Geologists believe they are getting close to identifying the locations of craters produced 470 million years ago, with new clues hinting at a variety of buried locations.
During the Ordovician Era, two asteroids with very different chemistry collided, pelting the inner solar system with smaller objects. It is thought that the rapid diversification of species that occurred during this time period was triggered by steady asteroid bombardment since many of these objects collided with Earth.
Earlier this year, two Swedish craters were identified as being formed by twin asteroids that slammed into the Earth within minutes of each other.
However, this pair and the ten other known impact craters from the period are thought to be the tip of a geological iceberg. Where 100,000 impact craters are visible on the moon, less than 200 are confirmed on Earth because erosion and tectonic forces have hidden the rest away. The further back in time a crater was formed, the less likely we will be able to make out its features today; 470 million years is ample time to bury evidence.
Nevertheless, the Geological Society of America annual conference was provided with new evidence that may add three U.S. geological anomalies to the list of Ordovician Craters.
Visual features of craters, such as the rim and central peak, become eroded easily. Conclusive evidence of impact craters, therefore, is usually in the form of shatter cones, rocks smashed by the force of the impact in ways seen nowhere else. Other evidence includes glassy tektites and deformation of minerals.
These have yet to be found at the proposed sites. However, Lawrence University Undergraduate Emily Zawacki pointed to a 2km wide site in Door County Wisconsin suspected of being an eroded crater. "This is a highly disturbed area in an otherwise flat-lying stratigraphy," Zawacki told Live Science. "It very clearly is anomalous and we feel a meteoritic impact best explains it."
Ohio University's Professor Keith Milam reported that the Howell Structure in Tennessee contains fragmented carbonate that shows signs of impact shocking, while Eric Gibbs, an undergraduate at the same university told the conference about the Jeptha Knob, Kentucky. "I don't think you can say for sure this is an impact structure yet," Gibbs said, but he hopes x-ray diffraction testing will resolve this.
All three sites are of an age to have been part of the Ordovician bombardment. While nations with strong geology programs may take the lead in identifying craters, the fact that all the known remnants of this event are in either North America, Sweden or Finland is puzzling. At the time, the areas that are now the US and Canada were equatorial, and Scandinavia lay to their south-east, raising the question of whether the concentration in what were then tropical latitudes was a coincidence, and if not, what could have united events up to 40 million years apart in this way.
Jens Ormö et al., Scientific Reports. The known impact craters from the Ordovician break-up all lie in areas that were close to the equator at the time, and the three new proposals would add to this pattern.