A multiproxy study of a basal till: a time‐transgressive accretion and deformation hypothesis

Abstract

A two-part basal till at Knud Strand, Denmark reveals a uniform fabric pattern and strength, petrographical composition and clay mineralogy. The nature of the contact with the underlying sediments, ductile deformation structures, partly intact soft sediment clasts, small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water. A time-transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds. It is suggested that, although strain indicators occur throughout the entire till thickness, the deformation at any point of time encompassed the uppermost part of the till only, allowing preservation of fragile clasts below. The substantial thickness of the till (up to 6 m) coupled with a much smaller (by more than one order of magnitude) inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition. The lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path.

Sedimentological Observations from the Tiskilwa Till, Illinois, and Sky Pilot Till, Manitoba

“…Similarly, these fabrics exhibited unimodal to spread unimodal shape similar to lodgement or melt-out tills suggesting that fabric shape is also not indicative of genesis. Based on the independent evidence for deformation (gradational basal contacts with reworked till and deformed sand inclusions), we interpret our fabric data from the Tiskilwa and Sky Pilot Tills as indicators of accumulated strain and local ice-flow direction (Bennett et al, 1999;Larsen et al, 2004) but do not attempt to infer genesis from them.…”

“…Studies of the sediment record provide valuable observations and insights by which to constrain inferences, assumptions, and hypotheses regarding basal conditions and processes (Clayton et al, 1989;Alley, 1991;Hicock, 1992;Hicock and Dreimanis, 1992;Johnson and Hansel, 1999;Boulton et al, 2001;Iverson, 2000a, 2002;Piotrowski et al, 2001Piotrowski et al, , 2004Larsen et al, 2004). Nevertheless, attempts to discern mechanisms of ice movement from the sediment record are complicated by the fact that processes beneath modern ice sheets are, to a first order, not directly observable, thus obviating a process-based approach to understanding sedimentation.…”

Basal till and subglacial conditions at the base of the Upper Odra ice lobe (southern Poland) during the Odranian (Saalian) Glaciation

“…How ever, the drop of the pore-wa ter pres sure af ter an ep i sode of drain age was prob a bly much larger than in the case of the till with few de for ma tions and with traces of subglacial wa ter flows at the ice-bed con tact. This in di cates that a de for ma tion till can ar chive the cy cles of changes in strain rate, in clud ing their di min ish ing, ceas ing and re ac ti va tion (see Boulton et al, 2001;Larsen et al, 2004;Piotrowski et al, 2004;Narloch et al, 2012Narloch et al, , 2013. This con clusion is con sis tent with the ob ser va tions of some mod ern glaciers, which in di cate tem po ral vari a tions of the basal move ment type: from subglacial de for ma tion to basal slid ing as a re sult of changes in the basal wa ter pres sure (Blake et al, 1992(Blake et al, , 1994Fischer and Clarke, 1994Iverson et al, 1994Iverson et al, , 1995Hooke et al, 1997;Fischer et al, 1999;Hart et al, 2009).…”

“…One of the ar gu ments that sup port this in ter pre ta tion is the sim i lar value of the clast fab ric in the whole till pro file (cf. Larsen et al, 2004). Unit B2 con tains in its lower part also sig nif i cant traces of de for ma tion.…”

“…10-30 cm), much smaller than the thick ness of unit B1 in its en tirety, and prob a bly its lower bound ary moved steadily up wards while pro gres sive de po si tion of till, took place. Grad ual up ward mi gra tion of a defor ma tion zone as a re sult of ac cu mu la tion of sed i ment be neath the gla cier was pos tu lated also by, among oth ers, Hart and Boulton (1991) and Larsen et al (2004). One of the ar gu ments that sup port this in ter pre ta tion is the sim i lar value of the clast fab ric in the whole till pro file (cf.…”