WHAT MIGHT THE PHYSICAL CHARACTERISTICS OF IMPACT EJECTA TELL US ABOUT METEORITE IMPACTS?

Physical properties of average ejecta grain size, ejecta thickness and maximum size of shocked quartz, may tell a great deal about impacts. The relations described here constrain the distance to and size of the impact associated with orphaned ejecta or evaluate whether a particular ejecta layer and impact are correlative. One factor determining maximum grain size of shocked quartz is annealing of PDFs as they pass through the atmosphere. Following Croskell et al. (2002) there is a good match between maximum grain size and distance for minimum launch angles of 45^o to 65^o and distances of 2,000 to 6,000 km. Applying this, the Manson Crater at a distance of 215 km from the impact the maximum shocked quartz diameter is 3.24 mm (Izett et al. 1998) which corresponds for a launch angle of 45^o to an upper bound distance from the crater of 700 km.

The other method was inspired by Collins et al (2005). What was done is to combine the relationships between ejecta thickness and average grain size. The relationship between average ejecta size and crater diameter is an empirical one based on observations of Venus. Thus the crater diameter is the final crater diameter. The relationship for ejecta thickness uses transient crater diameter. To make the two relationships compatible the transient crater diameter is used to calculate the expected final diameter. Three equations relating transient crater diameter to final crater diameter are considered, Croft (1985), McKinnon and Schenk (1985) and Holsapple (1993). The equations of Croft (1985) and McKinnon and Schenk (1985) give crater dimensions for Sudbury and Chicxulub consistent with field observations. Holsapple’s (1993) equation gives a final diameter for Chicxulub between 125 and 165 km and for Sudbury of 115-180 kilometer, which is too small.

The Archean was a time when many orphaned ejecta layers were deposited in the Transvaal (Africa) and Pilbara (Australia) cratons. The Paraburdoo (Australia) and Reivilo (South Africa) spherule layers appear to be correlative (Hassler et al., 2011). Average grain size and ejecta thickness of the Paraburdoo gives an impact crater 60 to 80 kilometers in diameter and crater to ejecta distance of 1250 to 1500 km. This result supports the hypothesis that the Transvaal and Pilbara cratons were once together in the super continent of Vaalbara.