Seismic‐reflection data show that most deepwater (>200 m water depth) basins are filled by sand and mud dispersed across clinoformal geometries characterized by gently dipping topsets, steeper foresets and gently dipping bottomsets. However, the entire geometry of these ubiquitous clinoforms is not always recognized in outcrops. Sometimes the infill is erroneously interpreted as “layer cake” or “ramp” stratigraphy because the topset‐foreset‐bottomset clinoforms are not well exposed. Regional 2‐D seismic lines show clinoforms in the Lower to Middle Jurassic Challaco, Lajas, and Los Molles formations in S. Neuquén Basin in Argentina. Time equivalent shelf, slope and basin‐floor segments of clinoforms are exposed, and can be walked out in hundreds of metres thick and kilometres‐wide outcrops. The studied margin‐scale clinoforms are not representing a continental‐margin but a deepwater shelf margin that built out in a back‐arc basin. Lajas‐Los Molles clinoforms have been outcrop‐mapped by tracing mudstones interpreted as flooding surfaces on the shelf and abandonment surfaces (low sedimentation rate) in the deepwater basin. The downslope and lateral facies variability in the outcrops is also consistent with a clinoform interpretation. The Lajas topset (shelf) is dominated by fluvial and tidal deposits. The shelf‐edge rollover zone is occasionally occupied by a 40–50‐m‐thick coarse‐grained shelf‐edge delta, sometimes incising into the underlying slope mudstones, producing oblique clinoforms expressing toplap erosion on seismic. A muddy transgressive phase capping the shelf‐edge deltas contains tidal sandbodies. Shelf‐edge deltas transition downslope into turbidite‐ and debris flow‐filled channels that penetrate down the mud‐prone Los Molles slope. At the base‐of‐slope, some 300m below the shelf edge, there are basin‐floor fan deposits (>200 m thick) composed of sandy submarine‐fan lobes separated by muddy abandonment intervals. The large‐scale outcrop correlation between topset–foreset–bottomset allows facies and depositional interpretation and sets outcrop criteria recognition for each clinoform segment.
The processes that transport sediment from the coastline to the shelf edge are key components of the sedimentary source-to-sink system, determining basin-margin building, deepwater deposition, organic-material accumulation, and the long-term carbon cycle. Research on shelf sediment transport has been aided recently by advances in modeling and marine technology. In this study we provide a much needed review of up-to-date findings on how sediment moves from the outer shelf onto the upper slope, and we summarize four dominant shelf-to-slope drivers: 1) river currents, 2) reworking storm waves and longshore currents, 3) strong tidal currents supplementing river outflow, and 4) small-scale to very large-scale gravity collapse of the shelf-edge area.
The three‐dimensional facies and architecture variability of shelf‐edge deltaic units cropping out at the transition between the Lower–Middle Jurassic Lajas and Los Molles formations of southern Neuquén Basin, Argentina, is presented here, as well as their stratigraphic relationship to uppermost deep‐water slope channel systems. Deep‐water, slope mudstones with thin turbidite beds merge upward with prodelta mudstones and thin sandstones, which are truncated by delta‐front to mouth‐bar sandstones. The latter sandstones are then downcut by large‐scale, trough cross‐stratified coarse‐grained sandstones and conglomerates of distributary channel systems and along‐strike, amalgamated with cross‐bedded sandy units showing evidence of tidal reworking. Proximal–distal facies and architecture variability within a shelf‐edge deltaic succession demonstrates that distributary channel‐complexes become wider and deeper basinward, forming channelized river‐dominated distributary fairways separated by tidally reworked inter‐distributary sand belts at the shelf edge. Evidence from depositional‐dip oriented outcrops shows a lack of collapsed and slumped strata at the shelf edge, and that the coarse shelf‐edge distributary channel fills continue far down the deepwater slope, and conglomerates transform to become high‐density turbidites to mainly thick‐bedded, sand‐matrix‐supported debrites. The interplay between flood tides and river currents is interpreted to have primarily modulated the focusing of river drainages, and consequently coarse‐grained sediment transport, along preferential routes on the outer‐shelf to shelf‐edge and down onto the slope. This contribution documents a unique example of coarse‐grained (mostly conglomeratic) shelf‐edge delta systems, tying bed‐scale facies and architecture data to a seismic‐scale shelf‐margin morphology, thus providing outcrop analogue data for the characterization of shelf‐edge delta systems in the subsurface.
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