Dorrik A. V. Stow

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of contourite vs. turbidite deposits, based on the analysis of many seismic profiles from both published and unpublished sources.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.

Deep-water facies, processes and models: a review and classification scheme for modern and ancient sediments

Abstract A review of previous work on modern and ancient deep-water facies, processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups. These facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes, through semi-permanent bottom-currents, to pelagic settling. In essence, the scheme is descriptive of the sedimentary attributes of sediments, although it is designed to aid interpretation of possible sediment transport/deposition processes. The classification scheme is three-tier with facies classes, groups and constituent facies, and is hierarchical to allow flexibility in its use. There are seven facies classes, with Classes A–E defined largely on the basis of grain-size differences, Class F on the basis of internal organization, and Class G on composition. The facies classes are: Class A, gravels, muddy gravels, gravelly muds, and pebbly sands, with ⩾ 5% gravel grade; Class B, sands, with ⩾ 80% sand grade and 80% mud, ⩾ 40% silt and 0–20% sand; Class E, muds and clays, with ⩾ 95% mud grade, For the purpose of large-scale mapping or reconnaissance fieldwork, either the level of facies classes or groups may be appropriate, whereas for more detailed sedimentology the more detailed facies level will be necessary.

Sequence of structures in fine-grained turbidites: Comparison of recent deep-sea and ancient flysch sediments

Abstract A comparative study of the sequence of sedimentary structures in ancient and modern fine-grained turbidites is made in three contrasting areas. They are (1) Holocene and Pleistocene deep-sea muds of the Nova Scotian Slope and Rise, (2) Middle Ordovician Sevier Shale of the Valley and Ridge Province of the Southern Appalachians, and (3) Cambro-Ordovician Halifax Slate of the Meguma Group in Nova Scotia. A standard sequence of structures is proposed for fine-grained turbidites. The complete sequence has nine sub-divisions that are here termed T0 to T8. “The lower subdivision (T0) comprises a silt lamina which has a sharp, scoured and load-cast base, internal parallel-lamination and cross-lamination, and a sharp current-lineated or wavy surface with ‘fading-ripples’ (= Type C etc. …).” (= Type C ripple-drift cross-lamination, Jopling and Walker, 1968). The overlying sequence shows textural and compositional grading through alternating silt and mud laminae. A convolute-laminated sub-division (T1) is overlain by low-amplitude climbing ripples (T2), thin regular laminae (T3), thin indistinct laminae (T4), and thin wipsy or convolute laminae (T5). The topmost three divisions, graded mud (T6), ungraded mud (T7) and bioturbated mud (T8), do not have silt laminae but rare patchy silt lenses and silt pseudonodules and a thin zone of micro-burrowing near the upper surface. The proposed sequence is analogous to the Bouma (1962) structural scheme for sandy turbidites and is approximately equivalent to Boumas (C)DE divisions. The repetition of partial sequences characterizes different parts of the slope/base-of-slope/basin plain environment, and represents deposition from different stages of evolution of a large, muddy, turbidity flow. Microstructural detail and sequence are well preserved in ancient and even slightly metamorphosed sediments. Their recognition is important for determining depositional processes and for palaeoenvironmental interpretation.

Classification and characterisation of deep-water sediment waves

Deep-water sediment waves can be classified using a combination of grain size and wave-forming process, although in some cases one or other of these criteria may be indeterminable. Sediment waves are generated beneath currents flowing across the seabed, in the form of either downslope-flowing turbidity currents or alongslope-flowing bottom currents. Waves formed by either process show varying characteristics, depending on whether they are constructed of coarse- or fine-grained sediments. Sediment wave studies over the last five decades are reviewed, and clear trends can be discerned. Early descriptive studies in the 1950s and 1960s relied almost exclusively on seismic reflection profiles, and the wave-forming process was often a subject of much debate. In the 1970s and 1980s the quality of sediment wave datasets increased, with sidescan sonar, deep-sea drilling and numerical modelling all applied to sediment wave studies. Consequently, the wave-forming process became more easily identifiable, and models for the growth of bottom current and turbidity current sediment waves were introduced. Most studies from the 1990s onwards have focussed on turbidity current sediment waves, in response to the increasing demand for data from turbidite systems from the hydrocarbon exploration and production industry. Studies of bottom current sediment waves during this period have focussed on the applications to palaeoceanography, in response to the recent boom in climate change studies. The main focus of this paper is the characterisation of both fine- and coarse-grained, turbidity and bottom current sediment waves, including the depositional environment, wave morphology, wave sediments and migration, and the wave-forming process. In addition, criteria for distinguishing between fine-grained bottom current and turbidity current waves are discussed, and also for identifying other wave-like features formed by different processes, such as creep folds. Although in many sediment wave studies the dominant wave-forming process is easy to determine, in others it is likely that a more complex combination of processes has occurred. Further studies should concentrate on methods for identifying these processes and how they interact, and also investigate the exact mechanisms for the initiation and evolution of sediment wave fields.

Sediment waves on the monterey fan levee: A preliminary physical interpretation

Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.

Deep-water sedimentary systems: New models for the 21st century

Abstract One of the principal scientific, technical and environmental challenges for the next century is undoubtedly the exploration and understanding of the deep oceans. Close collaboration between the hydrocarbon industry and scientific community is allowing us to push back this frontier and so to develop new models for deep-water sedimentary systems. The turbidity current paradigm is under scrutiny and refinements proposed for massive sands, megabeds and immature turbidites. Source area and sediment type are key controls. Bottom currents play an important part in the shaping of margins, the generation of hiatuses and bounding surfaces, the winnowing of sands and ventilation of ocean basins. It is at the level of architectural elements and their three-dimensional geometry that much activity is currently focused. Most advance has so far been made in terms of channel types, dimensions, aspect ratios, stacking patterns and hierarchies; to a lesser extent this is true for lobes, levee complexes, contourite drifts and sheet sands. It is only after this phase of study that we will be able to significantly improve our models for the larger-scale systems—fans, ramps, slope-aprons, basin plains and drifts.

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.

THE CHANGNING-MENGLIAN SUTURE ZONE; A SEGMENT OF THE MAJOR CATHAYSIAN-GONDWANA DIVIDE IN SOUTHEAST ASIA

Abstract In southwest China, the major Cathaysian-Gondwana divide (the Palaeo-Tethyan suture) is very well delineated by a narrow north-south zone of oceanic siliceous sedimentary rocks and dismembered ophiolite complexes including probable remains of reef-capped oceanic islands. The location of this zone, the Changning-Menglian suture, is not well appreciated in the English language literature, probably because of the failure to recognise the approximately 600 × 400 km Simao terrane which lies to the east of the suture. Many analyses mistakenly include the Simao element in the Sibumasu terrane thus placing a terrane of distinct Cathaysian affinities on the Gondwana side of the major palaeogeographic divide. A measure of the size and time span of Palaeo-Tethys is contained in the Changning-Menglian suture zone sedimentary rocks; deep marine cherts are dated between Early Devonian and Middle Permian, and limestones, interpreted as caps to seamounts, extend the age range of the ocean to Late Permian. These data agree with recent views of Cathaysian elements rifting from Gondwana in the Silurian or Devonian followed by a major rifting event on the north Gondwana margin early in the Permian. Subduction created an active continental margin on the western edge of the Simao terrane throughout much of the Triassic and the closure of this branch of Palaeo-Tethys is marked by the cessation of this activity in the early Late Triassic.