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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]

Abstract: A comparison between tidal bars from a deltaic to estuarine complex in the Middle Devonian Baltic Basin shows contrasts in sedimentary structures and bar preservation, related to changes from a regressive to transgressive setting. Detailed facies analysis of the 20–30 m thick tide-dominated estuarine Amata Formation is combined with previous analysis of the underlying deltaic Gauja Formation in order to develop specific criteria for differentiating between estuarine tidal bars and deltaic tidal bars. The estuarine tidal bars are characterized by: 1) preserved bar topsets; 2) dominantly landward palaeocurrent directions; 3) well to very-well-sorted, very fine- or fine-grained sandstones; and 4) overall retrogressive stacking with overlying marine mudstones. These bars, were deposited in the outer estuary, as elongated sand bodies. Tidal currents dominated the depositional regime, seen from the abundant tidal facies, and high energy is suggested by low abundance of mudstone drapes and mica drapes. The deltaic tidal bars are characterized by: 1) eroded bar topsets; 2) dominantly basinward palaeocurrent directions; 3) poorly sorted sandstone with grain-size ranging from very fine sands to granules; and 4) overall progradational stacking patterns. These tidal bars were situated seaward of the delta plain. Strong fluvial influence is reflected by coarse-grained, poorly sorted sandstone, mudstone and pebble clasts and the predominately basinward palaeocurrent directions. The criteria listed above can all be useful for separating between the two tidal-bar types in basins with similar settings as the Devonian Baltic Basin, however, to be used as a general facies model, the criteria must be filtered for local variations. We suggest that characteristics which occur due to the regressive to transgressive stacking patterns of tidal bars; the different palaeocurrent directions, and the preservation versus truncation of tidal bars, are broad diagnostic criteria that could be applied to other basins. [Copyright &y& Elsevier]