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_Journal of Structural Geology 32 (2010) 1873e1886_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Pure and shear-enhanced compaction bands in Aztec Sandstone_ _Peter Eichhubl*, John N. Hooker, Stephen E. Laubach_ _Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, TX 78758, USA_ _article info_ _Article history: Received 1 September 2009; Received in revised form 11 February 2010; Accepted 17 February 2010; Available online 26 February 2010_ _Keywords: Sandstone Compaction Deformation band Localization Aztec sandstone_ _abstract_ _We report on the occurrence of deformation bands in Jurassic eolian Aztec Sandstone at Valley of Fire, Nevada, that accommodated roughly equal amounts of shear and band-perpendicular compaction by grain rearrangement and porosity collapse. These bands, referred to as shear-enhanced compaction bands, differ in orientation, structural arrangement, and microtexture from pure compaction bands that form perpendicular to the shortening direction. Shear-enhanced compaction bands are planar over tens of meters, and commonly composed of multiple parallel thinner strands. Pure compaction bands are less commonly planar, typically wavy or chevron in geometry, and composed of single strands. Shear-enhanced compaction bands are inferred to form at 38e53 x14 relative to the maximum compressive principal stress, and thus differ from compactive shear bands that form at distinctly lower angles. While shear offsets along shear-enhanced compaction bands are only about 1/10 of the band thickness, by contrast, shear offsets may be large for compactive shear bands with formation of slip surfaces. Based on inferred timing and burial conditions, we interpret that the formation of shear-enhanced and pure compaction bands requires large initial porosity close to the loose packing porosity, good sorting, and high effective maximum compressive principal stress of about 20 MPa._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Deformation localization in sediment and porous sedimentary rock has received increasing attention in recent years, providing insights into localization processes in geologic systems in general (B?suelle and Rudnicki, 2004; Aydin et al., 2006; Fossen et al., 2007). Building on an earlier conceptual framework by Houlsby and Wroth (1980), Aydin et al. (2006) distinguished two fundamental modes of deformation localization: (1) Sharp modes of deformation localization are formally defined by a discontinuity in the displacement field and result in opening-mode fractures or joints, and slip surfaces. (2) Tabular modes of deformation localization are zones of finite width bound by two roughly parallel surfaces defined as discontinuities in the displacement gradient field. Deformation bands are a natural expression of the tabular mode of deformation localization. We use the term deformation band to include shear bands, compaction bands, and dilation bands based on the dominant component in displacement gradient across the band relative to the band orientation (sensu Du Bernard et al., 2002; Aydin et al., 2006). Based on theory and the observation that shear bands curve into both compaction and dilation bands, deformation bands may develop at 0 x14 through 90 x14 to the_ _? Corresponding author. Tel.: ?1 512 475 8829. E-mail address: peter.eichhubl@beg.utexas.edu (P. Eichhubl)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2010.02.004_ _shortening direction, involving appropriate combinations of shear, dilation and compaction (Mollema and Antonellini, 1996; Rudnicki and Olsson, 1998; Issen and Rudnicki, 2000; B?suelle, 2001; Du Bernard et al., 2002; Borja and Aydin, 2004; Aydin et al., 2006; Fossen et al., 2007). Pure compaction bands are oriented perpendicular to the maximum principal shortening direction, and pure dilation bands perpendicular to the least principal shortening direction within a five-fold division that includes pure compaction bands, compactive shear bands, isochoric shear bands, dilatant shear bands, and pure dilation bands._ _Despite the attention compaction localization has received in the engineering disciplines, field descriptions of compaction bands formed under geologic conditions are few. A description of compaction bands was first provided by Hill (1989) in Aztec Sandstone at Valley of Fire, Nevada. Later descriptions include those by Mollema and Antonellini (1996) and Schultz (2009) from the East Kaibab monocline, Utah, and by Sternlof et al. (2004), Holcomb et al. (2007), and Aydin and Ahmadov (2009) from Valley of Fire. However, different authors designated different sets of deformation bands as compaction bands. Sternlof et al. (2004) and Holcomb et al. (2007) designated bands as compaction bands that are oblique relative to the maximum principal shortening direction as inferred by Hill (1989). The compaction bands of Sternlof et al. (2004) and Holcomb et al. (2007) are at high angle to the bedding-parallel compaction bands described by Aydin and Ahmadov (2009). Mollema and Antonellini (1996) described two_ _1874_ _P. Eichhubl et al. Journal of Structural Geology 32 (2010) 1873e1886_ _sets of compaction bands occurring side-by-side from the East Kaibab monocline with same strike but 20 x14 difference in dip angle._ _This study was designed to (1) describe the field relationships of compaction and shear bands at Valley of Fire, (2) describe their microstructural characteristics, and (3) place them in a mechanical framework. We will demonstrate that planar pure compaction bands, oriented perpendicular to the direction of maximum principal shortening, form under specific loading conditions, providing a possible explanation why compaction bands have thus far been reported from only two field locations. In addition, we demonstrate that shear-enhanced compaction bands, observed previously in experiments (Fortin et al., 2006; Baud et al., 2006), occur as a distinct type of deformation bands which differ in orientation, textural properties, and geometric relations from pure compaction bands and compactive shear bands._ _2. Field structural characteristics of deformation bands_ _Deformation bands at Valley of Fire are most abundant in the upper section of the Jurassic Aztec Sandstone and the lower section of the Cretaceous Baseline Sandstone (Fig. 1). The Aztec Sandstone is a well-sorted, cross-bedded, and medium-grained sandstone deposited in a backarc setting (Marzolf, 1990). The Baseline Sandstone is a syntectonic molasse sequence of sandstone and conglomerate deposited in front of the Sevier thrust complex (Aschoff and Schmitt, 2008). Our detailed field descriptions of deformation bands focus on the Aztec Sandstone with a more uniform lithology than the Baseline Sandstone. Deformation bands are readily observed in outcrop due to their higher resistance to erosion resulting in positive relief of the bands. The upper section of the Aztec Sandstone is generally weakly cemented, increasing in induration toward its base. In addition to their positive relief, the observation of deformation bands in the upper Aztec Sandstone is aided by the preferred dissolution and reprecipitation of diagenetic iron oxides and hydroxides along bands (Eichhubl et al., 2004)._ _Deformation bands in the Aztec Sandstone exhibit large variations in band thickness, band spacing, slip amount, orientation, geometric properties, and microtextural properties. Band thickness ranges from w1 mm to >5 cm, with bands thicker than w1 cm typically composed of multiple parallel strands. Band spacing ranges from w5 mm to > 1 m, and slip from 0 to >10 cm._ _Structural cross-cutting relations indicate three stages of deformation band formation. Stage I are Late Cretaceous in age based on cross-cutting relations with Sevier-age thrust faults (Eichhubl and Flodin, 2005) and are the focus of this study. Stage I deformation bands occur in eight sets that differ in orientation, structural style, and microtexture (Table 1). Set 1 consists of bands with no discernible shear, which we interpret to be pure compaction bands. Sets 2e6 are bands with discernible slip of w1/10 band thickness, which we define as shear-enhanced compaction bands. Sets 7 and 8 are bands with slip that typically exceeds the band thickness; while they are compactive shear bands we refer to them as shear bands for simplicity._ _Fig. 1. a. Geologic map of Valley of Fire, after Eichhubl et al. (2004). b. Stratigraphic column of Valley of Fire after Bohannon (1977)._ _P. Eichhubl et al. Journal of Structural Geology 32 (2010) 1873e1886_ _1875_ _Table 1 Stage I deformation bands in upper Aztec Sandstone at Valley of Fire._ _Set Mode Relative orientation to bedding Strike, dip Structure Microtexture_ _1 Pure compaction High-angle Reverse Bedding-parallel Minor cataclasis_ _2 Shear-enhanced Reverse Left-lateral N20 x14E, 26 x14W Planar Strong cataclasis_ _3 Shear-enhanced Reverse Right-lateral N8 x14W, 24 x14E Planar Strong cataclasis_ _4 Shear-enhanced Strike, dip N70 x14E, 28 x14N Planar or chevron Planar Strong cataclasis_ _5 Shear-enhanced Strike, dip N61 x14E, 20 x14N Planar or chevron Planar Strong cataclasis_ _6 Shear-enhanced Strike, dip N15 x14W, 9 x14W Ramp-flat Planar Strong cataclasis_ _7 Compactive shear Reverse Left-lateral N85 x14E, 17 x14N Planar Poorly coordinated_ _8 Compactive shear Reverse Right-lateral N26 x14W, 44 x14E Planar Well coordinated_ _Stage I shear-enhanced compaction bands are the most ubiquitous brittle structures in the upper Aztec Sandstone. They are planar to curviplanar, and can be traced over 10e50 m along strike (Fig. 2a). Bands are up to 5 cm thick, and composed of up to 10e20 single strands. Where suitable markers are available, offset along these bands are about 1 mm for bands of 1 cm thickness (Fig. 2b)._ Ключевые слова: band formation, formed, letters, mathematical framework, valley, contact, shearenhanced compaction, author designated, compaction, force, fault, distinct structure, localization, macroscopic band, foresets, interpretation, surface, pure compaction, detrital grain, aydin, compressive, enhanced compaction band, preferred orientation, shear, paleo-uid, bulletin, cataclasis, oblique shortening, porosity loss, detrital shape, condition, marzolf, scale, wavy, location, journal structural, high degree, thickness, sternlof, vertical stress, particle movement, olsson issen, inferred, deformation localization, grain indentation, rudnicki, shear offset, dune, pollard, bedding, compaction band, journal, enhanced, shear displacement, sandstone, eichhubl, neighboring grain, b?suelle, band-oblique shortening, offset, nevada, differential stress, schultz, based, macroscopic, aztec sandstone, quartz, high angle, bedding-parallel slip, orientation, mechanics, sandstone journal, ?eld, shear component, perpendicular, principal, structural evolution, grain, geological, sem, late cretaceous, microtextural observation, oriented, scb, band set, strike, loading condition, geologic map, parallel, cement, pure compaction band, journal geophysical, large, particle, shear-enhanced compaction, band, strike-slip, issen, shear band, loading direction, eichhubl journal, host, positive relief, formation, composed, eld, angle, lders band, hill, stage, estimated based, porosity reduction, dune boundary, structural, deformation band, force chain, observation, slip, pure shear-enhanced, bohannon, shear enhanced, wang, deformation, shortening, oriented perpendicular, bedding plane, geology, relative, bedding-parallel, granular medium, mm, band orientation, geometric relation, rock mechanics, elsevier, loading, sediment, pure, wong, applied, conjugate set, shear-enhanced, shear enhanced compaction, zone, chevron pattern, cm, sample, form, shearenhanced, outer zone, enhanced compaction, microtextural ordering, rarely planar, chain, dune set, band relative, reduction, quartz cement, critical porosity, sedimentary, compactive, direction, neighboring band, baud, explanation, boundary, bedding arrow, dip, chevron-type, band thickness, initial porosity, shortening direction, survivor grain, form perpendicular, displacement, inferred based, slip surface, stress, porosity, set, reverse-slip, image, grain contact, olsson, compaction localization, geophysical, observed, stress magnitude, shear enhanced compaction band, shipton, compactive shear, planar, baseline sandstone, strike-slip sense, characteristically wavy, aztec, rock, structural geology, park nevada, table, maximum, oriented parallel, movement