Distinguishing sediment waves from slope failure deposits: field examples, including the ‘Humboldt slide’, and modelling results
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.
References (56)
- et al.
Spatial variability in sedimentary processes on the Eel continental slope
Mar. Geol.
(1999) - et al.
Sedimentation processes on the continental rise of northeastern South America
Mar. Geol.
(1977) A lee wave model for deep-sea mudwave activity
Deep-Sea Res.
(1988)- et al.
Humboldt slide – a large shear-dominated retrogressive slope failure
Mar. Geol.
(1999) - et al.
Geotechnical characteristics and slope stability in the Gulf of Cadiz
Mar. Geol.
(1999) - et al.
Regional variability of slope stability: application to the Eel Margin, California
Mar. Geol.
(1999) - et al.
Sedimentary features associated with channel overbank flow: examples from the Monterey Fan
Mar. Geol.
(2000) - et al.
Quaternary development of migrating sediment waves in the Var deep-sea fan: distribution, growth pattern, and implication for levee evolution
Sediment. Geol.
(2000) - et al.
River plume sedimentation modeling for sequence stratigraphy: application to the Eel shelf, California
Mar. Geol.
(1999) - et al.
Sediment waves on the Monterey Fan Levee: a preliminary physical interpretation
Mar. Geol.
(1980)
Tectonics, sedimentation, and erosion in northern California: submarine geomorphology and sediment preservation potential as a result of three competing processes
Mar. Geol.
INFLO1: A model predicting the behaviour of turbidity currents generated at a river mouth
Comput. Geosci.
Estimating river-sediment discharge to the ocean: application to the Eel Margin, northern California
Mar. Geol.
Observations on experimental, non-channelized, high-concentration turbidity currents and variations in deposits around obstacles
J. Sediment. Res. A
Weakly depositing turbidity currents on a small slope
J. Hydraul. Res.
Holocene slump on continental shelf off Malaspina Glacier, Gulf of Alaska
Am. Assoc. Petrol. Geol. Bull.
Styles of failure in late Holocene highstand prodelta wedges on the Adriatic shelf
J. Sediment. Res.
Migrating sediment waves created by turbidity currents in the northern South China Sea
Geology
Sediment ripples and dunes
Annu. Rev. Fluid Mech.
Earthquake-induced sediment failures on a 0.25° slope, Klamath River delta, California
Geology
Geometry and significance of stacked gullies on the northern California slope
Mar. Geol.
On the development of dunes in erodible channels
J. Fluid Mech.
Experiments on hydraulic jumps in turbidity currents near a canyon-fan transition
Science
Cited by (189)
Three-dimensional seismic evidence for depositional undulations nucleated around pre-existing relief, South China Sea
2024, Journal of Asian Earth SciencesFormation of undulating seafloor bedforms during the Minoan eruption and their implications for eruption dynamics and slope stability at Santorini
2023, Earth and Planetary Science LettersSediment redistribution processes in Baffin Island fjords
2023, Marine Geology