HR: 0800h
AN: T21A-0450    [Abstracts]
TI: Lithologic Characterization of the Deep Portion of the SAFOD Drillhole
AU: * Evans, J P
EM: jpevans@cc.usu.edu
AF: Dept. of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322 United States
AU: Moore, D E
EM: dmoore@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AU: Kirschner, D
EM: dkirschn@eas.slu.edu
AF: Dept. of Earth and Atmospheric Sciences, Saint Louis University, 329 Macelwane Hall 3507 Laclede Ave., St. Louis, MO 63103
AU: Solum, J G
EM: jsolum@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AB: Characterizing rock types encountered in the SAFOD drill hole is a fundamental component for developing a geologic model for the observatory, and helps guide subsequent geophysical experiments and later coring. We examined thin sections of grain mounts and loose, washed grains from cuttings from 10,100 to 12,900 feet measured depths (md), and used X-ray diffraction analyses to decipher the rock types in the deeper part of the borehole. The most significant lithologic change in the deep part of the drill hole occurs at 10,300 to 10,700 ft md, where the dominant rock type encountered in the hole changes from arkosic sandstone and shale above to dark green and grey siltstone and shale below. Locally in the lower section there are zones of sheared siltstone and shale. The lithologic transition is nearly complete by 10,800 ft md. Serpentine grains are evident starting at approximately 10,900 ft, some with mesh texture (pseudomorphic after olivine), and some pseudomorphic after pyroxenes. Minor amounts of serpentine are found at the 10,970 to 11,000 ft interval, which is associated with an increase in gas in the borehole. At depths >11,000 ft md, shale and siltstone fragments containing quartz, plagioclase, biotite, and K-feldspar are common, and calcite is abundant, occurring both as separate grains and as veins, cement, and alteration minerals in other rock fragments. Fossil fragments (Inoceramus) are observed in several of the samples, indicating a minimum Cretaceous age for these rocks. At some intervals the washed cuttings have 10 to 30 % green, sheared, lozenge-shaped grains of phyllosilicate-rich rock. Deeper in the hole (11,500 to 12,500 ft) some grains exhibit sheared phyllosilicate surfaces, and some sandstone fragments contain plagioclase, biotite, and K-feldspar. Serpentine grains are found at 12,700 to 12,900 ft md, along with shale and siltstone fragments. Rock types determined from this examination are consistent with the rocks being part of the Great Valley Sequence (or sedimentary rocks of the Franciscan Complex). This suggests the borehole successfully crossed the San Andreas fault, and that the fault zone may contain slivers of Great Valley and/or Franciscan rocks at depth. Petrographic, mineralogic, and geochemical analyses of sidewall and drill core are anticipated to help resolve the structure of the fault zone at depth.
DE: 3625 Petrography, microstructures, and textures
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8106 Continental margins: transform
DE: 9610 Cretaceous
SC: Tectonophysics [T]
MN: Fall Meeting 2005