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Impact breccias in carbonate rocks, Sierra Madera, Texas.Two main types of deformational breccia occur in the Sierra Madera cryptoexplosion structure: monolithologic breccias composed of shattered rock of a single lithology and mixed breccias composed of rocks of several lithologies. Monolithologic breccias generally show no mineralogic signs of shock deformation, but a few samples are shatter-coned in a manner suggesting simultaneous formation of breccias and shatter cones. Mixed breccias, forming irregular, cross-cutting bodies, consistently contain moderately to highly shocked material, with mineralogic evidence of shock pressures of 50 kb to more than 200 kb, which, with evidence from the structural geometry of Sierra Madera and orientation of shatter cones, indicate an impact origin of the breccias. The mode of occurrence of the breccias, petrographic characteristics, and association with shock features are shared by breccias in many other cryptoexplosion structures in both carbonate and crystalline rock terranes, suggesting that such breccias have a common origin.
Document ID
19720032372
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Wilshire, H. G.
Howard, K. A.
(U.S. Geological Survey Menlo Park, Calif., United States)
Offield, T. W.
(U.S. Geological Survey Denver, Colo., United States)
Date Acquired
August 6, 2013
Publication Date
April 1, 1971
Publication Information
Publication: Geological Society of America Bulletin
Volume: 82
Subject Category
Geophysics
Accession Number
72A16038
Funding Number(s)
CONTRACT_GRANT: NASA ORDER R-66
Distribution Limits
Public
Copyright
Other

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