Boulder‐gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south‐central Michigan, USA

Abstract

2003 (June): Boulder-gravel hummocks and wavy basal till contacts: products of subglacial meltwater flow beneath the Saginaw Lobe, south-central Michigan, USA. Boreas, Vol. 32, pp. 328-336. Oslo. ISSN 0300-9483.Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks ($100 m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50 m along the margin of a tunnel channel. The 0.4 m thick stratified till is overlain by up to 5 m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5 m amplitude and 3-5 m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.

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

“…Put simply, subglacial meltwater deposits are the product of a hydrological regime that is not unique to subglacial environments, with similar regimes documented from a range of other sedimentary environments including intertidal (Cheel & Middleton, 1986), fluvial (Cheel, 1990, Røe, 1987Alexander et al, 2001) and channelised turbidity currents (Ito & Saito, 2006). Upper plane bed flow regimes have also been recognised within glacial meltwater systems including subglacial (this study ;Brennand, 1994;Fisher et al, 2003), englacial (Delaney, 2001(Delaney, , 2002 proglacial (Duller et al, 2008) and subaqueous ice-contact environments (Russell & Arnott, 2003;Hornung et al, 2007;Lang & Winsemann, 2013).…”

“…Within a glacigenic context this includes subglacial drainage channels (Fisher et al, 2003) and outburst floods (jökulhlaups) debouching either subaerially (Duller et al, 2008) or onto subaqueous ice-contact fans (Lang & Winsemann, 2013). They have commonly been attributed to rapidly-aggrading stationary antidunes (Lang & Winsemann, 2010) that are supercritically climbing due to massive rates of sediment deposition under transitional dune to upper flow regimes (Cheel, 1990;Fielding, 2006).…”

“…Flow-roof coupling would logically generate turbulence, causing helicoidal flow to become destabilised driving bedform erosion and sediment reworking. Alternatively, Fisher et al (2003) have proposed flow-roof interaction coinciding with an abrupt drop in flow regime, could lead to the development of large-scale sinusoidal bedforms by the effect of ice-15…”

“…This is because the longitudinal (and vertical) development of the bedforms indicates that the flow regime was maintained during their formation -sinusoidal bedforms accreted during rapidly decelerating flow should climb with increasing steepness. Nevertheless, preservation of large-scale sinusoidal bedding does require turbulence to cease during or immediately after accretion (Fisher et al, 2003;Fielding, 2006;Lang & Winsemann, 2010).…”

“…Consequently, research in glacial environments has generally targeted deposits that form prominent sediment-landform assemblages such as eskers and tunnel valleys (Shreve, 1985;Brennand, 1994;Clark & Walder, 1994;Warren & Ashley, 1994;Delaney, 2001Delaney, , 2002. Studies relating to smaller meltwater features, or those that possess a more subtle or no surface expression, have proven more difficult to recognise and interpret for the reasons given above, but several studies have been published (Piotrowski & Tulaczyk, 1999;Fisher et al, 2003;.…”