Megaflutes in a continental shelf setting, Placentia Bay, Newfoundland John Shaw a, ⁎, Pere Puig b , Guoqi Han c a Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, Dartmouth, NS, Canada B2Y 4A2 b Institute of Marine Sciences (ICM-CSIC), Barcelona, Spain c Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, St. John's, NL, Canada abstract article info Article history: Received 5 September 2012 Received in revised form 7 January 2013 Accepted 9 January 2013 Available online 18 January 2013 Keywords: Megaflutes Placentia Bay Newfoundland Seafloor erosion Megaflutes – erosional scours normally found in deep water on continental slopes – were identified in 1978 on sidescan sonograms and seismic reflection profiles from Placentia Bay on the south coast of Newfoundland, Can- ada. Data from recent extensive multibeam sonar surveys provide an opportunity to describe the morphology and distribution of the megaflutes in detail, and to consider the formative processes. They occur on the east side of the outer bay, at a depth of ~200 m, in a 2–3 km-wide swath that continues to the south into Halibut Channel, over a total distance of ~100 km. The megaflutes have been formed by removal of a layer of postglacial mud, exposing underlying glaciomarine sediments and releasing a volume of 4.5 km 3 . They occur in a range of forms, including single, multiple, and coalescent types, and in some areas at least their inception was related to pre-existing pockmarks. Radiocarbon dates from piston cores are used to demonstrate that megaflute forma- tion post-dated ca. 9 ka. Megaflute formation in Placentia Bay has been attributed to a ‘reverse flow’ from the tsu- nami generated by the 1929 Grand Banks earthquake, and this is still the accepted hypothesis. We argue that the return flow from a tsunami did not generate the megaflutes, and suggest instead that they could be formed dur- ing south-flowing density currents generated when volumes of cold saline water stored in the deep (>250 m) basins at the head of Placentia Bay are intermittently displaced and spilled south in a shallow channel at the east side of the bay after intense coastal surface dense water formation events, perhaps during very cold winters. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. 1. Introduction The term ‘megaflute’ was used by Lonsdale and Hollister (1979) to describe scours larger than the true flutes described by Allen (1971, 1985). Megaflutes have steep upstream ends, and shallow in the down- stream direction. They have a wide-ranging size distribution. For exam- ple, megaflutes in the Carboniferous Ross Formation of western Ireland (Elliott, 2000; Macdonald et al., 2011a) that formed in deep water have lengths up to 15 m, widths up to 10 m, and depths up to 1.5 m. In the summary diagram published by Wynn et al. (2002), scours with megaflute morphology were commonly 1 km long, several kilometers wide, and up to 20 m deep. Furthermore, they occurred in fields that in- cluded amalgamated scours and erosional remnants. In modern environments, megaflutes occur in deep-water settings, commonly in channel–lobe transition zones, and occur on larger scales (see summary in Macdonald et al., 2011b). Macdonald et al. (2011b) appraised the occurrence and dynamics of megaflutes and other ellipti- cal erosional bedforms in deep-sea environments. They classified ellipti- cally shaped deep-sea scours into four groups (spoon, heel, crescentic, and oval). In the Mediterranean Sea, megaflutes have been also associat- ed with dense shelf water cascading into deeper areas. Lastras et al. (2007) report crescentic scours at depths of 2100 m on the distal- most reaches of Sète Canyon after joining with the Cap de Creus Canyon (Golfe du Lion), and Verdicchio and Trincardi (2006) describe crescentic scours on the SW Adriatic continental slope at depths of 500–700 m. They are several hundred meters to 1.5 km wide, flute-like along the long axes, and up to 25 m deep with steep upcurrent scarps. Features described as megaflutes occur in Placentia Bay, a large south-facing embayment on the south coast of Newfoundland, located between the Avalon and Burin Peninsulas (Fig. 1). Postglacial muddy sediments on the east side of the outer bay have been strongly imprinted by flutes. Submersible observations in the fluted zone by Syvitski et al. (1983) showed that each flute had a number of small caves lining its perimeter, with fecal debris and other ejecta surrounding the cave openings, from which it was inferred that the caves were occu- pied by fish and other burrowing organisms. Subsequent work showed that the flutes were commonly 1 km long and several hundreds of me- ters wide and were thus described as ‘megaflutes’ (Fader and Miller, 1986, 1988; Fader, 1989). They occurred in a northeast–southwest trending zone 100 km long and 3.4 km wide, in water depths ranging from 195 m in the north to 165 m in the south. Fader and Miller (1988) estimated that 4 km 3 of Late Pleistocene–Holocene Placentia Clay had been removed, exhuming the underlying (and more compact) Late Pleistocene Downing Silt (glaciomarine). Regarding formational mechanisms for the megaflutes in Placentia Bay, Syvitski et al. (1983) found no evidence of present-day active erosion, suggesting that formation may have been related to irregularly-spaced Geomorphology 189 (2013) 12–25 ⁎ Corresponding author. Tel.: +1 902 4266204. E-mail address: johnshaw@nrcan.gc.ca (J. Shaw). 0169-555X/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.01.010 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph