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Variations in Shelf-to-Slope Facies Distribution, Shelf-Margin Accretion Processes and Sediment Delivery in River-Dominated Shelf-Edge Deltas

Abstract

Shelf-edge deltas [SEDs] are the primary driver of shelf-margin accretion and a principle mechanism of deepwater sediment delivery. Fluvial-dominated SEDs are thought to be the most efficient at deepwater sediment delivery and for prograding the shelf margin. Yet, this notion prescribes that all fluvial-dominated SEDs accrete the shelf margin and deliver sediment to deepwater in the same manner. These interpretations spring from recognition criteria of fluvial-dominated SEDs derived from 2-D outcrops and subsurface data. Here we compare five fluvial-dominated SED–slope outcrops in the Permian Karoo-, Cretaceous Washakie-, and Eocene Ainsa- and Spitsbergen Central Basins. Our data highlights similar distribution of architectural elements from shelf edge to slope in each system and a repeated pattern of progradation, sediment bypass, and renewed progradation. We observe greater variability in the mechanisms of these patterns than previous studies, evidenced in the distinct facies within the architectural elements. This has implications for the timing and style of progradation, deepwater sediment delivery, and reservoir connectivity. In particular, we recognize three main types of fluvial-dominated SED dynamics: failure-dominated [FD], hyperpycnal flow-dominated [HD]; and episodic bypass-dominated [BD]. In FD systems mouth bar progradation at the shelf edge is a function of repeated buildup, collapse and evacuation, and reestablishment. Collapsed sediment is transported basinward but trapped at the upper slope, and accretes the margin. Coarse-grained sediment is only rarely delivered to lower slope or basin floor. In HD systems, hyperpycnal flows frequently erode mouth bars and transport the sediment to the slope, causing shelf-margin accretion by building thick turbidite aprons at the upper slope, attached to the SED. In BD systems, distributary channel avulsion halted progradation and forced the rapid and punctuated redistribution of sediment from the shelf edge to the basin floor. In FD systems, the juxtaposition of non-deformed and collapsed mouth bars limits the connectivity at the shelf edge, and preferential trapping on the upper slope results in low volumes of deepwater sands even under high sediment supply. In HD systems the constant sediment redistribution from shelf to slope results in well connected, extensive reservoirs on the slope. In BD systems, the episodic nature of sediment bypass results in spatially and temporally localized reservoirs.