The Block Composite Submarine Landslide, Southern New England Slope, U.S.A.: A Morphological Analysis
J. Locat, U.S. ten Brink, and J.D. Chaytor
Abstract Recent multibeam surveys along the continental slope and rise off southeast New England has enabled a detailed morphological analysis of the Block composite landslide. This landslide consists of at least three large debris lobes resting on a gradient less than 0.5°. The slide took place on gradients of between 1° and 5° in Quaternary sediments likely deposited at the time of low sea level and high sedimentation rates associated with glaciations. The slide debris lobes are very close to each other and cover an area of about 1.125 km2 of the sea floor. With an average thickness of 50 m, the total volume of the deposit is estimated at 36 km3. In some cases, the departure zone appears to be near the crest of the continental slope, at a water depth between 500 and 2,000 m with debris spreading over about 20 km at a depth ranging from 2,500 to 2,600 m. From preliminary analysis, at least one lobe of the Block Composite slide (lobe 2) would require further study to evaluate its tsunamigenic potential.
Submarine mass movement along the U.S. and Canadian Atlantic coasts have been the subject of recent and ongoing investigations. (e.g. Twichell et al. 2009; see ten Brink [2009] for a summary along the U.S. coast and Piper and McCall [2003] for the Canadian coast). As more multibeam data become available, these regions are
J. Locat (�)Department of Geology and Geological Engineering, Laval University, Québec, Canada, G1Y 2P1e-mail: [email protected]
U.S. ten Brink and J.D. ChaytorUnited States Geological Survey, Woods Hole Science Center, 384 Woods Hole Rd., Woods Hole, MA 02543, U.S.A.
found to exhibit extensive mass failure features showing prominent escarpments but little sediment on the failure plane. In many cases, the failed mass is seen as a significant source for turdidites for which deposition extends over very large areas of the continental rise and the abyssal plain. The southern New England (SNE) continental margin is a classic passive continental margin, characterized by an almost flat continental shelf (average gradient <0.5°), relatively steep (2–10°) and narrow continental slope incised by canyons, and a wide and shallowly sloping (average slope of ~1°) continental rise.
The geology of the margin comprises three major phases: (1) Early Jurassic-age rifting and salt deposition, (2) Middle Jurassic to Middle Cretaceous carbonate platform development (Poag 1991), and (3) Cenozoic deposition of marine and non-marine clastic sediment in marginal basins and on the rise (Poag and Sevon 1989; Ryan 1978). The most important period in regard to active submarine land-slide processes along the SNE-margin is the Cenozoic, particularly the Quaternary, when large volumes of river and glacially-derived sand and gravel were deposited onto 3 km thick sequence of stratified Tertiary sediment, composed largely of chalk (Tucholke and Mountain 1986; Robb et al. 1981; Poag and Sevon 1989). In the southern NE region, the Quaternary sedimentary section is approximately 200–300 m thick near the top of the continental slope (Austin et al. 1998), thinning seaward and becoming absent in places along the lower slope. The majority of landslides along the southern NE continental margin have occurred entirely within the Quaternary section along the slope, but truncated beds of Eocene chalk crop out in places, suggesting involvement of deeper margin strata (Twichell et al. 2009).
The Block Composite (BC) slide area off the coast of New England is located east of the Hudson Canyon, where the continental margin changes its orientation from northeast to east (Twichell et al. 2009).The Block Composite (BC) landslide (Fig. 1) is investigated here in more detail with the goal of understanding the nature of the mass movements that comprise it and their potential for generating tsunamis.
a
b
c
Fig. 1 General view of the sea floor around the Block composite submarine landslides shown here with the three major lobes (labeled 1, 2, and 3). Turbidity current or debris flow transport routes are shown feeding into active or inactive canyons. Major failure surfaces are also indicated by white arrows. (c): location map. (c) Details of the morphology in the BC slide area. In parenthesis are given the thickness in meters and volume in cubic kilometers. Inset location map from Google.com
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At this stage, the study presented here addresses the description of the morphology of the BC slide area and points towards elements that are to be explored further as part of an ongoing stability and mobility analysis. At this point, particular attention will be paid to the morphology of the various sloping surfaces in the area which may provide clues about the intact strength of the sediments mobilized by the various mass movements. In addition, possible failure scenarios and overall displacement of the failed masses involved in the BC slide are considered. Some of the interpretation presented here may change significantly as we complete the multibeam coverage of the area and acquire cores for geotechnical testing and for dating various failure sur-faces. At his time, when necessary, geotechnical properties will be inferred from a detailed studied of the Hudson Apron slide area carried out by Locat et al. (2003) and on cores of the ODP site 1073 on the Hudson Apron (Austin et al. 1998).
2 Methods
A Digital Elevation Model (DEM; Fig 1) with a cell resolution of 100 m was con-structed from bathymetric data set (Fig. 1) acquired by the National Oceanic and Atmospheric Administration and the University of New Hampshire Joint Hydrographic Center in support of the U.S. Law of the Sea Study (Gardner et al. 2006) and extracted from the NOAA National Geophysical Data Center bathymetry database to partly fill data gaps along the continental slope (see Fig. 1).
Using the hull-mounted Knudsen 320B/R deepwater echosounder on the R/V Oceanus, approximately 1,065 km of 3.5 kHz sub-bottom data were collected dur-ing a 6 day cruise between April 18–23, 2008. Power and gain settings were modi-fied as appropriate to maximize penetration, although they were predominantly fixed throughout the cruise at the following parameters: 12 kHz channel turned off, pulse length of 6 ms, manual gain between 145–205, AGC/TVG/processing gain/sensitivity turned off, and range set at 100 m (autophased). Sub-bottom penetration varied based on the composition of sediments and reached a maximum of 50 m.
3 Results
3.1 Geomorphology of the Block Composite Slide Area
The BC slide region extends from the continental shelf, at a depth of about 300 m, down to the upper continental rise at about 3,000 m (Figs. 1 and 2). Mass move-ment activity along the continental slope off New England has been intense as shown by various failure surfaces that are demonstrated in Fig. 1a. Most of these failure surfaces are devoid of displaced sediments (failure deposits), although there is some evidence on the continental rise that some of these slides may have generated debris flow deposits which can be traced down to a water depth of about 2,700 m (Figs. 1a and 2).
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3.2 The Block Composite Slide
Debris lobes of the BC slide are shown in Fig. 1. A detailed analysis of these lobes (Fig. 1c) reveals the presence of a failure escarpment 20–30 m high and inclined at about 2–3° which stretches sinuously for about 45 km in an east–west direction. The lobes are segmented into three primary lobes (1–3) of similar size (Fig. 1c) and three smaller secondary lobes (4–6). Lobes 1 and 3 are characterized by a series of com-pression ridges located downstream from the failure escarpment. For lobe 1, the compression ridges are spaced about 400–500 m apart with amplitudes less than 5 m. For lobe 3, compressions ridges are spaced about 700–800 m apart with amplitudes between 5 and 10 m. If the depression identified in Fig. 1c as a failure surface is valid, lobe 1 would have moved about 3 km on a surface inclined at less than 0.5°, which is close to the general gradient for the upper continental rise. Lobe 3 would have move less than 1 km over a gradient less than 0.2°. Interestingly, lobe 2 has no
a
b
c d
Fig. 2 (a) 3.5 kHz echosounder line across one of the lobes of the slide and morphological details of (b) position of seismic line in (a), (c) morphology of the BC slide on the upper continental rise, (d) an enlargement of the upper escarpment seen in (a), and (e) the morphological settings showing the potential extent of the BC slide and the location of the seismic lines (shown in white for the cross section in (a) and in red for the other ones)
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compression ridges but consist of a blocky deposit that cut across the compression ridges of both lobe 1 and 3, indicating a later event. From its morphological charac-teristics, lobe 2 likely is a debris flow deposit which may have come from higher elevation than lobe 1 and 3. These latter lobes may have been only displaced locally. In addition, lobe 2, much like lobe 4, contains large blocks up to 25 m high and about 500 m in diameter. Lobe 4 has morphology similar to lobe 2 and appears to cover parts of lobe 3. Lobe 5 consists largely of a field of blocks that appears smoothed by sediment accumulation. Lobe 6 is an out-runner block possibly from a previous slide. Like lobe 2, debris from lobe 6 must have come from higher elevation, (i.e., high enough to have gained the necessary momentum to reach its final position). The total volume of sediments included in all lobes is estimated at 36 km3.
With the limited data available, an attempt has been made to reconstruct a mor-phological cross section of the BC slide by coupling the multibeam and seismic data, as shown in Fig. 2. The first observation is the lack of penetration of the seis-mic signal in the sediments due to the near absence of high-frequency coherent layering in the sediments.
Some landslide debris is visible in Fig. 2c in the shallower parts of the continen-tal slope suggesting that some failed mass did remain on the failure plane. As shown in Fig. 2c, the failure plane appears to follow a stratigraphic layer which may represent sandy sediments as seen in core 1073 (Austin et al. 1998; Locat et al. 2003). Figure 2c shows that the upper headwall scarp is 30 m high and has an angle of 6° while the lower scarp has an angle of 14° and a height of 90 m. If this had been a single event, it would suggest that the total section of sediments involved in the slide was about 120 m thick.
As indicated by O’Leary (1993), many of the submarine mass movements in this area have a failure plane controlled by a stratigraphic layer and therefore, failures at various locations often tend to follow the same stratigraphic horizon. The locations of two escarpments are shown in Fig. 2b, c. Below the lower scarp (Fig. 2b, c) the gradient of the slope increases sharply to a value of 5.5° and that surface appears to be largely free of any debris. This area is likely acting as a bypass zone where any sliding mass would gain significant mobility and energy allowing it to disintegrate and flow down slope and eventually be deposited on the lower slope, or travel as a debris flow or a turbidity current to reach the continental rise. The BC slide deposit lies on a gradient of about 0.7° on the upper continental rise (Fig. 2a) and if the departure zone for the event was along the upper continental slope (i.e. at depths less than 700 m), then the farboschung of this event would be about 1.7° (Fig. 2a).
3.3 Morphology of Slopes and Strength
The BC slide area provides an opportunity to illustrate some of the processes by which a given slope can be generated. There are few classes of slopes: (1) sedimen-tation or accumulation slopes that are somewhat analogous to clinoforms (1a) and mass transport deposition (1b), (2) eroded slopes by processes like canyon formation
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(2a) and open slope instabilities related to scarp slopes (2b) and failure surface slopes (2c). A third group could correspond to a slope formed by tectonic processes such as plate tectonics (3a), diapirism (3b), or collapse (3c) and a forth type (4) associated with volcanism. It was shown by Locat and Lee (2002) and by Locat et al. (2009) that eroded slopes (type 2) reflect the intact strength of the material involved in a slide while accumulated slopes (type 1b) composed of debris flow deposits relate partly to the remolded strength of the material.
An analysis of height and surface slope data at specific points on the continental slope and rise in the BC slide area (located on Fig. 3a, c, d) is shown in Table 1 and Fig. 3f. For this analysis, geometric information was obtained for open slopes (white number in Table 1), landslide scarp slopes and heights (green numbers in Table 1), failure plane slopes (yellow number in Table 1), and canyon slopes and heights (red in Table 1). The data for eroded slopes are bound by an assumed fric-tion angle of 30° (taken here from the work on the Hudson Apron Locat et al. [2003]) and by slope angle of the continental slope (i.e. typically <5°). If geotech-nical properties of samples from the Hudson Apron area (Locat et al. 2003) are applied to the Quaternary sediments in the BC slide area, the drained shear strength parameters obtained from triaxial testing indicate that the cohesion is less than 5 kPa. This value suggests that cohesion is not significant for slopes higher than 20–50 m (Locat et al. 2009). On the other end, if the sediments become indu-rated by various cementation processes, than the actual cohesion will increase sharply and the maximum slope angle could be greater than the friction angle,
Fig. 3 (a, c, and d): Position of the various points where height and slope angle of escarpments and angle of planes listed in Table 1 were measured. (f): comparing eroded slopes with deposition slopes (Data is from Table 1)
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even for a high slope. It is seen from Fig. 1c that debris accumulates on slopes of about 5° and also on slopes less than 1° (Fig. 2a), indicating that the strength of the material does vary as a function of the degree of remolding achieved during a given failure process.
4 Discussion on Slopes, Strength, Triggering and Tsunamis
There may be situations where actual strength of sediments involved in a given slide are not readily known although a preliminary slope stability assessment may be necessary, as is the case here. Understanding slope formation in a given region, therefore, may help provide an estimate of strength properties.
In the case of the BC slide area, scarp slopes (class 2b) were considered separately for open slopes and for canyons. As indicated above, open slope failures seen along the southern NE sector of the US Atlantic margin have their failure plane largely controlled by bedding planes that can become weaker following various triggering mechanisms (e.g., earthquakes). In most cases, the height of scarp slopes are much less than those of canyon slopes and their gradients are also generally lower. For scarp slopes, it is also interesting to note that the scarp slope angle is higher for older strati-graphic units, i.e. scarp slopes located deeper on the continental slope are cut into older (and stronger) sediments. This is particularly the case for points 59, 58 and 56 in Figure 3a that are respectively at 7.7°, 12.3° and 20.2°, for which the slope angle is much higher than the dip of the bedding plane or the ‘unfailed’ nearby slope angles. The open slope failures are only covered with a thin layer of debris (Fig. 2c) indicating
Table 1 Values of slope heights (H) and angle of slopes and planes (S) at various sites around the BC landslides shown in. Red boxes are for canyon slopes, green for scarp slopes, yellow for fail-ure surfaces, and white for open slopes (apparently unfailed)
that most of the failed mass disintegrated and flowed down onto the lower part of the continental slope or onto the upper continental rise.
For canyon walls, there may be a continuous process between erosion and insta-bility development. For this reason, the failure plane angle would correspond more or less to the actual canyon wall gradient. By inspection of Fig. 1a, canyons that cut the continental slope generate a longitudinal profile more or less perpendicular to the general gradient of the continental slope. Therefore, the bedding plane, a pseudo-structural component of the sediment, was apparently unable to control the angle of the failure plane, since the direction of movement had to be towards the canyon, i.e. in a direction at about 90° from the dip of the bedding plane. On the upper rise, the canyons meander (Fig. 1a) but most of the canyon walls are steep and close to the friction angle. The potential bedding plane in that area is very close to the actual slope angle (i.e. <1°) which is typical for very steep slopes if cohesion becomes a significant strength parameter. It is interesting here to note that as the canyon becomes inactive, it is filled with sediments so that the slope height decreases while the slope angle remains more or less the same (these canyons are filled by debris flows which force them to become inactive). The crescent like shape seen along the edges of the canyons may be used to indicate that the sliding mechanism is closer to that of a circular failure (e.g. Haflidason et al. 2004). The increase in the scarp slope angle with depth may also indicate that the shear strength is also increasing with depth. Dugan and Flemming (2002) discuss the role of pore pressure on slope stability and concluded that significant excess pore pres-sures were present under the continental slope around the Hudson Apron. Some excess pore pressures may still exist on the slope as indicated by Robb (1984) who has observed spring sapping on the slope. That being said, the very low strength values obtained in ODP 1,073 cores (Austin et al. 1998; Locat et al. 2003) may result from a low degree of consolidation. However, the steep slopes of the canyons and the relatively high slopes in the area would support the argument that the strength values obtained in the ODP 1,073 cores were largely disturbed, possibly by the presence of gas (Kayen and Lee 1993), although these geotechnical test results may not be applicable in the BC slide region.
Regarding triggering mechanism, preliminary analysis of the BC slide area sug-gests that for most of the open-slope failures to take place, either significant excess pore pressures (or its equivalent, i.e., generated by gas hydrates Sultan et al. 2004), seismic acceleration, or both were required. When considering a slope angle of 8°, a scarp slope of 12°, a failure surface of 2°, a failure thickness of 45 m, and a slope height of about 700 m, an excess pore pressure equivalent to about 40% of the over-burden weight for a seismic acceleration coefficient of about 0.3 is required for instability to be generated (i.e., F = 1.0). With no seismic acceleration coefficient, the required excess pore pressure would need to be on the order of 90%. This is similar to what has been modeled for the Hudson Apron slide by Locat et al. (2003).
The tsunamigenic potential of a slide is mainly a function of the following parameters: slide volume, sliding mechanism, slide acceleration, and water depth (Geist et al. 2009). The slide volume calculated for the major lobes of the BC slide varies between 7.9 and 15.3 km3. As an example, for the Storegga (Nadim and Locat 2005),
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third party risks (i.e. those not related to the ‘owner’, e.g. coastal infrastructures) in the Storegga slide area, were considered to become significant for a slide volume greater than 5 km3, in a situation where the sliding mechanism was a circular failure and at a water depth of about 200 m. Similarly, for the Currituck area, Geist et al. (2009) have shown that a landslide volume of about 150 km3 could generate a coastal run up of 5 m. An important issue here with regards to the tsunamigenic potential of these slides, which is still to be resolved by dating and coring, relates to the sequence of events leading to the formation of these failure surfaces and of their initial volume. It is likely that some amphitheaters may be part of a single event, as concluded for the Currituck slide (Locat et al. 2009) and the Storegga slide (Haflidason et al. 2004), but others may not. It is still not clear yet as to where the slide was initiated.
The detailed morphology of the BC slide lobes (Figs. 1c and 2) indicates that at least lobe 2 could have originated from the slope (scenario 1) while for lobe 1 and 3, adjacent scarps may actually indicate a local source on the upper continental rise (scenario 2). As for lobe 1, it remains to be demonstrated how a mass of that size would acquire the necessary mobility to slide over a distance of about 3 km on a slope less than 1°, as it would require excess pore pressure greater than 0.9 or a very strong earthquake with seismic acceleration much greater than 0.3. Another sce-nario (scenario 3) may also consider a slide initiated on the continental slope that accumulated on the upper reaches of the continental rise, like lobe 2, but that quickly overloads (dynamic loading) the underlying sediments to trigger a second-ary slide that carries the debris over some distance. Scenario 2 would not generate a significant tsunami because of the depth (more than 2,000 m) and the limited acceleration which could be expected for a slide initiated on such a low slope angle. On the other hand, an analysis of the mobility of the slide responsible for lobe 2 may indicate a much greater potential than for a scenario 3 slide. The mobility analysis is still underway and the forthcoming results will also help evaluate the failure dynamics in the BC slide area.
Conclusions
The analysis of the seafloor geomorphology in the vicinity of the Block Composite (BC) slide has provided some insight on failure processes in this area that lead to the following conclusions:1. Slopes in canyons are much steeper than on failure scarps. It is speculated that the
generation of weak layers along bedding planes is more effective for cases where the dip of the slope is in the same direction as the dip of the bedding plane.
2. Eroded slopes in the area indicate that the in situ sediment strength increases significantly with burial depth to a point where values reach close to their friction angle (i.e. about 30°).
3. The BC slide is composed of three major lobes for a total of 36 km3, one of which may have originated from the continental slope with a potential for
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generating a tsunami. The other major lobes do not appear to have traveled over long distances and are not considered tsunamigenic.
4. A preliminary analysis of the slope stability of lobe 2 indicates that for that failure to occur, significant excess pore pressures needed to have been generated and that the main potential agent would likely have had to have been an earthquake.
5. More multibeam and seismic data along with coring and geotechnical testing is required to better understand slide initiation and timing of the many failure surfaces observed in this area.
Acknowledgments The authors would like to thank the U.S. National Regulatory Commission for their financial support.
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