Geology topics

J.G. Solum

Publications and source records attributed to J.G. Solum.

7 recordsLinked to original sources

USGS2010

Fault-related clay authigenesis along the Moab Fault: Implications for calculations of fault rock composition and mechanical and hydrologic fault zone properties

The presence of clays in fault rocks influences both the mechanical and hydrologic properties of clay-bearing faults, and therefore it is critical to understand the origin of clays in fault rocks and their distributions is of great importance for defining fundamental properties of faults in the shallow crust. Field mapping shows that layers of clay gouge and shale smear are common along the Moab Fault, from exposures with throws ranging from 10 to ???1000 m. Elemental analyses of four locations along the Moab Fault show that fault rocks are enriched in clays at R191 and Bartlett Wash, but that this clay enrichment occurred at different times and was associated with different fluids. Fault rocks at Corral and Courthouse Canyons show little difference in elemental composition from adjacent protolith, suggesting that formation of fault rocks at those locations is governed by mechanical processes. Friction tests show that these authigenic clays result in fault zone weakening, and potentially influence the style of failure along the fault (seismogenic vs. aseismic) and potentially influence the amount of fluid loss associated with coseismic dilation. Scanning electron microscopy shows that authigenesis promotes that continuity of slip surfaces, thereby enhancing seal capacity. The occurrence of the authigenesis, and its influence on the sealing properties of faults, highlights the importance of determining the processes that control this phenomenon. ?? 2010 Elsevier Ltd.

Journal of Structural Geology↗

USGS2007

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models

We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures in the fault zones and host rocks. Cutting samples represent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hundred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spacing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We identify Quaternary and Tertiary(?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760–1920 m measured depth; arkosic and lithic arenites, interbed-ded with siltstone sequences, from 1920 to ∼3150 m measured depth; and interbed-ded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least five fault zones, which include regions of damage zone and fault core on the basis of percent of cataclasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600–2000, 2200–2500, 2700–3000, 3050–3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80–85° southwest, and consists of multiple slip surfaces in a damage zone ∼250–300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity.

California↗

USGS2006

Frictional strength of cuttings and core from SAFOD drillhole phases 1 and 2

We investigated the frictional properties of drill cuttings and core obtained from 1.85-3.1 km true vertical depth in the SAFOD scientific borehole in central California. Triaxial frictional sliding experiments were conducted on samples from primary lithologic traits and significant shear zones, including the inferred active trace of the San Andreas fault. The samples were deformed at room temperature under constant effective normal stresses of 10, 40, and 80 MPa with axial shortening rates of 0.01-1.0 ??m s-1. The weakest samples were from shale, claystone, and siltstone units with friction coefficient ?? = 0.4-0.55. Stronger samples were from quartzo-feldspathic rocks with ?? ??? 0.6. Materials tested from two shear, zones at 2560 and 3067 m measured depth had ?? = 0.4-0.55 and velocity strengthening behavior consistent with fault creep at depths <4 km. The coefficient of friction for bulk samples from the inferred trace of the San Andreas fault was ???0.6. Copyright 2006 by the American Geophysical Union.

Geophysical Research Letters↗

USGS2006

Mineralogical characterization of protolith and fault rocks from the SAFOD Main Hole

Washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the San Andreas Fault Observatory at Depth (SAFOD). Both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology, P-T conditions, deformation mechanisms, and fluid composition and abundance. Regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults. In addition, both smectites occurring as mixed-layer phases and serpentine minerals are found in association with active strands of the San Andreas Fault that were intersected during drilling, although their rheological influence is not yet fully known. Faults containing these mineralogical phases are prime candidates for continuous coring during Phase 3 of SAFOD drilling in the summer of 2007. Copyright 2006 by the American Geophysical Union.

Geophysical Research Letters↗

USGS2006

Origin and significance of clay-coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California)

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 Namontmorillonite 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. Copyright 2006 by the American Geophysical Union.

Geophysical Research Letters↗

USGS2005

Neocrystallization, fabrics and age of clay minerals from an exposure of the Moab Fault, Utah

Pronounced changes in clay mineral assemblages are preserved along the Moab Fault (Utah). Gouge is enriched up to ???40% in 1Md illite relative to protolith, whereas altered protolith in the damage zone is enriched ???40% in illite-smectite relative to gouge and up to ???50% relative to protolith. These mineralogical changes indicate that clay gouge is formed not solely through mechanical incorporation of protolith, but also through fault-related authigenesis. The timing of mineralization is determined using 40Ar/39Ar dating of size fractions of fault rocks with varying detrital and authigenic clay content. We applied Ar dating of illite-smectite samples, as well as a newer approach that uses illite polytypes. Our analysis yields overlapping, early Paleocene ages for neoformed (1Md) gouge illite (63??2 Ma) and illite-smectite in the damage zone (60??2 Ma), which are compatible with results elsewhere. These ages represent the latest period of major fault motion, and demonstrate that the fault fabrics are not the result of recent alteration. The clay fabrics in fault rocks are poorly developed, indicating that fluids were not confined to the fault zone by preferentially oriented clays; rather we propose that fluids in the illite-rich gouge were isolated by adjacent lower permeability, illite-smectite-bearing rocks in the damage zone. ?? 2005 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗

USGS2005

Drill hole logging with infrared spectroscopy

Infrared spectroscopy has been used to identify rocks and minerals for over 40 years. The technique is sensitive to primary silicates as well as alteration products. Minerals can be uniquely identified based on multiple absorption features at wavelengths from the visible to the thermal infrared. We are currently establishing methods and protocols in order to use the technique for rapid assessment of downhole lithology on samples obtained during drilling operations. Initial work performed includes spectral analysis of chip cuttings and core sections from drill sites around Desert Peak, NV. In this paper, we report on a survey of 10,000 feet of drill cuttings, at 100 foot intervals, from the San Andreas Fault Observatory at Depth (SAFOD). Data from Blue Mountain geothermal wells will also be acquired. We will describe the utility of the technique for rapid assessment of lithologic and mineralogic discrimination.

Conference Paper↗