An obvious craterform is an excellent indicator of a possible impact origin; particularly, if it has the appropriate morphometry as illustrated in PART II of this series. But as noted, such features are rare and short-lived in the terrestrial environment. The burden of proof for an impact origin generally lies with the documentation of the occurrence of shock-metamorphic effects. Impacts produce distinctive “shock-metamorphic” effects that are found in situ within the crater and allow impact sites to be distinctively identified. Such shock-metamorphic effects, in addition to the shatter cones and slickenslides, include brecciated rocks, suevites, impact melts and pseudotachylites. They attest to the destructive power of the impact event.
BRECCIA – (from a Latin word meaning “broken”) is a rock that is composed of angular fragments of other rocks surrounded by a fine-grained “matrix” that may be of a similar or a different material. Breccias are extremely common in the central uplift, in crater-fill deposits, and in the ejecta blanket of meteorite impact craters.
IMPACT MELT – rock that has been made temporarily molten as a result of the energy released by the impact of a large colliding body. Impact melts include small particles, known as impact melt spherules, that are splashed out of the impact crater, and larger pools and sheets of melt that coalesce in low areas within the crater. They are composed predominantly of the target rocks, but can contain a small but measurable amount of the impactor.
PLANAR DEFORMATION FEATURES (PDF) –
The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions. This photograph of a thin slice of plagioclase, 0.03 millimetre thick, is seen here in cross-polarised light, with a ‘sensitive tint’ plate. The original plagioclase is coloured yellow and the shock-changed mineral is purple. This sample is from the Manicouagan impact structure. (Courtesy Denis W. Roy & MIAC).
PSEUDOTACHYLITE – is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench textures that began to crystallize from the melt. It is formed when a high pressure from an impact is applied to country rocks and then abruptly released. This causes the rock along and within fracture lines or faults to partly melt. The fractures or faults containing the pseudotachylite are welded shut as soon as the motion created by the impact stops.
Microscopic shock metamorphic features, shatter cones, impact glasses and pseudotachylites were formed during the contact and compression phase of the impact process. Polymict, clastic matrix breccia dikes, suevite, and bunte breccia contain fragments that were formed during the excavation and central uplift stage of the impact process when target rocks were in a cohesionless state allowing long-range fragment mixing. Subsequent stress is supported by the pseudotachylite as though it had never been active. The entire period of activity of a fracture or fault filled with pseudotachylite may be measured in minutes. (e.g., Pseudotachylite is a rock type formed by friction-induced melting, during very rapid deformation) Philpotts 1964; Maddock 1983.
SUEVITE – a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as impactites.
Barringer – comminution and facturing
Brent – breccia
Charlevoix – impact melt
This example of impact melt rock was extracted from within the Charlevoix impact structure. 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 compressability of the mineral itself. If the post shock 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.
Glover Bluff – breccia
Isle Rouleau – breccia
The day after exploring the Isle Rouleau impact structure we had an interview with the government geologist posted at Baie-du-Poste. After an excellent conversation regarding the local geology (and we did brag a bit about our explorations!), she was gracious enough to give us a sample of breccia recovered from the Isle Rouleau site. Thank you!
Manicouagan – breccia/melt rock/pseudotackylite
Approximately 214 million years ago an estimated 10 kilometre wide hypervelocity meteorite impacted at between 12 and 30 kilometres per second and formed the Manicouagan Impact Structure.
The resultant 100 kilometre diameter crater (image left – Courtesy NASA/LPI) is one of the largest impact craters still preserved on the surface of our planet. The Copernicus crater on the moon (image right – Courtesy NASA – A12) has a diameter of 93 kilometres. For comparison, the Manicouagan’s annular moat would fit comfortably within the rim of the Copernicus Crater.
Morphological elements of the Manicouagan structure are based on topographical expression and are:
- outer circumferential depression – ~150-km outer diameter;
- outer disturbed zone – ~150 km diameter;
- inner fractured zone – ~100 km outer diameter;
- annular moat – ~65 km outer diameter;
- inner plateau – ~55 km outer diameter;
- central region – ~25 km outer diameter. (Grieve, Head 1983)
In the summer of 2006, Eric Kujala and I explored the interior of the Manicouagan impact structure by canoe and on foot. We entered the structure from the east, crossing the all these morphological elements and concluding in the Memory Bay inlet. This inlet is on the east portion of the island forming the central peak of the structure. To read about our harrowing experience while in the crater please see my web site about the trip (O’Dale 2006).
At the point of impact, the country rocks were instantaneously evaporated/melted/shattered by the energy released leaving a 200 to 600 cubic kilometre sheet of impact melt directly on basement rocks. We observed changes in these impactite textures as a progressively increasing proportion of superheated melt and a decreasing fraction of cold fragmented country rock material toward the interior of the crater (Simonds 1976). The following images will illustrate these observations.
Outer Circumferential Depression, Outer Disturbed Zone and Inner Fractured Zone
In PART IV of this series I documented the shattered rock we encountered at approximately 40km from the central peak as we entered the outskirts of the crater. We did could not identify the outer circumferential depression.
The water filled circular annular moat that is prominent in space images is only one third of the size of the original crater. The water in the annular moat fills a ring where impact-brecciated rock was eroded away by glaciation. Before flooding of the reservoir, isolated outcrops of tilted and deformed limestone, siltstone and shale were found on the inner edges of the moat (Murtaugh, 1975).
The Inner Plateau of the Manicouagan structure is bounded by the annular moat, overlain by melt sheet, underlain by shocked basement rock (Orphal, Schultz 1978). We found a “lunar landscape” here containing various breccia types. The astronauts exploring the moon found that impact-melt breccias, similar to what we found here, were the most common rock types at the Apollo highland sites (Apollos 14, 15, 16 and 17) (Haskin 1998). We documented impact breccias formed by similar and very different country rocks like those found on the moon!
Impact breccias were melted, mixed, crushed and compressed by shock waves at various stages in the cratering process: (1) during the initial shock-wave expansion and transient crater formation; (2) during the subsequent modification of the transient crater. Even within the brief formation time of an impact crater, it is possible for the multiple generations of breccia to develop and to produce distinctive differences, even though the time between one breccia generation and the next may be measured in seconds or minutes (French 1998). The extremely small size of the grains within the matrix between the country rock fragments were formed by the very high pressure of the gas generated when the bolide impacted.
The Central Region of the Manicouagan Structure is a complex zone of uplifted, shocked and metamorphosed basement rocks with small tabular bodies of impact melt and pseudotachylite veins (Orphal, Schultz 1978). Recent U-Pb zircon dating of the impact melt gave an age of 214 ± 1 million years.
I tried to climb the talus slope up to the cliff face but it became very unstable the higher I climbed. I got to the point that I was creating dangerous rock slides without making any progress. I stopped to take this picture; looked down and found the “Manicouagan shatter cone” I documented in PART IV of this series. Serendipity at its best!
While we explored the impact melt cliffs on the north shore of Memory Bay we noticed an odd feature in one of the cliffs. Eric took this image of the feature. It shows a 10 m block of mafic gneiss (indicated in the image) suspended about 20 m above the base of the melt sheet. Such a block is 0.3g/cm³ denser than the melt and should settle at a minimum of 5 cm/sec (Stokes Law ) through a Manicouagan composition melt with 2% H2O (water) if it were still liquid at 1000°C. In order for that block to remain suspended, the melt must have begun to crystallize rapidly enough to trap the block before it settled to the bottom of the sheet (Simonds 1976).
 STOKES LAW: If the particles are falling in the viscous fluid by their own weight due to the Earth’s gravity, then a terminal velocity, also known as the settling velocity, is reached when this frictional force combined with the buoyant force exactly balance the gravitational force. The resulting settling velocity (or terminal velocity) is given by:
Vs = ( 2 (ρp – ρf ) / 9 η ) g R2 where:
- Vs is the particles’ settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf ),
- R is the radius of the spherical object (in metres),
- g is the Earth’s gravitational acceleration (m/s2),
- ρp is the mass density of the particles (kg/m3),
- ρf is the mass density of the fluid (kg/m3), and
- η is the fluid’s viscosity (in [kg m-1 s-1]).
Newporte – Breccia
Pingualuit – Impact Melt
The exploration history of the Pingualuit impact structure is a classic example of the difficulties for firm impact identification. Pingualuit Crater has a diameter of 3.44 km and a depth of 400 metres. It had kept its original shape over eons of erosion. Geologists were sure that this structure was the result of an impact, mainly because there are not any natural geological events that could explain how this structure formed. BUT, there was no firm evidence for impact. E.M. Shoemaker explored the area in 1961, and in his view, there remained little doubt of a meteoritic origin. He stated that obtaining critical evidence probably would require drilling through the crater floor (Shoemaker 1962). Boulger found a rounded vesicular pebble 1.75 cm across that was totally unlike any of the country rocks. The sample was sent to the Harvard-Smithsonian Centre for Astrophysics for petrographic examination. A thin section proved to be rich in quartz grains with multiple sets of planar features (Marvin, Kring, and Boulger 1988). Planar deformation features in quartz confirm an impact event (Grieve 2006).
Slate Islands – breccia
The ~7-km-wide Slate Islands group was created by a bolide impact 436 million years ago in northern Lake Superior. They represent the heavily eroded central peak of a ~32 km diameter (from bathymetric data) complex impact crater. It is not known if the present height of the central peak island is the result of stratigraphic uplift only or of uplift followed by partial collapse of the central peak and erosion. Target rocks consist of three main groups of Archean and Proterozoic supracrustal and intrusive rocks, about 2.7 Ga and 1.8 Ga and 1.1 Ga old respectively. Heterogeneous melt bodies are located within heavily brecciated units of the Slate Islands central uplift peak (Dressler et al, 1995). Specific impact breccia types in the target rocks are related to the various phases of the impact process.
Specific impact breccia types in the target rocks are related to the various phases of the impact process and are made up of fragments of the target rocks, containing various ratios of impact melt and shocked mineral inclusions. The breccia deposits illustrated here are typical for almost all the other breccia deposits I found on the islands.
Sudbury – breccia
The Sudbury Structure comprises a 200-250 km multi ring impact basin formed at 1.85 Ga. 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.
Immediately interfacing the upper SIC is the grey Whitewater breccia that contains many large angular rock fragments floating in a glass like amorphous rock. These fragments are the fallback particles from the surrounding Huronion supergroup country rock that were deposited immediately after the impact. These rock fragments were ejected ballistically hundreds of km into the atmosphere and then minutes to hours later “plopped” into this still molten rock. Note the large fragment in the lower right of the image that is hydrothermally altered and surrounded by a “chilled margin” (a mineralized area around the fragment caused by a hydrothermal vent). The size of the Sudbury structure implies that the hydrothermal venting continued for thousands of years after the impact.
Further into the structure is the darker Whitewater breccia containing smaller rock fall-back fragments originating from the igneous quarts granite north range footwall. Here the breccia indicates the introduction of carbon. A biogenic origin of the carbonaceous material (soot) found in the black Whitewater Group is theoretically caused by the evaporation/condensation from the hot impact fireball and/or from a later global cloud. The colour of the rock is not uniform indicating that the carbon is not uniformly distributed. Bucky balls (soccer-ball-shaped molecules of 60 carbon atoms) possibly of extraterrestrial origin and with traces of helium and argon gas trapped inside were found in this breccia.
The impact probably occurred in a shallow sea as there is evidence of water flow-back in the top layers of the Whitewater Group. The quantity of “breccia fall-back” specifies that the fall-back segment of the impact lasted a substantial amount of time (perhaps hours) before the appearance of the returning tsunami.
Sudbury – pseudotachylite
Sudbury pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of Sudbury pseudotachylite are found up to 100 km north of the SIC.
The black pulverized (by the impact) country rock injected into the pink gneiss, was formed when the high pressure from the meteorite impact was applied to these rocks and then abruptly released. This caused the rock along and within these dykes to partly melt. The dykes containing the pseudotachylite were welded shut as soon as the motion created by the impact stopped. Subsequent stress was supported by the fault as though it had never been active. The entire period of activity of a fault filled with pseudotachylite may be measured in minutes. (e.g., Pseudotachylite is a rock type formed by friction-induced melting, during very rapid deformation) (Philpotts 1964; Maddock 1983).
Around the “outskirts” of the Sudbury impact structure are many examples of pseudotachylite deposits. Pseudotachylite like this is also found around the large Vredefort impact structure in Africa. At Sudbury, the pulverized and melted country rock injected into the pink gneiss country rocks has similar chemistry to the derived gneisses. The pseudotachylite zones fall along structures inferred as super faults or fault-controlled structures.
Ten pseudotachylyte samples from the North Range of the 1850 Ma Sudbury impact structure have been analyzed by the 40Ar/39Ar laser spot fusion method. Field and petrological evidence indicate that the pseudotachylytes were formed at 1850 Ma by comminution and frictional melting due to impact-induced faulting. The cryptocrystalline to microcrystalline grain size (<30 μm) of the pseudotachylyte matrices and the predominance of orthoclase as the main K-bearing phase, have rendered the rocks particularly susceptible to Ar loss. The age determinations range from ∼1850 to ∼1000 Ma, with some samples yielding multiple ages that cannot be correlated with known geological events in the area. However, if the finite-difference algorithm of Wheeler (1996) is used to calculate combined Ar loss and the accumulation of radiogenic Ar for the K-bearing phases, it is possible to reproduce the range of observed ages. The model infers that the long-term volume diffusion of Ar has occurred and that, as a result, the Ar system cannot be treated with a conventional closure temperature approach. The algorithm requires burial of the impact structure to 5–6 km depth and 160–180 °C at 1850 Ma, followed by exhumation at ∼1000 Ma. These ages may be equated with two events: Penokean thin-skinned overthrusting in the North Range, immediately following impact, and exhumation ∼850 Ma later, coincident with the Grenville orogeny to the southeast. The results suggest that, contrary to previously accepted paradigms, the North Range has been affected by a protracted period of postimpact, low-grade thermal metamorphism. If these events also involved tectonic shortening within the North Range (as has been documented for the South Range), then the original size of the Sudbury impact structure has been underestimated.(Spray et al, Feb 2010)
Skelton Lake – breccia
Skeleton Lake is a generally circular lake provisionally classified as a simple meteorite crater. It is located in the Muskoka District of Ontario on the Canadian Shield slightly east of Georgian Bay. It is the largest open body of water within the Muskoka Lakes.
These images illustrate a breccia deposit on Opal Island situated in the area of the crater rim. This breccia may have been formed as the result of a hypervelocity impact.
As of this date (Feb. 2011), firm evidence of an impact has not been found in the Skelton Lake structure.
Wanapitei – suevite
The Wanapitei structure was confirmed as an impact crater by presence of coesite, which can be formed at pressures of 425-500 kilobars and temperatures near 1000°C (Dence et al. 1974). Lake Wanapitei is classified as a simple crater with an estimated diameter of 3 km (E. L’Heureux et al, 2003) to ~7-8 km. There is no evidence of a central uplift in the submerged crater (Dence and Popelar, 1972).