The Great Blurting

It is hard to resist curiosity when a phrase includes a superlative. Dickens knew this when he opened A Tale of Two Cities with the words, ‘It was the best of times, it was the worst of times…’. So impacted into post-Victorian English language are they that the Daily Mirror of 13 May 2012 used them to celebrate ‘The most scintillating finish in Premier League history’: referring of course to the footballing tales of the city of Manchester (UK, that is). So it was with some gaiety that I turned to a paper in the May 2012 issue of  Geology (Løseth. H. et al. 2012. World’s largest extrusive body of sand? Geology, v. 40, p. 467-470). Now, that is a title to conjure with, and I would advise any academic author to add a superlative adjective of some kind to their next manuscript title, to ensure more than 5 readers and at least one citation to add to her/his CV. Conversely, I caution against seemingly ultra-high impact, exclamatory single-word titles such as ‘Coelocanth!’, Porphyroblast!’, ‘Ignimbrite!’ or ‘Sphenochasm!’: they summon untoward visions of geoscientists much given to ‘snorting and pawing the air in salivating lust and groveling need’, in the manner of Hungry Joe’s reaction to a pornographic cameo brooch (Heller, J. 1961. Catch 22: Simon & Schuster).

The sand body in question lies in the Pleistocene subsurface of the Norwegian sector of the North Sea above the Snorre oilfield, and came to light through a 3-D seismic survey with extraordinarily good resolution that allowed the reconstruction of its base and top structure contours (in two-way time) and thus its overall volume and shape. At 10 km3, were it to have formed yesterday to cover Manhattan the paper’s abstract suggests that it would have reached the 37th floor of the Empire State Building. More parochially, had it engulfed  London’s old financial quarter centred on London Bridge (Post Codes EC1 to 4 and SE1) 30 St Mary Axe (‘The Gherkin’) and ‘The Shard’ would be buried in their entirety leaving one of capitalism’s iconic heartlands a curiously gnarled sandy plain.

English: Mud volcano, Romania Polski: Wulkan b...
Small mud volcano, Romania (Photo credit: Wikipedia)

That the sand is extrusive rather than being simply a sedimentary stratum is revealed by its extraordinary shape. Its thickest part is in a depression surrounded by mounds of the underlying unit – the former seabed – above which the body is absent. These mounds show marginal signs on the seismic sections of dykes that could have acted as feeders from stratiform sands deeper in the sequence, the dykes coinciding with the base of  ‘ditches’ in the body’s upper surface. In turn, the ditches have flanking ridges as if the ditches and the dykes below were feeders for the sand extrusion. Such an extrusive sand body is currently forming at the accidentally triggered Lusi sand volcano in Indonesia where a single vent exudes about 50 thousand m3 each day; a rate that would take 550 years to produce the Snorre field body. Pleistocene stratigraphy surrounding the vast North Sea ‘boil’ suggests that it formed during a period of rapid sedimentation from the huge North Sea ice shelf supplied by the Scandinavian ice sheet.

Helge Løseth and colleagues from Statoil and the University of Rennes ran a series of dry sandbox experiments to mimic the process of sand injection. By pumping air through interbedded sand, glass ballotini and silica powder, to represent two types of non cohesive sands and cohesive mudrocks, they found that increasing the overall air pressure in the box eventually fluidized the ‘sands’ which blurted through the ‘clays’ to form ‘volcanoes’ with plumes of sand that enlarged the area of deposition at the surface. Cutting into the sediments after the experiments revealed a remarkably real-looking system of intrusive sand bodies (dykes, sills and laccoliths) as well as the extrusive mass of sand. Chances are that such bodies may form more commonly in marine sequences, given encouraging over-pressuring through sudden increases in normal sedimentation. If so, the very open grain structure of the vented sands might provide superb petroleum reservoir characteristics.

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