HR: 16:15h
AN: T24B-02    [Abstracts]
TI: Source and Significance of the Sedimentary Rocks in the SAFOD Borehole: Preliminary Analysis
AU: * Draper, S D
EM: Sardendra@cc.usu.edu
AF: Utah State University, 4505 Old Main Hill, Logan, UT 84332 United States
AU: Boness, N L
EM: nboness@stanford.edu
AF: Stanford University, Department of Geophysics, Mitchell Building, Stanford, CA 94305 United States
AU: Evans, J P
EM: jpevans@cc.usu.edu
AF: Utah State University, 4505 Old Main Hill, Logan, UT 84332 United States
AB: Understanding the geologic and tectonic history of the rocks penetrated by the SAFOD borehole is an important step in understanding the evolution of the San Andreas fault in the vicinity of the drillsite. During phase one drilling, the SAFOD borehole encountered an unexpected, highly heterogeneous, sedimentary rock sequence from a measured depth of 6305 ft to 10360 ft. The presence of these rocks suggests that the San Andreas fault zone is significantly more complicated than originally hypothesized. We present two possible formation candidates that have similar characteristics to this sedimentary sequence: the Eocene Butano (or equivalent Point of Rocks Sandstone) or the Miocene Vaqueros (or equivalent Temblor Formation). These formations were both deposited as deep sea fans from a Salinian basement source and their gross lithology resembles the sedimentary rocks in the borehole. Boness and Zoback (2004) used a variety of datasets to initially characterize the physical properties of the sedimentary sequence: Geophysical logs including velocity, gamma, image logs, resistivity and porosity. These data showed the rocks to be significantly more conductive and less fractured than the granite and granodiorite above, although both units display similar seismic velocities. Core and cuttings samples show that the rocks are arkosic, grading from conglomerates to shales/mudstones. Clasts are generally angular to subangular, indicating a texturally immature sedimentary sequence. Point counts show a dearth of volcanic grains with abundant granitic clasts, indicating granitic provenance. To establish the source of the sedimentary sequence, and hopefully distinguish between the two candidate lithologies, we performed more detailed point counts at 50 ft intervals along with detailed thin section and microstructure analysis. Additionally, bulk mineralogy of the two candidate formations was analyzed using X-ray Diffraction of hand samples collected at the surface and then compared to mineralogy of the borehole. In the future we hope to undertake micropaleontological analysis of cuttings and core from shale rich layers. The geophysical properties of the units encountered will be integrated with these geologic data to understand the origin and history of the sedimentary units adjacent to the San Andreas fault at the SAFOD drillsite.
DE: 3625 Petrography, microstructures, and textures
DE: 3675 Sedimentary petrology
DE: 8010 Fractures and faults
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: Fall Meeting 2005