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Submarine channel-related thin-bedded turbidites are deposited in environments such as external levees, internal levees, depositional terraces and at times of channel abandonment. Thin-bedded turbidites are defined as beds that are less than 10 cm thick, but the described environments can at times contain beds up to 100 cm thick which would be classified as medium- or thick-bedded. This paper addresses examples of these environments from the modern seafloor, outcrop and the subsurface to suggest criteria that assist in the differentiation of levees and terraces from an architectural, sedimentological, ichnological and hydrocarbon reservoir perspective. External levees confine channel belts and are elongate sedimentary deposits that are a product of over-spill of turbidity currents from the channel belt they confine. External levees often have predictable vertical, lateral and downstream changes in thickness and sand content but are commonly modified by collapse of the inner external levee into the channel, by collapse on the outer external levee, by sediment waves, and by interaction of external levees with topographic features such as other channels, other external levees, basin margins or previous slump/slide blocks, which can greatly modify the sand distribution within them. A combination of internal levees, depositional terraces and slide blocks of external levee sediment make up thin-bedded turbidites within channel belts. We differentiate between wedge-shaped internal levees and topographically flat or subdued depositional terraces, whose differing geometries and sand distribution reflect the fact that the flow processes involved in the formation of these deposits are different. The characteristic wedge shape of an internal levee requires sufficient space within the channel belt for the over-spilling current to spread, decelerate and deposit the majority of its silt and sand grade suspended sediment before reaching the bounding topography of the channel belt. In the case of depositional terraces the space available in the channel belt is insufficient for the current to decelerate and deposit the majority of its sediment before reaching the bounding topography of the channel belt, creating confined sheet-like deposits. External levees, internal levees and depositional terraces have distinct sedimentological characteristics such as sand bed thickness trends and sedimentary structures that can be used to distinguish them. Together with sedimentological characteristics, in some systems these thin-bedded turbidite deposits contain distinctive trace fossil assemblages, where channel proximal deposits such as proximal external levees, internal levees and depositional terraces can have much higher ichnodiversity than sand-rich channel axes and more mud-dominated outer external levees. The depositional sites for internal levees and depositional terraces within channel belts can be formed by various processes such as entrenchment, point bar accretion, meander bend cut-off, channel margin failure, or changes in the flow parameters. These processes can result in elevated surfaces within the confines of the channel belt that subsequently become prone to the deposition of over-bank deposits. The development and preservation of levees and terraces is closely related to the evolution of the channel belt as a whole, which is controlled both by allogenic mechanisms (such as sea-level fluctuations, changes in turbidity current size and sediment calibre, and changes in the equilibrium profile of the channel), and by autogenic mechanisms (such as channel avulsion and resulting knick-point migration). Where preserved in the rock record thin-bedded turbidites have been uncommon primary targets for hydrocarbon field development, since most efforts have focused on the channel-fills which have the highest proportion of sand. However, thin-bedded turbidites can contain large amounts of sand, of which individual beds can be very laterally continuous, and hence can make significant secondary reservoir targets. [ABSTRACT FROM AUTHOR]

Abstract: The present study brings new insights on the small-scale characteristics of relatively distal turbidite deposits (classically defined as depositional lobes or sheets) from the deep-water clastic slope system of the Annot Sandstones (Grès d''Annot), in the Trois Evêchés area. This “seismic scale” outcrop has been studied in detail to characterize sedimentary geometries and facies associations within and between sheet-like sandstone-bodies. In terms of facies, their internal architecture is dominated by numerous erosional features, variable scale cross-bedding and dewatering structures. Facies range from gravelly sandstones to fine-grained sandstones and their spatial distribution inside sandstone-bodies is provided, allowing us to discuss the spatial changes in flow behaviour. A detailed dataset, made of very high-resolution correlations in thick to thin-bedded sheet-like sandstone-bodies, enables to identify five main types of architectural elements, composed of both channelised and unchannelised depositional bodies. Gravelly-filled channel-elements, laterally stacked channel-elements (interpreted as being related to sinuous channel migrations) and wing-like channel-elements constitute the channelised sheet systems whereas tabular sandstones (sheets sensu stricto), prograding and/or dome-shaped units constitute layered sheet systems. Sinuous channels are represented by the association of erosive-based, laterally stacked bedsets and very coarse-grained, low-angle cross-bedded facies. These latter facies are interpreted as the result of flow deconfinement and overbanks above channel margins. The stratigraphic relationships between these architectural elements permit establishing a conceptual model of the distribution and organisation of the components. This distribution scheme is based on the longitudinal differentiation of gravity processes, which are directly linked with architectural element types. This model is composed of proximal confined units where channelization processes, amalgamation and bypass dominate, to more distal and less confined units where compensation, constructive structures and high deposition rates control sedimentary architectures. The Trois Evêchés sub-basin may be considered as a good example of a channelised lobe to unchannelised lobe transitional environment, which explains the lateral and vertical high variability during its infill. The widespread apparition of residual facies, such as cross-stratified sandstones as well as basal lags, suggests significant bypass. The appearance of both channelised and unchannelised units is consistent with this transitional environment. The provided detailed dataset in this paper displays the associated wide range of architectural elements in these settings, and highlights the significant heterogeneities of sand-rich confined turbidite sheet systems. These heterogeneities are under the seismic resolution and may influence fluid migration in sand-rich deep-water reservoirs. [Copyright &y& Elsevier]