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Are Submarine Landslides an Underestimated Hazard on the Western North Atlantic Passive Margin? Alexandre Normandeau, D

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Are Submarine Landslides an Underestimated Hazard on the Western North Atlantic Passive Margin? Alexandre Normandeau, D https://doi.org/10.1130/G46201.1 Manuscript received 1 March 2019 Revised manuscript received 24 June 2019 Manuscript accepted 25 June 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 10 July 2019 Are submarine landslides an underestimated hazard on the western North Atlantic passive margin? Alexandre Normandeau, D. Calvin Campbell, David J.W. Piper, and Kimberley A. Jenner Geological Survey of Canada (Atlantic), Natural Resources Canada, 1 Challenger Drive, Dartmouth, Nova Scotia B2Y 4A2, Canada, ABSTRACT The dominant morphological feature along this The western North Atlantic passive margin is considered relatively stable, with few slope part of the margin is the Laurentian Fan (Fig. 1). instabilities recognized during the Holocene. However, new multibeam bathymetry mapping Recent seafloor mapping along the lower slope and sediment core acquisition off eastern Canada indicate that previously unidentified, large, of the eastern Canadian margin revealed a va- submarine landslide events occurred during the Late Holocene, between 4 and 1.5 ka. The riety of submarine landslide deposits (Mosher recognition of these new gravitational events, in addition to the well-known C.E. 1929 Grand et al., 2017) and the identification of a previ- Banks earthquake-induced landslide, indicates that approximately one large landslide event ously unmapped submarine landslide complex per 1000 years has occurred offshore eastern Canada within the past 4000 years, a much (>14 000 km2 area) on the western levee of shorter recurrence interval than hitherto reported. This Late Holocene recurrence rate is the Laurentian Fan (Normandeau et al., 2019; also similar to active margins around the world and is likely due to the under-consolidation Fig. 1) that is among the largest on the western and resultant instability of Scotian Slope sediments attributable to high glacial sedimenta- Atlantic margin (within the 10% largest in the tion rates. The discovery of these new Late Holocene landslides was made possible through world compared to a compilation by Moscardelli detailed examination of cores recovered from the lower slope. These results demonstrate that and Wood [2016]). submarine landslide hazard has been underestimated on the western North Atlantic mar- gin—home to significant submarine infrastructure and proximal to a large coastal population. METHODS Sediment cores (collected during RV Atlan- INTRODUCTION mented Holocene landslides are smaller and tis cruise AT22, and CCGS Hudson Expedition Submarine landslides are ubiquitous on the confined to regions of the Scotian margin cut 2016-011 Phase 1) (01, 02, 03, and 08; Fig. 1) North Atlantic seafloor, and have potential so- by steep-walled canyons (Piper, 2005; Jenner were analyzed using X-ray systems to image cietal impacts on public safety, the environment et al., 2007). internal structures. The cores were then photo- and the economy. Most large submarine land- An understanding of the precise age of graphed and put through a multi-sensor core slides preserved on the North Atlantic seabed, landslide recurrence, and stability of the sea- logger (MSCL) for bulk density (ρb), magnetic defined here as having reached distal locations floor over time, is crucial to correctly assessing susceptibility, and color reflectance measure- on the lower slope or rise (Clare et al., 2014), geohazards, especially proximal to large coastal ments. In addition, discrete shear-strength mea- were triggered during the Pleistocene or imme- populations and submarine infrastructure, such surements using a miniature vane shear (follow- diately following deglaciation (Maslin et al., as in the western North Atlantic (ten Brink et al., ing ASTM D4648/D4648M-16 [Standard Test 2004; Brothers et al., 2013). On a global scale, 2014). In this study, we analyzed sediment cores Methods for Laboratory Miniature Vane Shear Urlaub et al. (2013) observed significantly fewer collected on the lower Scotian Slope in water Test for Saturated Fine-Grained Clayey Soil]) landslides in the Late Holocene compared to the depths of 4000–5000 m for evidence of land- were made at 10 cm intervals and thin sections Late Pleistocene and Early Holocene, thus sug- slides that have affected a large area. Our study were prepared on selected intervals. Undrained gesting a decreased risk for coastal populations reveals that large landslide events are more fre- shear strength (Su) was used to estimate the two- and submarine infrastructure (Chaytor et al., quent during the past 4000 yr than previously dimensional (2-D) static factor of safety (FOS) 2009; ten Brink et al., 2014). reported, potentially representing a significantly for infinite slope approximation by In the western North Atlantic, the larg- underestimated hazard along the western North S est known Holocene submarine landslide was Atlantic. FOS = u , (1) triggered by the C.E. 1929 Grand Banks M7.2 γβ′H cossinβ earthquake (Kuenen, 1952). This earthquake, REGIONAL SETTING where γ′ is submerged unit weight (using mea- and subsequent submarine landslide, generated The study area encompasses the eastern sured bulk density and 1.024 g cm–3 for seawater a tsunami that killed 28 people (Ruffman and Scotian Slope, where the seafloor morphol- density), H is depth in meters below the sea- Hann, 2001). Additionally, 17 submarine cable ogy comprises deeply incised submarine can- floor (mbsf), andβ is seabed slope angle (cal- breaks were reported (Doxsee, 1948), costing yons—The Gully and Shortland canyons being culated from average slope of the levee beside millions of dollars in telecommunications dam- the most prominent—and relatively flat inter- the landslide). age. The widespread 1929 event is considered canyon areas that formed during Pleistocene Samples were collected over a 2–3 cm in- a rare event in the Holocene, since other docu- proglacial sedimentation (Mosher et al., 2004). terval for age dating of planktonic foraminifera CITATION: Normandeau, A., Campbell, D.C., Piper, D.J.W., and Jenner, K.A., 2019, Are submarine landslides an underestimated hazard on the western North Atlantic passive margin?: Geology, v. 47, p. 848–852, https:// doi .org /10 .1130 /G46201.1 848 www.gsapubs.org | Volume 47 | Number 9 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/9/848/4819423/848.pdf by guest on 02 October 2021 Figure 1. A: Location of Laurentian Fan (Eastern and Western valleys), the lower continental slope, and the western levee landslide deposit (red polygon) on the eastern Canadian continental margin. Red stars are de­ layed cable breaks during the C.E. 1929 landslide/ turbidity current (Doxsee, 1948); white dots are core locations from this study. B: Backscatter imagery of the western levee land­ slide deposit. C: Seismic profile illustrating the in­ ternal character of the western levee landslide. (Table DR1 in the GSA Data Repository1). Ages the FOS is ~7 on a slope of 1° and the critical deposited between 20 ka and 13 ka, when av- presented in this paper were calibrated using slope angle is ~7°. These values are considered erage sedimentation rates (0.06 cm yr–1) were Calib 7.1 (http:// calib .org /calib /calib .html), conservative since some sediments were likely higher than present day (Fig. 3B). At ~300 cm, a marine reservoir correction of 400 years destroyed during the piston coring operation and red-brown mud passes upward into bioturbated (ΔR = 0) (Reimer et al., 2013) and are presented were not included in this calculation. lighter red-brown mud with very few sandy tur- at 2σ as calibrated ages. Core 03, collected from the western levee bidites. Accordingly, sedimentation rates dimin- landslide deposit at 4500 mbsl (Figs. 1 and 3A) ish to ~0.02 cm yr–1. A 25-cm-thick turbidite, at RESULTS sampled a landslide deposit, which is consis- 130 cm, consists of laminated sand with fluid- Core 01, consisting of laminated red-brown tent with the seismic facies at the core site (Fig. ized sand at the top, possibly due to post-depo- mud and sand, was collected on the western DR1D). The base of the core comprises large sitional dewatering. Above this turbidite, biotur- levee above the landslide scarp at 4220 mbsl intact blocks (clasts) within the landslide. Up bated light red-brown mud is dated at 3.9 ka (95 (Figs. 1 and 2; Fig. DR1B). Undrained shear core, the sediment consists of highly disturbed cm) and 2.5 ka (60 cm). At 60 cm, bioturbated strength is low (<10 kPa) and the sediments are polymictic olive-gray, brown, red-brown and mud abruptly transitions up-core to laminated under-consolidated below 4 mbsf (Su/σv′ < 0.25, tan mud clasts. Planktonic foraminifera sam- red-brown and olive-gray sands and muds (Fig. where σv′ is the vertical effective stress; Ladd ples collected from olive-gray mud clasts are 3B). These two facies were deposited between and Foott, 1974) (Fig. 2). The sediments were dated at 3.6 ka (335 cm) and 3.9 ka (240 cm), 2.5 ka and 1.8 ka, at twice the regional sedimen- deposited between 18.2 and 17.7 ka, represent- which places a maximum age for the top-most tation rate (0.04 cm yr–1). In thin section, this ing relatively high Late Pleistocene sedimenta- part of the landslide at 3.6 ka. Two planktonic facies is characterized by sand and silt turbidites tion rates of 1 cm yr–1; sediments younger than foraminifera samples collected above the land- with sharp contacts and fine sand laminae (Figs. 17.7 ka were not recovered in the piston core. slide deposit were dated at 1.5 ka (55 cm) and 3D and 3E). From 20 cm to 8 cm, the sediment This core was used to estimate the FOS of the 0.6 ka (30 cm). The sedimentation rate between consists of bioturbated, light red-brown mud; surficial sediments because it represents unfailed these two ages (0.02 cm yr–1) is consistent with from 8 cm to the core top, a turbidite is inter- sediment above the landslide scarp.
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