Shelf-Edge Delta Regime as a Predictor of Deepwater Deposition

Shelf-edge delta regime, defined as the sum of the process effects of waves, currents and tides, impacting deltas as they arrive at the shelf edge is a key predictor for likelihood of sand transport off the shelf edge to deep water. Conventional sequence-stratigraphic concepts assumed that delivery of sand to the Deepwater occurs during falling and lowstand of relative sea level, and was associated with shelf-edge incision and sediment bypass. Recent analysis of outcrop and subsurface datasets has shown, however, that sediment can also be transported to the deep water during rising and highstand of sea level, provided sediment flux is high. In addition, some examples with documented shelf edge entrenchment associated with sea level fall did not deliver significant sand volumes to the deep water. In contrast to relative sea level and sediment flux, the role of the shelf-edge delta itself and its process regime in transporting sediment to the deep water has received relatively little attention. On wave-dominated coastlines, wave energy redistributes sediment in a shoreline-parallel direction reducing the amount that is transported basinward. Tidal deltas that occur on the outer shelf and shelf edge usually occur during transgression, and as such, much of the sediment is stored within the deltaic tidal channels and not transported basinward. River-dominated deltas, however, transport significant quantities of sediment basinward through a combination of turbidity currents and delta-front oversteepening and collapse. Shelf-edge delta regime is interpreted in outcrop following analysis of sedimentary structures, ichno-fossil assemblages and stratigraphic architecture at the bed and parasequence scale as these features are characteristically distinct in river-, tide- and wave-dominated deltas. In 3-dimensional seismic data, delta regime is interpreted from stratigraphic architecture and surface seismic attribute analysis methods, calibrated by well logs and core sampling. A dataset of more than 25 shelf-margin clinoforms is used to highlight how shelf-edge delta depositional style and particularly the presence of a strong river-drive for shelf-edge deltas is a more reliable indicator of Deepwater sand delivery than either traditional systems tract or shelf-edge trajectory approaches.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California