Impacts on Earth in the Late Triassic

Abstract

Sprayet al.¹ postulate that five widely dispersed terrestrial impact structures with very similar geological age estimates (about 214 million years ago, in the Late Triassic epoch) are evidence of a multiple impact event. Most notably, the three largest impact structures, Saint Martin in western Canada (∼40 km diameter), Manicouagan in eastern Canada (∼100 km diameter), and Rochechouart in France (∼25 km diameter), plot at virtually the same palaeolatitude in a continental reconstruction. Spray et al. suggest that this apparent crater chain was produced within hours as a series of coaxial projectiles collided in rapid succession with the rotating planet Earth, and drew analogies to the recent collision sequence of fragmented comet Shoemaker-Levy 9 with Jupiter.

Main

However, published palaeomagnetic data for the Manicouagan²,³ and Rochechouart⁴ impact structures argue strongly against such a closely timed origin for these ancient events. This is because the characteristic remanent magnetizations of the melt rocks, including the most rapidly cooled glassy phases, indicate formation in a Late Triassic palaeomagnetic dipolar field of normal polarity at Manicouagan but of reverse polarity at Rochechouart.

These impact events must therefore have been separated temporally by at least the few thousand years⁵ it takes for a geomagnetic polarity reversal to take place, a process which in any case occurred relatively infrequently (at an average rate of about twice per million years⁶,⁷) in the Late Triassic. Thus, although there is an interesting concentration of impact events in the Late Triassic, the opposite geomagnetic polarities recorded by the Manicouagan and Rochechouart melt rocks appear to preclude a synchronous multiple impact origin.

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