Sømme et al. (2009) present an important contribution in the ongoing development of ideas regarding clinoform geometries and the timing and volume of sediment supply to deep-marine depositional systems. Much progress has been made since the original overly simplistic lowstand models (e.g., Posamentier and Vail, 1988) and it is now clear that variations in sediment supply also play a key role in determining stratal architectures (e.g., Muto and Steel, 1997; Porębski and Steel, 2006). However, even recent updates on the sequence stratigraphic model and method tend to emphasize the role of accommodation and pay insufficient attention to the role of independent variations in supply, e.g., Catuneanu et al. (2009), as highlighted by Helland-Hansen (2009). Given this, there is clearly still a need for further discussion and research on this topic before consensus is reached. To this end, I would like to highlight a few additional points arising from Sømme et al. (2009).
Burgess and Hovius (1998) and Muto and Steel (2002) demonstrated that, given measured suspended sediment supply from modern river mouths, it is likely that even medium-size rivers can generate shelf-edge deltas and/or significant bypass of sediment into deep water during highstands that persist for 40 k.y. or more. This is especially relevant when amplitudes of glacioeustatic oscillations are low, during times when volumes of continental ice caps are also low (i.e., during greenhouse periods), which likely represents the largest proportion of Earth history. Sømme et al. illustrate the point nicely with their Dionisos models.
However, Sømme et al. went on to claim that “…during the Pliocene-Pleistocene, high-frequency, high-amplitude eustatic fluctuations maintained deep shelves prohibiting significant highstand progradation” (p. 590) Data and analysis from Burgess and Hovius (1998) tend to suggest that this is not the case; even during icehouse conditions, rivers with sufficient sediment supply (of which there were probably several globally at any time) would still have been able to create highstand shelf-edge deltas in exactly the same way that they are in the process of doing during the present Holocene interglacial highstand. It is also worth pointing out that various studies of Holcene shelves demonstrate significant sand-bypass across shelves into deep water during interglacial highstands (e.g., Weber et al., 1997; Covault et al., 2007; Boyd et al., 2008). All of which indicates that explanations or predictions of strata geometries should be made on the basis of both accommodation and supply, not accommodation alone, even during icehouse intervals.
Sømme et al. did not cite Burgess and Steel (2008), but their work repeated and vindicated many of the conclusions from that earlier work. However, Burgess and Steel went further than Sømme et al. and also investigated, albeit with a small number of model runs, the impact of variable sediment supply on clinoform topset width. They demonstrated that while other aspects of stratal geometry probably reflect a more equal balance of control by accommodation variations and supply variations, magnitude and variability of sediment supply is likely to be the primary control on shelf width, a result recently supported by Carvajal et al. (2009). Consequently, it is possible that initial progradation of clinoforms into unfilled accommodation will tend to set a topset width such that repeated progradation to the shelf edge is likely, even in the absence of relative sea-level falls. Burgess and Steel (2008) referred to this as the shelf-edge lock-in effect, and it is a hypothesis that should be further tested, especially given the predictions it makes for the nature of sediment delivery to deep-marine slope and basin floor systems.