Abstract
River systems play an important role in the filling of sedimentary basins and record the history of external forcing processes, such as climate, tectonics and sea-level change, acting on them. They are potential reservoirs for oil, gas and water, and can host coal and placer mineral deposits. Because of the
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complex interplay between the external forcing processes, however, understanding of the genesis of the stratigraphy of river systems and interpreting the stratigraphy correctly is far from straightforward. Current conceptual models are oversimplified, and more insight into the impact of external forcing processes must be gained to improve these models.
The aim of this study was to assess the impact of climate (i.e., discharge and sediment influx) on the development of the large-scale stratigraphic architecture of river systems, in isolation and in conjunction with sea-level fluctuations, through an analogue modelling approach. Analogue physical models reproduce the long-term average effects and products of the transport processes in a river system, rather than the transport processes themselves. An advantage of analogue modelling over numerical modelling is that it is hard to make the model fit preconceived notions about the results, making it possible to test and develop conceptual models.
The impact of climate (i.e., discharge and sediment influx) on the large-scale stratigraphic architecture of river-delta-shelf-basin systems appears not to be as dominant as the impact of sea-level change, but it does significantly affect the smaller scale stratigraphic architecture, such as the relative size of systems tracts and the rate and extent of erosion.
Furthermore, we found a fundamental difference between the impact of changes in discharge and the impact of changes in sediment influx on the yield and mass accumulation at the mouth of a river system. River systems can act as buffers for rapid changes in sediment influx, while they react very rapidly to changes in discharge. Thus, the small-scale stratigraphy at the river mouth is controlled mainly by changes in discharge, and the large-scale stratigraphy is controlled by changes in sediment influx (and sea-level fluctuations). Also, because the response of the river gradient to an increase in discharge is the opposite to its response to an increase in sediment influx (and vice versa), the mass accumulation at the river mouth, combined with the overall stratigraphic architecture of the system, can be used to constrain paleo-discharge and paleo-sediment influx scenarios.
Finally, our experiments show that a complex stratigraphic architecture is not necessarily the result of complex forcing, but can result from very simple changes in discharge.
To assess the development of the stratigraphy in physical models, a new method for processing the data obtained in the experiments was developed. Series of subsequent digital elevation models of the surface of the model are converted, using custom-made software, into synthetic three-dimensional stratigraphy, containing true isochronous surfaces. This data set contains the development of the system through time in three dimensions, and can be presented in various formats, such as geological maps, geological sections and Wheeler diagrams.
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