Paleoseismicity of the Continental Margin of Eastern Canada: Rare Regional Failures and Associated Turbidites in Orphan Basin GEOSPHERE; V

Home , Continental margin, Hemipelagic sediment, Till

Research Paper THEMED ISSUE: Exploring the Deep Sea and Beyond: Contributions to Marine Geology in Honor of William R. Normark

GEOSPHERE Paleoseismicity of the continental margin of eastern Canada: Rare regional failures and associated turbidites in Orphan Basin GEOSPHERE; v. 15, no. 1 David J.W. Piper, Efthymios Tripsanas*, David C. Mosher, and Kevin MacKillop https://doi.org/10.1130/GES02001.1 Geological Survey of Canada (Atlantic), Natural Resources Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada

17 figures; 2 tables; 1 supplemental file ABSTRACT land, Canada; Doxsee, 1948) slides. Smaller infrequent shallow failures that CORRESPONDENCE: david.piper@canada.ca appear synchronous in multiple valley systems on the continental slope are The eastern Canadian continental margin is a typical glaciated passive also most likely the result of seismic triggers (Jenner et al., 2007) In situ geo- CITATION: Piper, D.J.W., Tripsanas, E., Mosher, margin where historic earthquakes have triggered submarine landslides. This technical measurements confirm that most continental margin sediments on D.C., and MacKillop, K., 2019, Paleoseismicity of the continental margin of eastern Canada: Rare regional study compares seismological estimates of earthquake recurrence with the such formerly glaciated margins, except on slopes of more than a few degrees, failures and associated turbidites in Orphan Basin: geological record over the past 85 k.y. offshore of Newfoundland to assess the are stable and would require seismic accelerations associated with a large Geosphere, v. 15, no. 1, p. 85–107, https://doi.org/10 reliability of the geologic record. Heinrich layers in cores provide chronology at earthquake (likely magnitude M = 5 or greater) to fail (e.g., Mosher et al., 1994; .1130/GES02001.1. ~3–5 k.y. resolution in high-resolution seismic-reflection profiles across head- Nadim et al., 2005). However, failure may be triggered by erosional or tectonic scarps and mass-transport deposits. Landslide-generated turbidites on the steepening of slopes (e.g., Lamarche et al., 2008) or may occur as a result of Science Editor: Shanaka de Silva Guest Associate Editor: Andrea Fildani basin floor have distinctive petrology, sedimentology, and distribution, with loading failure on weak layers, particularly in areas of high sedimentation rate ~1 k.y. chronologic resolution. Large slope failures occurred synchronously in active deltas (Clare et al., 2016). Factors that precondition slope sediments Received 11 April 2018 over margin lengths of 50–300 km. Since 85 ka, four failures have affected a to be more liable to failure include high pore pressures due to high sedimen- Revision received 15 August 2018 >150-km-long sector of the slope and 18 failures were large enough to be rec- tation rates, dissociation of gas hydrates by increased bottom temperature or Accepted 29 October 2018 ognized in seismic-reflection profiles and/or cores. The widespread failures sea-level fall, or leakage of deep fluids (Masson et al., 2006). However, several Published online 20 December 2018 were earthquake triggered; other mechanisms for triggering laterally exten- studies have concluded that severe earthquakes are the likely triggers for fail- sive synchronous failure do not apply. A frequency-magnitude plot of length of ure on slopes of less than a few degrees (e.g., Nadim et al., 2005; Locat et al., failed slope was calibrated by the published relationship that an order-of-mag- 2009; Goldfinger, 2011), especially in areas of modest or low sedimentation nitude increase in failed slope length corresponds to two orders of magnitude rates. Severe historical earthquakes (with peak ground acceleration >0.3 g)

of earthquake energy, together with the published estimate of Mw = 8.0 for the have generally resulted in shallow retrogressive failures affecting the upper largest earthquake on the Canadian eastern continental margin. Mean recur- few tens of meters of sediment (e.g., Papatheodorou and Ferentinos, 1997; rence interval of M = 7 earthquakes at any point on the margin is estimated at Piper et al., 1999), but in cases where a buried weak layer is present, retrogres- 25–30 k.y. from both seismological models and the sediment failure record. On sion may affect a much thicker section (Kvalstad et al., 2005; Mosher and Piper, such a margin with modest sedimentation rates (~0.3 m/k.y.) and low seismic- 2007). In areas of high sedimentation rates, such as many deltas, failure may ity, sediment failures provide a robust estimator of past seismicity. be triggered by processes unrelated to earthquakes, particularly in shallow water where wave and tidal effects on pore pressure are important (Hampton OLD G et al., 1996; Flemings et al., 2008). INTRODUCTION The eastern Canadian continental margin was the site of the M = 7.2 Grand Banks earthquake in A.D. 1929 and the M = 7.4 A.D. 1933 Baffin Bay earth- The record of past seismicity can be inferred, under some circumstances, quake. Both epicenters were located beneath the continental slope, within OPEN ACCESS from a stratigraphic record that contains a proxy record of seismic activity the eastern continental margin zone of Mazzotti and Adams (2005), which is (Adams, 1990). Submarine failures result from a range of preconditioning fac- a 6000-km-long zone corresponding approximately to the continental slope tors (Mosher et al., 2010a). On formerly glaciated passive continental margins, from northern Baffin Bay to the southwest Scotian Slope. Estimated seismic

sediment failures in stratified proglacial and contourite drift sediments have moment release in this zone is equivalent to a Mw = 7 earthquake somewhere been generally ascribed to seismic triggering, as in the case of the Storegga within this zone every 50 yr, or one every 3 k.y. for each 100 km segment of the (offshore Norway; Kvalstad et al., 2005) and Grand Banks (offshore Newfound- margin if earthquakes were randomly distributed (Mazzotti and Adams, 2005). This paper is published under the terms of the This estimated frequency is incorporated in the 2005 Building Code of Can- CC‑BY-NC license. *Current address: Hellenic Petroleum, 4A Gravias, Marousi, Athens GR15125, Greece ada (Adams and Halchuk, 2003). The frequency of large submarine landslides

through the Quaternary in the vicinity of the 1929 Grand Banks earthquake In contrast, glacial ice extended only across the inner Grand Banks at the last and landslide, where paleotectonic structures have probably concentrated glacial maximum (Shaw et al., 2006). earthquakes (Mazzotti, 2007), suggests that earthquakes with effects similar The regional morphology of the continental slope is described by Mosher to those in 1929 have occurred approximately once every 50–120 k.y. (Piper et al. (2017). Specific to Orphan Basin, four geomorphic areas are recognized et al., 2005). on the continental slope (Fig. 2): the north slope, the slope off Trinity Trough, The purpose of this study is to investigate the relationship between seis- the south slope, and the slope off Sackville Spur. The north slope has gradients mological models and the record of sediment failure on the formerly glaciated of 1.5°–2.5° between 400 and 600 m water depth. The slope off Trinity Trough is eastern Canadian continental margin. We use a particularly well-known area smooth and gently inclined (1°–1.7°). In contrast, the south slope has regional of the continental margin, in Orphan Basin off northeastern Newfoundland, inclinations of 2°–4° between 300 and 1500 m water depth, and is highly dis- where there is an excellent stratigraphic record of large submarine failures sected by multiple canyons, with relief of as much as 500 m and local slopes on the continental margin. The work is based on extensive high-resolution of as much as 20°. These canyons are organized into two systems, the Sher- (sparker) seismic-reflection profiles (resolution in the order of 0.3–0.4 m), with idan and Bonanza canyon systems. The slope off Sackville Spur is also steep ground truth provided by a suite of piston cores. We use previously published (2°–3° from 1000 to 2500 m water depth) and is dissected by multiple shallow chronologic control back to 85 ka based on Heinrich event stratigraphy cali- gullies with a relief of 30–120 m that are separated from each other by depo- brated by radiocarbon dating (Tripsanas and Piper, 2008a) that is correlated sitional overbank areas. Canyons and gullies on the Grand Banks margin pass to the reference section from Integrated Ocean Drilling Program (IODP) Site downslope into basin-floor channels. U1302 (Channell et al., 2012; Mao et al., 2018). This chronologic control is used to date observed slope failure surfaces and overlying mass-transport deposits in seismic-reflection profiles. Basin-floor turbidites are classified as to whether Tectonic Setting and Modern Seismicity they were linked to failure events (Hampton, 1972; Tripsanas et al., 2008) or generated by ice-margin processes (Tripsanas and Piper, 2008b). Only the for- Orphan Basin formed initially during Early Cretaceous rifting of Iberia from mer are used to assess the record of slope failure. Limit equilibrium slope-sta- North America, with crustal hyperextension accentuated by the southeastward bility analysis using the known geometry of the slopes and strength and den- translation of the Flemish Cap microplate (Sibuet et al., 2007). Oceanic spread- sity data from piston cores is reported for typical continental slope areas that ing extended northwards to the Charlie Gibbs fracture zone by the Late Creta- have intermittently failed. We present arguments that the record of failures is ceous (Aptian–Santonian; Keen et al., 2014). a record of large-earthquake triggering. We compare this record with that else- The short instrumental historical record of seismicity (Fig. 1) shows a cluster where on the eastern Canadian margin. Finally, we compare the paleoseismic of earthquakes near the 1929 Grand Banks hypocenter and beneath the Lauren- record with estimates of earthquake frequency based on historical records and tian Fan. Otherwise there is a sparse record of small earthquakes on the south- strain rates, in order to draw conclusions about the reliability of the paleo-fail- eastern Canadian continental margin with little relationship to major tectonic ure record for estimating paleoseismicity. lineaments such as the Charlie Gibbs fracture zone in oceanic crust or along the Cumberland belt at the southern margin of Orphan Basin (Enachescu, 1987). The short length of the instrumental record means that it is a poor indication of GEOLOGICAL SETTING regional seismic hazard (Mazzotti, 2007).

Morphology of the Continental Margin Late Quaternary Sediments and Sedimentation Processes Orphan Basin is a bathymetric embayment in 2000–3000 m water depth on the northeastern Newfoundland continental margin. It is bounded to the The slope and floor of Orphan Basin are characterized by multiple and west, east, and southeast by the Grand Banks, Orphan Knoll, and Flemish Cap, widespread tongues of mass-transport deposits (MTDs) and turbidites. The respectively (Fig. 1). At times when the Laurentide Ice Sheet extended onto the MTDs include the glacigenic debris-flow deposits that extend from the slope continental shelf, western Orphan Basin was supplied with sediment through off Trinity Trough to the western basin floor and are capped by channels cut by an ice stream in Trinity Trough. Glacigenic debris-flow deposits accumulated hyperpycnal subglacial meltwater discharge (Tripsanas and Piper, 2008b). The on the floor of the basin during glacial maxima, forming the Trinity trough- near-seabed deposits date from the last glacial maximum in the area, from 28 mouth fan (Fig. 2; Tripsanas and Piper, 2008b). Both at glacial maxima and to 20 ka. Glacial meltwater also created hypopycnal muddy plumes that drifted during ice retreat, subglacial meltwater from ice streams in Trinity Trough and southeastward in the Labrador Current, depositing red-brown muds at sedi- Notre Dame Trough to the north supplied fine-grained sediment plumes that mentation rates of 0.3–0.5 m/k.y. between 28 and 15 ka on the south slope out were transported southeastward by the Labrador Current (Mao et al., 2018). to water depths of 2300 m (Tripsanas and Piper, 2008a). Episodic Late Pleisto

60°W 55°W 50°W 45°W 40°W FZ Quebec e bs Labrador Sea m Gib a lie h har D g C e u tr o o r Orphan N T 200 Knoll 50° U1302/3 N 0 4000

1000 3000 L Orphan a u Basin r Newfoundland ity e rin h n T ug t o s Figure 1. Map of the southeastern Cana- i . Tr Cumb. belt a F s n er dian continental margin showing the set- v a o FIG. 2 ting of the study area, relevant tectonic lin- C D P Flemish h eaments, and other locations discussed in a h Cap n s the text. Instrumental earthquake record n i

e m is from http://earthquakescanada .nrcan

l Grand e l .gc.ca /histor /caneqmap -eng .php. Marine F isotope stage 2 ice margin (dashed gray Banks 100 km line) is from Shaw et al. (2006). Bathymet- 45° ric contours in meters at 500, 1000, 2000, N 3000, and 4000 m. FZ—fracture zone; F.— fault; Cumb.—Cumberland; LGM—last 1929 epicenter LGM ice margin glacial maximum. pe Slo tian Sco

2000 Earthquake magnitudes 3000 A.D. 1985–2015 < 2.0 4000 Laurentian N 2.0–3.0 3.0–4.0 40° Fan >4.0 N

cene and Holocene failures on the south slope transformed into turbidity cur- foraminifera Neogloboquadrina pachyderma sinistral (Tripsanas and Piper, rents that flowed through channels in the southern part of the basin, most 2008a, 2008b). The greater bulk density of the detrital-carbonate beds inter- recently in the Sheridan failure at ca. 7 ka (Tripsanas et al., 2008). Interbedded bedded with muddy terrigenous sediment commonly results in corresponding with all of these sediments are thin detrital carbonate–rich beds, termed Hein- high-amplitude reflections in sparker seismic-reflection profiles (resolution in rich layers, which correlate with episodic discharge of glacial meltwater from the order of 0.3–0.4 m) where sedimentation rates are high (Fig. 3). Age control the Hudson Strait ice stream (Rashid et al., 2003; Channell et al., 2012). on the older section is provided by the presence of North Atlantic ash zone II just below Heinrich event H5a in several cores and by the isotope stratigraphy of core MD95-2025 (Fig. 2) (Hiscott et al., 2001) and IODP Sites U1302 and Regional Stratigraphic Control U1303 (Channell et al., 2012). We use the chronology of Heinrich events summarized by Olsen et al. (2014). For the age of the top of the detrital-carbonate Detrital carbonate-rich beds deposited during Heinrich events provide the interval in the Heinrich 1 event, we use the age of 16.3 ka of Gil et al. (2015). basis for widespread lithostratigraphic correlation, augmented by a distinctive Seismic markers for the past 85 k.y. correlate regionally throughout Orphan succession of red-brown muds deposited from meltwater plumes. The pre- Basin (Fig. 4), as documented by Tripsanas et al. (2007). Older markers that cor- cise identification of particular Heinrich events younger than 40 ka is based on relate to mid-Pleistocene marine isotope stages (Campbell, 2005) are generally numerous previously reported calibrated radiocarbon dates on the planktonic too deep to be imaged in most sparker profiles.

50°W 48°W 46°W

N Orpha 0km100 CORES glacigenic debris- n base >24 ka with flow deposits Spur failureor turbidite base >24 ka withno failureor turbidite

e base <24 ka 5 illustrated seismic profile

slop contour interval 50 mto 500 m, thereafter 200 m 2

300 800 North Orphan Orphan Knoll 200 NORTH

0 SECTOR 50°N 1000

2400 50 Bedrockhigh Figure 2. Bathymetric map of the north- 0 8 2004024-47 eastern Newfoundland (Canada) margin MD95-2025 including Orphan Basin, showing location 4D of data and regional gradients. Red lines TRINITY 4C indicate seismic-reflection profiles; bold Trinity trough- SECTOR segments are labeled with the figure num- mouth fan 9C 30 ber in which they are illustrated. MTD— 0 Trinity Trough 0 mass-transport deposit.

0 30 Basin 9B 4B SHERIDAN SECTOR

3 SACKVILLE 200 BONANZA 2003033-24 SECTOR SUPPLEMENTARY DATA: METHODS SECTOR Sou 4A High-resolution seismic-reflection data were acquired by the use of a Huntec DTS Sh sparkersystem, towed around 100 m below sea level. Power output was 480 Joules at 4 kV, fired eridan th 9A 7 s at 1-1.5 s, and a 7.3 m-long, 24 element (AQ-16 cartridges), oil filled streamer towed behind the lope 5 Gradient Cany fish was used for the signal recording. Signals were filtered at 500-3500 Hz and recorded in 6 o digital format and analogue hard copy on an EPC9800 thermal chart at a 0.25 s sweep. 48°N 1° n 3 2003033-19 Spur A total of 85 cores were collected from Orphan Basin using the AGC Long Corer 3° Bonanza Canyon Flemish 500 ckville (Driscoll et al., 1989), which recovered cores up to 12 m long, and a smaller Benthos piston 6° Grand B anks Sa Pass corer, which recovered cores <5 m in length. Trigger-weight gravity cores were used for better

recovery of the upper metre of sediment. Shipboard samples for water content and torvane shear

strength measurements were taken every 1.5 m to verify that laboratory measurements were not

compromised by storage. The cores were sealed with beeswax and stored upright at 4°C for 1–6 METHODS undrained shear-strength data were obtained on split cores every 0.1 m in months prior to analyses. P-wave velocity, bulk density (using gamma ray attenuation as a muddy sediment using an automated laboratory miniature shear vane. Con- proxy) and magnetic susceptibility data were obtained on whole-round core sections using the High-resolution seismic-reflection data were acquired by the use of a tinuous profiles of undrained shear strength S( u) relative to the effective ver- GEOTEK Multi-Sensor Core Logger (MSCL). Cores were split lengthwise, photographed, and

detailed visual sedimentological descriptions were made. Quantitative sediment colour was Huntec DTS 480 J sparker system, towed at ~100 m below sea level (Mosher tical overburden stress (σ′v), a measure of the stability of the sediment, were measured using a Minolta CM-2002 spectrophotometer at 0.05 m intervals along the core and and Simpkin, 1999). A total of 85 cores were collected from Orphan Basin us- calculated using normalized strength parameters (NSP) and a modified Mohr- plotted based on the CIE (Commission Internationale de l’Eclairage/International Commission ing the AGC Long Corer (Driscoll et al., 1989), which recovered cores up to Coulomb relationship (Bradshaw et al., 2000). Subsamples were analyzed for on Illumination) L*a*b* (L: luminance, a: from green to red, b: from blue to yellow) colour 12 m long, and a smaller Benthos piston corer, which recovered cores <5 m bulk density, moisture content, void ratio, and grain size. Atterberg limit deter- space model described by Debret et al. (2011). Peak undrained shear strength data were obtained

on split cores every 0.1 m in muddy sediment using an automated laboratory miniature shear in length. Trigger-weight gravity cores were used for better recovery of the mination, consolidation tests, and isotropically consolidated undrained triaxial vane (ASTM 4648-94). Measurements were omitted at intervals where drained conditions upper meter of sediment. P-wave velocity, bulk density, and magnetic suscep- tests were conducted at 4.6 m and 6.7 m for core 2003033-19 (Figs. 2, 3). The tibility data were obtained on whole-round core sections using the Geotek static factor of safety and the critical earthquake acceleration coefficient were 1 Supplemental Data. Details on methods. Please visit Multi-Sensor Core Logger. Sediment color was measured using a Minolta calculated following Lee and Edwards (1986) for 24 cores analyzed using the https://doi.org/10.1130/GES02001.S1 or access the full-text article on www.gsapubs.org to view the Sup- CM-2002 spectrophotometer at 0.05 m intervals and plotted in International miniature shear vane data (Table 1). Further details on methods are provided plemental Data. Commission on Illumination (CIE) L*a*b* color space (Debret et al., 2011). Peak in the Supplemental Data1.

2003033-019 2003033-024 The floor of the basin (Fig. 7) is also characterized by widespread sub Synthetic water depth 1224 m water depth 2300 m parallel reflections. These are also interrupted by rare MTDs, some of which seismogram 0 originate on the south slope and others that are glacigenic debris-flow deposits derived from Trinity Trough, described by Tripsanas and Piper (2008b). Channels on the basin floor are pathways for turbidity currents carrying sand H1 and gravel and may also contain MTDs. A buried channel near the bedrock high southwest of Orphan Knoll (Fig. 8) contains a record of at least two MTDs. H1 reflector H2 500 (ca. 16.3 ka) Cores

H2 reflector Cores on the basin floor commonly contain very fine-grained sand and silt H3 (ca. 24 ka) turbidites interbedded with muds. These appear temporally related to the rapid

Sub-bottom depth (cm ) supply of glacial meltwater plumes to the basin, and their red-brown color indicates a source from Trinity Trough (Tripsanas and Piper, 2008a). Downflow 1000 H3 reflector (ca. 29 ka) from the glacigenic debris-flow deposits on the Trinity trough-mouth fan are H7 reflector (?) H4 sandy turbidites that correlate with erosional surfaces on the fan interpreted H5 as resulting from infrequent hyperpycnal flow of meltwater (Tripsanas and H5a Piper, 2008b). These turbidites consist of normally graded sand beds capped H6 H7 reflector (ca. 85 ka) by red-brown silts and muds (Fig. 9C). Although the mean grain size decreases H7a H7b upwards, such beds are well sorted and have a very small mud component H8 except in the silt and mud cap (Figs. 10A, 10C). A second type of turbidite is found in channels draining from the south core stratigraphy from Tripsanas and Piper (2008a) slope and in a few cores elsewhere in the basin; it consists of massive gravel Figure 3. Correlation of Heinrich event (H) layers to high-resolution sparker profiles in two and sand (Fig. 9A, upper bed), or a bed of massive sand overlain by massive reference stratigraphic cores in Orphan Basin (located in Fig. 2). Synthetic seismogram is based on density and P-wave velocity measurements in the core. sandy gravel (Fig. 9A, lower bed) that in more distal settings is capped by a graded medium- to fine-grained sand at the top of the bed (Fig. 9A, lower bed). More proximal cores include gravelly diamicton (Fig. 9B). Proximal sands and RESULTS gravels are poorly sorted and include a fine muddy “tail” on a grain-size distribution curve (Figs. 10B, 10D). Tripsanas et al. (2008) showed that such a tur- Seismic-Reflection Profiles bidite was deposited above an erosional surface during a large regional failure on the south slope at ca. 7 ka and illustrated its proximal-to-distal evolution. Figure 5 shows a characteristic seismic-reflection profile at ~1000 m water Such turbidites were interpreted as “seismo-turbidites” initiated by submarine depth on the south slope of Orphan Basin. Subparallel reflections characterize landsliding triggered by an earthquake (Tripsanas et al., 2008). the slope, thinning slightly on ridges. Where failures have occurred, they are The age of turbidites in cores (right hand panels in Figs. 11, 12) has been recognized from the presence of headscarps and from stratified or acoustically estimated from their stratigraphic position relative to Heinrich layers (H1–H4), incoherent blocks above a décollement surface (Figs. 5, 6; also Tripsanas et al., Holocene detrital-carbonate layers (NI, LA), local meltwater plume beds (R1– 2008, their figure 6). Failures generally involve only the upper 10–20 m of sedi- R7), and hyperpycnal erosional layers (E1–E3) previously dated by Tripsanas ment (Fig. 6). Failures appear to be predominantly retrogressive and terminate and Piper (2008a, 2008b). The “seismo-turbidites” of Tripsanas et al. (2008) are upslope at the limit of glacial till at ~500 m water depth (Piper and Brunt, 2006; of value in precisely dating landslide events. The “hyperpycnites” of Tripsanas Tripsanas et al., 2008; Piper et al., 2012). Acoustically incoherent intervals, with and Piper (2008b) are distinctly different petrologically and unlikely to be asso- sharp bases that are commonly erosional, occupy some stratigraphic levels in ciated with landsliding. valleys and are interpreted as MTDs on the basis of sediment type recovered in cores (Tripsanas et al., 2008). Regional stratigraphic markers provide imprecise Geotechnical Properties of Cores from the Continental Slope chronology for these MTDs; e.g., the top of the MTD at ~5 m subbottom in the northwestern half of Figure 5 appears to be directly overlain by the H2 seismic Twenty-four (24) sediment cores taken on the slopes of Orphan Basin are marker (ca. 24 ka), suggesting an age of ca. 25 ka. On the lower slope, ~1300 m used to characterize the index properties of the sediments to a maximum depth, it is common to find a series of stacked MTDs (Fig. 6). subbottom depth of 12 m. Core 2003033-19 (Fig. 2) was chosen as representa

SSW NNE SACKVILLE SECTOR water depth 2373 m 0 m SHERIDAN SECTOR INTERPRETED H1 AGE MODEL MD95-2025 water depth 2925 m H3 MIS TRINITY SECTOR δ18O (‰) ) -1 1 MIS 543 GDF 2 1 10 m MIS H1 4-5 2 H2 H3 3-4 H4 MIS 3 H5

, v=1500 ms 5-6 H5a H6 4 H7 5 H8 5 20 m GDF 6 MIS 6-7 6 7 MIS 8 7 8-9 9 8 9 30 m 10

Depth (m below sea floor from Hiscott 11 et al. (2001). GDF 12 A 40 m 13-15 ? NW end Fig. 7 16

seismic character on Trinity trough-mouth fan B C D

~65 km ~70 km ~10 km

Figure 4. Correlation of deeper stratigraphy in core MD95-2025, including an oxygen isotope curve (Hiscott et al., 2001), to high-resolution sparker profiles in Orphan Basin, showing correlation to Heinrich (H) events and to marine isotope stages (MIS). Depth scale on seismic profile based on an assumed acoustic velocity,v , of 1500 m.s.–1. Interpreted seismic character on Trinity trough-mouth fan, up-dip from the illustrated basin floor stratigraphy, distinguishes stratified intervals (dashed pattern) and mass-transport deposits (gray tone; e.g., Fig. 7), some of which are glacigenic debris-flow (GDF) deposits. Sparker profiles are located in Figure 2.

TABLE 1. SUMMARY OF STABILITY ANALYSIS USING MINIATURE tive of slope sediments. The bulk density in hemipelagic sediment shows the VANE SHEAR DATA FROM 18 CORES, ORPHAN BASIN expected increasing trend with depth (dashed line in Fig. 13A) and is higher Factor of Critical slope Critical earthquake Critical thickness in intervals rich in ice-rafted detritus and in carbonate-rich Heinrich layers. safety (°) coefficient (m) Water content is also dependent on lithology and decreases slightly down Minimum 0.73.2 02.5 core. The vane shear strength values increase to 5 m downcore, but the trend Maximum83.618.327.9684.1 becomes negative to the bottom of the core (Fig. 13C). The maximum past Average138.1 11.7 65.7

pressure (P ′c) values obtained from consolidation tests (Table 2) suggest Notes: Factor of safety—the ratio of undrained shear strength of sediment to the that the sediments are normally consolidated to slightly overconsolidated shear stress on a slip surface, such that failure will occur when the value is <1. Critical slope—the slope at which the factor of safety is 1. Critical earthquake coefficient— (Fig. 13D). the ratio of the seismic acceleration to the acceleration of gravity that yields a factor Continuous undrained strength profiles were calculated for each core us- of safety = 1. Critical thickness—the depth of the slip surface at which the factor of safety = 1. For further explanation, see text section “Geotechnical Properties of Cores ing the triaxial vane shear data. The Su/σ′v ratio in the normal consolidation from the Continental Slope” and Supplemental Material (text footnote 1). stress range (NSP) was estimated to be 0.48 from the triaxial data. The co-

0 12 ca. 7 ka MTD 0.5° 1° km 2° NW 5° 1.2° 10° 950 1.7° ca. 25 ka MTD 2.2°

Figure 5. High-resolution sparker strike pro- H1 file from the south slope (Sheridan sector) of Orphan basin, showing seismic stratigra- H2 phy and multiple mass-transport deposits H3 (MTDs) (shown by gray semi-transparent fill). Also shown are examples of gradients of unfailed slopes above headwalls of failures. Profile is located in Figure 2. H1–H3— MIS4/5 Heinrich event layers; MIS—marine isotope MIS 5/6 stage. Depth below sea level (m)

1000

hesion (c′), pore pressure value (Af), and friction angles (ϕ′) used in the mod- The critical pseudo-static acceleration (kc) to cause failure was calcu- ified Mohr-Coulomb equation were obtained from the triaxial tests (Table 2). lated at various depths following Lee and Edwards (1986), assuming that

Multi-sensor core logger bulk density was used to calculate the effective over- Su/σ′v is constant with depth and that the sediments are normally consoli

burden stress (σ′v). In addition, σ′v values and miniature vane undrained shear dated. This calculation was done using Su/σ′v values of 0.35 (vane data)

strength measurements from the 24 cores were used to estimate Su/σ′v. The and 0.48 (triaxial data). The minimum earthquake coefficient k( x) calculated

averaged Su/σ′v from the 24 cores was 1.27 in the upper 2 m and 0.35 for sedi- for each core site was also included (Fig. 14B). This analysis suggests that ments at depths of greater than 2 m. The strength profiles are plotted with the on slopes of 6°, a seismic coefficient of >10% is required to cause failure vane measurements from core 2003033-19 (Fig. 13C) and show good correla- in most of the sediment, but that some intervals sampled only by vane tion to 5 m depth. Below 5 m, the vane data values are lower than those of the shear measurements might require a seismic coefficient of only 4%–5% three profiles. on slopes of 1°. There was only one site of the 24 studied where the cal- The normalized strength values (0.48) obtained from the triaxial tests culated static factor of safety was less than unity, and the slope angle for

(Table 2) are higher than the average Su/σ′v (0.35) obtained from the miniature this site was 11°. vane shear strength (Table 1; Fig. 13C). The undrained unconsolidated (UU) shear strength value matches the normalized strength parameter (NSP) profile

(Fig. 13C). These various measured values of Su/σ′v are consistent with esti- Maximum Unfailed Slope Angle on the Continental Slope mates made elsewhere. On the continental slope beyond the eastern periph-

ery of the 1929 Grand Banks landslide, Su/σ′v from three triaxial tests ranges The apparent greatest slope angle beyond which sediments have failed from 0.33 to 0.40 (Mosher et al., 2010b). Both east of the Grand Banks landslide was measured at 61 locations on canyon walls and steep continental slope

and in Orphan Basin, vane shear measurements suggest that Su/σ′v falls to from seismic-reflection profiles within ±30° of orthogonal to local strike. An 0.25 at some levels in cores and that the cores show normal consolidation. example is shown in Figure 5, where undisturbed sediment above headscarps

Karlsrud et al. (1985) suggested that a typical value of Su/σ′v for lean normally shows apparent slope angles of 1.7°, 1.2°, and 2.2°. The entire data set plotted consolidated Norwegian clay is 0.3–0.35, based on anisotropic consolidation as a histogram (Fig. 14A) shows a maximum angle of unfailed slope of ~3°,

triaxial tests. Lee et al. (2004) suggested that Su/σ′v is generally 0.3 for normally with higher than 3° sustained only in glacial till. Low critical-slope measure- consolidated sediments, whereas Skempton (1969) gave a range of 0.2–0.5 for ments may result from the apparent dip effect where profiles were not pre- normally consolidated clay. cisely orthogonal to the local strike.

1250

NW 0.5° H1 1° 2° H2 10°5° H3 2005033B 004

MIS 4/5

1300

ca. 7 ka MTD Figure 6. High-resolution sparker dip profile from the south slope (Bonanza sector) of Orphan basin, showing seismic stratigraphy and multiple mass-transport deposits (MTDs). Profile is located in Figure 2. H1–H3—Heinrich event layers; MIS— marine isotope stage. Depth below sea level (m) ca. 19.5 ka MTD

1350 ca. 37 ka MTD ? two events 2005-033B SE 003

ca. 25 ka (?) MTD

ca. 70 ka (?) MTD

0 12 1400 km

SE ca. 58 ka MTD 0.5° 7–20 ka sand-gravel deposits and MTDs 2350 1° 2°

5°10° NW H1 ca. 70 ka MTD H3 MIS 4/5

Channel Depth below MIS 5/6 Channel sea level (m) MIS 6/7 NWF-87 MIS 7/8 2400 MIS 8/9 ? MIS 10 MTDs (?)

MIS 12 MTDs (?) 0 12

Figure 7. High-resolution sparker strike profile from the floor of Orphan Basin (Sackville sector). showing seismic stratigraphy and multiple mass-transport deposits (MTDs). Profile is located in Figure 2. H1, H3—Heinrich event layers; MIS—marine isotope stage.

DISCUSSION extended to the shelf edge or upper slope. Seismic-reflection profiles show that in the last 100 k.y., glacigenic debris flows and associated hyperpycnal The Use of Turbidites to Recognize Failures flow erosion occurred only during the last glacial maximum, from 15 to 28 ka (Tripsanas and Piper, 2008a). Turbidites occurring before or after this time in- Previous work in Orphan Basin has suggested two principal mechanisms terval likely resulted from the flow transformation of landslides, or possibly as for the initiation of turbidity currents: hyperpycnal flow of subglacial meltwater storm-generated sediment gravity flows from the Grand Banks shelf. producing “hyperpycnites” (Tripsanas and Piper, 2008a) and transformation of In Orphan Basin, almost all sandy turbidites dating from the last glacial landslides on the continental slope likely triggered by earthquakes producing maximum share a similar petrography, dominated by iron oxide–stained quartz “seismo-turbidites”, as exemplified by the Sheridan failure at ca. 7 ka (Tripsanas grains derived from the Carboniferous strata eroded by the Trinity Trough ice et al., 2008). Distinguishing these two types of turbidite is necessary if turbidites stream. These sands were described and discussed in detail by Tripsanas and are to be used as indicators of paleoseismicity. Such a distinction is applicable, Piper (2008b). Muds eroded from the same source also have a red-brown color e.g., in fiord basins where turbidites have been used to interpret paleoseismic- that is imparted to the glacigenic turbidite beds. Such glacigenic turbidites ity (St-Onge et al., 2012). Therefore we discuss criteria based on sedimentologi- occur as amalgamated sand deposits in channels and lobes and as thin-bed- cal and petrographic character that may be applicable to other settings. ded (<5 cm thick) sand and mud turbidites in overbank settings (Tripsanas and The stratigraphic distribution of large hyperpycnal flows is inferred from Piper, 2008a). Conversely, seismo-turbidites have variable petrographic com- characteristic straight erosional channels extending downslope from the limit position due to their widespread sourcing principally from the Grand Banks, of till on the upper slope and interbedded with glacigenic debris-flow deposits as documented in detail for the 7 ka Sheridan failure by Tripsanas et al. (2008). (Tripsanas and Piper, 2008b; Piper et al., 2012). It is possible that some of the The presence of inverse grading at the base of a sand bed has been widely smaller beds that we previously classified as “hyperpycnal” were initiated at used as a criterion for hyperpycnites (Mulder et al., 2003), even though in- an ice margin by the plume settling process described by Hizzett et al. (2018) verse grading can also develop in high-density turbidity-current gravel-sand from the Squamish Delta (British Columbia, Canada). We therefore use the deposits due to kinematic sieving in a grain flow (Lowe, 1982). Furthermore, more general term “glacigenic turbidite” in this paper. Hyperpycnal flows not all hyperpycnal flows result in inverse grading (Lamb and Mohrig, 2009). In and glacigenic turbidites originated only when the Newfoundland ice sheet Orphan Basin, amalgamated channel-sand deposits interpreted as glacigenic

ESE Flank of bedrock high 0.5° 1° 2° 5°10° 2680

ca. 58 ka MTD

2004024-47 2690 ca. 85 ka MTD Figure 8. High-resolution sparker profile from the floor of Orphan Basin on the

WNW w flank of the bedrock high (north sector) southwest of Orphan Knoll, showing seis- H1 mic stratigraphy and multiple mass-transport deposits (MTDs). Profile is located in H2 Figure 2. H1–H3—Heinrich event layers; Depth belo

sea level (m) MIS—marine isotope stage. H3 Buried channel 2700 MIS 4/5

Buried channel

2710

0 0.5 1

flows range from medium to fine grained and are very well sorted, and some are commonly accompanied by a basal zone consisting of diamicton (muddy beds are normally graded (e.g., Fig. 10A). Reverse grading has not been recog- gravel). This characteristic is attributed to the limited hydraulic sorting that was nized (Tripsanas and Piper, 2008a). imposed on the wide grain-size range of the failed sediment masses. Thin-bed- On the other hand, thick proximal and channel-floor seismo-turbidite de- ded overbank and distal turbidite deposits, of both glacigenic and seismo-tur- posits have grain size ranging from gravel to fine-grained sand, are poorly bidite origin, display similar grain-size characteristics (Figs. 10C, 10D), so that sorted, and contain a considerable amount of mud (5%–35%; Fig. 10B). They their distinction is based on their detrital petrography (color) and age relative

Seismo-turbiditesHyperpycnite A 2004024-26 NWF-65 2004024-33 Mean grain B C water depth 2250 m 05size (mm) water depth 2495 m water depth 2833 m (m) (m) 0 (m)

Gravel (%) e 0 1.3 060

2.8

0.1 bed (H2) 1.4 detrital carbonat

2.9 d Holocene muddy ooze 0.2

1.5 gravel H2 Figure 9. Illustrations of different types

sand of turbidites in Orphan Basin. (A) Core chocolate- 3.0 brown mu 2004024-26 complete lithographic column; H3 photographs showing two slump-gener- H4 0.3 ated turbidites with erosional bases; and (E1 )

5 sand 1.6 eroded plots of mean grain size and percentage base of gravel (± sand). (B) Core NWF-65 show- 3.1 ing a distal slump-generated turbidite from the 7 ka failure. (C) Core 2004024-33 H5 eroded base

gravel showing glacigenic meltwater turbidites 0.4 Mean grain corresponding to the E1 erosion surface size (μm) 0 1000 of Tripsanas and Piper (2008a). H1–H5a—

Depth below sea floor (m) (m) H5a Heinrich event layers; IRD—coarse-grained 3.6 ice-rafted detritus.

0.5 n sand 3.7 eroded muddy base 0 Sand and100 diamicto 10 gravel (%) Unconformity Alternations of Hemipelagic plumites and sediment IRD-rich beds Pelagic Sand ooze turbidites Detrital- carbonate Gravel beds bed

to the occurrence of shelf-crossing glacial ice. Thus the origin of at least the erally be dated to better than a thousand years based on the precision of the thin-bedded turbidites can be quite elusive, and their usage as indicators of lithostratigraphy in Orphan Basin. The spatial distribution of smaller and shal- seismicity requires corroborating evidence. lower headscarps and MTDs is determined with confidence from seismic-reflection data. The along-slope extent of evidence of failures, including headscarps, Timing and Distribution of Failures MTDs, and seismo-turbidites, is shown schematically in Figures 15 and 16. For the youngest widespread slope failure in Orphan Basin at ca. 7 ka, The stratigraphic position of headscarps and MTDs within sparker profiles Tripsanas et al. (2008) demonstrated from cores and sparker profiles a clear allows their age to be estimated to within a few thousand years, depending on genetic relationship between retrogressive failure and its progressive transfor- sedimentation rates (Fig. 11). Intermittent turbidites on the basin floor can gen- mation into a turbidity current, which caused channel erosion and deposited

THICK (>0.5 m) CHANNEL-SAND DEPOSITS 15 top 15 A Hyperpycnite B Seismo-turbidite core 2004024-60 core 2004024-65 (2.75–5 m) (0–0.75 m)

10 10 2 5 (%) 4 3 6 8 base 6 3 1 5 5 5 1 9 2 7 ) 4 Figure 10. Sequence of grain-size distri- 0 0 −4 0 4 8 −4 0 4 8 butions through turbidite beds in Orphan Basin deposited from hyperpycnal flows or other glacial margin processes (glaci- THIN-BEDDED (<0.2 m) SAND DEPOSITS genic turbidites) and those initiated by 15 15 slumping (seismo-turbidites). Analyses Frequency (% C Hyperpycnite D Seismo-turbidite are numbered and color coded from base core 2004024-62 core 2004024-26 to top. (3.75–4 m) (4.7–4.85 m) 2 3 10 10 4 5 5 7 8 2 3 (%) 6 1 4 6 5 5 7 1 9 8 10

0 0 −4 0 4 8 −4 0 4 8 Grain size (phi)

first a diamicton and then a gravel and sand turbidite. The 7 ka failure was least a 120 km length of the south slope. If the failures in Flemish Pass and the recognized from both seismic-reflection and core data along a 280-km-long Grand Banks are correlative, the failure extends over 345 km. A 24 ka gravel- segment of the continental slope in the Bonanza, Sheridan, and Trinity sectors sand turbidite is present near the bedrock high southwest of Orphan Knoll, (Figs. 2, 15A; Tripsanas et al., 2008). A probable MTD within an older slide probably implying failure on the north slope, for a failure extent of ~400 km if complex on northwest Flemish Cap is dated as mid- to late Holocene in core all of these deposits represent a single event. 2011031-13 (Cameron et al., 2014), but dating is not sufficiently precise to be Other Late Pleistocene failures are of lesser extent. The ca. 19.5 ka failure sure that it correlates with the main ca. 7 ka failure of Tripsanas et al. (2008). If recognized in seismic-reflection data from the Bonanza sector (Fig. 6) has a it were correlative, the length of the failed segment would increase to 370 km. correlative gravel-sand turbidite in the Sackville sector, implying source fail- By analogy with the 7 ka failure, a widespread failure on the south slope at ure over at least 45 km of margin (Fig. 15B). A 22.5 ka MTD in the Sheridan ca. 25 ka (Figs. 5, 6) corresponds to a coarse sand turbidite with an erosional sector is of only local extent. Throughout the southern part of the basin below base (Fig. 9A, lower bed) on the basin floor, just below Heinrich event layer H2 the entire south slope, there are one or two gravel-sand turbidites (upper bed (Fig. 11). A large slide on northwest Flemish Cap (Fig. 15C) is also just below in Fig. 9A) dated at 12–14 ka and an additional turbidite at ca. 15 ka in the H2 (Cameron et al., 2014). MTDs of apparently similar age, just below H2, are Sackville sector (Fig. 11). No corresponding widespread failure on the south reported from the southeastern Grand Banks slope (Rashid et al., 2017) and slope is recognized in sparker profiles, perhaps because there is insufficient southern Flemish Pass (Rudolph et al., 2018). The 25 ka failure extends over at overlying draping sediment cover to allow it to be discriminated from the very

Stratigraphic Ages assessed from Ages assessed from column seismic-reflection sediment cores

Ag e (ka ) profiles 0 Large-scale MTD (>5 m thick)

Local-scale MTD (<5 m thick)

Diamicton

Gravel-sand LA turbidites NI Mud-silt 10 turbidites Unconformity Estimated error bar R1 H1 Figure 11. Summary of distribution Hemipelagic of mass-transport deposits (MTDs) in sediment high-resolution sparker profiles and cores from Orphan Basin and the occurrence of R2 Detrital-carbonate gravel-sand turbidites and erosional sur- bed R3 faces in cores back to 40 ka. Age scale is E3 based on Heinrich event layers (H1–H4), Plumites and R4 early Holocene detrital-carbonate layers IRD-rich beds (NI—Noble Inlet, LA—Lake Agassiz), local R5 Chocolate-brown meltwater-plume beds (R1–R7), and hy- R6 E2 mud turbidites perpycnal erosional layers (E1–E3) previ- H2 ously dated by Tripsanas and Piper (2008a, Sediment transported from 2008b). Error bars are estimated on a one sector to case-by-case basis depending on precision R7 another of stratigraphic markers. IRD—ice-rafted detritus. E1 H3 30

H4 40 a a n n ough ough sector sector sector sector sector sector sector sector Tr Tr Sackville Bonanz Sackville Bonanz Sherida Sherida inity inity North secto r North secto r Tr Tr Geographic location (see Fig. 2)

Stratigraphic Ages assessed from Ages assessed from column seismic-reflection sediment cores

Ag e (ka ) profiles H4 40 Legend Large-scale MTD (>5 m thick)

Local-scale MTD (<5 m thick)

Gravel-sand turbidites

Mud-silt H5 turbidites Closed error 50 bar Open error bar Hemipelagic sediment H5a Detrital-carbonate bed (Heinrich layer) Figure 12. Summary of distribution of mass-transport deposits (MTDs) in high-resolution sparker profiles and cores from Orphan Basin and the occurrence of gravel-sand turbidites and erosional sur- 60 faces in cores from 40 to 80 ka. Age scale is based on Heinrich event layers (H4–H6) correlated to core MD95-2025 (Hiscott H6 et al., 2001) and Integrated Ocean Drilling Program Sites U1302 and U1303 (Channell et al., 2012). Error bars are estimated on a case-by-case basis depending on precision of stratigraphic markers; older age limit of open error bars is unconstrained. MIS—marine isotope stage.

MIS 70 4/5

80 a a n n ough ough sector sector sector sector sector sector sector sector Tr Tr Sackville Sackville Bonanz Bonanz Sherida Sherida inity inity North secto r North secto r Tr Tr Geographic location (see Fig. 2)

3 A Bulk density (g/cm) B Water content (%) C Shear strength S u (kPa) D Shear stress (kPa) E Color F Lithology

1.52.0 2.5 020406080 100 010203040 020406080 100 0 Undrained uncon- Plastic limit solidated test calculated from Liquid limit Miniature shear bulk density vane measurement measured in consolidation 100 Mohr-Coulomb equation tests normal consolidation stress range H1

Su/σv′ = 0.35 200

300

400

CONSOLIDATION Depth (cm) AND TRIAXIAL TESTS H2 500

600

CONSOLIDATION AND TRIAXIAL TESTS

700 H3

0 4 core disturbance a* value Hemipelagic Hemipelagic Green to red sediment IRD-rich bed sediment with abundant IRD 800 30 60 Slope mud and Alternating chocolate- Detrital-carbonate L* value sand (contourites) brown mud turbidites bed Black to white and IRD-rich beds Sand bed

Figure 13. Downcore plots of geotechnical measurements from core 2003033-19, Orphan Basin. (A) Bulk density from multi-sensor core logger. Dashed line shows trend for low-carbonate muds. (B) Water content from discrete samples. (C) Shear strength from miniature shear vane and one undrained unconsolidated test; also shows predicted trends based on various assumptions dis-

cussed in text, using the Mohr-Coulomb equation, the normal consolidation stress range, and assuming the normalized strength ratio Su/σ′vo = 35. (D) Shear stress as calculated from downcore data and measured from two consolidation tests. (E) Downcore spectrophotometer values, L*—black to white; a*—green to red. (F) Lithology log, legend across bottom; IRD—ice-rafted detritus. Blue bars across all panels show correlative Heinrich event layers H1–H3.

TABLE 2. SUMMARY OF CONSOLIDATION AND TRIAXIAL TEST RESULTS FROM CORE 2003033-19, ORPHAN BASIN

Core depth LL PI Soil P′c c′ ϕ′ (m) (%) (%) classification Cc (kPa) OCR (kPa) (°) Af Su/σ′v 4.6 20.1 10.8 CL-ML 0.12 42.1–61.61.1–1.62 6.835.60.470.48 6.5 26.1 10.4 CL 0.2 52.2–90.00.87–1.5 0.136.10.250.48

Note:Su/σ′v is calculated using a correction of 0.80 to account for the effect of isotropic consolidation. LL—Liquid Limit; PI—Plasticity Index. Soil classification: CL—lean clay, ML—lean silt; Cc—Compressibility Index; P′c—preconsolidation pressure; OCR—overconsolidation ratio; c′—effective cohesion; ϕ′—effective friction angle; Af—Skempton’s pore pressure parameter at failure; Su/σ′v—normalized strength ratio. For further explanation, see text section “Geotechnical Properties of Cores from the Continental Slope” and Supplemental Material (text footnote 1).

widespread 7 ka failure, but the turbidites could be the result of failure on steep failure is also recognized on part of the north slope, but it is not certain that canyon walls. The 24 ka gravel-sand turbidites and erosional surfaces in the the two correlate (Fig. 15D). Failures of similar age were reported in central Bonanza and Sackville sectors appear more extensive than the 25 ka turbidites, Flemish Pass (Huppertz and Piper, 2009) and on the slope southwest of Flem- but 25 ka failures and MTDs are widespread in Flemish Pass (Fig. 15C; Cam- ish Pass (Toews, 2003), both areas where large failures are infrequent. The eron et al., 2014; Rashid et al., 2017). It is uncertain which event is represented conservative estimate of the extent of a single failure is 100 km on the south by the failures and MTDs on the south slope. Gravel-sand turbidites of limited slope; if all the failure were synchronous, then the extent would be 520 km. A extent are also found at 29 ka (Fig. 11), but as with the 12–14 ka turbidites, no gravel-sand turbidite was found in the Sackville sector only at 31.5 ka; the slope corresponding failure is recognized in seismic-reflection data in the Bonanza failures recognized in seismic-reflection data between Heinrich event layers H3 and Sackville sectors. and H4 in these two sectors might, therefore, represent two discrete events of The record of failure and corresponding gravel-sand turbidites is less pre- even shorter failure length. cisely dated prior to 35 ka because of the lower sedimentation rate and fewer A 45 ka gravel-sand turbidite in the Bonanza sector has no recognized cor- cores penetrating so deep. Furthermore, seismic-reflection imaging of slope relative failure on the south slope (Fig. 12). Failure at ca. 58 ka is recognized on failures is less complete. A ca. 37 ka failure in the Bonanza (Fig. 6) and parts the south slope (Fig. 7) and from a MTD adjacent to the bedrock high south- of the Sheridan sectors resulted in a gravel-sand turbidite (Fig. 11). A ca. 37 ka west of Orphan Knoll (Fig. 8) and corresponds to a gravel-sand turbidite in the

low slopes 12 due to apparent dip effect maximum 15 unfailed slope Figure 14. (A) Critical slope estimated from angle ~3° seismic profiles in Orphan Basin. Histo- 8 gram is based on measurements of maximum slope angle above failure headscarps, ficient (% ) 10 as illustrated in Figure 5. (B) Relationship between minimum seismic coefficients and slope angles. The minimum seismic coefficients are calculated using miniature shear Number of case s 4 n=61 vane data obtained from 24 piston cores Seismic coef and normalized shear strengths ratios

5 (Su/σ′v) using ratios of 0.48 and 0.35 (see Fig. 13). till S/uvσ′= 0.48

S/uvσ′= 0.35 0 Vane shear strength 01234567 Apparent failure angle (degrees) 0 024681012 A B Slope angle (degrees)

GEOSPHERE | Volume 15 | Number 1 Piper et al. | Paleoseismicity of the continental margin of eastern Canada Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/85/4619128/85.pdf 100 by guest on 27 September 2021 on 27 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/85/4619128/85.pdf Research Paper contours in meters, at 250 m intervals on the shelf and 1000 m intervals in deep water. BON.—Bonanza; MTD—mass-transport deposit. Piper (2009), with ages corrected after Rudolph et al. (2018); 3—Toews and Piper (2002); 4—Rashid et al. (2017); 5—Rudolph et al. (2018); 6—Marshall et al. (2014). Bathymetric additional control on the extent of failure. Ellipses define schematic along-slope extent of other failures from the literature, namely: 1—Cameron (2014); et al. 2—Huppertz and Figure 15. Maps showing possible regional extent of the larger failures in and near Orphan Basin. Colored bars show areas of failed slope from this study; core lithologies provide 45°N 50°N 45°N 50°N 45°N 50°N E C A 200 km

250 25 0

Grand Bank s

TH R NO

50°W TRINIT 50°W 50°W

SHERIDA 1000 ?

ca. 50 ka

N 2000 BON. 4 Orphan Basi n 4 2000

2 SACKVILLE

0 50 Orphan

3000 Knoll 0

ca. 58 ka failure Flemish Pass 300 5 ? 25 ka failur 45°W 45°W 45°W 7 ka failur e 1 Flemis h

Ca p

1000

0 400

500 0 other studie s Interpretations from path Possible transport on core interpretation Failure age based core by seismic profil Failure age tied to e 45°N 50°N 45°N 50°N 45° N 50° N F D B e turbidites Mud-silt Diamicton Thick MTD (>5 m) turbidites Gravel-sand Thin MTD (<5 m) 50° W 50° W 50°W 19 ka ? 19 and 20.5 ka failures 20.5 ka 6 2 2 ca. 85 ka failur e 37 ka failur 45°W 45°W 45° W 3 1 20.5 ka e

GEOSPHERE | Volume 15 | Number 1 Piper et al. | Paleoseismicity of the continental margin of eastern Canada 101 Research Paper

increasing Along-slope Bonanza sector (Fig. 12). This failure extends along at least 220 km of the south ice volume Sedimentation failure extent (km) slope, but whether this is the same as the failure on the bedrock high is uncer- Sea level (m) rate (cm/k.y.) 0 −120 n 050 0300 tain (Fig. 15E). A widespread 70 ka failure on the south slope (Figs. 6, 7) is rep- 0 “balanced” resented in cores by only a mud-silt overbank turbidite (Fig. 12); channel cores scenario do not penetrate to this stratigraphic horizon. A deeper MTD adjacent to the Peltier and Fairbank bedrock high (Fig. 8) has an extrapolated age of ca. 85 ka (Fig. 15F). It is approx- LA imately the same age as a major failure on the eastern side of Orphan Knoll (2006) NI (Toews and Piper, 2002), 150 km to the east, and as a failure in central Flemish

inity trough-mouth fa Pass (Campbell et al.,2002; Huppertz and Piper, 2009), 380 km to the south. s H1 Tr In order to evaluate whether the longer-distance correlations in Figure 15 Glaciogenic debris-flow deposits on are likely or not, three different interpretations of the failure record have been f

20 e compiled. The “maximum” scenario assumes that any large failures that have an overlapping age estimate within error are the product of the same earth- H2 quake trigger. The “conservative” scenario assumes that failures of similar ages separated by areas with no evidence for failure represent separate triggers. The “balanced” scenario (Fig. 16) applies geological judgement based on H3 advance on NE

Last maximum ic data available in intervening areas to correlate some of the features that were Newfoundland Shel separated in the conservative scenario. A plot of length of failed slope against cumulative failure rate or its inverse, the recurrence period (Fig. 17), is used to determine whether the conserva- 40 H4

maximum length of failed slope balanced length of failed slope H5 conservative length of failed slope minimum magnitude Age (ka) Chappell, 2002 to produce

) recognizable failure −1

H5a y. 6.0 4

10 0.1 ) w 60 H6 7.0 earthquake Speculative

1929 magnitude (M MIS 4/5 5 Recurrence period (yr)

10 0.01 recurrence 8.0

Cumulative failure rate (k. maximum magnitude on eastern Canadian 80 0 200 400 600 margin (Adams and Halchuk, 2003; J. Length of failed slope (km) Adams, 2010, H7 personal commun.) Figure 17. Cumulative frequency distribution for the failure length proxy for earthquake magni- H7a tude. Failure lengths follow the maximum, balanced, and conservative scenarios as discussed in the text. Red bar shows estimated recurrence interval of large failures around the epicenter Figure 16. Age distribution of widespread failures in Orphan Basin, also showing variation in of the 1929 Grand Banks earthquake (Piper et al., 2005). Gray dashed line is the best fit straight sedimentation rate on the south slope (from core 2004024-024 of Tripsanas and Piper [2008a]; line for the balanced scenario data. Earthquake magnitude scale is speculative; its positioning

>80 ka extrapolated from core MD95-2025) and variation in eustatic sea level (compiled from is based on the estimate by Adams and Halchuk (2003) of Mw = 8.0 for the largest earthquake Peltier and Fairbanks [2006] and Chappell [2002]); datum is present sea level. For discussion of on the Canadian eastern continental margin, further calibrated by the finding of ten Brink et al. uncertainties in failure length and definition of the “balanced” scenario see text. LA—Lake Agassiz (2009) that an order-of-magnitude increase in failed slope length corresponds to a two-point event; NI—Noble Inlet event; H1–H7a—Heinrich event layers; MIS—marine isotope stage. increase in earthquake magnitude. For further discussion, see text.

tive, balanced, or maximum estimates of failed slope length are more proba- due to a fall in sea level (Fig. 16) are thus the dominant control on gas hydrate ble. The length of failed slope is assessed from the distribution of correlative dissociation. (5) Almost simultaneous failure in multiple canyon systems could failures (either scarps or MTDs) along the continental slope. The 1929 Grand result from episodic release of subglacial meltwater from ice sheets grounded Banks failure occurred along 250 km of the slope (Mosher and Piper, 2007), on the upper slope, leading to erosion and undercutting of canyon walls (Piper and failures of similar size have an estimated recurrence interval of between and Gould, 2004). There is no evidence for this process on the south slope 50 k.y. and 120 k.y. based on a 1 m.y. record on the continental slope (Piper from the timing of failure correlating with meltwater events (Fig. 16) or from et al., 2005). The plot of recurrence period using maximum estimates of failed slope morphology showing canyon-widening events. We conclude that none slope length in Orphan Basin suggests a recurrence interval of only 25 k.y. of the five mechanisms discussed above account for the large synchronous for 200-km-long failed lengths of slope, the size of the 1929 Grand Banks fail- failures in Orphan Basin. A few smaller failures, such as the small MTD dated ure; this value is inconsistent with the record inferred from seismic-reflection at 22.5 ka in part of the Sheridan sector, might have been triggered by any of profiles and cores (Fig. 11). Use of the balanced and conservative estimates a number of factors, including (1) bioturbation leading to oversteepening on a for large failures results in more failures being recognized, but overall the fail- canyon wall or (2) higher pore pressures due to dissociation of gas hydrate fol- ures are of lesser extent. A scenario between the balanced and conservative lowing the prolonged fall in sea level. We consider, however, that earthquake estimates best predicts failure lengths consistent with the failure length and triggering remains the most likely cause. magnitude of the 1929 Grand Banks failure. Although the total number of recognized failures is small, failures appear to have been more abundant during maximum ice extent (30–20 ka) and during deglaciation (20–12 ka) (Figs. 11, 16) compared with the 40–80 ka time span Correlation of Failures with Potential Forcing Factors (Fig. 12). There is a similar observation along the Scotian margin (Mosher et al., 2004). This higher failure frequency is not considered to be solely an arti- The widespread distribution of the 7 ka regional failure surfaces and MTDs fact of the ability to image and core deeper horizons. There are two processes in multiple slope canyon systems led Tripsanas et al. (2008) to argue that a that might be responsible for this variability in failure frequency: (1) Loading large earthquake was the probable trigger for failure. Similar arguments can and unloading by glacial ice on parts of the outer continental shelf may have be made for most of the older failures and corresponding turbidites recognized led to greater seismicity on reactivated basement structures, as argued by Wu in this study. Several other mechanisms have been proposed in the literature (1998). In Fennoscandia, where ice kilometers in thickness overlay crystalline that might result in a widespread failure: (1) Lateral spreading deformation bedrock, an immediate deglacial burst of seismic activity is well documented at a weak décollement horizon, commonly a buried MTD horizon, can lead (Arvidsson, 1996). However, offshore of Newfoundland, much thinner ice to catastrophic failure (e.g., Mosher et al., 2004). Such décollement surfaces partly supported by marine buoyancy overlay shelf sediments, so loading and are limited by bathymetry and unlikely to occur in multiple canyon systems. consequent seismicity would have been less extreme. (2) Generation of high Where found on the southeastern Canadian margin, they generally underlie excess pore pressure, as a result of high sedimentation rates during glacial successions >30 m thick (Piper, 2001), in contrast to the <20 m thickness of maximum or from dissociation of gas hydrates during falling sea level prior most failures on the south slope of Orphan Basin. (2) Increased pore pressure to the last glacial maximum (Fig. 16) would result in smaller seismic acceler- in shelf-edge deltas generally formed at times of lowered sea level (e.g., Locat ations being required to trigger a failure (Mosher et al., 1994). A 0.3 decrease et al., 2009) can be ruled out because the upper slope underlain by glacial till in the magnitude of the earthquake required to create a failure of a particular has not failed. (3) Bearing-capacity failure as a result of ice loading on the outer size would double the apparent rate of earthquakes inferred from the failure shelf (Mulder and Moran, 1995) can also be ruled out by the lack of failure record, using the relationships of ten Brink et al. (2009). in the upper slope. (4) Many authors have proposed a relationship between dissociation of gas hydrates and failure, with a temporal relationship during glacial cycles as a result of either falling sea level or rising bottom-water tem- Estimating Seismic Magnitude from Failures perature (e.g., Maslin et al., 2004). A bottom simulating reflector is present on Sackville and Orphan Spur to the south and north of Orphan Basin respectively Whether continental slope sediment fails during an earthquake is a conse- (Mosher, 2011), but none has been recognized within Orphan Basin. Further- quence of the gradient, the shear strength of the sediment, and the magnitude more, there is no evidence that sufficient excess pore pressures (i.e., >30% of seismic accelerations (Mosher et al., 1994). Given that the gradient is highly hydrostatic) can be generated in the upper 20 m of the seabed (e.g., Christian variable on a slope incised by canyons and that most failures that we have and Heffler, 1993; Strout and Tjelta, 2005) to cause failure. The dissociation of studied appear retrogressive, any continental slope area with a mean gradient gas hydrates may, however, weaken slope sediments, and failure could result of >2° will have some local gradients >6° that would be sufficient to initiate from a lower-magnitude earthquake (Kvalstad et al., 2005). In Orphan Basin, retrogressive failure (Piper et al., 1999). Such areas are indicated by yellow or bottom waters have been cold (<3 °C) even in the Holocene; pressure changes red tones in Figure 2.

The maximum distance from an earthquake epicenter at which failure will submarine landslides will tend to underestimate the frequency and magnitude take place was estimated by ten Brink et al. (2009) for various earthquake mag- of earthquakes along the continental margin. If smaller failures in Figure 17 nitudes and seafloor gradients and was found to vary substantially with esti- were not triggered by earthquakes, but by some other process, it would not

mates of Su/σ′v for the sediments. For example, for a magnitude 7.2 earthquake, invalidate the interpretation of the larger more extensive failures for which an failure would take place on a 6° slope as much as 43 km from the epicenter for earthquake trigger is necessary to account for their distribution across multiple

Su/σ′v = 0.3. The extent of failure would decrease with higher values of Su/σ′v slope-drainage systems. and increase with earthquake magnitude.

The Mw = 7.2 1929 Grand Banks earthquake created widespread landslid- Regional Occurrence of Large Failures and Comparison ing over a distance of ~250 km along the continental slope (Piper et al., 1999; with the Instrumental Record Mosher and Piper, 2007). Like the 7 ka failure on the south slope of Orphan Basin, this event involved principally retrogressive failure. The two areas have There is no evidence that seismicity in Orphan Basin is regionally lower similar variability in local gradients, including slopes >6° on which failure likely than elsewhere on the southeastern Canadian margin. The south slope over- initiated (Piper et al., 1999). The central area of the Grand Banks landslide likely lies a major tectonic lineament, the Cumberland belt, and the north slope ter- had higher pore pressures, indicated by the abundant shallow gas in sedi- minates at the Dover fault, aligned with the Charlie Gibbs fracture zone (Fig. 1; ments in that region (Piper et al., 1999). Enachescu, 1987). The recurrence interval of large failures in Orphan Basin is Widespread sediment failure along a 100 km segment of the continental similar to that found elsewhere on the southern parts of the eastern continen- slope might therefore be interpreted in several ways. If the triggering earth- tal margin, as summarized by Piper (2005). For example, on the central Scotian quake were located beneath the slope, according to the calculations of ten Slope, Jenner et al. (2007) recognized four small failures in the past 15 k.y.,

Brink et al. (2009), it would have a magnitude of M = 6.8 for Su/σ′v = 0.2, M = comparable with the number on the south slope of Orphan Basin over the

7.5 for Su/σ′v = 0.3, or M = 6.4 if calibrated against the 1929 Grand Banks earth- same period (Fig. 16). quake. If the triggering earthquake were located beyond the failed area, e.g., There is no evidence that smaller earthquakes might act to cause seismic on the continental shelf, then the magnitude estimate would be a minimal esti- strengthening of slope sediment (Sawyer and DeVore, 2015). Such seismic mate. Thus the estimation of earthquake magnitude is difficult from sediment strengthening is apparent from cores on active margins as higher shear

strength data because of the inherent uncertainty about the strength proper- strengths, with Su/σ′v >0.4. Such strengthening is not apparent in cores from ties of the weakest layers and the location of the causative earthquake relative the south slope of Orphan Basin, or from many other areas of the eastern Ca- to the failed slope. Furthermore, variations in depth of earthquakes would in- nadian margin, where cores generally show normal consolidation (e.g., Mac fluence sediment response. Killop et al., 2004; Mosher et al., 2010b). With the low rates of seismicity on the In our study, we have recognized large failures occurring over a distance of Canadian passive margin, submarine failure thus provides a reliable estimator at least 200 and probably 300 km along the margin (conservative and balanced of seismic hazard. scenarios respectively) and small failures affecting a margin length an order of From seismological models, Mazzotti (2007) argued that random seismicity magnitude shorter. Using the calculations of ten Brink et al. (2009), a two-point under a typical intraplate strain rate of 10–10 a–1 would produce one M = 7 earth- increase in earthquake magnitude (e.g., from 6.0 to 8.0) results in an approx- quake per thousand years over the 106 km2 of the Canadian eastern continental imately ten-fold increase in the length of the failed slope for an epicenter on margin zone, or a recurrence interval of 33 k.y. for the 3.3% (200 km) of the east-

the slope, assuming constant Su/σ′v. It is therefore probable that the observed ern continental margin likely to affect Orphan Basin. Our best estimate for M = range of failure lengths in Orphan Basin has recorded about a two-point range 7 earthquakes on geological grounds is a recurrence interval of 25 k.y., implying of magnitudes. The maximum earthquake magnitude for the eastern conti- seismicity only a little higher than that of the random seismological model. The

nental margin is estimated as Mw = 8.0 (Adams and Halchuk, 2003; corrected seismological model and the submarine failure record are reasonably consis- to moment magnitude by J. Adams, 2010, personal commun.), implying tent in their estimates of seismic risk on the eastern Canadian margin, providing that earthquakes ranging in magnitude from near this maximum to magni- additional evidence that the large failures are earthquake triggered. tude ~6.0, giving a two-point range in magnitudes, are likely to be recorded by observed submarine landslides and turbidites. The scale of most-proba- CONCLUSIONS ble magnitude in Figure 17 has thus been positioned to cover this range of magnitudes and is consistent with the determined 7.2 magnitude of the 1929 1. With good coverage of high-resolution seismic-reflection profiles, Grand Banks failure, with an epicenter beneath the upper continental slope. chronologically tied to long cores, a correlation of submarine landslides Because not all epicenters are likely to lie beneath the continental slope, the is developed for Orphan Basin. estimate of the length of failed slope from an earthquake of known magnitude 2. Two main types of turbidite beds are recognized. Those with massive using Figure 17 represents a maximum length, or conversely the record from gravel and sand, with gravelly diamicton in more proximal cores, are

correlated with major retrogressive slope failures. Those with abundant Chappell, J., 2002, Sea level changes forced ice breakouts in the Last Glacial cycle: New results red-brown mud and iron-oxide-stained quartz sand were deposited from from coral terraces: Quaternary Science Reviews, v. 21, p. 1229–1240, https://doi .org/10.1016 /S0277-3791(01)00141-X. glacigenic flows that were at least in part of hyperpycnal origin. Christian, H.A., and Heffler, D.E., 1993, Lancelot—A seabed piezometric probe for geotechnical

3. Although the mean Su/σ′v ratio of typical sediment in Orphan Basin is studies: Geo-Marine Letters, v. 13, p. 189–195, https://doi.org/10.1007/BF01593193. Clare, M.A., Hughes Clarke, J.E., Talling, P.J., Cartigny, M.J.B., and Pratomo, D.G., 2016, Precon- ~0.35, implying considerable stability, Su/σ′v falls to as low as 0.25 in ditioning and triggering of offshore slope failures and turbidity currents revealed by most some weak layers. Slopes steeper than 3° show widespread failure, ex- detailed monitoring yet at a fjord-head delta: Earth and Planetary Science Letters, v. 450, cept where underlain by glacial till. p. 208–220, https://doi.org/10.1016/j.epsl.2016.06.021. 4. A frequency-magnitude plot of length of failed slope is calibrated by the Debret, M., Sebag, D., Desmet, M., Balsam, W., Copard, Y., Mourier, B., Susperrigui, A.-S., Arnaud, F., Bentaleb, I., Chapron, E., Lallier-Vergès, E., and Winiarski, T., 2011, Spectrocolori finding of ten Brink et al. (2009) that a two-point increase in earthquake metric interpretation of sedimentary dynamics: The new “Q7/4 diagram”: Earth-Science Re- magnitude results in an approximately ten-fold increase in the length views, v. 109, p. 1–19, https://doi.org/10.1016/j.earscirev.2011.07.002. of the failed slope, and the estimate by Mazzotti and Adams (2005) of Doxsee, W.W., 1948, The Grand Banks earthquake of November 18, 1929: Publications of the M = 8.0 for the largest earthquake on the Canadian eastern continental Dominion Observatory Ottawa, v. 7, p. 323–335. w Driscoll, A.H., Moran, K., Piper, D.J.W., Vilks, G., and Mayer, L.A., 1989, An improved deep ocean margin. This plot is consistent with the recurrence interval estimated for coring system, in Proceedings, 21st Annual Offshore Technology Conference, Houston,

the Mw = 7.2 Grand Banks earthquake, provided that balanced or conser- Texas, 1–4 May: Houston, OTC, v. 2, p. 55–64. vative estimates of failed slope length are used. Enachescu, M.E., 1987, Tectonic and structural framework of the northeast Newfoundland continental margin, in Beaumont, C., and Tankard, A.J., eds., Sedimentary Basins and 5. The frequency of earthquakes estimated from the record of large, gen- Basin-Forming Mechanisms: Canadian Society of Petroleum Geologists Memoir 12, erally retrogressive slope failures is similar to that estimated from seis- p. 117–146. mological modeling by Mazzotti (2007). It suggests that the slope failure Flemings, P.B., Long, H., Dugan, B., Germaine, J., John, C.M., Behrmann, J.H., Sawyer, D., and the IODP Expedition 308 Scientists, 2008, Pore pressure penetrometers document high record on passive margins with low seismicity can be used as a proxy overpressure near the seafloor where multiple submarine landslides have occurred on the for paleoseismicity. continental slope, offshore Louisiana, Gulf of Mexico: Earth and Planetary Science Letters, v. 269, p. 309–325, https://doi.org/10.1016/j.epsl.2007.12.005. Gil, I.M., Keigwin, L.D., and Abrantes, F., 2015, The deglaciation over Laurentian Fan: History of diatoms, IRD, ice and fresh water: Quaternary Science Reviews, v. 129, p. 57–67, https://doi ACKNOWLEDGMENTS .org/10.1016/j.quascirev.2015.10.006. This work was supported by the Geological Survey of Canada and the Canada Program of Energy Goldfinger, C., 2011, Submarine paleoseismology based on turbidite records: Annual Review of Research and Development. We thank Francky Saint-Ange and John Adams for internal review Marine Science, v. 3, p. 35–66, https://doi.org/10.1146/annurev-marine-120709-142852. that sharpened our arguments, and two anonymous reviewers whose reviews greatly improved Hampton, M.A., 1972, The role of subaqueous debris flow in generating turbidity currents: Jour- the manuscript. This is Geological Survey of Canada contribution 20100367. nal of Sedimentary Petrology, v. 42, p. 775–793. Hampton, M.A., Lee, H.J., and Locat, J., 1996, Submarine landslides: Reviews of Geophysics, v. 34, p. 33–59, https://doi.org/10.1029/95RG03287. Hiscott, R.N., Aksu, A.E., Mudie, P.J., and Parsons, D.F., 2001, A 340,000 year record of ice-raft- REFERENCES CITED ing, palaeoclimatic fluctuations, and shelf-crossing glacial advances in the southwestern Adams, J., 1990, Paleoseismicity of the Cascadia subduction zone: Evidence from turbidites Labrador Sea: Global and Planetary Change, v. 28, p. 227–240, https://doi.org/10.1016/S0921 off the Oregon-Washington margin: Tectonics, v. 9, p. 569–583, https://doi .org /10 .1029 -8181(00)00075-8. /TC009i004p00569. Hizzett, J.L., Hughes Clarke, J.E., Sumner, E.J., Cartigny, M.J.B., Talling, P.J., and Clare, M.A., Adams, J., and Halchuk, S., 2003, Fourth generation seismic hazard maps of Canada: Values 2018, Which triggers produce the most erosive, frequent, and longest runout turbidity cur- for over 650 Canadian localities intended for the 2005 Building Code of Canada: Geological rents on deltas?: Geophysical Research Letters, v. 45, p. 855–863, https://doi .org/10.1002 Survey of Canada Open File 4459, 155 p., https://doi.org/10.4095/214223. /2017GL075751. Arvidsson, R., 1996, Fennoscandian earthquakes: Whole crustal rupturing related to postglacial Huppertz, T.J., and Piper, D.J.W., 2009, The influence of shelf-crossing glaciation on continental rebound: Science, v. 274, p. 744–746, https://doi.org/10.1126/science.274.5288.744. slope sedimentation, Flemish Pass, eastern Canadian continental margin: Marine Geology, Bradshaw, A.S., Silva, A.J., and Bryant, W.R., 2000, Stress-strain and strength behavior of ma- v. 265, p. 67–85, https://doi.org/10.1016/j.margeo.2009.06.017. rine clays from continental slope, Gulf of Mexico: Presented at 14th Engineering Mechanics Jenner, K.A., Piper, D.J.W., Campbell, D.C., and Mosher, D.C., 2007, Lithofacies and origin of late Conference, American Society of Civil Engineers, 21–24 May 2000. Quaternary mass transport deposits in submarine canyons, central Scotian Slope, Canada: Cameron, G.D.M., Piper, D.J.W., and MacKillop, K., 2014, Sediment failures in northern Flemish Sedimentology, v. 54, p. 19–38, https://doi.org/10.1111/j.1365-3091.2006.00819.x. Pass: Geological Survey of Canada Open File 7566, 141 p., https://doi.org/10.4095/293680. Karlsrud, K., Aas, G., and Gregersen, O., 1985, Can We Predict Landslide Hazards in Soft Sen- Campbell, D.C., 2005, Major Quaternary mass-transport deposits in southern Orphan Basin, off- sitive Clays?: Summary of Norwegian Practice and Experiences: Norwegian Geotechnical shore Newfoundland and Labrador: Geological Survey of Canada Current Research 2005-D3, Institute Publication 158, 24 p. 10 p., https://doi.org/10.4095/220840. Keen, C.E., Dafoe, L.T., and Dickie, K., 2014, A volcanic province near the western termination Campbell, D.C., Piper, D.J.W., Douglas, E.V., and Migeon, S., 2002, Surficial geology of Flemish of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland: Pass: Assessment of hazards and constraints to development: Geological Survey of Canada Tectonics, v. 33, p. 1133–1153, https://doi.org/10.1002/2014TC003547. Open File 1454, 60 p., https://doi.org/10.4095/213857. Kvalstad, T.J., Andresen, L., Forsberg, C.F., Berg, K., Bryn, P., and Wangen, M., 2005, The Storegga Channell, J.E.T., Hodell, D.A., Romero, O., Hillaire-Marcel, C., de Vernal, A., Stoner, J.S., Mazaud, slide: Evaluation of triggering sources and slide mechanics: Marine and Petroleum Geology, A., and Röhl, U., 2012, A 750-kyr detrital-layer stratigraphy for the North Atlantic (IODP Sites v. 22, p. 245–256, https://doi.org/10.1016/j.marpetgeo.2004.10.019. U1302–U1303, Orphan Knoll, Labrador Sea): Earth and Planetary Science Letters, v. 317–318, Lamarche, G., Joanne, C., and Collot, J.-Y., 2008, Successive, large mass-transport deposits p. 218–230, https://doi.org/10.1016/j.epsl.2011.11.029. in the south Kermadec fore-arc basin, New Zealand: The Matakaoa Submarine Instabil-

ity Complex: Geochemistry Geophysics Geosystems, v. 9, Q04001, https://doi.org/10.1029 Mosher, D.C., Piper, D.J.W., MacKillop, K., and Jarrett, K., 2010b, Near surface geology of the /2007GC001843. Halibut Channel region of the SW Newfoundland Slope from GSC data holdings: Geological Lamb, M.P., and Mohrig, D., 2009, Do hyperpycnal-flow deposits record river-flood dynamics?: Survey of Canada Open File 6214, 72 p., https://doi.org/10.4095/261390 . Geology, v. 37, p. 1067–1070, https://doi.org/10.1130/G30286A.1. Mosher, D.C., Campbell, D.C., Gardner, J.V., Piper, D.J.W., Chaytor, J.D., and Rebesco, M., 2017, Lee, H.J., and Edwards, B.D., 1986, Regional method to assess offshore slope stability: Journal The role of deep-water sedimentary processes in shaping a continental margin: The North- of Geotechnical Engineering, v. 112, p. 489–509, https://doi.org/10.1061/(ASCE)0733-9410 west Atlantic: Marine Geology, v. 393, p. 245–259, https://doi.org/10.1016/j.margeo.2017.08 (1986)112:5(489). .018. Lee, H.J., Orzech, K., Locat, J., Boulanger, E., and Konrad, J.-M., 2004, Seismic strengthening, a Mulder, T., and Moran, K., 1995, Relationship among submarine instabilities, sea-level varia- conditioning factor influencing submarine landslide development: Extended abstract pre- tions and the presence of an ice sheet on the continental shelf: An example from the Ver- sented at 57th Canadian Geotechnical Conference, Quebec, Canada, 23–27 October, 14 p. rill Canyon area, Scotian Shelf: Paleoceanography, v. 10, p. 137–154, https://doi .org/10.1029 Locat, J., Lee, H., ten Brink, U.S., Twichell, D., Geist, E., and Sansoucy, M., 2009, Geomorphology, /94PA02352. stability and mobility of the Currituck slide: Marine Geology, v. 264, p. 28–40, https://doi.org Mulder, T., Syvitski, J.P.M., Migeon, S., Faugères, J.-C., and Savoye, B., 2003, Marine hyperpycnal /10.1016/j.margeo.2008.12.005. flows: Initiation, behavior and related deposits. A review: Marine and Petroleum Geology, Lowe, D.R., 1982, Sediment gravity flows: II. Depositional models with special reference to the v. 20, p. 861–882, https://doi.org/10.1016/j.marpetgeo.2003.01.003. deposits of high-density turbidity currents: Journal of Sedimentary Petrology, v. 52, p. 279– Nadim, F., Kvalstad, T.J., and Guttormsen, T., 2005, Quantification of risks associated with sea- 297, https://doi.org/10.1306/212F7F31-2B24-11D7-8648000102C1865D. bed instability: Marine and Petroleum Geology, v. 22, p. 311–318, https://doi .org/10.1016/j MacKillop, K., Jarrett, K., Mitchelmore, P., Mosher, D.C., and Campbell, D.C., 2004, Geotech- .marpetgeo.2004.10.022. nical characteristics of sediments on the western central Scotia Slope: Extended abstract Olsen, J., Rasmussen, T.L., and Reimer, P.J., 2014, North Atlantic marine radiocarbon reservoir presented at 57th Canadian Geotechnical Conference, Quebec, Canada, 23–27 October, 8 p. ages through Heinrich event H4: A new method for marine age model construction, in Austin, Mao, L., Piper, D.J.W., Saint-Ange, F., and Andrews, J.T., 2018, Labrador Current fluctuation W.E.N., Abbott, P.M., Davies, S.M., Pearce, N.J.G., and Wastegård, S., eds., Marine Tephro during the last glacial cycle: Marine Geology, v. 395, p. 234–246, https://doi .org/10.1016/j chronology: Geological Society of London Special Publication 398, p. 95–112, https://doi .org .margeo.2017.10.012. /10.1144 /SP398 .2 . Marshall, N.R., Piper, D.J.W., Saint-Ange, F., and Campbell, D.C., 2014, Late Quaternary history Papatheodorou, G., and Ferentinos, G., 1997, Submarine and coastal sediment failure triggered

of contourite drifts and variations in Labrador Current flow, Flemish Pass, offshore eastern by the 1995, Ms= 6.1 Aegion earthquake, Gulf of Corinth, Greece: Marine Geology, v. 137, Canada: Geo-Marine Letters, v. 34, p. 457–470, https://doi.org/10.1007/s00367-014-0377-z. p. 287–304, https://doi.org/10.1016/S0025-3227(96)00089-8. Maslin, M., Owen, M., Day, S., and Long, D., 2004, Linking continental slope failure to climate Peltier, W.R., and Fairbanks, R.G., 2006, Global glacial ice volume and Last Glacial Maximum change: Testing the clathrate gun hypothesis: Geology, v. 32, p. 53–56, https://doi.org/10 duration from an extended Barbados sea level record: Quaternary Science Reviews, v. 25, .1130/G20114.1. p. 3322–3337, https://doi.org/10.1016/j.quascirev.2006.04.010. Masson, D.G., Harbitz, C.B., Wynn, R.B., Pedersen, G., and Løvholt, F., 2006, Submarine land- Piper, D.J.W., 2001, The geological framework of sediment instability on the Scotian Slope: slides: Processes, triggers and hazard prediction: Philosophical Transactions of the Royal Studies to 1999: Geological Survey of Canada Open File 3920, 202 p., https://doi.org/10.4095 Society of London, Series A: Mathematical and Physical Sciences, v. 364, p. 2009–2039, /212710. https://doi.org/10.1098/rsta.2006.1810. Piper, D.J.W., 2005, Late Cenozoic evolution of the continental margin of eastern Canada: Norsk Mazzotti, S., 2007, Geodynamic models for earthquake studies in intraplate North America, in Geologisk Tidsskrift, v. 85, p. 305–318. Stein, S., and Mazzotti, S., eds., Continental Intraplate Earthquakes: Science, Hazard, and Piper, D.J.W., and Brunt, R.A., 2006, High-resolution seismic transects of the upper continental Policy Issues: Geological Society of America Special Paper 425, p. 17–33, https://doi.org/10 slope off southeastern Canada: Geological Survey of Canada Open File 5310, 77 p., https:// .1130/2007.2425(02). doi.org/10.4095/222863. Mazzotti, S., and Adams, J., 2005, Rates and uncertainties on seismic moment and deformation Piper, D.J.W., and Gould, K., 2004, Late Quaternary geological history of the continental slope, in eastern Canada: Journal of Geophysical Research, v. 110, B09301, https://doi.org/10.1029 South Whale Basin, and implications for hydrocarbon development: Geological Survey of /2004JB003510. Canada Current Research 2004-D1, 13 p., https://doi.org/10.4095/215107. Mosher, D.C., 2011, A margin-wide BSR gas hydrate assessment: Canada’s Atlantic margin: Piper, D.J.W., Cochonat, P., and Morrison, M.L., 1999, Sidescan sonar evidence for progressive Marine and Petroleum Geology, v. 28, p. 1540–1553, https://doi.org/10.1016/j.marpetgeo.2011 evolution of submarine failure into a turbidity current: The 1929 Grand Banks event: Sedi- .06.007. mentology, v. 46, p. 79–97, https://doi.org/10.1046/j.1365-3091.1999.00204.x. Mosher, D.C., and Piper, D.J.W., 2007, Analysis of multibeam seafloor imagery of the Laurentian Piper, D.J.W., MacDonald, A.W.A., Ingram, S., Williams, G.L., and McCall, C., 2005, Late Cenozoic Fan and the 1929 Grand Banks landslide area, in Lykousis, V., Sakellariou, D., and Locat, architecture of the St. Pierre Slope: Canadian Journal of Earth Sciences, v. 42, p. 1987–2000, J., eds., Submarine Mass Movements and Their Consequences: 3rd International Sympo- https://doi.org/10.1139/e05-059. sium: Dordrecht, Netherlands, Springer, Advances in Natural and Technological Hazards Piper, D.J.W., Deptuck, M.E., Mosher, D.C., Hughes Clarke, J.E., and Migeon, S., 2012, Erosional Research, v. 27, p. 77–88, https://doi.org/10.1007/978-1-4020-6512-5_9. and depositional features of glacial meltwater discharges on the eastern Canadian conti- Mosher, D.C., and Simpkin, P.G., 1999, Status and trends of marine high-resolution seismic re- nental margin, in Prather, B.E., Deptuck, M.E., Mohrig, D., van Hoorn, B., and Wynn, R.B., flection profiling: Data acquisition: Geoscience Canada, v. 26, p. 174–188. eds., Applications of the Principles of Seismic Geomorphology to Continental Slope and Mosher, D.C., Moran, K., and Hiscott, R.N., 1994, Late Quaternary sediment, sediment mass flow Base-of-Slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues: Society processes and slope stability on the Scotian Slope, Canada: Sedimentology, v. 41, p. 1039– for Sedimentary Geology (SEPM) Special Publication 99, p. 61–80, https://doi .org/10.2110 1061, https://doi.org/10.1111/j.1365-3091.1994.tb01439.x. /pec.12.99.0061. Mosher, D.C., Piper, D.J.W., Campbell, D.C., and Jenner, K.A., 2004, Near-surface geology and Rashid, H., Hesse, R., and Piper, D.J.W., 2003, Evidence for an additional Heinrich event be- sediment-failure geohazards of the central Scotian Slope: American Association of Petro- tween H5 and H6 in the Labrador Sea: Paleoceanography, v. 18, 1077, https://doi .org/10.1029 leum Geologists Bulletin, v. 88, p. 703–723, https://doi.org/10.1306/01260403084. /2003PA000913. Mosher, D.C., Moscardelli, L., Shipp, R.C., Chaytor, J.D., Baxter, C.D., Lee, H.J., and Urgeles, Rashid, H., MacKillop, K., Sherwin, J., Piper, D.J.W., Marche, B., and Vermooten, M., 2017, Slope R., 2010a, Submarine mass movements and their consequences, in Mosher, D.C., Moscar- instability on a shallow contourite-dominated continental margin, southeastern Grand delli, L., Shipp, R.C., Chaytor, J.D., Baxter, C.D., Lee, H.J., and Urgeles, R., eds., Submarine Banks, eastern Canada: Marine Geology, v. 393, p. 203–215, https://doi.org/10.1016/j.margeo Mass Movements and Their Consequences: 4th International Symposium: Dordrecht, Neth- .2017.01.001. erlands, Springer, Advances in Natural and Technological Hazards Research, v. 28, p. 1–8, Rudolph, N., Piper, D.J.W., and Campbell, D.C., 2018, Late Quaternary stratigraphy, sedimentol- https://doi.org/10.1007/978-90-481-3071-9_1. ogy and geohazards in Flemish Pass: Geological Survey of Canada Open File 8191 (in press).

Sawyer, D.E., and DeVore, J.R., 2015, Elevated shear strength of sediments on active margins: Toews, M.W., 2003, Late Cenozoic geology of Salar Basin, eastern slope of the Grand Banks of Evidence for seismic strengthening: Geophysical Research Letters, v. 42, p. 10,216–10,221, Newfoundland: Geological Survey of Canada Open File 1675, 68 p., https://doi .org/10.4095 https://doi.org/10.1002/2015GL066603. /214762. Shaw, J., Piper, D.J.W., Fader, G.B., King, E.L., Todd, B.J., Bell, T., Batterson, M.J., and Liverman, Toews, M.W., and Piper, D.J.W., 2002, Recurrence interval of seismically triggered mass-trans- D.J.E., 2006, A conceptual model of the deglaciation of Atlantic Canada: Quaternary Science port deposition at Orphan Knoll, continental margin off Newfoundland and Labrador: Reviews, v. 25, p. 2059–2081, https://doi.org/10.1016/j.quascirev.2006.03.002. Geological Survey of Canada Current Research 2002-E17, 10 p., https://doi.org/10.4095 Sibuet, J.-C., Srivastava, S.P., Enachescu, M., and Karner, G.D., 2007, Early Cretaceous motion of /213698. Flemish Cap with respect to North America: Implications on the formation of Orphan Basin Tripsanas, E.K., and Piper, D.J.W., 2008a, Late Quaternary stratigraphy and sedimentology of and SE Flemish Cap–Galicia Bank conjugate margins, in Karner, G.D., Manatschal, G., and Orphan Basin: Implications for meltwater dispersal in the southern Labrador Sea: Palaeo- Pinheiro, L.M., eds., Imaging, Mapping and Modelling Continental Lithosphere Extension geography, Palaeoclimatology, Palaeoecology, v. 260, p. 521–539, https://doi.org/10.1016/j and Breakup: Geological Society of London Special Publication 282, p. 63–76, https://doi.org .palaeo.2007.12.016. /10.1144/SP282.4. Tripsanas, E.K., and Piper, D.J.W., 2008b, Glaciogenic debris-flow deposits of Orphan Basin, off- Skempton, A.W., 1969, The consolidation of clays by gravitational compaction: Quarterly Journal shore eastern Canada: Sedimentological and rheological properties, origin, and relationship of the Geological Society, v. 125, p. 373–411. to meltwater discharge: Journal of Sedimentary Research, v. 78, p. 724–744, https://doi.org St-Onge, G., Chapron, E., Mulsow, S., Salas, M., Viel, M., Debret, M., Foucher, A., Mulder, T., /10.2110/jsr.2008.082. Winiarski, T., Desmet, M., Costa, P.J.M., Ghaleb, B., Jaouen, A., and Locat, J., 2012, Compar- Tripsanas, E.K., Piper, D.J.W., and Jarrett, K.A., 2007, Logs of piston cores and interpreted ultra- ison of earthquake-triggered turbidites from the Saguenay (Eastern Canada) and Reloncavi high-resolution seismic profiles, Orphan Basin: Geological Survey of Canada Open File (Chilean margin) Fjords: Implications for paleoseismicity and sedimentology: Sedimentary 5299, 339 p., https://doi.org/10.4095/223224. Geology, v. 243-244, p. 89–107, https://doi.org/10.1016/j.sedgeo.2011.11.003. Tripsanas, E.K., Piper, D.J.W., and Campbell, D.C., 2008, Evolution and depositional structure of Strout, J.M., and Tjelta, T.I., 2005, In situ pore pressures: What is their significance and how can earthquake-induced mass movements and gravity flows, southwest Orphan Basin, Labrador they be reliably measured?: Marine and Petroleum Geology, v. 22, p. 275–285, https://doi .org Sea: Marine and Petroleum Geology, v. 25, p. 645–662, https://doi .org/10.1016/j.marpetgeo /10.1016/j.marpetgeo.2004.10.024. .2007.08.002. ten Brink, U.S., Lee, H.J., Geist, E.L., and Twichell, D., 2009, Assessment of tsunami hazard to the Wu, P., 1998, Intraplate earthquakes and postglacial rebound in Eastern Canada and Northern U.S. East Coast using relationships between submarine landslides and earthquakes: Marine Europe, in Wu, P., ed., Dynamics of the Ice Age Earth: A Modern Perspective: Zurich, Trans Geology, v. 264, p. 65–73, https://doi.org/10.1016/j.margeo.2008.05.011. Tech Publications, p. 603–628.