Environmental signal propagation, preservation and shredding in sedimentary systems

Abstract

Sediment flux is an important control on the morphology of sedimentary landscapes and stratigraphy as it sets the rates and scales of landscape dynamics. For example, sediment flux influences the shape and style of river channels, the avulsion frequency of rivers, and the position of shorelines. Over long timescales, the sediment flux through sediment routing systems relates to environmental forcing conditions such as climate and tectonics, which set rates of catchment erosion and sediment transport. This makes stratigraphy accumulating in sedimentary basins record a potential archive of past climatic and tectonic conditions that could inform our understanding of current and future environmental change. However, we may not always be able to identify environmental signals in the stratigraphic record because of stochastic processes in sedimentary systems (autogenics) that have the potential to obscure the transfer of environmental signals (allogenics) to landscapes and stratigraphy. Previous work demonstrates that autogenic landscape processes act as a low-pass filter on the transfer of environmental signals to the stratigraphic record, but signal amplitude must play a role too. A quantitative understanding of the conditions required to archive environmental signals is essential to underpin interpretations of these signals in the stratigraphic record. The aim of this thesis is to develop a quantitative theoretical basis that can be used to assess the stratigraphic record as an archive for allogenic signals of varying sediment flux. First, I present a new theoretical framework that predicts a time-dependent magnitude threshold for the transfer of allogenic sediment supply signals to the stratigraphic record. The minimum signal amplitude is set by autogenic processes and decreases as an exponential function of signal duration. This new framework is supported by physical delta experiments specifically designed to test the framework. The threshold was constructed using an experiment forced with constant sediment supply rate and tested with four new experiments with similar forcing conditions, but cyclic sediment flux. Signals with a combination of cycle magnitude and duration that exceed the threshold are transferred to the stratigraphic record, while signals that fall below the threshold are not. The threshold framework relies on high-resolution measurements of autogenic sediment fluxes, which are usually not available in stratigraphic studies. However, the exponential decay of the threshold can be approximated from long-term accumulation rates and an estimate of timescales at which autogenic processes level out. This approximation is applied to field-scale examples of the Pleistocene Kerinitis delta (Greece) and the Eocene Escanilla sediment routing system (Spain) to test whether Milankovitch-scale sediment supply cycles could realistically have been preserved in the stratigraphic record of these systems. The delta experiments were also used to study how sediment supply cycles of different magnitudes and durations influence delta morphodynamics. The stochastic variability of autogenic processes obscures most theoretical relationships between sediment flux and morphodynamic processes. However, the experiments demonstrate that signals with a high rate of sediment supply change (acceleration) are likely to push existing morphology out of equilibrium, and thereby generate cyclicity in landscape evolution. Slowly accelerating signals increase channel depth, but do not affect floodplain morphology or the number of co-existing channels, like quickly accelerating channels do. Quickly accelerating highfrequency signals, however, may not leave thick enough deposits to withstand erosion prior to permanent burial, and so their preservation potential is limited. The results of this thesis: (1) provide a theoretical framework that predicts the scales of sediment flux signals we may expect to find in the stratigraphic record; (2) detail how the threshold framework can be applied to the rock record, using datasets of limited temporal resolution; and (3) inform earth scientists of landscape response to sediment supply signals with different combinations of signal magnitude, duration and acceleration. As such, this work contributes to a more quantitative understanding of the effects of environmental signals on landscapes and the stratigraphic record.