Elsevier

Marine Geology

Volume 192, Issues 1–3, 15 December 2002, Pages 79-104
Marine Geology

Distinguishing sediment waves from slope failure deposits: field examples, including the ‘Humboldt slide’, and modelling results

https://doi.org/10.1016/S0025-3227(02)00550-9Get rights and content

Abstract

Migrating sediment waves have been reported in a variety of marine settings, including submarine levee–fan systems, floors of fjords, and other basin or continental slope environments. Examination of such wave fields reveals nine diagnostic characteristics. When these characteristics are applied to several features previously attributed to submarine landslide deformation, they suggest that the features should most likely be reinterpreted as migrating sediment-wave fields. Sites that have been reinterpreted include the ‘Humboldt slide’ on the Eel River margin in northern California, the continental slope in the Gulf of Cadiz, the continental shelf off the Malaspina Glacier in the Gulf of Alaska, and the Adriatic shelf. A reassessment of all four features strongly suggests that numerous turbidity currents, separated by intervals of ambient hemipelagic sedimentation, deposited the wave fields over thousands of years. A numerical model of hyperpycnal discharge from the Eel River, for example, shows that under certain alongshore-current conditions, such events can produce turbidity currents that flow across the ‘Humboldt slide’, serving as the mechanism for the development of migrating sediment waves. Numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor (i.e. numerical steps), sediment waves can form and migrate upslope. Hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves. Physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms. The morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field, including those that were previously described as submarine landslides.

Introduction

Sediment waves with a turbidity current origin have been recognised in a variety of submarine environments worldwide. Most commonly the waves are associated with turbidity current levee and fan deposits (e.g. Normark et al., 1980, Nakajima et al., 1998). The waves often form on the back side (side away from channel floor) of levees, within a restricted range of slope gradients. For example, Normark et al. (1980) described an extensive field of turbidity-current sediment waves on the Monterey Fan that exist only in the area of steepest regional gradient where the levee slope is as much as three times greater than the adjacent channel floor and longitudinal levee gradients. Nakajima et al. (1998) found well-defined sediment waves in many locations along the Toyama Deep-Sea Channel System but only on levee backslopes with a gradient between 0.007 and 0.025. Turbidity-current sediment waves have also been found in other deep-sea environments, including above pre-existing debris flow deposits (Howe, 1996), along the sides of a sedimentary ridge (Embley and Langseth, 1977), and along the seaward wall of a deep-sea trench (Damuth, 1979). They are also found in a subaqueous delta within a fjord (Bornhold and Prior, 1990). The sediment waves have wavelengths of hundreds to thousands of metres and surface relief of up to 40 m or more.

Deposits that form sediment waves contain many individual turbidite event beds (Syvitski et al., 1987, p. 207), suggesting that these features developed over long time periods. The waves appear to be the result of many turbidity current events, undoubtedly each having travelled with somewhat different flow regimes, carrying differing grain-size populations. Sediment-wave deposits may also incorporate sediment deposited from other sources (i.e. nepheloid transport from shelf storms, plumes from river discharge). Thus the boundary conditions associated with the evolving seafloor morphology also evolve and impact on the dynamics of each subsequent turbidity current. Sediment waves commonly appear on seismic-reflection records to migrate upslope towards the current source. The waves are typically composed of fine-grained material (lutite) with silt laminae, and sediment accumulation is most rapid on the shorter upslope limb of each wave (e.g. Normark et al., 1980).

The timeliness of this topic relates to the interpretation problems facing geologists who interpret seismic-reflection records that contain these features. Many similar features are interpreted as slump compression waves (i.e. post-depositional failure) rather than current-related depositional features developed from the multiple passage of turbidity currents (e.g. Lee and Baraza, 1999). Others have tried to distinguish between such features developed from turbidity currents and those developed from contour-current influences (Howe, 1996). The STRATAFORM (Office of Naval Research programme to investigate STRATA FORMation on margins) community in the United States, for example, is divided on whether a particular feature on the Eel River margin, California (‘Humboldt slide’), is a landslide or a field of migrating sediment waves. Recent meetings with European geo-scientists who study continental margins show their community is also divided on the interpretation of similar features (F. Trincardi, S. Berné, personal communication, 1999).

Below, examples of deposits that have been described in the literature as being migrating sediment waves are provided and a set of common features is drawn from these examples that can be used to identify such deposits. Next, several similar deposits that have been identified as submarine landslides are described. However, these are likely additional examples of migrating sediment-wave fields. Finally, results of numerical and physical models are provided to show how turbidity currents can generate migrating sediment-wave fields, and the conditions under which such fields are formed.

The focus of this paper is on large-scale migrating sediment waves that can be resolved in acoustic and seismic seafloor images. It should be noted, however, that other kinds of migrating bedforms are observed at subresolution scale in outcrops and cores. These include the climbing ripples often observed in the upper units of turbidites (e.g. Reineck and Singh, 1975), as well as the dunes that are often found in sandy turbidites of sufficient thickness (Mohrig et al., 2001). The ripples in question typically have wavelengths in the order of 10 cm, and the dunes have wavelengths in the order of 1 m. Both of these small migrating bedforms have a characteristic asymmetry, such that the stoss side has a gentle slope and the lee side has a steep slope. Their small scale and strong asymmetry distinguish them from the large-scale, roughly symmetrical migrating sediment waves discussed below.

Section snippets

Examples of deposits accepted as migrating sediment waves

Numerous investigators have described turbidity current (TC) sediment-wave fields that have been found in a variety of environments. These environments include the backslopes of channel levees (Normark et al., 1980, Normark et al., 2002, Piper and Savoye, 1993, Gardner et al., 1996, Nakajima et al., 1998, Piper et al., 1999, McHugh and Ryan, 2000, Migeon et al., 2000), fjords (Bornhold and Prior, 1990), the continental rise near volcanic islands (Wynn et al., 2000), and submarine fans (Howe,

Common features

The above examples show that turbidity-current sediment waves have been observed in a variety of settings. However, there are typically several common features that characterise these waves, including:

(1) Differential deposition rates. The upstream flanks accumulate sediment more rapidly than the downstream flanks. This effect causes the sediment waves to migrate upslope.

(2) Continuous acoustic reflections through the features. Although spacing between reflections may vary as a result of item

Examples of deposits previously described as landslides

Several submarine deposits have many characteristics of turbidity-current sediment-wave fields, but have been described in the literature as landslides. One of the best documented examples is the ‘Humboldt slide’, located offshore Eureka, California.

Generation of turbidity currents by hyperpycnal flows on the Eel margin

If the ‘Humboldt slide’ (Lee et al., 1981, Gardner et al., 1999) is a field of migrating sediment waves, then it is essential that: (1) repeated turbidity currents flowed across the feature, and (2) the conditions of the flows were such that they formed sediment waves rather than parallel beds. One possible mechanism for generating turbidity currents that could reach the sediment-wave field is the development of hyperpycnal flows from the Eel River (Imran and Syvitski, 2000). Such a mechanism

Summary and conclusions

The existing literature and this special issue contain abundant examples of turbidity-current sediment waves. The wave fields can be found in a variety of settings including deep-sea fan systems, the floors of fjords, and along seafloor slopes. Diagnostic characteristics can be established to distinguish turbidity-current sediment-wave features from submarine landslide deposits. These include:

(1) The waves have upcurrent flanks that accumulate sediment more rapidly than the downcurrent flanks.

Acknowledgements

Funding for this research was provided by the Program of the Office of Naval Research (Code 32GS) under the direction of Joe Kravitz, Roy Wilkens and Jill Karsten, and the U.S. Geological Survey. This paper benefited from helpful reviews by William Normark, David Piper, Christopher Sherwood, Russell Wynn, and an anonymous reviewer.

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