‘Base profile’: a unifying concept in alluvial sequence stratigraphy

Abstract

Abstract The sequence stratigraphy of sedimentary strata is governed by the creation and removal of accommodation space. However, current sequence stratigraphic models do not properly account for changes in accommodation space in the alluvial environment. In order to rectify this, a concept is proposed here called base profile. Base profile can be used to describe and explain the deposition and removal of fluvial sediments analogous to the way the term base level (relative sea-level) is used in the sequence stratigraphic analysis of coastal and marine strata. Base profile is the surface measured relative to a chronostratigraphic datum within a drainage basin to which rivers would regrade were conditions to remain constant. It represents the continental extension of base level. Changes in accommodation space occur if base profile rises or falls as a result of (a) subsidence or uplift, (b) variations in sediment supply, (c) variations in river discharge, (d) eustatic fluctuations, and (e) progradation or retrogradation of the coast. During a rise in base profile, coarse-grained siliciclastic sediment is trapped on the alluvial plain due to fluvial aggradation and mostly only fine-grained sediment will reach the marine environment. A fall in base profile is associated with fluvial degradation and maximum sediment input to the marine environment. Base profile forms a simple basis for describing, correlating and interpreting alluvial strata. Simple models are developed here using base profile which show how different continental and marine processes can lead to the deposition of laterally extensive, coarse-grained fluvial strata.