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Sources of Clay in Fault Rock of the Moab Fault, Utah

Anyamele, Nwachukwu 1; Davatzes, Nicholas C.1; Solum, John G.2
1 Earth and Environmental Science, Temple University, Philadelphia, PA.
2 SIEP-EPT-RXN, Shell International Exploration and Production, Inc., Houston, TX.

Clay gouge in faulted sedimentary rocks has a significant influence on the sealing capacity and strength of brittle faults. Initial field studies of the source of clay in gouge largely focused on mechanical incorporation of protolithic clays and the geometry of folded, smeared clay-rich shale beds adjacent to faults. Recently, diagenesis has been recognized as an additional critical mechanism of clay enrichment. Here, we investigate the relative contributions of both mechanisms of clay enrichment focusing on the implications for fault permeability and strength.

Detailed mapping revealed distinct structural zones defined by the structures and distribution of fault rock at six sites of normally faulted sandstone and shale along the Moab Fault, Utah, including: (1) layers of clay-rich gouge separated by slip surfaces that include isolated sandstone breccia; (2) smeared shale adjacent to the gouge showing increasing bed parallel shearing nearer the fault; (3) faulted sandstone hosting deformation bands, slip surfaces, joints and veins. Fluid assisted alteration was revealed by a combination of high spatial resolution scan-lines and measured sections of elemental composition measured with a portable XRF augmented by SEM and XRD analysis of selected samples. Results to date include: (1) analysis of elemental concentration relative to immobile species (such as Ti) show that Ca, Sr, Ba are preferentially removed from the fault core, whereas Rb, K are added; (2) increased concentration of elements associated with illite such as K, occurs preferentially in the gouge; (3) the fault core hosts greatest alteration; (4) progressively greater alignment of clays from protolith to fault core to slip surface revealed by SEM; (5) formation of new clay poly-types in the gouge.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009