We thank Sambrook Smith et al. (2010) for their Comment and the opportunity to expand on points presented in our paper (Weissmann et al., 2010a). For further documentation and discussion of distributive fluvial systems (DFS) we refer readers to Hartley et al. (2010) and Weissmann et al. (2010b). As noted by Sambrook Smith et al., we avoided discussing marine marginal fluvial systems. We took this approach because we wished to focus on sedimentary basins exempt from direct marine influence and to remove ambiguity associated with accommodation generated by a marine base-level rise. Sambrook Smith et al. suggest that degradational terrains are preserved in the rock record and discuss a number of examples. We fully acknowledge the importance of incised valley fills due to sea-level change in the rock record (Weissmann et al., 2010b). It is not clear, however, whether the examples of the ancestral Mississippi and Tertiary of the Gulf Coast, highlighted by Sambrook Smith et al., were distributive or tributive in nature. Valley-fill deposits are preserved as the initial phase of aggradation in these basins; however, we would argue that as the basin fills, the depositional pattern becomes distributive. We also argue that, with the exception of incised valley fills, fluvial deposits in degradational terrains are very rarely preserved within fluvial basin-fill successions, and that DFS dominate the rock record.
Sambrook Smith et al. were concerned that large rivers are excluded from our study. Additionally, they make the point that many thick, regionally extensive fluvial deposits are preserved in the rock record and suggest that the largest system we illustrated, the Pilcomayo, is too small to develop the far larger rivers that they state are found in the rock record. The authors miss our point here in two ways. First, we did not focus on specific rivers but presented overall depositional patterns observed in continental sedimentary basins. We observed that many of the large rivers are not present in actively aggrading sedimentary basins and therefore have limited preservation potential. Where large rivers do enter sedimentary basins, they develop DFS (see examples documented in Hartley et al., 2010) and lie in axial positions in the basin (e.g., Brahmaputra and Ganges Rivers, Himalayan Foreland Basin; Paraná/Paraguay River system, Andes Foreland Basin). Thus, these large rivers may be represented in the rock record. Second, the dimensions of fluvial deposits preserved in the rock record will not equate directly to the present-day dimensions of an individual channel system. Thick, laterally extensive fluvial sediments will form as a result of the avulsion/migration of a channel system(s), and may be one of the reasons why “large” river deposits have not been identified. For example, it is clear from satellite imagery and fieldwork that past shifts of the Pilcomayo fluvial system have produced an extensive (up to 60-km-wide) belt of fluvial sands up to 100 km downstream of the apex. Further, avulsion and amalgamation with adjacent DFS will produce sheet-like sandstone bodies with dimensions greater than those discussed by Fielding (2007). We argue that most sandstone bodies described in the rock record and attributed to large fluvial systems were deposited either as part of a DFS or an axial system fed by DFS.
Sambrook Smith et al. suggest that we do not consider the issue of preservation potential, and refer to the half-graben facies model of Gawthorpe and Leeder (2000). A study of the Palomas half-graben by Mack et al. (2002) noted that the axial ancestral Rio Grande covered a relatively small part of the basin fill, as it was constrained by both hangingwall- and footwall-derived DFS. Thus, where rock record examples are available, they support our assertion that DFS will dominate the stratigraphic record of fluvial deposits.
The criteria we present for DFS recognition do have some overlap with those of tributive systems. The radial pattern of channels from an apical point is a distinctive characteristic, although we disagree that most workers would consider the DFS we illustrate to be alluvial fans. Importantly, the downstream decrease in channel size occurs in all climate regimes (not just semi-arid or arid systems) and is an order of magnitude larger than that described for tributive semi-arid rivers.
A key point that we maintain is that in order to understand fluvial systems preserved in the rock record, it is important when using modern analogues to examine areas of the Earth's crust that are subsiding and will therefore be preserved. We do not wish to imply that “the majority of current fluvial facies models are of limited relevance to the interpretation of ancient sediments” (Sambrook Smith et al., 2010). At the scale of the channels and their fill, these descriptions and interpretations provide a very valuable body of literature. However, what we believe is missing in the literature on fluvial systems is an understanding of the larger-than-channel belt and basinal context in which fluvial systems are developed, and it is at this larger scale that we believe the DFS paradigm has its greatest value.
