Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier, Alaska, USA

Abstract

The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.

Submarginal debris transport and till formation in active temperate glacier systems: The southeast Iceland type locality

“…Of special interest in the Skaftafellsjökull till sequences is the pseudo-stratified zone at site Sk1 (inset photographs in Figure 11a), because it strongly resembles the outcrop characteristics of melt-out tills recently reported from Alaska by Larson et al (2016). The normally poor preservation of melt-out till appears to have been improved in the Alaska case study because of a significant thickness and debris content of the parent (stratified) basal ice facies, thought to be the product of glaciohydraulic supercooling (cf.…”

An erosional origin for drumlins of NW Poland

“…Strong and north-east (up-ice) dipping fabrics in the lower till (Dmm(1)) suggests that this is a subglacial traction till (Evans et al, 2006) containing glaciotectonically deformed stringers of outwash sand. Shaw, 1979;Lawson, 1981;Larson et al 2016;Evans, 2018: pp. These sediments are interpreted as a successively deposited infill of a subglacial R-type meltwater channel (Röthlisberger, 1972;Flowers, 2015) whereby the till is a melt-out till (cf.…”

“…These sediments are interpreted as a successively deposited infill of a subglacial R-type meltwater channel (Röthlisberger, 1972;Flowers, 2015) whereby the till is a melt-out till (cf. Shaw, 1979;Lawson, 1981;Larson et al 2016;Evans, 2018: pp. 202-221) released from the base of a stagnant ice during an event of channel closure and basal re-coupling.…”

Glacial Geomorphology

“…As the grasp of subglacial rheological conditions has been better understood due to microscopic examination of undisturbed samples of these tills; so older till classifications, and with that their stratigraphies, have had to be reviewed and revised. For some time now, it has been apparent that all subglacial tills other than those tills formed under virtually zero stress (melt-out tills in some instances (Larson et al, 2016)) and very high stress conditions (lodgement tills, Meer and Menzies, 2011) ( Fig. 5) have been deformed in some manner (tectomicts, Menzies, 2012) and as such need to be re-evaluated and reclassified-evidence of a paradigm shift in thinking!…”

“…Glacial erosion of bedrock surfaces, intact bedrock units and sediments involve a range of processes, at times referred to as wear and attrition, that require a broad grasp of several closely allied components such as bedrock, glacial ice, glacial meltwater, sediment Larson et al, 2016). (B) Is a typical Pleistocene subglacial till from Oakville, Ontario, Canada, exhibiting the common microstructures of a deformed till (Eyles et al, 2011), and (C) is a subglacial till from near Moneydie, Perthshire, Scotland, that shows a unistrial plasmic fabric indicative of high stress deformation (Menzies and van der Meer, 1998).…”