Precambrian impact structures and ejecta on earth: A review

Impact cratering is an important geological process on Earth. This review summarizes the state of knowledge of the Precambrian (Hadean, Archean, and Proterozoic) impact record on Earth. After an early collision that may have led to the formation of the Moon, both Earth and Moon suffered intense post-accretionary bombardment at about 4.5 and 3.9 billion years ago. Evidence for a “late heavy bombardment” phase at about 3.85 Ga is currently debated because the lunar rock record might be biased, and no relevant impact record has yet been confirmed on Earth. Several 3.5 to 2.5 Ga old spherule layers in South Africa and Australia, and two large impact structures, Vredefort and Sudbury, at about 2 Ga, represent most of the oldest actual terrestrial impact record. The impact record for more than half of the geological history of the Earth is incomplete, and, as a result of the lack of old continental crust on Earth, there is also only limited evidence for impact processes during the first 2.5 billion years of Earth history. Some more (mostly badly dated) impact structures are (partly) preserved for the Proterozoic period, as are a couple of ejecta layers. Given that the rock record preserved on Earth is very restricted for this early time period, the limited impact record is not surprising, but as recent discoveries show, there is still room for more research and new findings.