Tributary, distributive and other fluvial patterns; what really represents the norm in the continental rock record (invited review)

This paper evaluates the recent claim made by Weissmann et al. (2010) that deposits of “Distributive Fluvial Systems” (DFS) may form the bulk of the continental fluvial record. Weissmann et al. (2010) define DFS as “a pattern of channel and floodplain deposits that radiate outward from an apex that is located where the river enters the sedimentary basin”. As such, DFS are fan-like systems on which multiple channels coexist and distribute water and sediment across the fan. Published criteria for the recognition of DFS (Weissmann et al., 2010) are 1) a radial pattern of channels from the DFS apex; 2) downslope decreases in channel size; 3) down-DFS grain-size decreases; and 4) a lack of lateral channel confinement. However, in Weissmann et al. (2010) and subsequent papers, only the first of these criteria is applied rigorously, thus allowing a variety of types of fluvial system (including avulsive, incised, anabranching, and coastal plain distributary systems) to potentially be classified uncritically as DFS. An ancient succession formed by DFS should preserve mostly small channel bodies, with a limited range of dimensions and no outsized channel bodies. Channel bodies in DFS-dominated successions should also decrease in size in a down-palaeoslope direction and show palaeocurrent relationships indicative of radial dispersal. In order for the term DFS to be applied, these characteristics should also be established within stratigraphic bodies that formed coevally. However, very few examples have yet been published that satisfy these criteria. If the claim that DFS dominate the alluvial stratigraphic record is valid, then few, if any, sedimentary bodies formed by large channels should be preserved. A review of published research demonstrates, however, that the alluvial stratigraphic record contains a broad diversity of fluvial network styles, including abundant representatives of tributary, avulsive, anabranching, and incised, in addition to distributary types. It is also clear that the deposits of large, main stem rivers that are comparable to the big rivers of the modern world are abundantly preserved in the stratigraphic record. An analysis of the planimetric area of modern tributary and distributary fluvial systems is presented, demonstrating that tributary systems are also likely to represent a far larger proportion of the ancient sedimentary record than DFS. The body of evidence presented herein from modern and ancient alluvial systems thus suggests that the hypothesis of Weissmann et al. (2010) should be wholly rejected. DFS do not dominate the continental stratigraphic record