William D. McCaffrey

Velocity structure, turbulence and fluid stresses in experimental gravity currents

Gravity currents are of considerable environmental and industrial importance as hazards and as agents of sediment transport, and the deposits of ancient turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distribution of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcritical, brine underflows in a rectangular lock-exchange tank. Laser-Doppler anemometry was used to construct a two-dimensional picture of the velocity structure. The velocity maximum within the gravity current occurs at y/d ≈ 0.2. The shape of the velocity profile is governed by the differing and interfering effects of the lower (rigid) and upper (diffuse) boundaries and can be approximated with the law of the wall up to the velocity maximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the head consists of a single large vortex and an overall motion of fluid away from the bed, and this largely undiluted fluid becomes rapidly mixed with ambient fluid in the wake region. The distribution of turbulence within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent energy spectrum and scale with flow thickness. Turbulent kinetic energy reaches a maximum in the shear layer at the upper boundary of the flow where the large eddies are generated and is at a minimum near the velocity maximum where fluid shear is low.

Oblique reflection of turbidity currents

Turbidity currents meeting obstacles, for example, the margins of a confined basin, are subject to reflection. The consequent change in flow direction is expressed in the sequence of depositional structures of the resulting bed of sediment. Putative examples of orthogonal reflection have been described, based on 180° opposed current directions. We present field evidence for the more general case of oblique reflection of turbidites, and we report the results of flume experiments indicating a mechanism involving generation of internal solitary waves at an oblique ramp. These propagate normal to the ramp, regardless of the angle of incidence. Flow directions in reflected turbidites may indicate the orientation of reflecting surfaces, such as basin margin slopes, and thus may be of considerable help in paleogeographic and tectonic reconstructions.

Process controls on the development of stratigraphic trap potential on the margins of confined turbidite systems and aids to reservoir evaluation

Stratigraphic trapping at pinch-out margins is a key feature of many turbidite-hosted hydrocarbon reservoirs. In systems confined by lateral or oblique frontal slopes, outcrop studies show that there is a continuum between two geometries of pinch-out configuration. In type A, turbidites thin onto the confining surface--although the final sandstone pinch-out is commonly abrupt--and individual beds tend not to erode into earlier deposits. In type B, turbidite sandstones commonly thicken toward the confining slope, and beds may incise into earlier deposits. These two types may occur in combination, to give a wide spectrum of pinch-out characteristics. Our analysis suggests the principal control in determining pinch-out character is flow magnitude, with smaller flows producing type A and larger flows producing type B. In areas of poor seismic control it can be difficult to assess either pinch-out character or the proximity of wells to confining slopes. Because estimates of paleoflow magnitude can be made from core or high-quality log image data, however, it is possible to make reasonable estimates of pinch-out character even from wells such as exploration wells, which may be placed conservatively, away from the field margins. Furthermore, systematic paleoflow variations and thickness trends are commonly seen in individual turbidite sandstones as they approach confining slopes. For example, dispersal directions indicate flow deflection parallel with the strike of confining topography; beds thin toward type A onlaps and thicken toward type B onlaps. These relationships can be exploited via analysis of vertical successions to constrain well position with respect to the slope. Similarly, the presence, location, and frequency of locally derived debrites can provide information on the presence and proximity of confining slopes. (Begin page 972)

Velocity and turbulence structure of density currents and internal solitary waves: potential sediment transport and the formation of wave ripples in deep water

Abstract Laser Doppler anemometry (LDA) was used to measure the instantaneous downstream and vertical velocities in a series of simple and reflected saline density currents in a lock-exchange flume tank. All the currents were turbulent and subcritical. Mean downstream fluid velocities were in excess of the head velocity by up to 30%, and instantaneous velocities were greater by up to 50%. Turbulence intensities were highest within the head, and generally greatest in the middle part of the current, but did not correspond with the level of highest mean velocities. The maximum Reynolds stress also occurred within the head; large negative values were associated with shear along the upper boundary of the current. Peaks of turbulence, Reynolds stress and shear velocity occurred in association with the arrival of reflections. In large-scale turbidity currents, such reflections would be capable of re-entraining and resuspending sediment deposited by the forward current. Some reflections take the form of solitary waves within a residual flow with a velocity vector in the opposite direction. In nature, these could produce symmetrical ripples in environments below storm-wave base.

Controls on sinuosity evolution within submarine channels

The planform geometry of submarine channels commonly exhibits a spatiotemporal stability generally not observed in fluvial channels. As such, submarine channels tend to lack the meander loop cutoffs and frequent avulsion history typical of fluvial channels. Fluvial sinuosity develops through inner-bend deposition and outer-bend erosion. Inner-bend deposits have also been recognized in submarine channels, from subsurface and seafloor images and from ancient channel outcrops, and have been demonstrated within physical models. However, outer-bend sediment accumulations are a feature thought to be unique to submarine channels. We report on physical experiments on channelized, subaqueous, particle-driven turbidity currents that demonstrate that channel-fill architecture relates directly to the degree of flow bypass, in turn largely determined by the degree of confinement. In general, weakly bypassing flows deposit at the outer bend, whereas strongly bypassing flows deposit at the inner bend. Therefore flows within aggradational channel systems whose axes are bypass dominated may preferentially deposit at the inner bend, ultimately having the effect of increasing channel sinuosity through time; this is an evolution pattern commonly observed in seismic images. Once developed, the apparent spatio-temporal longevity of sinuosity within many systems may be explained by the passage of turbidity currents of varying magnitude (and consequently bypass potential) depositing preferentially at either the inner or outer bank of the channel, maintaining a quasistable morphological equilibrium. Fluvial channels do not have the ability to reduce or maintain their sinuosity in this way, which plausibly explains why they tend to develop cutoffs at higher rates than subaqueous channels.

Facies architecture of the Gres de Peira Cava, SE France: landward stacking patterns in ponded turbiditic basins

Basins in which turbidity currents are completely or partially trapped are common in many tectonically active, deep-water settings. Field study of an Eocene–Oligocene turbiditic system in the Peïra Cava area, a sub-basin of the Alpine foreland in southeastern France, allows spatial characterization of a ponded basin fill on the basis of a correlation framework derived from measured outcrop sections and photomosaics. The basin-fill architecture comprises a sand-rich, proximal scour-and-fill facies and a downstream transition to mud-rich, basin-plain turbidite sheet facies. The proximal facies is interpreted to have formed directly downstream of a slope break, where currents were highly erosional during some periods and highly depositional during other periods, as a result of the interacting effects of turbulence enhancement and rapid deceleration. Both the proximal facies and the downstream transition to distal basin-plain facies occur in progressively landward positions at higher stratigraphic levels. The landward shift in depositional facies is likely to have resulted from the basin-floor aggradation and a landward migration of the slope break. This ‘back-stepping’ process may be expected to occur in many ponded turbiditic basins and to produce a similar type of sedimentary architecture.

Global (latitudinal) variation in submarine channel sinuosity

Current classifications of submarine channels and fans link channel sinuosity to gradient, and in turn to sediment caliber, with end members being high-sinuosity, low-gradient, fine-grained systems and low-sinuosity, high-gradient, coarse-grained systems. However, the most sinuous modern submarine channels, such as the Amazon, Bengal, Indus, and Zaire, along with ancient sinuous submarine channels, are located in equatorial regions. Here we quantitatively compare slope versus latitude controls on submarine channel sinuosity and show that the latitudinal control is strong, while that of slope is weak. Variation in sinuosity with latitude is shown to occur uniquely in submarine channels; no comparable relationship is observed for terrestrial river channels. Possible causal mechanisms for this latitudinal variation are explored, focusing on the influence of the Coriolis force, flow type, and sediment type. Although climate does not vary straightforwardly with latitude, climatic controls on flow and sediment type may explain some of the latitudinal variation; Coriolis force, however, varies with latitude alone and produces an excellent fit to the observed sinuosity-latitude distribution. Regardless of which control predominates, latitudinal global variation in channel sinuosity should have changed over geologic time. Since deposit architecture and facies are linked directly with sinuosity, submarine channel deposits should also systematically vary in space and time.

A database approach for constraining stochastic simulations of the sedimentary heterogeneity of fluvial reservoirs

Quantitative databases storing analog data describing the geometry of sedimentologic features are commonly used to derive input for geostatistical simulations of reservoir sedimentary architecture; however, geometrical information alone is inadequate for the detailed characterization of sedimentary heterogeneity. A relational database storing fluvial architecture data has been developed and populated with literature- and field-derived data from modern rivers and ancient successions. The database scheme characterizes fluvial architecture at three different scales of observation—recording style of internal organization, geometries, and spatial relationships of genetic units—classifying data sets according to controlling factors (e.g., climate type) and context-descriptive characteristics (e.g., river pattern). The database can therefore be filtered on both architectural features and boundary conditions to yield outputs tailored on the system being modeled to generate input to object- and pixel-based stochastic simulations of reservoir architecture. When modeling heterogeneity with stochastic simulations, the choice of input parameters quantifying spatial variation is problematic because of the paucity of primary data and the partial characterization of supposed analogs. This database-driven approach permits the definition of various constraints referring to either genetic units (e.g., architectural elements) or material units (i.e., contiguous volumes of sediment characterized by the same value of a given categorical or discretized variable; e.g., same lithofacies type, clay and silt content, and others), which permit the realistic description of fluvial architecture heterogeneity. Applications of this database approach include the computation of relative dimensional parameters and the generation of auto- and cross-variograms and transition-probability matrices, which are necessary to effectively model spatial complexity.

The influence of a lateral basin-slope on the depositional patterns of natural and experimental turbidity currents

Abstract Understanding topographic effects upon the depositional processes of turbidity currents and the resulting deposit characteristics is key to producing reliable depositional models for turbidity currents. In this study, the effect on depositional patterns of a lateral slope whose strike is parallel to the principal direction of flow is explored using field and experimental results. This type of basin topography is commonly found in confined turbidite systems. Field data from the Peïra Cava turbidite system of the Tertiary Alpine Foreland Basin (SE France) and experimental data show that a characteristic depositional pattern is produced by surge-type waning flows that interact with a lateral slope. This pattern comprises beds that thin (and fine in the field study) not only downstream but also markedly away from the lateral slope (Type I beds). In the Peïra Cava system, this pattern is also observed in average values of sandstone bed thickness, sandstone percentage and grain-size, derived from measured sections, demonstrating that the processes responsible for this pattern also control gross properties within this sheet system. The characteristic thinning-away-from-slope deposit geometry is interpreted as an effect of the lateral slope via its influence on spatial variations in flow properties and on the suspended load fallout rate (SLFR) from currents. Flow velocity non-uniformity cannot explain thinning into the basin because flow has a higher deceleration along streamlines away from the slope that should cause higher SLFR and thicker deposits away from the slope instead of close to the slope. A concentration non-uniformity mechanism is invoked that has the effect of maintaining relatively high flow concentrations and hence SLFR in medial and distal locations close to the slope. Experiments suggest that this may arise due to different rates of flow expansion on the obstructed and unobstructed sides of the current in proximal regions. Velocity non-uniformity can, however, explain the geometry of deposits that thicken away from slope. Beds of this type do occur occasionally in the Peïra Cava system (Type II beds). Flow velocity non-uniformity patterns have been used previously to successfully explain the spatial distributions of depositional facies of turbidity currents that have interacted with topography. The analysis in this study demonstrates that velocity non-uniformity, by itself, cannot explain depositional patterns in all basin settings. Future depositional models need to incorporate the effects of spatial changes in other flow properties, such as flow concentration, upon deposition to be able to predict turbidite facies in many different types of basin setting.

Submarine channel response to intrabasinal tectonics: The influence of lateral tilt

Lateral tilting is a common deformation style in extensional basins; its influence on subaerial channels is, to a degree, understood and may be significant, controlling the style of channel development and the resultant sand-body architecture. Growth faulting and lateral tilting in turbidite channel systems have been demonstrated from three-dimensional seismic data, but the resultant architecture of channels within these settings has not yet been documented. In the Carboniferous of northern England, a sand-rich slope channel, developed within a basin undergoing late-stage extension, underwent progressive and unidirectional migration toward a topographic low on a laterally tilting block. The resultant sandstone body is wedge shaped in cross section and composed dominantly of sigmoidal lateral accretion deposits. The channel returned to an axial course before undergoing lateral migration in the same direction, creating a multistory, multilateral channel sandstone body. The repeated unidirectional migration combined with evidence of syndepositional deformation suggests that active tectonism strongly influenced channel evolution and deposition. A model of submarine channel evolution in extensional basins is presented; in systems where large displacements occur, the channel system may avulse, creating isolated sand ribbons, which are connected updip; where the lateral dip is always more influential than the regional dip, the system may pond in the hanging-wall syncline. The model is compared to a subsurface channel within the Pliocene of the Nile Delta slope, which was influenced by syndepositional fault movement; application of the outcrop-derived model allows some simple architectural interpretations to be made.