Impact superfloods

The scale of flooding unleashed by a large asteroid or comet crashing into the ocean would dwarf all the above superfloods. The gigantic waves thrown up by bombardment would produce floods that could truly be called super (Huggett 1989a, 1989b).

Earth-crossing asteroids and comets possess enormous kinetic energy. Should they crash into the ocean, they would create an enormous wave system that would flood continental lowlands (Figure 5.4; see also Hills et al. 1994). Tolerably firm sedimentary evidence exists for the production of superwaves by asteroidal or cometary impacts. The Chicxulub structure on the Yucatán Peninsula, Mexico, is almost certainly an impact crater dating from the Cretaceous-Tertiary boundary. A plausible scenario is that an extraterrestrial object about 10 km in diameter smote the Earth, triggering massive earthquakes and the collapse of soft sediments down nearby continental slopes (Hildebrand et al. 1991). Giant tsunamis would have radiated from the Yucatán Peninsula, scouring sediments from the sea floor and coursing over surrounding lowlands, depositing a jumble of fine and coarse sediments. An outcrop at Mimbral, which lies across the Gulf of Mexico from Chicxulub, records the sequence of events during the impact (Swinburne 1993). A spherule bed overlies Cretaceous deep-water sediments. Molten droplets of rock thrown out of the impact crater and then cooled would have produced these spherules. On top of the spherule bed lies a wedge of sediments deposited by the train of tsunamis that would have rocked back and forth over the site. On top of the tsunami bed are ripples in fine-grained sediments that represent the last stages of tsunami dissipation. Similar tsunami deposits are found in Texas, where a wave amplitude of 50-100 m is indicated (Bourgeois et al. 1988). The 700-m-thick clastic sequence of the Cacarajicara Formation, Cuba, which includes breccia and

Figure 5.4 Tsunami envelope from 30 min to 9.5 h after the impact of a 1.1-km diameter asteroid (1950 DA), which has a 0.0-0.3 per cent probability of hitting the Earth in the year 2880, 600 km east of the United States coast. Waves hundreds of metres high at the impact site disperse quickly. After 2 h, 100-m waves hit the coast from Cape Cod to Cape Hatteras, and within 12 h 15-20-m waves arrive in Europe and Africa. Source: Adapted from Ward and Asphaug (2003).

Figure 5.4 Tsunami envelope from 30 min to 9.5 h after the impact of a 1.1-km diameter asteroid (1950 DA), which has a 0.0-0.3 per cent probability of hitting the Earth in the year 2880, 600 km east of the United States coast. Waves hundreds of metres high at the impact site disperse quickly. After 2 h, 100-m waves hit the coast from Cape Cod to Cape Hatteras, and within 12 h 15-20-m waves arrive in Europe and Africa. Source: Adapted from Ward and Asphaug (2003).

boulders, may be a tsunami deposit from the Chicxulub impact (Kiyokawa et al. 2002), but the exact age of the deposit is uncertain.

Tektite fields are strong evidence of impact events. However, the terrestrial record of impact cratering suggests that more than 3 km of impact-crater ejecta deposits should have been produced during the last two billion years. The question is: where has all the ejecta gone? A possible answer is that it remains as diamictites. Diamictites are poorly sorted mixtures of sediment, commonly with boulder-sized clasts in a fine-grained matrix, which textural characteristics are similar to those of impact crater deposits predicted by an impact model (Oberbeck et al. 1993; see also Rampino 1994). According to the model, even modest impacts in shallow seas, forming craters with diameters as small as 5 km, would have ejected large amounts of material and produced characteristic sequences of sediments. The sedimentary productions would include thick bodies of sediments traditionally recognized as tillites (a variety of diamictite), on the ocean floor, in deltas, and on land (Figure 5.5). At present, all tillites, even those 10 times thicker than Pleistocene glacial deposits, are construed as glacial deposits. If they should be of glacial origin, then a difficulty arises: what process leads to the preservation of ancient glacial deposits, but removes all traces of impact crater deposits that, in theory, should total at least 3 km.? If the reinterpretation of tillites and diamictites as impact crater deposits laid down within a few hours should be correct, then the implications are truly astounding. It would mean, for instance, fewer glaciations in the past, a fact that would demand a fresh look at theories of geological climates. In addition, it would explain some palaeoclimatic anomalies, to wit, the occurrence of Lower Proterozoic tillites when the global climate was warm, and the low-latitude distribution of some Upper Proterozoic glacial deposits (Rampino 1994). It would also lend credibility to the diluvial hypothesis of landscape development. There are many Miocene breccias, thousands of metres thick with clasts that are tens of metres in diameter, that appear to have formed violently, or at least chaotically (J. R. Marshall, personal communication 1993). These deposits are enigmatic and do not appear to be impact ejecta. The possibility that they are impact-induced superflood deposits seems worth considering.

Superflooding following oceanic impacts is not a process confined to the remote geological past. Several researchers believe that it occurred at the transition of the Pleistocene and Holocene epochs (e.g. Spedicato 1990; Kristan-Tollmann and Tollmann 1992). This might explain the flood myths found in nearly all cultures. The suggestion is that Noah's Flood occurred around 9,545 years ago when the Earth collided with a comet several kilometres in diameter that had broken into seven large pieces and several smaller bits. The cometary fragments generated truly gigantic waves, the waters from which gushed out from the sites of impact, streamed over mountain chains, and poured deep into continents (Kristan-Tollman and Tollman 1992). It has also been posited that a comet or asteroid might have struck the Laurentide ice sheet 11,000 years ago creating enough water to submerge Canada and the north-eastern and northern midwestern United States to depths of 1-2 km, and to produce the Alberta erratics train and many drumlin fields (Hunt 1990, 169). That is certainly a novel explanation of rapid deglaciation!

If these impact-induced superwaves should have occurred at the end of the last ice age, what would the likely effects have been on reaching lowland areas in western Europe? A few speculations seem credible, if unproven (Huggett 1994). Superwaves might have produced the very extensive spreads of Late Pleistocene gravels found, for instance, in Cheshire and Lancashire. Equally, vast quantities of water rapidly draining off lowland areas might produce gorge-like valleys such as those found in Cornwall, Devon, and Dorset. In Cornwall, several workers have noticed steep-sided valleys cutting into wide plateaux. Rejuvenation following uplift of an upland plain currently standing at about 130 m above sea-level might have formed these river-gorges in the Tintagel area, including the Rocky Valley and its picturesque St Nectan's Kieve (Dewey 1916; see also Hendriks 1923). A catastrophic origin by receding superflood waters is another possibility. The draining of super-flood waters might also account for the chines of Bournemouth - narrow gorges in which

Figure 5.5 A hypothetical sequence of events following the impact of mantle-penetrating bolide in a shallow sea. The full sequence would take several hours to run. The vertical scale is exaggerated. Source: Reprinted by permission of The University of Chicago Press from: V. R Oberbeck, J. R Marshall and H. Aggarwal (1993) Impacts, tillites, and the break up of Gondwanaland. Journal of Geology 101, 1-19. © 1993 The University of Chicago. All rights reserved.

Figure 5.5 A hypothetical sequence of events following the impact of mantle-penetrating bolide in a shallow sea. The full sequence would take several hours to run. The vertical scale is exaggerated. Source: Reprinted by permission of The University of Chicago Press from: V. R Oberbeck, J. R Marshall and H. Aggarwal (1993) Impacts, tillites, and the break up of Gondwanaland. Journal of Geology 101, 1-19. © 1993 The University of Chicago. All rights reserved.

streams from the Bournemouth Plateau tumble into the sea. Like the river-gorges in Cornwall, the Bournemouth chines are generally thought to have been created by rejuvenation (e.g. Arkell 1947, 318), but superflood waters could have produced the same features. Interestingly, the river-gorges and chines of southern England are reminiscent of the drowned valleys or calas found on the island of Mallorca. The calas are a prominent landscape feature around the eastern and southern coast of the island. The ephemeral streams that feed the calas originate in the Sierra de Levante. They cut into a plain formed in horizontal Miocene limestones. There is no evidence that water running off the Sierra de Levante has reached the sea within living memory, and analysis of fluvial gravels confirms minimal transport along the streams in the recent past. The calas were not produced by marine submergence during the Holocene epoch, but are fossil forms created at the end of the Tertiary period or the beginning of the Pleistocene epoch (Butzer 1962). It is possible that they are not purely fluvial forms, but were largely fashioned by the passage of super-flood waters.

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