Gordon R. Osinski, and Ludovic Ferrière
Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and “double” cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationship Dsc = 0.4 Da, where Dsc is the maximum spatial extent of in situ shatter cones, and Da is the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.
Fractured bedrock and pseudotachylite at the “outskirts” of impact structures is NOT firm evidence of an impact. But these finds would indicate that further investigation to gather evidence of in impact may be warranted. One of the evidences used to confirm an impact is the discovery of shatter cones.
As the science of impact crater identification progressed in the 1950’s, it was soon realized that fractured and brecciated rocks in combination with a circular structure could not be used as absolute proof of an impact crater. Even though deformation of this type is consistent with meteorite impact, the equivalent deformation can also be produced by tectonic and volcanic activity (Grieve, Robertson 1975). It is then necessary to establish an alternative set of criteria to confirm identification of old impact craters. Shock-metamorphic effects in rocks have been established as the distinctive mark of an impact crater.
Curious cone-shaped rocks called shatter cones were first found in 1905 at the Steinheim Basin in southern Germany, an eroded meteorite crater about 3 km in diameter. In 1947, R.S. Dietz established that the recognition of shatter cones was a reliable geologic criterion for the recognition of impact structures (in the absence of meteorites). Dietz was the first to suggest that shatter cones only occur in impact (or explosion) craters. By 1964 shatter cones were found around the Sudbury ore body in Ontario, providing the first evidence for the meteoritic origin for the structure (Dietz 1947).
Shatter cones have been observed in rocks shocked in nuclear test explosions and produced experimentally in the laboratory. The required shock pressure to produce a shatter cone is estimated between roughly 2 and 20 GPa. In general, the apex of the cones points to the shock source, but irregular orientations and even counter orientation are frequent. Research proposes that shatter cones form from the passage of an impact pressure shock-wave.
The identification of shatter cones (especially poorly-developed ones) depends considerably on the experience and the eye of the beholder. They have several characteristics that distinguish them from non-impact features (French, Koeberl, 2009):
- they can form in all the rock types present in an impact structure: carbonates, shales, clastic sediments, granites, gabbros, and other crystalline rocks;
- they consist of penetrating fracture surfaces, along which the rock can be broken to reveal new cones or partial cones;
- the surfaces have positive and negative relief, and concave negative surfaces (“casts”) of the convex cone surfaces can commonly be found; and
- the striations on cone surfaces are distinctive and directional; they consist of alternating positive and negative grooves that radiate downward and outward from the apex of the cone. Secondary radiating striations commonly develop along the primary ones, forming a distinctive structure.
Shatter cones have been described from many meteorite impact structures and are widely regarded as a diagnostic macroscopic (can be seen with the naked eye) recognition feature for impact. They are a dead give-a-way for the amateur crater hunter (like us) to confirm that the structure being explored is the result of an impact (if a LARGE man-made explosion can be eliminated). In many cases, the initial discovery of shatter cones has spurred successful searches for other shock effects.
As of May 2010, there are 174 confirmed impact craters found on our home planet. Of 57 discovered in North America, 29 are in Canada, 27 are in the United States and 1 is in Mexico (Spray 2010). I would venture that there are several more remaining to be discovered, just look at the moon! Finding a formerly undiscovered impact crater here on our planet would be good science and a heck of a lot of fun!!
As amateur crater hunters tromping through the bush hoping to find the results of a large meteorite impact, we should realize that: (1) most rocks on Earth have never been involved in a meteorite impact; (2) even in impact structures, most of the rocks will not look shocked; (3) no matter how “circular” a structure may look, definite and unquestionable impact-produced features will only be found in the rocks (French 2005).
Shatter cones that ARE firm evidence of a hypervelocity impact (or a very large man made explosion). Some of the craters discovered over the years were first identified after finding shatter cones in the immediate area. So, as amateur impact crater hunters, to find shatter cones with these unique features we have to get down on the ground and root around.
Shatter cones have unique features visible to the naked eye and can be identified with the following characteristics: (1) shatter cones form in all rock types, the best ones in fine-grained rocks; (2) the orientation of shatter cones are relative to the centre of the impact structure; (3) freshly-broken samples will show shatter-coned interior surfaces; and (4) shatter-cones show grooves with positive and negative relief (French 2005).
In this article I will show the shatter cones I have found in situ that led to the respective structure being identified as an impact crater.
Charlevoix, a complex impact structure, is visible as a heavily eroded semicircular area located on the north-shore of the St.Lawrence River. Shatter cones were discovered in the course of regional mapping of the area (Rondot, 1966). They are widespread and abundant in the gneisses and limestone of the central uplift at La Malbaie (Charlevoix) crater. Virtually all rocks within a radius of 12 km from Mont des Eboulements are shatter-coned, with the exception of those on Isle aux Coudres (Robertson 1968).
This bedrock outcrop contains in situ shatter cones. The hypothesis that the Charlevoix structure might be the result of a cosmic impact originated here in 1965 when this outcrop was first studied. Jehan Rondot was in the midst of routine regional mapping in the Charlevoix region (Rondot 1966) when he recognized unusual fracture patterns here in a well exposed roadside outcrop. He later learned from John Murtaugh (who was mapping Manicouagan at the time) that he had discovered shatter cones (Dence 2004).
*Charnockite is a granofels that contains orthopyroxene, quartz, and feldspar. It is frequently described as an orthopyroxene granite.
Granites are felsic rocks that usually contain no or very little pyroxene.
The Isle Rouleau impact structure is in Lac Mistassini in central Quebec. Regional mapping in 1973 confirmed the structure as impact related (Caty et al., 1976). It is assumed that the island represents the eroded remnant of a central peak.
The Manicouagan impact structure in central Quebec contains a large ~65km diameter annular moat (filled by a water reservoir) which is very obvious in satellite imagery. The crater itself extends outside of the reservoir to a diameter of over 150 km. A geophysical survey was carried out in 1963 to confirm the structure as an impact crater (Grieve 2006).
Pingualuit – Slickensides?
During my exploration of the Pingualuit Crater, I specifically searched for any evidence of shatter cones. At the time I documented geologic “striations” in some rocks that were not shatter cones. These formations that I could not identify are documented in the images below.
I am suggesting that the impact at Pingualuit caused sudden fractures along local fault lines creating these features that might be identified as slickensides. Further I am suggesting that slickensides may form as a result of a cosmic impact, but they can not be used as confirmation of an impact.
This slickenside hypothesis of mine is going through the “peer review” process. I am inviting input on identifying what these structures are and if they are connected to the impact at Pingualuit.
Lac de la Presqu’ile impact structure is locate in Central Quebec and includes a roughly annular lake ~6km in diameter with a large promontory on its eastern shore. The structure was recognized as impact related with the discovery of shatter cones during the course of regional mapping (Higgins and Tate, 1990).
The Slate Islands impact structure is visible as a circular series of islands almost totally submerged in Lake Superior. The Discovery of extensive breccias and shatter cones during geological mapping of these islands in 1974 led to their recognition as the central peak of an impact structure (Sage, 1974).
The Sudbury Impact Structure comprises a 200-250 km multi ring impact basin formed 1.85 billion years ago. The core of the structure is elliptical, 60 x 30 km, containing a layered 2.5 km thick impact melt sheet, referred to as the Sudbury Igneous Complex (SIC). The SIC was formed by differentiation of the impact melt pool at the probable main contact point of the impactor.
Shatter cones have been reported up to 15 kms away from the periphery of the SIC. The cones commonly point toward the centre of the Sudbury basin, indicating that the Sudbury crater structure has undergone considerable erosion since the impact occurred 1.85 billion years ago.