New evidence for the impact origin of the Ries Basin, Bavaria, Germany

The Ries basin is a shallow, nearly circular depression about 17 miles in diameter that lies between the Swabian and Franconian plateaus of southern Germany. Great masses of breccia and a system of thrust sheets associated with the Ries have been studied by German geologists for about a century. E. Werner and Otto Stutzer suggested that the Ries was an impact crater, but the consensus of the principal investigators has been that it was formed by some sort of volcanic explosion. The only direct evidence of magmatic activity at the Ries is the presence of glass in scattered patches of a breccia called suevite. Some of the glass has long been recognized as sintered fragments of old crystalline rocks. We have found that glasses of various composition coexist in single specimens of suevite. In addition, coesite, a high-pressure polymorph of SiO2, and lechatelierite, SiO2 glass, occur in the sintered rocks in the suevite. The presence of the same phases in sintered rock fragments at Meteor Crater, Arizona, and the coexistence of glasses of different composition suggest that the glassy components of suevite are of impact rather than volcanic origin.