Jean-Claude Faugères

Seismic features diagnostic of contourite drifts

Abstract The sedimentary construction of oceanic margins is most often carried out by the combined action of gravitational processes and processes related to bottom (contour) currents. One of the major difficulties encountered in the interpretation of seismic profiles crossing such margins is the differentiation of these two types of deposit, especially where they display very complicated imbricated geometries. The aim of this paper, therefore, is to derive criteria for the recognition of contourite vs. turbidite deposits, based on the analysis of many seismic profiles from both published and unpublished sources. The following features are the most diagnostic for the recognition of contourite drifts. At the scale of the basin, four different drift types can be distinguished according to the morphostructural context, their general morphology and the hydrodynamic conditions. These are: contourite-sheeted drifts (including abyssal sheets and slope-plastered sheets), elongate-mounded drifts (detached and separated types), channel-related drifts (including lateral and axial patch drifts and downstream contourite fans), and confined drifts trapped in small, tectonically active basins. At the scale of the drift, three features provide the best diagnostic criteria for recognising contourite deposits on seismic profiles: major discontinuities that can be traced across the whole drift and represent time lines corresponding to hydrological events, lenticular, convex-upward depositional units with a variable geometry, and a specific style of progradation–aggradation of these units that is influenced by interaction of the bottom current with Coriolis force and with the morphology. At the scale of depositional units, the seismofacies show a wide variety of reflector characteristics, many of which are very similar to those observed in turbidite series. Distinction between sediment wave seismofacies deposited by turbidity currents and bottom currents still remains ambiguous.

Bottom currents, contourites and deep-sea sediment drifts: current state-of-the-art

Abstract This paper provides both an introduction to and summary for the Atlas of Contourite Systems that has been compiled as part of the International Geological Correlation Project - IGCP 432. Following the seminal works of George Wust on the physical oceanography of bottom currents, and Charley Hollister on contourite sediments, a series of significant advances have been made over the past few decades. While accepting that ideas and terms must remain flexible as our knowledge base continues to increase, we present a consensus view on terminology and definitions of bottom currents, contourites and drifts. Both thermohaline and wind-driven circulation, influenced by Coriolis Force and molded by topography, contribute to the oceanic system of bottom currents. These semi-permanent currents show significant variability in time and space, marked by periodic benthic storm events in areas of high surface kinetic energy. Six different drift types are recognized in the ocean basins and margins at depths greater than about 300 m: (i) contourite sheet drifts; (ii) elongate mounded drifts; (iii) channel related drifts; (iv) confined drifts; (v) infill drifts; and (vi) modified drift-turbidite systems. In addition to this overall geometry, their chief seismic characteristics include: a uniform reflector pattern that reflects long-term stability, drift-wide erosional discontinuities caused by periodic changes in bottom current regime, and stacked broadly lenticular seismic depositional units showing oblique to downcurrent migration. At a smaller scale, a variety of seismic facies can be recognized that are here related to bottom current intensity. A model for seismic facies cyclicity (alternating transparent/reflector zones) is further elaborated, and linked to bottom current/climate change. Both erosional features and depositional bedforms are diagnostic of bottom current systems and velocities. Many different contourite facies are now known to exist, encompassing all compositional types. We propose here a Cl-5 notation for the standard contourite facies sequence, which can be interpreted in terms of fluctuation in bottom current velocity and/or sediment supply. Several proxies can be utilized to decode contourite successions in terms of current fluctuation. Gravel lag and shale chip contourites, as well as erosional discontinuities are indicative of still greater velocities. There are a small but growing number of land-based examples of fossil contourites, based on careful analysis using the recommended three-stage approach to interpretation. Debate still surrounds the recognition and interpretation of bottom current reworked turbidites.

Deep-water contourite systems: modern drifts and ancient series, seismic and sedimentary characteristics

Countourites are a widespread but poorly known group of sediments linked to the action of powerful bottom currents in deep water. Although we know they are especially common along continental margins and through oceanic gateways, they have been surrounded by contoversy since they were first recognized in the early 1960s. Where correctly recognized and decoded they can provide one of the keys to our better understanding of bottom water circulation and of the ocean–climate link. They are part of the spectrum of deposits that confronts the oil industry as exploration moves into progressively greater water depths. This memoir is an important outcome of the International Geological Correlation Project 432 on Bottom Currents, Contourites and Palaeocirculation . It includes 30 papers involving over 75 key scientists from around the world. Following an introductory state–of–the–art paper by the editors, there are 25 separate case studies on modern drifts and four on ancient contourite series. Each contribution highlights the specific geological and oceanographic setting, bathymetry, physiographic and stratigraphic context, seismic attributes and sedimentary characteristics of that drift. Case studies range from some of the well-documented North Atlantic drifts to those much less known from the Mediterrenean, from important syntheses of the Gulf of Cadiz and Vema Channel Gateway, to completely new data on South Atlantic, Pacific and Antartic margin systems. The four papers on ancient series from Japan, China and Cyprus serve to emphasise the complex nature and subtle characteristics of contourites, which make their identification a scientific challenge. This volume is dedicated to the memory of Charlie Hollister (1936–1999), one of the founding fathers and pioneers of countourite research.

Bottom-current-controlled sedimentation: a synthesis of the contourite problem

Abstract An overview of the main items concerning deep bottom-current-controlled deposits is presented. These include: the definition of contourites, their processes of deposition and facies characteristics; contourite drift types and seismic patterns; the interplay of turbidity and bottom-current processes; and the correlation between bottom-current processes and global changes in climate/sea-level. From this discussion, several of the more significant problems are highlighted, which we believe should be priority targets for future research: (1) identification of contourites, especially in ancient series, where they are interbedded with turbidites and other facies; (2) refinement of the numerous parameters used as tracers of palaeocirculation patterns; (3) understanding the interaction between the different types of bottom currents, and also between bottom-current and other deep-water processes; (4) distinction in the sedimentary record between the effects of short time-scale bottom-current variations due to regional causes like benthic storms, and geological-scale current fluctuations related to climatic or astronomic control; (5) understanding the control of glacial/interglacial climatic cycles on the nature and rate of contourite deposition.

HYDROLOGY, MORPHOLOGY AND SEDIMENTOLOGY OF THE CAMPOS CONTINENTAL MARGIN, OFFSHORE BRAZIL

Abstract Slope sand deposits have accumulated from at least the Neogene to the Present on the southeastern Brazilian continental margin (Campos Basin area). This region shows sand accumulations concentrated on the upper portion and on the base of the continental slope with a middle to lower slope bypass zone. A synthesis of preliminary results, supported by recent cores, high-resolution geophysical surveys, geotechnical investigations and environmental research, is presented and permits a prelitrunary analysis of the sedimentological mechanisms operational in this area. These point toward a temporal and spatial multiscale set of phenomena responsible for sand deposits. At any sea-level stand these deposits are dependent on: (1) a suitable sediment source; (2) offshelf transport mechanisms; (3) a morphostructural and hydrodynamic context responsible for the deposition of these sands in the upper portion of continental slopes. The proposed scenario of depositional processes concerns: (1) a set of hydrological processes such as surface currents and counter-currents, waves, tides and eddies with sufficient energy to form submarine sand dune fields at the outer shelf; (2) the offshelf export of this sediment under a combined action of spillover, internal waves, eddies ‘seafloor polishing effect’ and gravity processes (turbidity currents); and (3) the slope sand deposits and their distribution controlled by the action of contour currents, mass movements and the morphological context, such as canyons, gullies or scarps.

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.

Bottom-current-controlled sand deposits — a review of modern shallow- to deep-water environments

Abstract Different examples of modern marine-sand accumulations generated or strongly influenced by the action of bottom currents, are here presented. They are drawn from a variety of tectonic and morphological settings and grouped into three water-depth zones: deep-water (>2000 m), mid-water (300–2000 m), and outer-shelf/upper-slope (50–300 m). Deposits in the first two of these depth zones are normal contourites, according to their original definition (Heezen and Hollister, 1971) being those sediments that have been transported and deposited by contour currents in deep-water environments. Those deposited at shallower depths, under the influence of surficial geostrophic currents combined with other hydrodynamic factors (shelf currents induced by wind, tide and waves, gyres, internal waves, etc.), are more properly referred to as outer-shelf/upper-slope bottom-current sands (or shallow-water bottom-current sands). We have elaborated a facies model for each bathymetric zone. Deep-water sandy contourites are relatively rare, thin- and very thin-bedded, highly bioturbated and mainly of bioclastic composition. They are interbedded with muddy contourites and pelagites or, in some areas, with turbidites. In the latter case, thin bottom-current-reworked, sandy tops of turbidites provide a different and distinct facies. Mid-water sandy contourites are more common, ranging up to a metre in thickness, and may form extensive sandy sheets in a variety of slope, bank and channel settings. They are mainly of mixed siliciclastic—bioclastic composition, typically bioturbated, and associated with muddy/silty contourites in coarsening-up/fining-up complete or truncated sequences. Shallow-water bottom-current sands occur in particular outer-shelf/upper-slope settings, where they may develop relatively thick (1–20 m), laterally extensive sheets covered by fields of sandwaves, megaripples and ribbons. Internal structures may be preserved along with much bioturbation. Their composition varies from mainly siliciclastic to bioclastic, and they may be interbedded with both inner-shelf facies and slope hemipelagites. The principal factors that control the deposition of sandy contourites and shallow-water bottom-current sands are the hydrodynamic regime of the basin, the availability of coarse-grained (sandy) sediments and the physiographic context of the area swept by the currents. The greater the depth, the finer and rarer the bottom-current or sandy contourite deposits. Global sea-level and climatic changes and the time involved in the depositional history play an ultimate role in the development of important sand accumulations of this sort by controlling the ocean-circulation pattern and its long-term persistence. From the present analysis, we conclude that mid-depth sandy contourites are the most commonly found in modern environments, and that shallow-water bottom-current sands constitute the most significant potential oil reservoirs to be found in the geological record.

Facies distribution and textural variation in Faro Drift contourites: Velocity fluctuation and drift growth

Abstract Detailed study of seismic profiles, piston cores and bottom photographs from the Faro Drift on the southern margin of Portugal has led to a better understanding of drift development and its relationship to bottom current circulation. Data on the contourite facies characteristics and the surface microphysiography have been published elsewhere; here, we concentrate on sediment distribution and geometry. Longitudinal trends in facies types, mean grain size, sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the Drift. These are generally more intense in the marginal channels and at the upstream or eastern end of the drift. Three different scales of vertical variation of facies can be identified. At the large scale, 300–500 m of sediment has accumulated over 4–6 Ma in a regular vertical succession due to the northward progradation of the Drift. At the medium scale, the upper 20–30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea-level stands respectively over the past 0.3 Ma. At the small scale, the topmost 2–3 m of sediment deposited in approximately 0.03 Ma shows three zones of coarser-grained sediments separated by finer-grained contourites. This sequence can also be interpreted in terms of long-term fluctuation in bottom current activity. Although the signal is clearly complex, this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns.

Fossil contourites: a critical review

Abstract Despite three decades of study, there is still great controversy over the recognition and interpretation of fossil contourites exposed in ancient series on land. In order to best examine this problem, we briefly review the evidence from modern systems, including the many examples of Cenozoic contourites that have been recovered from DSDP/ODP drilling on major drifts in the present-day oceans. The range of contourite facies described from both deep-water (>2000 m) and mid-water (300–2000 m) drifts are mostly fine-grained, bioturbated and homogeneous, often with a distinct bedding cyclicity, and with some coarser-grained sandy contourites developed under higher-energy bottom currents. There are also a number of current-controlled sediment bodies that have formed in outer shelf/upper slope settings (50–300 m) under the influence of counter currents, underflows and major surface currents. These are not considered contourites sensu stricto, but may be mistaken as such in ancient examples. The most commonly described fossil contourites in the literature have been interpreted by the authors concerned as bottom-current reworked turbidites. However, a critical review suggests that these are the facies most subject to misinterpretation and many of the sediments claimed as fossil contourites are almost certainly fine-grained turbidites, whereas others were more likely formed under outer shelf/upper slope current systems. There remain very few ancient examples that are more closely comparable to modern contourites; these include the Cretaceous Talme Yafe Formation in Israel, the Ordovician Jiuxi Drift in China, and parts of the Paleogene Lefkara Formation, Cyprus and the Neogene Misaki Formation in Japan. We present a set of possible criteria for the recognition of fossil contourites and bottom-current reworked turbidites.

Quaternary development of migrating sediment waves in the Var deep-sea fan: distribution, growth pattern, and implication for levee evolution

Abstract A field of sediment waves built by turbidity currents on the Var deep-sea fan has been studied using a large amount of seismic-reflection data and cores. To understand the spatial organisation and evolution of the sediment waves, maps of both surficial and an older subsurface wave field were prepared. Three different sediment wave geometries (symmetrical to asymmetrical cross-section) were recognised and can be linked to particular styles of evolution through time (from simple vertical aggradation to upslope and upcurrent progradation). Each geometry appears to be linked with: (i) a particular location on the Var Sedimentary Ridge; (ii) the type of turbidite deposits; (iii) the local gradient slope; and (iv) the height of the Ridge. Several factors control the nature and distribution of sediment waves and these factors do not change significantly through time, as suggested by the common geometries, evolution and distribution of the sediment waves between the present and fossil fields. Supercritical flow conditions and high sediment supply allow the development of well-developed and prograding sediment waves, while subcritical flow conditions and low sediment supply allow the formation of smaller and aggrading sediment waves. The impact of these two factors is also influenced by the morphology of the fan valley, the height of the Ridge, and the type of transported sediment. The evolution of sediment wave amplitude is also strongly influenced by the distance of sediment waves from the Ridge crest, while wavelength evolution is more influenced by slope gradient. The building of the Var Sedimentary Ridge and particularly the asymmetry and the overall morphology of the levee are directly controlled by the growth pattern of the sediment waves.