Ejecta from the Sudbury impact

Sudbury in Ontario, Canada hosts one of the largest nickel and platinum-group metal deposits, and it in turn is associated with the world’s second largest impact structure (260 km diameter), dated at 1850 Ma.  About 650 km to the WNW is another of Canada’s Precambrian treasures, the Gunflint Chert beds that contain the earliest incontrovertible fossil cells.  Those cherts are also roughly the same age as the Sudbury impact structure, so what better place to seek material excavated and ejected by the offending meteorite? No need either to thrash around the bush to collect rocks; the succession has been penetrated by 5 drill cores near Thunder Bay and in northern Minnesota.  Sure enough, all the cores show signs of impact ejecta (Addison, W.D. et al. 2005.  Discovery of distal ejecta from the 1850 Ma Sudbury impact event.  Geology, v. 33, p. 193-196).  The proof takes the form of shocked quartz and feldspar grains and melt spherules, but in a sequence of silicified carbonates above the level of the Gunflint Chert.  Ejecta material is about 0.6 m thick.  Because the carbonates contain no volcanic horizons, establishing the age of the ejecta depends on a thin volcanic ash 5 m above it, which yielded zircon U-Pb ages between 1827 to 1832 Ma.  There are no other known impacts around this time, so Sudbury is the most likely source of the ejecta.  Apart from being the oldest impactite layer known that can be tied to a source, there are a couple of intriguing features.  The ejecta layer occurs almost at the top of the Gunflint Formation famous for its cellular remains, yet the overlying strata contain no sign of fossils.  The authors wonder if this might represent mass extinction, but these slightly younger sediments are clastic rocks in which cell microfossils are unlikely to have been preserved.  However, they do show signs of anoxia, including high organic carbon content and sulfide minerals.  Hopefully carbon isotope data from the section might throw light on how impacts in a world exclusively that of single-celled organisms affected the biota: an interesting comparison with the K-T boundary.  The other puzzle is that the ejecta are in shallow-marine sediments.  Being only a few hundred km from the linked impact structure, some sign of disturbance by tsunamis or water-release by huge seismic shocks might be expected within the sediments.  No signs of such disturbances have been reported.

One thought on “Ejecta from the Sudbury impact

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s