Thierry Mulder

Classification of Offshore Mass Movements

More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments. The processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized. The three types are differentiated mainly by motion, architecture, and shape of failure surface. For slides, the morphology of deposits can usually be linked to a process, but for plastic flows and turbidity currents, information about the motion is mainly provided by the sedimentary record. A static classification based on these features is given, and is related to a dynamic classification system to try to underline the morphological transformation of an offshore event from initiation to deposition.

Predicting the terrestrial flux of sediment to the global ocean: a planetary perspective

A new model for predicting the long-term flux of sediment from river basins to the coastal ocean is applied to a global data set of 340 river basins. The model is based on relief, basin area (or, averaged discharge), and basin-averaged temperature. Basinaveraged temperature is determined from basin location (latitude, longitude) and the lapse rate across the basin relief (hypsometric approximation). The sediment flux model incorporates climate through basin temperature and hydrologic runoff. Solutions are provided for each of the major hemispheric climate regions (polar, temperate and tropic). The model successfully predicts the pre-anthropogenic flux of sediment to within the uncertainties associated with the global observations (within a factor of two for 75% of rivers that range across five orders of magnitude in basin area and discharge). Most of the ‘‘problem’’ rivers are associated with low observational loads (often smaller rivers where anthropogenic impacts are often magnified, and temporal variability is high). Model predictions provide a baseline for researchers: (1) to question the quality of observational data where available and disagreement is greatest, (2) to examine a river basin for unusually large anthropogenic influences (i.e. causes of erosion or causes of hinterland sediment retention), and (3) to uncover secondary factors not addressed by our model (lithology, lakes). The model provides a powerful tool to address the impact of paleo-climate fluctuations (warmer/colder; wetter/drier) on the impact of sediment flux to the coastal ocean. D 2003 Elsevier B.V. All rights reserved.

Estimating fluvial sediment transport: the rating parameters

Correlations between suspended sediment load rating parameters, river basin morphology, and climate provide information about the physical controls on the sediment load in rivers and are used to create predictive equations for the sediment rating parameters. Long-term time-averaged values of discharge, suspended load, flow duration, flow peakedness, and temporally averaged values of precipitation, temperature, and range in temperature were coupled with the drainage area and basin relief to establish statistical relationships with the sediment rating parameters for 59 gauging stations. Rating parameters (a and b) are defined by a power law relating daily discharge values of a river (Q) and its sediment concentration Cs, where Cs = aQb. The rating coefficient a (the mathematical concentration at Q = 1 m3/s) is inversely proportional to the long-term mean discharge and is secondarily related to the average air temperature and the basins topographic relief. The rating exponent b (the log-log slope of the power law) correlates most strongly with the average air temperature and basin relief and has lesser correlations with the long-term load of the river (which is related to basin relief and drainage area). The rating equation describes the long-term character of the suspended sediment load in a river. Each river undergoes higher-frequency variability (decadal, interannual, and storm event) around this characteristic response, controlled by weather patterns and channel recovery from extreme precipitation events.

Detailed seismic-reflection and sedimentary study of turbidite sediment waves on the Var Sedimentary Ridge (SE France): significance for sediment transport and deposition and for the mechanisms of sediment-wave construction

Abstract Sediment waves have been observed on the backside of levees in deep-sea turbidite systems where they are built by turbidity currents that spill out of channels and spread sediment over the levees. In an attempt to understand the way in which, sediment waves are initiated, prograde and/or aggrade, two sediment waves were selected within the present Var turbidite system (Ligurian Sea, north-western Mediterranean) for a detailed sedimentary study. The data include high-resolution seismic-reflection profiles, 3.5-kHz echo-sounder profile, high-resolution side-scan imagery and cores collected from the upstream flank, crest, and downstream flank of both sediment waves. Core-to-core correlations allow interpretation of the dynamic of gravity-flows across the sediment waves. The asymmetrical internal structure of the waves results from higher rates of sediment deposition on the upstream flank and lower rates of deposition and erosion on the downstream flank and further results from active progradation stages during periods of greater gravity-flow activity. The strong differences of depositional processes across the sediment waves result through time in the individualisation of large and interconnected sand bodies in the distal part of the levee.

Climatic and Morphologic Relationships of Rivers: Implications of Sea-Level Fluctuations on River Loads

The characteristics of 279 rivers that discharge into the world oceans are analyzed in terms of their basin hydrology (river discharge), morphometry (basin slope and area, river length, extension of the continental shelf seaward of the river mouth), and climate (precipitation). Statistically reliable relationships are found between discharge and basin area, and between sediment load and a combined function of basin area and slope. These functions are used to demonstrate how river hydrologic features would be strongly influenced by sea-level fluctuations, particularly under the influence of continental shelf emergence. A fall in sea level toward a glacioeustatic lowstand would induce the merging of rivers on the subaerial continental shelf, thereby allowing giant rivers to form. For example, rivers of western Europe would reorganize themselves into two or three very large rivers. Sediment concentration carried by these mega-rivers would decrease, and thus the number of hyperpycnal plumes generated at river mouths would be reduced. There would, however, be a strong increase in global sediment delivery and thus in the frequency of undrained delta-front failures because of both the progressive concentration of depocenters at the mouths of giant rivers and delta migration toward the shelf breaks. The global increase of sedimentation rate should then be empha-sized at giant river mouths. Associated with a global increase of hypsometry would be a significant increase in the frequency and volume of turbidity currents, since high slopes facilitate flow acceleration and slope erosion.

Twentieth century floods recorded in the deep Mediterranean sediments

Flood-generated turbidity currents represent an important process of marine sedimentation. However, no deposit related to this process has yet been described at sea. Turbiditic sequences cored at 2000 m water depth in the Mediterranean show tractive structures and superposition of reverse and normal grading. They are related to floods triggered during the twentieth century. Deposits from flood-generated turbidity currents have a great impact on the interpretation of deep-ocean paleoclimate records obtained near continental margins by linking deep-marine sedimentary records to continental climatic changes through flood frequency and magnitude. Implications of this study should help redefine the reservoir character of oil fields in fine-grained turbidites.

The Var submarine sedimentary system: understanding Holocene sediment delivery processes and their importance to the geological record

Abstract The Var system extends off Nice in the Western Mediterranean. It comprises a river, a delta and a submarine valley leading to a deep-sea fan that together have been in operation since the Early Pliocene. The Var system is an area experiencing active sediment transport, where at least three major types of sediment transfer process are identified: hyperpycnal turbid plumes, surge-like turbidity currents generated by shallow failures induced by excess pore pressure during river flood periods, and by large earthquake-triggered slides. The last two processes might generate higher-density turbidity currents, but at different return intervals. Hydrological data, direct observations of the sea floor, geotechnical testing and numerical modelling confirm the very high frequency of these sediment transfer events. Some of the processes have catastrophic surge behaviour, others are continuous during periods of river flooding. In the latter case, all the sediment supplied to the vicinity of the river mouth is transferred seaward without or with only brief periods of deposition. The geological record of such continuous activity remains difficult to identify. The palaeo-events identified in sedimentary series are often widespread, high-magnitude events with return periods close to a millennium, i.e. usually beyond historical records. Normal ‘background’ processes provide only thin deposits that are not interpretable in the geological record.

Abrupt change in slope causes variation in the deposit thickness of concentrated particle-driven density currents

Abstract Abrupt changes in slope occur frequently in some marine environments. Laterally confined laboratory experiments show that when dense surge-like particulate flows travel over abrupt slope reductions their deposits may have an asymmetric bell-shaped thickening near the break of slope. This thickening is herein called a slope-break deposit and is related to the change in kinetic energy of the flow. Slope-break deposits are particularly pronounced under relatively high-velocity flows. In our experiments these are flows with high particle concentrations ( c =5–10% by volume silicon carbide) moving down slopes steeper than 3°. The experiments were designed such that currents generated by a lock-exchange mechanism flowed down a slope (variable slope angle) onto a flat surface. The bodies of the flows became thicker and slower just downstream of the slope break. Because the flow body slows very rapidly, particles are dumped forming the slope-break deposit. The peak in sediment thickness of the slope-break deposit is downstream of the break in slope because falling particles continue to move forward in the flow during settling. The slope-break deposits may be several tens of percent thicker than deposits from an equivalent flow in which no change in slope occurred. The parameters of the slope-break deposit correlate with slope angle change.

Multi-process generated sediment waves on the Landes Plateau (Bay of Biscay, North Atlantic)

Detailed analyses of recently collected bathymetric and sparker seismic data, support a new interpretation of the Landes Plateau field of sediment waves located on the Aquitaine upper continental slope (Bay of Biscay). The wave geometry, previously described as the result of a major sediment failure, is interpreted as a structure with a complex origin including the interaction of depositional and gravity deformation processes. Depositional processes are mainly recorded by the upslope migrating pattern of the waves resulting from oblique or sigmoid downlap reflections on the upslope flank of the waves and by toplapping and truncated reflections on the downslope flank. Hemipelagic and turbiditic sedimentation may be involved in the wave building as well as contouritic processes that could be related to the existing northward polar current and internal waves. Gravity deformations are syndepositional, discontinuous and of low amplitude, affecting thick layers which alternate with undeformed layers. They seem to correspond to gentle sediment creeping or stretching associated with minor listric or compaction-like faults, and possible limited back rotation of sediment blocks. These multi-process generated sediment waves could be rather common on the continental margins as they could have been mistaken with either depositional or deformational structures.

Chapter 3 – Contour Currents and Contourite Drifts

Abstract This chapter provides a current state-of-the-art on the contourite systems with a major focus on the modern systems (drifts), the processes of deposition and the diagnostic sedimentological and seismic features. A short overview of the ancient contourite problem is also provided. Firstly, the history of contourites is briefly reminded since the pionner works during the sixties. The major stages in the advances of the knowledge of these deposits are underlined, and a consensus view is proposed on the definitions of contourites (“deep or shallow contourites”) and drifts, and on the large diversity of bottom currents that can be involved in contourite deposition. The second part of the chapter is then dedicated to the Ocean geostrophic circulation and the main characteristics of the surficial and the thermohaline circulation. It drives to the contour current definition and physical features. The sedimentary processes (erosion, transport and deposition) related to contour currents form the third part with an emphasis on the processes occurring in the nepheloid layer and in the Benthic Boundary Layer as demonstrated by the results of the HEBBLE project. The two following parts concern the contourite deposits: facies, sequences, bedforms and geometry within drift sedimentary body. Examples of detailed studies of contourite core using combined physical methods are necessary to identify this deposit. Similarly a multi-scale approach is necessary to recognize drift construction from seismic lines only. Some sedimentological and seimic diagnostic features are proposed to identify Contourites. The last part point out how difficult is the recognition of ancient contourite. The lack of recent progress is due in part to diagenesis, outcrop discontinuities and tectonic deformation. Two types of ancient contourites, the bottom-current-reworked turbidites and the shallow-water ancient contourites, are presented in more details in order to highlight the debate that still surrounds their recognition and interpretation.