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

Abstract

This paper presents the results of a detailed study of a complex hydrofracture system and host diamictons exposed within a longitudinal section through an elongate drumlin located to the west of Cemlyn Bay, Anglesey, NW Wales. This complex, laterally extensive sand, silt and clay filled hydrofracture system was active over a prolonged period and is thought to have developed beneath the Late Devensian (Weichselian) Irish Sea Ice Stream as it overrode this part of NW Anglesey. The sediment-fill to the hydrofracture system is deformed with kinematic indicators (folds, thrusts, augen) recording a SW-directed sense of shear, consistent with the regional ice flow direction across this part of the island. The lack of any geomorphological evidence for active retreat of the Irish Sea ice across Anglesey, has led to the conclusion that hydrofracturing at the Cemlyn Bay site occurred within the bed of the Irish Sea Ice Stream whilst this relatively faster flowing corridor of ice was actively overriding the island. Shear imposed by the overriding ice led to the development of a subglacial shear zone which facilitated the propagation of the hydrofracture system with the laterally extensive feeder sills occurring parallel to Y-type Riedel shears. Although a subglacial setting beneath the active Irish Sea Ice Stream can be argued for the Cemlyn Bay hydrofracture system, its relationship to the formation of the 'host' drumlin remains uncertain. However, evidence presented here suggests that hydrofracturing may have occurred during the later stages or post landform development in response to the migration of overpressurised meltwater within the bed of the Irish Sea ice; possibly accompanying the local thinning and shutdown of the Irish Sea Ice Stream on Anglesey.

Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK

“…[ACCEPTED MANUSCRIPT] flow associated with fluctuating porewater availability (Phillips and Merritt 2008;Phillips et al, 2013b). Hydrofractures provide clear evidence for the passage of over-pressurised meltwater through glacial environments and are thought to be mainly developed in ice-marginal, sub-marginal to subglacial settings where the overburden pressure exerted by the ice leads to the required periodic over-pressurisation of the hydrogeological system (van der Meer et al, 1999Meer et al, 2009Roberts et al, 2009;Phillips et al, 2013b;Phillips & Hughes, 2014). This suggests that Lithofacies C was probably developed in an ice-marginal to proglacial setting with the periodic build-up of the hydrostatic pressures possibly reflecting a seasonal (spring-summer) increase in meltwater production.…”

“…Erratic clasts from the basal and upper diamictons (Section 3; Greenly, 1919) are identical to those identified from Lithofacies B (Section 3 -Appendices 1 & 2) and derived from local-and far-travelled bedrock outcrops, and identify common sources for the entire Lleiniog sequence situated within and adjacent to the Irish Sea Basin . The simplest interpretation is that the basal diamicton was deposited during an initial southwards advance of Irish Sea Ice across Anglesey (Phillips et al, 2013a) with a subsequent retreat and readvance across the island that deposited the upper diamicton associated with the overriding of the island by the Irish Sea Ice Stream (Thomas & Chiverrell, 2007;Phillips et al, 2010;Phillips & Hughes, 2014).…”

Ductile and brittle styles of subglacial sediment deformation: An example from western Ireland

“…Furthermore, sediment deformation by the non-uniform application of strain results in an increase in pore water pressure within the sediment body and the development of strain-induced pore water pressure gradients (Iverson et al, 1994;Truffer and Harrison, 2006). In turn, this can lead to the formation of water escape structures which are directed down this pore water pressure gradient (Phillips and Hughes, 2014).…”

“…This means that these structures are to a much lesser extent influenced by the properties of the medium such as grain size and grain sorting than those produced by ductile deformation. Brittle deformation structures including faults, shears/décollements, and fracture fills such as clastic dikes are commonly described in the glacial literature (e.g., Burbidge et al, 1988;Dreimanis and Rappol, 1997;van der Meer et al, 1999van der Meer et al, , 2009Le Heron and Etienne, 2005;Phillips et al, 2013b;Phillips and Hughes, 2014) (Table 1) but their physical mechanisms of formation in the glacial environment are poorly known either from laboratory or theoretical perspectives (Rathbun et al, 2008;Altuhafi et al, 2009;Rathbun and Marone, 2010;Tarplee et al, 2011). In the subglacial environment, especially near to the ice margin, extensional and/or compressional glacier regimes can lead to both normal and reverse faulting within subglacial sediments (Phillips et al, 2007(Phillips et al, , 2013b.…”

Proceedings of the Geologists’ Association (PGA) Volume 126, Issue 1

“…Brew et al (2015), as a result of detailed work in the Fenland provide a new and illuminating model of coastal evolution and sea-level change in that part of eastern England. Finally, the subject of glacial and periglacial geology is examined in papers by Phillips and Hughes (2014) who look at hydrofracturing as a result of overpressurised subglacial meltwater during glaciation of Anglesey; Miller et al (2014) who use LiDAR imagery to determine the glacial and paraglacial history of part of the southeast Lake District, England; Busfield et al (2015) who use the lithology and derived palynology to determine the ice flow paths of the last glaciers to cross east Yorkshire and Paul (2014) provides a critical examination of the factors that have determined the fluvial geomorphology of part of the Cotswold escarpment and in so doing provides insight into the processes that formed this landscape and the patterns of settlement and water-powered industry in the area Over the past five years or so, the PGA has hosted a number of important papers on the topic of geoconservation, Earth heritage and Earth management, and papers in this field have been well used with high citation values. Over the period covered by this Editorial the journal has received papers from a variety of locations around the world with Rocha et al (2014) who assess the geological heritage of Cape Mondego Natural Monument, Central Portugal, which is the GSSP for the base of the Bajocian Stage and the ASSP for the base of the Bathonian Stage; Enniouar et al (2014) who describe and evaluate a Middle Jurassic sauropod tracksite in the Western High Atlas, Morocco in terms of sustainable geotourism, and Veress et al (2014) who present an analysis of the tor-like features that define the Lena Pillars Nature Park in the Republic of Sakha, East Siberia, Russia.…”