Micromorphology of iceberg scour in clays: Glacial Lake Agassiz, Manitoba, Canada

Quaternary Science Reviews 55 (2012) 125e144

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Micromorphology of iceberg scour in clays: Glacial Lake Agassiz, Manitoba, Canada Lorna D. Linch a, *, Jaap J.M. van der Meer a, John Menzies b a b

Centre for Micromorphology, School of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom Earth Sciences Department, Brock University, 500 Glenridge Avenue, St. Catharines, L2S 3A1 Ontario, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 March 2012 Received in revised form 10 July 2012 Accepted 12 July 2012 Available online 4 October 2012

Icebergs plough through unconsolidated lake/sea sediments gouging out kilometre long grooves, 100s of metres wide and tens of metres deep. Although the surface morphology of iceberg scours is welldocumented, little is known about what scours look like in stratigraphic section, particularly where surface characteristics are absent (e.g. through decay or burial). This investigation establishes a definitive suite of diagnostic criteria for identifying iceberg scours in clays by macroscopically and microscopically (2D thin sections) examining sediment deformation below iceberg scours in former Glacial Lake Agassiz, Manitoba, Canada. Structures unequivocally attributed to iceberg scour in this investigation include subhorizontal microfabrics, folds, sheath folds, augen-shapes, normal faults, reverse/thrust faults, discrete shears, partially destroyed clasts, intraclasts type III, multiple domains, water escape structures, flow structures, unistrial plasmic fabric, dropstones, realigned bedding, and specific structural sequences. This research will be particularly valuable in palaeoenvironmental reconstruction and in predicting glacial dynamics. In addition, it may eventually aid structural engineering on polar shelves, which could be of great value to oil and gas companies. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Iceberg keel scour Micromorphology Deformation Glacial Lake Agassiz

1. Introduction Iceberg scouring occurring in offshore water zones (30e500 m) (Dredge, 1982) is a post-depositional mechanical process caused by free-drifting icebergs that periodically contact unconsolidated lake floor or seabed sediment when their draft exceeds water depth (Gipp, 1993). If a grounded iceberg continues to be driven forward by waves and currents (Woodworth-Lynas, 1996), wind (Carlson et al., 2005), and the energy released during calving events (Bass and Lever, 1989), its keel will plough through sediment in an identical manner in both glaciolacustrine and glaciomarine environments (Woodworth-Lynas, 1996) creating curvilinear or straight grooves (Fig. 1). Using sonograph records and even manned submersibles (e.g. Josenhans, 1987; Hodgson et al., 1988) great efforts have been made to understand offshore grounding and scouring processes of icebergs because of their influence on topography and geological properties of sediment (Lien, 1983; Longva and Bakkejord, 1990), which are decisive factors when offshore man-made structures (e.g. oil/gas pipelines) are installed. Recently Kilfeather et al. (2010)

* Corresponding author. Tel.: þ44 (0)7533 946 738. E-mail address: [email protected] (L.D. Linch). 0277-3791/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2012.07.008

microscopically examined one iceberg-turbated sediment sample from a core from the East Greenland continental shelf. In addition, several studies have investigated relict onshore iceberg scours, documenting a range of sub-scour deformation structures to several metres in depth (e.g. Woodworth-Lynas and Landva, 1988; Longva and Bakkejord, 1990; Woodworth-Lynas and Guigné, 1990; Woodworth-Lynas, 1992; Rocha-Campos et al., 1994; Eyles et al., 1997; Eden and Eyles, 2001; Eyles et al., 2005; Eyles and Meulendyk, 2008: see Eden and Eyles (2001) and Graham (2007) for a summary of ice keel scour investigations to date). Already some of these findings are incorporated into the design and planning of offshore structures in polar regions. However, there remains lack of detail on vertical and horizontal displacement of sediment as well as preferred style and intensity of sub-scour deformation and how this correlates to scour processes. Such information is critical to future planning and installation of offshore structures where active scouring by icebergs occurs. This concern is particularly relevant with the onset of climate change and the predicted increase in number and abundance of icebergs in Arctic seas (Bond and Lotti, 1995; Bigg, 1999). In addition, although differentiating deformation by iceberg scour and subglacial processes has been problematic in the past (Kilfeather et al., 2010), recognising iceberg scouring in sediment may lead to the reinterpretation of sediments now seen as subglacial (Dowdeswell et al., 1994; Eden and Eyles,

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Fig. 1. Salient features of a typical continental shelf iceberg scour mark (modified from Woodworth-Lynas and Dowdeswell, 1994).

2001), which could change current ideas of glacial geomorphology and sedimentology, providing critical insights into ice sheet and glacier reconstructions (Gilbert et al., 1992; Dowdeswell et al., 1993, 1994; Eden and Eyles, 2001; Woodworth-Lynas and Guigné, 2003). Additional environmental reconstruction from this research includes identification of water bodies deep enough for wind and currents to drive ice floes forward during ice/bed interaction (Dredge, 1982; Woodworth-Lynas and Guigné, 2003); as well as inferences on water depth (Gilbert et al., 1992; Eden and Eyles, 2001), and palaeocurrent and palaeowind directions (Longva and Thoresen, 1991; Gilbert et al., 1992; Gipp, 1993; Eden and Eyles, 2001). Finally, this research may eventually contribute towards understanding the potential for ice keel scours related to conceptual models such as Late Neoproterozoic “Snowball Earth” (Hoffman and Schrag, 2000). This investigation examines the macroscopic and microscopic structure of iceberg scours in a fine-grained lacustrine sedimentary sequence in former Glacial Lake Agassiz (GLA), Manitoba, Canada. Particular attention is paid to the style and intensity of sub-scour deformation from both single scours (over the berms and in the area between the berms) and multiple scour (where two or more scours intersect); the latter remain completely unrecognised in the geological record (Woodworth-Lynas, 1996; Eyles et al., 1997). 2. Setting The area formerly occupied by GLA, near the small town of Lorette (southern Manitoba), was chosen primarily because it is heavily iceberg scoured (Clayton et al., 1965; Dredge, 1982; Woodworth-Lynas and Landva, 1988; Woodworth-Lynas and Guigné, 1990; Woodworth-Lynas, 1992) (Fig. 2). In southern Manitoba GLA was as deep as 213 m (Elson, 1967). The lake was supplied with water from melting of the Laurentide Ice Sheet (LIS) some 12.3 ka ago via meltwater channels or directly from the ice front (Teller, 1976; Teller and Clayton, 1983). Stratigraphy reflects changes in lake level in response to advance and retreat of the LIS. In southern Manitoba three stratigraphic clay-rich units overlie

carbonate-rich till (Teller, 1976). The lowermost unit (Unit 1), correlated to the Brenna Formation (BF), was deposited during the deep water Cass-Lockhart phase when ice advance blocked drainage outlets in northern Manitoba between c. 11.5e10.3 ka ago (Teller, 1976; Teller and Clayton, 1983), and is identified as a greye blue clay. Subsequent ice retreat unblocked outlets allowing the lake to drain and levels to fall (Teller and Clayton, 1983). Continued drainage during the low water Moorhead phase exposed the basin floor to subaerial processes between c. 10.3e9.9 ka ago (Teller and Clayton, 1983; Woodworth-Lynas and Guigné, 1990). The Moorhead phase ended when the LIS re-advanced and eastern outlets were dammed during the Emerson phase when the middle unit (Unit 2), which is correlated to the Lower Sherack Formation, was deposited over the northern part of the basin (Teller, 1976; Teller and Clayton, 1983). Observations from this investigation and from a study by Woodworth-Lynas and Guigné (1990) suggest Unit 2 is absent in the Lorette area. About 9.9 ka ago lake levels rose to as much as 110 m depth (Teller and Thorleifson, 1983) allowing deposition of the uppermost unit (Unit 3), which correlates to the Upper Sherack Formation (USF) (Teller, 1976; Teller and Clayton, 1983), and is identified as the brown clay in this investigation. This rise in lake level is thought to have triggered a surge of the glacier that subsequently disintegrated and produced numerous icebergs that scoured the previously exposed lakebed sediments (WoodworthLynas and Matile, 1988; Woodworth-Lynas and Guigné, 1990). Woodworth-Lynas and Guigné (1990) suggest deposition of the USF, which fills or partly-fills many scour troughs around Lorette, was underway during the time of scouring because the USF is incorporated into deformed clays of the underlying BF. Silt, thought to be transported by bottom currents (density flows) subsequent to scour formation, partially in-fills the iceberg scours and is identified as a yellow Silt Facies (SF). The silt is believed to be responsible for the scours’ post-emergence as low, positive-relief, ground surface ridges (Nielson and Matile, 1982; Teller et al., 1996). However, the timing, nature and distribution of silt deposition and reasons for ridge formation are not fully understood. The Emerson phase ended c. 9.5 ka ago with catastrophic outbursts, which allowed lake

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Fig. 2. Site location: A) Aerial photograph of Lorette, Manitoba, Canada, and scours investigated in this study; B) Overlay map on an aerial photograph showing local topographic relief, and locations of pits and slope transects. Note that light grey tracks are iceberg scours but apparent scour boundaries on the aerial photograph do not necessarily correlate to those boundaries identified on the ground. Differences in elevation between Global Positioning System (GPS) waypoints were no more than 5 m. Reproduced with the permission of Natural Resources Canada 2012, courtesy of the National Air Photo Library (A23700-46).

drainage into Lake Superior during the Nipigon phase (Fenton et al., 1983) and final drainage into the Tyrell Sea between c. 8.5e7.5 ka ago (Klassen, 1983; Teller and Thorleifson, 1983). Iceberg scours in GLA provide an opportunity to examine subscour sediment because: i) drainage of GLA, which was probably too rapid for littoral processes to destroy deep water features, and the nature of the sediment (fine-grained silt and clay) means scour morphology is defined and intact, appearing well-preserved at the ground surface (Dredge, 1982); ii) laminated lacustrine sediment provides ideal markers to display deformation structures associated with scouring (Woodworth-Lynas and Guigné, 1990; WoodworthLynas, 1996) and clays are ideal for the development and identification of microscopic plasmic fabrics (Linch, 2010); iii) sub-scour features are preserved below 2e3 m of silty, clayey sediment, thick enough to protect against the effects of agricultural activities and deformation related to seasonal freeze-thaw of the ground (Woodworth-Lynas and Landva, 1988); iv) regional slope in this part of the Lake Agassiz basin is insignificant, which precludes debris flow in this area (Woodworth-Lynas and Landva, 1988); and v) there was no further re-advance of glacial ice into the area after the glaciolacustrine sequence was deposited, which suggests that any observable deformation can only be associated to scouring. In addition, GLA remains one of the only known sites in the World where single scour events can be examined. Examining single scour events is necessary before attempting to identify multiple (superimposed) scours or ice keel turbated sediment (Vorren et al., 1983; Barnes and Lien, 1988). 3. Materials and methods 3.1. Scour and pit selections By using aerial photography, high-resolution mapping, and relief profiling, two straight, intact and well-preserved single scours were chosen for examination (Single Scour 2 and Single Scour 3) (Fig. 2). In addition, an area of multiple scour, where two or more scours intersect (Multiple Scour 1), was identified in order to

compare the sub-scour structure to those sites scoured by single events (Fig. 2). Scour relief was measured using a pantometer (after Pitty, 1968) along a series of transects that crossed scour widths and were spaced 20 m apart along scour lengths (Linch, 2010). A series of four 15 m2 pits, 6 m þ deep were excavated on the three chosen scours. Pits were built both between scour boundaries and across scour boundaries to check for differences transverse to the scour direction (after Woodworth-Lynas and Guigné, 1990). Pit positions were as follows: i) Pit 1 on Multiple Scour 1; ii) Pit 2 over the northwest boundary/berm of Single Scour 2 (scour width is w84 m in the field); iii) Pit 3 over the northeast boundary/berm of Single Scour 3 (scour width is w171 m in the field); and iv) Pit 4 between the boundaries of Single Scour 3 (Fig. 2). Finally we ensured pits were positioned away from the start/end of scours where equilibrium between the ice and sediment (Woodworth-Lynas et al., 1991) probably had been reached. Pits were macroscopically described and sampled for thin section micromorphological analysis (Linch, 2010). 3.2. Micromorphological sampling and description Fifty undisturbed samples were collected for micromorphological analysis from pit exposures following the methodology detailed in van der Meer (1995). Samples were selected based on macroscopic structure of the sediment or anywhere considered representative of the sediment section as a whole. Non-random sampling in this way ensured that structures were not missed. Samples were air and/or acetone dried, impregnated with resin, then cut and mounted following the procedures described in Lee and Kemp (1993), Carr and Lee (1998), and Menzies (2000). Thin sections were mostly cut transverse to iceberg scour direction on the basis that style and intensity of stress is less likely to change longitudinal to scour direction once equilibrium between the sediment and ice keel is reached (assuming grain size remains the same along the length) (Woodworth-Lynas et al., 1991). One thin section was cut longitudinal/parallel to iceberg scour direction and

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Table 1 Micromorphological deformation structures in Multiple Scour 1 (Pit 1). Key: Yellow ¼ SF; Light brown ¼ USF; Dark Brown ¼ BF; Comp ¼ Compression; P/dep ¼ Post ¼ High abundance, welldepositional; DA ¼ Dominant Angle; Ba ¼ Banding; Be ¼ True sedimentary bedding; S ¼ Shear; F ¼ Flow; BO ¼ Boudinage; L ¼ Lamination; ¼ Moderate abundance, well-developed; CC ¼ Moderate developed; CCC ¼ High abundance, moderately developed; BBB ¼ High abundance, weakly developed; abundance, moderately developed; BB ¼ Moderate abundance, weakly developed; ¼ Low abundance, well-developed; C ¼ Low abundance, moderately developed; B ¼ Low abundance, weakly developed.

another was cut horizontal/plan view to iceberg scour direction to examine: i) potential deformation of the pit face from penetration of the backhoe bucket during excavation; ii) potential deformation of the sides of the sample during the insertion of the sampling tin; iii) three-dimensional structure of the sediment, longitudinal to scour direction (Linch, 2010). Results from these two thin sections demonstrate that the backhoe did not penetrate more than 3 cm into the pit face and consequently any thin section cut at least 3 cm in from the pit face (standard practice throughout this study) was not affected by backhoe-induced stress and deformation (Linch, 2010). All thin sections were studied using a Petroscope and Leica M420 petrographic microscope. Descriptions followed classifications developed by Brewer (1976), van der Meer (1993, 1997) and Menzies (1998, 2000). Micromorphological data are presented in Tables 1e4 (after Menzies, 2000; Carr, 2004; Tarplee, 2006; Linch, 2010). 3.3. Bulk sediment analyses Grain size, carbonate content and clay mineralogy and plasticity indices are important factors in the context of the microscopic examination of sediment because they can affect the detectability, style and intensity of sediment deformation. A number of bulk samples considered texturally representative of each facies at each site were collected in sealed, airtight sample bags. The methods employed follow those of Sheldrick and Wang (1993) for grain size analysis; De Kimpe (1993) for clay mineralogy; Goh et al. (1993) for carbonate content; and McBride (1993) for plasticity indices.

4. Descriptions Surface mapping confirms that iceberg scours in this area generally comprise wide, inconsistent low-relief ridges (Fig. 3). This is probably attributable to a combination of agricultural practices and irregular deposition of the SF, believed to be responsible for ridge formation within scours (Nielson and Matile, 1982). 4.1. Silt Facies (SF) Macroscopically the SF comprises poorly sorted, sandy silt (<15e25% clay, 60e80% silt and <10e25% sand; 2.5 Y 6/2 to 2.5 Y 6/ 4) with a few sub-rounded pebbles (<1e4 cm) and some clayey silt intraclasts (<2 cm) (Linch, 2010). It occurs sporadically. For example, in Pit 2 the SF is a 1 m thick bed while in Pit 3 it occurs as sharply bounded lenses, 30 cm to several metres in length, embedded within the clay (Fig. 4). Clay mineralogy and the plasticity index are typical of non-expansive sediment and at >25% carbonate content indicates plasmic fabric may be more significant in the SF than observed in this investigation (Linch, 2010). Microscopically the SF comprises sandy silt interspersed with some clayey laminae (>50e200 mm thick laminae) and/or clayey inclusions. Sand grains (<100e500 mm) ranging from well-rounded to very angular occur randomly or in indistinct sub-vertical (140e160 ) bands and small clusters. An isolated limestone clast (6.5 mm) is half-bound by a halo of grey silt. Voids are limited comprising planar jig-saw fissures e.g. between boundaries,

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Table 2 Micromorphological deformation structures in Single Scour 2 (Pit 2). Key: Yellow ¼ SF; Light brown ¼ USF; Dark Brown ¼ BF; Comp ¼ Compression; P/dep ¼ Post depositional; ¼ High abundance, well-developed; DA ¼ Dominant Angle; Ba ¼ Banding; Be ¼ True sedimentary bedding; S ¼ Shear; F ¼ Flow; BO ¼ Boudinage; L ¼ Lamination; ¼ Moderate abundance, well-developed; CC ¼ Moderate abunCCC ¼ High abundance, moderately developed; BBB ¼ High abundance, weakly developed; dance, moderately developed; BB ¼ Moderate abundance, weakly developed; ¼ Low abundance, well-developed; C ¼ Low abundance, moderately developed; B ¼ Low abundance, weakly developed.

rounded to elongated packing voids (250e500 mm), and rounded vesicles (<250 mm). Deformation structures are sporadic (Linch, 2010) and include the following: boudinage; a chaotic pattern of concave-up and convex-up single and nested folds (Twiss and Moore, 2007), some rootless, and oriented without preference; low angle normal faults (offsets of w200 mme3.5 mm) that either cut across horizontal laminae and entire folds or develop on outer fold edges; fracture cleavages; intraclasts type III (silt or clay); silty multiple domains; flame-shaped clay inclusions and other wavy clay layers that appear to have undergone flow; and steep (60 ) SFUSF boundaries characterised by discontinuous clayey laminae, normal faults, silt lobes, angular silt blocks, and water escape structures (WES). An overview of all microscopic deformation structures is presented in Tables 1e4. 4.2. Upper Sherack Formation (USF) Macroscopically the USF (2.5 Y 4/2 to 2.5 Y N3), occurring to variable depths (tens of centimetres to several metres) over the whole of the investigated area, comprises horizontal (80e90 ), silty clay laminae (>80% clay, 5% silt and <10% sand) with a few small (0.5e1 cm), sub-rounded pebbles (without striae); softsediment (mostly carbonate or silt, and a few sand), sub-rounded clasts; and silty, discontinuous bands and lenses (Linch, 2010). In the upper reaches laminae, that alternate between light brownyellow (10 YR 5/6) and dark grey (10 YR 4/1), are w1.0 cm thick narrowing with depth (<0.3 cm). The upper USF is characterised by weak to moderate fissuring, forming columns that extend into the overlying topsoil mix where the SF is absent. Elsewhere fissuring

makes it difficult to distinguish where the USF meets underlying BF. Continuous (up to 2 m in horizontal length), vertical to sub-vertical shears and polished slickensided surfaces arranged in complex and cross-cutting orientations frequently traverse the USF-BF boundary (Fig. 5). Clay mineralogy reveals relatively high smectite (w5e6 : after Velde, 1992), which is noted for its high plasticity and cohesion, and marked swelling when wet and shrinkage on drying (Brady and Weil, 1999). This is consistent with the plasticity indices, which range from 45 to 75 where values >25 are typical for expansive clays (Brady and Weil, 1999); and complements the fact that the subsurface is renowned for causing problems during and after construction, particularly subsidence (pers. comm. R. LeBrun, 2007). Consequently, some deformation structures in thin section, particularly skelsepic plasmic fabric (Dalrymple and Jim, 1984), may be attributed to shrink/swell of sediment subsequent to the most recent lake retreat and subaerial exposure. Carbonate clasts observed in the field suggest eroded limestone probably exists within the matrix, which is consistent with laboratory analysis that reveals carbonate contents >20% indicating that plasmic fabric may be more significant in the USF than observed in this investigation (Linch, 2010). Microscopically the USF comprises horizontal clay laminae (<100 mm thick) (Fig. 6B); isolated, horizontally aligned angular sand grains (<200 mm) randomly distributed or in small clusters; and intact, horizontal to sub-horizontal sub-angular to subrounded carbonate clasts (500 mme14 mm) randomly distributed, in small clusters, or in a line/bed, and increasing in number with depth. Below clasts, laminae (and respective delineating fissures) are depressed and birefringence is enhanced while overlying

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Table 3 Micromorphological deformation structures at the boundary of Single Scour 3 (Pit 3). Key: Yellow ¼ SF; Light brown ¼ USF; Dark Brown ¼ BF; Comp ¼ Compression; ¼ High P/dep ¼ Post depositional; DA ¼ Dominant Angle; Ba ¼ Banding; Be ¼ True sedimentary bedding; S ¼ Shear; F ¼ Flow; BO ¼ Boudinage; L ¼ Lamination; ¼ Moderate abundance, well-developed; abundance, well-developed; CCC ¼ High abundance, moderately developed; BBB ¼ High abundance, weakly developed; CC ¼ Moderate abundance, moderately developed; BB ¼ Moderate abundance, weakly developed; ¼ Low abundance, well-developed; C ¼ Low abundance, moderately developed; B ¼ Low abundance, weakly developed.

laminae (and fissures) are draped. Horizontal planar jig-saw fissures (<500 mm to millimetres in width) that increase with depth occur between laminae as well as delineate laminated structure where laminae are not obvious. Deformation structures are sporadic (Linch, 2010) and include the following: normal faulting (120 ) (Fig. 6CeE); clusters of irregularly-shaped sandy silt inclusions (<500 mme1 mm) that are not as coherent as intraclasts; a linear silt deposit (Fig. 6D); a sub-rounded clayey intraclast (<1 mm) and an irregularlyshaped silt intraclast (Fig. 6E); small (<200e500 mm), finelylaminated elongated and horizontal golden clayey deposits; moderately developed domains comprising silty material as well as two inclusions (up to 4 mm) of grey sandy silt, and the occasional silty stringer or wisp (<1 mm); short (1 mm), discontinuous and sub-vertical lines of weakly developed unistrial plasmic fabric cutting across horizontal beds of birefringence; kinking plasmic fabric (adjacent to a clast); bedding-parallel plasmic fabric and/or thick beds (several millimetres) of birefringence, displaced to form lozenge/augen-shaped pockets; and steep USF-BF boundaries (Tables 1e4). 4.3. Brenna Formation (BF) 4.3.1. Macroscopic The BF (5 Y 4/1 to 5 Y 3/2 to 2.5 Y N3) comprises silty clay laminae (70e95% clay; <10% silt; and <20% sand) <0.1 cm thick and of uniform grain size (upper reaches) or alternating light grey silt (10 YR 6/8) and grey clay (5 Y 4/1) with depth (Linch, 2010).

Laminae drape over, and are depressed under, the few sub-rounded pebbles (centimetres) and striated cobbles that occur (upper reaches). In Multiple Scour 1 (Pit 1) a sandy gravelly band (0.75 cm thick) overlies heavily deformed silt and clay. Deformation (millimetres to centimetres), prevalent mostly in the lower reaches of the BF, includes: distortion of primary sedimentary features; attenuated carbonate clasts; folding and overturned laminae; normal and reverse/thrust faulting; sediment domains; and blocks of reoriented laminae (Fig. 8). In addition, complex, cross-cutting vertical to sub-vertical slickensided surfaces and shears (Fig. 7) are common throughout, particularly in the upper BF. There are more soft-sediment carbonate clasts (0.5e3 cm) in the BF than in overlying USF and calcareous concretions and vertical veins (calcitans) are common. Similar to the USF, clay mineralogy and plasticity values suggest the BF is prone to shrink/swell, and the presence of carbonate may mask plasmic fabric (Linch, 2010). 4.3.2. Micromorphology Microscopically (see Linch, 2010) the BF comprises clay laminae (<100 mm thick) largely undisturbed in the upper reaches but reoriented and deformed with depth. Angular sand grains (typically <500 mm) are random, clustered, or in bands. Angular to subrounded carbonate clasts (<500 mme15 mm), occurring throughout the BF, are random or arranged in layers and are partially destroyed where deformation increases. Clasts are typically sub-horizontal with some horizontal or vertical. Planar jigsaw fissures, which increase with depth, delineate folded laminae and sometimes faults.

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Table 4 Micromorphological deformation structures between the boundaries of Single Scour 3 (Pit 4). Key: Yellow ¼ SF; Light brown ¼ USF; Dark Brown ¼ BF; Comp ¼ Compression; ¼ High P/dep ¼ Post depositional; DA ¼ Dominant Angle; Ba ¼ Banding; Be ¼ True sedimentary bedding; S ¼ Shear; F ¼ Flow; BO ¼ Boudinage; L ¼ Lamination; ¼ Moderate abundance, well-developed; abundance, well-developed; CCC ¼ High abundance, moderately developed; BBB ¼ High abundance, weakly developed; CC ¼ Moderate abundance, moderately developed; BB ¼ Moderate abundance, weakly developed; ¼ Low abundance, well-developed; C ¼ Low abundance, moderately developed; B ¼ Low abundance, weakly developed.

Bedding realignment in several directions is a significant feature in the BF, particularly below w4.42 m where deformation is prevalent (henceforth referred to as ‘zones of high deformation’) (Fig. 9). For example, in Multiple Scour 1 (Pit 1) bedding remains horizontal to w5.17 m depth and deformation structures are few, while below this bedding is sub-horizontal (60e135 ) in a heavily deformed zone. Clast haloes occur only in Multiple Scour 1 (Pit 1) and between the boundaries of Single Scour 3 (Pit 4) in zones of high deformation (Fig. 10A). Asymmetric, upright to recumbent folds (over tens of centimetres) occur throughout the BF (Fig. 10BeD) but are most abundant and well-developed in the zones of high deformation in Multiple Scour 1 and on the berm and between the berms of Single Scour 3. Folds occur in association with: overlying undisturbed laminae; realigned bedding; within lenses; and normal, reverse and thrust faults. Sheath folds occur in Multiple Scour 1 (Pit 1) and between the boundaries of Single Scour 3 (Pit 4) (Fig. 10E) in zones of high deformation. Augen-shapes, including tapered-ended carbonate clasts, multiple domains, and silt and clay lenses (Fig. 10F) occur in low abundance in the BF. The BF is heavily faulted (Fig. 10B, G and H) but faults are laterally discontinuous (on a scale of tens of centimetres). Normal faults occur throughout, while reverse faults (with dips >45 ) and the more common thrust faults (with dips <45 ) (Twiss and Moore, 2007) occur only in Multiple Scour 1 (Pit 1) where they decrease with depth, and between the boundaries of Single Scour 3 (Pit 4). Faults can either i) cut across structural features on a variety of angles (0e175 ); ii) develop on the outside hinge of folds (i.e.

normal faults); and iii) develop on the inside hinge of folds (i.e. reverse/thrust faults). While many faults are sharp and closed others appear diffuse and ‘watery’ (Fig. 10G). Sharply-defined fracture cleavages, with a normal sense of direction, occur occasionally. Discrete shears occur on a variety of angles (20e145 ) either randomly or within folds. Intraclasts type III are infrequent. However, irregularly-shaped silt and carbonate inclusions (Fig. 10I), and the occasional silt lens (Fig. 10J) occur within zones of high deformation. Multiple domains, although discontinuous over tens of centimetres, are frequent particularly in zones of high deformation (Fig. 10K) and where brittle structures (e.g. faults) are less frequent. Some multiple domains are overprinted by faults, while others actually incorporate the remains of faulted laminae, or lie adjacent to isolated blocks of laminated sediment. WES are well-developed but fairly infrequent occurring in zones of high deformation (Fig. 10B, C, G, and L). Moderately developed to well-developed flow structures occur throughout (Fig. 10E) typically alongside multiple domains and/or WES. Skelsepic plasmic fabric is infrequent and moderately developed mostly occurring in the upper reaches of the BF where other deformations are few and bedding is largely horizontal. Skelsepic plasmic fabric typically forms thick (tens of microns) diffusely bounded bands around rounded carbonate clasts. Unistrial plasmic fabric is generally well-developed and frequent. It is most prevalent where deformation structures are abundant and well-developed but may also be identified within and below clast layers (e.g. Multiple Scour 1 (Pit 1) at 5.17 m depth). Unistrial plasmic fabric is

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Fig. 3. Ground surface relief of scours: A) Multiple Scour 1; B) Single Scour 2; C) Single Scour 3. Locations of transects are illustrated in Fig. 2.

identified as individual thin straight lines of birefringence (sub) perpendicular to bedding (Fig. 10M), bundles of branching and merging lines, merging into a fissure, and along fault lines. Bedding-parallel plasmic fabric is abundant and well-developed, identified as continuous and regular lines (micrometres thick) and beds of birefringence (millimetres thick). Where bedding and brecciated, discontinuous, augen-shaped (Fig. 10F), blocky beds of birefringence are locally reoriented; bedding-parallel plasmic fabric is also reoriented (Fig. 10N). In thin section, specific sequences of structures can be identified below 6.42 m depth in Multiple Scour 1 (Pit 1) and below 4.42 m depth between the boundaries of Single Scour 3 (Pit 4) in zones where deformation structures are well-developed and abundant. Sequences comprise three units characterised by particular structures: the top of the sequence comprises either undisturbed or disturbed laminae; the

mid-sequence comprises clay beds and/or a layer of carbonate/sand clasts (or vice versa); and the bottom of the sequence comprises disturbed laminae. Fig. 11 illustrates one example of one of these sequences. Finally, it is worth mentioning that in thin section sediment structure both longitudinal and transverse to scour direction is similar, except where heavily folded laminae are absent in thin section longitudinal to scour direction. 5. Discussion 5.1. Silt Facies (SF) Microstructures observed in the SF are not unique to lacustrine settings and thus should be tested against different groups of processes, such as gravity-driven and subglacial. However, we can

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coherent enough to allow preservation of these structures without homogenising sediment as slurry (Hiemstra, 2001). In addition to deformation by high energy flow, there is evidence of local displacement and/or redistribution of material down scour slopes in the form of boudins as well as faults cutting across entire structures (e.g. folds) and multiple domains. Certainly, a deformed boundary with a steep incline of 60 in the SF suggests slip of overlying material. At these depths (<3 m) it is probably also not uncommon for farming practices and/or frost action to cause minor displacement of sediment. In addition, shrink/swell of underlying and adjacent clay may have caused minor displacement in the SF. Other deformation structures present a more ambiguous history such as an isolated clast halo, which may have been deposited through melt-out from seasonal/perennial floating ice. Ice rafted debris (IRD)-like sand grains within silty bands from seasonal/ perennial floating winter ice (van der Meer and Warren, 1997; Tomkins et al., 2009) is not unknown, but not widespread in the SF. 5.2. Upper Sherack Formation (USF)

Fig. 4. Typical stratigraphy of the investigated area (Pit 4). Dashed lines mark lithofacies boundaries.

dismiss subglacial deformation and debris flows on natural slopes because this part of the basin was not overrun by glacial ice after deposition of lake sediments, and the overall slope is negligible. Furthermore, the suite of structures (style and intensity) observed in the SF is totally unlike those observed in subglacial and masswasting sediments, both of which are discussed later. Poorly-sorted, partially deformed, pebbly sandy silt is consistent with deposition of the SF by sporadic density currents acting as a source of coarse-grained sediment in deep lakes (Reading, 1998). Its spatial discontinuity suggests the SF does not fill all scours along their entire lengths as previously suggested (Nielson and Matile, 1982; Teller et al., 1996) while lenses of silt within clay imply the SF and some of the upper USF were deposited simultaneously, which is consistent with observations by Woodworth-Lynas and Guigné (1990). Deformation by high energy turbulent flow, which reworked and disaggregated the SF and some underlying/adjacent clay is demonstrated by a multitude of structures (Linch, 2010): i) boudin-shapes that formed under rotational shear during ductile deformation (van der Meer, 1993; Menzies et al., 1997; van der Wateren et al., 2000) or liquefaction and water escape (Phillips et al., 2007); ii) folds of semi-coherent laminae in all directions that indicate drag (Hendry and Stauffer, 1975); iii) low angle normal faults associated with extension of folds (Twiss and Moore, 2007) after a reduction in water content (Phillips and Auton, 2000); iv) multiple domains that indicate fluidisation/liquefaction (van der Meer, 1993; Menzies, 1998, 2000; Phillips and Auton, 2000) and sediment mixing; v) flow structures that developed under elevated water pressures; and vi) isolated clay laminae, inclusions and intraclasts type III. However, turbulence must have remained low enough and/or sediment

The fine-grained, laminated structure of the USF together with few features that indicate reworking suggest it was deposited in distal low energy standing water (van der Meer et al., 1992) with any subsequent deformation occurring under a low stress regime (Carr et al., 2000). Clasts, many with depressed/draped laminae and enhanced birefringence, support an origin as IRD (Carr et al., 2000) (Fig. 12), although it is also possible that smaller grains were derived from wind-blown sources (van der Meer et al., 1992). Those that are soft yet remain intact support minimal sediment disturbance after their deposition. The fissile columnar structure is postdepositional, probably in response to dewatering during subaerial exposure (Jim, 1990). The USF (known locally as “sticky gumbo” pers. comm. R. LeBrun, 2007) is infamous for causing movement during construction and agricultural management. Therefore, highly polished slickensides, typical of expansive (smectite) clays (e.g. vertisols, see Nieuwenhuis and Trustrum, 1977; Blokhuis et al., 1990; Tessier et al., 1990; Brady and Weil, 1999), probably indicate internal movement during shrink/swell (e.g. during subaerial exposure), rather than shear by iceberg scouring (cf. WoodworthLynas and Guigné, 1990). Other deformation structures indicative of shear such as normal faults, unistrial plasmic fabric, kinking plasmic fabric, beddingparallel plasmic fabric, and blocky beds and lozenge/augenshaped pockets of birefringence probably developed in response

Fig. 5. Perpendicular polished slickensided surfaces in the USF (Pit 3). Pen is 14.5 cm long.

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Fig. 6. Thin section 1M1aa and accompanying microphotos taken from the USF (Pit 1): A) thin section 1M1aa; B) continuous, horizontal laminae; C) normal faults in horizontal laminae; D) silt structure that has undergone flow and some normal faulting; E) irregularly-shaped silty intraclast showing normal faulting.

to dewatering, compaction and compression during shrink/swell (Nieuwenhuis and Trustrum, 1977; Dalrymple and Jim, 1984; Blokhuis et al., 1990), farming practices, frost action, overloading (van der Meer and Warren, 1997; Phillips et al., 2007) and minor gravity-driven sediment redistribution down scour slopes (Linch, 2010). In addition, little other evidence of high external stress suggests domains reflect matrix packing density variations (Menzies and Zaniewski, 2003), and/or shear-induced volumetric changes (Hiemstra and Rijsdijk, 2003) e.g. during shrink/swell.

5.3. Brenna Formation (BF) The following discusses the textural and structural features of the BF, highlighting those that can be attributed to iceberg scour

Fig. 7. Shear plane in the BF (Pit 2). Spatula is 24.9 cm long.

(Linch, 2010). A summary of deformation structures attributed to iceberg scour are illustrated in Table 5 and in Fig. 12. 5.3.1. Matrix and skeletal grains The fine-grained, laminated structure suggests deposition of the BF in low energy, standing water (van der Meer et al., 1992) distal from the ice margin (Carr et al., 2000). Absent silt-clay laminae in the upper BF indicate that supply of coarse-grained silt ceased over time. In the upper BF absence of features that indicate reworking suggest any subsequent deformation was localised or occurred under a low stress regime (Carr et al., 2000). However, iceberg scouring is believed to have begun only after deposition of the BF and at the start of deposition of the lowermost USF (Woodworth-Lynas and Matile, 1988; Woodworth-Lynas and

Fig. 8. Folding, faulting, silt domains, Fe (iron) stain and carbonate clasts in the BF (Pit 4). Pencil is 14.5 cm long.

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Fig. 9. Realigned bedding in the top half of thin section 4B21aa, taken from the BF (Pit 4). All unlabelled boxes indicate carbonate clasts. NF ¼ normal faults; RF ¼ reverse faults; TF ¼ thrust faults.

Guigné, 1990), suggesting that all the BF should be deformed. Large striated cobbles resting in what is believed to be largely undisturbed upper BF support the presence of icebergs after the main scouring event. Underlying lowermost BF examined in this investigation is heavily deformed and reoriented by iceberg scour. Clasts, many with depressed/draped laminae and enhanced birefringence, support an origin as IRD (Carr et al., 2000) (Fig. 12). Carbonate clasts, common to this glaciolacustrine sequence, probably originate from carbonate-rich till from central or northeastern Manitoba. Their increase in the BF supports an increase in the number of debris-laden floating ice masses, while partial destruction and distortion of carbonate clasts suggests deformation by iceberg scouring (Fig. 12). Variable grain shapes reflect transport processes such as grain abrasion and edge rounding (Carr et al., 2000) to crushing (Hiemstra and van der Meer, 1997; Carr et al., 2000; Carr, 2001). Low grain density and the understanding that iceberg scour occurs in an unconfined environment, suggests observed grain shapes indicate previous (glacial) processes (Mahaney et al., 2004). Nevertheless, if iceberg scour does influence grain shape, abraded and crushed grains can no longer be considered exclusive to subglacial environments. Similarly, sub-horizontal grains indicating high magnitude unidirectional lateral stress (Carr et al., 2000; Carr, 2001, 2004; Carr and Rose, 2003), no longer remain exclusive to subglacial environments either (Fig. 12). 5.3.2. Rotation, compression and slump There are a number of microstructures in the BF (which are absent in overlying USF) that are related to rotation, compression and slump. Firstly, clast haloes, also known as ‘clast/intraclast

casings’ (Hiemstra, 2001), ‘grain coatings’ (Kilfeather et al., 2010); and ‘aggregates’ with coatings of fine-grained material (van der Meer et al., 2010); are not well understood and could reflect a number of processes, which is why they are not included in Fig. 12. Their absence in overlying USF however, suggests an origin of rotation (van der Meer, 1993; Lachniet et al., 1999, 2001; Hiemstra, 2001; Phillips, 2006; Kilfeather et al., 2010) induced by iceberg scour in the BF. Secondly folds and sheath folds, the latter of which develop under high strain in sections perpendicular to shear direction (Kluiving et al., 1991), are attributed to iceberg scour. This is supported by their presence in zones of high deformation and realigned bedding attributed to iceberg scour, their absence in overlying USF, and the unlikelihood of their development by other processes. It also complements previous investigations that assign (macroscale) folds and sheath folds to ice keel scour (e.g. Eyles and Clark, 1988; Woodworth-Lynas and Landva, 1988; Longva and Bakkejord, 1990; Woodworth-Lynas and Guigné, 1990; van der Meer et al., 1992; Rocha-Campos et al., 1994; Eyles et al., 1997; van der Meer and Warren, 1997; Eyles et al., 2005). If clast haloes develop by iceberg scour they may do so during sediment rotation (Fig. 12): i) similar to subglacial environments by direct downward stress from the scouring iceberg keel; ii) related to the initial impact of a scouring iceberg when stress is high enough to penetrate the sediment and allow intense keel modification before equilibrium is reached (WoodworthLynas et al., 1991) i.e. cumulative strain (Benn and Evans, 1998);

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Fig. 11. Thin section 1B21aa demonstrating a structural sequence in the BF (Pit 1) comprising 1) an upper unit of preserved laminae, 2) a mid-sequence unit of carbonate clasts (sandy limestone layer) and underlying clay, and 3) a lower unit of disturbed (heavily folded and faulted) laminae. All unlabelled boxes indicate carbonate clasts.

iii) indirectly as a result of debris flows by: a) impact during calving and by the ploughing process itself when large amounts of sediment are transported from the trough to the berms as a result of combined squeeze and push (Lien et al., 1989); b) the forward movement of the iceberg, which can have a pumping effect that sets up strong local currents that initiate continuous debris slide (Lien et al., 1989); c) turbulence created by the ice keel in the same way it is created around flow-deflecting boulders (Phillips, 2006; Kilfeather et al., 2010); d) gravity-induced collapse of the surcharge during the ‘conveyor belt’ action (Woodworth-Lynas et al., 1991); e) redistribution of cohesive blocks or non-cohesive sediment down the inner scour slopes back into the trough or down the outer slopes after the keel has passed (Lien et al., 1989; Woodworth-Lynas et al., 1991); and f) collapse of sediment in the space left by melt-out of blocks of ice broken from the keel and driven into the sediment (Woodworth-Lynas et al., 1991). Compression and high strains necessary for development of folds and sheath folds respectively may occur if confined conditions are created below the keel during iceberg scour (Fig. 12). However, folds are more likely to develop in response to horizontal pressures by the lateral displacement of sediment during passage of the keel (Woodworth-Lynas and Landva, 1988; Woodworth-Lynas and Guigné, 1990) as well as in front of the iceberg where resultant structures will be later overridden (Fig. 12). Certainly, Woodworth-Lynas and Guigné (1990) noted

folds immediately below a scour incision surface (the latter of which was not modified by the folds) implying the keel may be in contact with sediment either during fold generation or immediately after so that folds become truncated (Woodworth-Lynas and Landva, 1988; Woodworth-Lynas and Guigné, 1990). In addition, in our investigation chaotic fold geometry lacking in symmetry suggests continuous strain and progressive rotation of the principal strain axes as the keel advanced, overrode and moved away from the folded zone (Woodworth-Lynas and Landva, 1988). Moreover, rootless folds indicate high strain reworking whereby overturned and attenuated fold limbs are disrupted (van der Wateren et al., 2000). Folds by slump in front (in the surcharge) and to the sides of the forward ploughing keel must also be common in sediment that is unconstrained and capable of moving in all directions (Fig. 12). Preserved laminae overlying folds in a variety of orientations, suggest several flow directions, precluding slump down scour slopes after the keel has passed. Finally, rotation/compression/slump structures associated to other deformation structures may indicate several deformation events. For example, folds within realigned bedding must first have slumped or been compressed into shape before the block rotated, and folds that incorporate faults suggest that folds either predate faults or develop simultaneously (Phillips, 2006). Either way, the combination of ductile and brittle structures in the BF indicates polyphase deformation histories (Menzies, 2000; Phillips et al., 2007). 5.3.3. Planar shear Given that the BF is very similar in sedimentary characteristics to the USF, slickensides and shears may also represent shrink/

Fig. 10. Microstructures in the BF: A) halo (dashed line) around a limestone clast; B) folded and faulted (normal, reverse and thrust faults) laminae, and water escape structure (dashed line); C) water escape structure and folded laminae; D) folded sandy carbonate deposit; E) sheath fold (oval), reoriented laminae and clay flow structures (at top of image); F) clayey augen-shaped lenses only detectable under cross-polarised light; G) diffuse normal fault and adjacent water escape structure where horizontal laminae have collapsed (top right); H) laminated silt lens separated by a reverse fault; I) irregularly-shaped carbonate, reoriented droplet structure, disturbed overlying laminae and bed of displaced birefringence (dashed lines); J) silt lens with internally folded laminae; K) interfingering silt and clay multiple domains and stringers; L) water escape structure; M) unistrial plasmic fabric perpendicular to laminae; N) displaced, blocky beds of birefringence.

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Fig. 12. Micromorphological deformation structures attributed to iceberg scour processes in Glacial Lake Agassiz, Manitoba, Canada.

swell as described above, but an origin by iceberg scour cannot be dismissed. In contrast, augen-shapes, normal and reverse/thrust faults (including fracture cleavages) as well as discrete shears all indicate planar shear (van der Meer, 1993; Menzies, 1998, 2000; Carr, 1999; Phillips et al., 2007) by iceberg scour. This is supported by their presence in zones of high deformation and realigned bedding also attributed to iceberg scour, their general absence or local development in overlying USF, and the unlikelihood of their development by other processes. It also complements previous macroscale investigations that assign faults and fracture cleavages to ice keel scour (Eyles and Clark, 1988; Woodworth-Lynas and Landva, 1988; Woodworth-Lynas and Guigné, 1990; Longva and Bakkejord, 1990; Eyles et al., 2005). It has long been recognised that horizontal and vertical displacement of sea/lake bed sediment occurs as the keel moves through it, thus allowing development of failure planes and planar shear structures (Woodworth-Lynas and Landva, 1988; Woodworth-Lynas and Guigné, 1990; Woodworth-Lynas et al., 1991). Failure planes may occur by (Fig. 12):

i) compression from downward vertical motion of the keel (Poorooshasb et al., 1989); ii) compression some distance ahead of the scouring keel, particularly reverse/thrust faulting rather than normal faulting (Woodworth-Lynas and Landva, 1988); iii) compression between slabs of sediment sheared and displaced away from the advancing keel (Woodworth-Lynas and Guigné, 1990); iv) extension during shear and drag between the moving keel and the sediment at the sides and below the keel, causing tensional dilation and cracking (Woodworth-Lynas et al., 1991); v) extension as a result of stress release during sediment expulsion from beneath the edges of the keel (WoodworthLynas et al., 1991); vi) extension during gravity-induced shear on slopes of a resultant iceberg scour, particularly normal faulting rather than reverse/thrust faulting (Woodworth-Lynas and Guigné, 1990);

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Table 5 Summary of micromorphological deformation structures in Multiple Scour 1 and Single Scours 2 and 3. Key: Tick ¼ Present and attributed to iceberg scour; Grey box ¼ Absent and/or not attributed to iceberg scour; CCC ¼ High abundance; CC ¼ Moderate abundance; C ¼ Low abundance.

vii) compression or extension in response to crustal shortening/ extension of fold hinges, which indicate folds either predate or develop simultaneously to faults and shears (note that augen-shapes were not observed in association to folds in this study) (Woodworth-Lynas and Landva, 1988; Longva and Bakkejord, 1990); and viii) compression or extension in response to the weight and/or impact of clasts during distal deposition of IRD (Hiemstra, 2001) or in response to direct emplacement of IRD from the keel (Josenhans, 1987). In addition, normal faults may originally have had a reverse sense of movement yet subsequent passage of the keel allowed relaxation of the sediment due to stress release that caused reactivation of the fault with an opposite, normal sense of motion (i.e. behind the keel: Fig. 12) (Woodworth-Lynas and Landva, 1988). In the BF, multiple domains that have developed into augen-shapes, and faults that cut across multiple domains, provide examples where fluidisation is followed by dewatering and shear (Phillips and Auton, 2000; Phillips et al., 2007). To further complicate matters however, diffuse faults in the BF indicate water movement (Hiemstra, 2001) and fluidisation after planar shear, probably where earlier developed faults controlled the sites of water escape (Phillips et al., 2007). 5.3.4. Abrasion Partially destroyed soft-sediment carbonate clasts in zones of high deformation and realigned bedding indicate abrasion by iceberg scour probably below, lateral to and in front (including in the surcharge) of the keel (Fig. 12). Intact carbonate clasts in overlying USF, which is unaffected by iceberg scouring, support this. 5.3.5. Sediment mixing Intraclasts type III, which represent progressive incorporation of new material into deforming sediment by either rotation (van der Meer, 1993, 1995; Carr, 1999, 2001; Lachniet et al., 1999, 2001; Hiemstra, 2001) or IRD (van der Meer and Warren, 1997; Tomkins et al., 2009), and multiple sediment domains and stringers, which represent fluidisation/liquefaction typically initiated by an external source (van der Meer,1993; Menzies,1998, 2000; Phillips and Auton, 2000; Mahaney et al., 2004; Phillips, 2006) are indicative of

reworking by iceberg scour. This is supported by their presence in zones of high deformation and realigned bedding, and the unlikelihood of their development by other processes. It also complements previous observations of fluidisation/liquefaction in iceberg scoured sediment (Hodgson et al., 1988; Longva and Bakkejord, 1990), which is of little surprise given that fluidisation/liquefaction, fluid escape, and sediment domain interdigitation is typical where deformation occurs in sediment that is water-saturated (Mahaney et al., 2004; Phillips, 2006; Phillips et al., 2007). Rotation responsible for intraclast formation may occur by iceberg scour in the same ways described for clast haloes (above). Intraclasts are illustrated ubiquitously in Fig. 12 however, because they may also indicate IRD. Presence of soft-sediment intraclasts remain evidence that a sediment is generally immature in deformation history as opposed to a maturely deformed sediment (e.g. by subglacial deformation) that will be completely homogenous through reworking and comminution over prolonged periods of time (van der Meer, 1995). Strains sufficient to fluidise sediment and produce multiple domains are likely to occur either immediately below the scouring keel (particularly if confined conditions similar to a subglacial environment are created), and/or in high strain debris flows of noncohesive sediment in the surcharge directly in front of the keel and indirectly down steep-sided scour slopes (Fig. 12). 5.3.6. Pore water WES and flow structures indicate localised pore water movement under high pressures by rapid and forceful dewatering (Eyles and Eyles, 1983; Carr, 1999; Lachniet et al., 1999, 2001; Carr et al., 2000; van der Meer et al., 2010) by iceberg scour. This is particularly true where laminae appear collapsed, which complements Longva and Bakkejord (1990) who attributed collapsed laminae to water escape and fluidisation by iceberg scour. In addition, WES and flow structures only occur in zones of high deformation and realigned bedding attributed to iceberg scour, and are unlikely to be developed by other processes. It is no coincidence that multiple domains, WES, and flow structures typically occur side-by-side on the basis that they require similar conditions to develop. Water escape and flow may occur in high confining conditions during iceberg scour (e.g. below a large iceberg) in much the same way as in a subglacial environment (van der Meer, 1993; Menzies,

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1998, 2000; van der Meer et al., 2010) (Fig. 12). Pore water may also move in front of an advancing zone of high stress (Menzies, 2000) like in front of a scouring iceberg including the surcharge (Fig. 12). In addition, the initial impact of an ice keel will exert great stress below and lateral (Fig. 12) to the keel before equilibrium is reached, and will expel a lot of water. Water escape and flow may also develop during scour-induced sediment flow directed immediately away from the scouring ice keel (in front and to the sides) or as a result of gravity-induced flow (Lachniet et al., 1999, 2001) down the slopes of the scour after the iceberg has passed (Fig. 12). Finally, forceful dewatering will also develop in response to falling IRD (Fig. 12), which can account for some localised anomalies in the compaction of sediment (Hiemstra, 2001). Thus we interpret WES and any sign of forced water escape within laminated sediment as being caused by iceberg scour. 5.3.7. Plasmic fabric Plasmic fabric is an excellent way of recording stresses imparted on sediments as these stresses realign clay particles into oriented domains that are visible under the microscope. Flow materials rarely produce plasmic fabrics, but where they do it is typically poorly developed (Bertran and Texier, 1999; Lachniet et al., 1999, 2001; Hiemstra, 2001). Skelsepic plasmic fabric, which is indicative of rotational movement (van der Meer, 1993; Menzies, 1998, 2000; Carr, 2004), may indicate a wetting and drying regime (Dalrymple and Jim, 1984), which is why it is omitted from Fig. 12. Wetting and drying could be the cause of skelsepic plasmic fabric development in the upper BF where other deformation is limited, bedding remains relatively undisturbed, and high smectite and plasticity indices indicate the potential for shrink/swell. However, shrink/swell usually results in thin, tight-fit skelsepic plasmic fabric that fills all irregularities in the outline of a grain, while in the BF skelsepic plasmic fabric forms a wide zone, which is thought to indicate rotation (Hiemstra and Rijsdijk, 2003). Moreover, the absence of skelsepic plasmic fabric in overlying USF suggests shrink/swell is not necessarily responsible for its development. Skelsepic plasmic fabric in zones of high deformation and realigned bedding attributed to iceberg scour and rounded clasts around which it develops suggest it may indicate rotation by iceberg scour. If skelsepic plasmic fabric does indicate rotation and compression as it does in subglacial environments, then its increased abundance and development on the boundaries of Single Scours 2 and 3 (Pits 2 and 3) suggests rotation is common at the boundaries of single scours. Either way, soft and crumbly carbonate clasts must have maintained some cohesion to develop skelsepic plasmic fabric around their edges in the first place, suggesting that in GLA sediment skelsepic plasmic fabric developed easily and probably under low stress. Unistrial plasmic fabric is characterised by long and narrow oriented domains and indicates unidirectional planar shear reflecting failure associated with dewatering (van der Meer, 1995). Although processes by shrink/swell, overloading of overlying material, gravity redistribution and localised sediment displacement by deposition of IRD (Fig. 12) cannot be entirely dismissed as the cause of unistrial plasmic fabric, overall increase and development of unistrial plasmic fabric in zones of high deformation and realigned bedding suggests planar shear directly by iceberg scour (Fig. 12). This is supported by its development within folds and realigned bedding, which are attributed to compression/slump and rotation induced by iceberg scour. However, unistrial plasmic fabric in overlying USF and in the BF where bedding remains horizontal and relatively undisturbed suggests some development is independent of scouring. Branching and merging unistrial plasmic fabric is related to increasing strain, which Hiemstra and Rijsdijk (2003) suggest is partly controlled by sediment properties.

Bedding-parallel plasmic fabric is a response to compaction and initial dewatering of the sediment (Phillips and Auton, 2000; Phillips et al., 2007), which is typically attributed to lithostatic pressure (van der Meer and Warren, 1997; Phillips et al., 2007). In the BF beddingparallel plasmic fabric is most abundant where deformation structures are least abundant, which suggests it probably developed by compaction rather than from iceberg scouring. However, blocky beds and lozenge/augen-shaped pockets of birefringence indicate minor shear displacements (Hiemstra, 2001), which could be related to shrink/swell, overlying mass or shear. Because we cannot be sure bedding-parallel plasmic fabric is omitted from Fig. 12. 5.4. Additional characteristics of iceberg scoured sediment 5.4.1. Dropstones Dropstones, which are extant in the BF, point to floating ice (IRD) (van der Meer and Warren, 1997; Tomkins et al., 2009) (Fig. 12), although smaller particles may be derived from wind blown sources (van der Meer and Warren, 1997). Large striated cobbles certainly reflect icebergs. Depressed laminae and enhanced birefringence below clasts indicate localised compaction, which supports an origin as IRD. Substantial presence of IRD in the upper BF indicates an abundance of floating ice and/or ice containing high quantities of debris. 5.4.2. Realigned bedding Realigned bedding indicates excavation and subsequent rotation and overturn of cohesive blocks of preserved laminae by iceberg scour. Such blocks form the iceberg berm (e.g. Woodworth-Lynas et al., 1991; Woodworth-Lynas and Dowdeswell, 1994). Bedding that is realigned on a variety of orientations over short distances indicates chaotic redistribution of blocks that have been pushed away in all directions including immediately in front and to the sides of a scouring keel, as well as those that roll back down into the scour trough (Fig. 12). Reoriented bedding often includes a suite of well-developed deformation structures, which may be directly or indirectly related to iceberg scouring. 5.4.3. Structural sequences Structural sequences are combinations of two or more of the following units: undisturbed laminae, fluidised sediment, clast layers, and deformed laminated sediments; and they are typical for the lowermost BF. Their identification in zones of high deformation and realigned bedding, and association with well-developed clast layers that represent IRD, suggest development in response to iceberg scour. Woodworth-Lynas and Landva (1988) observed deformation immediately below a clast layer and normal sedimentation above it, which they interpreted as the iceberg scour incision surface followed by normal sedimentation. In this instance, clasts must be deposited directly at the ice-sediment contact (Josenhans, 1987) resulting in deformation of underlying sediment (see ‘Structural Sequence I’ Fig. 12). Partially destroyed carbonate clasts in IRD layers suggest stress was applied either during the emplacement of the IRD or after its deposition. However, where overlying laminae remain relatively undisturbed it is obvious that carbonate clasts were not affected after their deposition. In this investigation, underlying deformation that does not truncate the IRD layer suggests stress was transmitted from the clast layer down through the sediment. Preservation of the IRD layer suggests icebergs may simply glide across the scour surface after reaching equilibrium allowing the scour to remain of equal depth and width along its length (Woodworth-Lynas et al., 1991). Fluidised silt may represent flow of non-cohesive sediment from the banks of the scour down on top of the scour incision surface after the iceberg passed (see ‘Structural Sequence II’ Fig. 12). Realigned bedding on top of

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a carbonate IRD layer and underlying deformation (folds etc.) probably indicates a block of coherent sediment excavated out of the scour trough, which later rolled down the banks of the scour after the iceberg passed, landing on top of the scour incision surface (Woodworth-Lynas et al., 1991) (see ‘Structural Sequence III’ Fig. 12).

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Table 6 Suites of microstructures related to subglacial tills, mass-wasting deposits and iceberg scoured sediment. Key: Grey box ¼ Absent and/or not attributed to subglacial/mass-wasting/iceberg scour processes; Rare ¼ Rarely identified; CCC ¼ High abundance; CC ¼ Moderate abundance; C ¼ Low abundance.

5.4.4. Sediment structure longitudinal to iceberg scour The fact that we cannot see folds below the IRD clast layer in the longitudinal/parallel direction to iceberg scour suggests folds were compressed/slumped laterally and are better observed transverse/ lateral to iceberg scour direction. 5.4.5. Multiple scour vs. single scour Table 5 compares and summarises structures most likely to be attributable to iceberg scour in GLA. All investigated scours in GLA share a number of structural characteristics including: folds, augenshapes, normal faults, partially destroyed clasts, multiple domains, WES, flow structures, unistrial plasmic fabric, dropstones, realigned bedding, and sub-horizontal microfabrics. This demonstrates a range of brittle and ductile structures in both multiple and single scours. Of these structures, the most abundant and well-developed are folds, normal faults, multiple domains, unistrial plasmic fabric, and realigned bedding. In contrast, iceberg scoured sediment at the boundary pits of single scours demonstrates overall fewer structures. Sheath folds, reverse/thrust faults, intraclasts type III, and structural sequences are missing from scour boundaries yet are present in both Multiple Scour 1 (Pit 1) and between the boundaries of Single Scour 3 (Pit 4). This implies total stress endured in multiple scours and below single scours is probably higher than the stress at the boundaries of single scours. 5.5. Subglacial, mass-wasting and iceberg scour deformation Table 6 is a comparison of microstructures as present in subglacial tills, mass-wasting deposits and iceberg scour sediments. The frequency at which microstructures occur ranges on a scale from: ‘absent and/or cannot be attributed to subglacial/masswasting/iceberg scour processes’ (blank-grey box), to ‘rare’, to ‘increasing frequency’ (number of dots). However, quantification of micromorphological features is in its infancy and only test methods have been developed (Neudorf, 2008; Menzies and Whiteman, 2009; Patel, 2011; Linch and van der Meer, in press). Consequently the frequencies listed in Table 6 are based on the experience of the authors and thus are subjective (see Leighton et al., 2012). Because of the number of thin sections on which these estimates are based (Linch, 2010; van der Meer and Menzies, 2011) we do think that this is a valid approach. Although there are many microstructures that occur in all three environments, the subglacial and mass-wasting environments show the strongest overlap, especially in shear and rotation related microstructures. However, because of the differences in strength of the stresses imposed on the sediments in subglacial and masswasting environments, there are also differences between both environments in resultant microstructures, like the development of plasmic fabrics. In addition, differences in grain size will undoubtedly produce differences in microstructures in both environments. Iceberg scoured sediments show a distinctly different suite of microstructures (to subglacial and mass-wasting sediments), where folding, faulting, fluidisation and water escape are most important. The clear differences in suites of microstructures between the different environments indicate that micromorphology is a valid technique in the recognition of such sediments. The fact that micromorphology is particularly applicable to cores makes it an excellent tool in the study of the past presence and frequency of scouring icebergs.

6. Conclusions  Minor abundance and development of deformation structures in the upper BF suggest deposition of relatively undisturbed glaciolacustrine sediment. However, the lower BF demonstrates many deformation structures attributable to (direct/ indirect) iceberg scour processes including: sub-horizontal microfabrics, folds, sheath folds, augen-shapes, normal faults,

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reverse/thrust faults, discrete shears, intraclasts type III, partially destroyed clasts, multiple domains, WES, flow structures, unistrial plasmic fabric, dropstones, realigned bedding, and structural sequences. This suite of microstructures is distinctly different from the suite of microstructures related to subglacial tills and mass-wasting deposits; and can now be applied to detect the occurrence of former iceberg scour in cores (subject to grain size), where macroscopic structures are hardly detectable. Structures that are identified but cannot necessarily be assigned to iceberg scour in the BF include: slickensides and shears, clast haloes, skelsepic plasmic fabric, and bedding-parallel plasmic fabric. Presence of both ductile and brittle structures indicates polyphase deformation. Planar shear structures are more prevalent in iceberg scoured sediment than rotational structures. In addition, structures associated with fluidisation/liquefaction and high pore water are generally well-developed and abundant. Dropstones and sediment dumps on a range of scales characterise the BF, which supports the presence of large icebergs. Specific structural sequences and realigned bedding are exclusive to, and indicate, iceberg scour in clays. IRD layers underlain by deformation probably indicate the iceberg scour incision surface, which implies the incision surface is not very thick where the iceberg simply glides through the sediment. The iceberg scour environment is highly complex, comprising deformation i) distal to the scour; ii) lateral and in front of the scour, both in contact with the keel and more distally; iii) directly below the scour; and iv) in the area behind the scour after the keel has passed. Zones of realigned bedding and deformation begin at depths of between 3 and 8 m in the different pits in this investigation. Thus, it seems reasonable to assume that the iceberg scour incision surfaces occur at the depth where deformation starts. Deformation is then identified down to the bottom of all pits and thus, extends to depths of 0.34 m in Pit 1, 3.29 m in Pit 2, 0.48 m in Pit 3, and 2.15 m in Pit 4. Deformation extends to an unknown depth below the pit floors. Stress in a multiple scour and between the boundaries of a single scour is higher than the stress at the boundary of a single scour.

Role of the funding sources This research was primarily funded by NERC (NER/S/A/2006/ 14030), with supplementary funds awarded by the Foundation for Canadian Studies in the UK, International Association of Sedimentologists, Quaternary Research Association, and British Society for Geomorphology. The funding sources had no involvement in study design, in the collection, analysis and interpretation of data, in the writing of the report, and in the decision to submit the article for publication. Acknowledgements Thanks are due to Adrian Palmer and Dave Alderton (Royal Holloway University of London), and Candy Kramer and Daryl Dagesse (Brock University) for help in the lab, and Lori Decker for help in the field. Thanks also to Chris Woodworth-Lynas (PETRA International Ltd), Gaywood Matile (Manitoba Geological Survey), Jim Rose (Royal Holloway University of London), Julian Dowdeswell (University of Cambridge), Brian Jeremy Todd (Geological Survey of Canada), and Colm O’Cofaigh (Durham University) for input during discussion. Special thanks are due to Rolly and Eva LeBrun and

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