic researchers, as well as anyone interested in the landscape around them evolved. Free-todownload (A0 size, pdf format) maps and photo gallery wall posters are available as part of this new BGS product.
CRONUS-EARTH PROJECT: RESULTS Fred Phillips. New Mexico Tech, United States E-mail address: [email protected]
The CRONUS-Earth Project was funded by the U.S. National Science Foundation in 2005, with the objective of obtaining a definitive set of geological calibration samples to quantify the production rates of terrestrial cosmogenic nuclides and to resolve discrepancies in cosmogenic nuclide systematics and scaling. Samples have been obtained and analyzed from five primary calibration sites, and numerous secondary sites, yielding a data set that is much more consistent than those from previous studies. One important finding is that scaling methods based on neutron-monitor counting data over-predict nuclide production at high elevation. This at least partly results from response multiplicity in the neutron monitors. Low-energy neutron production of 36Cl has been calibrated using depthprofile data. Calculations indicate that scaling may vary among different nuclides, in particular for 3He. We have developed an on-line application to facilitate the calculation of internally consistent and globally applicable cosmogenic chronologies. 40
AR/39AR GEOCHRONOLOGY OF FELDSPAR MEGACRYSTS FROM LATE PLEISTOCENE VOLCANOES, NEWER VOLCANIC PROVINCE, SOUTHEAST AUSTRALIA David Phillips. The University of Melbourne, Australia
E-mail address: [email protected]