Deformation bands in nonwelded ignimbrites: Petrophysical controls on fault-zone deformation and evidence of preferential fluid flow

Abstract

The impact of faults on fluid flow and transport through thick vadose zones depends in part on the nature of fault-zone deformation. Both fractures and deformation bands occur in ignimbrite sequences at Los Alamos, New Mexico, and Busted Butte, Nevada. The primary controls on mode of failure are grain-contact area and strength, which are directly related to degree of welding and crystallization and inversely proportional to porosity. Low-porosity welded units deform by transgranular fracture; high-porosity, glassy, nonwelded units deform by cataclasis within deformation bands. Moderately high porosity, nonwelded units that have undergone devitrification and/or vapor-phase crystallization form either deformation bands or fractures, depending on local variations in the degree and nature of crystallization. Grain-and pore-size reduction in deformation bands commonly produces indurated, tabular zones of clay-sized fault material. Many of these bands are locally rich in smectite and/or cemented by carbonate. Preferential wetting of deformation bands is inferred to promote alteration and cementation. We therefore interpret variably altered fault-zone material as evidence of preferential fluid flow in the vadose zone, which we infer to result from enhanced unsaturated permeability due to pore-size reduction in deformation bands.

Observations on Faults and Associated Permeability Structures in Hydrogeologic Units at the Nevada Test Site

“…Fault characteristics observed in VTAs at the NTS vary according to the strength and degree of consolidation of the rock. Typical VTAs, consisting of porous and weakly consolidated vitric nonwelded tuff, have fault zone characteristics similar to those observed in nonwelded tuffs at Yucca Mountain and northern New Mexico (Wilson et al, 2003) and in porous and weakly consolidated sediments (Rawling and Goodwin, 2003;Rawling et al, 2001;Antonellini and Aydin, 1994;. Fault core zones within typical VTA rocks at the NTS were observed to be very narrow zones, typically less than 15 cm (6 in.)…”

“…In saturated conditions, and particularly if cemented, deformation bands most likely act as very narrow tabular zones of reduced permeability and can be three to four orders of magnitude less permeable than the protolith (Antonellini and Aydin, 1994;. However, under unsaturated conditions in semiarid to arid climates, deformation bands can be up to six orders of magnitude more permeable than the protolith (Wilson et al, 2003).…”

“…Deformation bands are millimeter-thick tabular zones accommodating millimeter-to centimeterscale slippage in rock units (Fossen and Bale, 2007;Wilson et al, 2003). Slippage is accomplished by grain boundary sliding, grain fragmentation, and porosity reduction.…”

Phase II Groundwater Flow Model of Corrective Action Unit 98: Frenchman Flat, Nevada Test Site, Nye County, Nevada, Rev. No.: 0

“…Together, the two fault elements tend to inhibit cross-fault flow and enhance fault-parallel flow, imparting an overall hydraulic anisotropy to the fault. In contrast, porous sedimentary and volcanic rocks tend to accommodate small amounts of strain by cataclasis and shear-induced compaction along deformation bands rather than by fracturing Aydin, 1994 andWilson et al, 2003). In sandstones, these deformation bands contain finely ground particles that have substantially lower permeability (one to four orders of magnitude) and higher capillary pressures than adjacent undeformed rock, factors that allow deformation band faults to trap oil and sometimes compartmentalize oil reservoirs (Antonellini et al, 1999).…”

“…Recently published studies of fault-zone architecture in consolidated rocks have made distinctions between deformation behavior in low-porosity sedimentary and crystalline rocks, and in more porous sedimentary and volcanic rocks (Forster and Evans, 1991;Caine et al, 1996;Antonellini and Aydin, 1994;Wilson et al, 2003). Faults in low-porosity rocks generally have two primary architectural elements that can be developed to varying degrees depending on protolith material, location within the fault, and extent of displacement and cementation.…”