Paleo-fl uid fl ow and deformation in the Aztec Sandstone at the Valley of Fire, —Evidence for the coupling of hydrogeologic, diagenetic, and tectonic processes

Peter Eichhubl† W. Lansing Taylor‡ David D. Pollard Atilla Aydin Department of Geological and Environmental Sciences, Stanford University, Stanford, 94305-2115, USA

ABSTRACT of structural heterogeneities for focused fl uid in southeastern Nevada. The Aztec Sandstone fl ow in a porous sandstone . has undergone multiple episodes of deforma- Paleo-fl uid fl ow conditions are recon- tion and diagenetic alteration including the structed for an exhumed faulted and frac- Keywords: fl uid fl ow, sandstone, , precipitation, dissolution, and remobilization tured sandstone aquifer, the Aztec , deformation, thrust. of iron oxides and hydroxides. These oxida- Sandstone at Valley of Fire, Nevada. This tion and reduction reactions provide a record of reconstruction is based on detailed map- INTRODUCTION paleo-fl uid fl ow that is readily identifi able in the ping of multicolored alteration patterns that fi eld, allowing the spatial reconstruction of fl ow resulted from syndepositional reddening of Understanding the spatial and temporal distri- migration pathways and fl ow direction. Based the eolian sandstone and repeated episodes of bution of fl uid fl ow in the subsurface is of funda- on crosscutting relations with structural fea- dissolution, mobilization, and reprecipitation mental importance to the successful management tures, we demonstrate that the timing and focus- of iron oxide and hydroxide. A fi rst stage of of and hydrocarbon resources. In ing of fl uid fl ow across this sandstone unit was bleaching and local redeposition of hematite addition, the interaction of formation fl uids with strongly infl uenced by deformation and tectonic is attributed to upward migration of reduc- the hydrosphere and atmosphere is of increas- processes. This paper emphasizes map-scale ing basinal fl uid during and subsequent ing environmental concern (Moore, 1999). Of processes. The relationships between alteration to Late Sevier thrusting and particular signifi cance to these processes are the and deformation on the scale and the in foreland deposition of clastic . A effects of structural heterogeneities such as faults situ hydraulic properties of structural features second stage of bleaching and iron remobili- and fractures in controlling diagenetic precipita- were described earlier in separate publications zation, precipitating predominantly goethite tion and dissolution and the combined effects of stemming from this and related research efforts and minor iron sulfates, occurred during deformation and diagenesis in controlling the (Taylor et al., 1999; Taylor and Pollard, 2000; Miocene strike-slip faulting associated with fl uid migration pathways and their permeability. Jourde et al., 2002; Flodin et al., 2004; Sternlof Basin and Range . This second stage This study followed a paleohydrologic et al., 2004; Myers and Aydin, 2004). is explained by mixing of reducing sulfi de- approach to investigate these effects in an rich basinal fl uid with meteoric water enter- exhumed faulted and fractured sandstone res- GEOLOGIC SETTING ing the aquifer. The distribution of alteration ervoir and aquifer. This investigative approach patterns indicates that regional-scale fl uid reconstructs the spatial and temporal variation Valley of Fire State Park is situated 60 km migration pathways were controlled by of fl uid fl ow based on crosscutting relations northeast of Las Vegas, Nevada (Fig. 1), within stratigraphic contacts and by thrust faults, between structures and diagenetic features that the Basin and Range physiographic province, whereas the outcrop-scale focusing of fl ow are observable in outcrop or with analytical tech- 50 km west of the western edge of the Colorado was controlled by structural heterogeneities niques. Diagenetic features that are indicative of . The park derives its name from the mul- such as joints, -based faults, and defor- past fl uid fl ow include alteration boundaries that, ticolored sandstone formations that range in hue mation bands as well as the sedimentary in some cases, allow inference of the direction of from orange-red to purple, yellow, and white architecture. The complex interaction of fl uid fl ow. Outcrop analog studies of paleofl ow and that are part of the 1400-m-thick Upper structural heterogeneities with alteration systems are particularly useful in providing and Jurassic Aztec Sandstone (Bohan- is consistent with their measured hydraulic a three-dimensional view of the infl uence of non, 1977, 1983), an eolian deposited properties, demonstrating the signifi cance structures on fl uid fl ow at scales ranging from in a backarc setting (Marzolf, 1983, 1990). The microscopic to regional. In addition, outcrop Aztec Sandstone is part of a Mesozoic clastic analog studies can assess temporal variations in sequence that overlies upper Paleozoic - †E-mail: [email protected]. ‡Present address: Anadarko Petroleum Corpora- fl ow regimes over geologic time scales. ates and (Bohannon, 1983). Triassic red tion, 1201 Lake Robbins Drive, The Woodlands, We investigated paleo-fl uid fl ow in the Juras- beds of the Moenkopi, Chinle, and Moenave Texas 77380, USA. sic Aztec Sandstone at Valley of Fire State Park Formations, with a combined thickness of up

GSA Bulletin; September/October 2004; v. 116; no. 9/10; p. 1120–1136; doi: 10.1130/B25446.1; 11 fi gures; 1 table; Data Repository item 2004123.

For permission to copy, contact [email protected] 1120 © 2004 Geological Society of America PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE

114°33' 114°32' 114°31' Tertiary & Quaternary Cretaceous Willow Jurassic Aztec Sandstone Tank upper red and banded red and white units Thrust

600 middle alteration units lower red unit 662 Tearfault + NEVADA 20 Triassic (undiff.) BH White Dome + (dotted where inferred)

600 Las Vegas 600 39 roadway 22 34 20 655 36°28' + 600 L Figure 1. Geologic map of 32 the northern part of Valley C 34 of Fire State Park, modifi ed WP BH B from Flodin and Aydin (2004). 700 600 W WP—Waterpocket fault; BH— 36 Bighorn fault; W—Wall fault;

700 L—Lonewolf fault; C—Classic 900 600 fault; M—Mouse’s Tank fault;

33 36°27' B—Baseline fault. M Silica Dome 28 + 800 WP 20 600 700 Figure 2 700 25 700 700 20 Mouse's Tank 600

900 700 700 36°26' 30 N 800

36°26' 36°27' 36°28'700 36°29' 36°30' 700 1 km B M 600 UTM zone 11 NAD 1983 contour interval 25 m 4034000 4036000 4038000 4040000 4042000 114°33' 114°32' 114°31' 114°30' 114°29' 720000 722000 724000 726000

to 2100 m (Bohannon, 1977), are composed Tank Formation includes locally derived red eastern margin of this klippe (Longwell, 1949), of -, -, and with Aztec Sandstone components (Longwell, 1949) indicating that the eastern edge of the klippe layers (Marzolf, 1990). The Aztec Sandstone is among far-traveled components, indic- represents the leading edge of the thrust sheet. unconformably overlain, with a discordance of ative of partial exhumation and of the Upper portions of the Baseline Sandstone are locally up to 10°, by a distinctive Aztec Sandstone subsequent to its deposition deposited on top of the Willow Tank thrust that forms the basal member of the Cretaceous (Bohannon, 1983). sheet, constraining the age of thrusting to Late Willow Tank Formation. The Willow Tank For- The Willow Tank Formation and lower por- Cretaceous () (Longwell, 1949; mation, composed of predominantly mudstone, tions of the Cretaceous Baseline Sandstone are Bohannon, 1983). and the overlying sandstone and conglomerate overthrust by Aztec Sandstone of the Willow With a hiatus during Paleogene times, depo- layers of the Baseline Sandstone have a com- Tank thrust sheet (Figs. 1 and 2), the lowest of sition of clastic units continued conformably bined thickness of about 1300 m (Bohannon, the Sevier thrust sheets in the study area (Long- during Oligocene to Pliocene times (Bohan- 1977). These units are interpreted as synoro- well, 1949; Bohannon, 1983). Tearfault Mesa in non, 1983). An ~25° angular genic foreland deposits of the eastward-directed the northern part of Figure 1 forms a klippe of at the base of the Upper Miocene (10–4 Ma) Cretaceous Sevier thrust sheets (Bohannon, the Willow Tank thrust sheet. Cretaceous strata Muddy Creek Formation (Bohannon et al., 1983). The basal conglomerate of the Willow are steepened and locally overturned along the 1993) constrains the time of tilting of the Aztec

Geological Society of America Bulletin, September/October 2004 1121 EICHHUBL et al.

722000 114°31'

** 600 * 36°29' BH Fig. 7

600 Holocene Cretaceous (undiff.) 600

4040000 Jurassic Aztec Sandstone red white 4040000 B yellow orange 600 purple A banded red and white banded purple and white banded orange and white banded purple and yellow 36°28' BH kaolinite cemented layer L * A B paleoflow direction measurement (Fig. 11) C fault (dashed where inferred)

600 M fault name (see Fig. 1) W 4038000 thrust fault road 4038000 N

700

M 1 km

36°27' UTM zone 11 NAD 1983 contour interval 25 m 36°27' 36°28' 36°29' 700

114°32' 722000 114°31'

Figure 2. Map of diagenetic alteration units for area shown in Figure 1.

1122 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE

Sandstone, presently dipping 20°–30° north- Two sets of oblique-slip faults with pre- 4°–65° 2θ at 1°/min on a Rigaku powder diffrac- east. The Muddy Creek Formation, in turn, dominant strike-slip and lesser normal-slip tometer. Samples for elemental analysis were dips 5°–8° northeast, indicating continued components occur as two sets, a left-lateral fault prepared by lithium-tetraborate fusion and ana- tilting throughout the Late Miocene. The Aztec set striking NNE and a right-lateral set striking lyzed by ICP-AES at Actlabs-Skyline, Tucson, Sandstone and overlying Cretaceous strata are NW (Flodin and Aydin, 2004; Myers and Aydin, . Two samples were analyzed by X-ray folded into a gentle NE-plunging syncline 2004) (Fig. 1). The NNE-striking set is more fl uorescence at Washington State University, referred to as Overton syncline by Carpenter prominently developed on the regional scale, Pullman, Washington, following the methods of and Carpenter (1994) (Fig. 1). but both sets show mutually abutting geometries Johnson et al. (1999). Ferrous iron was analyzed on a local scale, suggesting both sets were active by Actlabs-Skyline, Tucson, Arizona, by titration DEFORMATION concurrently (Flodin and Aydin, 2004). Faults of with dichromate after HCl-HF acid digestion. both sets dip steeply with mean dip angles of Deformation structures in the Aztec Sand- 80°–90°. Following Flodin and Aydin (2004) Compositional Changes stone include deformation bands, joints and and Myers and Aydin (2004), the evolution of sheared joints, and faults that are composed of these faults includes the formation of joints, slip The Aztec Sandstone is a fi ne to medium- deformation bands, slip surfaces, sheared joints, along joints, formation of splay fractures, and grained subarkose (Marzolf, 1983) with up to and zones. linkage of sheared joints. 8% (Flodin et al., 2003) and a smaller Deformation bands are tabular features of These faults offset the Aztec/Willow Tank amount of lithic components. The sandstone localized deformation that are frequently more Formation contact. Following the maps of is generally friable with a of 15–25% resistant to erosion and thus observed on the Bohannon (1977) and Carpenter (1989), the and permeability of 100–2500 md (Flodin et outcrop as ridges ranging in thickness from 1 youngest strata offset by faults of the Baseline al., 2003). Grains are subrounded to rounded, mm to 10 cm, with a typical width of about 1 cm fault system are the Miocene Horse Spring characteristic of the eolian depositional environ- (Antonellini and Aydin, 1994). At Valley of Fire, Formation and probably lower sections of the ment. Throughout the section grains are weakly Hill (1989) distinguished three earlier sets of Upper Miocene Muddy Creek Formation (10– indented as a result of , and pore deformation bands, striking northwest, north- 4 Ma, Bohannon et al., 1993). These faults are space is partially fi lled by cement (Fig. 3A). northwest, and north-northeast, that are charac- thus considered to be associated with Basin and The predominant feldspar is orthoclase, observed terized by predominant band-parallel compac- Range tectonics (Flodin and Aydin, 2004). in in various stages of replacement by tion. Following Mollema and Antonellini (1996) kaolin . With the exception of one sam- we refer to these deformation bands as compac- DIAGENETIC ALTERATION ple of high stratigraphic position (sample 04/04/ tion bands. Repeated mutual crosscutting sug- 01–3) that contained , the dominant kaolin gests that these three sets formed concurrently. Methods was determined by X-ray diffraction to The three sets of bands are crosscut be kaolinite, following criteria given by Lanson by three sets of deformation bands (Flodin and The distribution of diagenetic alteration of et al. (2002) (Table 1). Other clay minerals are Aydin, 2004) that exhibit macroscopic shear off- the Aztec Sandstone (Fig. 2) was mapped on mixed-layer illite/smectite. Kaolinite forms pore- sets and are therefore referred to as shear bands. color aerial photographs on a scale of 1:4000 fi lling clay and partially replaces K-feldspar. One set of shear bands, with slip ranging from 1 and extensively fi eld-checked. color was Kaolinite are locally abundant. A to 3 cm, parallels depositional boundaries of the determined using the Munsell color charts notable occurrence is within an approximately cross-bedded strata (Hill, 1989). The other two (Munsell Color, 1994). For hues outside the 2-m-thick bed of kaolinite-cemented sandstone sets of shear bands usually occur as subvertical range of the charts, the rock color (Fig. 2) that may represent a paleosoil horizon. zones of multiple shear bands and associated charts (Rock-Color Chart Committee, 1970) Kaolinite concretions are also found localized slip surfaces, with slip on the order of decimeters were used. Munsell and rock color names are along faults indicative of syn- to postfaulting (Flodin and Aydin, 2004). The top-to-the-east given in combination with their numeric charac- kaolinite remobilization. Mixed-layer illite/ sense of shear of the bed-parallel shear bands terization of hue (shade), value (lightness), and smectite is 90% illite with a Reichweite of 3. In is kinematically consistent with Sevier thrust- chroma (saturation). The fi eld designations of the stratigraphically lowest sample (99-20), illite ing, suggesting that these shear bands formed alteration units do not follow these color names, replaces mixed-layer illite/smectite. concurrently with thrusting. however, because they do not distinguish among Minor amounts of quartz overgrowth cement Taylor et al. (1999) distinguished four sets differences in hue that are signifi cant in the con- were observed in stratigraphically lower parts of joints in the Aztec Sandstone. The fi rst set text of this study. of the Aztec Sandstone. Quartz cement was also is composed of parallel vertical joints that Petrographic descriptions are based on 59 found locally within the damage zone and to both strike roughly north-south. This set is locally mostly polished sections of samples impreg- sides of the Bighorn fault (Fig. 2), cementing replaced by two sets of joints that also strike nated with blue epoxy under vacuum. Eleven allochthonous Aztec Sandstone of the Willow north-south but form two intersecting sets with representative samples of diagenetic alteration Tank thrust sheet as well as fault rock that is part an acute angle of about 30°. Joints of the fourth zones were collected for compositional analysis of the autochthonous Aztec Sandstone. Carbonate set are slightly sinuous and do not show a strong by powder X-ray diffraction (XRD), inductively cement that is not attributed to modern caliche is preferred orientation but may favor a roughly coupled plasma–atomic-emission spectrom- found locally as pore-fi lling cement along joints east-west orientation. Joints have been observed etry (ICP-AES), and X-ray fl uorescence (XRF). and as concretions, both occurrences restricted to to consistently crosscut compaction and shear XRD samples were disaggregated in an agate the lower part of the Aztec Sandstone. bands and are thus interpreted to form after and the <2 µm size fraction separated fol- The characteristic hues of red, orange, purple, deformation bands and concurrently with, and lowing standard centrifuge techniques (Poppe et yellow, and white of the Aztec Sandstone result possibly also prior to, strike-slip faulting (Flodin al., 2001). Oriented sample mounts of glycolated from varying amounts and forms of iron oxide and Aydin, 2004; Myers and Aydin, 2004). and unglycolated samples were scanned over and hydroxide cement, the dominant pigments

Geological Society of America Bulletin, September/October 2004 1123 EICHHUBL et al. being hematite and goethite. The uniform red A B color (Munsell 10R 6/8 light red) of the lower kaol red and upper red alteration units of the Aztec Sandstone results from thin grain coats of hematite. In thin section, hematite forms a mottled brownish coloration of grain surfaces (Fig. 3A). In samples of lower red sandstone containing quartz overgrowth cement, the hematite coat predates the quartz overgrowth, indicative of an early diagenetic or syndeposi- 100 µm 50 µm tional origin of the hematite coat. Compared to grain coats in red sandstone, grain coats in yellow sandstone (Munsell 10YR C D 7/6 yellow to 10YR 8/2 very pale brown) are more patchy and partly recrystallized to ≤1 µm sized (Fig. 3B). Based on X-ray dif- fractograms, the dominant Fe oxide is goethite (Table 1). This is consistent with goethite being the predominant pigment in yellow (Schwert mann, 1993). In orange-colored sandstone (Munsell 2.5YR 5/8 red), hematite 100 µm forms 5–10 µm sized globules that are attached 100 µm to grain surfaces (Fig. 3C). The equally spaced distribution of hematite globules along grain E F surfaces is suggestive of globule formation by local dissolution of earlier grain coats and reprecipitation of hematite with accompanying coarsening. Purple coloration (rock color chart 5RP 6/2 pale red purple to 5RP 4/2 gray- ish red purple and Munsell 10R 6/3 pale red) is caused by 1–3 µm sized grains of Fe oxide (Fig. 3D), identifi ed by XRD to be dominantly 100 µm goethite (Table 1). Some samples of purple and 200 µm yellow sandstone also contain smaller amounts of the sulfates alunite KAl (SO ) (OH) and 3 4 2 6 Figure 3. Photomicrographs (plane-polarized light) of altered Aztec Sandstone. (A) Hema- jarosite KFe (SO ) (OH) . Jarosite provides a 3 4 2 6 tite grain coat (arrow) in upper red unit. Pore space is partially cemented by kaolinite (kaol mottled brownish color to the otherwise uniform at top left of image). (B) Patchy goethite grain coat (arrow) in yellow Aztec Sandstone. yellow sandstone. White sandstone ( Munsell (C) Hematite globules 5–10 µm in size in orange Aztec Sandstone. (D) Goethite recrystal- 10YR 8/1 to 7.5YR 8/1 white) is devoid of grain lized to 1–3 µm-sized grains (arrow) in purple band. (E) White Aztec Sandstone is devoid of hematite grain coats. (F) Pore-fi lling hematite adjacent to a joint.

TABLE 1. MINERALOGICAL COMPOSITION OF ALTERED AZTEC SANDSTONE Sample Sample description Location Longitude Latitude qtz K-fdspr kaol dick smec ill I/S R hem goe alun jar other number (W) (N) 04/04/01-3 Red sandstone in upper 200 m E of Bighorn fault 114°31.47′ 36°29.09′ xx –x––903xx ––– red unit 04/04/01-5 White sandstone below 200 m E of Bighorn fault 114°31.47′ 36°29.09′ xx x–––903–––– upper red unit 04/04/01-7 Purple sandstone in 200 m E of Bighorn fault 114°31.44′ 36°28.93′ xx x–––903– x x– purple and yellow unit 04/05/01-4 Yellow sandstone Fire Road 114°30.34′ 36°27.25′ xx x–––903–x–– 11/18/01-1C Hematite-stained Colorock 114°41.16′ 36°23.80′ x–(x) – – (x) 90 3 xx x – – sandstone next to joint 11/18/01-5 Yellow sandstone Buffi ngton area 114°41.14′ 36°23.13′ x (x) – – – – 903 – xxx x 11/20/01-1 Orange sandstone in 250 m E of White Dome 114°31.51′ 36°28.55′ x– x–(x) – 90 3 (x) – x – banded orange and road white unit 99-72 Purple sandstone S White Dome 114°31.98′ 36°29.00′ x– x–––903– x xx 99-20 Red sandstone in lower S Mouse’s Tank 114°31.11′ 36°25.98′ x– x––x––xxx ––chlorite red unit 99-65 Red sandstone in lower S Rainbow Vista 114°30.86′ 36°26.98′ xx x–––903xx (x) – – red unit Note: qtz—quartz; K-fdspr—potassium-feldspar; kaol—kaolinite; dick—dickite; smec—smectite; ill—illite; I/S—illite/smectite mixed-layer clay; R—Reichweite; hem— hematite; goe—goethite; alun—alunite; jar—jarosite; x—present; (x)—present in traces; xx—abundant (for hematite and goethite only); – indicates mineral below detection.

1124 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE coats (Fig. 3E) but contains sparse fl akes of The boundary between the lower red to yellow as middle units. The middle alteration units coarse crystalline hematite. and purple sandstone of the overlying middle include bands of white, yellow, purple, orange, Joints and faults are frequently associated alteration units is a relatively sharp transition and smaller amounts of red sandstone. Alteration with haloes of dark red (Munsell 10R 4/3 to in coloration (Fig. 4A) that occurs over 1–5 cm. bands too thin to be mapped as separate units 10R 4/4 weak red) pore-fi lling hematite. This Within this transition, the boundary appears were categorized as banded orange and white, type of hematite resembles hematite globules frayed due to preferred reddening of fi ner- banded purple and white, and banded purple found in orange sandstone but occurs at higher grained sandstone layers and the absence of red and yellow (Fig. 2). The dominant portion of density, completely occluding intergranular stain within coarser layers (Fig. 4B). Along the the banded units consists of banded orange and pores (Fig. 3F). boundary the red sandstone is frequently some- white sandstone. Similar to the banded red and Elemental analyses of Aztec Sandstone what darker (arrow in Fig. 4B) due to a larger white unit at the top of the Aztec Sandstone, the (Table DR11) indicate that concentrations in of hematite. Rather than being planar, banding is manifest as an orange stain in the

Fe2O3 are <1%, including samples of lower the upper boundary of the lower red unit has a fi ner-grained sedimentary layers and the absence and upper red sandstone. The only exception lobate-cuspate geometry, with lobes of purple or of pigment in the coarser-grained layers. This is one sample shown in Figure 3F containing yellow sandstone penetrating the lower red sand- banding results in an alternating sequence of

pore-fi lling hematite with ~3% Fe2O3. The stone (Fig. 4C). The boundary is parallel to bed- about 10–20-cm-thick orange and white layers elemental analyses generally lack systematic ding over distances of 10–100 m but frequently that follow depositional bedding. In addition,

trends in Fe2O3 for samples of different hue. The climbs, and occasionally drops, across bedding locally abundant deformation bands affected the differences in hue apparently refl ect differences over distances of 10–50 m. In the Rainbow Vista distribution of pigment, acting as boundaries that in mineral composition and grain size of Fe area (lower right section of Fig. 2), the average separate compartments with internal color gradi- minerals rather than Fe concentration, in agree- orientation of the boundary dips to the northeast ents from orange to white (Fig. 4F). The distri- ment with fi ndings in soil science (Torrent and by up to 49°, compared to bedding at 22°. bution of pigment in banded orange and white Schwert mann, 1987; Schwertmann, 1993). sandstone is thus controlled by both depositional The Upper Red Alteration Unit layering and deformation bands. Characterization of Diagenetic Alteration The designation upper red unit is used to Banded purple and white alteration resembles Units describe a 30–50-m-thick section within the the banded orange and white alteration in the stratigraphically upper part of the Aztec Sand- pattern of alteration except that orange pigment Flodin et al. (2003) and Myers and Aydin stone that resembles the lower red unit in its uni- is replaced by purple. The boundary between (2004) subdivided the Aztec Sandstone at Valley form red alteration color. Similar to the upper banded orange and white and the banded purple of Fire State Park into three informal members, boundary of the lower red unit, the lower bound- and white alteration is gradational. Purple and referred to as “lower red,” “middle beige” or ary of the upper red unit is parallel to bedding white banding is observed in the vicinity of and “middle yellow,” and “upper orange” members. over distances of 10–30 m, then cuts up section in contact with the yellow and purple alteration Although the boundaries of these rock units over similar distances at an angle of up to 20° units discussed next (Fig. 4G). are subparallel to bedding on a regional scale, relative to bedding. On average, this boundary Purple, yellow, and banded purple and yellow we will show that they are diagenetic and not dips 40° to the northeast, compared to a bedding alteration units are frequently observed to cut primarily depositional in origin and thus not dip of 20°–30° to the northeast. obliquely across banded orange and white and members in a stratigraphic sense. Instead, we banded purple and white alteration with a sharp subdivide the Aztec Sandstone into diagenetic The Banded Red and White Unit boundary. Purple bands in yellow sandstone alteration units to describe mappable sections The upper red unit is overlain by a 30–60 m frequently do not follow bedding. These bands of the formation of similar alteration color and thick unit, referred to as banded red and white are rather irregular with diffuse boundaries color distribution. unit (Fig. 2), that is composed of red layers alter- resembling schlieren or marbling in metamor- nating with white layers (Fig. 4D). The bound- phic or metasomatic rocks. Remnants or ghosts The Lower Red Alteration Unit ary between the banded red and white unit with of stratigraphically controlled orange and white In modifi cation of the previous nomenclature, the underlying upper red unit frequently follows or purple and white banding may be locally rec- we use the term lower red unit to designate the bedding but locally cuts obliquely across bed- ognizable in yellow and purple alteration units. stratigraphically lower 800–900-m-thick sec- ding. At those locations the white bands fade These remnants, as well as the crosscutting rela- tion, of the formation that is stained uniformly with an orange intermediate color into the tions of the alteration unit boundaries, indicate red (Fig. 1). The red hue of the lower red unit is uniform red stain. Approaching the Willow that banded orange and white alteration predates uniform without regard to grain-size variations Tank thrust, the red and white banding becomes the yellow and purple alteration. Based on the among cross-stratifi ed layers. The only varia- irregular, with white patches and streaks cutting spatial association of banded purple and white tion in color is associated with darker red bands across bedding (Fig. 4E). The top 10–20 m alteration along the boundary to the yellow and resembling Liesegang bands. These bands are of the banded red and white unit south of the purple alteration unit, we infer that the purple found locally within and adjacent to faults and Willow Tank thrust is bleached white to pink and white alteration is a secondary alteration of are oblique to bedding and thus identifi ed as a (Munsell 10R 8/3 to 2.5R 8/3 pink) (Fig. 2). the orange and white alteration. secondary alteration of the uniform red stain. In addition, the allochthonous Aztec Sandstone Bands of uniform white and orange altera- along the base of the Willow Tank thrust sheet is tion colors that are up to 20 m thick and thus bleached white (Fig. 4E, arrows). mapped as separate units (Fig. 2) were found 1GSA Data Repository item 2004123, elemental along and parallel to the base of the upper red composition of altered Aztec Sandstone, is avail- able on the Web at http://www.geosociety.org/ The Middle Alteration Units alteration unit. In the northern part of the map pubs/ft2004.htm. Requests may also be sent to Units of various colors between the lower area (Fig. 2), a 10–20-m-thick white band lies [email protected]. and upper red units are referred to collectively below the upper red unit, followed by an equally

Geological Society of America Bulletin, September/October 2004 1125 EICHHUBL et al.

A B

Figure 4. Alteration of Aztec Sandstone at Valley of Fire. (A) Boundary of lower red unit to overlying yellow unit at Silica C Dome. Field of view about 0.5 km. (B) Close-up of lower red boundary. Arrow points at layer of coarser hematite yield- ing a darker stain. (C) Detail of lower red boundary at Silica Dome. Cusps (arrow) point C D toward yellow unit, lobes point toward lower red unit. Field of view about 50 m. (D) Banded red and white unit. Basal conglomerate of Willow Tank Formation in background. Map case (30 × 40 cm, circled) for scale. (E) White Aztec Sandstone (foreground) below allochthonous Aztec Sandstone of Willow Tank thrust sheet (background). Dashed line indicates position of thrust. Arrows point at bleached Aztec E F Sandstone along the base of the thrust sheet. The autoch- thonous and allochthonous sections of the Aztec Sandstone are separated by shale of the Willow Tank Formation (Kwt). Kwt Field of view about 0.5 km. (F) Banded orange and white unit. Note alteration compartmen- talization due to deformation deformation bands. (G) Banded orange and bands white unit crosscut by yellow and purple unit. Along contact, banded orange and white unit G H conglomerate is altered to banded purple and white. Notebook (15 × 22 cm, circled) for scale. (H) Bound- ary of white sandstone to upper red sandstone, cutting upsec- tion across bedding. Banded red and white unit and basal yellow conglomerate of Willow Tank Formation in background. purple/white Notebook (circled) for scale.

orange/white

1126 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE

A B

orange joints white

sheared joint Figure 5. Crosscutting relations of alteration with faults and fractures. (A) Lobes of orange unit with purple rim (arrows) extending into underlying white sandstone, preferentially C D following joints. Pocket knife (circle) is 9 cm long. (B) Lobes fault of orange unit into white sand- stone are offset by 2–3 cm due red to shear along joints 5–10 m yellow west of the Bighorn fault red (Fig. 1). (C) Offset of the lower yellow red to yellow boundary along compaction the Mouse’s Tank fault (Fig. 1), band right side (E side) away from viewer by 240 m of left-lateral strike slip. Field of view about 40 m. (D) Apparent offset of lower red to yellow boundary E F along compaction band. Offset is explained by relative retar- shear band dation of alteration front on the left side of the deformation joints band. (E) Preferred bleaching (arrows) of banded red and white unit along shear bands about 100 m south of Willow Tank thrust. (F) Bleaching of shear band purple sandstone along joints. Field of view ~3 m. (G) Tail of purple band around (arrow) indicates direction of front advancement (in direc- G H tion of arrow) to the SE and downsection. (H) Kaolinite concretions preserved red stain concretions due to preferred bleaching of surrounding sandstone. Kaolin- ite-cemented layer (Fig. 2).

Geological Society of America Bulletin, September/October 2004 1127 EICHHUBL et al. thick orange band, and a second white band. symmetric (Fig. 6), with yellow and banded Bighorn fault, the yellow and purple lobe climbs The boundary between white and the upper orange and white units in the center, over- and stratigraphically (Figs. 2 and 6). red unit is locally sharp, changing from white underlain by purple and yellow and the red to red within 1 cm (Fig. 4H). At other places, sandstone of the upper and lower red units. The CROSSCUTTING RELATIONS this boundary appears washed out and grada- symmetry is broken by the occurrence of white AMONG ALTERATION UNITS AND tional over 1–2 m. The orange band below the and orange bands along the boundary to the DEFORMATION STRUCTURES upper red unit is roughly parallel to the lower upper red unit, and by the banded red and white boundary of the upper red unit. Whereas the sandstone above the upper red unit. The spatial distribution of alteration units is upper boundary of the orange band is rather dif- Purple and yellow alteration thus occurs affected by faults, joints, and deformation bands fuse, its lower boundary is typically sharp and in three horizons within the middle alteration due to (a) structural offset of diagenetic bound- accentuated by a 1–3-cm-thick band of purple units: (1) in contact with the lower red alteration aries and (b) localized alteration along these sandstone (Fig. 5A). This lower boundary forms unit, (2) in the center of the middle alteration structural features. Structural offset of diage- lobes that protrude into the underlying white or units, and (3) at the top of the middle alteration netic boundaries is equal to the structural offset yellow sandstone for 5–10 m (Fig. 5A). These units. The central occurrence of yellow and pur- of stratigraphic markers if the diagenetic bound- lobes frequently follow joints (Fig. 5A), and ple sandstone forms a lobe that occupies most of ary predates fault slip, or less than the offset of where joints are reactivated in shear, the bound- the middle alteration units to the southeast and stratigraphic markers if the diagenetic boundary aries of these lobes are offset (Fig. 5B). tapers off to the northwest. The top boundary of formed while the fault was active. Localized this lobe is less inclined than the other alteration alteration along faults or deformation bands Distribution of Alteration Units bands, allowing the overlying banded orange can result in an apparent offset of diagenetic and white unit to increase in thickness toward boundaries that can exceed the structural offset The distribution of alteration units within the the northwest (Fig. 6). To the northwest of the of stratigraphic markers along these structures. mapped area as seen in cross section is roughly Localized alteration indicates that deformation

Southwest Bighorn Fault Northeast Cretaceous (undiff.) Jurassic Aztec Sandstone Willow Tank Thrust Pt. 662 Tearfault Mesa red Lonewolf Fault white White Dome * * * Classic Fault Pt. 655 yellow orange purple banded red and white banded purple and white banded orange and white banded purple and yellow * quartz cementation fault thrust fault bedd ing dip ~ 22° 100 m

0 1 km

Figure 6. Schematic composite cross section across diagenetic alteration units in Figure 2, based on fi eld sketches taken from several van- tage points. Due to the northwestward plunge of the alteration units, the top parts of the section refl ect observations in the northern portion of the map area, and lower parts are taken from observations in the southern portion of the map. Because alteration features do not project along strike over the extent of the map area, this section represents an idealized composite. Although unit thicknesses and inclination are approximately to scale, horizontal distances are condensed to about half of the vertical scale.

1128 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE preceded alteration. Both types of crosscutting of the alteration halo by 2 cm. These haloes are offset. The shallowly inclined upper surface relations provide constraints on the relative tim- along the lower boundary of the orange band of the central yellow lobe has small structural ing of diagenetic alteration and fl ow processes that underlies the upper red unit. The forma- offsets on the order of 1–10 m along these that caused alteration. In addition, localized tion of these joints thus predates this alteration faults, about one order of magnitude less than alteration provides evidence for the effects of boundary, but shear along these joints postdates their total apparent slip, indicating that fault slip structural heterogeneities on fl uid fl ow through the alteration boundary. outlasted the formation of alteration bands. The this sandstone unit. The interaction of faults with the purple, upper boundary of the lower red unit is, simi- Examples of structurally offset diagenetic orange, and yellow alteration zones is more larly to the lower boundary of the upper red unit, boundaries are seen along the upper boundary complex. Rather than being physically offset, structurally offset along these strike-slip faults. of the lower red unit and the lower boundary of these alteration bands frequently turn to become Along the Mouse’s Tank fault, the lower red unit the upper red unit. Both boundaries are structur- parallel to these faults. The best example of is structurally offset by 240 m of apparent left- ally offset by the two sets of strike-slip faults in this interaction is the apparent offset of the lateral slip (Fig. 5C). the area, the NNE-striking left-lateral set and orange, white, yellow, and purple bands along the NW-striking right-lateral set (Figs. 1, 2, the Bighorn fault immediately below the upper DISCUSSION and 5C). The structural offset of these alteration red unit (Figs. 2 and 6). Approaching the fault boundaries agrees with the offset of stratigraphic from the southeast, part of the orange band Interpretation of Alteration Units markers including boundaries and the basal turns parallel to the fault and follows the fault conglomerate of the Willow Tank Formation for about 15 m (Figs. 7A and 7C). Similarly, Syndepositional Reddening and First (Flodin and Aydin, 2004). Thus, strike-slip fault- the yellow and purple bands turn and follow Bleaching and Remobilization Stage ing postdates these alteration boundaries. the fault trace before crossing the fault (Figs. 2 Following the petrographic studies of red Examples of apparent offsets due to localized and 6). This change in attitude of the alteration beds in modern and ancient depositional set- alteration were observed along deformation bands is not due to drag of bedding. Bedding tings by Walker et al. (1978, 1981), we interpret bands that appear to offset the top boundary of does not noticeably change attitude adjacent to the hematite grain coats resulting in the uniform the lower red alteration unit (Fig. 5D). However, the fault. Rather, it appears that the purple and red alteration of the lower and upper red units these deformation bands lack any observable yellow bands approximately maintain a fi xed as an early syndepositional diagenetic alteration shear offset and are thus recognized as compac- distance relative to the upper red unit, which under near-surface conditions immediately fol- tion bands. Taylor and Pollard (2000) explained is structurally offset along the Bighorn fault by lowing deposition. This syndepositional diage- the apparent offset of alteration boundaries 600 m of apparent left-lateral slip (Flodin and netic origin may have included multiple cycles across compaction bands by the relative retar- Aydin, 2004). The position of these bands is of deposition, alteration, and redeposition (Wil- dation of a moving reaction front on one side therefore affected by the presence of this fault, son, 1992). A syndepositional origin of the red of these bands relative to the other side. This and the diagenetic and fl uid fl ow processes that stain is consistent with the abundant occurrence interpretation implies that the alteration caus- resulted in these bands postdate most if not all of clasts of red Aztec Sandstone in the basal ing the red to yellow boundary is affected by of the structural offset of the upper red unit. This conglomerate of the Willow Tank Formation the presence of the compaction band. Thus, the also suggests that the orange, purple, and yellow (Longwell, 1949). A syndepositional origin of alteration boundary postdates the formation of bands are secondary diagenetic modifi cations the red stain also is consistent with the uniform the compaction bands. of the upper red boundary. Final stages of fault alteration color irrespective of grain size and The asymmetric alteration around deforma- slip appear to have outlasted the formation of porosity variations in cross-beds or deformation tion bands in the banded orange and white unit the orange band, as indicated by the left-lateral bands within the red alteration units. Variations (Fig. 4F) described above is interpreted in a offset of the lower boundary of the orange band in permeability corresponding to variations in similar way. No slip is discernible across these along sheared joints (Fig. 5B). These joints porosity would likely result in color variations bands (Hill, 1989) that would create a structural appear to be part of the damage zone of the if hematite precipitation were secondary within juxtaposition of orange next to white sandstone. Bighorn fault or of an immediately adjacent an initially white sandstone that had been red- Rather, diagenetic alteration of the sandstone subsidiary fault. dened by advective transport of ferrous iron appears localized adjacent to these bands. Purple and yellow sandstones of the central and subsequent oxidation. Instead, the uniform Shear bands are observed to infl uence the lobe within the middle alteration units appear to distribution of hematite is consistent with the in bleaching within the banded red and white unit be defl ected upward along the southeast side of situ breakdown of iron-containing in an south of the Willow Tank thrust (Fig. 5E). These the Bighorn fault and subsidiary faults (Fig. 6). oxidizing environment and local precipitation shear bands offset bedding planes by as much as Again, this defl ection indicates that the faults of iron oxide coatings on the surfaces of chemi- 10 cm. Because bleaching is confi ned to certain were present at the time of the alteration. cally more resistant quartz grains, without the layers of the cross-bedded sandstone, the red Crosscutting relations similar to those involvement of any signifi cant postdepositional and white layers appear to be offset by the shear observed along the Bighorn fault are seen advective transport. band. Bleaching is localized along these shear along the Lonewolf and Classic faults to the This alteration likely affected the entire for- bands (arrows in Fig. 5E), however, indicating east (Figs. 2 and 6). In both cases, the upper red mation, resulting in a continuous sequence of that alteration at least in part postdates the for- unit, including the banded red and white unit, red-stained sandstone (Fig. 8A). We consider mation of these bands. is structurally offset by an amount consistent red kaolinite concretions within the middle alter- Joints can be associated with either localized with the offset of the overlying Willow Tank ation units (Fig. 5H) as vestiges of this initial red precipitation of Fe oxides or localized bleaching Formation. The apparent offset of the purple hematite stain, assuming that the tight kaolinite (Figs. 5B and 5F). Figure 5B depicts hematite and yellow bands underneath the upper red unit cementation of these concretions prevented the haloes around joints that have been subse- can be explained by their tendency to follow the dissolution of hematite from within the con- quently sheared, thus offsetting the boundaries upper red unit without or with little structural cretions whereas the surrounding permeable

Geological Society of America Bulletin, September/October 2004 1129 EICHHUBL et al.

A Bighorn fault B West East faults

B

concretions 50 cm C faults Figure 7. Defl ection of orange and purple bands along the Bighorn fault. (A) Sche- matic cross section. (B) Kaolinite concre- tions along the fault (detail of A). (C) Pur- C ple-red stain along fault where juxtaposed against the upper red sandstone (detail of A). Map case (30 × 40 cm) for scale.

5 m

banded red & white purple-red upper red yellow orange with purple rim white bleached orange concretions bleached fault rock bedding cover

A. Late Jurassic B. Late Cretaceous C. Tertiary

Figure 8. Stages of diagenetic altera- tion, fl uid fl ow, and deformation in the Aztec Sandstone at Valley of Fire. (A) Syndepositional reddening of eolian ? sandstone. (B) First stage of bleaching and iron remobilization in upper sec- tion of the Aztec Sandstone associated with Sevier thrusting. Field observations suggest local fl ow along strike of the orogenic front, but a regional direction of fl ow away from the orogen is inferred. (C) Second stage of bleaching and iron remobilization associated with mixing of basinal and meteoric fl uids during Basin and Range tectonics. meteoric fluid basinal fluid

1130 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE sandstone was preferentially bleached. Hematite white sandstone- dark red band- uniform grain coats also provided the precursor iron all hematite fine hematite red sandstone- minerals to reprecipitated hematite and goethite unstable unstable, coarse all hematite cements in the middle alteration units. hematite grows (meta-) stable The secondary origin of the middle alteration units relative to the homogeneous red of the lower and upper red units is deduced from the fine nature of the lobate-cuspate shape of the bound- relative ary between the red and middle alteration units. propagation Lobes along the lower red boundary (Fig. 4C) of reaction point into the red sandstone, whereas cusps front or "side" coarse point into the overlying yellow sandstone. This Fluid saturation suggests that the reaction boundary moved into

the red sandstone, presumably due to fi ngering Solubility of hematite of a reducing alteration front or “side.” The occurrence of slightly darker sand- stone along this diagenetic alteration bound- Distance ary (Fig. 4B) is consistent with coarsening of Figure 9. Schematic profi le of fl uid saturation relative to hematite as a function of distance hematite immediately ahead of the reducing across a coarsening reaction front or “side.” On the upstream side of the reaction front, the front. Coarsening, also referred to as Ostwald reducing fl uid is undersaturated relative to fi ne and coarse hematite, causing dissolution of ripening, results from the tendency of form- all hematite and bleaching of the sandstone. Within a transition zone, the fl uid is undersatu- ing energetically more favorable large crystals rated relative to fi ne hematite that gets dissolved but supersaturated for coarse crystalline during repeated dissolution and reprecipitation hematite. Dissolution of fi ne hematite brings Fe into solution that leads to the growth of (Lasaga, 1998). The dependence of hematite coarser crystalline hematite. Beyond the downstream side of the reaction front, the fl uid is in solubility on crystal size, with small crystals metastable equilibrium relative to both coarse and fi ne hematite and no alteration reaction being more soluble than larger crystals, was occurs. The thermodynamically favored replacement of fi ne hematite by coarse hematite is recently reviewed by Cornell and Schwertmann kinetically suppressed under these conditions. (2003). Thus, coarsening would result from local supersaturation of the fl uid for hematite of large crystal size even though the fl uid is under- saturated relative to the bulk of smaller, and pos- tion of the larger hematite grains and the forma- part of the fi rst stage of bleaching and remobili- sibly amorphous (Walker et al., 1978), hematite tion of the second white band below and behind zation (Fig. 8B). The banded red and white unit forming the grain coats. Immediately ahead of the orange band. We thus interpret the orange is mechanically offset by the strike-slip faults the advancing reduction front, some initially band below the upper red boundary to have that also offset the Willow Tank thrust. Bleach- larger hematite crystals grew at the expense of initially formed during the fi rst alteration stage ing at the top of the upper red unit is spatially smaller crystals that got dissolved (Fig. 9). Due as the byproduct of bleaching of the upper red associated with the Willow Tank thrust. This to the effect of hematite crystal size on alteration unit. The orange band was partially remobilized bleaching also postdates shear bands. We attri- color (Torrent and Schwertmann, 1987), this during a later second stage of alteration, with its bute the bleaching within the banded red and zone became darker red. As the reduction front lower boundary migrating preferentially along white sandstone to its proximity to the overlying advanced farther, the fl uid eventually became joints (Fig. 5A). gray and organic-rich of the Creta- undersaturated even for larger hematite crystals The boundaries of the lower and upper red ceous Willow Tank Formation, with the bleach- and all hematite went into solution. When the units to the middle alteration units clearly pre- ing being enhanced below and in the vicinity of reduction front stopped propagating, a darker date faulting but, as shown above, postdate com- the Willow Tank thrust sheet. rim of hematite remained. paction bands. The crosscutting of the banded Bleaching of the red sandstone by the dissolu- A similar process can be invoked for the orange and white alteration with strike-slip tion of hematite grain coats requires that the fl uid bands of white and orange following the lower faulting is not clear, although faults do not affect composition shifted toward more reducing and/ boundary of the upper red unit. The difference the intensity of the banding, which thus appears or acidic conditions (Fig. 10). Such a shift likely from the above scenario lies in the coarsening to predate strike-slip faulting. The alteration resulted from the infl ux of basinal fl uids that had of the few remaining larger hematite crystals of the banded orange and white unit postdates been in contact with organic-rich sediment or behind, rather than ahead of, the reduction front. deformation bands because these bands form the hydrocarbons. We thus infer the fi rst bleaching Coarsening is consistent with the larger size of boundaries to the alteration compartments that and remobilization stage to be associated with hematite crystals in orange sandstone compared are characteristic of this unit. No younger set the fl ow of reducing and/or more acidic basinal to red sandstone as observed in thin section. of deformation bands is observed that does not fl uids. High concentrations of organic acid, meth- The fi rst band of white sandstone separating interact with the banded orange and white altera- ane, or hydrogen sulfi de that are common in such uniform red and banded orange sandstone is tion. The hematite resulting in the orange color fl uids would have provided favorable conditions thus interpreted as the result of a kinetic lag of the banded orange and white unit is similar to for hematite dissolution (Chan et al., 2000). between dissolution and reprecipitation. With the hematite in the orange unit and thus is con- continued advancement of the reduction front, sidered a secondary alteration product. Second Bleaching and Remobilization Stage undersaturation was reached even for those We also consider the alteration reactions The second bleaching and remobilization larger grains, resulting in the secondary dissolu- observed in the banded red and white unit to be stage resulted in remobilization of iron oxides

Geological Society of America Bulletin, September/October 2004 1131 EICHHUBL et al.

20 A reddened face B bleached fac 15 O 2 10 H e 2 0 deformation ε 5 bands p

0 d face H 20 d face –5 H 2 reddene bleache –10 0 2 4 6 8 10 12 14 pH C. Location A inferred flow direction D. Location B inferred flow direction (trend/plunge) (trend/plunge) Hematite Goethite NN 050 ± 10°/42 ± 2° 017 ± 12°/20 ± 19° Wüstite Amorphous Fe(OH)3 bedding Jarosite Pyrite

Figure 10. Diagram of pε vs. pH depicting the stability of iron oxides, iron hydroxides, 2− sulfi des, and sulfates in the presence of SO4 and K+. Both hematite and goethite are shown as potentially stable phases. Jarosite and amorphous Fe(OH)3 are shown for the absence of hematite, goethite, and , e.g., due to kinetic reasons. Recalculated after bedding Doner and Lynn (1989) using Act2, Release 4.0.2 (Bethke, 2002), and the “thermo.dat” database. Solubility products for hematite and goethite after Trolard and Tardy (1987). Diagram is calculated for 80 °C, 16 MPa, reddened face equal angle, lower hemisphere projection 2+ −4 2− and activities of H2O = 1, Fe = 10 , SO4 = 5 × 10−3, and K+ = 5 × 10−4. bleached face great circle of deformation band

Figure 11. (A) Compartments of sandstone formed by two sets of intersecting deformation bands resulting in systematic reddening and bleaching of sandstone adjacent to deformation and iron hydroxides within and along the bands and “ponding” of red stain next to the deformation band intersection. Coin (circled) boundaries of the fi rst alteration stage (Fig. 8C). for scale. (B) Interpretation of A. Arrow indicates inferred fl ow direction of fl uid. (C and The products of this alteration stage include the D) Stereographic projections of deformation bands with associated alteration haloes for yellow and purple alteration colors as well as the two locations (A and B, Fig. 2) within the banded orange and white unit. Shading indicates local remobilization of the orange layer below deformation band compartments that are bordered solely by bleached (blue) or reddened the upper red unit. All these alteration products (red) faces. Compartments bordered by a combination of bleached and reddened faces are overprint or crosscut the alteration products unfi lled. Bedding refl ects local orientation of cross-bedded strata. See text for interpretation attributed to the fi rst bleaching and remobiliza- of inferred fl ow directions. tion stage. Based on the crosscutting relations with faults, this second alteration stage occurred during strike-slip faulting. The subhorizontal upper boundary of the yellow lobe in the center pH of about 4 and high pH of about 12, whereas rence of goethite under subsurface conditions is of the middle alteration units also suggests that a neutral pH of about 8 favors the formation of the laboratory observation that hematite precipi- this alteration occurred after tilting of the Aztec hematite (Schwertmann, 1985). Under partially tation requires the precipitation of ferrihydrite as Sandstone to the northeast. saturated conditions, hematite is favored rela- a precursor phase, a poorly ordered iron hydrox-

This second stage of bleaching and remobi- tive to goethite with decreasing water activity or ide of composition Fe1.55O1.66(OH)1.33 (Cornell lization is characterized by the common occur- humidity (Torrent et al., 1982; Schwertmann and and Schwertmann, 2003), whereas goethite can rence of goethite and minor alunite and jarosite. Taylor, 1989). In soils, low temperatures, high precipitate directly from solution (Schwertmann

At temperatures below 100 °C and unit water Al and H2O activity, and higher organic content and Taylor, 1989). Goethite will thus be favored activity, coarse-grained goethite is considered favor goethite precipitation (Schwertmann and over hematite due to sluggish reaction kinet- the thermodynamically stable phase (Cornell Taylor, 1989). Small particle size favors the ics even under conditions where equilibrium and Schwertmann, 2003). The precipitation of stability of hematite (Schwertmann and Taylor, considerations would predict hematite stability. goethite rather than hematite is favored at low 1989). Perhaps most signifi cant for the occur- While the complexity of the hematite-goethite

1132 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE system makes it diffi cult to ascribe the replace- Inferred Paleofl ow Direction characterized by consistently inward-facing red ment of hematite by goethite within the Aztec faces (Fig. 11B). If the compartment is oriented Sandstone to a specifi c environmental factor, it Criteria that constrain the direction of paleo- in an oblique direction relative to the fl ow vec- can be concluded that this replacement would be fl uid fl ow in the Aztec Sandstone include the tor, the faces defi ning the compartment would favored by a decrease in temperature, a decrease asymmetry of alteration adjacent to deforma- consist of both red and bleached faces. in pH, and a decrease in water saturation under tion bands in the banded orange and white unit At both locations that were investigated, mul- undersaturated conditions. (Figs. 4F, 11A, 11B). The orientation of pre- tiple compartments consistently had reddened or

The sulfates alunite, KAl3(SO4)2(OH)6, and ferred reddening or bleaching adjacent to defor- bleached faces (Figs. 11C and 11D). These are jarosite, KFe3(SO4)2(OH)6, frequently form as mation bands in the banded orange and white marked in Figure 11C and D by light red or light the result of pyrite oxidation in organic-rich unit appears locally uniform. This consistency in blue shading, respectively. A unique fl ow orien- that are in contact with acidic oxidizing the orientation of asymmetric alteration around tation is obtained at both locations, however, by water (Langmuir, 1997). The Aztec Sandstone, deformation bands suggests the local orienta- additionally requiring that these compartments deposited subaerially under oxygenated condi- tion of reactive fl uid fl ow. We hypothesize that are enclosed in a larger compartment with the tions, clearly did not contain primary or early the asymmetric reddening and bleaching, and same consistent reddened or bleached faces. For diagenetic sulfi de. Any sulfur that is now con- in particular the “ponding” of red alteration at each of the two locations, we fi nd only one com- tained in alunite and jarosite must have been intersecting deformation bands (Figs. 11A and partment that meets this criterion, marked in brought into the system by migrating fl uids. 11B), resulted from solute sieving across defor- Figures 11C and 11D, as containing the inferred Jarosite is stable under acidic and strongly oxi- mation bands that acted as geologic membranes fl ow direction. At both locations, the fl ow direc- dizing conditions (Fig. 10). In the sulfate system for ions in solution. Such membrane behavior, tion obtained using this graphical technique containing Al, K, and Fe, alunite is stable under suggested by Whitworth et al. (1999) for faults trends NE-SW and plunges to the NE. Whereas somewhat lower-pε (more reducing) conditions in sandstone, would increase the activity of dis- at location A this direction corresponds to a compared to jarosite. solved ions on the upstream side relative to the downward fl ow of reducing fl uid, the opposite A possible scenario that combines reducing downstream side. For the reaction fl ow direction is inferred for location B. Appar- conditions resulting in bleaching with suitable ently, this technique yields consistent results on + − ↔ 2+ conditions for sulfate precipitation involves Fe2O3 + 6H + 2e 2Fe + 3H2O an outcrop scale, but inferred fl ow directions are the upward migration of reducing sulfi de-rich variable at the kilometer scale. basinal fl uids and mixing with oxidizing mete- the activity for Fe2+ ions is expected to be higher Flow direction indicators for other alteration oric waters. As shown in the pε-pH diagram on the upstream side of deformation bands, units include oxidation trails around oxide con- (Fig. 10) the reaction of hematite with sulfi de- resulting in preferred hematite growth, or a cretions and lobate alteration boundaries. Tails rich low- to intermediate-pε fl uids would lead to lesser extent of hematite dissolution, compared around iron oxide concretions (Fig. 5G) indicate the dissolution of hematite without necessarily to the downstream side of deformation bands downward and SE-directed migration of a pur- precipitating pyrite, provided pH conditions where hematite precipitation would be impeded ple alteration front south of Point 655 (Fig. 1). are suffi ciently low and the system is not too or dissolution enhanced. The lower boundary of the orange layer that strongly reducing. Under those conditions iron To test if the direction of preferred reddening underlies the upper red unit forms lobes that had goes in solution as Fe2+. The abundance of hema- and thus the inferred direction of reactive fl uid migrated downsection (Fig. 5A). In both cases, tite may have buffered the system suffi ciently fl ow is uniform over the mapped area, we mea- the direction of the tails or lobes is interpreted to to prevent formation of pyrite, which is not sured the orientation of deformation bands and refl ect the direction of advective mass transfer observed as mineral phase or pseudomorph. We the associated orientation of preferred reddening and thus the fl uid fl ow direction. suggest that oxidizing conditions necessary for or bleaching at two locations within the banded the precipitation of jarosite and alunite were met orange and white unit. Data were collected over Fluid Flow, Alteration, and Correlation during subsequent mixing of the reduced sulfi de- an outcrop area of approximately 500 m2 at loca- with Regional Tectonics rich basinal fl uid with oxidizing meteoric water. tion A and 200 m2 at location B (Fig. 2). Data The associated increase in pε would have moved collection was aimed at obtaining a uniform spa- The correlation of deformation bands with the system into the jarosite fi eld, and, under high tial distribution of deformation band orientations Sevier thrusting suggested by Hill (1989) and Al activities, alunite stability fi eld, whereas an by including less common bands with orienta- Flodin and Aydin (2004) provides a lower increase in both pε and pH would have resulted tions that deviate from the three dominant sets time constraint for the onset of basinal fl uid in the precipitation of amorphous iron hydroxide described by Hill (1989). Nonetheless, the data expulsion during the fi rst stage of bleaching that may then have been converted to goethite or sets fall into two distinct sets that strike NNW- and remobilization (Fig. 8B). In addition, the hematite. Under kinetically favorable conditions, SSE and ENE-WSW, respectively, when plotted spatial correlation of bleaching in the upper goethite would have precipitated directly from as great circles on an equal-angle stereographic red unit with the Willow Tank thrust requires solution without the amorphous iron hydroxide projection (Figs. 11C and 11D). that the thrust sheet be in place by the time precursor, thus competing with the precipita- For each great circle in Figures 11C and of alteration. An upper time constraint for the tion of jarosite. The pε-pH conditions may have 11D, the side corresponding to the bleached fi rst stage of bleaching and remobilization is varied locally, resulting in varying amounts of face of the deformation band is marked blue provided by strike-slip faulting and tilting of the sulfates being precipitated. and the side corresponding to the red face is sequence during Miocene times. Basinal fl uids Based on the crosscutting relations of sulfate- marked red. Assuming that fl uid fl ow, and thus were likely to be expelled due to the increase containing yellow and purple alteration bands with the diagenetic alteration around bands, are not in burial associated with the emplacement of faulting, we thus ascribe the second bleaching and infl uenced by neighboring bands, we hypoth- the thrust sheets and concurrent to subsequent remobilization stage to meteoric water entering esize that deformation band compartments that deposition of detrital units in the foreland. We the aquifer and mixing with basinal fl uid. contain the approaching fl ow vector should be estimate that the maximum burial depth of the

Geological Society of America Bulletin, September/October 2004 1133 EICHHUBL et al.

Aztec Sandstone top corresponds closely to the direction of fl ow was locally variable. On a that the migration of basinal fl uid continued thickness of the post-Jurassic units to the east, regional, i.e., basinwide, scale fl uid fl ow would during the second bleaching and remobiliza- amounting to about 1600 m. The occurrence of be expected to be directed away from the axis of tion stage. quartz overgrowth cement and diagenetic illite in greatest rate of increase in overburden thickness We interpret the lower and upper yellow and the lowermost part of the Aztec Sandstone, typi- and topographic elevation and thus from the oro- purple bands within the middle alteration units cally requiring burial temperatures in excess of genic front toward the foreland (Fig. 8B) (Oliver, to be secondary modifi cations of the boundaries about 80 °C (Worden and Morad, 2000, 2003), 1986; Sverjensky and Garven, 1992). separating the lower and upper red units from appears consistent with this estimate. The geo- The syn- to post-strike-slip faulting age of the the middle alteration units. The upper yellow thermal gradient could have been elevated along second bleaching and remobilization stage and and purple bands locally replace the white and the thrust front due to fl uid fl ow, consistent with the subhorizontal orientation of the top bound- orange bands that are interpreted to have formed the occurrence of quartz cement and dickite in ary of the middle yellow unit place the infl ux during the fi rst bleaching and remobilization close proximity and below the Willow Tank of meteoric fl uids in Miocene to post-Miocene stage. Compared to hematite in the homoge- thrust sheet. With the exception of the Willow times (Fig. 8C). A younger cutoff is provided neous lower and upper red units, hematite in the Tank thrust, the post-Jurassic sequence on top by the formation of modern topography that orange bands may have been more conducive to of the Aztec Sandstone clearly indicates that the truncates the yellow and purple alteration units secondary dissolution and reprecipitation during thrust sheets did not extend to the eastern part of of the second alteration stage. The infl ux of the second bleaching and remobilization stage, the study area. The exposed leading edge of the meteoric fl uids is likely the result of an increase resulting in remobilization of the boundaries of Willow Tank thrust sheet and a right-lateral tear in topographic head with the exhumation of the orange unit and, at a more advanced stage, fault along its southern edge (Longwell, 1949) the Muddy Mountains and the incision of the alteration to purple and yellow goethite-contain- makes it likely that most of the area shown in Colorado River, the latter inferred to initiate at ing sandstone. Figure 2 was not covered by the Willow Tank about 5 Ma (Lucchitta, 1987). A topographic thrust. Continuous Cretaceous on drive for meteoric water fl ow is consistent with Effect of Structural Heterogeneities on top of the Aztec Sandstone south of the Willow fl ow direction indicators described above that Fluid Flow Tank thrust excludes that the Aztec Sandstone suggest that the direction of fl uid movement shown in Figure 2 was covered by the next was downdip and to the southeast. While the regional distribution of the lower higher thrust sheet, the Muddy Mountain The distribution of yellow and purple units red, middle, and upper red alteration units is thrust sheet. Connecting the closest erosional likely refl ects the dependence of hematite controlled by stratigraphic contacts, structural remnants of the Muddy Mountain thrust sheet, dissolution and goethite precipitation on the features have a strong effect on the focusing as defi ned by Bohannon (1983), to the west hematite distribution inherited from the fi rst of fl ow and alteration on scales ranging from and southeast of the study area places the front alteration stage as well as the focusing of mete- <1 km to the outcrop and hand sample scale. of the Muddy Mountain thrust approximately oric water and basinal fl uid within the sandstone The effect of map-scale faults on fl uid fl ow and along the western boundary of the detailed map formation. The crystallinity and abundance alteration is best illustrated along the Bighorn area (Fig. 2). With the study area located ahead of hematite formed during the fi rst bleaching fault and associated faults, defl ecting the purple of the thrust sheets, expulsion of basinal fl uids and remobilization stage would control the and yellow alteration units (Fig. 6). We suggest could thus have started with the advancement of saturation state and local Fe activity in solu- that this defl ection resulted from impeded fl ow the orogenic front and reached a maximum with tion and thus the remobilization of iron during and thus limited transport of solutes across maximum burial prior to regional tilting and the second bleaching and remobilization stage. these faults. Fault hydraulic properties were deposition of the Miocene Muddy Creek For- Focusing of fl uid fl ow would affect the mixing determined by Flodin et al. (2004), who mea- mation. Following Bohannon (1983) the upward ratio of meteoric water and basinal fl uid and sured a one to three orders of magnitude reduc- coarsening of the Cretaceous sequence recorded thus the local fl uid composition, including pε tion in fault permeability for fl ow across these the eastward progression of the Sevier thrust and pH, the water/rock ratio, and the velocity faults compared to the host sandstone. Although sheets, with the coarsest deposits of the Upper and residence time of the migrating fl uid within faults apparently acted as aquitards for cross- Cretaceous Overton Conglomerate Member of the formation. fault fl ow, the locally enhanced precipitation of the Baseline Sandstone corresponding to the The lobate shape of the central yellow unit iron oxides and hydroxides along these faults fi nal stages of thrusting. tapering off to the northwest could refl ect the within otherwise bleached sandstone that is The two paleofl ow direction analyses for this interfi ngering of the two fl uids present during juxtaposed against red sandstone suggests that fi rst fl uid fl ow stage as determined in the banded the second bleaching and remobilization stage, iron in solution may have “bled” through these orange and white unit indicate NE-SW–trending i.e., reducing basinal fl uid moving updip, and faults. Along the Bighorn fault, this localized fl ow vectors but opposite polarity. Restoring the meteoric water moving downdip. The position alteration is observed in the central yellow and Aztec Sandstone into its untilted pre-Miocene of this lobe probably follows the most permeable purple alteration units as well as in the upper position results in subhorizontal formation con- part of the middle alteration units. The shape orange unit (Figs. 7A and 7C), in both cases on tacts and subhorizontal boundaries of the lower of the lobe could mimic the plume of denser the SE side of the Bighorn fault. We consider red to middle alteration units and of the middle basinal fl uid interfi ngering with lighter meteoric this preferred bleeding of iron onto the SE alteration units to upper red alteration unit. This water. Thus, whereas the overall confi guration side of the fault as additional evidence for the restoration rotates the fl ow vectors into subhori- of the alteration units is, to fi rst approximation, SE-directed fl ow of meteoric fl uid during the zontal positions that are approximately normal to controlled by the tilted formation boundaries, second alteration stage. the southeasterly thrusting direction of the Wil- we interpret the subhorizontal upper boundary The partial sealing character of these faults low Tank thrust and approximately parallel to the of the central yellow lobe to follow a hydraulic has been attributed by Flodin et al. (2004) to inferred trend of the orogenic front. The opposite equipotential surface. This interpretation of the the signifi cant grain size reduction associated polarity of the fl ow vectors suggests that the central lobe as a plume of basinal fl uid requires with fault slip. Diagenetic sealing effects were

1134 Geological Society of America Bulletin, September/October 2004 PALEO-FLUID FLOW AND DEFORMATION IN THE AZTEC SANDSTONE, VALLEY OF FIRE considered to be minimal, although localized not exclude fl uid fl ow across the lower red unit. age, acted as aquitards for cross-fault fl ow due to kaolinite precipitation along these faults is Alteration features are, in fact, observed in the extensive grain size reduction in the fault core, clearly observed (Fig. 7B). Permeability paral- lower red unit as described above. Nevertheless, and as fl uid pathways for along-fault fl ow due to lel to these faults within the fault damage zone fl ow velocity and thus fl uid volume may have well-developed damage zones. was found by Jourde et al. (2002) to be higher been restricted suffi ciently across the lower red The complex interaction of alteration bands by up to one order of magnitude. Enhanced fl ow unit to prevent signifi cant hematite dissolution with structural heterogeneities and thrusting along these faults would have contributed to the and bleaching of the formation. illustrates the coupling among fl uid fl ow, dia- defl ection and parallel alignment of alteration genesis, regional-scale tectonism, and brittle zones along these faults. CONCLUSIONS deformation in subsurface fl uid fl ow systems. Flodin and Aydin (2004) and Myers and Aydin (2004) described in detail the process of Red, orange, yellow, and purple alteration ACKNOWLEDGMENTS fault formation by the opening of joints, subse- bands in the Aztec Sandstone are associated This study was fi nancially supported by U.S. Depart- quent shearing of these joints and formation of with distinct forms of iron oxide and hydroxide. ment of Energy grant DE-FG 03-94ER14462. We thank splay fractures, and linkage of sheared joints. Red and orange colors result from hematite of the rangers of the Valley of Fire State Park for logistic Slip along these joint-based faults resulted in different crystal size, yellow and purple colors support and Eric Flodin, Kurt Sternlof, Jim Boles, and cataclasis and thus in a reduction in cross-fault from goethite. A uniform red stain resulting Rod Myers for discussions. Reviews by Martha Gerdes, Wanda Taylor, Peter Vrolijk, Peter Mozley, and Nancy permeability. The reactivation of single joints in from thin grain coats of hematite is attributed Riggs are thankfully acknowledged. shear, on the other hand, tends to increase the to syndepositional dissolution of Fe-bearing permeability of these joints due to the dilatant detrital mineral phases and hematite precipita- REFERENCES CITED increase in joint aperture and the linkage with tion. Based on mutual crosscutting relations other joints, resulting in increased fl ow pathway of orange, yellow, purple, and white alteration Antonellini, M.A., and Aydin, A., 1994, Effects of faulting on fl uid fl ow in porous sandstones: Petrophysical prop- connectivity. Sheared joints are thus frequently bands and their interaction with faults, joints, erties: American Association of Petroleum Geologists found to be associated with haloes of Fe oxides and deformation bands, we distinguish two Bulletin, v. 78, p. 355–377. (Fig. 5B), whereas adjacent unsheared and iso- stages of dissolution or bleaching, remobiliza- Bethke, C.M., 2002, The Geochemist’s Workbench. For Microsoft Windows. Release 4.0.2: Golden, Colorado, lated joints are barren (Taylor et al., 1999). The tion, and reprecipitation of iron oxides and Distributed by RockWare Science Software, 1 signifi cance of these processes for the perme- hydroxides. The fi rst bleaching and remobiliza- CD-ROM. 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Geological Society of America Bulletin, September/October 2004 1135 EICHHUBL et al.

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1136 Geological Society of America Bulletin, September/October 2004