DOWNSTREAM CHANGES IN ALLUVIAL ARCHITECTURE - AN EXPLORATION OF CONTROLS ON CHANNEL-STACKING PATTERNS

Various, but related, models have been proposed to explain the archite ctural arrangement of channel stacking patterns in avulsion-dominated alluvial sequences. The early models published by Leeder, Alien, and B ridge (LAB) addressed the role of changes in sedimentation rate (a pro xy for subsidence rate) as a control on stacking patterns. The models decouple avulsion frequency from sedimentation rates, resulting in an inverse relationship between stacking density (or interconnectedness) and sedimentation rates. A key element missing from these models is th e likely dependence of avulsion frequency on local sedimentation rate within the active channel belt. We consider a simple model whereby avu lsion takes place only when a minimum, critical, relief is developed b etween a channel bank and the adjacent flood plain. If avulsion freque ncy increases at rates slower than the increase in sedimentation rate, then stacking density increases with decreasing sedimentation rate, s imilar to that predicted by the LAB models. However, if avulsion frequ ency increases linearly with sedimentation rate, then there is no chan ge in stacking pattern with changes in sedimentation rate. If avulsion frequency increases faster than sedimentation rates, as seen in some data sets, then stacking patterns become more dense with increasing se dimentation rates, a result that is the exact opposite of that predict ed by the LAB models. Therefore sensitive dependence on the relationsh ip between avulsion frequency and sedimentation rate calls into questi on the veracity of some previous interpretations of relative subsidenc e made in alluvial architecture studies. We provide an alternative, si mple geometric model that links changes in subsidence rate to downstre am rate of change in stacking pattern as seen in three dimensions with in sedimentary basins. Other controls that are considered include: the geometry of subsidence; whether avulsions take place locally along a river or regionally affect the basin; whether local sedimentation rate or flow depth controls the thickness of sand bodies; and the exact re lationship between avulsion frequency and sedimentation rate. The prim ary result of the model is that subsidence strongly influences the rat e at which alluvial architecture changes in the downstream direction, but other controls dictate whether the stacking pattern becomes more d ense or less dense downstream. Hence, we suggest that subsidence exert s an influence on stacking patterns not necessarily evident in individ ual vertical sections, but may be recorded in three dimensions as down stream changes in alluvial architecture. Unfortunately any model of al luvial architecture in avulsion-dominated sequences is limited by our lack of understanding of the processes controlling avulsion. As a resu lt any model of alluvial stacking patterns is at best a working hypoth esis that should not be taken as proof of changes in tectonic subsiden ce rates or sea-level changes.