Micromorphological evidence for subglacial multiphase sedimentation and deformation during overpressurized fluid flow associated with hydrofracturing

Abstract

evidence for multiphase sedimentation and deformation during hydrofracturing Keywords: micromorphology, hydrofractures, subglacial to ice marginal environments Hydrofracture systems are being increasingly recognised within subglacial to ice marginal settings and represent a visible expression of the passage of pressurised meltwater through these glacial environments. Such structures provide a clear record of fluctuating hydrostatic pressure, leading to brittle fracturing of the host sediment/bedrock, and penecontemporaneous liquefaction and introduction of the sediment-fill. A detailed macro and microstructural 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 several phases of fluid flow. The main conduits which fed the hydrofracture system are located along bedding within the sandstone, with the site of the wider, steeply inclined to subvertical, transgressive linking sections being controlled by the contemporaneous development of high-angle fractures and normal faults; the latter occurring in response to localised extension within the bedrock. A comparison with published engineering hydraulic fracturing data indicates that the various stages of sediment-fill deposited during a flow event can be directly related to the fluctuation in overpressure during hydrofracing. A model is proposed linking the evolution of this hydrofracture system to the retreat of the overlying Findhorn glacier. The results of this study also indicate that the development and repeated reactivation of subglacial hydrofracture systems can have a dramatic effect on the permeability of the bed, influencing the potential for overpressure build-up within the subglacial hydrogeological system, and facilitating the migration of meltwater beneath glaciers and ice sheets.Emrys Phillips (erp@bgs.ac.uk; British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK); Jez Everest (British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK); Helen Reeves (British Geological Survey, Sir Kingsley Dunham Centre, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK) Hydrofracture systems, also referred to as water-escape features or clastic dykes, represent a visible expression of the passage of pressurised meltwater through subglacial to ice marginal environments (Dionne & Shilts 1974;Christiansen et al. 1982;von Brunn & Talbot 1986;Burbridge et al. 1988;Dreimanis 1992;Larsen & Mangerud 1992;McCabe & Dardis 1994;Dreimanis & Rappol 1997; van der Meer et 2 al. 1999; Rijsdijk et al. 1999;Le Heron & Etienne 2005;Boulton 2006;Goździk & van Loon 2007;van der Meer et al. 2008;Phillips & Merritt 2008). Previous work has demonstrated that hydrofractures record marked fluctuations in hydrostatic pressure within the glacial hydrogeological system, leading to brittle fracturing of the preexisting sediment and/or bedrock, and the penecontemporaneo...

Hydrofracturing in response to the development of an overpressurised subglacial meltwater system during drumlin formation: an example from Anglesey, NW Wales

“…This supports the interpretation of Phillips et al, (2013) that the bedrock-rich diamicton on Anglesey represents a glacially reworked periglacial head deposit with the schistose rock fragments being derived locally from the underlying New Harbour Group. No Siluro-Devonian and/or Carboniferous strata are exposed in the Cemlyn Bay area, but do crop out offshore to the N/NW of Anglesey (Jackson et al, 1995) and could have been transported and incorporated into this predominantly locally derived diamicton by the overriding Irish Sea ice.…”

“…3a) with the striae trending towards 220˚, consistent with the SW-directed flow of ice across the area. Phillips et al, (2013) describe a bedrock-rich diamicton directly overlying brecciated New Harbour Group bedrock in the area between Porth Penrhyn-mawr (SH 287 852) and Porth Tywyn-mawr (SH 286 837), to the south of the present study area. These authors interpreted this diamicton as a periglacial head deposit which was reworked by the overriding Irish Sea Ice.…”

“…It is possible that the lower bedrock-rich diamicton exposed in the Cemlyn Bay section is similarly a glacially reworked, pre-existing periglacial (head) deposit. Palynological data from both diamictons published by Phillips et al, (2013) demonstrate that the palynomorph assemblage is dominated by Carboniferous Densosporites spp. and Lycospora pusilla, as well as undifferentiated Quaternary pollen grains, with the lower grey diamicton containing Mesozoic (Classopollis) fauna (see Table 1 of Phillips et al, 2013).…”

“…Palynological data from both diamictons published by Phillips et al, (2013) demonstrate that the palynomorph assemblage is dominated by Carboniferous Densosporites spp. and Lycospora pusilla, as well as undifferentiated Quaternary pollen grains, with the lower grey diamicton containing Mesozoic (Classopollis) fauna (see Table 1 of Phillips et al, 2013). The presence of Carboniferous palynomorphs within the very locally derived New Harbour Group-rich lower diamicton is consistent with the incorporation of more 'far travelled' material into this reworked head deposit by the overriding Irish Sea ice.…”

Assessing the Long-Term Behaviour of the Industrial Bentonites Employed in a Repository for Radioactive Wastes by Studying Natural Bentonites in the Field

“…The resin is then cured by allowing the solvent to evaporate. This methodology has previously been used very successfully to minimise drying-induced artefacts in petrographic studies of water-sensitive soils and unconsolidated sediments [47], and in the analysis of glacio-tectonic fabrics in clay-rich sediments [48]. Unfortunately, this thin section preparation technique is very time consuming; the diffusive solvent-replacement process can take 6 to 12 months for completion, depending on the permeability of the material.…”