This research investigates the character and significance of shelf-edge deltas within the sedimentary source-to-sink system, and how variability at the shelf edge leads to different styles of deep-water deposition. Because the shelf-edge represents one of the key entry points for terrigenous sediment to be delivered into the deep water, understanding of the sedimentary processes in operation at these locations, and the character of sediment transported through these deltas is critical to understanding of deep-water sedimentary systems. The research was carried out using three datasets: an outcrop dataset of 6000 m of measured sections from the Permian-Triassic Karoo Basin, South Africa, a 3D seismic data volume from the Eocene Northern Santos Basin, offshore Brazil and a dataset of 29 previously published descriptions of shelf-edge deltas from a variety of locations and data types. The data presented highlight the importance of sediment instability in the progradation of basin margins, and deep-water transport of sediment. The strata of the Karoo Basin shelf margin represent river-dominated delta deposits that become more deformed as the shelf-edge position is approached. At the shelf edge, basinward dipping, offlapping packages of soft-sediment-deformed and undeformed strata record repetitive collapse and re-establishment of shelf-edge mouth bar packages. The offlapping strata of the Karoo outcrops record progradation of the shelf margin through accretion of the shelf-edge delta, for over 1 km before subsequent transgression. The Eocene Northern Santos Basin shelf margin, in contrast, exhibits instability features which remove kilometers-wide wedges of the outer shelf that are transported to the basin floor to be deposited as mass-transport packages. In this example, shelf-edge progradation is achieved through „stable. accretion of mixed turbidites and contourites. The data also emphasize the importance of the role of shelf-edge delta processes in the delivery of sediment to the basin floor. A global dataset of 29 examples of shelf-edge systems strongly indicates that river domination of the shelf-edge system (as read from cores, well logs or isopach maps) serves as a more reliable predictor of deep-water sediment delivery and deposition than relative sea level fall as traditionally read in shelf-edge trajectories or sequence boundaries. / text

The interaction of numerous sedimentary processes at key transition points along the depositional profile results in a complex heterogeneity in ancient basin margin successions. This complexity is generally well studied along depositional dip sections, but lateral (strike) variability and consequent implications for sediment distribution and stratigraphic architecture is commonly less well constrained. In the Karoo Basin, continuous NW-SE-oriented exposure over 80 km has been characterized by 53 logs with 9910 m of cumulative thickness, >2500 palaeocurrent measurements, and ground-, drone- and helicopter-based photo panels. Palaeoflow indicators suggest dominant sediment transport was to the N-NE, with E-W and NE-SW bidirectional components. These are consistent with a strike orientation of the outcrop belt relative to the NE-N margin progradation direction and a NE-SW reworking by waves orientation. In the south of the study area, upper slope and shelf edge parasequences (50-75 m-thick), show current ripples and inverse-to-normal grading in micaceous and organic-rich siltstones and sandstones. They are interpreted as river-dominated prodelta and mouth bar deposits, locally incised by distributary channels (100 m-thick, 1.5 km-wide). Overlying shelf parasequences are thinner (15-50 m) with symmetrical ripple tops, HCS and low angle cross bedding, interpreted as wave-influenced deltaic or shoreface deposits. They transition upward into erosive-based, fining-up sandstones and isolated sharp-based tabular climbing-rippled sandstones, interpreted as channels and crevasse splays within delta plain mudstones. Along strike to the north, upper slope parasequences show more wave reworking indicators and no evidence of gullying or incision. Overlying shelf parasequences are sandier, more amalgamated and strongly influenced by wave action. They are interpreted as offshore, shoreface, foreshore and strandplain deposits. Southern nearshore environments were therefore more river-dominated with bypass and sediment delivery to deeper parts of the basin across a steep, more erosive margin. Wave and storm current redistribution along strike to the northern, lower gradient margin resulted in higher net-to-gross and sand connectivity on a wider shelf, without major incision, bypass and sand supply to the upper slope. No evidence of major avulsions in the upstream tributary and distributary systems are interpreted because the bypass and fluvial-dominated characteristics are persistent in the southern areas through time, whereas the northern margin maintained a sand-starved upper slope and a wave dominated shelf succession. The overall thicker and delta- dominated succession in the south, and the thinner, more condensed and wave dominated stratigraphy in the north are interpreted to be controlled by a combination of basement and basin configuration and differential basin margin physiography. However, relative sea level fluctuations controlled the stacking patterns, with an overall shallowing-upward profile that can be subdivided into two prograding phases, separated by a transgressive phase. At parasequence scale, climate, autocyclicity and coastal processes influenced the equilibrium between sediment input, redistribution and compensational stacking. This study demonstrates that although basin margin successions may be consistently progradational, the interaction of mixed coastal processes and differential spatial configuration can result in a complex along-strike sedimentary architecture, with major implications for sediment distribution through time and space.