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

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 margin—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

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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. At ~7 mbsf, reported Holocene sedimentation rates in the preted to represent the 1929 event. region (Skene and Piper, 2003). A graded sand Core 08 was collected on the levee of Short- 1GSA Data Repository item 2019308, Figure DR1 at the top of the core likely corresponds to the land Canyon, in 3850 m water depth, upslope (geographic and stratigraphic locations of cores 01, 1929 event (Fig. 3). from the other cores (Fig. 1). It consists mainly 02, and 03) and Table DR1 (radiocarbon age informa- tion) , is available online at http://www .geosociety .org Core 02 was collected beyond the distal of bioturbated olive-gray mud (Fig. 3). At 60 cm, /datarepository/2019/, or on request from editing@ edge of the landslide in 5000 mbsl (Fig. 1; Fig. a thin sand layer with overlying red-brown mud geosociety.org. DR1C). Late Pleistocene sandy turbidites were is tentatively correlated to the 25-cm-thick sand

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/9/848/4819423/848.pdf by guest on 02 October 2021 turbidite in core 02. The red-brown turbidite is overlain by bioturbated olive-gray mud to the top of the core, where a sand bed is interpreted to record the 1929 earthquake.

LATE HOLOCENE LANDSLIDE RECURRENCE We interpret Holocene turbidites on the lower Scotian Slope as generated by landslides for the following reasons: (1) unlike shore-con- nected canyons (e.g., Sweet and Blum, 2016), Figure 2. Physical properties of piston core 01 (01PC) col the Scotian margin is located far from the coast lected on an unfailed part of (>200 km); (2) river-mouth related flows are the western Laurentian Fan impossible in the Holocene due to the absence levee crest (eastern Cana of nearby rivers; and (3) large storm-induced dian continental margin). turbidity currents are unlikely because the sub- Bulk density (ρb) and un marine canyon heads are deeper than 60 mbsl. drained shear strength (Su) were used to estimate the Therefore, large landslides are the most plausible static factor of safety. σv′ is explanation for the rare occurrence of Late Holo- the vertical effective stress. cene distal turbidites on the lower Scotian Slope. CT—computed tomography. The 25-cm-thick distal turbidite in core 02 dated ca. 4 ka is interpreted to represent a significant landslide event (Fig. 3B). The tentative correlation of this turbidite to the turbidite in core 08 suggests a landslide source triggered in the Shortland Canyon region more than 100 km away. Alternatively, the two turbidites could represent two distinct events triggered at (approximately) the same time. In this scenario, the distal turbidite may represent the first stage of the western levee landslide, because it is the

Figure 3. Physical properties of cores 03 (A; 03PC piston core), 02 (B; 02PC piston core) and 08 (C; 08TWC trigger weight core) containing Holocene landslide deposits and turbidites (D and E), on the eastern Canadian continental margin. The cores reveal four events related to

landslides between 4 ka and 1929. CT—computed tomography; ρb— bulk density; MS—magnetic susceptibility; a*—color from green (nega tive values) to red (positive values); WLL—western levee landslide.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/9/848/4819423/848.pdf by guest on 02 October 2021 thickest turbidite in the core and is located near MODERN SLOPE STABILITY CONCLUSIONS AND SOCIETAL the western levee landslide (Fig. 4). The current In a review of submarine landslides world- IMPLICATIONS dataset cannot confirm this hypothesis because wide, Urlaub et al. (2013) showed that signifi- Our study shows that four large landslide core 03 only records the last stage of the land- cantly fewer landslides occurred during the past events occurred offshore eastern Canada since slide event. Nonetheless, a significant landslide 6 k.y. than since the Late Glacial Maximum 4 ka, sourced from a giant channel levee on occurred at ca. 4 ka. (LGM). Only four large failures (>1 km3) were the lower slope and from the upper slope. This Core 02 has a thin red-brown turbidite at a recognized, with the highest frequency on the ~1000 yr, Late Holocene recurrence rate of large core depth of ~60 cm, deposited at ca. 2.5 ka, Iberian margin (1 every 1000 yr) (Gràcia et al., submarine landslides is more frequent than pre- interbedded with laminated olive-gray Holocene 2010). Recently, Li et al. (2017) also recognized viously reported, potentially indicating a higher sediment. Red-brown sediment on the Scotian a Late Holocene landslide (2 ka) event in the probability of a submarine landslide occurring in Margin is only found in Pleistocene and older eastern Atlantic. Our study shows that at least the future in the study area. Two of the world’s deposits (Piper et al. 2007). In order to deposit four landslide events, likely originating from largest Late Holocene failures (C.E. 1929 and the red-brown turbidite, a process is required the western levee of the Laurentian Fan, oc- 1.5 ka on the Laurentian Fan levee) occurred in that erodes the surficial (generally 2 m) olive- curred during the past 4000 yr on a small area the Laurentian Fan region, one of which (1929) gray sediment in order to reach and mobilize of the Scotian margin. The total volume of the had dramatic consequences on offshore infra- the underlying Pleistocene red-brown mud. A multi-staged landslide on the western levee was structure and loss of life; the other (1.5 ka) is landslide is the most likely process to achieve estimated at ~300–400 km3 (Normandeau et al., potentially unstable to this day. this. This event potentially represents the reac- 2019). These results are significant in that they Although these landslide events occurred tivation of the western levee landslide, since no indicate that the recurrence and distribution of far from the coast, there are considerations for age-equivalent turbidite is recorded in core 08 medium to large submarine landslides might impacts to seabed infrastructure. Three sub from Shortland Canyon. well be underestimated in the Holocene due to marine cables cross the Laurentian Fan region, Core 03 constrains the minimum and maxi- the absence of appropriate mapping and dating. all located on the large Laurentian Fan levee mum ages of the last stage of the western levee The discovery of these Late Holocene land- landslide described here. Additionally, the Cana- landslide to 1.5 ka and 3.6 ka, respectively. slide events also raises questions surrounding dian Atlantic margin is an area of active oil and Radiocarbon data show that the 20-cm-thick the stability of the modern slope, especially the gas exploration, with recent exploration wells laminated olive-gray mud and silt in core 02 Laurentian Fan levee. FOS analysis indicates a drilled in water depths >2000 m and a potential (1.8 ka) is concomitant with the last stage of minimum critical slope of 7° (Fig. 2), a gradient for deepwater oil production. The tsunamigenic the >25-m-thick landslide deposit observed in commonly found where escarpments are pres- potential of these newly identified landslides is seismic reflection data and cored in core 03. ent. Evidence for instability along escarpments unknown, but the potential threat to coastal com- This suggests that the top of the olive-gray lam- is shown by the presence of crown cracks in munities of eastern North America should not inated mud facies represents the distal deposit the upper reaches of the levee (see imagery in be discounted. A reevaluation of submarine- of the large western levee landslide (Fig. 3). Normandeau et al. [2019]). These slopes could landslide risk across the western North Atlan- These results indicate that the western levee thus fail without an external trigger, and may tic margin is recommended, and would require landslide did not produce a widespread turbi- currently be under slow lateral spreading. more systematic seafloor mapping, analysis of dite during its last stage of retrogression. The Away from escarpments, the slope is statically the distal record of large events, targeted slope graded olive-gray mud deposited between 2.5 stable over much of the study area, suggesting an stability analysis, and numerical modeling of and 1.5 ka likely represents thin mud turbidites external mechanism is required to trigger slope landslide tsunamigenic potential. originating from sediment in suspension dur- failure. Seismic activity is a strong candidate ing multi-staged landslide retrogression. There- for triggering the landslides in the Laurentian ACKNOWLEDGMENTS fore, the last stage of landslide retrogression Fan region, especially because there is an active We thank the RV Atlantis cruise AT22–1 and the CCGS likely represents a series of smaller events de- seismic zone on the Laurentian Slope (Hasegawa Hudson 2016–011 Phase 1 participants for their help during data acquisition. Support for this project was posited sequentially within 1000 yr, from 2.5 and Kanamori, 1987). In addition, fluid overpres- provided by the Geological Survey of Canada. We to 1.5 kyr B.P. These new ages show that the sure due to high Pleistocene sedimentation rates thank Lorena Moscardelli, Jason Chaytor, Derek western levee landslide is among the largest have led to the under-consolidation and low Sawyer, and Morelia Urlaub for their constructive Holocene submarine landslides in the western shear strength of the sediments, making them comments that improved the quality of this manuscript. North Atlantic. more susceptible to failure (e.g., Sawyer et al., Finally, the well-known C.E. 1929 landslide 2017). 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