Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, Southeastern Utah, U.S.A

Abstract

Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges-Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth.Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Crosssectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a singlestory body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in an ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.

The sedimentology of river confluences

“…() and Ullah et al . () in the initial stages of fill. However, while set thickness may be greatest at such sites, the deposits of channel confluences may be reworked, so that single sets will not dominate the fill, and average set thickness will be an order of magnitude less than channel depth.…”

“…This may result in a profile similar to that suggested by Ullah et al . (), although generated in a different way.…”

The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery

“…Therefore, in actively migrating confluence zones, the sedimentary product may likely comprise multiple stacked, truncated deposits of differing orientations that may prove difficult to interpret except for the most recent depositional phase. Such a complex, overlapping sequence of scour and fill would suggest the recent model proposed by Ullah et al (2015), where the scour fill comprises a single large set, is not necessarily representative of potential confluence scour preserved in alluvial stratigraphy. The mobile confluences described herein share a sedimentological character more in common with the model proposed by Siegenthaler and Huggenberger (1993), where multiple erosion surfaces are viewed as a defining characteristic.…”

“…River confluences have the potential to create some of the points of deepest incision into underlying sediments (Mosley, 1976;Best, 1988;Bristow et al, 1993;Salter, 1993;Siegenthaler and Huggenberger, 1993;Best and Ashworth, 1997;Miall and Jones, 2003;Ullah et al, 2015) and hence their subsequent fill has been argued to possess the highest preservation potential of fluvial channels (Huber and Huggenberger, 2015). Since the depth of junction scour and mobility of the confluence are determined by flow processes in the confluence hydrodynamic zone (Best and Rhoads, 2008), it can be argued that differing junction dynamics may produce a range of characteristic confluence zone sedimentology from sandy bar development to mudfilled scours.…”

Architecture, paleosols and cyclicity of the Middle- Late Pennsylvanian proximal fluvial system (Nyřany Member, Pilsen Basin, Czech Republic)

“…Medium-scale tangential trough cross-beds reflect migration of curved crested gravel-sand dunes that cover lower bar accretional surfaces (Gustavson 1978, Miall 1996, Best et al 2003, whereas sigmoidal trough crossbeds and planar cross-beds reflect avalanche surfaces of unit bars (Bridge 1993b, Bridge & Tye 2000. Large-scale sets of exceptionally thick planar cross-beds represent chute and tributary mouth bar surfaces or delta lobes at channel confluence zones (e.g., Bristow & Best 1993;Bridge 1993a, b;Ullah et al 2015). Smaller scale concave-up channel shape bodies filled by simple sets of large-scale inclined strata (Fig.…”