CAN-AM STRUCTURE

CAN-AM (PROBABLE) IMPACT STRUCTURE

by: Charles O’Dale

  • Type: Central peak
  • Age (ma): ~500 (Geological dating)a
  • Diameter: 100 km
  • Location: Under Lake Huron N 43° 14’ W 82° 20’

a 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).

Regional shaded relief map of residual magnetic anomaly field of the Can-Am structure located within Lake Huron.
The Can-Am probable impact structure in Lake Huron, looking west from GOZooM.

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.

 

There is a strong similarity between the profiles of the Can-Am structure and the Manicouagan Impact Crater. The coincidence between magnetic and gravity signatures 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 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 looking north from GOZooM at ~2500 AL.
The Can-Am structure is directly north of the city of Sarnia, and its position is indicated by the superimposed circle. 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

Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education

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.

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.

Additional Articles

With the help of magnetic sensors, scientists have detected a rimmed circular structure, 30 miles in diameter, more than a mile beneath the floor of Lake Huron. They believe the magnetic ring marks a buried crater — potentially one of the largest known — blasted by a meteorite at least 500 million years ago.

Four researchers with the Geological Survey of Canada unexpectedly discovered the circular pattern while examining new, high-resolution magnetic images of a southern Ontario region previously labeled as “essentially featureless.” They noticed that the patterns of gravitational and magnetic forces emanating from the structure resembled those of verified impact craters. The Ottawa-based researchers, led by David A. Forsyth, temporarily named their discovery the Can-AM structure because it straddles the Canadian-U.S. border. They describe the find in the August GEOLOGY.

Scientists have taken particular interest in impact craters over the last decade, in light of new theories linking meteor and comet strikes with ancient climate disruptions and mass extinctions. These theories suggest that massive meteorites collinding with Earth have periodically clouded the atmosphere with dust, triggering climate changes, photosynthetic failures and subsequent mass extinctions, including the great dinosaur demise some 65 million years ago (SN: 5/19/90, p.311).

But so far, the impact data have been too scattered to prove any correlation between the collisions and the species extinctions, says Jay Melosh, a planetary scientist at the University of Arizona in Tucson. With more craters like Can-Am to study, he says, “we can test those correlations.”

A hole the size of the Can-Am anomaly would have required a projectile about 3 miles in diameter, Melosh estimates. “We would expect 100 craters this size or greater during the last 500 million years,” he says. “We know of about six.”

Petroleum engineers might also want a closer look at Can-Am, since impact structures sometimes trap hydrocarbons. “The Can-Am structure may fall into this category,” Forsyth says.

The only way to establish whether the Can-Am structure actually resulted from a meteorite collision is to examine the rock beneath it. “Impact pressure [approximates] the pressure in the core of the Earth,” Melosh explains. Under the tremendous heat and pressure of a meteorite impact, rocks melt, shatter or metamorphose into strange forms that not even volcanic explosions can generate. Some of these forms — like stishovite, a phase of quartz produced under extremely high pressure — are “generally considered indisputable evidence [of an impact] when found near the surface,” Melosh says.

“We need somebody to drill [Can-Am],” Forsyth says. At an average depth of only a mile or so, this could prove light work for modern drilling techniques. The U.S. scientific drilling program, however, has stalled in recent years, and the fledgling Canadian drilling program is just getting off the ground. Forsyth says he has heard of no plans to probe the secrets of the Can-Am structure.

  • Equinox magazine, March/April 1991.