DISCOVERY OF MICROSCOPIC EVIDENCE FOR SHOCK METAMORPHISM AT THE SERPENT MOUND STRUCTURE, SOUTH-CENTRAL OHIO - CONFIRMATION OF AN ORIGIN BY IMPACT

The origin of the Serpent Mound structure in south-central Ohio has be en disputed for many years. Clearly, more evidence was needed to resol ve the confusion concerning the origin of the Serpent Mound feature ei ther by endogenic processes or by hypervelocity impact. A petrographic study of 21 samples taken from a core 903 m long drilled in the centr al uplift of the structure provides evidence of shock metamorphism in the form of multiple sets of planar deformation features in quartz gra ins, as well as the presence of clasts of altered impact-melt rock. Cr ystallographic orientations of the planar deformation features show ma xima at the shock-characteristic planes of {10 (1) over bar 3} and {10 (1) over bar 2} and additional maxima at {10 (1) over bar 1}, {21 (3) over bar 1}, and {51 (6) over bar 1}. Geochemical analyses of impact breccias show minor enrichments in the abundances of the siderophile e lements Cr, Co, Ni, and Ir, indicating the presence of a minor meteori tic component. (C) 1998 Elsevier Science B.V. All rights reserved.