a K-Ar ages of 372 million years (m.y.) and 342 m.y. for impactites and 335 m.y. for pseudotachylites. Age recalculated to 342 ± 15 using post 1977 decay constants (J. Whitehead, 2002).
The rubidium-strontium age of the crystalline country rocks at the Charlevoix crater is 1280 m.y., suggesting that the Rb-Sr clock was unaffected by the impact (Rondot 1971).
I took this panoramic image of the Charlevoix crater looking east from about 2000 feet above the western most area of the impact modified zone. In the immediate foreground are the terraces and in the mid foreground is the annular peripheral trough (ring graben) visible as the ring of light coloured vegetation surrounding the central uplift region. The central peak, Mont des Eboulements, is silhouetted in the background against the St. Lawrence River.
General Area: The Charlevoix Impact Crater, a multi ringed basin with a central uplift (Rondot 2000), is located in southern Quebec on the north shore of the St. Lawrence River, 105 km NE of Quebec City and lies on the southern edge of the Canadian Shield. The structure was identified as a meteorite crater in 1965 with the discovery of shattercones in the area.
The initial dimensions of the crater were estimated to have been 28 km in diameter (the inner circle in the landsat image) and 10 km in depth immediately after the impact. The subsequent post impact crater collapse created the central peak and peripheral modification resulting in a final crater structure of 54 km in diameter (outer circle in the landsat image). Erosion has removed the original crater rim, some of the central uplift and the crater-fill products but the crater floor has been preserved.
Specific Features: Structure is dominated by a 1.5 km wide semi-circular peripheral valley, which lies interior to a ring of hills defining a diameter of -46 km. Interior to the valley lies a plateau and a central peak which rises to -750 m above sea level. The western half of the crater has been tectonically removed by a major fault system which runs down the St. Lawrence Valley.
Illustrated in the schematic:
The St. Lawrence River separating the crystalline, Grenville age, Precambrian shield rocks to the north and west, from the northeast-trending Palaeozoic sedimentary rocks of the Appalachian region to the south and east;
Under the St. Lawrence River is the Palaeozoic sedimentary rock that buried the south-eastern part of the crater;
The deepest part of the St. Lawrence River is immediately next to the crater.
It is proposed that the meteor impact weakened the rift faults and introduced its own fractures. The present earthquake activity probably occurs along these weak fault surfaces. The effect of the impact crater on the type of faulting versus depth is not readily discernable from available data. In general, meteor impacts do not leave neotectonic seismic signatures therefore the Charlevoix impact crater might represent a different case because of the presence of weakened paleo-rift faults (Lamontagne et al. 2000).
Earthquakes are generated at the present times as the broken crust continues to move in response to movement of the continents (R. Eyles; ONTARIO ROCKS).
Charlevoix Earthquake Zone
The structure coincides with the most seismically active area in eastern Canada. It is not known whether there is a physical connection between the impact deformation and seismic activity.
The Charlevoix crater region is historically the most active earthquake zone in Eastern Canada, illustrated in this Digital Elevation Model with Epicentres (Lamontagne et al. 2000). The region has been monitored by a microseismic array since 1977, yielding accurate locations of the earthquake’s hypocenters. Previous analyses of data from the array indicated a relationship between the earthquakes and the St. Lawrence Valley paleo-rift faults (Note 6). Within the impact structure, the highly fractured basement releases strain energy in small earthquakes but it is unclear why earthquakes do not occur over the whole impact structure. The larger earthquake events (stars in the Digital Elevation Model with Epicentres chart) tend to be concentrated at both ends of the Charlevoix Seismic Zone located at the periphery of the impact structure. Most of the smaller earthquake events tend to be concentrated to the south east of the crater under the St. Lawrence River where the crater was obliterated and buried by early Palaeozoic sedimentary rock.
This is a chromo-stereoscopic image (about 80 km by 90 km; 30-m pixel size) that integrates the RADARSAT-SAR ortho-image with terrain elevation and seismicity (each data set with its own colour range). For the elevation, the colour range varies from 0 m in blue to 1100 m in red. The texture of the land surface comes from the RADAR data. While the south shore is a gently rolling landscape, the north shore is a mixture of rugged highlands, plateaus and valleys, separated by dramatic changes in elevation. The earthquake hypocenters (circles) for January 1978-September 1999 are overlain, with colors related to focal depth. White triangles are the stations of the Charlevoix Local Seismograph Network, part of the GSC’s Canadian National Seismograph Network: (www.seismo.nrcan.gc.ca).
At the time of the impact during the Alleghenian Orogeny, America was colliding with Africa and Europe resulting in mountain building in the area. Fish are dominant in the oceans, the first land animals appear and plant life on land becomes highly developed.
As illustrated in this Aerial Radar image, the morphology of the Charlevoix crater consists of:
Country rock at the outer modification zone of the crater;
Terraces (created from the initial crater collapse and modification);
Annular peripheral trough (ring graben);
Inner ring of hills;
Central peak (Mont Des Eboulements, is 780 metres above sea level).
Autochthonous breccias in the gneiss (material brecciated but still virtually in place) are prominent along and inside the peripheral trough near the margin of the crater at about 15 to 17 km from the center of the structure. Not all exposures in this zone are brecciated but all rocks at this distance from the center do exhibit an abnormally high degree of fracturing.
Shatter cones occur both in the Precambrian foliated crystalline basement and in the Ordovician sedimentary cover within 12 km of the central peak.
The paleo-rift faults consist of at least four fault lines running parallel in the St. Lawrence River and along its north shore through the Charlevoix crater. The break up of Pangea at about 150 million years ago saw the widespread formation of faults called rifts or grabens. Some widened to become new oceans (North and South Atlantic) but many others “failed” to develop and are preserved today as aulacogens deeply buried below younger strata. Large rivers follow these lines of structural weakness (such as the Ottawa and St. Lawrence rivers).
Thrusting along the St. Lawrence River fault lines during either the Taconian or Acadian Orogenies, or both, obliterated the south-eastern portion of the crater. An unknown thickness of early (pre-Charlevoix impact) Palaeozoic sedimentary rock was then transported over the obliterated portion of the crater. Finally the St. Lawrence River flooded the area leaving the semi-circular crater remnant. The deepest parts of the St. Lawrence in this area are along its interface with the crater. The missing part of the crater has not been detected under the St. Lawrence River. The rocks of Isle aux Coudres, visible at the extreme lower right (south-west) of the image immediately to the south of the crater in the St. Lawrence River, are Palaeozoic sedimentary rocks that have been transported to this position from the southeast after the impact (Robertson, 1968).
The water depth of the St. Lawrence River here is relatively shallow but between Isle aux Coudres and the crater it reaches 55 metres or more (Robertson, 1968).
The relatively flat Isle aux Coudres does not share any of the Charlevoix impact crater’s geomorphology. The early Palaeozoic limestone deposits on Ile aux Coudres are older than the impacted Ordovician limestone within the crater but they do not show any evidence of shatterconing or brecciation of the types found on the mainland that are at an equivalent distance from central peak Mont des Eboulements. Isle aux Coudres is therefore not a remnant of the missing part of the structure but is an assemblage laid down elsewhere prior to the meteorite impact and transported to its present location by tectonism.
The size of the Charlevoix meteorite crater and the magnitude of the events that created this structure were impressed on me when it took almost 10 minutes to fly the diameter of the crater zone. As well, I am still struck by the strength of the geological forces that obliterated almost half of this crater and then made it virtually disappear!
In the summer of 2005 Ron St. Martin and I spent a few days exploring the Charlevoix impact structure.
As the central uplift of the structure was being formed, rocks around the periphery of the transient crater collapsed downward and inward to form one or more depressed rings (ring grabens or troughs). Here I am standing on the north-east portion of the annular plateau that was formed during the crater periphery collapse. The inner ring of hills and the central peak are visible behind me. If I was standing here at the time of impact, I would certainly be having a very bad day (but I am enjoying myself here)!
The airport is located on the annular plateau that is visible in the background to the north-east in this image. In the center of the image is the trough that separates the annular plateau from the inner ring of hills. This image was taken from the south-western section of the annular plateau facing north-east.
*Charnockite is a granofels that contains orthopyroxene, quartz, and feldspar and is frequently described as an orthopyroxene granite. Granites are felsic rocks that usually contain no or very little pyroxene.
In the small trough dividing the inner ring of hills from the annular plateau is a series of shattercone deposits like the one illustrated here. Verification of an impact origin requires the discovery of unique impact-produced features. At present, one of a few of the accepted impact features is the presence of shatter cones. In the field, well developed and indisputable shatter cones are the best indicators of impact if they are distinctive and widely distributed, especially in the basement rocks of deeply eroded structures.
Unfortunately for both of us, the gross weight allowance of my airplane dictated that we could only take on so much additional weight in order to safely return home. For this reason we had to leave many excellent shattercones specimens behind. Ron once asked me if we could take back a very large shattercone we had found, and I regretfully answered, “Yes, but you will have to stay behind”.
Another feature that verifies an impact origin is impact melt rock. At pressures in excess of about 60 GPa, rocks undergo complete (bulk) melting to form impact melts. The melts can reach very high temperatures due to the passage of shock waves that generate temperatures far beyond those commonly encountered in normal crustal processes or in volcanic eruptions. Each mineral grain is instantaneously raised to a post-shock temperature that depends on the shock-wave pressure and on the density and compressibility of the mineral itself. If the postshock temperature produced in a mineral exceeds its normal melting temperature, each grain of that mineral in the rock will melt, immediately and independently, after the shock wave has passed. The melt will have approximately the same composition as the original mineral before any flow or mixing takes place, and the melt regions will initially be distributed through the rock in the same pattern as the original mineral grains. Note the country rock fragment in the inclusion.