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Keywords:

  • Basin modelling;
  • experimental sedimentology;
  • sequence stratigraphy;
  • unconformity

Abstract

Experiments have been carried out in a model basin 16 × 1·2 × 0·9 m to address the effect of base-level and discharge changes on actively growing alluvial-shoreline wedges. Two distinct types of erosive surfaces were investigated: one produced by base-level fluctuations in the coastal zone and the other by discharge and supply fluctuations in the upstream alluvial basin. In the first experiment, three similar base-level cycles were simulated keeping sediment supply and basin tilting constant within each cycle, and changing discharge from one cycle to the next. In the second experiment, rises and falls of base level were instantaneous and discharge changes were in phase (high discharge linked to high water levels in the basin). In the third experiment, base level and discharge changed gradually, at different rates and they were out of phase, resembling a typical glacio-eustatic cycle in which sea-level rises and falls are linked to increased and decreased discharge, respectively. The resulting stratigraphy of the alluvial to deltaic sedimentary wedge was analysed in terms of the development of unconformities and the evolving depositional geometry. An intervening decoupled zone between parts of the model basin dominated by alluvial processes and that at the coastal zone is identified. Within this decoupled zone, unconformities in the alluvial succession tend to vanish basinward, and base-level generated coastal unconformities disappear landward. The two types of unconformity can be generated at different times during a glacio-eustatic cycle, and it is thus erroneous to correlate them, even though they may appear to form a continuous surface. Unconformities within the modelled stratigraphy do not constitute time lines nor do they consistently separate younger from older beds, as they require a significant time to form and they have thick sedimentary packages as depositional correlatives. The experiments also support a fourfold division of sequences, showing the development of a significant sedimentary package during base-level falls.