CRATER REFERENCE

IDENTIFICATION OF AN IMPACT EVENT 

Charles O’Dale

1. IDENTIFICATION TOOLS/METHODS

megascopic (overview – bird’s eye / satellite scale);

macroscopic (shatter cones, breccia, fractured rock – can be seen easily seen with the naked eye); and

microscopic (requires a microscope to see)

The principal criteria for determining if a geological feature is an impact structure formed by the hypervelocity impact of a meteorite or comet are listed below:

  1. Presence of shatter cones that are in situ (macroscopic evidence).
  2. Presence of multiple planar deformation features (PDFs) in minerals within in situ lithologies (microscopic evidence).
  3. Presence of high pressure mineral polymorphs within in situ lithologies (microscopic evidence and requiring proof via X-ray diffraction, etc.).
  4. Morphometry. On other planetary bodies, such as the Moon and Mars, we rely on the shape of the impact structure to determine its presence and type (simple versus complex, etc.). This is a megascopic quality (i.e., too big to be seen unaided by the human eye, thus requiring remote sensing, aerial photography, detailed mapping of multiple outcrops to assemble and view the typically km- or multiple km-size structure). On Earth, recognizing impact structures solely by their morphometry is complicated by two factors: (a) weathering, erosion, burial processes and tectonic deformation can obscure and/or destroy the original shape; (b) certain terrestrial features generated by means other than impact can have comparable circular form (e.g., volcanoes, salt diapirs, glacigenic features), such that a circular structure alone is not sufficient to claim impact structure status. Some buried craters have been revealed solely by geophysical techniques, although drill core is typically required to reveal macro- and microscopic evidence to prove an impact origin.
  5. Presence of an impact melt sheet and/or dikes, and impact melt breccias that were generated due to hypervelocity impact (macroscopic). These bodies typically have a crustal composition derived by the fusion of target rocks (i.e., there is no mantle contribution to the melt). Such melts may be contaminated by meteoritic (projectile) components (the latter requires specialized geochemical analysis to detect the projectile components). Melt sheets may be overlain by so-called fallback breccias (referred to as “suevite” by some workers), and material blasted out of the crater may form ejecta blankets about the original central cavity. For large impact events, ejecta can be distributed globally. Impact melt sheets are recognized by careful mapping and rock sampling followed by microscopy and geochemical analysis.
  6. Pseudotachylyte and Breccias: Pseudotachylyte is a rock type generated by faulting at either microscopic or macroscopic scales. However, pseudotachylytes are also associated with seismic faulting due to endogenic processes (e.g., earthquakes due to isostatic rebound and plate tectonics), so they are not exclusively impact generated. However, in association with features listed above, they can be a contributory criterion. Pseudotachylyte associated with impact structures may form in radial and concentric fault systems that help to define the megascopic structure of the crater. Pseudotachylytes can be included in a family of rocks referred to as breccias. Many different types of breccia can be developed as part of the impact process (including impact melt breccias listed in (5) above), but breccias can also form by endogenic processes. The interpretation of breccias therefore requires considerable care and experience. Moreover, they should not be considered diagnostic of impact, but rather contributory evidence.

In terms of relative importance, it is generally considered that criteria 1-3 above are definitive (they all relate to the passage of a shock wave through rock and resulting modification processes), with contributory evidence being added by 4-6 (which result from secondary effects, such as gravitationally driven crater modification). For buried structures that cannot be directly accessed, but are well-preserved as revealed by detailed geophysical techniques (especially seismic data), some workers consider this as strong evidence in favour of an impact origin. Normally, buried craters are verified by drilling and sampling the material directly for evaluation using criteria 1-3 above.

GEOMORPHOLOGY – One of the first indicators of a possible impact site is “circular geology”.
SHATTER CONES/SLICKENSIDES – Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events.
BRECCIA – Breccias are extremely common in the central uplift, in crater-fill deposits, and in the ejecta blanket of meteorite impact craters.
FRACTURED ROCK – While travelling toward impact sites I documented fractured rocks increasing in magnitude as we neared the crater site.
GRAVITY ANOMALIES – Gravity contours illustrate anomalies caused by fractured country rock under an impact site.
MAGNETIC ANOMALIES – Magnetic studies  document the magnetic disturbances within impact structures.
SHOCK METAMORPHOSIM – The extreme pressures and temperatures at hypervelocity impacts have caused shock metamorphic effects on target rocks.
CRATER EJECTA – When the crater formation process ends, the resulting circular structure and the surrounding area is covered by an ejecta blanket.
TEKTITE – Tektites are small, glassy pebble-like objects that form during meteorite impact. They represent droplets of molten target rock that are ejected up into the Earth’s atmosphere, which then fall back to the surface up to several hundred kilometers from where their source impact crater. They often acquire aerodynamic shapes as they fly through the atmosphere.
CRATER RIM – Documentation of surviving crater rims on this planet.
CRATER STRUCTURE – Why are impact craters almost always round? (or polygonal)?

2. IMPACT CRATER FORMATION

3. IMPACT CRATER CLASSIFICATION

4. METEORITE

5. CANADIAN IMPACT CRATERS – Documented in 1967 & 1975.

Reference:

Halliday, I. & Griffin, A. A.  Application of the Scientific Method to Problems of Crater Recognition Meteoritics, volume 2, number 2, page 79.

Osinski G. The Geological Record of Meteorite Impacts Canadian Space Agency 2018

Philpotts, A.R. 1964, Origin of Pseudotachylites, American Journal of Science, Vol 262.

ANN M. THERRIAULT – RICHARD A. F. GRIEVE – MARK PILKINGTON The recognition of terrestrial impact structures Natural Resources Canada