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Origin and significance of clay‐coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California)
Author(s) -
Schleicher Anja M.,
van der Pluijm Ben A.,
Solum John G.,
Warr Laurence N.
Publication year - 2006
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2006gl026505
Subject(s) - geology , clay minerals , borehole , mineralogy , nucleation , montmorillonite , texture (cosmology) , scanning electron microscope , fault (geology) , geochemistry , materials science , composite material , chemistry , geotechnical engineering , paleontology , image (mathematics) , organic chemistry , artificial intelligence , computer science
The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X‐ray‐diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca‐K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Na‐montmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault.