United States Meteorite Impact Craters

Only a tiny fraction of the scientific work that needs to be done on Earth's known impact craters has been accomplished, and only a small fraction of the planet's craters have been discovered.  Even more work remains to be done on other surfaces in the solar system.  With the following list of questions, topics, and references to examples of specific kinds of work, I hope to save others a bit of time and effort if they are beginning to consider the field from the perspective of a researcher.

The order of the topics in the list is somewhat arbitrary, though I've tried to place them more-or-less sequentially in a manner such that each informs the following: ie:  Prove there is a crater, describe its rough location, place it in time and in a contemporary geological setting, characterize the impactor, understand the geological and geographical setting at the time of impact, and so on.  The list is far from exhaustive, and will grow with time.

Crater confirmation - Is the structure a crater?

• field search for shatter cones
• mapping of shatter cones, breccia, and topographical disturbance
• field mapping of ejecta/glass/meteorites for very small craters
• instrumental identification of high pressure mineral polymorphs
• drilling and core section analysis
• petrographic search for grain-scale indicators of sock metamorphism - planar fractures (PFs), planar deformation features (PDFs), diaplectic glass, ballen quartz, and so on.
• instrumental identification of elemental or isotopic indicators of impactor components preserved in melts or breccias

Crater Morphometry - What is the structure's physical shape and size?

• diameter, depth, rim and uplift height, depth and nature of fill or removal
• topographical results of weathering and decay
• dimensions of uplift or annular basin and associated faulting
• asymmetries and implications for impact angle or ejecta emplacement
• computation of likely impactor size/velocity
• volume of altered or displaced rock.

Context - What rocks were here and where have they moved to?

• regional geologic mapping
• crater structural mapping
• crater geological mapping

Impactites - How did impact alter the rocks?

• pre-impact rock units
• displacement of pre-impact rock units 
• petrology and petrography of pre- and post-impact-alteration rocks
• grain-scale altertion of specific minerals
• evolution of new minerals by shock and heat
• brecciation - distinguishing types of breccia, mechanism of origin, and timing of emplacement (e.g.: Bjornerud M. G., 1998)

Constraints on Peak Shock Pressure

• shatter cones
• coesite and stishovite
• PFs and PDFs in quartz
• XRD indicators in carbonates (e.g.: Henderson and Milam, 2014, 2015; Bell M. S., 2010; Ská- la R. and Jakeš P., 1999; Skála R. and Hörz F., 2002; Huson S. A., et al., 2009)

Other High Pressure Polymorphs and Shock Effects (Petrographic)

• Twinning, kink banding, evolution of diamond, rutile, etc.

Historical / Paleo- Setting and Effects of the Impact: 

• geologic and geopgraphic setting at time of impact
• biological effects
• destructive radius of heat, seismic, wind blast, and shock
• marine or land impact

Crater age:

• conodonts or other biostratigraphic
• radioisotope studies
• fission-track age (Koeberl et al., 1993; Weber et al., 2005; Crowley et al., 1989)
• paleomagnetic
• stratigraphic constraints on crater or ejecta
• timing of regional mineralization relative to fracture/breccia formation

Characterizing the impactor:

• dimensions,
• type (from isotopes or bulk elemental ratios in target rocks)
• velocity
• vector

Surface crater mapping: GIS topography, faults and rock units, Morphology and topography?

Subsurface and Remote Studies

• gravity
• seismic profile
• drilling
• magnetic

Contribution to Global Understanding

• relation to other known craters in age, size, impactor type, target material, etc.
• identification of trends in global data sets
• projections of unidentified crater numbers.

Post-impact processes -

• crater degradation - burial/erosion
• geochemical changes to excavated or impact aletered materials
• hydrothermal processes
• mineralization of crater associated faults 
• resource concentration or potentials 
• regional changes to target rocks: dolomitization, displacement, metamorphism, tilting, etc.

There are also non-crater-specific studies - impact modelling, high pressure mineral phases, crater accumulation rates, crater to impactor scaling, morphology to impact energy scaling, morphology to rock type scaling, hypervelocity impact physics, extinctions studies.

And non-traditional cratering questions: effects of impacts in water and in air (meteors).

And of indirect changes: proximal and distal ejecta studies (sometimes without an associated crater), Ir anomalies, sperules and spherule beds, tsunamite and paleoseismic deposits.

Inferred impact risk and impact risk abatement.

Bibliography:

Bell, M.S. (2010) GSA Special Papers, 1-20.

Henderson, T. and Milam, K. A. (2014) Initial XRD analysis of Silurian carbonates from the central peak of the Kentland impact structure, Newton County, Indiana, USA (abstract). Abstracts of the 2014 GSA Annual Meeting, Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.760.

Henderson, T. and Milam, K. A. 2015. XRD Analyses of Silurian Dolostones from the Central Uplift of the Kentland Impact Structure, Newton County, Indiana, USA (abstract). 46th Lunar and Planetary Science Conference.

Ská- la, R. and Jakeš, P. (1999) GSA Special Papers, 339, 205-214.

Skála, R. and Hörz, F. (2002) GSA Special Papers, 356, 3070.

Huson, S.A., et al. (2009) Meteoritics and Planetary Science, 44, 1695-1706.

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below this line are website development notes to myself:

update and upgrade summary graphics with appropriate citations

finish integrating basic bibliographies / annotations

produce single-page 'information sheet' downloadable PDF summaries for each site

convert all references to consistent format

Update LPSC, GSA, and AAPG abstracts:

Search GSA abstracts: https://gsa.confex.com/gsa/htsearch.cgi

Search LPSC abstracts:http://www.lpi.usra.edu/publications/abstracts.shtml

Search AAPG abstracts: http://www.searchanddiscovery.com/

get copies of those publications not already on hand

provide well cited summary paragraph of basic metrics for each crater

cross-link to Odale pages, eg: http://ottawa-rasc.ca/wiki/index.php?title=Odale_articles_Glover_Bluff

Build and provide summary metrics of US craters on intro page

Note the notable gaps in knowledge about each crater on the associated individual crater pages.

Somehow distinguish key articles and field guides.

It would be nice to do a mini stratigraphic profile for each, with depth of cover, depth of sedimentary target rock, depth to basement, uplift height, and known depth of disturbance.  Could relate this to diameters as well, to create a nice picture of the set.

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There is occasionally some humor in attempts to describe impact megabreccia or central uplifts by geologists using conventional mapping techniques, particularly when these descriptions were written before the impact paradigm had fully emerged or been accepted.  The frustration, at times, must have been considerable.  I've collected a few humorously poignant examples here:

This one is from Robert Boyer, 1953. "A rotational type of fault of greater size separates the eastern and western portions of the quarry. A rotation of 90 degrees resulted in the attitude of bedding of the two halves of the quarry being essentially at right angles." < This is how you cuss in a top hat. >

Another from the same work: "The exact locations of the fault planes are often difficult to map, and intense shattering of all of the rock aids in disguising them." < I paraphrased this in marginalia as "#$@&%*!" >

Raymond Gutschick (1983) wrote: "More than 1000 feet (305 m) of stratigraphic section or succession is exposed in fault blocks in the quarry, even though the maximum depth of the quarry is only 330 feet (101 m)!" <It's bigger on the inside.>