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Inherent Organisational Time Scales of Sedimentary Systems


On geological timescales, sedimentary systems generate geometrically simple stratigraphic successions that essentially balance the subsidence profile of basins. This contrasts with the internal organisation of such successions, which display complex stacking patterns of depositional elements that are governed by the morphological controls on the sedimentary system. Yet at some scale their stacking becomes organised in accordance with the subsidence of the basin. Short-term complexity of sedimentary systems is therefore embedded within the geometrically simple patterns of long-term stratigraphic evolution. Developing an understanding of the controls upon the transition from short-term complexity to long-term simple geometrical style and upon characteristic transition time scales would allow us to define over what time-ranges geological models require sedimentological short-term complexity to remain realistic, and the time-ranges when simple stratigraphic modelling suffices. Recently, statistical techniques have been developed that quantify how the variability in deposition and stacking pattern of depositional elements changes with increasing size of the observation window. These new techniques thus offer a means to verify the validity of the hierarchical approach of stratigraphy, when based upon controlled stratigraphic laboratory experiments. Here we apply these statistical techniques to four high resolution analogue modelling datasets of progradational shelf clinoforms that formed under different combinations of tectonic and sea level forcing. The models were generated at the Eurotank Facility, Utrecht University, the Netherlands. The aim is to establish the robustness of the novel statistical techniques, and probe some of the intricacies of their application to stratigraphic patterns. The statistics indicate that the model systems self-organised from a 0.5 h random avulsion input signal to long-term simple geometric stratigraphy at the organizational timescale of 10 – 30 h, which suggests an inherent timescale to the sedimentary system. This random input signal also changed structure as the fluvial system became increasingly compensational downdip. Additionally, base level variation influenced the organizational time scale of stratigraphy: high aggradation rates during transgression decreased the required timescales, whereas low aggradation rates during regression increased them.