A meltwater origin for Antarctic shelf bedforms with special attention to megalineations
Introduction
Deep troughs cross the Antarctic continental shelf where distributary ice streams from the Antarctic Ice Sheets reached the ocean (e.g., Wellner et al., 2006). The shelf is subdivided along these troughs into inner, middle, and outer zones on the basis of geology and geomorphology. Outcrops or subcrops of igneous and sedimentary rock with high relief characterize the inner zone. Offshore-dipping sedimentary strata underlie thick surficial deposits on the outer shelf. The middle shelf marks the transition between the two. The geomorphology of the shelf, which is to a large extent controlled by the geological context, is considered here.
Numerous authors have written recently on bedforms in formerly glaciated Antarctic shelf-crossing troughs (e.g., Shipp et al., 1999, Canals et al., 2000, Wellner et al., 2001, Lowe and Anderson, 2002, Ó Cofaigh et al., 2002, Lowe and Anderson, 2003, Gilbert et al., 2003, Dowdeswell et al., 2004, Ó Cofaigh et al., 2005, Heroy and Anderson, 2005, Wellner et al., 2006, Domack et al., 2006). A remarkably consistent sequence of trough bedforms extends from the inner shelf to the continental slope (Fig. 1). Megascale glacial lineations (MSGL; Clark, 1993; Fig. 2) are extensive on the shelf and understanding them is basic to an understanding of processes beneath paleo-ice streams.
Although MSGL are generally interpreted as products of subglacial deformation of soft sediments, we show that similar megalineations form in broad, turbulent flows in water and air. These other lineations are worth considering as analogs for MSGL. If form and pattern are similar for the two sets of lineations and there is a case for broad, turbulent flows beneath former Antarctic shelf ice streams, a meltwater origin for MSGL is a promising alternative to formation by deforming deforming-bed processes. This possibility is explored here.
Section snippets
Tunnel channels
Tunnel channels (e.g., Brennand and Shaw, 1994) are subglacial channels cut into the glacier substrate. Water in the channels commonly flowed uphill, giving them an undulating long profile. Tunnel channels may anastomose and they start and end suddenly. Other than boulders (Cutler et al., 2002), they rarely have deposits at their mouths. Kristensen et al. (2007) noted adverse gradients in channel end slopes and argued that this indicates high discharges with the generation of viscous heat.
Abundant meltwater interpretation
This discussion on abundant meltwater is based on evidence from tunnel channels, drumlins, gullies and slope morphology, and sediment. It establishes meltwater discharge estimates that are used later in the interpretation of megascale glacial lineations.
Minimal meltwater interpretation
Although Lowe and Anderson (2003) attributed a number of morphological features at Pine Island Bay to the action of abundant meltwater and Gilbert et al. (2003) suggested that streamlined bedforms in the Greenpeace Trough originated in outburst floods, this standpoint is not popular these days, and the required amounts of water are minimized. For example, Wellner et al. (2006) reinterpreted crescentic scours as p-forms and suggested that they need not be formed by meltwater. However, this view
Meltwater discharge continuity
Several authors (Wellner et al., 2001, Ó Cofaigh et al., 2002, Lowe and Anderson, 2003, Evans et al., 2005) realised that abundant meltwater on the inner shelf has to drain across the middle and outer shelves and that the nature of the flow is problematic. As a solution, they proposed that drainage was by Darcian flow through fine-grained till. Surface till softening was also attributed to this process. By contrast, Shoemaker (1991), Piotrowski (1997), and Carlson et al. (2007) showed that
Megalineations and broad, turbulent flow
Geomorphic and sedimentary evidence are expected if high magnitude, broad meltwater flow swept across the outer shelf. With the discharges estimated for tunnel channel and drumlin formation, flows over the whole shelf would have been fast with high bed shear stresses and the competence and capacity to erode soft beds and to transport coarse sediment. Under these flow conditions and an erodible bed, bedforms would have been inevitable on the middle and outer shelves. Since MSGL are the only
Discussion
Clark (1994), Benn and Evans, 1998, Benn and Evans, 2006, Clarke et al. (2005), Cofaigh et al. (2005) and Evans et al. (2006) presented arguments against the megaflood model. We respond to these criticisms where they relate to this paper. One of our main purposes was to show how lineations with the characteristics outlined by Clark, 1993, Clark, 1994 may form in turbulent flows. Benn and Evans, 1998, Benn and Evans, 2006 objected mainly to the use of form analogy, and we consider these
Conclusions
Antarctic Peninsula shelf bedforms are arranged systematically and their explanation requires a unified set of processes.
Tunnel channels and drumlins on the inner shelf, gullies eroded into the outer shelf and the upper parts of the slope, channels on the slope and turbidites on the slope, rise, and shelf indicate abundant meltwater. The large scale of tunnel channels and drumlins with crescentic scours around their heads suggest formation by megafloods. Comparisons with estimated discharges in
Acknowledgements
An NSERC Canada Discovery Grant to John Shaw is gratefully acknowledged. We thank Ian Brookes, Robert Gilbert, and David Sharpe for their helpful and supportive comments. We benefited from reviews by David Piper and an anonymous reviewer who were both critical and constructive. John Anderson and Larry Poppe kindly provided illustrations.
References (105)
Macroforms, large bedforms and rhythmic sedimentary sequences in subglacial eskers, south-central Ontario: implications for esker genesis and meltwater regime
Sedimentary Geology
(1994)Aeolian erosional lineations in the Libyan Desert, Dakhla Region, Egypt
Geomorphology
(2001)- et al.
Modeling the subglacial hydrology of the James Bay lobe of the Laurentide Ice Sheet
Quaternary Science Reviews
(2007) Large-scale ice-molding — a discussion of genesis and glaciological significance
Sedimentary Geology
(1994)- et al.
Extent and basal characteristic of the M'Clntock Channel Ice Stream
Quaternary International
(2001) - et al.
Fresh arguments against the Shaw megaflood hypothesis. A reply to comments by David Sharpe on “Paleohydraulics of the last outburst from glacial Lake Agassiz and the 8200 BP cold event
Quaternary Science Reviews
(2005) - et al.
Sedimentological evidence for outburst floods from the Laurentide Ice Sheet margin in Wisconsin, USA: implications for tunnel-channel formation
Quaternary International
(2002) - et al.
Subglacial morphology and glacial evolution of the Palmer outlet system, Antarctic Peninsula
Geomorphology
(2006) - et al.
Continental slope morphology and sedimentary processes at the mouth of an Antarctic palaeo-ice stream
Marine Geology
(2004) - et al.
Late Quaternary glacial history, flow dynamics and sedimentation along the eastern margins of the Antarctic Peninsula Ice Sheet
Quaternary Science Reviews
(2005)
A critical assessment of sub-glacial mega-floods: a case study of glacial sediments and landforms in south-central Alberta
Quaternary Science Reviews
Subglacial meltwater origin and subaerial meltwater modifications of drumlins near Morley, Alberta, Canada
Sedimentary Geology
Significance of fine-grained sediment lofting from meltwater generated turbidity currents for the timing of glaciomarine sediment transport into the deep sea
Sedimentary Geology
On the formation of tunnel valleys of the southern Laurentide ice sheet
Quaternary Science Reviews
Outburst flood origin of the central Kalamazoo River valley, Michigan, USA
Quaternary Science Reviews
Reconstruction of the West Antarctic Ice Sheet in Pine Island Bay during the last glacial maximum and subsequent retreat history
Quaternary Science Reviews
The Blackspring Ridge Flute Field, south central Alberta: evidence for subglacial sheetflow erosion
Quaternary International
Conditions for ignition of catastrophically erosional turbidity currents
Marine Geology
Subglacial hydrology in northwestern Germany during the last glaciation: groundwater flow, tunnel valleys and hydrological cycles
Quaternary Science Reviews
Stratigraphic and sedimentological evidence for late Wisconsinan outburst floods to Laurentian Fan
Palaeogeography, Palaeoclimaology, Palaeoecology
Ice sheets retreat from the Antarctic Peninsula Shelf
Continental Shelf Research
Subglacial tunnel channels, Porcupine Hills, southwest Alberta, Canada
Quaternary International
Evidence for rapid sedimentation in a tunnel channel, Oak Ridges Moraine, southern Ontario, Canada
Sedimentary Geology
Icelandic jökulhlaup impacts: implications for ice-sheet hydrology, sediment transport and geomorphology
Geomorphology
Hairpin erosional marks, horseshoe vortices and subglacial erosion
Sedimentary Geology
Drumlins and catastrophic subglacial floods
Sedimentary Geology
The Athabasca fluting field, Alberta, Canada: implications for the formation of large-scale fluting (erosional lineation)
Quaternary Science Reviews
Incipient tunnel channels
Quaternary International
Palaeo-ice streams
Quaternary Science Reviews
Late Pleistocene ice export events into the Arctic Ocean from the M'Clure Strait Ice Stream, Canadian Arctic Archipelago
Global and Planetary Change
Short-distance variability in slope bed-forms along the south western Adriatic Margin (central Mediterranean)
Marine Geology
Outburst flooding and the initiation of ice-stream surges in response to climatic cooling: a hypothesis
Geomorphology
Weddell Fan and associated abyssal plain, Antarctica: morphology, sediment processes, and factors influencing sediment supply
Geo-marine Letters
Meltwater production due to strain heating in Storglaciaren, Sweden
Journal of Geophysical Research
Hydraulic modeling of subglacial tunnel channels
Hydrological Processes
Glaciers and Glaciation
Subglacial megafloods: outrageous hypothesis or simply outrageous
Channel networks carved by subglacial water — observations and reconstruction in the Eastern Puget Lowland of Washington
Geological Society of America Bulletin
Glacial tunnel valleys and Quaternary history of the Scotian Shelf
Nature
Bedrock megagrooves in Assynt, NW Scotland
Geomorphology
Ice sheet dynamics and subglacial meltwater regimes inferred from the sedimentology of glaciofluvial systems: Victoria Island, District of Franklin, Northwest Territories
Canadian Journal of Earth Sciences
Subglacial tunnel channels and associated landforms, south-central Ontario — their implications for ice-sheet hydrology
Canadian Journal of Earth Sciences
The Lake Missoula floods and the Channeled Scabland
Journal of Geology
Deep water currents form megafurrows near US Gulf's Sigsbee Escarpment
Offshore
Deep sea-floor evidence of fast ice streams of the Antarctic Peninsula
Geology
Mega-scale glacial lineations and cross-cutting ice flow landforms
Earth Surface Processes and Landforms
Glaciodynamic context of subglacial bedform generation and preservation
Annals of Glaciology
A groove-ploughing theory for the production of mega-scale glacial lineations and implications for ice-stream mechanics
Journal of Glaciology
Flow dynamics and till genesis associated with a marine-based Antarctic palaeo-ice stream
Quaternary Science Reviews
Cited by (31)
Global Late Quaternary Megafloods
2022, Treatise on GeomorphologyLandform evolution of Oudemans crater and its bounding plateau plains on Mars: Geomorphological constraints on the Tharsis ice-cap hypothesis
2021, IcarusCitation Excerpt :However, the new observations from Mars based on the above discussion may provide a fresh perspective on this important issue in glacier physics. Current models invoke basal-ice irregularities ploughing into softer subglacial tills (Tulaczyk et al., 2001; Clark et al., 2003), subglacial mega-floods (Shaw et al., 2008), kinematic-wave and/or rheological-contrast triggered instability at the ice-till interface (Hindmarsh, 1998; Fowler and Chapwanya, 2014), and coeval erosion and deposition of subglacial tills below fast-moving ice streams (Barcheck et al., 2020). Establishing which of the above mechanisms was responsible for the formation of the striated terrain on Mars may allow us to relate spacing, length, and length-width ratio of MSGLs to the dynamics and size/thickness of the involved ice sheet.
The origins of s-forms: Form similarity, process analogy, and links to high-energy, subglacial meltwater flows
2020, Earth-Science ReviewsCitation Excerpt :Such extreme flows, which are not encountered in open channels, signify high subglacial pressure gradients. Hairpin scours also typify large-scale bedrock fluting or megalineations (Fig. 3F and H; Shaw and Gilbert, 1990; Shaw, 1994a, 1994b, 1996; Pollard et al., 1996; Ó Cofaigh et al., 2002; Gilbert et al., 2003; Shaw et al., 2008; Kor and Cowell, 1998; Shaw et al., 2010; Eyles, 2012). Megalineations may be kilometres long, with tapering and reduction in height probably reflecting widening and weakening vortex legs downflow (Fig. 1B).
Bedrock mega-grooves in glaciated terrain: A review
2018, Earth-Science Reviews