Peridocity and impactflux changes in the Phanerozoic

We have noted that Seyfert and Sirkin (1979) (Figure 8.1) have suggested that there is evidence of periodicity as regards the maxima of impacts. However, Grieve et al. (1977) have argued that, at this time, the number of known 'certain' impacts and the lack of precision in dating of these impact events are such that it is not possible to demonstrate, with any degree of certainty, that periodicity of impacts exists. Accordingly, we took the dates of the impact events that are represented in Table 8.3, which, it will be recalled, relate to the tracks of seven different reference points, but date back only 255 to 260 Ma.

We shall now look briefly at impact events that may have developed in the age range from 260 to 600 Ma as inferred from the tracks shown in Figures 8.27 to 8.29, relating to Fennoscandia and the Pacific.

Figure 8.22 The track from 98 to 50 Ma. (a) shows minor deflections in a generally westward track, which continues in (b) until 65.35 Ma, where a major (K/T boundary) track change takes place which continues until 58.3 Ma, and thereafter continues with only minor changes to 50 Ma.

Figure 8.23 (a) The general track from 50 to 0 Ma, starting with an abrupt change at 50 Ma. (b) to (d) This track is shown on a larger scale. The most dramatic change, in the period from 50 to 0 Ma, occurred 5 Ma ago, all other changes in track could be classified as modest, minor or trivial.

As before, we selected impacts cited by Hodge (1994) for this period. There were 21 events which could be clearly ascribed to this period, of which two had a diameter of 54 and 55 km respectively. Three craters had diameters respectively of 32, 15 and 12 km. The remaining 16 events had craters which were less than 10 km in diameter.

All these impacts contribute to the known flux. However, it is probable that none, except perhaps the 54 and 55 km craters, could be classified as likely to give rise to a detectable global or regional disturbance. These impacts, aged 357 and 368 Ma, occurred in Quebec and Sweden respectively. Both these structures are now situated at relatively low altitude. Only one half of the Charlevoir, Quebec, structure is exposed, the

Figure 8.23 (a) The general track from 50 to 0 Ma, starting with an abrupt change at 50 Ma. (b) to (d) This track is shown on a larger scale. The most dramatic change, in the period from 50 to 0 Ma, occurred 5 Ma ago, all other changes in track could be classified as modest, minor or trivial.

As before, we selected impacts cited by Hodge (1994) for this period. There were 21 events which could be clearly ascribed to this period, of which two had a diameter of 54 and 55 km respectively. Three craters had diameters respectively of 32, 15 and 12 km. The remaining 16 events had craters which were less than 10 km in diameter.

All these impacts contribute to the known flux. However, it is probable that none, except perhaps the 54 and 55 km craters, could be classified as likely to give rise to a detectable global or regional disturbance. These impacts, aged 357 and 368 Ma, occurred in Quebec and Sweden respectively. Both these structures are now situated at relatively low altitude. Only one half of the Charlevoir, Quebec, structure is exposed, the

Figure 8.24 (a) General track for marker on the west coast of the USA from 0 to 260 Ma. (b) Detail of track from 0 to 55 Ma and (c) from 48 to 100 Ma.
Figure 8.25 Track (a) from 95 to 195 Ma and (b) from 190 to 260 Ma.

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