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World’s Largest Extrusive Sand Body?

Løseth, Helge *1; Rodrigues, Nuno 1; Cobbold, Peter R.2
(1) Statoil ASA, Trondheim, Norway.
(2) Geosciences-Rennes, Rennes, France.

Using 3D seismic and well data from the northern Norwegian North Sea, we describe a large (10 km3) sand body and interpret it as extrusive. To our knowledge, this is by far the world’s largest such body reported in the literature. It would bury Manhattan (60 km2) under 160 m of sand, or all of London (1579 km2) under 6 m of sand.

The sand vented to the seafloor, when this was more than 500 m deep, during the Pleistocene glacial period. Currently, the sand (1) covers an area of more than 260 km2, (2) is up to 125 m thick, (3) fills low areas around mounds, which formed when underlying sand injectites lifted the overburden, (4) wedges out, away from a central thick zone, (5) is locally absent along ditches, 20 km long and up to 50 m deep, which overlie feeders on the flanks of the mounds, and (6) consists of fine-grained to medium-grained, sub-rounded to rounded grains. Such sand grains indicate a mature sand source, which is typically absent from contemporaneous glaciomarine sediment. These sands have intruded polygonally faulted smectite-rich mudstone of Eocene to Oligocene age and diatom-rich mudstone of Oligocene to Miocene age. We compare the distribution of the sand in nature with the results of experiments and see strong similarities. For the final stage of one experiment, a cross section of the model revealed a complex arrangement of structures, including a depleted parent bed, laccoliths and sills with lateral wings feeding the vents, and extruded sand. The longer the experiment lasted, the more sand came to the surface, by depletion of parent layers at depth. Scaled physical modelling has also helped to reveal the processes forming injectites and extrudites. In our interpretation, high fluid pressure fractured the regional Hordaland Group seal in the study area, so that fluidized sand, mixed with seawater and glaciomarine sediment, moved rapidly to the seafloor through fissures on the flanks of underlying mounds and was carried up to 8 km by sea currents, before it settled. The source of the intrusive and extrusive sand is not yet certain, but Paleocene turbidite sand is a likely candidate. Large extrusive sands represent a new type of reservoir, which traditional models of deposition do not predict.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California