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Mud dominates volumetrically the fraction of sediment delivered and deposited in deep-water environments, and mudstone is a major component of basin-floor successions. However, studies of basin-floor deposits have mainly focused on their proximal sandstone-prone part. A consequent bias therefore remains in the understanding of depositional processes and stratigraphic architecture in mudstone-prone distal settings beyond the sandstone pinchouts of basin-floor fans. This study uses macroscopic and microscopic descriptions of over 500 m of continuous cores from research boreholes from the Permian Skoorsteenberg Formation of the Karoo Basin, South Africa, to document the sedimentology, stratigraphy, and ichnology of a distal mudstone-prone basin-floor succession. Very thin- to thin-bedded mudstones, deposited by low-density turbidity currents, stack to form bedsets bounded by thin packages (< 0.7 m thick) of background mudstones. Genetically related bedsets stack to form bedset packages, which are bounded by thicker (> 0.7 m thick) background mudstones. Stratigraphic correlation between cores suggests that bedsets represent the distal fringes of submarine fan lobe elements and/or lobes, and bedset packages represent the distal fringes of lobe complexes and/or lobe complex sets. The internal stacking pattern of bedsets and bedset packages is highly variable vertically and laterally, which records dominantly autogenic processes (e.g., compensational stacking, avulsion of feeder channels). The background mudstones are characterized by remnant tractional structures and outsize particles, and are interpreted as deposited from low-density turbidity currents and debris flows before intense biogenic reworking. These observations challenge the idea that mud accumulates only from hemipelagic suspension fallout in distal basin-floor environments. Thin background mudstones separating bedsets (< 0.7 m thick) are interpreted to mainly represent autogenically driven lobe abandonment due to up-dip channel avulsion. The thicker background mudstones separating bedset packages (> 0.7 m thick) are interpreted to dominantly mark allogenically driven regional decrease of sand supply to the basin floor. The recognition of sandstone-prone basin-floor fans passing into genetically linked distal fringe mudstones suggests that submarine lobes are at least ∼ 20 km longer than previously estimated. This study provides sedimentological, stratigraphic, and ichnological criteria to differentiate mudstones deposited in different sub-environments in distal deep-water basin-floor settings, with implications for the accurate characterization of basin-floor fan architecture, and their use as archives of paleoenvironmental change.

The inception and evolution of channels in deep-water systems is controlled by the axial gradient and lateral confinement experienced by their formative flows. These parameters are often shaped by the action of tectonic structures and/or the emplacement of mass-transport deposits (MTDs). The Arro turbidite system (Aínsa depocenter, Spanish Pyrenees) is an ancient example of a deep-water channelized system from a bathymetrically complex basin, deposited in an active tectonic setting. Sedimentologic fieldwork and geologic mapping of the Arro system has been undertaken to provide context for a detailed study of three of the best-exposed outcrops: Sierra de Soto Gully, Barranco de la Caxigosa, and Muro de Bellos. These locations exemplify the role of confinement in controlling the facies and architecture in the system. Sedimentologic characterization of the deposits has allowed the identification of fifteen facies and eight facies associations; these form a continuum and are non-unique to any depositional environment. However, architectural characterization allowed the grouping of facies associations into four depositional elements: i) weakly confined, increasing-to-decreasing energy deposits; ii) progradational, weakly confined to overbank deposits; iii) alternations of MTDs and turbidites; iv) channel fills. Different styles of channel architecture are observed. In Barranco de la Caxigosa, a master surface which was cut and subsequently filled hosts three channel stories with erosional bases; channelization was enhanced by quasi-instantaneous imposition of lateral confinement by the emplacement of MTDs. In Muro de Bellos, the inception of partially levee-confined channel stories was enhanced by progressive narrowing of the depositional fairway by tectonic structures, which also controlled their migration. Results of this study suggest that deep-water channelization in active tectonic settings may be enhanced or hindered due to: 1) flow interaction with MTD-margin topography or; 2) MTD-top topography; 3) differential compaction of MTDs and/or sediment being loaded into MTDs; 4) formation of megascours by erosive MTDs; 5) basin-floor topography being reset by MTDs. Therefore, the Arro system can be used as an analog for ancient subsurface or outcrop of channelized deposits in bathymetrically complex basins, or as an ancient record of deposits left by flow types observed in modern confined systems.

Low-accommodation deltaic systems are often challenging to interpret due to their condensed, low-gradient nature, which often results in extensive, sheet-like sandstone bodies. As a result, detailed studies of such systems are scarce, and their internal depositional architecture is still poorly understood. We analyze one such system, the Cenomanian deltaic Mesa Rica Sandstone (Dakota Group), which was deposited in the Western Interior Seaway, in east-central New Mexico, USA. A > 20-km-long escarpment, subparallel to the main delta progradation direction, allows a detailed analysis of facies distribution, depositional architecture, and the spatial extent of stratigraphic surfaces. Results reveal an arrangement of laterally variable shallowing-upward facies successions with three depositional cycles preserved. The first cycle is characterized by deltaic sheet-like sandstone bodies that are consistently overlain by sand-filled amalgamated distributary-channel deposits. The two successive cycles record a progressive reduction of sediment supply into the basin. Vertical and lateral relationships between facies associations and architectural geometries allow the recognition of regional key stratal surfaces, incised-valley fills, and the presence of lagoonal deposits at a sub-regional scale.The Mesa Rica deltaic system represents a river-dominated delta with multiple distributary channels. The sheet-like delta-front sandstone bodies are interpreted as the result of the combined effect of high sandy-sediment supply and low accommodation. The latter acted as an accelerator for autogenic depositional mechanisms such as mouth-bar deposition and abandonment, and for the highly avulsive character of distributary channels. After deposition, minor wave reworking facilitated lateral sand redistribution and favored bioturbation. This study demonstrates that sheet-like delta-front sandstone geometries from low-accommodation systems can be formed without the dominance of wave redistribution processes. This cautions against interpretations of amalgamated shoreline systems based solely on apparent sandstone geometries, without taking into account the preservation potential and postdepositional modification of primary deltaic characteristics.

Clinoform geometries and trajectories are widely used to predict the spatial and temporal evolution of sand distribution, but most analytical approaches underplay the significance of topset and shelf process regime in determining how and when sediment is conveyed downdip or stored on the continental shelf. We present an integrated study of clinoform rollover trajectory and detailed grain character analysis to assess the role of topset process regime in determining sand distribution and sediment character across clinothems. This study targets the topset, foreset, and bottomset deposits of four successive Miocene intrashelf clinothem sequences, which represent deposition under either river-dominated or wave-dominated conditions. Seismic reflection data was combined with core analysis and grain character data derived from 664 samples collected from three cored research boreholes. In river-dominated clinothems, the transfer of coarse-grained sediment occurs under both rising and flat-to-falling clinoform rollover trajectories, suggesting that process regime is more important in determining sediment delivery than clinoform trajectory; river-dominated systems are effective conveyors of sediment into deeper water. Wave-dominated clinothems deposited exclusively under rising clinoform rollover trajectories largely retain sand within topset and foreset deposits; wave-dominated systems are effective sediment filters. Notably, deposition under either river- or wave-dominated topset process regimes results in quantifiable differences in grain character attributes along clinoform profiles. Sediments in river-dominated systems are coarser, less well-rounded, and more poorly sorted, and show greater intersequence and intrasequence variability than those in wave-dominated systems; prediction of sediment character is more challenging in river-dominated systems. This study highlights the need for caution when attempting to predict downdip sand distribution from clinoform trajectory alone, and provides a novel perspective into downdip grain character profiles under end-member topset process regime conditions. The results of this study can be used to better constrain sediment grain-size and grain-shape distributions in process-based forward models, and have widespread applications in prediction of reservoir quality in both frontier and mature hydrocarbon basins.