Progressive Shock Metamorphism of Quartzite Ejecta from the Sedan Nuclear Explosion Crater

Cambrian and Mississipian orthoquartzites, present as fragments in alluvium, experienced shock-wave pressures up to 500+ kb during the Sedan (100 kiloton) nuclear cratering explosion. Ejecta samples display diverse shock-damage effects correlative, in part, with increasing peak pressures that establish a sequence of progressive shock metamorphism having these principal characteristics: (1) Lower pressure effects include cataclasislike shattering of individual quartz grains by irregular microfractures and subparallel fractures cutting across grains. (2) Shock-induced discontinuities (planar features) in quartz show systematic variations with increasing shock_damage. As planar feature sets per grain increase from 1.18 to 4.75, their orientations coincident with $$\omega{10\bar{1}3}$$ decrease in frequency from 60+ percent to 35 percent and $$\Xi{11\bar{2}2}$$ sets decrease from 12 percent to 3 percent, whereas $$\pi{10\bar{1}2}$$ increase from 0 percent to 35 percent. Basal features, another shock criterion, form in relatively few samples. Examination by scanning electron microscope reveals planar features to be structural discontinuities rather than open fractures. Lack of preferred orientation of quartz c-axes or planar features relative to possible principal stress axes indicates that, at higher shock pressures, a nearly isotropic stress field was produced. (3) X-ray diffraction and asterism measurements demonstrate a progressive breakdown of crystal structure that increases directly with number and density of planar features. (4) Selective phase transformations, leading to disordered silica pseudomorphs (diaplectic glass or thetomorphs), are evident after planar features exceed ~4 sets/grain. X-ray diffraction analysis confirms major structural breakdown at this stage. (5) The refractive indices of isotropized quartz range between 1.463 and 1.478 (except one coesite-bearing sample having an average index for quartz of 1.496), whereas that of iron-rich black glass in vesiculated quartzite ranges between 1.510 and 1.546. Most shock effects produced by meteorite impact into quartzose crystalline rocks and sandstones are duplicated to varying degrees in the sedan quartzites.