Bizarre impact structure beneath North Sea

The increasing use of finely-resolving 3-D seismic surveys in offshore exploration for hydrocarbons reveals exquisite detail of structure in strata beneath the sea floor.  So it is no surprise that oil-company geophysicists are able to image features that would otherwise remain hidden to researchers in universities.  If such discoveries are of little interest commercially, their finders are free to publish.  During routine surveys in the southern North Sea, an array of seismic profiles gradually built up a picture of something more reminiscent of the surface of an icy moon of Jupiter than a sequence of basinal sediments (Stewart, S.A. & Allen, P.J.  2002.  A 20-km-diameter multi-ringed impact structure in the North Sea.  Nature, v. 418, p. 520-523).  The circular feature found in strata at the top of the Cretaceous, might have been passed off as the product of deeper rise of salt diapirs from the widespread Permian evaporites of the North Sea basin, but for several features.  The surveys revealed no signs of the low-density Permian salt having bulged upwards below the structure, and disruption stops at depth.

The feature consists of at least 10 concentric rings extending to 20 km diameter, and at its centre is a bowl-shaped depression around a clear peak.  Not only is it an impact structure, but one of a particular class known as multi-ringed basins.  Those known from the Moon, are vastly bigger and are thought to have formed by such immense energy that the lunar surface rippled to fail along large concentric faults.  Lunar and terrestrial craters of the size of the North Sea structure usually have no concentric structure, being circular pits with rims and occasionally a central peak cause by rebound of the crust after impact.  The only similar features known are from moons of the Giant Planets that are made mostly of ice.  It is surprising that the North Sea example closely resembles them.  Modelling of such craters on Callisto suggests that they form when surface materials are underlain at depth by weaker ones; possibly an ice-liquid slush on ice moons.  The North Sea impact was into the Upper Cretaceous Chalk, whose upper strata are more homogeneous than those at deeper stratigraphic levels, which contain layers of mudstone.  Had impact occurred while the strata were not completely lithified, then the clays would have allowed inward movement to fill the crater excavated by impact, the more rigid upper Chalk having fractured during this movement.

Whether or not the impact accompanied the Chicxulub crater, implicated in the end-Cretaceous mass extinction, is not certain, although it does seem to predate Tertiary sedimentation in the North Sea.  There are probably many more impact structures on the sea floor, buried by marine sediments, but only in hydrocarbon-rich basins are they likely to be unmasked by seismic surveys.

Evidence builds for major impacts in Early Archaean

Following the discovery that anomalous tungsten isotope compositions of some Early Archaean rocks suggest a major component of extraterrestrial material in them (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment), geochemists from Louisiana State and Stanford universities report evidence of debris from very large impacts in the same period (Byerly, G.R. et al. 2002.  An Archean impact layer from the Pilbara and Kaapvaal cratons.  Science, v. 297, p. 1325-1327).  Their case rests on the occurrence of layers of rock containing spherules of what formed as molten silicate droplets, in Early Archaean greenstone belts of the Barberton and Warrawoona areas of South Africa and Australia.  Zircons from a single layer in both areas yield identical ages of 3470 Ma, suggesting that the layers formed during a single impact event.  The authors speculate that a major unconformity in the Archaean of the Pilbara province in Australia, which is around the same age, may be the result of tsunamis induced by the impact.  It seems as if the responsible impact had a global effect, and may have released 1 to 2 orders of magnitude more energy than that responsible for the K/T event.  Judging by the lunar cratering record, this and previous finds help confirm expectations of similar bombardment on Earth during the Early Archaean.

Very early differentiation of planetary bodies

The radioactive decay of 182hafnium to 182tungsten seems likely to resolve the influence of impacts on the Earth ‘s evolution (See Earth Pages News, August 2002, Tungsten and Archaean heavy bombardment).  It is even more useful in refining ideas about the evolutionary pace of the parent bodies of meteorites.  The half-life of 182Hf is only 9 million years (all of it has decayed away in the Solar System by now), so the amount of radiogenic 182W associated with hafnium in a meteorite is a guide to pervasive geochemical processes early in the history of their parent bodies.  Hafnium has an affinity for silicates, whereas tungsten is siderophile and likely to enter planetary cores, should they form.  Because 182Hf decays so quickly, it is not easy to work out its original abundance, relative to stable 180Hf, in the source material for the Solar System.  That is a prerequisite for estimating when the hafnium-tungsten differentiation took place in a planetary body.  Two papers in the final August 2002 issue of Nature agree on this initial ratio (Yin, Q. et al. 2002.  A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites.  Nature, v. 418, p. 949-952.  Kleine, T. et al. 2002.  Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry.  Nature, v. 418, p. 952-955), which has important connotations; it is less than half the previously assumed value.  They determined this initial ratio using Hf-W data from independently dated carbonaceous-chondrite meteorites, whose parent bodies were never fractionated.

The two research groups, from Harvard University and the French Laboratoire des Sciences de la Terre, and the universities of Münster and Köln, Germany, respectively, use the new initial ratio to estimate the age of core formation from a range of meteorites.  Their estimates dramatically shorten the time between original accretion and core formation in a variety of bodies whose Hf-W isotopes have been studied previously.  The parent of the eucrite class of meteorites, probably the asteroid Vesta, differentiated within only 3 to 4 Ma, whereas the cores of the Earth and Mars took a little longer – about 29 and 13 Ma respectively.  In geological terms, accretion and core formation probably accompanied one another.  Of course, such estimates based on isotopic decay systems assume that the initial ratios existed at the time of accretion.  That may not be valid if the pre-Solar nebula took millions of years to evolve to the stage of self-collapse under gravity, which is the prerequisite for the formation of a planetary system.  However, there is evidence from short-lived decay systems involving other radioactive isotopes, such as 26Al, in meteorites, that points to the influence of a nearby supernova that triggered the formation of our Solar System.  Such an event is required to synthesize short-lived isotopes anyway.  Moreover, the shock from a supernova could accelerate collapse to mere few tens of thousand years.

See: Cameron, A.G.W. 2002.  Birth of a Solar System.  Nature, v. 418, p. 924-925.

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