Confined Turbidite System Research Articles

Impact of sediment provenance and depositional setting on chlorite content in Cretaceous turbiditic sandstones, Norway

AbstractChlorite minerals, mainly in the form of clay coats, play a critical role in determining the reservoir quality of siliciclastic rocks. They can positively influence reservoir quality by preserving porosity during deep burial, but they can also play a negative role by reducing permeability through pore filling. The main aim of this research is to determine the optimal conditions for chlorite growth in sedimentary basins. This study investigates the Lower Cretaceous turbidite sandstone of the Agat Formation in the North Sea. We used a source‐to‐sink approach to investigate the impact of sediment source composition, chemical weathering and depositional environment on chlorite formation. Understanding the interplay between these processes can help refine exploration and exploitation strategies, optimise hydrocarbon recovery, and reduce exploration risks. Representative samples from two hydrocarbon fields (the Duva and Agat fields) were investigated using petrography, geochemistry, heavy mineral identification and quantification, and U–Pb geochronology of detrital zircons. Our results show a strong heterogeneity in the sediment provenance between the two turbidite systems. In the Duva field, the sandstone is derived from a mixture of mafic and felsic sources, producing Fe‐rich sediments. Intense chemical weathering generates fine fraction materials rich in kaolinite, vermiculite, and hydroxy‐interlayered clays, which are transported into shallow marine settings. Subsequent interaction with seawater results in the formation of glauconitic materials, Fe‐illite, and phosphatic concretions. These Fe‐rich materials are remobilised into deep marine settings, providing precursors for the development of authigenic Fe‐clays such as berthierine and chlorite. Conversely, in the Agat field, the sandstone is predominantly sourced from felsic rocks that underwent low chemical weathering, producing sediment rich in quartz and feldspar with a low amount of clays. With few Fe‐rich materials transported into the basin, the development of chlorite in the Agat field was less pervasive. Basin configuration and depositional environment exerted additional control on chlorite distribution. In the confined turbidite system (e.g. Duva field), chlorite is typically found as coating, whereas in less confined turbidite systems (e.g. Agat field) chlorite shows complex distribution related to depositional environment and dewatering processes. Our findings demonstrate the importance of considering the entire sediment routing system, from source to sink, when predicting chlorite occurrence and its impact on reservoir quality in deep marine settings. This integrated approach can guide exploration and development efforts in deepwater clastic reservoirs.

Confined turbidite system of the upper Paleozoic Itararé Group, Paraná Basin, Brazil: Beyond the topographic control paradigm

Recent studies in the eastern margin of the Paraná Basin, about the initial deposition of the Taciba Formation (Itararé Group) revealed a preserved, partially exhumed glacial trough as indicated by the morphology of its basal nonconformity. In this work, we reinforce the importance of the glacially-carved topography controlling the subsequent sedimentation by emphasizing turbidity currents and their deposits. This glacial trough is large enough to confine part of the following deglacial sediments, including a sand-rich turbidite system that represent phases of more consistent sand supply, a result from high sedimentation rates associated either with direct influx of meltwater-derived jets and/or with a rapid delta progradation at the trough head, causing over-steepening, unsteadiness, and eventual failure of the delta-front deposits. This turbidite system consists of three sand-rich stages (Ts1 to Ts3) intercalated with three fine-grained intervals (F1 to F3). Ts stages form bedsets with an overall lenticular geometry, thinning-out toward the trough flanks where the individual sandstone beds pinch-out close to the metamorphic Precambrian bedrock. They are ascribed to sinuous, laterally extensive channelized bodies representing different phases of channel activity through time. In general, each stage consists of an initial deposition derived from long-lived turbidity currents triggered by flood-driven hyperpycnal flows followed by short-lived, surge-type turbidity currents. F1, F2 and F3 deposits consist of fine-grained intervals that separate Ts1, Ts2 and Ts3 stages by an abrupt change in grain size and bed thickness. They comprise thin, sand-to-mud couplets showing granule-to-boulder sized basement-affinity dropstones, and several minor slumps, which are interpreted as overbank/levee sedimentation taking place during the successive passage of turbidity currents. A paleogeographic reconstruction of the glacial landscape reveal some similarities with modern fjords of British Columbia, Canada, that can be used to geometries and facies distribution prediction as an analog to assist seismic and well-log data.

Early post-rift confined turbidite systems in a supra-detachment basin: Implications for the early to middle Miocene basin evolution and hydrocarbon exploration of the Pannonian Basin

Understanding the interplay of sedimentation and deformation style is essential to develop an adequate model of the evolutionary history of an extensional sedimentary basin. Due to their volume and temporal extent, late post-rift deposits are often in the focus of research; however, early post-rift deposits are less studied. The aim of the study is to reconstruct the tectono-sedimentary evolution of a syn -rift to early post-rift succession in the hanging wall of a low-angle detachment zone and to recognize the effects of this deformation style on turbidity current pathways and depocenters. The small Őri sub-basin of the western part of the Miocene Pannonian basin is used as a study area where the oldest early post-rift turbidites of ~15–12.8 Ma occurred. These sediments were deposited in a supradetachment setting, where inherited and actively forming structures controlled the sediment pathways and accommodation of both the distally and locally sourced marine turbidites. Seismic data, well logs and core information were integrated to identify and analyze unconformities, structural elements, and turbidite deposits. Syn-rift, early post-rift and late post-rift system tracts were identified. The syn -rift/post-rift unconformity is a result of combined eustatic sea level drop and the change in deformation intensity. The main low-angle detachment fault, orthogonal hanging wall transfer faults and related folds together controlled the complex synformal shape of the basin, which was related to major corrugations of the footwall detachment. Four turbidite phases were distinguished based on upward increasing sand content, areal extent, reservoir quality, and degree of confinement as the turbidity currents reached the basin margins. (1) Small fans deflected by transfer faults developed, followed by (2) an axial turbidite system that reached an anticline and was deflected by it. (3) Lateral confinement by the anticline increased significantly and thick sandstone bodies with abrupt pinch-outs formed, while the influence of smaller faults diminished. (4) The axial turbidite systems covered the basin floor and their interaction with basin margins created continuous sand bodies of confined character. The sand fairways and types of confinement of a supra-detachment basin differ from high-angle normal fault systems. The exhumed metamorphic rocks provided the sediment, and the extension-parallel antiformal and synformal corrugations acted as lateral confinement. The gentle back-tilt of the detachment hanging wall block exerted a frontal confinement for the extensive turbidite system at the distal part of the basin, but the gentle NE-ward dip over the detachment still permitted long run-out distances. • Combination of eustasy and tectonics leads to syn-rift-early post-rift unconformity. • Corrugation fold of a detachment creates an intricate topography for sediment flows. • Deformation activity decreases during deposition of early post-rift turbidites. • Lateral, frontal confinement of turbidity currents controls reservoirs quality. • Differences between sand fairways in high- and low-angle extensional systems.

Tracking sand‐fairways through a deformed turbidite system: the Numidian (Miocene) of Central Sicily, Italy

AbstractUnderstanding ancient deep‐water sedimentary systems that accumulated at complex plate boundaries requires confronting the stratigraphic record of deformed sedimentary successions by tracking sand‐fairways and identifying original relationships in later deformed sequences. Here, we investigate the Numidian turbidite system (early to mid‐Miocene) of Central‐East Sicily to explore a deep‐water sedimentary system deposited at an active thrust belt on the Central Mediterranean. Turbidites include multi‐metre thick‐bedded, ultra‐mature quartz sandstones that were sourced from North Africa and are now deformed and dismembered within the Apennine‐Maghrebian orogen. To date, much research has focused on the little‐deformed sections that sample discrete parts of the original turbidite pathways. Yet the bulk of these systems are represented by deformed successions and these have attracted little modern sedimentological and stratigraphic investigation. We present new data based on field mapping, sedimentological/structural fieldwork, and biostratigraphy (planktonic foraminifera and nannofossils) that focus on the Numidian turbidites of Central‐East Sicily. Thickness and facies variations, together with evidence of large‐scale sediment bypass and local substrate reworking, characterize the Numidian turbidites of the study area, consistent with a partially confined turbidite system. Our work demonstrates that the Numidian turbidite system accumulated across active structures and these provided tortuous, evolving corridors through which turbidity currents were routed, transporting coarse sand over many hundreds of km. These results provide insight on structurally confined turbidites in analogous tectonic settings and demonstrate the need to seek sedimentological and stratigraphic data from deformed and dismembered parts of deep‐water systems.

Annot Sandstone in the Peïra Cava basin: An example of an asymmetric facies distribution in a confined turbidite system (SE France)

High-resolution physical stratigraphy and detailed facies analysis have been carried out in the foredeep turbidites of Annot Sandstone in the Peïra Cava basin (French Maritime Alps) in order to characterize the relationship between facies and basin morphology. Detailed correlation patterns are evidence of a distinction between a southern bypass-dominated region, coincident with a channel-lobe transition and a north-eastern depositional zone, represented by sheet-like basin plain. These depositional elements are characterized by three main groups of beds related to the downcurrent evolution of bipartite flows. These facies groups are: 1) pebbly coarse-grained massive sandstones with rip-up mudstone clasts and impact mudstone breccias (Type I and II beds) deposited by basal dense flows, 2) coarse-grained massive sandstone overlain by tractive structures (Type III and IV beds) indicating the bypass of overlying turbulent flows and 3) massive medium-grained and fine-grained laminated sandstones related to the deposition of high and low density turbidity currents (Type V and VI beds). Ponding and reflection processes, affecting the upper turbulent flows, can characterize all type beds, but especially the beds of the third group. As described in other confined basins of the northern Apennines (Italy), the lateral and vertical distribution of these type of beds, together with other important sedimentary characteristics, - such as the sandstone/mudstone ratio, bed thicknesses, amalgamation surfaces and paleocurrents - reveal that the deposition of the Annot Sandstone in the Peïra Cava basin was controlled by an asymmetric basin with a steep western margin. This margin favored, on the one hand, basal dense flow decelerations and impacts, as well as bypass and deflection of the upper turbulent flows towards the north east.

The Numidian of Sicily revisited: a thrust-influenced confined turbidite system

Understanding whether a system was unconfined, and deposited on a relatively unstructured basin-floor, or was confined by actively deforming substrate is important for the prediction of turbidite stratigraphy. Here we consider the Numidian turbidite system (Oligocene-Miocene) of Sicily - for many the type example of thick structureless submarine sandstones. New mapping and detailed sedimentology in the Nebrodi and Madonie Mountains (northern Sicily), allied to existing and new biostratigraphic data, challenge conventional interpretations for the Numidian system as a whole. Rather than having been deposited within an unstructured foredeep by relatively unconfined flows, we show that Numidian deposition was confined by active structures. These governed the routing of turbidity currents to create sand fairways and associated facies variations. The controlling structures include thrust-related folds together with inherited basin-floor faults. Existing models suggest that facies variations between adjacent outcrops on Sicily (and elsewhere) result from long-range stratigraphic variations being juxtaposed by later large-displacement thrusts. Our research reveals a much simpler tectonic structure but a more complex stratigraphic arrangement for the Numidian on Sicily - a characteristic of confined turbidite systems.

Palynofacies classification of the depositional elements of confined turbidite systems: Examples from the Gres d'Annot, SE France

Turbidites deposited in confined mini-basins may demonstrate extremely complex stratigraphic architectures. We hypothesize that particulate organic matter preserved in turbidites will not be randomly distributed and may be used to assist the identification of architectural elements. An integrated sedimentological and palynological study was conducted on confined turbidites in the Peïra Cava sub-basin of the Eocene to Oligocene Grès d'Annot system, SE France; this provides a natural laboratory, where certainty in the stratigraphy allows confidence in sampling of sub-environments. Elements reflect deposits from high and low density flows including: thin-beds that onlap the basin margin and heterolithics spread across the basin; base of slope megabeds; and thick-beds that have a proximal to distal expression from south to north across the basin. One hundred samples were collected from logged sections across the basin, with 10 g of mudstone per sample being processed for a count of three hundred pieces of organic matter. Both allochthonous terrestrial and relatively autochthonous marine matter were recovered, with results showing a progressive fining of material from proximal to distal areas. Base of slope megabeds and proximal ponded thick-beds are dominated by dense humic matter, medial areas become dominated by light plant material, and distal samples are dominated by amorphous matter, interpreted as a result of density sorting of organic matter in turbidity currents. Exploratory ordination analysis and fuzzy cluster analysis were used to examine these results. Based on this study, we provide evidence of density fractionation of organics in turbidity currents, which is implied to be a major control on the distribution of particles in deep-marine depositional systems. This allows a palynofacies classification scheme to be developed to recognise architectural elements, which may be applied to sub-surface samples to assist the characterization of deep-water mini-basin architecture, understanding of which is crucial for hydrocarbon exploration and development.

Ichnodiversity and ichnoabundance: Revealing depositional trends in a confined turbidite system

AbstractIchnological studies are still in their infancy when it comes to the interpretation of deep‐marine deposits. The Eocene–Oligocene turbidite system of the Grès d'Annot Formation in south‐east France is well‐studied sedimentologically, but its trace‐fossil content is poorly known. Here, an integrated ichnological–sedimentological study is presented from the Annot sub‐basin for the first time, which demonstrates its value for interpreting proximal to distal and axial to marginal trends in confined turbidite systems. A comprehensive trace‐fossil data set was collected from seven outcrops situated in the southern part of the basin. These data are presented following a morphology‐based classification scheme to allow easy recognition and characterization of ichnotaxa. Ichnodiversity and the abundance of ichnotaxa are regarded as important parameters in such interpretations. Instead of simply counting ichnotaxa per outcrop or stratigraphic unit, an equation has been developed in which the ‘ichnoabundance’ (new term) of each counted ichnotaxon is calculated. An exponential growth factor is applied to the increase of the frequency of trace fossils, and is assumed in this equation to better reflect the population dynamics of benthic organisms. A comparison of the solution for pre‐turbidite and post‐turbidite trace‐fossil suites seems to be more suitable for revealing regional and stratigraphic trends compared with conventional approaches. Despite varying size and conditions of the studied outcrops, the results achieved from the Grès d'Annot Formation can help in the reconstruction of sedimentary processes acting in this confined turbidite basin.

Three-dimensional paleomorphologic reconstruction and turbidite distribution prediction revealing a Pleistocene confined basin system in the northeast Nankai Trough area

Integrated three-dimensional (3-D) paleomorphologic and sedimentary modeling was used to predict the basin architecture and depositional pattern of Pleistocene forearc basin turbidites in a gas hydrate field along the northeast Nankai Trough, off central Japan. Structural unfolding and stratigraphic decompaction of the targeted stratigraphic unit resulted in successful modeling of the paleobathymetry at the time of deposition. This paleobathymetry was characterized by a simple U-shaped paleominibasin. Subsequent turbidity current modeling on the reconstructed paleobathymetric surface demonstrated morphologically controlled turbidity current behavior and selective turbidite sand distribution within the minibasin, which strongly suggests the development of a confined turbidite system. Among three candidate inflow patterns, a northeasterly inflow pattern was determined as most likely. In this scenario, flow reflection and deflection caused ponding and a concentration of sandy turbidite accumulation in the basin center, which facilitated filling of the minibasin. Such a sedimentary character is undetected by seismic data in the studied gas hydrate reservoir formation because of hydrate-cementation–induced seismic anomalies. Our model suggests that 3-D horizon surfaces mapped from 3-D seismic data along with well-log data can be used to predict paleobasin characteristics and depositional processes in deep-water turbidite systems even if seismic profiles cannot be determined because of the presence of gas hydrates.

Constraining the origin and evolution of confined turbidite systems: southern Cretan margin, Eastern Mediterranean Sea (34°30–36°N)

Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN), seismic reflection and sediment-core data are used in the analysis of two tectonic troughs south of Crete, Eastern Mediterranean Sea. Here, up to 1.2 s two-way travel time (TWTT) of strata have accumulated since the Middle Miocene in association with extension in the South Aegean region. The study area comprises >100-km- long by >25-km-wide basins filled by sediments subdivided into two seismic units: (1) an upper Unit 1 deposited in sub-basins which follow the present-day configuration of the southern Cretan margin; (2) a basal Unit 2, more than 500 ms (TWTT) thick, accumulated in deeper half-graben/grabens distinct from the present-day depocentres. Both units overlap a locally stratified Unit 3 comprising the pre-Neogene core complex of Crete and Gavdos. In this work, the interpreted seismic units are correlated with the onshore stratigraphy, demonstrating that denudation processes occurring on Crete and Gavdos in response to major tectonic events have been responsible for high sedimentation rates along the proximal southern Cretan margin. Consequently, topographically confined sedimentary units have been deposited south of Crete in the last 12 Ma, including turbidites and other mass-flow deposits fed by evolving transverse and axial channel systems. Surface processes controlling facies distribution include the direct inflow of sediment from alluvial-fan systems and incising mountain rivers onto the Cretan slope, where significant sediment instability processes occur at present. In this setting, seismic profiles reveal eight different types of stratigraphic contacts on basin-margin highs, and basinal areas show evidence of halokinesis and/or fluid escape. The acquired data also show that significant changes to the margin’s configuration occurred in association with the post-Alpine tectonic and eustatic episodes affecting the Eastern Mediterranean.

Sandy modern turbidite lobes: A new insight from high resolution seismic data

The present study is focused on the latest Pleistocene distal lobe of a confined turbidite system. Several scales of depositional structures are seen using very high-resolution seismic data, which are quite comparable with those of onland outcrops. The lobe is a sedimentary complex and was built by numerous successive events. This study allows for the first time to describe a comprehensive model for the inter-relationship of different architectural elements within a lobe complex. We demonstrate that the three-dimensional geometry of the lobe is directly controlled by the degree of confinement of gravity currents and thus to the pre-existing morphology that evolved during lobe growth. The confinement and lithology of the lobe has a great impact on the reservoir physical properties.

JOSEPH, P. & LOMAS, S. A. (eds) 2004. Deep-Water Sedimentation in the Alpine Basin of SE France. Geological Society Special Publication no. 221. viii + 448 pp. London, Bath: Geological Society of London. Price £105.00, US $175.00; GSL/IGI members' price £52.50, US $88.00; AAPG/SEPM/GSA/RAS members' price £63.00, US $105.00 (hard covers). ISBN 1 86239 148 3 LOMAS, S. A. &

The latest two special publications from the Geological Society both derive from an international workshop on Confined Turbidite Systems held in Nice (France) in September 2001. The particular focus of this workshop was the Gres d’Annot turbidite system: the subject of seminal early studies by Bouma and Stanley, and still a mecca for turbidite sedimentologists. Faced with a total number of papers too great for a single volume, the editors have pragmatically confined those on the Gres d’Annot into one ` publication and allowed the residue to spill …

Confined turbidite systems

Abstract Deep-water sedimentation is currently a major focus of both academic research and industrial interest. Recent studies have emphasized the fundamental influence of seafloor topography on the growth and morphology of submarine ‘fans’: in many turbidite systems and turbidite hydrocarbon reservoirs, depositional system development has been moderately to strongly confined by pre-existing bounding slopes. This publication examines aspects of sediment dispersal and accumulation in deepwater systems where basin-floor topography has profoundly affected deposition, and the associated controls on hydrocarbon reservoir architecture and heterogeneity. The papers herein offer a global perspective which is wide-ranging in terms of both approach and location, including contrasting case studies of outcrop, subsurface, modern and experimental systems.

The character and origin of thick base-of-slope sandstone units of the Peïra Cava outlier, SE France

Abstract Many hydrocarbon reservoirs occur within confined turbidite systems in which the depositional pattern of turbidity currents has been strongly influenced by basin-floor topography. In certain settings basin-floor topography may cause the development of anomalously thick (tens of metres) sandstones that are potentially excellent reservoir units. Southern exposures of the Peïra Cava outlier (Eocene-Oligocene; Annot Sandstones) provide well-exposed outcrops of such decametre-thick sandstone bodies. These units are located close to basin margins and downstream from an inferred topographic break-in-slope. Several base-of-slope sandstone bodies are examined that illustrate a common sedimentary theme of a complex basal unit, comprising laterally pinching or inter-fingering debrite and turbidite, abruptly overlain by a single, thick normally graded turbidite deposit. One of these sandstone bodies pinches out laterally over less than several hundred metres and sits within a deep (>20 m) ‘spoon shaped’ erosional scour. The scour is similar to morphological features observed in modern base-of-slope settings recently imaged using high-resolution submarine bathymetric surveys. Several different process interpretations may explain the occurrence of such sandstone bodies including remobilization of newly deposited sediment off basinmargins and enhanced deposition due to flow across a break-in-slope. A submarine channel interpretation is not consistent with the field observations. However, these units do share a number of similar features to channels that could lead to the misinterpretation of reservoir geometry.

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.

Spatial variability of Hurst statistics in the Castagnola Formation, Tertiary Piedmont Basin, NW ITaly: discrimination of sub-environments in a confined turbidite system

Abstract The purpose of this paper is to investigate the potential of the use of the Hurst test as a statistical tool for quantifying the degree of clustering of high and low values of field measurements in vertical turbiditic sections within a small and confined tectonically mobile basin. The paper (1) documents the fact that significant clustering occurs in almost all sections, (2) shows that there are systematic spatial variations in the degree of clustering such that different parts of the basin have different Hurst statistics, (3) discusses the origin of those spatial trends, and (4) compares Hurst H with Chen and Hiscott’s Hurst K . The case study focuses on one well-exposed reservoir-scale turbiditic unit cropping out in the eastern part of the Tertiary Piedmont Basin (NW, Italy) and representing the lower portion of an Upper Oligocene-Lower Miocene turbidite system (Castagnola Formation). The sedimentological model that has been adopted is based on the description of 41 sedimentological logs (approximately 25–35 m apart) physically correlated bed by bed. Three variables were studied: thickness of sandstones and siltstones, grain size score, and sand and silt thickness percentage (i.e. the ratio of coarse division to overlying mudstones, in percent). The Hurst test has revealed the existence of common clustering for the three studied variables (only 2 of 41 logs failed to pass the test at the 10% significance level for all the studied variables). This clustering is associated with lateral and vertical facies variations in response to changes in depositional sub-environments within the Castagnola Basin. A geostatistical analysis has been performed in order to estimate the spatial distribution of the Hurst statistics within the studied area. The analysis makes possible predictions at unsampled locations.

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)

ABSTRACT: Changes in architectural style as a confined turbidite system aggrades: the Peïra Cava system, south-east France

ABSTRACT: Changes in architectural style as a confined turbidite system aggrades: the Peïra Cava system, south-east France