THE ROLE OF CROSS-BED ORIENTATION AND THE RELATED ANISOTROPY IN THE DISTRIBUTION OF COMPACTION BANDS AND JOINTS IN AEOLIAN SANDSTONE

Shang Deng and Atilla Aydin Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, E-mail: [email protected] (S. Deng)

Utah (Fig. 1). The objectives of this paper are twofold: Abstract (1) to describe the spatial relationships among multiple sets of joints, cross-beds, and dune boundaries in the We propose a close relationship between the Aztec Sandstone and Navajo Sandstone; and (2) to orientations of cross-beds and the cross-bed package place this new information in a conceptual framework confined joints in the Jurassic aeolian Aztec Sandstone that may shed light on the formation, orientation, and cropping out in the Valley of Fire State Park (NV) and spatial distribution of the joints therein. Navajo Sandstone in Zion National Park (UT). The In this paper, we first introduce the geologic settings field data demonstrates that the orientation of cross- of the two study areas, VoF and ZNP. Then, we review bed package confined joints is related to the orientation our previous work on compaction bands in Aztec of cross-beds, suggesting that in addition to the Sandstone exposed at the VoF. In the main body of the distribution of compaction bands, cross-bed orientation manuscript, we deal with the spatial and geometric and the associated anisotropy also exert a strong relationships between joints sets and dune architecture control on the formation, and orientation of the joints. at both the VoF and ZNP. These data imply that in These results may have important implications for fluid addition to the influences of cross-beds on the flow through aeolian sandstones in reservoirs and distribution of compaction bands in aeolian sandstones, aquifers. the orientation and distribution of joints are also

affected by the cross-bed orientations. Keywords: Compaction bands, joints, cross-beds, anisotropy, Aztec Sandstone, Navajo Sandstone.

Introduction We have studied the relationship between cross-bed orientation and the distribution of compaction bands in aeolian sandstone for several years (Deng and Aydin, 2012, 2013), a short summary of which will be provided in this paper. This research lead to the notion that if the cross-bed related anisotropy affects the orientation of compaction bands, there is probably a similar effect on the formation and orientation of the joints which occur in the same rock at the same location. In comparison to compaction bands, joints are relatively simple structures that exhibit dominantly opening displacements (Pollard and Aydin 1988). They often occur perpendicularly to bedding and are confined by layer boundaries or mechanical layer packages in sedimentary rocks (Hodgson, 1961a, b; Price, 1966; Hancock, 1985; Helgeson and Aydin 1991; Gross and Engelder, 1995). In this study, we focus primarily on the distribution and orientation of joints and their relationships to the Fig. 1. Distribution of the Jurassic Aztec/Navajo cross-beds in the Jurassic Aztec Sandstone exposed at Sandstone in western USA (slightly modified from the Valley of Fire State Park (VoF), Nevada and Navajo Verlander, (1995)). The approximate locations of Sandstone in Zion National Park (ZNP) in southwestern Valley of Fire State Park (VoF) in Nevada and Zion National Park (ZNP) in Utah are labeled.

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Fig. 2. Simplified geologic map of the Valley of Fire State Park (slightly modified from Taylor, 1999). The locations of three study areas (labeled A, B and C) and some other prominent geologic and geographic sites are also labeled. Area C is the main area for the past research on the occurrence and characteristics of compaction bands.

Geological Framework Aztec Sandstone at Valley of Fire State Park The Aztec Sandstone is a 1400-meter-thick, cross- bedded aeolian deposit (Marzolf, 1983) and consists of a number of dune units with NW-NNE oriented dune boundaries with moderate dip angles (less than 30°). The dune boundaries can be defined as second order bounding surfaces, which are commonly present in ancient aeolian sandstones and are attributed to the migration of dunes across draas (Brookfield, 1977). The Aztec Sandstone was subjected to at least two significant deformation phases: (1) east-southeast- directed compressive deformation during the Sevier Orogeny (e.g., during the late Mesozoic) and (2) Basin Fig. 3. Image created using Google Earth showing and Range extension and strike-slip faulting beginning the field area at Zion National Park. The traces of the NNW trending joint zones separating the mesas are labeled with red dashed lines.

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Fig. 4. Compaction bands of different orientations and their distributions in dunes. a) Attitudes of cross- beds and compaction bands of different orientations measured in the field. Mean values (framed by black rectangular lines) of different sets of bands are highlighted and labeled. b) A ground photograph showing the juxtaposition of bed-parallel and high-angle to bedding compaction bands. Note strikingly different cross-bed orientations. c) Cropped map of the major study area (see location “C” in Fig. 1) on an enlarged aerial photograph, showing dunes and structural domains with various types of compaction bands (cb) (color-coded). in the mid-Miocene time and continuing to the present the western margin of the Colorado Plateau (Rogers et (Bohannon, 1979, 1983; Sternlof, 2006). al., 2004). The data on the distribution and orientation of joints The spatial and geometric relationships between were collected primarily from the main study area (Site joints and dune architectures were studied in the Navajo A in Fig. 2), although some observations from Site B Sandstone exposed in the Checkerboard Mesa and are also used for confirmation of the results drawn from Crazy Quilt Mesa areas in eastern Zion National Park Site A. Site C is the main study area for previous (ZNP) (Fig. 3). research on the distribution and orientation of compaction bands. Methodology Navajo Sandstone at Zion National Park Mapping for this research was carried out at scales Navajo Sandstone is a chronostratigraphic ranging from 1:50to 1:170. Maps were made by using equivalent of the Aztec Sandstone (Marzolf, 1983). enlarged aerial photographs taken at a height of around The 610m thick Navajo Sandstone in the ZNP region is 20m from a camera mounted on an unmanned aerial characterized by a set of widely spaced, vertical and vehicle (Engel et al., 2011). Orientation measurements NNW trending sheared joint zones or small faults that were made using a Branton compass. In cases where the erode to rounded cliffs and slot canyons (Fig. 2) of the cliff surface is steep and can’t be reached, park (Rogers and Engelder, 2004). These structures are measurements were taken by projection and interpreted as evidence for a change of the stress system approximation at a distance from the surface using an and the resulting modest Basin and Range extension in electronic compass-inclinometer embedded in a smart phone (Pavlis, 2010). For all the measured structural

Stanford Rock Fracture Project Vol. 25, 2014 A-3 Fig. 5. Comparison between the model results and the field data of distribution of compaction bands of different orientations in cross-bed domains. (a) Distribution of strikes of cross-beds within Ha1 compaction band domain (red bars in the middle) and strikes of all measured cross-beds (gray shade). Notice that the former is imprinted on the latter, therefore the strikes of cross-beds without Ha1 compaction bands are shown by the portions of bins without red bars. (b) Strength of localized compaction corresponding to the orientation of Ha1 in cross-beds of various orientations for various dip angles. (c) Distribution of strikes of cross-beds within Bp domain (red bars (Bp1) and blue bars (Bp2 and Bp3) in the middle) and strikes of all measured cross-beds (gray shade). (d) Strength of localized compaction occurring in the orientations (strike and dip angle given in the title) of Bp1, Bp2, and Bp3 in cross-beds of various orientations. orientation data we use the right-hand-rule convention bands (Aydin et al., 2006; Borja and Aydin, 2004; (Pollard and Fletcher, 2005). Fossen et al., 2007; Issen and Rudnicki, 2000; Rudinicki and Rice, 1975). The earliest example for The role of cross-bed orientation in the compaction bands was reported by Hill (1989), who distribution of compaction bands in described multiple sets of compaction bands at high angles to bedding in the Jurassic Aztec Sandstone

Aztec Sandstone exposed in the Valley of Fire State Park in southeastern Compaction bands are characterized by reduced Nevada (USA). Sternlof (2006) carried out the most porosity and represent one kinematic end-member of a extensive analysis to date of high-angle to-bedding family of deformation bands, which form by compaction bands. Finally, Aydin and Ahmadov (2009) localization of volumetric strain into narrow tabular reported compaction bands parallel to bedding, called

Stanford Rock Fracture Project Vol. 25, 2014 A-4 bed-parallel compaction bands, in aeolian sandstone In general, the calculated strength of localized with sub-horizontal to low-angle bedding. Eichhubl et compaction correlates well to sets of compaction bands al. (2010) documented the occurrence of various sets of observed in the Aztec Sandstone. For instance, good compaction bands in the park. Deng and Aydin (2012) correlations exist between the cross-beds (with strikes further evaluated the role of the dune architectures, ranging from 180°-220° and dip angles ranging from defined by both cross-beds of varying orientations and 10° to 40°) without Ha1 compaction bands (Fig. 5a) dune boundaries, in compaction band formation and and the cross-beds with relatively higher strength of orientation. In this section we provide only a short localized compaction (Fig. 5b). Good correlations also summary of previous studies on the role of cross-bed exist between the cross-beds within the Ha1 orientation in the distribution of compaction bands in compaction band domain with strikes mainly ranging Aztec Sandstone. See Deng and Aydin(2012, 2013) for from 0°-90° and 230°-360°(dip angles ranging from 10° the full description of the field data and the model. to 40° (Fig. 5a)),and cross-beds with relatively lower The field data reported in Deng and Aydin (2012) strength of localized compaction (tendency for were collected from Site C in Fig. 2. The aeolain localized compaction is enhanced (Fig. 5b)).In the same deposits in this study area consist of dune units with fashion, clear correlations are observed among the predominantly northwest-trending dune boundaries cross-beds with(or without) Bp compaction bands and (Fig. 4). Field observations and measurements by Deng the cross-beds with relatively lower (or higher) strength and Aydin (2012) demonstrated two important of localized compaction (Figs. 5c, d). phenomena on the occurrence, geometry, and distribution of compaction bands in the Aztec The role of cross-bed orientation in the Sandstone at the Valley of Fire State Park, Nevada: (1) distribution and orientation of joints there are multiple sets of non-orthogonal compaction bands formed coevally (Fig. 4a);and (2) the low-angle Aztec Sandstone at Valley of Fire bed-parallel compaction bands and the high-angle-to- 1. Depositional architecture bedding compaction bands occur only in cross-beds with certain ranges of bedding orientations (Figs. 4b, c). The aeolian deposits in the study area (Site A) Based on these two observations, two possible briefly described under the Geological Framework, interpretations can be reasonably suggested. First, at consist of a number of dune units with NNW-NNE least one set of the compaction bands may have oriented dune boundaries with moderate dip angles accommodated compaction oblique to one of the (less than 30°) (Figs. 6 and 7). According to the principal planes implying shear-enhanced compaction truncating relationship between these bounding localization. In this case, the anisotropy and surfaces, 18 dune units (D1-D18) were identified and heterogeneity associated with cross-beds induce named from older to younger within the mapping area strength anisotropy of localized compaction. That is, (Site A). cross-beds with certain orientations are conducive for Morphologic, depositional, and structural the formation of a certain set of compaction bands in a resemblances and differences between these 18 dune particular orientation but unfavorable for the formation units on the enlarged aerial photograph are checked by of compaction bands of other orientations. Second, and visual inspection and field measurements. Depending alternatively, if each set of the compaction bands on how striking the change of cross-bed orientations is represents the principal stress planes, the stress within a dune, the number of measurement stations orientation may vary spatially and temporarily, perhaps (MS) was selected and the orientation measurements due to the anisotropy and heterogeneity of the aeolian were made. The changes in cross-bed orientations medium with variable cross-bed orientations. Of course, among different stations and dunes were shown by rose these two interpretations are not mutually exclusive and diagrams in Fig. 6. For example, in the northern part of a combination of both scenarios is also possible. D4, the nearly WSW-striking cross-beds (red in the Following the experimental studies indicating the rose diagram for MS 1-2 in the figure) change to nearly strength anisotropy of localized compaction for WNW-striking cross-beds in the southern part (MS 3- Rothbach Sandstone(Louis et al., 2009) and Dimelstadt 4). While clearly visible in the rose diagrams, the Sandstone(Baud at al., 2012), Deng and Aydin (2013) differences in cross-bed orientations are also reflected applied a quadratic failure criterion for anisotropic by the trends of white-orange stripes in the background materials (Tsai and Wu, 1971) to describe the strength mosaic map (Fig. 6). A larger degree of changes in anisotropy of localized compaction under axisymmetric cross-bed orientations within a single dune was also compression and compared the results with the observed, for instance, in D13 and D17 (see rose observed relationship among cross-bed orientations and diagrams MS 1-4, 5-7, 1-3, and 4-5 in Fig. 6). In the presence and orientation of compaction bands. addition to the variation of cross-bed orientations within a single dune, differences between the

Stanford Rock Fracture Project Vol. 25, 2014 A-5 Fig. 6. Map of the main study area (see location “A” in Fig. 1) on a mosaic of low attitude aerial photographs. The map shows dunes (numbered for identification purposes) and various sets of joints (traced in yellow color). Rose diagrams showing the measured strikes of cross-beds (red) and joints (black) are used for visual comparison on the changes in cross-bed orientation and joint orientations among different dunes and measurement stations. Notice that the cross-beds are characterized by the hues of orange and white stripes. The location of Fig. 8a is labeled with a white frame. orientations of cross-beds from one dune to the next can roughly W-striking. In contrast, the cross-beds in D13 be significant. For instance, the cross-beds in D3, D6, immediately adjacent to this boundary are nearly N- D7, and D8 below the lower boundary of D13 are

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striking. Similar changes can also be observed from D8, D10, D12, D13to D9, D11, D15, and D17, respectively.

2. Joint assemblages Based on the orientation, geometry, and relationship with sedimentary architectures (cross-beds and dune boundaries), we have identified three distinct joint sets across the mapping area: 1) joint zones that are approximately north-south trending; (2) joints orthogonal to the joint zones; and (3) cross-bed package confined joints. Joint zones in the mapping area are clusters of sub- parallel joints with strikes ranging from 355º to 15º and from 161º to 200º (Fig. 7). Dip angles of these zonal joints vary between 64º to 88º. Many of the joint zones occur as through-going features in cross-beds of various orientations. A good example of this can be found in the western part of the map (Fig. 6), where a joint zone occurs as a continuous feature through D1 to D8. Though the joint zones appear to be not influenced by the cross-bed orientations, certain dune boundaries exert an impact on the joint zone. For instance, the aforementioned joint zone ends adjacent to the lower boundary of D1 and conjoins with a segment parallel to Fig. 7. Attitudes of cross-beds, dune boundaries, that dune boundary. Similarly, segments of the joint and three sets of joints measured in the field area. zone in the north part of D18 are bounded by the lower and 8). The data also show that the cross-bed packages dune boundary of D18. Besides the joint zones that are of different orientations also have different orientations continuous and across cross-beds of various of CBP confined joints therein (Fig. 9). In order to orientations, some relatively less continuous joint zones better understand the potential relationships between were observed and mapped. They occur in the north cross-beds and the CBP confined joints, trace part (MS 2), west part (MS 3), and south part (MS 11) geometries of these on erosional scarps were mapped in of duneD13. These joint zones occur locally in cross- several locations in the study areas. The orientations of beds of various orientations, but are all adjacent to the cross -beds and the corresponding CBP confined joints lower boundary of D13. were measured at each station and the data are plotted Joints orthogonal to the joint zones are in Figs.6 and 10. The results of analysis on the systematically confined or truncated against the through geometric relations between CBP confined joints and going joints of the joint zones (Fig. 8). Based on this cross-beds are shown in Fig. 11. These figures (6, 10, truncation relationship, this joint set is a younger set and 11) show several systematic features as than the approximately north-south trending joint summarized below. zones. (1) In the rose diagrams plotted in Fig. 6, the bins Cross-bed package confined joints are bounded by representing the joints (black) (mostly CBP the bedding interfaces. The concept of cross-bed package (CBP) is equivalent to mechanical layer (Narr, confined joints) are at high-angle (approaching 1991; Gross, 1995), which lies between bed-parallel right angle) to those representing the cross-beds surfaces. Joints are thought to terminate at the package (red). Such a pattern in the strikes of cross-beds boundaries. Comparing to the joint zones, CBP and CBP confined joints remains almost consistent confined joints are less continuous (Fig. 8). The as the cross-bed orientations change. For example, orientations of CBP confined joints have an overlap corresponding to the transition from WSW-striking with the joint zones. However, the orientations of the cross-beds in the northern part of dune D4 (MS 1-2) former are characterized by a much broader distribution with strikes ranging from 0º to 80º and from 170º to to nearly WNW-striking cross-beds in the southern 356º. Dip angles of the CBP confined joints vary part (MS 3-4), the strikes of CBP confined joints between 58º and 89º (Fig. 7). change from NNW-striking to NNE-striking. The field observations suggest that the CBP Significant variations of strikes of CBP confined confined joints occur at high-angle to bedding (Figs. 7 joints within a single dune unit are also observed,

Stanford Rock Fracture Project Vol. 25, 2014 A-7 Aside from the variation in CBP confined joint orientations within a dune, significant differences are also observed between orientations of CBP confined joints in two adjacent dune units. For instance, the CBP confined joints in D3, D6, D7,and D8 adjacent to the lower boundary of D13 have approximately N-S strikes, whereas similar to the cross-beds’ change to a N-strike in D13, the CBP confined joints change to a W-strike. Contrasting differences in the orientations of CBP confined joints can also be observed in D8 and D9, D10 and D11, D12 and D17, and D13 and D15. (2) The data presented in the map in Fig. 6 are plotted Fig. 8. Ground photograph taken in D18 (MS 1) in a different way to show the different trends of showing different sets of joints (view due north- the mean values numerically in Fig. 10. The plot east). shows two roughly sub-parallel trends of strikes of for instance, in D13. In the northern part of D13 cross-beds (red) and CBP confined joints (blue)

(MS 1-4), the cross-beds have NE-strikes and the over all measurement stations. Notice that the CBP confined joints therein have NW-strikes. outliers are due to certain high-angle joints dipping Following the change of cross-beds to a northern to the opposite direction resulting in a nearly 180º strike in the western part of D13 (MS 5-7), the difference in strike values of the same joint set. CBP confined joints therein change to a western (3) Using the mean values of orientations calculated strike. from the data collected from each measurement

Fig. 9. Dune units with contrastingly different cross-bed orientations and the variation of joint orientations. a). Ground photograph including multiple dune units in which dune boundaries (red lines), cross-bed package boundaries (white dashed lines), and joints (white arrows) are labeled.See Fig. 5. for location of the photograph. b). Stereonet showing the variation of cross-bed orientations across these dune units and the variation of joint orientations therein.

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Fig. 10. Plot showing the mean value and the ranges of measured strikes of cross-beds and CBP confined joints at each measurement station. station, the dihedral angles between the cross-beds azimuths of CBP confined joints are calculated and

and CBP confined joints as well as the difference plotted in Fig. 11. In general, the dihedral angles between the dip azimuths of bedding and strike between the CBP confined joints and cross-beds

range from 70º to 88º with a mean value of about 80º (Fig. 11a). The differences between the dip azimuths of bedding and strike azimuths of CBP confined joints vary from 0º to 26º and more frequently from 147º to 177º (Fig. 11b). These results suggest that (1) CBP confined joints are at high-angle to bedding but not strictly perpendicular to bedding, and (2) CBP confined joints are trending roughly parallel to the dip direction of the cross-beds.

Navajo Sandstone in Zion National Park. 1. Checkerboard Mesa Due to the steep cliff face in the upper part of the northern face of the Checkerboard Mesa, only the Fig. 11. Distribution plots of dihedral angles and bottom three dune units (D1-D3) are accessible. The azimuth differences between cross-beds and CBP orientations of cross-beds and selected CBP confined confined joints. (a) Dihedral angles between cross- joints are measured and plotted for each dune unit in beds and CBP confined joints therein at each Fig. 12. measurement station. (b) Differences between the The measured orientations of cross-beds in the three dip azimuth of cross-beds and strike azimuth of dune units have different distributions (inset in Fig. 12). CBP confined joints at each measurement station. The mean values (strike/dip angle) of cross-bed

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Fig. 12. Ground photograph showing the north face of Checkerboard Mesa. The orientations of cross- beds and selected joints in the bottom three dune units are measured and shown in the inset. Man (circled) for scale in the lower right corner of the figure. orientations in D1, D2, and D3 are 174º/23º, 149º/21º, In general, the greatest variation occurs between the and 194º/30º, respectively. In comparison, the mean cross-beds in D5 and the remaining dune units. The orientations of the measured CBP confined joints in D1 cross-beds in D5 have NW-dips whereas the cross-beds and D3 are 53.8º/76.2º and 41.6º/71.3º. However, the in other dunes have SW-dips. In comparison, the CBP orientations of CBP confined joints in D2 display two confined joints in D5 also have contrastingly different clusters. The mean values of these two clusters are orientations from the joints in other dunes (Fig. 13). 44.6º/76.4º and 279º/80.2º suggesting that the CBP confined joints are trending roughly parallel to the dip Discussion direction of the cross-beds in D1 and D2, but not in D3.

The notion that joints are normal to bedding 2. Crazy Quilt Mesa The notion that joints are commonly perpendicular The eastern face of Crazy Quilt Mesa is to bedding is supported by many previous studies on characterized by a relatively consistent topography. The jointing in sedimentary rocks (Hodgeson, 1961, Price, dip angle of the cliff face is about 60º (Fig. 13a). In the 1966, Kulander et al., 1979; Gross and Engelder, 1995; selected area of study, it is easy to observe the Odonne et al., 2007). A commonly accepted reason for differences in the trace geometry of cross-beds and this notion is that sedimentary rocks are originally flat- CBP confined joints in different dune units (Fig. 13b). lying and the regional principal stresses are orthogonal The orientations of cross-beds and joints were to bedding when the joints formed before any tilting of measured by projection and approximation using the bedding. However, for aeolian sandstones in which electronic inclinometer as described in the methodology the cross-beds have an originally significant dip angle section of this paper and plotted in stereonet (Fig. 13c). due to the depositional processes (Kocurek, 1991), the

Stanford Rock Fracture Project Vol. 25, 2014 A-10 Fig. 13. Ground photograph showing the focused area of study on the east face of Crazy Quilt Mesa and the measured orientation data of the cross-beds and joints therein. (a) East face of Crazy Quilt Mesa (the location of (b) is indicated by the white frame). (b) Focused area of study. The dune boundaries are labeled with red lines. Joints are labeled with arrows. (c) Measured (approximation) orientation of cross- beds and joints in six dune units defined in (b). reason why joints formed at high-angle to bedding The traces of the CBP confined joints together with remains intriguing. the cross-beds form a remarkable rectangular pattern Assuming that rocks are homogeneous, the tensile (Figs. 12, 13b). Regarding such a pattern, National Park strength along various orientations should be similar. Service (2005) and Chan et al. (2007) considered what However, the aeolian sandstones in our study areas we termed as the CBP confined joints as structures have a strong cross-bedding and it is likely that these genetically similar to polygonal crack networks which cross-beds introduce a significant anisotropy. The formed as a result of temperature change (thermal modeling studies based on the strength of localized joints) or moisture change (desiccation joints). Chan et compaction with weak (favorable) and strong al. (2007) also pointed out that bedding anisotropy (unfavorable) orientations are very promising to plays a role in forming the rectangular pattern rather explain the distribution and orientation of compaction than polygonal pattern. However, we propose that the bands. Based on this, it is likely that the strength of CBP confined joints formed under the influences of opening-mode fracture formation can also be defined as cross-bed related anisotropy, but have a tectonic origin a function of cross-bed orientation. The relationship rather than having desiccation or thermal origins. An between the orientation of cross-beds and the CBP example illustrating the differences in the morphology confined joints documented in this study demonstrates and distribution of tectonic joints (CBP confined joints) that orientation of the CBP confined joints vary and desiccation joints (polygonal joints) is given in Fig. consistently with the change of cross-bed orientations. 14. In dune D1 the desiccation joints occur as a This suggests that cross-beds and the associated polygonal joint network perpendicular to the erosional anisotropy in aeolian sandstones may potentially impact surface, whereas in D2 and D3, the CBP confined joints the stress state within the dune units and consequently occur at high-angle to bedding (Fig. 14). Though the may control the orientation of the CBP confined joints cross-beds in D1, D2 and D3 have similar orientations described earlier. (probably similar anisotropy), desiccation joints clearly

Stanford Rock Fracture Project Vol. 25, 2014 A-11 Fig. 14. Comparison between tectonic joints and desiccation joints. a) A low attitude aerial photograph taken from the north face of Crazy Quilt Mesa showing the desiccation joints in D1 and tectonic joints in D2 and D3. Man in the middle for scale. b) Measured orientations of cross-beds and joints of different types in D1, D2, and D3. have a more scattered distribution of orientation than associated with a higher degree of inhomogeneity. This the CBP confined joints. suggests that boundaries of various hierarchies exert different influences on joint sets formed under different Boundary hierarchy for different joint sets mechanisms. This hierarchy reflecting various degrees of the influences on joint formation in the aeolian The joint zones in Aztec Sandstone documented in sandstones is not well known. this study have a north-south orientation. This is the same orientation as the joint zones studied by Taylor et al. (1999) and Myers and Aydin (2004). In particular, Conclusions Myers and Aydin (2004) observed that many of the We propose a close relationship between the joint zones originate as splay fracture arrays emanating orientations of cross-beds and the cross-bed package from slipped bedding planes or inclined faults (Aydin confined joints in the Jurassic aeolian Aztec and Navajo and de Joussineaou, 2014). Accordingly, the joint zones sandstones cropping out in the Valley of Fire State Park studied in this paper might have a similar origin. There and Zion National Park, respectively. Field data is no clear evidence indicating the age relationship presented in this study demonstrate that cross-bed between the CBP confined joints and the joint zones. package confined joints are generally at high-angle to The relatively consistent orientation of joint zones bedding and are mostly trending roughly parallel to the suggests that they might be formed under a different dip direction of the cross-beds. In light of these stress regime than the CBP confined joints. observations, cross-beds and the associated anisotropy, We note that cross-beds exert a strong influence on in addition to the distribution of compaction bands, may the CBP confined joints and the dune boundaries have a also exert a strong control on the formation and strong influence on the joint zones as evidenced by orientation of the joints. Laboratory study of anisotropy commonly observed truncation of the joint zones at due to cross-beds in the Aztec sandstone is underway to dune boundaries. Compared to the bedding interfaces test this hypothesis. that bound the CBP confined joints, dune boundaries These results may have significant implications for can be considered as boundaries of a hierarchy fluid flow through sandstones with depositional and

Stanford Rock Fracture Project Vol. 25, 2014 A-12 structural architectures similar to those of the study Fossen, H., Schultz, R.A., Shipton, Z.K., Mair, K., 2007. area. Deformation bands in sandstonea review. Journal of the Geological Society, London 164, 755e769. Gross, M. R., 1995. Fracture partitioning: Failure mode as a Acknowledgments function of lithology in the Monterey Formation of The authors wish to thank Rui Jiang for his coastal California. Geological Society of America assistance in the field. This research has been supported Bulletin, 107(7), 779-792. by the DOE Basic Energy Sciences, Division of Gross, M.R., Engelder, T., 1995. Fracture strain in adjacent Chemical Sciences, Geosciences and Biosciences, units of the Monterey Formation: Scale effects and Geosciences Research Program, grant #DE-FG02- evidence for uniform displacement boundary conditions. 04ER15588. Journal of Structural Geology 17, 1303±1318. Hancock, P.L., 1985. 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