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Small-scale and microscopic structures

Figure 5.22 Snowball fracture pattern superimposed on Bosumtwi drainage system. The line AA marks the position of the best exposed vertical tension fracture, which, it will be seen, extends at least 2.0 crater diameters from ground zero (after G.H.S.Jones).

provides important and supportive evidence that the structure under investigation was formed by impact. To these can be added the identification of shock-induced features within individual mineral grains, particularly quartz and feldspars.

Remnants of meteorites are only rarely found associated with impact features. Those which are most easily recognised are the iron meteoritic fragments, such as are found in and around the Meteor Crater, Arizona. However, it has recently become possible to relate the geochemical signature of the impacting body by identifying the siderophile elements, such as iridium, osmium and platinum, within melt-rocks and breccias associated with the impact structure.

Intermediate-scale structures in the rock, as opposed to the mineralogy, which are diagnostic of impact events, are shatter cones (Figure 5.24a) with characteristic converging striations. These cones are features which result from shear failure, as indicated in Figure 5.24b. Such structures were formed in high-energy experiments conducted in the USA It was inferred that the differential stress necessary to produce such shatter cones is equal to or in excess of 20 kb.

The presence or absence of these various small to microscopic-scale features is the diagnostic evidence that proves (if they are present) that the structure under investigation is the result of an impact. However, it must be emphasised that the finding of such evidence in or adjacent to terrestrial craters requires careful study. When the crater is on the ocean floor, the task becomes many orders of magnitude more difficult. Hence, if such high-stress minerals are not found, this does not prove that the structure under investigation is a non-impact feature.

These by-products of a moderately large impact event are usually much easier to detect, or infer, than it is to establish the site or even the existence of the impact scar. Based on a study by Alvarez et al. (1980), specimens taken from close to the Cretaceous/Tertiary (K/T) boundary were obtained from various widespread sites. These small samples contained quantities of tektites, quartz with deformation lamellae and high concentrations of iridium. The authors claimed that such a concentration of items near the K/T boundary was evidence that the boundary itself was the direct or indirect result of a major meteoritic impact or impacts that must have had an influence that was world-wide. Indeed, evidence for one or more events

Figure 5.23a Manicouagon impact structure with a diameter of 100 km, showing well-developed peripheral graben as defined by two lakes (courtesy of NASA).

ranges from the Southern States of the USA to Italy far to the east, and the Pacific Ocean far to the west. However, Robin et al. (1993) suggest that the evidence they found in the Pacific points to separate events and that several events are necessary to explain the global distribution of spinel-bearing nodules. The postulated event(s) soon attracted attention, for it was also suggested that it contributed to the extinction of the dinosaurs. The hunt for the site of the event which caused such world-wide havoc was immediately set in motion. Currently, the favoured site is that of the Chicxulub crater in N. Yucatan, Mexico, which has a diameter of about 180-200 km. (Hildebrand, 1991). However, as will be seen in subsequent chapters, we concur with Robin et al. and suggest that other events contributed to, or even exceeded, the influence of the Mexican event at the time of the K/T transition.

Figure 5.23a Manicouagon impact structure with a diameter of 100 km, showing well-developed peripheral graben as defined by two lakes (courtesy of NASA).

ranges from the Southern States of the USA to Italy far to the east, and the Pacific Ocean far to the west. However, Robin et al. (1993) suggest that the evidence they found in the Pacific points to separate events and that several events are necessary to explain the global distribution of spinel-bearing nodules. The postulated event(s) soon attracted attention, for it was also suggested that it contributed to the extinction of the dinosaurs. The hunt for the site of the event which caused such world-wide havoc was immediately set in motion. Currently, the favoured site is that of the Chicxulub crater in N. Yucatan, Mexico, which has a diameter of about 180-200 km. (Hildebrand, 1991). However, as will be seen in subsequent chapters, we concur with Robin et al. and suggest that other events contributed to, or even exceeded, the influence of the Mexican event at the time of the K/T transition.

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