Subglacial hydrology and drumlin sediments in connemara, western ireland

Abstract

Sediment-cored drumlins are uncommon across the glacially eroded granite bedrock of Connemara (west County Galway, western Ireland). This paper describes the internal sediments from two significant drumlin exposures at Ardmore and Ballyconneely. Both drumlins show a lowermost lithofacies association comprising overconsolidated subglacial diamicton with clast lithology, strong fabrics and overfolds indicative of regional east/southeast to west/northwest late Weichselian ice flow. This lithofacies association is overlain at both sites by west-dipping, stratified, graded gravel and diamicton beds that downlap onto laminated silts. These beds reflect episodic debris flows into a leeside cavity developed following creation of relief by deposition and deformation of a subglacial diamicton nucleus located up-ice. Variations in substrate hydrological processes are identified in both drumlins based on the presence of clastic dikes, flame structures and debris flowdeposited gravels which are present at different locations spatially within the drumlin and at different stratigraphic levels. These features reflect the close connection between subglacial hydrological and sedimentary processes and their changes over time as the drumlin form develops.

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

“…Bedrock strike, however, has not exerted a strong influence on ice flow direction, with striae and s-forms, where observed, commonly cross-cutting strike (Knight, submitted for publication). During the late Pleistocene glaciation, regional ice flow was dominantly westward through Clew Bay, similar to patterns elsewhere in western Ireland embayments (Knight and McCabe, 1997;Knight, 1999Knight, , 2011Knight, , 2012aKnight, , 2014. In Clew Bay this ice flow was in part directed by uplands to the north (Murrisk Mountains) and south (Croagh Patrick).…”

“…The wide range of deformation structures present at Roonah Point is typical of those found within many Irish drumlins (e.g., McCabe and Dardis, 1994;Rijsdijk et al, 1999Rijsdijk et al, , 2010Knight, 2014). The sediments within the drumlin at Roonah Point are of subglacial origin; and therefore it is most likely that all of the deformation structures are also of subglacial origin since they are contained within the drumlin form (e.g., Larsen and Mangerud, 1992;Phillips and Hughes, 2014).…”

“…Table 2 shows that the deformation structures evolved during different periods, and are associated with different subglacial and hydrological conditions. The genetic relationship between changes in subglacial processes and environments, and development of different types of deformation structures, is known to be manifested during drumlinisation when overlying ice flow is exerting strain on subglacial sediments and thus changing pore water gradients within the substrate (Knight, 2012b(Knight, , 2014Phillips and Hughes, 2014). As a result, some structures may have undergone several phases of deformation, resulting in overprinted morphological or sedimentary signatures.…”

Subglacial processes from drumlins in Clew Bay, western Ireland

“…This is seen in many areas of western Ireland (e.g. McCabe and Dardis, ; Knight, , , ). In addition, variations in bedrock type and sediment thickness overlying bedrock have significant impacts on drumlin size (Greenwood and Clark, ) and location (Raukas and Tavast, ).…”

Glacial and paraglacial environments

“…These loose sediments can be reworked locally by mass wasting or meltout of buried ice, or transported potentially long distances from source by rivers which are invigorated by high rates of glacial meltwater production. Since Church and Ryder's seminal papers, consideration of the paraglacial process domain has encompassed a range of alpine geomorphological processes, including different mass wastage processes (including landslides, debris flows/torrents, soil creep, rockfalls), cambering, glacial lake outbursts, soil development, weathering, and biogeochemical processes, amongst others (see Knight andHarrison 2009, 2014).…”