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Subaqueous mass‐transport processes are one of the mechanisms for transport of sediment into the deep sea. Internal structures and depositional processes of carbonate mass‐transport deposits are relatively poorly understood relative to siliciclastic facies due to their comparative paucity in the rock record. A variety of carbonate mass‐transport deposits, including slumps, debrites and deep‐channel‐confined density flow deposits, occur in Middle–Upper Ordovician slope deposits in western Inner Mongolia (Wuhai), China. These provide a rare opportunity to illustrate the emplacement history of carbonate mass‐transport deposits at the outcrop scale. The slumps and debrites host remarkable folds, chaotic beds and imbricated beds that reflect differences in both rheology and position on the slope. Individual slump sheets show gradations between undulating laminae, inclined and recumbent folds, highly deformed folds, and chaotic textures upslope from the toe region. Debrites are commonly interbedded with slump deposits, whereas imbricated beds are present in the middle and lower parts of the toes of slump sheets near the terminal wall. In the study area, thin‐bedded limestone with slump deposits of the Kelimoli Formation are overlain by fine‐grained, siliciclastic‐dominated, slope deposits of the Wulalike Formation. A thick breccia of the Wulalike Formation was deposited in a main feeder channel in south‐east Wuhai, but to the west‐north‐west the breccia was deposited in distributary channels possibly represented as a unique lower‐slope pattern of gullies. At the latter locality, the breccia was deposited solely within the channels on a steep west‐north‐west dipping slope under density‐driven flows. The mass‐transport deposits documented herein records passive to foreland basin tectonic transitions, and associated platform foundering and steepening of the slope. A slope facies model was constructed to demonstrate the spatial and temporal variations of mass‐transport deposits during basin evolution, and as such it provides a template for the interpretation of the deposits of ancient slopes that underwent passive to active tectonic transitions. [ABSTRACT FROM AUTHOR]

Monitoring of modern deep‐water channels has revealed how migrating channel‐floor features generate and remove stratigraphy, improving understanding of how channel morphologies relate to their deposits. Here, seafloor and subsurface data are reconciled through an integrated study of high‐resolution bathymetry and three‐dimensional seismic data imaging a ca 150 km stretch of the trench‐axial Hikurangi Channel, offshore New Zealand. On the seafloor, terraced channel‐walls bound a flat, wide, channel‐floor, ornamented with three scales of features that increase then decrease in longitudinal gradient downstream, and widen downstream: cyclic‐steps, knickpoints and knickpoint‐zones (in increasing size). Mass‐transport deposits derived from channel‐wall collapse, are bordered by wide and flat reaches of channel‐floor upstream and by knickpoint‐zones (reaches containing multiple knickpoints) downstream. In the subsurface, recognition of ten seismofacies and five types of surface enables identification of four depositional elements: channel‐fill, sheet or terrace, levée, and mass‐transport deposits. Integration of subsurface and seafloor interpretations reveals that knickpoint‐zones initiate on the downstream margins of channel‐damming mass‐transport deposits; they migrate and incise through the mass‐transport deposits and weakly‐confined deposits formed upstream, as the channel tends towards equilibrium. Downstream of a knickpoint‐zone, a flat channel‐floor is bounded by newly‐formed terraces. Knickpoints migrate by eroding upstream and depositing downstream, generating filled concave‐up (cross‐sectional) surfaces in their wake. Within knickpoint‐zones, knickpoint‐generated surfaces are re‐incised by subsequently‐passing knickpoints to produce a composite bounding surface; this surface does not delineate the morphology of any palaeo‐conduit. The Hikurangi Channel's subsurface architecture records the localized erosional response to mass‐transport deposit emplacement via knickpoint‐zone migration, showcasing how transient seafloor features can build channelized stratigraphy. This model provides an additional mechanism to conventional models of channel deposit formation through 'cut‐and‐fill' over long stretches of channel. These findings may aid subsurface interpretation in systems lacking a contemporary self‐analogue or with poor data coverage. [ABSTRACT FROM AUTHOR]

Submarine slope channel systems have complicated three‐dimensional geometries and facies distributions, which are challenging to resolve using subsurface data. Outcrop analogues can provide sub‐seismic‐scale detail, although most exhumed systems only afford two‐dimensional constraints on the depositional architecture. A rare example of an accessible fine‐grained slope channel complex set situated in a tectonically quiescent basin that offers seismic‐scale, down‐dip and across‐strike exposures is the Klein Hangklip area, Tanqua‐Karoo Basin, South Africa. This study investigates the three‐dimensional architecture of this channel complex set to characterise the stratigraphic evolution of a submarine channel‐fill and the implications this has for both sediment transport to the deep‐oceans and reservoir quality distribution. Correlated sedimentary logs and mapping of key surfaces across a 3 km2 area reveal that: (i) the oldest channel elements in channel complexes infill relatively deep channel cuts and have low aspect‐ratios. Later channel elements are bound by comparatively flat erosion surfaces and have high aspect‐ratios; (ii) facies changes across depositional strike are consistent and predictable; conversely, facies change in successive down depositional dip positions indicating longitudinal variability in depositional processes; (iii) stratigraphic architecture is consistent and predictable at seismic‐scale both down‐dip and across‐strike in three‐dimensions; (iv) channel‐base‐deposits exhibit spatial heterogeneity on one to hundreds of metres length‐scales, which can inhibit accurate recognition and interpretations drawn from one‐dimensional or limited two‐dimensional datasets; and (v) channel‐base‐deposit character is linked to sediment bypass magnitude and longevity, which suggests that time‐partitioning is biased towards conduit excavation and maintenance rather than the fill‐phase. The data provide insights into the stratigraphic evolution and architecture of slope channel‐fills on fine‐grained continental margins and can be utilised to improve predictions derived from lower resolution and one‐dimensional well data. [ABSTRACT FROM AUTHOR]

Abstract Initial porosity and permeability in deep-water systems are controlled by primary sedimentary texture and mineralogy. Therefore, understanding the sedimentary processes that control changes in primary texture is critical for improved reservoir quality predictions. A well-constrained, exhumed submarine lobe in the Jaca Basin, and a submarine channel-fill element in the Aínsa Basin, northern Spain, were studied to characterize the depositional reservoir quality in axial to marginal/fringe positions. Construction of architectural panels and strategic sampling enabled analysis of the spatial changes in textural properties, and their relationship to reservoir quality distribution. Samples were analyzed in thin-section to establish how depositional processes inferred from outcrop observations affect textural properties. Results show that high-density turbidites are concentrated in lobe- and channel-axis positions and exhibit good depositional reservoir quality. Lobe off-axis deposits contain high- and low-density turbidites and have moderate depositional reservoir quality. Conversely, low-density turbidites dominate lobe fringe and channel-margin positions and have relatively poor depositional reservoir quality. There is a sharp decrease in depositional reservoir quality between the lobe off-axis and lobe fringe due to: 1) an abrupt increase in matrix content; 2) an abrupt decrease in sandstone amalgamation; and 3) a decrease in grain-size. There is an abrupt increase in depositional reservoir quality from channel margin to channel axis corresponding to: 1) an increase in total sandstone thickness and amalgamation; 2) an increase in grain-size, 3) a decrease in matrix content. Rates of change of key properties are up to two orders of magnitude greater between channel-fill sub-environments compared to lobe sub-environments. Spatial variability in properties of discrete architectural elements, and rates of changes, provides input to reservoir models during exploration, appraisal, and development phases of hydrocarbon fields. Highlights • Bed- and grain-scale properties mapped in individual architectural elements. • Sediment gravity flow-type controls depositional reservoir quality. • Process-based approach to reservoir quality aids prediction. [ABSTRACT FROM AUTHOR]