Shelf-Edge Delta Types and Their Sequence-Stratigraphic Relationships
Ron Steel, Szczepan J. Porębski, Piret Plink-Bjorklund, Donatella Mellere, Maija Schellpeper, 2013. "Shelf-Edge Delta Types and Their Sequence-Stratigraphic Relationships", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Shelf-edge deltas are the main driver for the delivery of sediment to the deep water lowstand systems tracts. However, the mere presence of deltas at the shelf margin does not guarantee accumulation of deep-water sands. The two main reasons for this are: (1) deltas that develop at the shelf edge during relative sea-level fall generally need to be significantly incised by their own distributaries for sand delivery to be focused down to a basin-floor fan system, and (2) deltas that develop when sea level is rising (late lowstand) tend to be inefficient sand-delivery systems, and disperse sand mainly onto the slope as sheet-like turbidite lobes, with few or no basin-floor fans. Thus, given the presence of deltas at the shelf-edge, both the likely magnitude and direction of sea-level change at the shelf edge needs to be estimated, before significant time-equivalent, deep-water sand can be predicted on the basin floor.
Shelf-edge deltas are generally thicker, significantly more unstable, and markedly more turbiditeprone than inner/or mid-shelf deltas. These major differences are due to longer run-out slopes (greater water depths), steeper mud-prone slopes, and greater accommodation at the shelf margin compared to deltas in more proximal shelf settings. There are four main types of shelf-edge deltas that have been documented from a database developed mainly from the Eocene shelf margin on Spitsbergen and the Miocene shelf margin of the Carpathian Foredeep:
Type A deltas develop on the outer shelf/shelf-margin transition but without significant progradation beyond the shelf edge onto the slope. These deltas usually form during the falling stage of a fall-to-rise cycle on the shelf.
Type B deltas develop at the shelf margin but are significantly cannibalized by fluvial-feeder erosion. Such deltas also form during falling stage, but base level falls below the shelf edge. The deltas are fairly sharp based on the outer shelf, are sand prone, and are deeply eroded by their own river distributaries. Because of the fluvial incision, only remnants of these deltas are preserved. However, their main significance and legacy is their time-equivalent, downslope suite of deep water, lowstand deposits including basin-floor fans.
Type C deltas develop at the shelf edge, produce significant basinward growth of the shelf margin but rarely link down to basin-floor fans. They form during a late, rising stage of the fall-to-rise cycle, as they overlie earlier cannibalized deltas and older basin-floor fans of the same sequence. They are many tens of meters thick and consist of stacked, well-developed upward-coarsening and thickening units..
Type D deltas are progradational to aggradational delta complexes at the shelf margin, without underlying shelf-edge erosion, and only rare, linked basin-floor fans. Type A and C deltas simply amalgamate during a fall-to-rise cycle to become a single, thick (many tens of meters) deltaic wedge that is perched at the shelf margin and drapes far out onto the slope.