--> The Dark Art of Palaeobathymetry: How Can We Reconstruct the Shape of the Sea Floor in Structurally Active Regions?
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The Dark Art of Palaeobathymetry: How Can We Reconstruct the Shape of the Sea Floor in Structurally Active Regions?

Abstract

Seafloor topography is an important control on turbidite reservoir distribution. However, there are major problems with techniques used to reconstruct seafloor topography in structurally active settings. We commonly assume that palaeobathymetry of a surface mirrors the isopach of the overlying interval (thicks = lows, thins = highs). But the present day seafloor within major sediment fairways commonly shows little or no evidence of the structures active beneath them, which are effectively swamped. Where sediment flux is lower, structures may have sea floor expression. Worse, the isopach approach is philosophically flawed – we assume that the upper surface of the interval was unstructured to deduce the topography of the lower surface, then we use the isopach of the interval above that to deduce the structure of the upper surface; but this invalidates our previous assumption! A way forward requires a deeper understanding of the structural and depositional history. Structure growth is typically continuous, whereas sedimentation tends to be pulsed. Forward modelling of these processes gives a synthetic stratigraphic architecture and predicted bathymetry. By adjusting the rates so that synthetic and observed stratal architectures match, we can derive a model for the palaeobathymetry through time. This can be refined using seismic facies and images of depositional systems. Modelling results show that simple isopach-based bathymetry is a poor approximation. The shape of the seafloor changes through the depositional pulse; our models predict that during the peak of reservoir deposition, basins may have near-flat bottoms, a gently-dipping onlap/offlap fringe, and a more steeply-dipping perimeter.