Weissmann et al. (2010) discuss what they term “distributary fluvial systems” (DFS) to address a key question in the earth sciences: what modern systems are appropriate analogues to the rock record? They conclude “Since DFSs dominate depositional patterns in all continental sedimentary basins, this style of deposition may represent the norm in the continental rock record, with axial and incised river deposits composing a relatively minor proportion of the succession” (p. 42). This bold claim implies that most current fluvial facies models have limited relevance to the interpretation of ancient deposits. We argue herein that this significant assertion is unsupported.
The Weissmann et al. paper lacks clarity concerning the term “continental sedimentary basins,” stating “We include only purely continental sedimentary basins, excluding review of fluvial systems in marine marginal basins” (our emphases; p. 39). Marine marginal is not defined, and the qualifier “purely continental” is not used in either title or text. Thus the impression is given that the conclusions relate to the vast majority of river basins. The term marine marginal appears to include any river system that exits to the sea, and examination of their figure 1 and table DR1 shows their analysis excludes all of Earth's large rivers. Which rivers are considered is central to their conclusions regarding what constitutes the norm in the rock record.
The idea that degradational terrains do not equate to preservation is flawed, as preservation is also linked to local/regional subsidence and other base-level changes (this is true in both upland and marine marginal environments). In the Nile Valley, incision due to the Messinian salinity crisis triggered formation of the Eeonile, a large-scale degradational valley. According to Weissmann et al., the degradational nature of this basin would preclude preservation, yet subsequent sea-level rise led to ∼1500 m of fluvial aggradation (Said, 1993). Downstream reaches of the Brahmaputra River, where the river is clearly tributive, have preserved ∼80-m-thick Holocene fluvial deposits above underlying lowstand gravels (Goodbred and Kuehl, 2000) and thus also show preservation in non-DFS settings. Up to 100 m of Quaternary fluvial sediments are preserved in the lower Mississippi Valley (Saucier, 1994) and large underlying thicknesses of ancestral-Mississippi fluvial sediments are present that have accumulated over ∼300 m.y. (Potter and Hamblin, 2006). Late Tertiary to late Quaternary fluvial sediments, up to 500 m thick, are also found along the Texas Coastal Plain (Galloway, 1981). Thus the reality of preservation in the Earth's large river basins in tributary settings does not match the author's claim that “…tributary rivers set in degradational settings….have very limited preservation potential” (p. 39).
The largest river Weissmann et al. analyze, the Pilcomayo megafan, has a channel width of ∼300–1000 m and a depth of ∼10 m. However, far larger rivers are found in the rock record (Fielding, 2007) that would, adopting an uniformitarian approach, not be present as DFS. It is hard to reconcile this question of large ancient channel size with the authors’ claim that DFS would be the norm in the continental rock record. The issue of preservation potential is not considered by the authors, and only snapshots from Google Earth© are presented, yielding extrapolations about the relative importance of DFS over axial rivers. For the Basin and Range Province, Weissmann et al. state “Axial rivers, often held between opposing DFSs, cover a relatively small area of the basin…” (p. 41) yet no consideration is given to temporal evolution (i.e., the volumetric rather than areal extent is key). Gawthorpe and Leeder (2000, their figures 6A–6D) show sequences of basin evolution that demonstrate how fans influence axial river location (as described by Weissmann et al.), but, importantly, they also illustrate how the axial river can migrate laterally, eroding fan deposits and reducing their preservation potential.
The authors give four diagnostic criteria for the recognition of DFS, and argue these are “…different from those of rivers in degradational settings…” (p. 39). However, these criteria are not unique to DFS and are thus difficult to apply. Of the criteria listed, only the first—“radial pattern of channels from the DFS apex”— (p. 42) would appear to be truly diagnostic of DFS, and indeed what all previous studies have called simply an alluvial fan. The other criteria also apply to other rivers in tributive and degradational settings: (1) downstream decreases in channel size are common in semi-arid rivers; (2) most rivers show a downstream decrease in grain size; and (3) many rivers possess broad floodplains thus are not laterally confined, with the Earth's largest rivers largely unconfined over thousands of square kilometers. It is unclear what the “…significant differences in sedimentary patterns…” (p. 42) that exist between DFS and other rivers would be, as these overlapping criteria invalidate the differences they contend may be so important.
In summary, we contend the conclusion of Weissmann et al., that deposition from DFS represents the norm in the continental rock record, cannot be sustained.
