Sequence stratigraphy of a glaciated basin fill, with a focus on esker sedimentation

Abstract

A large integrated data set of cores, outcrop data, and seismic transects from the mud-buried Vars-Winchester esker in the Champlain Sea basin, Canada, was studied to gain insight into how muddy glaciated basins fi ll with sediment, and how esker sedimentary systems contribute to this process.Three stratigraphic units-a till sheet over carbonate bedrock, the Vars-Winchester esker , and overlying Champlain Sea mudare identifi ed in the data set. The till is massive, mud rich, carbonate rich, and drumlinized. The esker is also carbonate rich, and rests erosively on till or bedrock. It consists of two elements, a narrow gravelly central ridge and a broad sandy carapace. Three units comprise the overlying mud package: gray carbonate-rich rhythmites, massive bioturbated mud, and carbonate-poor, red-andgray rhythmites. A sequence stratigraphic model is proposed to explain these observations. Emphasis is placed on gradual ice-front translation superimposed by rapid meltwater events. The esker is interpreted to have been derived from the underlying till by water that fl owed through a subglacial conduit (R-channel), within which the narrow gravelly central ridge was deposited. Most mud and fi ner sand bypassed the conduit and was deposited proglacially on the fl oor of the Champlain Sea, fi rst as sandy outwash and, farther basinward, as muddy carbonate-rich rhythmites. Gradual ice-front retreat superposed distal facies over proximal facies, generating the upward-fi ning succession that starts with the esker gravel and ends with muddy rhythmites. Most esker sediment appears to have been deposited during rapid, jökulhlaup-like fl oods that punctuated grad-ual retreat. Discharges are estimated to have been high, possibly on the order of several hundred to, perhaps more commonly, several thousand cubic meters per second. The chaotic and random-looking appearance of the resultant sedimentological signatures in the esker sensu stricto is sharply contrasted with the regularity of the muddy rhythmites. If the rhythmites are indeed correlative to the esker, which seems reasonable given their geochemistry and the fact that their volume scales to the volume of mud in the till, the fl ood events that deposited the esker must have been seasonally mediated, and the basin water must have attenuated the fl ood signal, resulting in a rhythmic "on-off" signature in more distal portions of the system. The regularity of the rhythmites does not betray the chaotic nature of the esker sensu stricto, and vice versa. Studying either one in isolation would lead to a very different "end-member" impression of how eskers form and how esker sedimentary systems operate during the infi lling of glaciated basins.

The role of sediment supply in esker formation and ice tunnel evolution

“…These observations are echoed in the landform record and suggest that dynamic esker deposition is fundamentally controlled by variations in ice tunnel geometry (accommodation space) and sedimentation rate (Brennand, 1994;Brennand and Shaw, 1996;Burke et al, 2012a). Over the short timescales of transient events often recorded in eskers (Shulmeister, 1989;Brennand, 1994Brennand, , 2000Brennand and Shaw, 1996;Cummings et al, 2011), ice tunnel geometry is a balance between growth by frictional melting and mechanical excavation, and ice tunnel closure through sedimentation, rather than ice creep (Burke et al, 2012a). It is imperative that numerical models are consistent with the landform record, thus they should incorporate both the transient conditions, as well as seasonal drainage patterns (Burke et al, 2012b).…”

“…19). Like for many eskers (e.g., Brennand, 1994Brennand, , 2000Brennand and Shaw, 1996;Fiore et al, 2002;Burke et al, 2008Burke et al, , 2010Cummings et al, 2011), the architecture of those in south-western Alberta suggests a highly dynamic depositional environment. Although many of the criteria suggested as diagnostic of high magnitude deposition within eskers are present (c.f.…”

“…It is imperative that numerical models are consistent with the landform record, thus they should incorporate both the transient conditions, as well as seasonal drainage patterns (Burke et al, 2012b). However, most investigations of eskers have focused on regions containing the largest examples and the largest examples within regions (Shilts, 1984;Brennand, 1994;Burke et al, 2008Burke et al, , 2010Burke et al, , 2012aCummings et al, 2011;Storrar et al, 2014) where there was both a ready supply of meltwater (Brennand, 2000;Storrar et al, 2014) and sediment (Brennand, 2000). The lack of large eskers on the Prairies has resulted in only limited investigation of these landforms (Stalker, 1960), yet understanding what controlled their architecture and apparent regionally deranged distribution (Brennand, 2000) could provide further insight into the fundamental controls on esker formation.…”

“…Consequently, eskers have been used to infer former ice sheet dynamics (e.g., Shilts, 1984;Aylsworth and Shilts, 1989;Clark and Walder, 1994;Brennand, 2000;Storrar et al, 2014) and to test numerical models (e.g., Boulton et al, 2009). However, much debate remains as to the processes responsible for esker formation (Cummings et al, 2011) and, although recent work has shown that the glacial hydrologic system is never in steady state (e.g., Gray, 2005;Wingham et al, 2006;Fricker et al, 2007;Bell, 2008;Stearns et al, 2008;Bartholomaus et al, 2011), and that eskers may actually record relatively short lived and dynamic events (e.g., Brennand, 1994;Burke et al, 2008Burke et al, , 2010Burke et al, , 2012aCummings et al, 2011), numerical models have typically assumed steady state esker formation (e.g., Hooke and Fastook, 2007;Boulton et al, 2009). Detailed work on glacial lake outburst flood (GLOF) eskers at contemporary glaciers has demonstrated that the balance between water supply and sediment supply drive esker formation (Burke et al, , 2010.…”

Empirical Geophysical/Geotechnical Relationships in the Champlain Sea Sediments of Eastern Ontario

“…The focus has been on understanding the evolution of basin sedimentation (e.g. Cummings et al 2011 ;Medioli et al 2012 ), detecting the presence of ground disturbance (e.g. Pullan et al 2011 ), and measuring overburden properties for geohazard and groundwater studies (Benjumea et al 2003 ;Pugin et al 2007Pugin et al , 2009Hunter et al 2010 ;Crow et al 2011 ).…”