Defining the shelf edge and the three-dimensional shelf edge to slope facies variability in shelf-edge deltas

作者：Laugier Fabien J^*; Plink Bjorklund Piret

来源：Sedimentology, 2016, 63(5): 1280-1320.

DOI：10.1111/sed.12263

• Clinoforms
• Karoo Basin
• Kookfontein Formation
• shelf-edge deltas
• slope channelization
• tidal influence

Shelf-edge deltas play a critical role in shelf-margin accretion and deepwater sediment delivery, yet much remains to be understood about the detailed linkage between shelf edge and slope sedimentation. The shelf edge separates the flat-lying shelf from steeper slope regions, and is observable in seismic data and continuous outcrops; however, it is commonly obscured in non-continuous outcrops. Defining this zone is essential because it segregates areas dominated by shelf currents from those governed by gravity-driven processes. Understanding this linkage is paramount for predicting and characterizing associated deepwater reservoirs. In the Tanqua Karoo Basin, the Permian Kookfontein Formation shelf-slope clinothems are well-exposed for 21km along depositional strike and dip. Two independent methods identified the shelf-edge position, indicating that it is defined by: (i) a transition from predominantly shelf-current to gravitational deposits; (ii) an increase in soft-sediment deformation; (iii) a significant gradient increase; and (iv) clinothem thickening. A quantitative approach was used to assess the impact of process-regime variability along the shelf edge on downslope sedimentation. Facies proportions were quantified from sedimentary logs and photographic panels, and integrated with mapped key surfaces to construct a stratigraphic grid. Spatial variability in facies proportions highlights two types of shelf-edge depositional zones within the same shelf-edge delta. Where deposition occurred in fluvial-dominated zones, the slope is sand rich, channelized with channels widening downslope, and rich in collapse features. Where deltaic deposits indicate considerable tidal reworking, the deposits are thin and pinch-out close to the shelf edge, and the slope is sand poor and lacks channelization. Amplification of tidal energy, and decrease in fluvial drive on the shelf, coincides with a decrease in mouth bar and shelf-edge collapse, and a lack of channelization on the slope. This analysis suggests that process-regime variability along the shelf edge exercised significant control on shelf-edge progradation, slope channelization and deepwater sediment delivery.

• 出版日期2016-8