. Collected reprints, Essa Institute for Oceanography. Oceanography Fig. 2. Photomicrographs, magnified about 40 times, of (a) normal unfractured Coconino sandstone and (b) its shock-fractured equivalent from Meteor (Barringer) Crater, Ariz. Rhombohedral sets of planar In addition, meteoritic spherules in silica glass, but not ponderable meteorites, are present. Criteria. The term cryptoexplosion structure pertains to a highly disturbed geological feature resulting from a natural explosion of uncertain cause. Such structures are fairly common, but only some of them appear to be a
. Collected reprints, Essa Institute for Oceanography. Oceanography Fig. 2. Photomicrographs, magnified about 40 times, of (a) normal unfractured Coconino sandstone and (b) its shock-fractured equivalent from Meteor (Barringer) Crater, Ariz. Rhombohedral sets of planar In addition, meteoritic spherules in silica glass, but not ponderable meteorites, are present. Criteria. The term cryptoexplosion structure pertains to a highly disturbed geological feature resulting from a natural explosion of uncertain cause. Such structures are fairly common, but only some of them appear to be astroblemes. Others are generally believed to be cryptovolcanic structures caused by phreatic (steam) or volcanic explosions but without the extrusion of volcanic products or hydrothermal alteration. The distinctive hallmarks of astroblemes result from intensive shock-wave damage to the rocks. At least four types of criteria are now generally recognized. First is the presence in the rock for- mations of a distinctive type of conical fracture, called shatter cones, which display striated mark- ings like "horsetails" (Fig. 1). These are apparently the result of shock fracturing and therefore seem to be a useful criterion. Second is the presence of shock-induced polymorphs of silica or feldspar, such as coesite, stishovite, and maskelynite. Third is the occurrence of shock-induced microfractur- ing of crystals whereby, for example, certain planar sets of fracture appear in quartz (Fig. 2) and biotite becomes kink-banded. The fourth criterion is the presence of lechatelierite, high-temperature silica glass. Discoveries. Putative astroblemes contain one or more of the above criteria. Seventeen are now known from shatter coning alone. This is often the first hallmark to be identified as it is a field criterion rather than a laboratory one. The Stein- heim Basin, a twin structure to the Ries Basin formed at the same time, is the prototype shatter- coned structure. The Wells C
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