Abstract
AbstractFormation of alluvial stratigraphy is controlled by autogenic processes that mix their imprints with allogenic forcing. In some alluvial successions, sedimentary cycles have been linked to astronomically‐driven, cyclic climate changes. However, it remains challenging to define how such cyclic allogenic forcing leads to sedimentary cycles when it continuously occurs in concert with autogenic forcing. Accordingly, we evaluate the impact of cyclic and non‐cyclic upstream forcing on alluvial stratigraphy through a process‐based alluvial architecture model, the Karssenberg and Bridge (2008) model (KB08). The KB08 model depicts diffusion‐based sediment transport, erosion and deposition within a network of channel belts and associated floodplains, with river avulsion dependent on lateral floodplain gradient, flood magnitude and frequency, and stochastic components. We find cyclic alluvial stratigraphic patterns to occur when there is cyclicity in the ratio of sediment supply over water discharge (Qs/Qw ratio), in the precondition that the allogenic forcing has sufficiently large amplitudes and long, but not very long, wavelengths, depending on inherent properties of the modelled basin (e.g. basin subsidence, size, and slope). Each alluvial stratigraphic cycle consists of two phases: an aggradation phase characterized by rapid sedimentation due to frequent channel shifting and a non‐deposition phase characterized by channel belt stability and, depending on Qs/Qw amplitudes, incision. Larger Qs/Qw ratio amplitudes contribute to weaker downstream signal shredding by stochastic components in the model. Floodplain topographic differences are found to be compensated by autogenic dynamics at certain compensational timescales in fully autogenic runs, while the presence of allogenic forcing clearly impacts the compensational stacking patterns.
Highlights
Alluvial deposition is controlled by both autogenic and allogenic controls (Abels, Kraus, & Gingerich, 2013; Hajek, Heller, & Schur, 2012), which are difficult to be disentangled since they act at overlapping spatial and temporal scales (Bridge, 1993; Stouthamer & Berendsen, 2007)
We evaluate the impact of cyclic and non-cyclic upstream forcing on alluvial stratigraphy through a process-based alluvial architecture model, the Karssenberg and Bridge (2008) model (KB08)
Our objective is to evaluate the effect of cyclic and non-cyclic upstream forcing on alluvial stratigraphy, and we follow a simple relation between sediment load and water discharge based on Syvitski, Morehead, Bahr, and Mulder (2000): Qst = aQbw+t 1 (4)
Summary
Alluvial deposition is controlled by both autogenic and allogenic controls (Abels, Kraus, & Gingerich, 2013; Hajek, Heller, & Schur, 2012), which are difficult to be disentangled since they act at overlapping spatial and temporal scales (Bridge, 1993; Stouthamer & Berendsen, 2007). Allogenic controls on a fluvial depositional environment refer to changes in upstream and downstream conditions, such as climatically driven water discharge and sediment supply variation, tectonically driven basin subsidence and exhumation, and climatically- and/or tectonically driven base-level fluctuation. Based on the equilibrium scenario, we create long-term accommodation through steadily rising the base level at a rate of 0.4 m/kyr, which matches the long-term depositional rate that can be approximated by dividing Qs0 (1.0 × 106 m3/year) over the basin size (60 km by 40 km) In this scenario, stratigraphy is built up under full autogenic controls, since there are no variations in the water discharge and sediment supply at the basin inlet. A transport system is able to visit every spot in a basin repeatedly during an interval longer than this timescale (Straub et al, 2009; Wang et al, 2011)
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