The seismic expression of synsedimentary channel features within the Chalk of southern England

Abstract

Onshore seismic reflection data to the north of Bournemouth, reveal a series of troughs or depressions (concave-up) and mounded (concave-down) structures within the Upper Chalk. Onlap, downlap and truncation of reflections is observed and the structures lie in close proximity to faults in the underlying Jurassic strata. It is probably the first time that such prominent and significant erosive channel structures have been imaged so clearly and described on seismic reflection data from onshore UK. Similar stratal geometries observed in the Chalk at crop are interpreted as channels formed by submarine erosion and redeposition, during relative sea-level falls. We interpret the structures described here as of primary syndepositional submarine erosion of the Chalk, producing channels, the infill to which may be slumped. Eustatic global sea-level changes are often cited as the cause of anomalous Chalk thicknesses and structures. Increasingly, however, such features are being linked to syndepositional tectonics which were clearly affecting Chalk deposition in surrounding basins. The structures described here may provide important information in the debate about whether late Cretaceous tectonics locally contributed to sea-level changes which affected Chalk sedimentation in the Anglo-Pads Basin, and that considering just one external control (such as eustasy) is oversimplified.