[meteorite-list] Corossol Impact Structure, Quebec, Canada

[Paul H.]

Back in 2011, Higgins et al. (2011) published an extended
abstract about what they called the Corossol Impact 
Crater, which is a 4-km diameter impact structure that 
lies off of the coast of Sept-Iles in Gulf of St Lawrence, 
Canada. They suggested that it might be as young as 
12,900 BP based upon calibrated radiocarbon dates 
from shells in marine sediments that directly overlie the 
structure. 

However, I am skeptical of it being a terminal Pleistocene
crater in that the colored relief map shown in that extended
abstract show a badly eroded impact structure that has 
been strongly modified by erosion. The feature shown in 
Figure 1 looks nothing like a pristine impact crater that was 
created close to 12,900 years ago. In addition the calibrated 
radiocarbon date corresponds exactly to the point in time 
that various maps showing reconstructing the deglaciation 
of North America indicate that this part of Canada became 
ice-free as the Laurentide Ice Sheet melted back. Thus, the 
12,900 BP date could very well be a minimum date that 
indicates only when the site of the impact structure became
ice-free and sediments containing datable could 
accumulate after being scoured by a continental ice sheet.
Finally, Higgins et al. (2011) pointed out that this impact 
structure could be any age between 12,900 BP and local
470 million year old sedimentary rocks. This left it as an 
uncertain candidate for a terminal Pleistocene impact.

More recently, I found an abstract by Locat et al. (2012).
After an analysis of "excellent" multibeam sonar coverage
of the area, they concluded on the basis of its surface 
morphology that this impact structure exhibited the
characteristics of glacial erosion. Based upon other 
morphological criteria, they estimated the minimum age
of this impact structure to about 20 million years. They 
assigned a tentative age to it that ranged between 20 to
470 million years. Although this is still quite a range of
age, the estimated minimum age of 20 million years,
if it holds, would make it far too old to be the result of
an terminal Pleistocene impact. Also, a 20 million year
minimum age seems to be far more consistent with the 
eroded appearance of this impact structure than a 
12,900 BP age. if this impact structure is glacially 
eroded, then it would even more unlikely that this is
the result of a terminal Pleistocene impact given the 
glacial history of the region. However, further research 
into the age of this impact structure is still needed.

Reference Cited

Higgins M. D., P. Lajeunesse, G. St-Onge, J. Locat, M. 
Duchesne, J. Ortiz, and R. Sanfacon, 2011, Bathymetric and 
Petrological Evidence for a Young (Pleistocene?) 4-km 
Diameter Impact Crater in the Gulf of Saint Lawrence, 
Canada. 42nd Lunar and Planetary Science Conference 
(2011), Abstract #1504.

Locat, J., P. Lajeunesse, G. St-Onge, M. Duchesne, 
M. D. Higgins, R. Sanfacon, and J.  Ortiz, 2012, A
morphological analysis for estimating the age of a 
possible impact structure; the Corossol Structure 
on the seafloor of the northwestern Gulf of St 
Lawrence Eastern Canada. Congres Geologique 
International, Resumes. vol. 34, pp. 3440.

Yours,

Paul H.