We thank Burgess (2010) for his Comment to our Geology paper (Sømme et al. 2009b). We will address the issues raised by Burgess, and at the same time use this opportunity to emphasize and expand on some of our main ideas and arguments presented in Sømme et al. (2009b).
Burgess refers to earlier work by Burgess and Hovius (1998) and Muto and Steel (2002) who calculated the minimum and maximum time required for modern deltas to prograde across their respective shelves during the present highstand. Although such calculations are associated with uncertainties related to redistribution of sediments by wave and current processes on the shelf, it seems quite reasonable that several modern deltas theoretically could reach the outer shelf during highstand conditions, as seen by the most recent lobe of the Mississippi Delta. Evidently, many delta systems bypass sandy sediment to the slope and basin floor during the present highstand. The majority of this sediment flux is promoted by shelf-indented submarine canyons (e.g., the Congo system; Eisma and Kalf, 1984), although sediment is also transported across the shelf by current processes (see discussion and list of some Holocene bypass systems in Sømme et al., 2009a).
Despite large variations, the “average” depth of the modern shelf edge is ∼130 m below present sea level (Shepard, 1973), and there is little evidence from the outer shelf sedimentary record suggesting that Quaternary deltas frequently prograded to the outer shelf during previous highstand periods. If they did, outer shelf bathymetry should be significantly more variable, and in places quite shallow due to the drowning of thick delta lobes. This is not to say that icehouse highstand shelf-edge deltas cannot form, but the relatively consistent depth of the present shelf edges indicate that shelf-edge deltas primarily formed during falling and lowstand of sea level.
The many factors controlling overall delta progradation and shelf morphology can be visualized as a balance with multiple arms of different lengths, each arm corresponding to an auto- or allogenic-forcing mechanism that is likely to affect the system in any way. Each system has a unique combination of force applied to the different arms, and the ratio between these will change at all times. On a global scale, high Holocene sea level combined with a relatively long period (∼1 m.y.) of high-amplitude eustatic fluctuations has resulted in high shelf volumes that are not representative of the geological past (Mesozoic and Cenozoic; Miller at al., 2005), thus putting so much weight on the eustatic sea level arm of the balance that other parameters have been overshadowed. Exceptions may include many of the systems discussed by Burgess and Hovius (1998) and Muto and Steel (2002), in addition to others, where the sum of one or several forcing parameters override the effect of eustasy.
What we attempt to demonstrate with our paper is that the many arms of the multidimensional balance may have been more competitive during greenhouse times. That is, lower eustatic amplitudes operating at perhaps different frequencies may have allowed other controlling factors to play a more significant role in controlling the behavior of delta systems. A consequence of this is a different stratigraphic architecture during greenhouse times, and it may therefore be difficult to apply models of system behavior that have been conditioned to icehouse systems to the greenhouse world.
Burgess (2010) emphasizes a recent paper (Burgess and Steel, 2008) that nicely demonstrates how the delta topset width (coastal plain and shelf) is closely related to sediment supply. We expand on this and argue that the first-order morphology of every segment in the source-to-sink system (i.e., the fluvial system, the shelf, the slope, and the basin floor fan) are genetically related, as shown by Sømme et al. (2009a) who compared parameters from a number of sub-modern, source-to-sink systems. As such, the shelf should not be considered as a separate geomorphic feature, but it should be regarded as part of the larger integrated source-to-sink system. Large drainage basins tend to have high sediment supply and wide, low-gradient alluvial/coastal plains. As the shelf merely represents the drowning of a lowstand alluvial/coastal plain, it then follows that the shelf width is closely related to the hypsometry and the sediment supply in the associated drainage basin.
On geological time scales, the width of a sedimentary shelf is conditioned to the average position of the “balance” that links all factors controlling shelf morphology and delta progradation. Hypothetically, if the Quaternary icehouse period was to be followed abruptly by a greenhouse period characterized by eustatic amplitudes of, say, ∼40 m, one might expect a sudden narrowing of the shelf. But because the shelf width is always related to the sediment supply and gradient of the upstream alluvial/coastal plain, greenhouse progradation and retrogradation across a shallow shelf is expected to result in flattening of the alluvial/coastal plain-shelf gradient until the system adapts to the new boundary conditions (assuming quasi-steady sediment supply). However, as eustatic amplitudes were less dominating relative to other forcing mechanism during greenhouse times, greenhouse delta-shelf systems are expected to be more variable, and more susceptible to local variations and small perturbations in forcing parameters other than eustasy. We therefore agree with Burgess (2010) and Burgess and Steel (2008) that the interaction between the various controlling parameters determining shelf stratigraphy, and how the balance between these have changed over time, probably is more complex than is often assumed.