MOLYNEUX, S. J. M., J. A. CARTWRIGHT, and L. LONERGAN, T. H.

Huxley School of the Environment, Earth Sciences and Engineering, Imperial College of Science, Technology and Medicine, South Kensington, London

Abstract: Giant Sedimentary Cone Sheets: Evidence for Major Post-Depositional Sandstone Remobilisation

We describe a new class of giant transposition structure(sedimentary intrusion), recognised on 3-D seismic surveys from the North Sea, specifically UK blocks 21/4 and 5. These concave upward conical sandstone intrusions are termed sedimentary cone sheets. They are typically circular to elliptical in map-view with diameters of single cones ranging from 0.5 to 2 km. Their sides are 100-300 m tall, and dip between 5 and 15 degrees (compacted). The cone sheets can be isolated or developed as compound structures amalgamated in clusters covering hundreds of square kilometres.

We considered several possible genetic models for these structures, including palaeopockmark craters, collapsed gas hydrate diapirs, and even isolated fan channel ribbon deposits. However, the preferred explanation is that upward migration of thermogenic methane generated from Jurassic source rocks released during earthquake or by auto-triggered failure of pressure seals led to inflated fluid pressures in isolated Palaeocene or Eocene deep water sandbodies. Catastrophic pressure inflation (internal blow-out) then led to large volume liquefaction of the sands, and upward conical injection which exploited pre-existing fault and fracture networks (polygonal faults), in the low permeability claystones of the Lower Eocene. We believe that these Lower Eocene intrusions are the largest reported examples of clastic remobilisation of transposition, which by their very scale may have been overlooked or misinterpreted in outcrop in other basins.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California