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Journal Article

Searching for Distal Ejecta on the Craton: The Sedimentary Effects of Meteorite Impact

Andrew Schedl
The Journal of Geology
Vol. 123, No. 3 (May 2015), pp. 201-232
DOI: 10.1086/681624
Stable URL: http://www.jstor.org/stable/10.1086/681624
Page Count: 32

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Topics: Ejecta, Impact craters, Earthquakes, Tsunamis, Seas, Diameters, Waves, Velocity, Water depth, Liquefaction
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Searching for Distal Ejecta on the Craton: The Sedimentary Effects of Meteorite Impact
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Abstract

AbstractDistal ejecta from meteorite impacts are rare and hard to spot in the field because of their thinness (<1 m). This article suggests looking for distal ejecta in association with larger sedimentary structures, seismites, slope failure features (slumps, turbidites, and submarine landslides), and offshore tsunamites (water depths ≥25 m) in former epeiric seas. Distal ejecta tend to overlie seismites and slope failure features because seismic waves travel faster than ejecta. In epeiric seas, impact-generated tsunamis travel slower (25–85 m/s) than ejecta, so the tsunamis rework ejecta deposits. However, if a tsunami moves ‘from shallow to deeper water, then it buries the ejecta deposit, preserving it, as observed for Acraman. Impact-generated offshore-tsunami deposits (≥25 m water depths) are potentially distinct from those generated by cratonic earthquakes. The impact tsunamis can transport >5-mm-diameter sediments and produce high-flow-regime bedforms, megaripples, dunes, upper-plane beds, and possibly antidunes, whereas earthquake tsunamis cannot produce these features in an offshore environment. The last section of the article discusses the relationship of the physical character of the ejecta (thickness, accretionary lapilli diameter, and maximum grain size of shocked quartz) and distance to and size of impact. This information could be used (1) to link a particular ejecta layer to a particular crater, (2) to put constraints on where to look for a crater associated with a particular ejecta layer (Alamo, 250–325 km from the center of the crater; Stac Fada, 225–300 km from the center of the crater), and (3) to provide constraints on the size of a crater, which may have been destroyed by erosion and tectonics (Alamo, 40–70 km diameter; Stac Fada, 80–150 km).

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