GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 139-4
Presentation Time: 2:20 PM


SCHMITZ, Birger1, TERFELT, Fredrik1 and BERGSTRÖM, Stig M.2, (1)Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, 22100, Sweden, (2)School of Earth Sciences, The Ohio State University, Columbus, OH 43210

The approximately 190 impact craters currently known on Earth represent only a small fraction of the craters originally formed on Earth’s surface. Available data suggest that there is an overabundance by one order of magnitude of craters in the age interval 470-440 Ma. These craters are interpreted to have been caused by the impacts of small asteroids from the breakup of the L-chondrite parent body (LCPB) in the asteroid belt, which is the largest documented breakup event during the past 3.5 Gyr. There are currently documented eight such craters in North America, six in Baltoscandia, and one in Australia that appear to have formed within ca. 30 Myr after the LCPB.

For North America, the Earth Impact Database lists the Ames, Clearwater East, Slate Islands, Brent, Pilot, and Couture craters. However, the lower part of the crater filling of the Rock Elm impact crater in Wisconsin has yielded Sandbian conodonts and should be included. The Decorah crater has recently been dated to have formed within a few million years after the LCPB. In Baltoscandia, six out of ca. 20 known Phanerozoic impact craters are of Middle-early Late Ordovician age based on biostratigraphy. These are the Tvären, Lockne–Målingen, Hummeln, Granby, and Kärdla craters. Recently, the Middle Ordovician Lawn Hill crater was described from Australia.

The high numbers of post-LCPB craters in two restricted areas, Baltoscandia and eastern North America, likely reflect ideal preservation conditions, including high sea-levels on cratons during the Ordovician Period. Extrapolating from the regional crater densities to a global situation yields in the range ca. 500-2000 crater-forming impacts related to the LCPB, depending on extrapolation approach. Even the lowest estimate is significantly higher than estimates predicted by modelling studies. We argue that there is compelling evidence for a major asteroid shower to Earth after the LCPB, but the impacting asteroids appear to mostly have been small, typically 100-1000 meters in diameter.