Discovery of distal ejecta from Sudbury impact event

Paul H bristolia at yahoo.com
Tue Mar 8 23:26:46 EST 2005

Discovery of distal ejecta from the 1850 Ma Sudbury
impact event
from "March Geology and GSA TODAY" media highlights
at:

http://www.eurekalert.org/pub_releases/2005-02/gsoa-mga022805.php
http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613

Addison, W. D., and others, 2005, Discovery of
distal ejecta from the 1850 Ma Sudbury impact
event. Geology: Vol. 33, No. 3, pp. 193–196.

Addison et al. announce the discovery of impact
ejecta from the Sudbury, Ontario, Canada,
structure, the second largest and third or fourth
oldest extraterrestrial Earth impact site. At 1.85
billion years old, these Paleoproterozoic ejecta
are three times older than the previous oldest
dated ejecta linked to a specific impact (Acraman,
Australia, 0.59 billion years old). It is also larger
than the well-known Chicxulub, Mexico (0.065
billion years old) impact linked to the extinction
of the dinosaurs and many other species. The
young Chicxulub impact, particularly its well-
preserved worldwide ejecta debris layers, have
produced criteria to judge other large ejecta
deposits. Foremost is the occurrence of sets of
microscopic parallel lamellae in quartz and
feldspar grains produced by the intense shock
generated at the point of impact. Secondarily,
the impact generated a megaplume of vaporized,
melted, and crushed crustal rocks, creating molten
droplets containing bubbles of gas, and larger
accreted balls of dust and rock shards called
impact accretionary lapilli. These features, and
more, are seen in the Sudbury debris. The debris
(ejecta) studied here, landed 650 km west northwest
of Sudbury near Thunder Bay, Ontario, Canada, and
875 km west of Sudbury near Hibbing, Minnesota,
United States. This huge impact likely deposited
debris all around Earth, but it is very difficult to
find because so much of the evidence has been
destroyed in the recycling of Earth's crust by plate
tectonics. Life at the time of the Sudbury impact
was confined to the oceans and consisted of
unicellular and colonial unicellular organisms. So
far, Addison et al. have found no evidence of
extinction of this life. However, future studies may
link this impact and its ejecta with changes in the c
lassic Gunflint Iron Formation unicellular organisms
and their photosynthetic microbial mats, which
helped produce Earth's atmospheric oxygen.