DOES THE MARINE-TARGET CRATER RECORD REFLECT AN INCREASED COSMIC BOMBARDMENT DURING THE MIDDLE ORDOVICIAN?

Many attempts have been made to find times in the Earth’s history with anomalously high cosmic bombardment and even to identify periodicity for these peaks, although with little success (Grieve and Pesonen, 1996).

As a rule marine-target craters offer the best means of dating because of the biostratigraphic control on deposits that formed before and after the impact (cf., Grahn et al., 1996). Recent years work on such craters has revealed that most of them seem to concentrate to two levels in the Middle Ordovician, either lowermost Caradoc (approx. 455 Ma; Brent, Kärdla, Lockne, Tvären) or the lowermost Llanvirn (approx. 470 Ma; Ames, Granby, Neugrund). The proposed (and very likely) impact crater Hummeln, Sweden, does also belong to the group of Llanvirnian craters.

In addition, an extraordinary concentration of fossil meteorites from several separate falls have been reported from the uppermost Arenigian - lowermost Llanvirnian beds in southern Sweden (Schmitz et al., 1997; 2001). These falls indicate a one to two orders of magnitude increase in the influx of asteroidal material during a few million years.

The Canadian list of proven terrestrial impact craters gives ages for an additional number of craters that may fall into the interval of interest (Calvin 450 ±10 Ma, Couture 430 ±25 Ma, Pilot 445 ± 2 Ma, Slate Islands 450 Ma). Comstock et al. (2004) writes that the recently proposed Peerless crater formed about 430-450 Ma., and Whitehead et al. (2003) argue for a Llanvirnian age for Charlevoix. The environment and age of these craters is apparently still a more or less open matter.

Despite the anomalously large number of Middle Ordovician craters it is the meteorite falls reported by Schmitz et al. that give reliable support for an increased bombardment at that time. The stratigraphic relation between the falls can not be explained by other causes. However, apparent clustering in time of impact craters may be due to geological causes: The Middle Ordovician was a time with large, relatively shallow epicontinental seas that favored both the formation of craters in the seafloor (Deeper oceans require larger, more rare impacts) and their preservation (depositional instead of erosional environment; thick and stable continental lithosphere instead of lithosphere more prone to deformation and destruction through collision).