Large-scale Sediment Waves Research Articles

Genesis and evolution of large-scale sediment waves in submarine canyons since the Penultimate Glacial Maximum (ca. 140 ka), northern South China Sea margin

Sediment waves are common on the seafloor, but large-scale ones developed in submarine canyons are rarely reported. In this study, we document for the first time the Quaternary deep-water canyon-confined large-scale sediment waves developed on the northern South China Sea margin based on the analysis of a high-quality 3-D seismic reflection volume combined with a 2-D multichannel seismic reflection profile and three wells. Results show that the onset of these canyon-confined large-scale sediment waves can be dated back to the Penultimate Glacial Maximum (PGM)(ca.140 ka). This was the last period for the Pearl River Delta to prograde over the shelf edge. And these sediment waves have continued to develop and migrate upslope until present. The large-scale sediment waves pertaining to the PGM have a dominant down-slope asymmetrical 2-D morphology with wavelengths of 1.059–6.090 km and wave heights of 3.3–32.5 m. The present large-scale sediment waves show an up-slope asymmetrical 2-D morphology with dimensions of 0.667–5.628 km wavelength and 2.7–14.0 m wave height, which are smaller relative to those at the PGM. Grain sizes of the sediment waves at the PGM and present are interpreted to be coarse, inferred from seismic reflection data by high amplitude reflections (HARs). After a comprehensive analysis of the types of sediment-laden flows, the sediment provenances, the seafloor topography and the features of the sediment waves, these large-scale sediment waves might be interpreted as cyclic steps generated by the down-slope flowing supercritical turbidity currents and associated internal hydraulic jumps along high gradient (approximately greater than 1.28°) canyon thalwegs characterized by numerous slope breaks. The evolution of the large-scale sediment waves from partially depositional at the PGM to erosion-dominated at the present is controlled by variations of the sediment supply and the submarine slope accommodation along the canyon thalweg, which is manifested in the co-evolution of the sediment waves with the canyon. The findings of this study tell us that in addition to axial channels and mass-transport complexes, large-scale sediment waves can also develop on the canyon floor, which helps to improve our understanding of the complex canyon processes. The ‘large-scale sediment waves’ described here may be a new potential deepwater-reservoir element for the deep-water hydrocarbon exploration associated with submarine canyons.

Seismic geomorphological reconstructions of Plio-Pleistocene bottom current variability at Goban Spur

High-resolution single channel sparker reflection seismic data revealed the presence of large-scale sediment waves nearby DSDP Site 548, located on Goban Spur. They developed in a gentle terraced environment which contrasts with the canyon-incised Celtic margin, and the relatively smooth Porcupine Seabight to the north. Based upon the morphological characteristics of the observed seabed and buried sediment waves, energetic alongslope bottom currents are thought to be the driving mechanism for the sediment wave development. These currents are driven on their turn by an enhanced internal tide regime that could be attributed to the introduction of the Mediterranean Outflow Water. The DSDP Site 548 downhole geophysical data and the seismic stratigraphic analysis allowed the differentiation of three sequences that relate to evolutionary stages since the lower Pliocene. The sequences are bounded by local erosional events, associated with mass wasting events, which seem to occur roughly synchronously to major northern hemisphere glaciations, respectively during the Lower Pleistocene (~2.5 to 2.15Ma), and the Middle Pleistocene (~0.45Ma). The lower sequence (from ~4.5 to ~2.15Ma) shows no morphological evidence of bottom-current driven sedimentation. It is however settled over a smooth erosional surface which could indicate the introduction of the Mediterranean Outflow Water. The intermediate sequence is characterised by large-scale sediment waves that have gradually developed in close association with palaeo-seafloor irregularities. It is inferred that the sedimentation resumed with a relative bottom current energy increase. The youngest sequence recorded active sediment wave formation, similar to the previous sequence. Although the Goban Spur sediment waves cannot be regarded as contourite drifts as such, their stratigraphic evolution corresponds to other well-documented contourite depositional systems, influenced by the Mediterranean Outflow Water.

Spatio-temporal evolution of sediment waves developed on the Gulf of Valencia margin (NW Mediterranean) during the Plio-Quaternary

Several fields of large-scale sediment waves have been observed along the Gulf of Valencia continental margin (NW Mediterranean). The largest sediment waves develop on the continental slope, extending from 250 to 850m water depth, with wavelengths ranging between 500m and 1000m and wave heights from ~2m to ~50m. On the lower part of the slope, sediment waves are quasi-stationary “vertically accreting”, becoming up-slope migrating towards the mid- and upper part of the slope. A second group of sediment waves have developed over the outer continental shelf, with wavelengths of 400 to 800m and heights of 2 to 4m, also displaying an up-slope migrating pattern. Multi-channel seismic lines crossing the continental margin show that the sediment waves over the continental slope region have been continuously developed on the foreset region of the prograding margin clinoform. Several units of sediment waves have been identified in the sedimentary record, evolving in accordance with the margin progradation. Detailed analysis of single-channel (sparker) seismic profiles revealed the presence of several sediment depositional subunits over the outer continental shelf, some of them with successive development of sediment waves being truncated by erosive surfaces, likely related to Quaternary eustatic sea-level oscillations. These erosional surfaces can be followed downslope into paraconformable strata of the sediment waves on the continental slope, where constant bedform growth is observed, without being affected by sea level changes. Based on geophysical data, the thickness of the sediment waves mapped units show that the largest sediment waves (in wave ratio, length and height) develop where sediment deposition rates are the highest, coinciding with the upper part of the continental slope (foreset clinoforms), confined by the presence of structural highs. The development of these sediment waves has been previously explained by the interaction of internal waves over the continental slope. Because sediment waves are preserved in the sedimentary record since the Lower/Pliocene, internal waves activity could have been present in this part of the margin shortly after the Zanclean reflooding of the Mediterranean Basin, following the Messinian desiccation event ~5.6 My ago. Deep water hydrodynamic conditions were re-established at that time, modulating sediment transport and deposition over the continental slope and outer continental shelf.

Large-scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows

Large-scale (20m to 7km wavelength) bedforms are common on the seafloor, yet there is a lack of consensus on how they form and thus what to call them. We conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments. The data form three distinct groups: 1) small-scale (20–300m wavelength) sediment waves with mixed relief made of medium sand to cobble-sized sediment that form in confined settings, which we call small sediment waves; 2) large-scale (300–7000m wavelength) sediment waves with mixed relief made of fine-grained sediment that form in relatively unconfined settings, which we call large sediment waves; and 3) large-scale fully enclosed depressions in the seafloor, which we call scours. There is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact. This data gap probably results from the environments in which sediment waves form being either confined (e.g. channel or canyon) or unconfined (e.g. open slope). Bedform migration direction is available for 36% of the data and includes small and large-scale sediment waves; of these examples all are shown to migrate up-current. Up-current migration is indicative of supercritical flow; thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves. Therefore, we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size. The migration direction for scours remains unknown. Scours may form from similar processes to small and large sediment waves, or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre-existing defects in the seafloor. This novel statistical analysis of a global database shows that up-current migrating bedforms associated with supercritical flow are unusually widespread, and are recognised at two distinct scales.

Seismic evidence of current-controlled sedimentation in the Belgica mound province, upper Porcupine slope, southwest of Ireland

The Porcupine Seabight is an embayment that takes a particular position in the NE Atlantic slope. Sonographs, a few current measurements and hydrodynamic modelling suggest the presence of a strong northward-flowing bottom current, locally enhanced by internal tides, affecting the eastern slope of the Seabight. At this location a province of coral banks is described, expressed as mounds lined up in along-slope-trending ridges. In this paper, very high-resolution single-channel seismic profiles are used to evaluate to what extent the bottom currents influenced the deposition of the sediments surrounding the mounds throughout the Late Cenozoic. Three seismostratigraphic units (P1, P2 and P3) can be identified in the Belgica mound area, separated by two margin-wide discontinuities (RD2 and RD1). Within Unit P1 (probably Early to Middle Miocene) upslope-migrating sediment waves are observed, suggesting strong bottom currents were already active in the Miocene. After an early Middle Miocene erosion event, represented by reflector RD2, an acoustically transparent layer (Unit P2) of as yet unknown lithology was deposited in the studied area. A second margin-wide erosional event, marked by the Late Pliocene RD1 reflector, removed a large part of Unit P2 and has cut deeply into Unit P1. Subsequently, the Belgica mounds were constructed spectacularly fast on topographic irregularities on the RD1 paleobathymetry. The onlap within the Quaternary Unit P3, which surrounds these mounds, suggests that the mounds were already present before the deposition of P3 and were big enough to affect the intensity of the currents around them. Furthermore, the channels and the mounds are, together with the complex oceanographic regime, the key morphological elements responsible for the shaping of a contourite system in the Belgica mound area during the Quaternary. One drift body is formed by an inferred south–north-directed current, with a drift levee and associated channel located on its western side. Between this channel and the mounds, large-scale sediment waves suggest an intensified bottom current running along the foot of the steep flanks of the mounds. The Belgica mounds are embedded in another drift body. Here, an interaction of bottom and turbidity currents is suggested, creating short turbidite channels at the southern and northern flanks of the mounds. Locally, small confined drifts can be observed where Unit P3 is deposited in a narrow passage made by the paleobathymetry of RD1 and the mounds.

Generation and migration of coarse-grained sediment waves in turbidity current channels and channel–lobe transition zones

Large-scale sediment waves, composed of gravels and sands, have been studied using deep-water sidescan systems. New data are presented from submarine channels off the Canary Islands and from canyon mouths off Portugal. Data from other areas are briefly reviewed, including a re-interpretation of data from Laurentian Fan, in order to summarise the varied morphology and setting of these bedforms. Coarse-grained sediment waves are found in the proximal, dominantly bypassing areas of deep-water turbidite systems, within canyons, channels and channel–lobe transition zones. Wave heights are in the region of 1–10 m, and wavelengths are up to several hundred metres. The distribution of waves, and sparse sedimentological evidence from modern and ancient sediment wave fields, suggests that initial transport and deposition of coarse sediment occurs within a high-density turbidity current, and not as a non-Newtonian debris flow. In some cases the development of pronounced wave asymmetry, and evidence of wave disruption and reworking, suggests that the wave morphology is at least partially controlled by a later phase of low-density turbidity flow. Grain size also appears to exert some control on wave morphology, for example, gravel-rich waves have a greater height for the same wavelength than sand-rich waves. Coarse-grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents, and are equally difficult to recognise in outcrop because of their large size.

ABSTRACT: A New Process Model for Turbidity Current Mudwaves (Sediment Waves) : Evidence from Toyama Deep-Sea Channel Levees, Japan Sea

Mudwaves are large-scale sediment waves known from the levees of submarine channels, as well as from abyssal plains and continental slopes. In spite of the common occurrence of mudwaves on the deep-sea floor, their origin have been controversial for the last thirty years due to the insufficient evidence. A new process model for turbidity current mudwaves can be proposed here, based on the detailed investigation of the mudwave fields on the levees of the Toyama deep-sea channel. The present data constitute abundant sediment core samples from stoss and lee slopes of the mudwaves, high resolution (3.5kHz) echosounding and single-channel airgun seismic reflection profiles. These data allow to reveal 3-D anatomy of the mudwave fields. A layer-by-layer correlation of the deposits across the mudwaves shows that the turbidites are markedly thicker (~twenty times) on the stoss sides of the mudwaves, which reflects differential accretion and has resulted in retrogradational climbing of the aggrading mudwaves. The stoss sides have apparently acted as sites of flow deceleration, whereas the lee slopes have accelerated the flow and often promoted sediment bypassing. Seismic reflection profiles show that the mudwaves have been instigated by pre-existing large sand dunes that are up to 30m thick and were created by spillover turbidity currents with estimated thicknesses of the order of 500m and velocities of 1-10m/s at the initial phase of the levee development. The differential draping of the sand dunes by subsequent spillover turbidity currents, with a finer-grained load and estimated velocities of <35cm/s, is thought to have instigated the development of the mudwaves. Although mudwaves are chiefly composed of muddy sediments, the present model predicts the large substratum sand dune fields that would be a potential reservoir.

Shallow-water imaging multibeam sonars: A new tool for investigating seafloor processes in the coastal zone and on the continental shelf

Hydrographic quality bathymetry and quantitative acoustic backscatter data are now being acquired in shallow water on a routine basis using high frequency multibeam sonars. The data provided by these systems produce hitherto unobtainable information about geomorphology and seafloor geologic processes in the coastal zone and on the continental shelf. Before one can use the multibeam data for hydrography or quantitative acoustic backscatter studies, however, it is essential to be able to correct for systematic errors in the data. For bathymetric data, artifacts common to deep-water systems (roll, refraction, positioning) need to be corrected. In addition, the potentially far greater effects of tides, heave, vessel lift/squat, antenna motion and internal time delays become of increasing importance in shallower water. Such artifacts now cause greater errors in hydrographic data quality than bottom detection. Many of these artifacts are a result of imperfect motion sensing, however, new methods such as differential GPS hold great potential for resolving such limitations. For backscatter data, while the system response is well characterised, significant post processing is required to remove residual effects of imaging geometry, gain adjustments and water column effects. With the removal of these system artifacts and the establishment of a calibrated test site in intertidal regions (where the seabed may be intimately examined by eye) one can build up a sediment classification scheme for routine regional seafloor identification. When properly processed, high frequency multibeam sonar data can provide a view of seafloor geology and geomorphology at resolutions of as little as a few decimetres. Specific applications include quantitative estimation of sediment transport rates in large-scale sediment waves, volume effects of iceberg scouring, extent and style of seafloor mass-wasting and delineation of structural trends in bedrock. In addition, the imagery potentially provides a means of quantitative classification of seafloor lithology, allowing sedimentologists the ability to examine spatial distributions of seabed sediment type without resorting to subjective estimation or prohibitively expensive bottom-sampling programs. Using Simrad EM100 and EM1000 sonars as an example, this paper illustrates the nature and scale of possible artifacts, the necessary post-processing steps and shows specific applications of these sonars.