Odale-Articles-CanAm

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IMPACT CRATER EXPLORATIONS

by: Charles O'Dale

CAN-AM (PROBABLE) IMPACT STRUCTURE

The Can-Am probable impact structure, Lake Huron.
Regional shaded relief map of residual magnetic anomaly field of the Can-Am structure.
  • Age (ma): ~500
  • Diameter: 100 km
  • Location: Under Lake Huron N 43° 14’ W 82° 20’
  • Dating Method: The interruption of the Grenville trends, the clear, regular, circular signature of the feature, and the interpreted magnetic source depths at the Precambrian surface, suggest an event post-Grenville and possibly as young as early Paleozoic (see text).

A study of the Grenville front (G.F.), in the southern part of Lake Huron, revealed an interruption of the north-east “Grenville” aeromagnetic trends by a clear circular anomaly pattern that is due to structural and metamorphic changes associated with a post-Grenville event. This suggests that a 100 km wide crater is concealed in that position beneath approximately two kilometres of Paleozoic sedimentary rock at the bottom of Lake Huron. The authors of the paper describing the find have proposed the name “Can-Am Crater” for the structure as it is situated on the Canadian/US border (Forsyth et al, 1990).


MAJOR CIRCULAR STRUCTURE BENEATH SOUTHERN LAKE HURON DEFINED FROM POTENTIAL FIELD DATA (Forsyth et al, 1990);

Abstract

Newly compiled and merged Canadian and United States potential field data for the area of southern Lake Huron reveal a large circular structure that has a diameter that may extend to 100 km. The aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43°14.21′N, long 82°19.88′W, surrounded by a principal ring with a diameter of 50 km and indications of conformable, arcuate structure extending to a diameter of about 100 km. The prevailing, northeast-striking aeromagnetic signature of the Grenville front and adjacent Grenville front tectonic zone is interrupted and may be overprinted by the circular feature. The Bouguer gravity field shows a strong positive anomaly that extends southeast from the Grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field. The gravity field illustrates the circular structure with an annular low and central high, as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature.

There is no strong evidence of the feature in the bathymetry data of southern Lake Huron, although the general coastline of the southern end of Lake Huron closely follows the outline of the main circular feature. The interruption of the Grenville trends, the clear, regular, circular signature of the feature, and the interpreted magnetic source depths at the Precambrian surface, suggest an event possibly as young as early Paleozoic. In the absence of basement samples, the origin of the feature is unconfirmed. However, the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the Canadian Shield.


Can-Am vertical derivative of residual magnetic anomaly field. G.F. = Grenville front.
Can-Am bouguer anomaly field. G.F. = Grenville front.
The Bouguer anomaly field image (left) documents the circular gravitational anomaly in the area. The gravity data are from the data bases of the U.S. Geological Survey and the Geological Survey of Canada. In the image the Can-Am structure gravity anomaly pattern is superimposed on the local area gravity anomalies that were caused by the original density contrasts of the target rock types.

This image of the vertical derivative of residual magnetic anomaly field (right) is marked by a central magnetic high and a principal magnetic ring with a diameter of approximately 50 km. The principal ring encloses a magnetically quiet zone surrounding the 6 km diameter central magnetic peak. Beyond the ring, conformable arcuate anomalies outline semilobate features extending to a diameter of about 100 km. The magnetic data are from data bases at the Geological Survey of Canada, the National Geophysical Data Centre, Michigan State University and the United States Geological Survey. The aeromagnetic anomaly pattern of impact structures commonly feature a distinct central high and annular rings (Coles and Clark, 1978).

The gravity signature of an impact structure is primarily a low, owing to fracturing and brecciation of the impact target rocks (Grieve, 1988). The gravity low surrounding a central high, which is usually lower than the regional field, is the specific anomaly form of an impact crater. Other similar gravity anomalies are documented in the Brent, Wanapitei and West Hawk structures that are identified as impact structures in the Canadian Shield.


Odale-canam-Gravity profiles.jpg
In this image, the residual gravity profile of the Can-Am structure is compared with that of the Manicouagan impact structure (Sweeney, 1978). There is a strong similarity between the profiles. The coincidence between magnetic and gravity signatures of the Can-Am structure strongly suggests a common source for both fields. This data documents that the Precambrian basement rocks are interrupted by anomalies that clearly outline the circular nature of the structure and provides evidence that the remnants of a complex meteorite crater is situated in the south end of Lake Huron.


The Can-Am probable impact structure in Lake Huron east (looking west).
The superimposed circle in the aerial image documents the approximate center of the Can-Am structure in Lake Huron. The city of Sarnia is in the left of the image. The Can-Am structure is buried under at least 2 km of Paleozoic sedimentary rock, thus eliminating any evidence of the structure by hydrographic studies. The Ontario shoreline conforms to the southeastern part of the principal ring in the aeromagnetic data and this may indicate that the structure has had a secondary effect on the deposition or erosion of Paleozoic strata.
The Can-Am probable impact structure in Lake Huron south (looking north).
Odale-canam-Aeromagnetic.jpg
The Can-Am structure is directly north of the city of Sarnia, and its position is indicated by the superimposed circle (right). No intrusive (non-impact) structure of a similar scale, featuring both a central peak and peripheral ring, has yet been identified in the potential field data of the exposed Grenville province. This reinforces the hypothesis that the Can-Am structure is possibly the remnant of an impactor.


Further geophysical surveys and sampling are required to define the structural features, define the origin of the arcuate anomalies and precisely date the Can-Am structure.


Reference

D.A. Forsyth, M. Pilkington, R.A.F. Grieve, D. Addinett. Major circular structure beneath southern Lake Huron defined from potential field data. GEOLOGY, v18, p 773-777, August 1990.



Additional Articles

  1. Stolzenburg, W.; "Impact Crater May Lie beneath Lake Huron," Science News, 138:133, 1990.
  2. Equinox magazine, March/April 1991.


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