6 the Morphology and Sedimentology of Landforms Created by Subglacial Megafloods
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6 The morphology and sedimentology of landforms created by subglacial megafloods MANDY J. MUNRO-STASIUK, JOHN SHAW, DARREN B. SJOGREN, TRACY A. BRENNAND, TIMOTHY G. FISHER, DAVID R. SHARPE, PHILIP S.G. KOR, CLAIRE L. BEANEY and BRUCE B. RAINS Summary marks produced by turbulently flowing water. He there- Subglacial landforms across various scales preserve fore proposed that they may have been formed by melt- the history of movement, deposition and erosion by the last water erosion and deposition. Since the publication of this great ice sheets and their meltwater. The origin of many paper an enormous volume of research has been under- of these landforms is, however, contentious. In this chapter taken addressing a meltwater genesis for a broad suite of these forms are described both individually and as suites subglacial landforms including: s-forms, drumlins, fluting, that make up entire landscapes. Their interpretations are hummocky terrain, Rogen moraine, megaripples, tunnel discussed with reference to the megaflood hypothesis. A channels, eskers, streamlined hills, rises and re-entrant val- description is provided of individual forms via their size, leys. As a continuum, these landforms define broad land- shape, landform associations, sedimentology and the rela- scape unconformities which on the largest scale indicate tionship between landform surfaces and internal sediments. meltwater flowpaths. Adding credence to the meltwater The possible origins of each are then discussed. To sim- hypothesis is the recognition of other erosional landforms plify the chapter the landforms are categorised by their size of similar form that are produced by turbulent flow. These (micro, meso, macro and mega), although, importantly, include: yardangs (Figure 6.1a), which are widely accepted it should be noted that several landforms show similari- as the work of wind (Greeley and Iversen, 1985); sastrugi ties across scales. Also discussed is the relevant subglacial in snow and ice, which also result from wind sculpting hydrology associated with the described forms, especially (e.g. Herzfeld et al., 2003) (Figure 6.1b); erosional marks the volume and discharge rates of megaflood flows, and produced by tsunami waves (Bryant and Young, 1996) where water may have been stored prior to the megaflood (Figure 6.1c); and the loess hills of the Channeled Sca- events. blands (Figure 6.1d), which were formed by enormous outpourings of water from glacial Lake Missoula and the 6.1 Introduction Cordilleran Ice Sheet system (Bretz, 1959; Baker, 1978; As early as 1812, Sir James Hall interpreted the Shaw et al., 2000). Common to the forms illustrated in Fig- famous Castle Rock in Edinburgh, Scotland, a crag and tail, ure 6.1, and the majority of the subglacial forms discussed as a landform created by immense, turbulent floods. Liken- in this paper, is that they are considered as the products ing the hill to features carved in snow by wind, he could of turbulent flow that contained vortices. For the forms only hypothesise that water was responsible; probably giant described here the inferred turbulent flow is subglacial tidal waves, as, at that time, he knew of no other mecha- meltwater. nism that could conceivably create such streamlining. It is In this chapter, a synopsis is provided of the sub- now very clear that the streamlined forms first noted by glacial landforms that are interpreted to be produced Hall are part of a continuum containing landforms of many by meltwater, and together they document evidence for shapes and sizes. While many researchers have attributed megafloods. Their size, shape, landform associations, sedi- their formation to subglacial processes involving deforma- mentology, and the relationship between landform surfaces tion of the glacier bed, in the last few decades attention has and internal sediments are all described. It is argued here moved back to water as a major landforming agent. that all the features described, except for Livingstone drum- This shift in ideas began when Shaw (1983) noted lins, Rogen moraine and eskers, are exclusively the product that drumlins were similar in form to inverted erosional of subglacial meltwater erosion. Livingstone drumlins and 78 Megaflooding on Earth and Mars, ed. Devon M. Burr, Paul A. Carling and Victor R. Baker. Published by Cambridge University Press. C Cambridge University Press 2009. Morphology and sedimentology of landforms 79 Figure 6.1. Examples of landforms produced by turbulent flows. (a) Yardangs, Chad, beginning at prominent, bedrock escarpments. Note the strong linearity of the bedforms (image from Google Earth); (b) Sastrugi in snow (courtesy the Royal Geographical Society); (c) Erosional marks produced by tsunami waves (courtesy of E. Bryant); (d) Streamlined loess hill in the Channeled Scablands. Rogen moraine are related to erosion; however, the erosion Table 6.1. Classification of subglacial landforms was up into the base of the ice rather than down into the created by meltwater flows (D: depositional, substratum. The eroded cavities were filled subsequently E: erosional) with sediment during waning flows. Eskers are believed to represent the final phases of floods. Because several of the Scale Landform features discussed cross the boundaries of erosional ver- sus depositional, they are categorised by size (micro, meso, Microforms s-forms (E) macro and mega) (Table 6.1). Importantly, though, some of Mesoforms Drumlins (E & D) these landforms show similarities across several orders of Fluting (E) magnitude in scale and this fact is noted where appropriate. Hummocky terrain (E) Rogen moraine (E & D) All of the described forms are observed as surface or buried Megaripples (E) features in formerly glaciated areas. Tunnel channels (E) Eskers (D) 6.1.1 The megaflood controversy Macroforms Streamlined hills (E) Before discussing megaflood landforms, it is Megaforms Landscape unconformities (E) pointed out that the megaflood hypothesis is not accepted Flowpaths (E) universally. There is a significant debate in the literature 80 Mandy J. Munro-Stasiuk et al. as to whether many of the features described herein were a few millimetres in size to a few tens of metres (e.g. formed by ice or by water. Arguments in opposition to the Kor et al., 1991; Munro-Stasiuk et al., 2005). Ljungner¨ meltwater theory has been summarised by Benn and Evans (1930) first described and classified these features, now (2006) and Evans et al. (2006). In particular, Evans et al. commonly referred to as s-forms (sculpted forms) after (2006) state, ‘It is regrettable that (the megaflood hypoth- Kor et al. (1991) (Figure 6.2). Many microforms, espe- esis) has not been subject to systematic scrutiny or testing cially muschelbruche,¨ sichelwannen, spindle flutes, and at local scales of field based enquiry.’ This statement is a other scour marks have been reproduced in flumes (e.g. misrepresentation of the megaflood research as the major- Allen, 1971; Shaw and Sharpe, 1987) and identical forms ity of evidence, in the form of dozens of studies, is from have been noted in fluvial environments (e.g. Maxson, field-based work. 1940; Karcz, 1968; Baker and Pickup, 1987; Wohl, 1992; Perhaps the biggest cause for concern among critics Tinkler, 1993; Baker and Kale, 1998; Hancock et al., 1998; of the meltwater hypothesis is the use of form analogy (e.g. Wohl and Ikeda, 1998; Gupta et al., 1999; Whipple et al., Benn and Evans, 2006), one of the original lines of evi- 2000; Richardson and Carling, 2005). It is known, there- dence that Shaw (1983) used to develop his hypothesis. For fore, that these can be eroded by water. Also, the presence instance, Benn and Evans (1998, p. 446) wrote, ‘The use of sharp upper rims on many of these forms typically is of sole marks beneath turbidites as analogues for drumlins representative of flow separation (e.g. Allen, 1982; Sharpe involves a major leap of faith, given the huge difference in and Shaw, 1989) and the ever-present crescentic and hair- the scale of the features.’ This assertion is simply wrong. pin furrows represent the generation of horseshoe vortices The analogy between drumlins and erosional marks does around obstacles encountered by the flow (Peabody, 1947; not always involve huge differences in scale. For instance: Dzulynski and Sanders, 1962; Karcz, 1968; Baker, 1973; Normark et al. (1979) described 500 m wide crescentic Allen, 1982; Sharpe and Shaw, 1989; Shaw, 1994; Lorenc scour marks on the Navy Submarine Fan; Fildani et al. et al., 1994). (2006) described scours along the Monterey East Channel A recent thorough synopsis of sculpted forms by that are up to 4.5 km wide, 6 km long and 200 m deep; Richardson and Carling (2005) divides strictly fluvial forms and Piper et al. (2007) described a turbidity current scour into three topological types: (1) concave features that on the Laurentian Fan, 2 km long and 100 m deep. More include potholes and furrows; (2) convex and undulating importantly, the analogy is strong because there are a large forms such as hummocky and undulating forms; and (3) number of points of similarity between Livingstone drum- composite forms such as compound obstacle marks. How- lins (the ones originally compared to the scour marks) and ever, here a simpler classification is used, based on Kor et al. inverted sole marks. Lorenz (1974) stated that the improb- (1991), that groups forms into three types (Table 6.2) based ability of coincidental similarity is 1:2 (1−n), where n is the on their orientation relative to the flow direction: (1) trans- number of points of similarity. In this case, there are con- verse s-forms are usually wider than they are long, where servatively eight points of similarity: (1) orientation with length is measured parallel to the formative flow direction; flow; (2) appearance in fields of similar form; (3) steeper (2) longitudinal forms are generally longer than they are at the upstream end; (4) spindle form; (5) parabolic form; wide, with their long axes lying parallel to the flow direc- (6) transverse asymmetrical form; (7) barchanoid form; and tion; and (3) non-directional forms have no obvious rela- (8) infilling by current-bedded sediment.