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Two other criteria of shock impact are both high-pressure polymorphs of silica, coesite and stishovite, which can be produced in the laboratory only under extremely high pressures and fairly high temperatures (Stoffler 1971). Other specialized types of melted rocks, suevite and pseudotachylite, are also key indicators of meteorite impacts (Dennis 1971; Wilshire et al. Lunar exploration has confirmed another unique feature of astroblemes, whether terrestrial or lunar; namely a central uplift area which occurs only in the larger impacts (Cohen et al. This uplift can be easily observed in such confirmed impact sites as the Wells Creek Basin, Tennessee, where Ordovician strata have been raised 750 m, and the Sierra Madera of Texas, where Permian rocks have been uplifted 1200 m (Wilshire and Howard 1968).The presence of coesite, which can be formed at pressures of 425-500 kilobars and temperatures near 1000C, has confirmed the meteoritic origin of Lake Wanapitei Crater in Ontario (Dence et al. Associated with central uplift is the actual forcing of adjacent strata inward and upward accompanied by intense brecciation of the uplift area (Wilshire et al. The dating of the meteorite impact events is one of the most intriguing aspects of this study, since it has application in determining the frequency of meteorite impacts both on earth and on the moon (Baldwin 1971).In Mauritania, the Tenoumer Crater has a vast age gap between shocked and unshocked rocks. The Rb-Sr clock for the same basement rocks reads 24 m.y.The K-Ar ages of unshocked Precambrian basement rocks are 2010, 1770, and 1820 m.y., while the shocked rocks that were melted by impact show K-Ar ages of 2.4, 2.6, 4.2 and 9.2 m.y. while for the shocked rocks it shows slightly reduced ages of 20 m.y.ago, while the mean age of the Charlevoix Crater is published as 380 m.y.The discrepancy is usually attributed to argon escape.The tremendous pressures (425-500 kilobars with coesite) and the high temperatures (1000C with coesite and 400 with stishovite formation) result presumably in a resetting of the potassium-argon and the fission-track radiometric clocks.

effects high pressure radiometric dating-71effects high pressure radiometric dating-75

Located in Siberia some 600 km from the nearest recent volcanoes, its nearly perfect circularity 18 km in diameter has been studied both by satellite and ground observation. impact structures have been suggested over the years: Meteor (Barringer) Crater, Arizona (Figure 1); Kentland structure, Indiana; Japhtha Knob, Kentucky; Wells Creek Basin, Tennessee; Serpent Mound, Ohio; and Crooked Creek, Missouri (Middlehurst and Kuiper 1963).

Each strewn-field has provided samples of harmonious composition as well as harmonious radiometric dates (Table 1).

The most clear-cut case of a common origin for an astrobleme and nearby tektites is the Bosumtwi Crater (Ghana) and the Ivory Coast tektites. Samples from both localities also lie on the 1970 m.y.

The dating of sanidine, which is argon retentive, in the central uplift region shows a K-Ar age of 133 m.y. Many such erratic blocks are found along the Pliocene surface outside the crater area proper (Dennis 1971).

Concordant K-Ar and fission-track ages of 14.8 and 14.0 m.y. Brecciated Mesozoic rocks (Triassic and Jurassic) are found within the crater while undisturbed upper Miocene rocks fill the basin of the crater up to 300 m deep.

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