GYPSUM VEINS IN THE TRIASSIC MOENKOPI FORMATION,
SOUTHERN UTAH: DIAGENETIC AND TECTONIC
IMPLICATIONS AND ANALOG
RELEVANCE TO MARS
by
Brennan William Young
A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of
Master of Science
in
Geology
Department of Geology and Geophysics
The University of Utah
August 2016 Copyright © Brennan William Young 2016
All Rights Reserved The University of Utah Graduate School
STATEMENT OF THESIS APPROVAL
The thesis of ______Brennan William Young has been approved by the following supervisory committee members:
Marjorie A. Chan______, Chair 4/25/2016 Date Approved
Brenda Bowen______, Member 4/25/2016 Date Approved
Thure E. Cerling______, Member 4/25/2016 Date Approved
and by ______John M. Bartley______, Chair/Dean of the Department/College/School o f ______Geology and Geophysics
and by David B. Kieda, Dean of The Graduate School. ABSTRACT
Gypsum vein geometry and chemistry are evaluated in the Triassic Moenkopi
Formation in order to determine the source of mineral fill and conditions and timing of vein emplacement. Moenkopi veins are similar to veins at Endeavour and Gale craters on
Mars. Mapping, geochemical, geometric, and basin analysis techniques are employed to better understand the Moenkopi vein network, assess them as an analog to Mars veins, and update a geologic map of the study area at the 1:24,000 scale.
A three-part classification scheme organizes observations by vein geometry, vein distribution, and vein mineralogical characteristics to hypothesize vein network generation and evolution processes. The Moenkopi vein network is geometrically complex, stratigraphically distributed, and exhibits multiple varieties of gypsum.
Moenkopi veins have similar Sr and S isotope ratios as primary Moenkopi gypsum beds and are interpreted to be sourced from with the unit. Moenkopi veins are dominantly horizontally oriented and cross-cut other diagenetic features and are interpreted to have been emplaced via hydraulic fracturing at <1 km depth during Colorado Plateau uplift <8
Ma.
Mars veins share many geometric, textural, and mineralogical features with
Moenkopi veins. Endeavour Crater veins appear to be lithologically distributed and are oriented to the crater rim and are interpreted to be sourced from early diagenetic sulfate and to have been emplaced during topographic collapse of the crater rim. Gale Crater veins appear to be stratigraphically distributed and to be subtly dominantly horizontally
oriented and are interpreted to be sourced from an as-yet unobserved stratigraphic unit
and to have been emplaced during exhumation. Mars and Moenkopi veins were emplaced
via hydraulic fracturing at low confining pressures and low temperatures and have been
minimally altered since emplacement.
An updated map of the Torrey 7.5’ quadrangle, Wayne County, Utah includes
mapping and descriptions of extensive Triassic through mid-Jurassic sedimentary,
Noegene igneous, and covering Quaternary units. The Torrey map provides context for understanding Moenkopi veins and provides a more nuanced picture of geomorphic
evolution in the Pleistocene, including alluvial surfaces that were were emplaced ~100-
550 ka and represent episodes of mass wasting and fluvial activity.
iv CONTENTS
ABSTRACT...... iii
LIST OF TABLES...... vii
ACKNOWLEDGMENTS ...... ix
TRIASSIC MOENKOPI FORMATION GYPSUM VEINS: DIAGENESIS AND STRUCTURE, SOUTHERN UTAH...... 1
Abstract...... 1 Introduction...... 2 Methods...... 10 Results...... 12 Discussion...... 31 Conclusions...... 44 Acknowledgments...... 45 References...... 46
CALCIUM SULFATE VEINS IN TRIASSIC MOENKOPI MUDROCKS OF SOUTHERN UTAH: ANALOGS TO SULFATE VEINS ON MARS...... 51
Abstract ...... 51 Introduction ...... 52 Methods...... 61 Results...... 64 Discussion...... 73 Conclusions...... 85 Acknowledgments...... 86 References...... 86 GEOLOGIC MAP OF THE TORREY 7.5’ QUADRANGLE, WAYNE COUNTY, UTAH...... 93
Abstract...... 93 Introduction ...... 93 Description of Map Units...... 98 Structure...... 122 Diagenesis...... 124 Geologic History...... 127 Hazards...... 131 Conclusions...... 134 Acknowledgments...... 134 References ...... 134
Appendices
A: KEY LOCALITIES...... 139
B: GYPSUM TEXTURES...... 146
C: FRACTURE TYPES...... 148
D: SAMPLES...... 150
E: 2D VEIN MEASUREMENTS...... 154
F: GEOCHEMICAL DATA...... 159
G: QEMSCAN RESULTS...... 166
H: REDUCTION COLORATION OF THE MOENKOPI FORMATION...... 168
I: IGNEOUS DIKE AR-AR AGE DATA...... 171
vi LIST OF TABLES
1. Moenkopi lithofacies descriptions...... 13
2. New vein class terminology, based on the presence of a plane of mineral fabric interruption (including clay and wall rock inclusions) that is parallel or sub-parallel to the vein wall, and by mineralogical changes along the length or width of the vein...... 18
3. Vein intersection classes...... 23
4. Vein network classification...... 32
5. Moenkopi-Mars vein comparison...... 84
6. Inverted topography age order for boulder terraces in the Torrey quadrangle...... 103
7. Details of key localities, with abbreviations, relevant figures, and directional view azimuth of figure images...... 139
8. Fracture terminology relevant to this study...... 148
9. SG samples with summary descriptions and corresponding analyses...... 151
10. TO and SB samples with summary descriptions and corresponding analyses...... 152
11. Whole-rock geochemistry for gypsum samples collected in the SG and TO areas, presented in percent weight...... 161
12. Whole-rock geochemistry for host rock samples collected in the SG and TO areas, presented in percent weight...... 162
13. Trace-element geochemistry for gypsum samples in the SG and TO areas, presented in percent weight...... 163
14. Trace-element geochemistry for host rock samples in the SG and TO areas, presented in percent weight...... 164
15. Stable-isotope geochemistry for samples collected in the SG and TO areas...... 165 16. QEMScan results presented in percent normalized area covered by top-pick species...... 167
viii ACKNOWLEDGMENTS
An enormous thank you to my committee members, Drs. Marjorie Chan (chair),
Brenda Bowen, and Thure Cerling, for providing invaluable advising and mentoring that
influenced every aspect of the research presented in this thesis.
My fellow graduate students, Casey Duncan, Peter Steele, and David Wheatley
provided invaluable discussions that helped me refine ideas, figures, and text. I
acknowledge Tyler Huth as co-mapper for the Torrey quadrangle, Mike Jury as our field
assistant in the mapping effort, and Dr. David Marchetti for guidance with mapping
Quaternary units and supplying some cosmogenic age dates.
I acknowledge the invaluable support of Alex Fackrell, Michelle Williams,
friends, my wife, Lucy, and family for field assistance, arguments over terminology, and the encouraging push to finish.
Funding: Geological Society of America (GSA), United States Geological Survey
(USGS EDMAP initiative), and the University of Utah Department of Geology and
Geophysics. TRIASSIC MONEKOPI FORMATION GYPSUM VEINS:
DIAGENESIS AND STRUCTURE,
SOUTHERN UTAH
Citations and references are in the style of the Journal of Sedimentary Research
Abstract
Gypsum vein geometry and chemistry are evaluated in the Triassic Moenkopi
Formation in order to determine the source of mineral fill and conditions and timing of vein emplacement. Vein fill consists of satin spar and selenite varieties of gypsum from rehydration and mobilization of evaporite minerals derived from intraformational primary calcium sulfate beds. A three-part classification scheme organizes observations by vein geometry, vein distribution, and vein mineralogical characteristics to hypothesize vein network generation and evolution processes.
The Moenkopi vein network is complex and was formed via hydraulic fracturing at low confining pressures. Veins cross-cut bedding-parallel gray-green banding and reduction spots, which formed between two likely stages of burial ~170-160 Ma and ~95-
80 Ma. Vein distributions and basin modeling suggest vein emplacement at low temperatures at <1 km depth, coincident with Colorado Plateau uplift <8 Ma. Veins have been subject to minimal alteration during uplift to current exposure. 2
Introduction
Gypsum veins cross-cut the lowermost and uppermost mudstones, siltstones, and
sandstones of the tidal, marginal marine Lower Triassic Moenkopi Formation. Moenkopi veins have been referred to as “gypsum stringers” (McLelland et al. 2007; Sorber et al.
2007), inferring vein composition as gypsum (CaSO4^2H2O) though they have not been
previously tested for the presence of anhydrite (CaSo4) or bassanite (CaSO4^.5H2O).
Although recognized as diagenetic features, Moenkopi veins have remained enigmatic
due to a lack of detailed mineralogic and geometric analyses to constrain vein source and timing.
The purpose of this study is to characterize the context, chemistry, and geometry
of the Moenkopi vein network in order to better understand the diagenetic, burial, and uplift history of the Moenkopi Formation.
Geologic Context
The Moenkopi Formation is well exposed in the Colorado Plateau. The unit
thickens from 0 thickness at the Utah-Colorado border to more than 300 m near Torrey,
Utah in the western Capitol Reef area, and is thickest near St. George, Utah at ~900 m
(Fig 1; Blakey 1973, 1974; Sorber 2007; Biek et al. 2009, 2010; Doelling et al. 2010;
Morris et al. 2010). Stratigraphic equivalents to the Moenkopi Formation are the
Woodside Shale, Thaynes Formation, and Mahogany Member of the Ankareh Formation
in northern Utah (Blakey 1974; Sprinkel 1994). The Moenkopi Formation rests
unconformably between the Permian Kaibab Formation and the Upper Triassic Chinle
Formation.
The Moenkopi Formation is a marginal marine unit of Early Triassic (Olenekian- 3
Figure 1. Regional stratigraphy across southern Utah. The Moenkopi Formation approaches 0 thickness at the Colorado-Utah border to the east and is thickest near St. George in the west. 4
Anisian) age deposited at the edge of the Early Triassic Sea (Fig. 2; Blakey 1973, 1974;
Morales 1987). It is characterized by predominantly muddy and silty sediment and fine sandstone beds. The terrigenous material was derived from Precambrian gneiss, schist, and granite from the Uncompahgre element of the Ancestral Rocky Mountains (Blakey
1973). Gypsum beds 0.1 to 1 m thick, as well as gypsum veins, are present in the
Moenkopi Formation’s lower and upper mudstones (Van Deventer 1974; Morris et al.
1897; Biek et al. 2009). Locally gray-green bedding-parallel banding and reduction spots are present (Blakey 1974; Van Deventer 1974; Morales 1987; Sorber 2007; Biek et al.
2009; Morris et al. 2010).
Figure 2. Paleogeographic map of western North America during the Early Triassic, ~245 Ma (map credit Ron Blakey ©Colorado Plateau Geosystems; after Blakey and Ranney 2008). Arrows show general transport of sediment to the Moenkopi system. Black lines = marine source, gray lines = fluvial deltaic source. SG = St. George, TO = Torrey, SB = location of Sorrel Buttes 33-1 core. 5
The majority of the Moenkopi Formation represents various tidal shoreface and marine environments deposited at the margin of an Early Triassic sea, evidenced by the predominance of silica mud, abundant wave ripples and symmetric ripples, bidirectional cross stratification, presence of muddy and nodular beds of gypsum, and interfingering limestone units (Blakey 1973, 1974; Morales 1987; Biek et al. 2009, 2010; Doelling et al.
2010; Morris et al. 2010).
East-west facies changes within the Moenkopi Formation results in two member nomenclatures in southern Utah (Fig. 1). Where possible, south-central Utah nomenclature is used here which includes, in ascending order: the Black Dragon (Trmb),
Sinbad Limestone (Trms), Torrey (Trmt), and Moody Canyon (Trmm) members. The
Black Dragon and Moody Canyon members are both tidal flat- to sabkha-deposited gypsiferous mudstones and sandstones. The upper redbeds (Moody Canyon) are best exposed in the study areas and are the focus of this study. The Sinbad Limestone Member is algal boundstone and fossiliferous grainstone and packstone and is not gypsiferous.
The Torrey Member is lithologically similar to the Moody Canyon Member but lacks gypsum.
Overlying the Moenkopi Formation is the Chinle Formation, whose lowest member, the fluvial Shinarump Conglomerate Member, commonly forms the caprock for upper Moenkopi cliffs. The Shinarump Conglomerate is regionally discontinuous but is present at all field sites. More than 600 m of terrestrial Mesozoic strata (the Chinle
Formation, Glen Canyon Group, and Temple Cap Formation) overlies the Moenkopi
Formation before the next gypsiferous unit, the terrestrial-shallow marine Middle Jurassic
Carmel Formation, which contains <10 m thick gypsum beds (Morris et al. 2010; 6
Doelling and Sprinkel 2016, personal communication).
Study Areas
Two main areas and one core in southern Utah are evaluated in this study (Fig. 3), which are addressed and generally presented from west to the east.
St. George (SG)
A roadcut along Banded Hills Drive was chosen to represent the St. George (SG), southwestern Utah field area for >1 km-long cliffs of upper red member (Moody Canyon equivalent) mudstone, siltstone, and sandstone in ~100 m of relatively unbroken stratigraphy. The upper red member was the only unit studied at SG, where it is underlain by the slope-forming Shnabkaib Member (Fig. 1; Biek et al. 2009; Doelling et al. 2010).
Two key localities are designated at SG. The St. George-North (SG-N) locality uses the majority of the roadcut to measure stratigraphic section and make detailed observations. The St. George-South (SG-S) locality exposes a dense network of gypsum veins.
Torrey (TO)
The Torrey (TO), south-central Utah field area consists of various natural outcrops of the Moenkopi Formation near the towns of Torrey and Teasdale, Utah in the western Capitol Reef area. TO was selected for study because of its exposure of all of the
Moenkopi Formation except the lowest portion of the Black Dragon Member. Other
Mesozoic strata, including the gypsiferous Carmel Formation, are also well exposed in
TO. Gypsum also occurs in some Holocene soils.
Four key localities collectively capture the Black Dragon and Moody Canyon TO-VR Gilbert Bench' mmtt ' w: -
Trmsh SG-N
Torrey SB133-1 Trms « _ . ^aa&SniSfids CapitollRe er ig7 f
/ Bryce Canyon MSt onGeorge s ------■------/ I Moenkopi or equivalent Trmm Meters State and National Parks Kitometers
Figure 3. Maps of the St. George (SG, left) and Torrey (TO, right) field areas with locations of key localities highlighted in yellow. Stratigraphic columns are condensed versions of those in Fig. 5 and show the stratigraphic positions of key localities. 8
members of the Moenkopi Formation in TO. The Torrey-Velvet Ridge (TO-VR) outcrop
is a >100 m tall gypsiferous cliff of Moody Canyon mudstone and sandstone capped by
the Shinarump Conglomerate. The Torrey-West (TO-W) locality is a 10-12 m tall cliff of
Moody Canyon mudstone and siltstone west of Teasdale. The Torrey-South (TO-S)
locality is a ~6 m cliff of Moody Canyon mudstone and sandstone located southeast of
Torrey and Teasdale. The Torrey-North (TO-N) locality is a slope-forming exposure of
Black Dragon mudstone located south of Torrey and east of Teasdale. Vein expressions
at TO-N are irregular, jagged, and hidden in the slope of the outcrop, so no reliable
geometric data were documented.
Sorrel Buttes Core (SB)
The Sorrel Buttes 33-1 core (SB) was collected by the Nova Natural Resource
Corporation. SB exhibits 49 m of gray-green Moenkopi mudstone and siltstone located
between Capitol Reef and Canyonlands national parks, ~77 km east of Torrey (Fig. 3).
The SB core provides a comparison of the Moenkopi Formation at depth in southern
Utah, although the exact stratigraphic position of the core was previously unknown and is
correlated as part of this study.
Hypotheses
This project focuses on four hypotheses to explain the occurrences and origins of veins in the Moenkopi Formation:
1. Vein minerals are gypsum with no significant inclusions of other minerals.
2. Vein minerals are sourced from primary calcium sulfate beds within the
Moenkopi Formation. 9
3. Fracture generation was assisted by natural hydraulic fracturing.
4. Fracturing and vein fill occurred during last-stage diagenesis coincident with
Colorado Plateau uplift.
Terminology
Vein, fracture, and structure terminology largely follows published work
(Sorkhabi 2014a, 2014b). Mineral terms to describe calcium sulfate and its varieties are used inconsistently in the literature. Gypsum is calcium sulfate dihydrate (CaSOr2H2O), with less hydrous or anhydrous phases of bassanite (CaSOr0.5H2O) and anhydrite
(CaSO4). Gypsum is morphologically diverse, with different crystal morphologies referred to in this study as mineral varieties. Varieties of gypsum important in this study are (1) satin spar, with opaque fibrous to elongate blocky crystals; (2) selenite, with translucent to transparent glassy or blocky crystals; and (3) alabaster, which is sugary to massive microcrystalline gypsum (Fig. 4). The controls on the formation of different varieties of gypsum are not well understood and require laboratory experimentation that falls outside of the scope of this project.
Textures fibrous, antitaxial sheets (~fibrous), blocky, hexagonal massive, sugary, nodular
Occurrence ubiquitous, veins and nodules jq j rm™r«Scontact (TO) gypsum beds, nodules Figure 4. Gypsum varieties observed in the field with important textures and general occurrence context. 10
Methods
Field Methods
Field methods included measuring stratigraphic section, measuring structural geometries with a Brunton compass (bedding, faults, veins, mineral fabric), collecting samples for petrologic and chemical analyses, and capturing images for detailed 2D analyses of vein geometries. Field measured geometries were analyzed using the program
Stereonet for Windows (Allmendinger et al. 2013; Cordozo and Allmendinger 2013).
Image Analysis
Images were captured with digital cameras. High-resolution panoramic images were captured by mounting a digital camera on a GigaPan EPIC 100 robotic arm and stitched using GigaPan software. Several scale references allow for scaled image analysis and oblique-face angle correction. Panoramic images were overlain with a 1 m grid oriented to bedding and 2 m wide and then used to measure vein lengths, thicknesses, general vein orientation (horizontal or vertical), intersection types, and intersection angles. Measurements are accurate to 0.5 cm for most measured sections and potential error does not exceed 1 cm, producing a resolution bias toward thicker veins when veins are thinner than 1 cm.
Some 2D vein measurements were made of cliff faces with a variety of orientations to capture representation of the 3D system. A total of 873 veins and 784 vein intersections were measured at SG and 1477 veins and 1845 vein intersections were measured at TO. Vein density is the fraction of surface area covered by veins or nodules and does not include gypsum beds. 11
Laboratory Methods
A total of 14 mineral samples were identified by X-ray powder diffraction. A total of 21 thin sections were polished for petrologic and QEMScan analysis. QEMScan
(Quantitative electron microprobe scan) scans the surface of a prepared thin section or plug with an electron beam, returning values that roughly correspond to elements and their relative abundances. QEMScan is not sensitive to hydrogen, so cannot differentiate between hydrous and anhydrous species.
Bulk rock and trace element geochemistry was performed on 35 samples (12 SG,
22 TO, 1 SB). Strontium stable isotope geochemistry of 37 samples (13 SG, 23 TO, 1
SB) was analyzed. Sulfur stable isotope geochemistry of 37 samples (13 SG, 22 TO, 2
SB) was analyzed and standardized to Diablo Canyon Troilite. Small (<2 mm) pyrite crystals from SB were analyzed for S isotopes.
Burial History Diagram
A burial history diagram was created using PetroMod software and measured stratigraphy supplemented with published burial history, exhumation history, unit thickness, and other timing data and constraints (Dumitru et al. 1994; Sprinkel 1994;
Mathis 2000; Davis and Bump 2009; Sprinkel et al. 2010; Karlstrom et al. 2014). There are many assumptions in the construction of a burial history diagram, such as a general lack of resolution for the many heterolithic units of Colorado Plateau. However, the model provides some insight to the evolution of the system and placement of structural and diagenetic events. Even though hydrocarbon maturity is not the focus of this study, assumed heatflow values were provided that are consistent with modern Colorado Plateau heatflow, typical continental heatflow, and the onset of lithospheric delamination 12 following the models of Dumitru et al. (1994) and Karlstrom et al. (2014). The model assumes a linear exhumation rate derived from lithospheric delamination and dynamic topography models (McQuarrie and Chase 2000; Moucha et al. 2009; Lavendar et al.
2011).
Results
Moenkopi Lithology
Lithologic Facies
Lithologic facies of the Moenkopi Formation are broadly divided into three categories: sandstone, mudstone, and limestone (Table 1; Fig. 5). Three sandstone (and siltstone) facies are cross-stratified sandstone, ripple-laminated sandstone, and climbing- ripple sandstone. Mudstone facies were difficult to characterize because primary structures are commonly very fine and obscured by weathering, except via examination in thin section or core, which is not useful for field evaluation. However, two distinguishable mudstone facies in the field include laminated mudstone and silty mudstone. Three limestone facies are distinguished as algal boundstone, fossiliferous packstone, and oolitic grainstone facies.
Sorrel Buttes Core
The SB core contains 49 m of Moenkopi section (drilling depths 2070-2232 ft) consisting of gray-green, dark brown, and tan siliceous mudstone, siltstone, and sandstone (Fig. 5). Siltstone and sandstone tend to be lighter colors. Mudstones are ripple- to planar-laminated, and siltstone and sandstone bodies are commonly ripple cross-stratified. Many intervals are lightly bioturbated (BI <3). Authigenic pyrite crystals 13
Table 1. Moenkopi lithofacies descriptions. Although there is a column for vein density, vein density was not always highly correlated with certain lithologic facies, but more with particular stratigraphic units. Vein densities are qualitative, but correlate to estimates of the area covered by visible vein material on a rock surface, with high vein density >10%, medium (med) >3%, and low <3%.______Facies Description Vein Unit Environment Density Ss: Cross Reddish orange to pale yellow fSs and Slt with med Trmu, Tidal channel stratified high-angle, commonly bi-directional cross Trmm (delta), Sandstone stratification, climbing ripples, tool marks, and proximal to channel forms. Occasional lightly contorted shore bedding and dewatering structures. Ms fragments common at base of channelized sandstone, especially where underlain by mudstone. Includes <1 m beds and pockets of dark reddish brown Cs, and Ms. Yellow hue occasionally present a lithologic boundaries, penetrating as far as 1-2 m into underlying Ms. Ss: Ripple- Reddish orange to yellowish tan fSs and Slt with Trmt Tidal beach laminated symmetric ripples, tool marks, mud cracks, and Sandstone occasional to rare reptilian trackways. Some lightly contorted bedding and dewatering structures. Interbedded with dark reddish brown rippled Ms. Ss: Reddish orange to tan fSs and Slt with high, Trmu, Tidal channel Climbing- asymmetric ripple laminations, climbing ripples, nodules Trmm rippled and flute marks. Sandstone Ms: Dark reddish brown to reddish orange Cs, Ms, high Trmu, Tidal flat Laminated and Slt with very fine parallel and ripple Trmb, Mudstone laminations. Light bioturbation (BI <3) and Trmt, occasional dewatering structures. Weathers in Trmm massive globules. Some irregular gray-green horizons and locally present mm-scale reduction spots. Ms: Silty Reddish brown to brown Ms and Slt with fine high Trmu, Tidal flat (high mudstone parallel and ripple laminations. Light bioturbation Trmm energy event) (bioturbation index <3) and occasional dewatering structures. Irregular green horizons associated with some especially silty layers. Ls: Algal Gray algal mat and shallow Ls mound structures. - Trms Shallow marine Boundstone Ls: Gray gastropod and bivalve packstones, notably Trms Shallow marine Fossiliferous meekoceras and lingula. Packstone Ls: Oolitic Yellowish sand-sized oolitic grainstones Trms Shoreface Grainstone interbedded with fine, slope-forming Ms, Slt, and SS. Ss = sandstone, fSs = fine sandstone, Slt = siltstone, Cs = claystone, Ms = mudstone, Ls = limestone. Trmu = upper red mbr (SG), Trmm = Moody Canyon Mbr, Trmt = Torrey Mbr, Trms = Sinbad Limestone Mbr, Trmb = Black Dragon Mbr. SG TO SB 33-1 Vein Vein Depth Bioturbation Lithology Age Fm Mbr m Density Age Fm Mbr Density (m) Index clastic I | conglomerate 635- m I | sandstone I I siltstone selenite pyrite Q mudstone 640-1 evaporite TO-VR □ gypsum carbonate
645- I boundstone | | grainstone I | mudstone
clay clasts Primary Structure
TO-W El gravel TO-S 655- PTI cross-stratification mud dasts 0 ripple cross-stratification E3 ripples 660- [ 0 ripple lamination pyrite | parallel lamination |B| biorturbated lamination calcite/ mud drapes dolomite 665 - [FI highly bioturbated cement
6 7 0 -1 oily
mud clasts
TO-N mud lenses sand lenses t t ~i m 036 C- ,5-
Figure 5. Measured stratigraphic columns for the SG and TO field areas and SB core. 15
are locally present. Some intervals are dark and oily. The SB core contains no gypsum or
anhydrite.
Mineralogy
Mudstone, sulfate bed, and vein samples are presented here in the order of
petrographic observations, XRD results, QEMScan results, and geochemical results.
In the both field areas, mudstones are composed primarily of gypsum-cemented
quartz and feldspar (45-90%), iron oxides (5-30%), and clay minerals (<1-30%), with trace amounts of muscovite and pyrite (<1%). Because of the fine grain size, cement is
difficult to see even in thin section, but appears to be primarily gypsiferous and mixed with calcite and/or dolomite. Red mudstones lack pyrite and contain more iron oxides than the green mudstones.
XRD in both vein and mudstone samples shows gypsum, with minor amounts of
calcite, suggesting gypsum as the principal cement with calcite inclusions (Fig. 6). Other
minerals include quartz, illite, and kaolinite, and a weak signal for swelling clays.
QEMScan shows a dominance of sulfates (5-59% in mudstones, 94-100% in
gypsum samples) and quartz (11-75% in mudstones, 0-5% in gypsum samples), followed by feldspars and micas (~16% in mudstones), with trace amounts of carbonates, Cl- bearing species, and other oxides. In gypsum samples, all species except sulfates are
concentrated in mudstone inclusions. Green mudstones are enriched in quartz and
feldspar compared to red mudstones. Red mudstones have more Fe-bearing species and
other oxides. The bulk rock geochemical composition of Moenkopi mudstones shows
high concentrations of Ca and S (likely gypsum cement), Fe (Fe-oxides), Ag (micas and a
few clays), Mg (dolomite), and K (feldspars) (Fig. 7). 16
Figure 6. Summary XRD results for the SG and TO areas, with key mineral peaks indicated (gyp = gypsum, qtz = quartz). 17
Ag Al As B Ba Be Bi Ca Cd Co Cr Cu Fe Ga Hg K LaMgMnMo Na IMi P Pb S Sb Sc Sr Th Ti Tl U V W Zn Figure 7. Bulk trace element geochemical results, with averages. Values below detection level were set to 0.1 ppm.
Gypsum Varieties
Satin spar (Fig. 4a) is the most abundant variety of gypsum in both study areas and fills most veins and nodules. Most nodules appear massive but, when broken open, they commonly reveal a radial fibrous texture. Selenite (Fig. 4b) occurs in few veins. One vein at the Moody Canyon-Shinarump Conglomerate contact in the Torrey area is selenite. Alabaster is only in gypsum beds and some nodules.
Veins are commonly of a composite nature, meaning that they are composed of one or more distinct veins that join and share the same fracture aperture, and some veins change mineral variety along their length or width. Veins are organized into classes
(Table 2, Fig. 8) because of the composite and polymineralogical characteristics of the veins. This classification scheme describes veins as either simple (1 vein part) or composite (2 adjacent vein parts) and monophase (single mineral or mineral variety) or 18
Table 2. New vein class terminology, based on the presence of a plane of mineral fabric interruption (including clay and wall rock inclusions) that is parallel or subparallel to the vein wall, and by mineralogical changes along the length or width of the vein (supporting images in Fig. 8).______Class Group Description Significance Vein Part General A distinct, simple vein Term adjoined to others in a composite vein. Composite Class Veins that have an internal Multiple adjacent veins; multiple crack- parting surface or plane of and-seal events; a vein formed adjacent mineral fabric interruption to a pre-existing vein. (i.e., veins with multiple vein parts). Monophase Class Veins exhibit only one Stable subsurface environment. mineral phase or variety. Polyphase Class Veins exhibit more than one Changing subsurface environment, mineral phase or variety. temporally (simple and composite veins) or spatially (simple veins). Simple Class Veins that have no internal Vein formed in a single crack-and-seal parting surface or plane of event. mineral fabric interruption (i.e., veins with only one vein part). Bidissonant Composite All vein parts veins exhibit a Temporal and spatial changes in Polyphase change in mineral phase or subsurface environment during variety, but changes are not emplacement of all vein parts. paralleled between vein parts. Discordant Composite No apparent transition along Vein parts emplaced in different Polyphase vein length, but vein parts environments. exhibit different mineral phases or varieties. Harmonic Composite Same mineral transition Chemical changes post-emplacement. Polyphase across all parting surfaces. Unidissonant Composite Mineral transition in one or Temporal and/or spatial changes in Polyphase some, but not all vein parts. subsurface environment during emplacement of polyphase vein parts; stable environment during emplacement of monophase vein parts. 19
Figure 8. Graphical description of vein class terminology. Classes are based on the number of discrete vein parts represented in a single fracture aperture, and number of mineral types, phases, or varieties present in the vein. Four classes are conceivable for composite polyphase veins, though only discordant composite polyphase veins are observed in the field areas. polyphase (multiple minerals or mineral varieties). Image resolution was not sufficient to consistently distinguish between vein classes in 2D measurements.
Rarely at the SG-N locality, veins are simple polyphase or discordant composite polyphase (Fig. 8). Veins weather out in positive relief in mudstones and negative relief in sandstones and some siltstones.
Vein Texture
Gypsum textures include the fibrous texture of aligned satin spar crystals within a vein, unorganized satin spar fibers (only at TO-N), glassy sheets of selenite crystals, 20
blocky selenite crystals (rarely and only at SG-N), polygonal texture on some selenite
vein surfaces, radial fibers of satin spar nodules, massive or granular texture of alabaster
nodules and primary beds, and the “chicken-wire” texture of nodular gypsum beds mixed
with clay, mud, and silt. Veins generally appear to be antitaxial (outward-growing), with
fibers oriented subperpendicular to the vein walls and crystal coarsening from near the
center of the vein toward the vein margins (Bons and Montenari 2005; Bons et al. 2012).
Veins exhibit two internal fabrics: a linear fabric parallel to gypsum fibers that are
subperpendicular to the fracture wall, and a planar fabric that is parallel to the fracture wall. The linear fabric is ubiquitous but not obvious in most selenite veins, in which the
planar fabric is more obvious. The linear fabric is near a 90° angle to the fracture wall, with preferential orientation being roughly vertical. From the perpendicular to the vein
face to the vein fiber, rotation is usually clockwise around a horizontal axis that trends
S35E, but with considerable scatter in the data (Fig. 9a). The planar fabric is less obvious
or absent in satin spar crystals and consists of changes in crystal size, orientation (shear
fabrics), opacity, or clay or wall rock inclusions.
Some vein fibers are distorted into sigmoid shapes with finer crystals near the
center of the vein, suggesting cataclasis and pure shear across the vein. The sense of vein
shear is generally reverse. Other indicators of shear include tension gashes and faults.
Tension gashes are more common in TO than in SG. The tension gash enveloping surface
dips 20-30° E (sometimes as shallowly as 5°) and the individual gashes tend to be
subhorizontal (Fig. 9a). Reverse faults in SG dip 31-36° SE, some with fragile
slickenlines that rake ~90° on the fault surface, and have 2-3 m net slip (Fig. 9c). There 21
Figure 9. Lower-hemisphere stereonets representing data from the Torrey (TO, n = 118) and St. George (SG, n = 236) field sites. Shown are right-hand-rule axes of rotation (looking down the axis, rotation is clockwise) from the vein pole to vein fabric and tension gash orientations in the TO field area (a) and orientation data from the TO (b) and SG (c) field areas. are few to no gypsum veins in sections with faults, but this may not be related to the presence of faults as vein density decreases in the uppermost Moenkopi at both field sites with and without nearby faults.
Vein Geometry
Gypsum veins show no correlation between thickness and length. Vein thickness peaks at ~1 cm, but thinner veins also exist although they are below detection at the resolution of images used in 2D measurements. Most veins (from field observations) are between 0.5 and 1 cm thick with, mean thickness at 0.86 cm. Exposed vein length 22 occurrence peaks at 12 cm, with mean vein length of 45 cm. The occurrence of vein length and thickness then decrease logarithmically. Veins that appear to be thick (>1.5 cm) are almost invariably composite veins. Collectively, the thickest and most continuous veins are horizontal, with H/V (horizontal/vertical ratio) of 3.1 for both metrics.
The orientations of veins and cross-cutting relationships are important in strain analysis, but a systematic grouping a veins according to orientation and cross-cutting relationships proved ineffective because veins commonly change orientation and have inconsistent cross-cutting relationships. Classifying vein intersections proved more effective, and all vein intersections were characterized as either horizontally dominant
(the more vertical vein at the point of intersection is cut off by the more horizontal vein), vertically dominant, cross-dominant (both veins continue past the intersection), or as bends (vein changes orientation). When vein lengths were measured, their vertical and horizontal parts were counted separately and joined with the bend intersection (Table 3;
Fig. 10).
Vein intersection classes are, in order of decreasing abundance, splits (where a composite vein separates into two or more simpler veins; 49%), terminations (where one vein abuts against another; 47%), bends (where a vein abruptly changes orientation; 3%), and splays (where a simple vein divides into many smaller veins; <1%). Splits are less common with increasing intersection angle, terminations have most intersection angles of
20-40° and 75-90°, and elbows (sharp bends) are more common with increasing intersection angle. H/V dominance is similar for all intersection classes (Fig. 11). Most cross-dominant intersections are terminations (77%), meaning that one vein cross-cuts the other but both continue in both directions past the intersection, and elbows (19%). If 23
Table 3. Vein intersection classes (supporting images in Fig. 10). Geometry Group Description Significance Bend Bend A significant change in vein orientation (i.e., Indicates mechanical vertical to horizontal) where mineral fabric is anisotropy, a locally continuous through the bend. While measuring variable stress field, or vein geometries, the vein on either side of the complex dilation. bend is counted as a separate vein. Curve Bend A bend in a vein from one orientation to Indicates a locally another over a smooth arc. variable stress field. Elbow Bend, A bend in a vein from one orientation to Indicates mechanical Intersection another at a sharp angle. If a bend occurs such anisotropy, a locally that one or more veins that make a composite variable stress field, or vein (Table 2; Fig. 8). complex dilation. Parallel Intersection Veins are parallel to sub-parallel of each other, Indicates mechanical with minimal intersections. anisotropy or dominate orientation of dilation. Splay Intersection A simple vein (Table 2; Fig. 8) separates into ? two or more veins. Split Intersection A composite vein (Table 2; Fig. 8) separates Multiple crack-and-seal along a parting surface into two or more veins. events; a vein formed If the divergence is at a sharp angle, it is adjacent to a pre termed an elbow split. existing vein. Termination Intersection One vein terminates against another. Multiple crack-and-seal events. elbow splits (51% of measured splits) are counted with bends, 29% of measured intersections are bends, 3% of which are curves (mode angle of 75-85°, radius of curvature 8.6 cm in sandstones, 9.9 cm in mudstones) and 88% are elbow splits (mode angle of 80-90°). Some vertical veins are very dominant, where most horizontal veins will join at a vertical “trunk” with elbow split intersections. The vein NE of the trunk tends to bend upward to join the composite trunk and the vein SW of the trunk tends to bend downward. Vertical vein trunks tend to be thin (<1 cm).
Vein Distribution
From 2D measurements, mean sandstone/siltstone vein density is 7.1% in the SG area and 8.0% in the TO area. Mean mudstone vein density is 6.8% in the SG area and
11.5% in the TO area. Commonly, fracture intensity is highest and vein thickness is lowest in siltstone and sandstone. Many sandstone bodies have few or no veins. Silty 24
Figure 10. Graphical description of vein intersection classes, with typical intersection angles and dominance ratios (vertical : horizontal : cross). 25 [ [| | j | :
Figure 11. Vein intersection statistics, by intersection class. A vein is horizontally dominant if, at the intersection of two veins, the more horizontal vein is more continuous. Vertical dominance is where a vertical vein is more continuous where two veins intersect. Cross-dominance is where both veins continue past the intersection. Bends are not intersections between different veins, but where one vein significantly changes orientation (i.e., from horizontal to vertical). mudstone and siltstone bodies tend to have many very thin veins that could not be detected by image analysis. Overall, vein density is not well correlated to lithology or bed thickness (Fig. 5).
Vein density decreases dramatically in the upper 10-20 m of the Moenkopi
Formation. One selenite vein undulates subparallel to the Moenkopi-Chinle contact near the TO-W locality, but no other gypsum veins were detected in the upper ~40 m of the
Moenkopi at that location.
SG gridded area covered 56.5 m2 of the SG-N roadcut, of which 3.87 m2 is veins and nodules and 0.28 m2 measured for gypsum beds. Grid cells for 2D measurements were arranged such that each meter in stratigraphic height was measured in a 2 x 1 m area, so the 0.28 m2 of gypsum beds is best interpreted as one or more gypsum beds totaling 0.14 m in thickness. TO gridded area covered 67.5 m2 of cliff face between the
TO-W, TO-VR, and TO-S localities, of which 7.16 m2 is veins (no nodules). TO gypsum beds tend to be in slope-forming sections that were not measured, but are <1 m thick. 26
Geochemistry
Whole Rock and Trace Element Geochemistry
Gypsum samples have similar ratios of CaO, SO3, and H2O (~LOI) to pure
gypsum. TO gypsum samples include more impurities than SG gypsum samples.
Mudstones are mostly silica by weight (SiO2, 40-60%), followed by aluminum (AhO3,
~11%), H2O (~LOI, 7-15%) and calcium (CaO, 4-9%). Trace element concentrations are
similar between gypsum and mudstone samples, with generally high concentrations of
Al, Ca, Fe, K, Mg, Na, S, and Sr, except gypsum incorporates little to no Be, Co, and Ti
whereas mudstone tends to incorporate at least some of these elements (Fig. 7). TO
samples are slightly more enriched in Na than SG samples.
Some trace elements are sensitive to redox conditions. Redox proxies include
V/Cr, Ni/Co, and V/(V+Ni) (Ross and Bustin 2009; Xu et al. 2012; Wang et al. 2015,
2016). Samples are too poor in certain elements (i.e., U, Th, Re, Mo) to exploit other
redox proxies. All samples tend to have low Ni/Co, V/Cr, and V/(V+Ni) ratios and plot
as oxic, but some mudstones and gypsum samples are dysoxic (Fig. 12).
Stable Isotope Geochemistry
Most Moenkopi samples have isotopic values in the range of 87Sr/86Sr 0.708
0.709 and S34S +10-30 (Fig. 13). Three outliers are a sample of SG green mudstone
(87Sr/86Sr 0.711), SB mudstone (87Sr/86Sr 0.710 and S34S +0), and a TO selenite vein at
the Moenkopi-Chinle contact. The TO selenite vein has similar Sr and S isotope ratios as
Shinarump Conglomerate sandstone from the same area (87Sr/86Sr 0.709, S34S -6).
Moenkopi gypsum is distinct in Sr and S isotope values from Jurassic Carmel Formation
gypsum and modern pedogenic gypsum. 27
Ni/Co V/(V+Ni) Figure 12. Redox condition indicators V/Cr, Ni/Co, and V/(V+Ni) for 30 samples in the Moenkopi Formation and comparative samples.
Figure 13. 87Sr/86Sr and S34S c d t stable isotope results for 37 samples from the Moenkopi Formation and comparative samples. Three samples are not shown because of unknown S34S or 87Sr/86Sr: TO Sinbad Limestone (87Sr/86Sr 0.7079), SB pyrite (S34S +6.73), and SG Shinarump Conglomerate (87Sr/86Sr 0.7103). The ranges of Early Triassic, Middle Jurassic, and modern seawater are shown (Veizer 1989; Strauss 1997; Worden 1997; Strauss 1999; McArthur et al. 2001; Kampschulte and Strauss 2004; Strauss 2004). 28
Other Diagenetic Features
Most of the Moenkopi Formation is a distinctive orange to dark red color, with scattered, lighter red- to yellow-colored sandstone bodies. Many bedding-parallel horizons are gray-green to yellow-green, where the red-green contact is irregular or spotty and rarely gradational (Fig. 14). Green horizons tend to be associated with higher silt content, ripple cross-stratification, climbing ripples, flute casts, and mud cracks.
Though a sedimentary layer may be a different color from adjacent layers, the contact between colors is rarely identical to the lithologic contact. Green horizons are distorted in a zone of disturbed mudstone near near faults (Fig. 14d).
Green reduction spots are also present in many places of the upper Moenkopi
(Fig. 14b) and their presence, size, and spatial density are related to laterally continuous packages of fine red mudstone. Reduction spots are ellipsoidal, with the long axis (S1) subhorizontal and the short axis (S3) subvertical to bedding-perpendicular. Mean
Figure 14. Patterns of gray-green pigmentation, with typical spotty bedding-parallel horizons (a), reduction spots are cross-cut by gypsum veins (b), spotty green pigmentation adjacent to nodules in gypsum beds (c), and distribution of green pigmentation association with reverse faults (d). 29
reduction spot S3/S1 ratio is 0.90 at SG (n = 22) and 0.82 at TO (n = 24).
Gypsum veins cross-cut green host rock coloration. Where gypsum veins are not present in SG uppermost Moenkopi, green coloration follows fractures, which does not occur elsewhere at either field site. Mudstone adjacent to gypsum bed nodules is sometimes green (Fig. 14c). Some green coloration follows primary structures, including soft-sediment deformation, burrows, and mudcracks. In both field areas, mudstones in the uppermost 1-10 m of the Moenkopi (just below the Chinle Formation) are pale yellow to dark green. The yellow coloration sometimes also occurs in mudstones below and adjacent to thick Moenkopi sandstone deposits.
Burial History
The Moenkopi Formation likely experienced two episodes of rapid burial and a relatively fast, uninterrupted episode of uplift (Fig. 15). The first burial episode occurred
~170-160 Ma with the deposition of the Middle Jurassic San Rafael Group and is modeled to have accomplished ~0.6 vertical stretch in the Moenkopi Formation via compaction (~680 m original thickness to ~400 m). The second burial episode occurred
~95-80 Ma with the deposition of the Late Cretaceous Mancos Shale and accomplished
~0.9 vertical stretch. Exhumation began ~30-35 Ma (Pederson et al. 2002; Moucha et al.
2009; Lavendar et al. 2011).
Calcium sulfate undergoes the anhydrite-gypsum phase transition at ~55°C, which correlates to a depth of 1.7-2.4 km (Yamamoto 1969; Freyer and Voigt 2003; Klein and
Dutrow 2008), assuming a range of geothermal gradients from typical continental lithosphere, ~25°C/km to the modern Colorado Plateau, 25-35°C (Blacket and Wakefield
2004). 30
Pore fluid pressure is also modeled based on porosity, permeability, and grain size estimates given rock type. Between the first and second rapid burial events, pore pressure in the Moenkopi Formation is ~100 MPa. Pore pressure grows to ~150 MPa after the second burial event, falls to ~50 MPa in the past 20 Myr and to 30-20 MPa in the past 10
Myr (Fig. 15).
The model buries Permian rocks below 7 km depth, which is likely 100-200 m too deep based on vitrinite reflectance data (Dumitru et al. 1994). However, the model requires these depths with given modern stratigraphic thickness and estimated rock properties. The shape of the burial curve (where major burial and exhumation events occur) is more important to this study than the few hundred meters of overburial, as it describes rapid burial events, periods of relative stability, exhumation, approximate
Contours are pore fluid pressure Figure 15. Burial history diagram, using general southern Utah stratigraphy applicable to burial in the Torrey area. White contours show modeled pore fluid pressures. The red hashed area indicates where gypsum veins have most likely been emplaced. 31
amount of compaction, and an estimate of pore fluid pressure.
Discussion
SB Core Correlation
The SB core seems to fit well into the tide-dominated beach system of the Torrey
Member because of its lack of gypsum, mild to moderate bioturbation, ripple-cross bedded sandstone interbedded with ripple- and planar-laminated mudstone and siltstone
(Fig. 5). The lack of SB gypsum may also be attributed to the depth of the core (perhaps
~650 m depth is too deep for Moenkopi fracture), the thickness of the Moenkopi in the
SB area (<200 m?, not extensive enough to capture full variability of lithologies and
fractures), and/or the chemistry of the extracted section of core (biased towards the
reduced/green mudstone and oily intervals).
Vein Network Classification
A series of broad characteristics, including vein mineralogy, mineral variety, vein
fabrics, mineral texture, vein length, vein thickness, vein class, vein orientation, vein
intersection class, vein intersection angle, vein intersection dominance, host medium, and vein distribution played an important role in interpreting the Moenkopi vein network.
Therefore, a three-part (i.e., 3 letter/number) classification scheme is defined to organize
observations and produce hypotheses on vein network emplacement and alteration
processes. The observational components of the classification are the geometry,
distribution, and mineralogical variations in the vein network (Table 4) and should be
supported with petrologic, chemical, and other data.
The first part uses observations of vein geometries expressed in the network, 32
Table 4. Vein network classification. This classification scheme is a synthesis of many vein characteristics into a simpler and more intuitive classification scheme of vein networks. This scheme utilizes three parts, which are combined into class B3a networks, C1c networks, etc. The first part (A, B, C) describes the complexity of vein geometry, the second part (1, 2, 3) describes how veins are distributed, and the third part (a, b, c, d) describes the mineral varieties within the vein network. Vein Diagnostic Characteristics Significance Network Class A _ _ Simple veins; thickness, length, Not hydraulic and controlled by the regional stress and intersection dominance H/V 4 state. 1; no elbows or curves; consistent intersection angles. B _ _ Composite veins; thickness, Hydraulically induced, with first-order geometry length, and intersection controlled by the regional stress state. Pre-existing dominance H/V ~ 1; common fractures can be opened hydraulically, filled with vein splits, some elbows and curves; minerals, and form elbows with veins of other variable intersection angles. orientations. C __ Composite veins; thickness, Hydraulically controlled or complex deformational length, and intersection history. dominance H/V = 1; ubiquitous splits and elbows, common curves; highly variable intersection angles. _ 1 _ Lithologic distribution Vein distribution controlled by the mechanical properties and composition of the host rock. _ 2 _ Stratigraphic distribution Mineralizing fluids are probably sourced from within the stratigraphic unit or from an adjacent unit. _ 3 _ Heterogeneous or isolated Vein distribution controlled by spatial proximity to distribution vein mineral source. Mineralizing fluids had limited connectivity to other parts of the rock and mineral source is highly localized (i.e., a pluton, drop-stone, biota, etc.) and/or is sourced from a fluid conduit transporting authigenic material (i.e., a fault, dike, etc.). a Monophase veins Vein network emplaced in a stable environment. b Discordant polyphase veins Environment changed between crack-and-seal events. __ c Discordant, unidissonant, and/or Environment changed during crack-and-seal events. bidissonant polyphase veins Crack-and-seal events are closely spaced temporally. __ d Simple polyphase or harmonic Veins altered post-emplacement. composite polyphase veins 33 including the distinction between simple and composite veins, bends, and intersection type, angle, and dominance, to infer degree of overpressurization or other controls on the network geometry. ‘A’ describes a network that is geometrically simple, ‘B’ a system that is geometrically complex, involving composite veins, bends, and complex cross cutting relationships, and ‘C’ is a highly geometrically complex or disordered system.
The second part uses observations of the spatial distribution of veins to infer vein mineral source, where ‘1’ describes a lithologically distributed network, ‘2’ describes a stratigraphically distributed network, and ‘3’ describes a heterogeneous or highly local distribution of the vein network.
The third part uses observations of the presence of multiple minerals, mineral phases, or mineral varieties (i.e., gypsum vs. selenite) in the vein network to infer the environmental (chemical, temperature, and pressure) evolution of the system during vein emplacement, where ‘a’ describes a network with monophase veins, ‘b’ describes a network with discordant composite polyphase veins, ‘c’ describes a network with many polyphase vein classes, and ‘d’ describes a network with mostly simple polyphase or harmonic composite polyphase veins.
The Moenkopi vein network is a B2c network in the SG field area because it is geometrically complex, distributed stratigraphically, and has a variety of polyphase veins.
The TO vein network is a B2a network because it is geometrically complex, distributed stratigraphically, and is all satin spar (with one exception near the Moenkopi-Chinle contact). 34
Vein Mineralogy
An initial hypothesis to test was that the vein minerals are gypsum. Petrography,
XRD, and geochemical results presented here indicate that veins, nodules, and sulfate beds are indeed gypsum, with trace inclusions of calcite and/or other phases of calcium
sulfate. Satin spar is the dominant variety of gypsum, followed by alabaster in gypsum beds, and locally present selenite.
In pure gypsum (CaSOr2H2O), Ca/S should be 1.25 by weight and H2O/S (LOI
as proxy for H2O) should be 1.13 by weight. Ca/S tends to be slightly high (~1.3) and
LOI/S tends to be slightly low (~ 1.17) in gypsum samples. This is consistent with the
presence of trace solid inclusions of calcite (also indicated by XRD; Fig. 6), dolomite,
bassanite, anhydrite, and other trace impurities. QEMScan indicates almost pure calcium
sulfate in all vein samples and indicates minor, randomly distributed impurities of
elevated Ca, Mg, K, and other trace elements (Fig. 7).
Changes in mineral or mineral variety indicate a probable change in
environmental condition, such as pressure, temperature, and chemistry, so polyphase
veins indicate environmental changes, though controls on the formation of specific
gypsum varieties remain largely unknown. Gypsum crystals have been demonstrated to
grow larger and faster in low Ca2+/SO42-, low Mg2+, and high Al3+ (Freyer and Voigt
2003; Abdel-Aal et al. 2004; Rashad et al. 2004). Crystals tend to be needle-like in a pure
CaSO4 solution, with elevated Mg2+, and possibly in the presence of hydrocarbons
(Freyer and Voigt 2003; Rashad et al. 2004). In the presence of Al3+, crystals tend toward
thick rhombic clusters (Rashad et al. 2004).
Selenite crystals are larger than satin spar crystals, suggesting that they either 35
grew more slowly or that the precipitating brine was enriched in SO42- and/or Mg2+.
Elevated levels of Mg seem likely because of the presence of dolomite and elevated Mg content of analyzed samples (Fig. 7). Given the presence of calcite solid inclusions in tested samples, it seems reasonable to assume high Ca2+/SO42-, which favors small, slow- growing crystals. Low Al/Mg should result in small, needle-like crystals, but TO samples have high Al/Mg (mean 1.20) compared to SG samples (mean 0.34), yet more selenite was observed at SG.
It seems unlikely that changes in gypsum variety (satin spar vs. selenite) are related to chemistry or temperature because changes occur along the length of some veins and SG satin spar and selenite veins are chemically similar. Near- or at-surface alteration of gypsum does not explain lateral changes in variety as variety transitions occur only locally, do not carry across vein parts in composite polyphase veins, and have no significant chemical differences between satin spar and selenite veins. Subtle differences in pressure are therefore the most likely control on gypsum variety in the SG and TO areas. Gypsum’s solubility increases with greater pressure (Freyer and Voigt 2003), so should favor slower crystal growth, which should favor the larger selenite crystals, so perhaps selenite veins formed early in the development of the Moenkopi vein network, before lithostatic pressure was relieved by unroofing and water pressure was relieved by crystallization and fluid escape during fracturing. A high pressure gradient at the
Moenkopi-Chinle contact may have been favorable for retarding gypsum crystal growth, favoring the emplacement of a selenite vein near the Moenkopi-Chinle contact.
Laboratory investigations (beyond the scope of this study) would be required to test the pressure and pressure gradient control hypotheses. 36
Source of Calcium Sulfate
Moenkopi gypsum is hypothesized to be sourced from primary calcium sulfate beds within the Moenkopi Formation. Strontium and sulfur isotopes trace the divalent cations (Ca2+) and sulfur in the system. Stratigraphic relationships also indicate a probable source.
Isotopic Signature
The 86Sr isotope is stable in geologic time, and 87Sr increases by beta-decay of
87Rb with a half-life of 48.8 Gyr (Veizer 1989). The isotope 87Sr is concentrated in Rb- rich minerals more typical of continental crust than oceanic crust, so a higher 87Sr/86Sr ratio is typical of a granitic source (White 2013). Sr is not considered to significantly fractionate, and it is assumed that seawater Sr is relatively homogeneous through geologic time (Veizer 1989; Denison et al. 1998). Therefore, the Sr composition in minerals is dependent on the chemistry of the precipitating brines, with negligible fractionation due to crystallization or biologic processes. Diagenesis in marginal marine and continental settings tends to increase the concentration of 87Sr (Denison et al. 1998;
Veizer 1989). The concentration of 34S and 32S is likewise assumed to be homogeneously distributed in seawater and does not fractionate by calcium sulfate phase transitions or chemical equilibrium reactions (Worden et al. 1997; Seal et al. 2000). The isotope ratio of Sr is more sensitive to diagenesis than is the S isotope ratio of a sample (Denison et al.
1998).
Moenkopi Sr does not agree well with Early Triassic seawater (Fig. 13; Veizer
1989; Strauss 1997; Worden 1997; Strauss 1999; McArthur et al. 2001; Kampschulte and
Strauss 2004; Strauss 2004). The high 87Sr/86Sr of Moenkopi gypsum suggests significant 37 continental, nonmarine input or recrystallization in diagenetic fluids enriched in 87Sr. The terrestrial signature agrees with paleogeographic interpretations of the region, with the inlet for the Early Triassic epeiric seaway far to the northwest, and Moenkopi sediment derived from the Uncompahgre Uplift (Fig. 2; Blakey 1973; Blakey and Ranney 2008). A diagenetic signature is less clear—if evaporite Sr isotope ratio generally increases with greater diagenetic alteration, then 87Sr/86Sr should be higher in veins than in primary sulfate beds, but instead they are similar (Fig. 13). The mudstone diagenetic signature therefore appears to be dominated by gypsiferous cement and any Sr-altering diagenesis occurred prior to vein formation.
Moenkopi S34S agrees with Early Triassic seawater except for SB mudstone and a selenite vein at the Moenkopi-Chinle contact near TO-W, which have low S34S (Strauss
1997, 1999; Kamschulte and Strauss 2004; Strauss 2004). If S was sourced from Early
Triassic seawater, Moenkopi gypsum has undergone little microbial reduction (lowering
S34S), or bacterial reduction is countered by abiotic processes (increasing S34S) (Thode and Monster 1965; Seal 2000; Bruchert 2004; Balci 2007; Mazumdar et al. 2008).
Sulfate-reducing bacteria are only active in anoxic environments (Seal et al. 2000; Balci et al. 2007), but green (reduced) mudstones have about the same S34S as red (oxidized) mudstones except for SB mudstone. Mudstone adjacent to gypsum bed nodules is commonly colored green, perhaps indicating some microbial reduction at the mudstone- gypsum interface.
Gypsum beds, gypsum veins, and mudstones have similar Sr and S isotopic ratios
(Fig. 13), supporting the notion that they share a source, or that the gypsum in veins and in mudstone cement are sourced from primary calcium sulfate beds within the Moenkopi 38
Formation. The selenite vein at the Moenkopi-Chinle contact has similar Sr and S isotopic ratios as Shinarump Conglomerate sandstone. Either the Moenkopi-Chinle selenite vein was formed from fluids prevalent in the Chinle Formation, waters mixed significantly at the Moenkopi-Chinle interface, or both. The former is unlikely because the Chinle Formation bears no gypsum veins and the latter is supported by both the decrease in vein density toward the Moenkopi-Chinle contact and the abnormal yellow coloration of Moenkopi mudstone adjacent to the contact. Isotopic values of Jurassic
Carmel Formation gypsum agree well with Middle Jurassic seawater and are distinct from Moenkopi gypsum, so the Carmel Formation is not a likely source for Moenkopi veins (Vezier 1989; Strauss 1997; Worden 1997; Strauss 1999; McArthur 2001;
Kampschulte and Strauss 2004; Strauss 2004).
In summary, Moenkopi Sr appears to be principally derived from a continental source and S from a marine source, and neither show much evidence of diagenetic alteration. Moenkopi veins are similar in Sr and S isotopic ratios to Moenkopi gypsum beds and are distinct from other extraformational gypsum, so veins are likely sourced from within the Moenkopi Formation.
Stratigraphic Distribution
Moenkopi gypsum veins seem to be stratigraphically constrained rather than correlated simply to lithology. There are no gypsum veins in the Sinbad Limestone and
Torrey members at TO, even though the Torrey Member is lithologically very similar to the gypsiferous Moody Canyon Member (Fig. 5). Vein density decreases to 0 in the upper
10’s of meters of the uppermost Moenkopi redbeds at both field sites. The lack of gypsum veins in the uppermost Moenkopi is related either to distance from source or 39 chemical mixing with fluids in the overlying Chinle Formation. Because the selenite vein at the Moenkopi-Chinle contact has similar Sr and S isotopic ratios as the Shinarump
Conglomerate sandstone and Moenkopi mudstone adjacent to the contact is colored yellow, chemical fluid mixing here is likely a leading factor in the diagenesis of the uppermost Moenkopi.
When anhydrite alters to gypsum, it can experience a 30-70% volume increase
(Heard and Rubey 1966; Rauh and Thuro 2007; Klein and Dutrow 2008; Jaworska 2012).
If the 0.14 m total gypsum bed thickness in SG were all originally anhydrite that altered to gypsum, it would only yield 0.1-0.3% of the gypsum in the 2D measurements by volume increase. The remaining amount of original anhydrite required to supply SG veins equates to ~1.3 m sum bedding thickness. The calcium sulfate, then, did not originate in the SG field area. However, the Shnabkaib Member, which underlies the SG upper red member, is gypsiferous with many, thin beds of gypsum (Biek et al. 2009;
Hayden and Willis 2011). Gypsum beds are also often laterally discontinuous over 10’s of meters, so gypsum beds that exist or existed laterally from the interrogated area may also account for the missing primary calcium sulfate. TO gypsum veins require hydration of ~2.6 m sum thickness of anhydrite. Several <1 m-thick gypsum beds in Moody
Canyon stratigraphy, if originally anhydrite and hydrated with a volume increase to gypsum, more than satisfy the TO mass balance.
In summary, Moenkopi veins (1) are restricted to stratigraphic units rather than lithologies, (2) can be the product of calcium sulfate mobilization during anhydrate alteration to gypsum, (3) have similar Sr and S isotopic ratios to primary calcium sulfate beds within the unit, and (4) are distinct from other gypsum in the same area by Sr and S 40 isotope ratios. Thus, Moenkopi veins are most likely sourced from primary calcium sulfate beds within the Moenkopi Formation.
Mode of Vein Creation
Veins are hypothesized to be generated via hydraulic fracturing. The composite and antitaxial nature of the veins is indicative of multiple crack-and-seal events, in which the growth of vein fill kept pace with the opening of fractures, where the fracture aperture or fracture fluid conduit was never as wide as the current vein is today. When a new crack forms adjacent to a pre-existing vein, a thin remnant of wall rock often remains on the edge of a vein, between the two veins, and is incorporated into the composite vein.
Hence, antitaxial veins separated with inclusions of wall rock, clay partings, or discontinuous mineral fabric are classified as composite veins—the vein is two independent veins that have grown parallel to each other. The composite nature of veins is most obvious when one or both vein parts undulate or separate, including more host rock between them.
It is possible that vertical fractures developed during burial (when vertical stress was at its greatest) but vertical dilation was favored at the time of vein genesis, exploiting only parts of pre-existing fractures. However, high confining pressure is required to keep preexisting fractures closed during hydraulic fracturing (Engelder and Lacazette 1990), and it is certain that some vertical fractures formed later in the vein formation process.
Most vertical veins are very short and terminate against horizontal veins, even if other horizontal veins terminate against them. Mudstones are relatively clay-poor, so the dominant horizontal orientation of the veins is largely due to principal stress orientation and overpressurization of the system rather than to mechanical anisotropy from the 41 compaction of clay minerals.
Antitaxial fabric, multiple crack-and-seal events, sharp elbows, vein curvature, and complex vein cross-cutting relationships are suggestive of natural hydraulic fracturing at low confining pressures (Engelder and Lacazette 1990; Phillip 2008).
Timing of Vein Formation
Fracturing and vein fill are hypothesized to occur during late-stage diagenesis associated with unroofing of the Colorado Plateau. A progression of age constraints, in order of increasing specificity, is presented here.
Gypsum veins in both field areas cross-cut diagenetic gray-green coloration in the host mudstone. Gray-green reduction spots show 0.8-0.9 bedding-perpendicular stretch, which is consistent with reduction after the first rapid burial event ~170-160 Ma, followed by deformation during the second rapid burial event ~95-80 Ma (Fig. 15).
Mudstones are quartz- and feldspar-rich and clay-poor, so original reduction spot ellipiticity due to clay mineral alignment during compaction is unlikely. Furthermore, modeled compaction during the second burial event is very similar to measured reduction spot stretch, which places reduction responsible for green coloration somewhere between
160 and 95 Ma. The gypsum veins, then, must have been emplaced <95 Ma.
Veins tend to be satin spar with fibrous texture, and even some selenite crystals have a fibrous texture orientated subperpendicular to fracture walls. The pristine texture of the veins is inconsistent with alteration of anhydrite veins, which can result in as much as a 70% volume increase that would destroy the pristine fibrous texture of the veins
(Heard and Rubey 1966; Raugh and Thuro 2007; Klein and Dutrow 2008; Jaworska
2012), though vein components (Ca and SO4) may be derived from the rehydration of 42 anhydrite located elsewhere. The veins must have precipitated at a temperature and pressure where gypsum is stable (Yamamoto 1969; Klein and Dutrow 2008). Given a typical geothermal gradient of 25-35°C for the Colorado Plateau (Blackett and Wakefield
2004) and lithostatic stress (~27 MPa/km), gypsum may have been stable at ~2 km depth as early as 8 Ma. Some Moenkopi veins exhibit shear fabrics. Experiments find that the yield strength of gypsum is as high as 15 MPa when dry and as low as 2 MPa when wet
(Castellanza 2008; Plachy 2009), which relates to depth conditions of ~75-560 m.
Moenkopi veins could have been emplaced <1 Myr.
Most veins are subparallel to bedding, and all tension gashes have horizontal gashes in a dipping shear plane (Fig. 9), indicating that the majority of veins formed during vertical dilation of the Moenkopi Formation. The modern stress field through much of the Colorado Plateau has generally ENE-WSW oriented tension and NNW-SSE oriented compression (Flesch and Kreemer 2010). Vertical veins tend to strike ~N-S in both field areas, which is roughly consistent with the modern stress field (Fig. 9b and c).
TO vertical veins strike NW-SE, which is still consistent with E-W extension, but perhaps is indicative of older fractures. Strongly dominant vertical veins almost always have many “branches” of horizontal veins that split of in elbows. Horizontal veins that approach the vertical vein tend to bend downward when approaching from the S-SW and upward when approaching from the N-NE, suggesting that (1) long-length vertical veins predate horizontal veins and (2) dilation during vein emplacement was in both the vertical and NE-SW directions, where the NE side moved up relative to the SW side.
Gypsum veins seem to fit well within the modern stress field, with dilation in the vertical and ENE-WSW directions, which supports vein timing coincident with Colorado Plateau 43 uplift < 8 Ma (Flesch and Kreemer 2010).
Rock Mechanics
Veins were generated via hydraulic fracturing < 8 Ma between 75 and 2000 m depth with confining pressures between 2 and 56.5 MPa, with stress orientations similar to the modern stress field. Differential stress in the modern Colorado Plateau is 2-7 MPa
(mode ~5 MPa; Flesch and Kreemer 2010).
Moenkopi rocks were probably simultaneously brought to shear and tensile failure because both shear and tensional features are part of the vein network. Sandstones do not exhibit shear fabrics associated with veins, so sandstones were only brought to tensile failure. Mean effective stress, then, must have been near 0 MPa. With the modern
Colorado Plateau stress state, the mudstone shear strength must be ~2.5 MPa. Tensile strength tends to be less than shear strength (about half), so mudstone tensile strength must be ~1.25 MPa (Fig 16; Labuz and Zang 2012). The sandstone only failed in tension, so tensile strength is ~2.5 MPa and shear strength ~5 MPa.
The failure envelope of Moenkopi mudstone can be estimated using the strength of the rock in addition to the angle of internal friction for these rocks. Tension gashes are useful for measuring the angle of internal friction of a material because tension gashes exhibit both tensile fractures, oriented to the minimum and maximum compressive stresses, and a shear plane. The angle between gashes and the shear plane varies between about 5 and 30°, with a mode of ~20°. Tensile fractures (gashes) propagate in the direction of 0 1, so the angle between the normal to the shear plane and 0 1, 0, is 70°. The angle of internal friction, 9 , is related to 0 by 9 = 20 - 90°, so 9 = 50° (Fig. 16).
Driving the rocks to simultaneous tensile and shear failure requires ~0 mean 44
Figure 16. Mohr Circle diagram for mudstone hydraulic fracturing. Pore pressure (Ppore) was assumed to be ~20 MPa from burial history estimates (Fig. 15). Modern differential stress (odiff) in the Colorado ranges from 2 to 7 MPa, with mode ~5 MPa (Flesch and Kreemer 2010). The angle of internal friction (9 ) was estimated based on the angular relationship between gashes and shear planes in tension gashes. effective stress, which requires pore fluid pressures at least equivalent to the sum of lithostatic stress (confining pressure) and deviatoric stress (Fig. 16). The estimate of pore fluid pressure in the Moenkopi Formation 8-0 Ma is ~20-30 MPa (Fig. 15), which corresponds to 16.5-29 MPa lithostatic stress, or 0.6-1 km depth.
Conclusions
A product in this study is a classification scheme that describes the geometry
(simple to complex), distribution, and mineralogical variety of a vein network. The purpose of the vein network classification is to organize observations in a way that promotes the generation of hypotheses on vein network history. The combined use of ancillary datasets (burial history, modern stress regime, petrographic studies of veins and host rocks, bulk and stable isotope geochemistry, QEMScan, etc.) results in the interpretations presented here. The Moenkopi vein network is classified as a B2a-c vein network because of complex vein geometries, stratigraphic distribution, and the presence 45
of multiple mineral varieties.
Moenkopi veins are typically fibrous satin spar or blocky selenite gypsum with
trace inclusions of calcite. The source of vein fill is primary calcium sulfate beds within
the Moenkopi Formation, indicated by stable isotope similarity between veins and
primary sulfate beds and the stratigraphic distribution of the vein network.
Moenkopi veins were emplaced via hydraulic fracturing, as indicated by the
antitaxial fabric of the veins, composite nature of the veins, and complex geometries and
cross-cutting vein relationships. The Moenkopi vein network was emplaced <8 Ma,
perhaps as early as <1 Ma at <1 km depth, coincident with Colorado Plateau uplift,
indicated by horizontal vein dominance, general orientation of veins with the modern
stress field, vein fibrous texture, deformational (shear) fabrics in vein fibers, and
comparison with a burial history model. As estimated from tension gash kinematic
analysis, Moenkopi mudrocks have ~1.25 MPa tensile strength and ~2.5 MPa shear
strength, with an angle of internal friction of ~50°. Moenkopi sandstones have ~2.5 MPa
tensile strength.
Acknowledgments
I thank the United States Geological Survey, the Geological Society of America,
and the University of Utah Department of Geology and Geophysics for their generous
financial support, and to the Utah Geological Survey’s Core Research Lab for access to
Moenkopi core. Thanks to Drs. Brenda Bowen, Thure Cerling, Peter Lippert, John
Bartley, and Lauren Birgenheier for advice and instruction, and to Drs. Charlotte
Schreiber and Kathleen Benison for discussions on calcium sulfate. I thank Alex Fackrell
and Mark Young for field assistance. 46
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MUDROCKS OF SOUTHERN UTAH: ANALOGS
TO SULFATE VEINS ON MARS
References are written in the style of the Journal of Geophysical Research: Planets
Abstract
Well-exposed gypsum veins in the Triassic Moenkopi Formation in southern
Utah, USA, are similar to veins at Endeavour and Gale craters on Mars. Both Moenkopi and Mars veins are hydrated calcium sulfate, have fibrous textures, and cross-cut other diagenetic features. Moenkopi veins are stratigraphically localized and have similar Sr and S isotope ratios as primary Moenkopi sulfate beds and are interpreted to be sourced from within the unit. Endeavour veins seem to be lithologically localized and may have a local source. Gale veins are stratigraphically distributed and appear to have been sourced from another stratigraphic unit.
Evaluation and comparison of vein networks indicate horizontal Moenkopi veins are longer and thicker than vertical veins and the veins are generally oriented with the modern stress field, so are thus interpreted to have formed in the latest stages of exhumation. Endeavour veins appear to be generally vertical and oriented parallel to
Cape York, and are interpreted to have formed in response to topographic collapse of the crater rim. Horizontal Gale veins appear to be slightly more continuous than vertical veins and may have formed during exhumation. Abrupt changes in orientation, complex 52 cross-cutting relationships, and fibrous (antitaxial) texture in Moenkopi and Mars veins suggest emplacement via hydraulic fracture at low temperatures. Both Moenkopi and
Mars veins are interpreted as late-stage diagenetic features (<8 Ma for Moenkopi, <3 Ga for Mars). Moenkopi and Mars veins show little evidence for alteration and thus have remained at low temperatures since emplacement and survived due to dry desert conditions conducive for preservation.
Moenkopi veins are good terrestrial analogs for Endeavour and Gale crater veins because their geometry and textures record information about crustal deformation, the emplacement and alteration history of the veins, and their mineralogy constrains them to a calcium- and sulfate-enriched source.
Introduction
Hydrous calcium sulfate veins cross-cut tidal, marginal marine mudrocks of the
Lower Triassic Moenkopi Formation and show remarkable similarities to sulfate veins that cross-cut the fluvial and lacustrine sedimentary rocks at the rim of Endeavour Crater and inside Gale Crater, Mars [Squyres et al., 2012; Nachon et al., 2014]. The geometry, chemistry, mineralogy, and distribution of the well-exposed Moenkopi vein network may be valuable to interpreting similar Mars veins (Fig. 17) [Squyres et al., 2012; Grotzinger et al., 2014; Nachon et al., 2014].
The purpose of this study is to assess the usefulness of hydrous calcium sulfate veins in the Moenkopi Formation as analogs to those found at Endeavour and Gale craters, Mars, to better understand the diagenetic and deformational history of fine grained deposits on the red planet.
The Moenkopi Formation was selected for an analog study to Mars because it (1) 53
Figure 17. Calcium sulfate veins on Earth (a-b) and on Mars (c-d) (Mars images courtesy NASA/JPL-Caltech). (a) Veins in the upper red member of the Moenkopi Formation in St. George, Utah. (b) Veins in the Moody Canyon Member (upper red equivalent) near Torrey, Utah. (c) “Homestake” vein at the base of Cape York, Endeavour Crater (Opportunity, Pancam, sol 2769), (d) “Missoula” contact zone in the Pahump Hills (Curiosity, MAHLI, sol 1031). is cross-cut by light-toned calcium sulfate veins, (2) has extensive exposure, (3) is fine grained like the most gypsiferous of vein-bearing Mars rocks, and (4) is set in the well- exposed context stratigraphy of the Colorado Plateau, where basin history can be examined and compared to Mars rocks.
Hypotheses
This study focuses on four hypotheses to explain the occurrence and origin of hydrated calcium sulfate veins on Earth and Mars, as well as to assess the usefulness of
Moenkopi examples as analogs to Mars veins.
1. Moenkopi and Mars veins are sourced from local primary deposits.
2. Moenkopi and Mars veins are products of hydraulic fracturing at low confining
pressures. 54
3. Moenkopi and Mars veins are late-stage diagenetic features that formed during
regional uplift and unroofing.
4. The extent of Moenkopi examples provides valuable textural and geometric data
to interpret Mars gypsum veins.
Geologic Context
Earth, Moenkopi Formation
The Moenkopi Formation is well exposed on the Colorado Plateau within parts of
Utah, Nevada, and Arizona. The unit thickens westward from 0 m at the Utah-Colorado border to >300 m near Torrey, Utah, in the western Capitol Reef area, and is thickest in southern Utah at ~900 m [Blakey, 1973, 1974; Sorber, 2007; Biek et al., 2009; Biek et al.,
2010; Morris et al., 2010]. The Moenkopi Formation is characterized by predominant mudstone interbedded with siltstone, sandstone, and gypsum beds that represent tidal marginal marine and sabkha environments at the edge of the Early Triassic epeiric sea
(Fig. 18) [Blakey, 1973, 1974; Morales, 1987; Biek et al., 2010]. Gypsum beds that vary in thickness from 0.1 to 1 m and gypsum “stringer” veins are present in the Moenkopi
Formation’s lower and upper mudstones [Morris et al., 1897; Biek et al., 2009].
The Moenkopi Formation is studied at two field localities: in St. George (SG) in southwestern Utah and near Torrey (TO), south-central Utah (Fig. 18). The SG area utilizes a roadcut along Banded Hills Drive that exposes 100+ m of upper Moenkopi redbeds, and is located at 37.045°N, 113.519° W (NAD 27). The TO area is located near the towns of Torrey and Teasdale, Utah, located near 38.28° N, 111.44°W (NAD 27) and includes exposures of all but the lowest portions of the Moenkopi Formation.
Facies changes result in many nomenclatures for Moenkopi strati graphic 55
Figure 18. Measured stratigraphy of the Moenkopi Formation at the St. George (SG) and Torrey (TO) field sites. The SG column represents the equivalent of the TO Moody Canyon Member. 56 members, but this study will utilize nomenclature used in the TO area in south-central
Utah, near Capitol Reef National Park, briefly described in ascending order. The Black
Dragon Member is composed of pebble conglomerates at its base and red, gypsiferous mudstone and siltstone. The Sinbad Limestone is composed of algal, fossiliferous, and oolitic limestones and is not gypsiferous. The Torrey Member is composed of calcite- and dolomite-cemented sandstone and siltstone and is not gypsiferous. The Moody Canyon
Member is composed of gypsiferous mudstone, siltstone, and sandstone and is the focus of this study.
Regional unconformities bound the bottom and top of the Moenkopi Formation.
The Late Triassic Chinle Formation overlies the Moenkopi Formation and is composed of a basal coarse lithic sandstone to pebble conglomerate, argillaceous mudstones, siltstones, and terrestrial carbonates. About 2 km of eolian and fluvial Jurassic strata separate the Moenkopi Formations from the Carmel Formation. The Carmel Formation is a marginal marine Middle Jurassic unit composed of mudstone, siltstone, sandstone, limestone, and gypsum.
Mars, Endeavour Crater
The Mars Exploration Rover (MER) Opportunity landed on Meridiani Planum in
2004. Opportunity has examined Meridiani sandstones and mudstones at localities such as Eagle, Endurance, and Victoria craters [Grotzinger et al., 2005; McLennan et al.,
2005] and reached the rim of Endeavour Crater in 2011 (Sol 2764). Endeavour is a simple crater, 22 km in diameter, 100-200 m deep, and located at 2.1° S, 354.8° W (Fig.
19a, b). Opportunity has yet to enter Endeavour Crater (it began its descent into
Endeavour via Marathon Valley in 2015), but has made numerous observations along the 57
—vc ‘ . - N. a '' °iryse ■: r ^ |; \\ Jtopia Planum Ely^tum n a m tia v-c.v- . niquidA rabia kciid ' ZUn kiIN ■ , i k - . - ■ " V uoie crater , Endeavour Crater • - 11 StK-'fcj;.; (Curiosity) 'i 0 Valles ‘ ■' . ■ (Opportunity) ' : V ‘.>v w vy> X . Kr :.V. . , ■M’ iesperia' - ’ /" ' -Ul 1 3 Planita lerra :0S D . s Chnmeri a > S' ' i p . .1 It 1.91678 S \ Landing
Cape York Endeavour MER Opportunity Crater
r-s lo lo 2.51363 S 5 Figure 19. Mars “field” locations (b) Endeavour Crater and (c) Gale Crater. (courtesy Google Mars with imagery from NASA’s Mars Global Surveyor satellite and HiRISE imagery, NASA/USGS/JPL-Caltech). (a) Endeavour and Gale are in opposite hemispheres and near the Mars equator (courtesy JMars from Arizona State University with a shaded relief map from the Mars Orbital Laser Altimeter). crater rim. Notable localities are Homestake, at the base of the crater’s western rim, and on Cape York, a ~1.5 km-long mountain on the crater’s western rim (Fig. 19b) [Squyres et al., 2012; Farrand et al., 2014; Crumpler et al., 2015].
Meridiani stratigraphic units, in ascending order, include the Matijevic,
Shoemaker, Grasberg, and Burns formations (Fig. 20) [Grotzinger et al., 2005;
McLennan et al., 2005; Arvidson et al., 2014a; Arvidson et al., 2014b; Crumpler et al.,
2015]. The Matijevic and Shoemaker formations appear to be Noachian in age [Squyres et al., 2012; Crumpler et al., 2015]. The Grasberg and Burns formations were probably deposited during Noachian or Hesperian time. 58
Figure 20. Mars stratigraphy of Endeavour (left) and Gale craters (rover, center and CRISM, right). Gale stratigraphy from rover observations is arranged by elevation, although the arrangement may not be representative of stratigraphic relationships. Red areas are qualitative estimates of vein density, based on published data and observations from this study). 59
The Matijevic formation is a >1 m-thick sulfate-rich unit of fine-grained, finely layered, light-toned rocks with dark veneers (interpreted as surface varnishes) [Arvidson et al., 2014a, 2014b]. The Shoemaker formation is a 3-5 m-thick unit composed of coarse, unbedded, or massively bedded ejecta blocks and breccias, sometimes with clast alignment and fine-scale, sulfate-rich laminations in the matrix [Squyres et al., 2012;
Arvidson et al., 2014a, 2014b; Crumpler et al., 2015]. Brecciation probably occurred during the Endeavour impact and breccias were transported or reworked through fluvial processes [Squyres et al., 2012; Arvidson et al., 2014a; Crumpler et al., 2015]. The
Grasberg formation (“Deadwood”) is a 2.5 m-thick fine-grained, platy unit for which lithology is not well constrained but it is darker near its base and lighter and more resistant near its upper contact [Squyres et al., 2012; Crumpler et al., 2015]. The
Grasberg formation is probably derived from Shoemaker formation material and formed after most of the erosion of Endeavour Crater [Squyres et al., 2012; Crumpler et al.,
2015]. The Burns formation is the principal sandstone that forms the rocks of Meridiani
Planum and is composed of coarse to medium, well-sorted, sulfate-rich basaltic sandstones with mm-scale laminations and eolian cross-bedding [Grotzinger et al., 2005;
McLennan et al., 2005; Squyres et al., 2012; Crumpler et al., 2015].
Hydrous, fibrous (reportedly antitaxial) calcium sulfate veins cross-cut the
Matijevic, Grasberg, and Burns formations [Squyres et al., 2012; Crumpler et al., 2015].
Gale Crater
The Mars Science Laboratory (MSL) rover Curiosity landed inside Gale Crater in
2012. Gale is a complex crater, 154 km in diameter, ~1800-2750 m deep, and located on the border of Terra Cimmeria at 5.37° S, 137.81° W (Fig. 19a, c) [Grotzinger et al., 60
2014]. The mountain at the crater center is Aeolis Mons but may be better known as
Mount Sharp and is ~1905-3070 m higher than the crater floor [Grotzinger et al., 2014;
Stack et al., 2016]. Most observations through Curiosity are near and on the northwestern side of Mount Sharp. Key localities include Yellowknife Bay and the Pahrump Hills
[Nachon et al., 2014; Stack et al., 2014; Vaniman et al., 2014; Kronyak et al., 2015; Stack et al., 2016].
Broad spectral units discerned for Gale Crater with Compact Reconnaissance
Imaging Spectrometer for Mars (CRISM) satellite are the sulfate-rich lower formation and the upper formation of undetermined composition [Milliken et al., 2010; Stack et al.,
2016]. The lower formation is separated into the sulfate-rich lower member, sulfate and clay-rich middle member, and sulfate-rich upper member [Stack et al., 2016].
Data from Curiosity are used to describe Gale rocks by lithology and texture. The only formation designated in Gale Crater is the informal Yellowknife Bay formation, subdivided into the Sheepbed, Gillespie Lake, and Glenelg members (Fig. 20). Overlying the Yellowknife Bay sediments are units distinguished by visual description only, including “bright outcrop,” “bright bedded outcrop,” “bright striated outcrop,” and
“resistant cratered outcrop” [Stack et al., 2016]. The Yellowknife Bay formation appears to be close to the Noachian-Hesperian boundary, with overlying sediments likely Early
Hesperian in age [Grotzinger et al., 2014].
The Yellowknife Bay formation is a fluvio-lacustrine unit of hydrated clay- and sulfate-bearing basaltic mudstone, sandstone, and pebble conglomerate. The composition of Yellowknife Bay formation rocks is consistent with derivation from the upper Martian crust [Grotzinger et al., 2013; Grotzinger et al., 2014]. Hydrated calcium sulfate veins 61 cross-cut nearly every depositional unit in Gale Crater but are most intense in the
Sheepbed member of the Yellowknife Bay formation [Nachon et al., 2014; Kronyak et al., 2015].
Methods
Terrestrial Field Methods
Field data were collected by measuring stratigraphic section, structural geometries, and mineral fabrics with a Brunton compass. Vein and mudrock samples were collected for petrologic and chemical analyses.
Two-dimensional vein geometries were evaluated in the Moenkopi Formation though scaled image analysis. Measurements are accurate to 0.5 cm for most measured sections, and potential error does not exceed 1 cm. This error results in a resolution bias toward thicker veins when veins are thinner than 1 cm. Vein density is a 2D measurement and is the fraction of vein and nodule area over total measured area. A total of 873 veins and 784 vein intersections were measured in 56.5 m2 of SG cliff face. A total of 1477 veins and 1845 intersections were measured in 67.5 m2 of TO area cliff face. In both field areas, measured cliffs were at a variety of orientations to represent the 3D system.
Intersections were divided into three general groups: terminations are where one vein abuts or terminates against another, splits are where a composite vein divides into simpler veins, and bends are where one vein significantly changes orientation, such as a horizontal vein becoming a vertical vein. 62
Terrestrial Laboratory Methods
Minerals were identified by X-ray powder diffraction. A total of 21 thin sections were created and polished for QEMScan (Quantitiative Electron Microprobe Scan) analysis for host rock, vein, and microfabric characterization. QEMScan passes an electron beam over a prepared thin section or rock plug that scatters when it strikes the target, returning a value to a sensor that roughly corresponds to the types and relative quantities of atoms in each pixel. QEMScan is not sensitive to hydrogen so cannot determine mineral hydration state.
Bulk rock and trace element geochemistry was performed on 35 samples (12 SG,
22 TO, 1 core). The Sr isotope geochemistry was analyzed for 36 samples (13 SG, 23
TO) and S isotope geochemistry for 35 samples (13 SG, 22 TO).
Terrestrial Burial History Diagram
A burial history diagram of the TO area was created using PetroMod, using data collected as part of this work, supplemented with published literature on stratigraphic thickness, depositional history, exhumation history, and other timing data and constraints
[Dumitru et al., 1994; Sprinkel, 1994; Mathis, 2000; Davis and Bump, 2009; Karlstrom et al., 2014]. Many assumptions go into the construction of a burial history diagram (i.e., simplification of lithology and thickness of units that are missing above the modern topographic surface), but the model provides some insight to the evolution of the system and placement of tectonic and diagenetic events.
Mars Methods
Previous work on Mars rocks and calcium sulfate veins, including descriptions of their occurrence, textures, and chemistry, has been accomplished through the Opportunity 63 and Curiosity rovers [Madden et al., 2004; Grotzinger et al., 2005; McLennan et al.,
2005; Bish et al., 2006; Milliken et al., 2010; Grotzinger et al., 2013; Leshin et al., 2013;
Arvidson et al., 2014a, 2014b; Grotzinger et al., 2014; McLennan et al., 2014; Ming et al., 2014; Nachon et al., 2014; Stack et al., 2014; Vaniman et al., 2014; Vasavada, 2014;
Crumpler et al., 2015; Kronyak et al., 2015; Marshall, 2015]. Analysis of Mars data is therefore largely a literature review, with some new comparisons of Mars vein textures and geometries.
Opportunity, at Endeavour Crater, is equipped with a panoramic camera mounted on a short post or mast (Pancam), and a Microscopic Imager camera (MI). Opportunity is also equipped with a suite of spectrometers, including the Mossbauer spectrometer
(MIMOS II) and alpha particle X-ray spectrometer (APXS) for mineralogical and geochemical tests.
Curiosity, at Gale Crater, is equipped with a camera mounted on a short post or mast (Mastcam), and a Mars Hand Lens Imager camera (MAHLI). Curiosity is also equipped with a series of spectrometers, among which are the Sample Analysis at Mars
(SAM) instrument and the Chemistry and Mineralogy X-ray power diffraction and fluorescence instrument (CheMin).
Two-dimensional measurements were attempted using Mars imagery but were restricted to vertical faces. Vertical outcrops have thus far been rare on Mars, so the number of observations is small (180 veins and 125 vein intersections were measured).
Some Mars images are difficult to scale and orient, so most vein lengths, thicknesses, etc. were measured relative to each other rather than to an absolute scale. 64
Results
Comparative results of Earth and Mars veins are presented in the order of (1) vein and encasing host rock chemistry and mineralogy, (2) vein geometry, and (3) other diagenetic features and their relationships with veins. For each body of results, the
Moenkopi Formation on Earth will be addressed first, followed by Endeavour and Gale craters on Mars.
Calcium Sulfate
Anhydrite, bassinite, and gypsum are respectively increasingly hydrous phases of calcium sulfate (CaSO4, CaSO4^.5H2O, and CaSO4^2 H2O). The calcium sulfate crystal lattice requires Ca2+ and SO42-. The Ca2+ in the crystal lattice is commonly replaced with another divalent cation (i.e., Ba2+ or Sr2+). The SO42- can be a product of sulfide oxidation and iron reduction [Bain, 1990; Seal, 2000; Balci et al., 2007; Mazumdar et al., 2008].
The formation of gypsum or anhydrite on Earth requires abundant S and high salinities
[Murray, 1964; Bain, 1990]. The low surface temperature, pressure, and humidity at
Mars’ surface favors anhydrite [Yamamoto, 1969].
Earth, Moenkopi Formation
Mineral analysis from XRD (n = 14), bulk geochemistry (n = 34), QEMScan (n =
8), and thin section petrography (n = 21) reveal that calcium sulfate present in the
Moenkopi Formation is gypsum with trace inclusions of anhydrite, bassinite, and/or calcite. The principal cement in vein host rock is gypsum with trace inclusions of calcite.
Mudrocks are quartz- and feldspar-rich and clay-poor.
Three common varieties of gypsum are opaque fibrous to elongate blocky satin 65 spar (Fig. 21a), translucent to transparent glassy or sheet-like selenite, and opaque microcrystalline to sugary alabaster (Fig. 22a). Almost all gypsum veins and nodules are
satin spar, few veins are selenite, and gypsum beds are alabaster. Vein fibers show antitaxial (outward growing) texture, with crystal coarsening toward the vein margin, fibers continuous across the center of the vein, and occasionally incorporation of fragments of wall rock [Bons andMontenari, 2005; Bons et al., 2012].
Isotopic analyses are used to trace S and divalent cations (Sr2+, Ca2+) between gypsum vein, gypsum bed, encasing host rock, and other samples. Gypsum veins, nodules, and mudstones in the Moenkopi Formation have similar Sr and S isotopic ratios as primary calcium sulfate beds within the formation (Fig. 23). Sandstone from the Late
Triassic Chinle Formation, gypsum from the Jurassic Carmel Formation, and modern pedogenic gypsum are each different from Moenkopi samples in Sr and S isotopic ratios.
Mars, Endeavour Crater
At Endeavour Crater, the Homestake vein, other light-toned veins, and mineralized nodules are predominantly CaSO4 in a hydrated or semihydrated state with a
Earth,TO-S Mars, Endeavour Crater,"Homestake” Mars, Gale Crater, Pahrump Hills,"Garden City” Opportunity, Pancam (inset Ml), sol 2764-2766 Curiosity, MAHU, sol 946 Figure 21. Fibrous textures in (a) Moenkopi veins at the TO field site (TO-S locality), (b) the Mars “Homestake” vein at the base of Cape York, Endeavour Crater (Opportunity, Pancam, inset MI, sols 2764-2769), and (c) Mars veins at “Garden City” in the Pahrump Hills, Gale Crater (Curiosity, MAHLI, sol 946) (Mars images courtesy NASA/JPL-Caltech). 66
Figure 22. Nodular “chickenwire” texture in gypsum beds at the SG field site (a) and diagenetic nodule clusters at the “Selwyn” target site in Gale Crater, Mars (Curiosity, MAHLI, sol 154, courtesy NASA/JPL-Caltech).
o-1' o-‘ o-1' o-1' o-1' o- 87Sr/86Sr Figure 23. 87Sr/86Sr and S34S c d t stable isotope results from the Moenkopi Formation and comparative samples. Two samples do not appear on this graph because of unknown S34S. An algal boundstone from the Sinbad Limestone has 87Sr/86Sr 0.7079. A sample of Shinarump Conglomerate from the St. George area has 87Sr/86Sr 0.7103. The ranges of Early Triassic, Middle Jurassic, and modern seawater are shown [after Veizer, 1989; Strauss, 1997; Worden, 1997; Strauss, 1999; McArthur et al., 2001; Kampschulte and Strauss, 2004; and Strauss, 2004]. 67 disorganized, heterogeneous distribution of hydration phases [Squyres et al., 2012;
Arvidson et al., 2014a; Crumpler et al., 2015]. The Grasberg formation has subtly elevated levels of CaO, FeO, and SO3 as measured by Opportunity’s APXS instrument, and contains gypsiferous cement and gypsum veins [Squyres et al., 2012; Arvidson et al.,
2014a; Crumpler et al., 2015]. Veins are fibrous and light-toned, so appear to be satin spar (Fig. 21b). Veins are reportedly antitaxial [Squyres et al., 2012], though definitive evidence for antitaxial texture, beyond fibrous texture, is currently lacking.
Endeavour veins were precipitated from relatively dilute (water activity ~0.98), sulfate-rich fluids at a low temperature, and were precipitated closest to the Noachian source rocks rather than other sulfates or chlorides [Squyres et al., 2012; Arvidson et al.,
2014a].
Mars, Gale Crater
Veins that cross-cut much of the Yellowknife Bay formation in Gale Crater are light-toned and composed principally of calcium sulfate minerals that are often, but not always, hydrous (anhydrite and bassanite) [Grotzinger et al., 2014; McLennan et al.,
2014; Ming et al., 2014; Nachon et al., 2014]. CheMin and SAM were used to perform geochemical and XRD analyses on Yellowknife Bay sedimentary rocks (n = 1405), and it was found that they contain an elevated level of SO3 as well as calcium sulfate, and that the rocks, which are primarily clastic basalts, have experienced some alteration
[McLennan et al., 2014]. Veins are fibrous and light-toned, so appear to be satin spar or fibrous anhydrite (Fig. 21c). Gale veins are reportedly antitaxial [Nachon et al., 2014], though definitive evidence for antitaxial texture, beyond fibrous texture, is lacking. 68
Vein Geometry
Earth, Moenkopi Formation
Moenkopi veins are stratigraphically distributed with a weaker lithologic control of vein extent. Veins are not present in the uppermost ~10 m of the Moenkopi Formation at both field sites.
Veins in the Moenkopi Formation are generally subhorizontal or vertical, with a generally N-S strike in both field areas (n = 355). Most veins are sub-bedding-parallel, but rarely perfectly bedding-parallel. Total horizontal vein length and vein thickness is three times that of vertical vein length, with a H/V (horizontal/vertical) ratio of 3.1 for both metrics (n = 2350).
Most Moenkopi vein intersections (47%) are where one vein terminates against another (n = 2565). Many Moenkopi vein intersections (24%) are splits, where two adjoined veins separate from each other. Many Moenkopi vein “intersections” are bends, where a vein changes orientation (Fig. 24a). Vein intersections are subtly horizontally dominant, or the more horizontal vein at the point of intersection tends to be the more continuous of the two intersecting veins with a H/V dominance ratio of 1.2.
Figure 24. Bends in in Earth (a) and Mars (b and c) examples: (a) Moenkopi veins at the SG field site, (b) “Ortiz” target at the “Whitewater Lake” locality on Cape York, Endeavour Crater (Opportunity, MI, sol 3189), and (c) ”Missoula” contact zone in Pahrump Hills, Gale Crater (Curiosity, MAHLI, sol 1031) (Mars image courtesy NASA/JPL-Caltech). 69
Veins are commonly composite veins, meaning that two or more distinct veins are coupled or adjoined such that they appear to share the same fracture aperture or opening.
The composite nature of many veins becomes apparent at vein intersections where one vein splits from the other, sometimes at gentle angles (splits, 24% of vein intersections), and sometimes at sharp elbows (51% of split intersections). A thin layer of clay or mud commonly separates vein parts in a composite vein. Vein partings vary in thickness from
<1 mm to incorporating >1 cm-wide bodies of wall rock.
Crystal fibers are continuous across simple veins (Fig. 21a), but not across vein parts in a composite vein. Most fibers are not perpendicular to the vein wall by <10°.
Some veins exhibit shear fabrics (Fig. 25).
Mars, Endeavour Crater
Veins on Cape York have a mean width of 2 cm, exposed length of 33 cm, and are commonly segmented every 5 cm (n = 37) [Crumpler and Athena Science Team,
2012; Squyres et al., 2012; Crumpler et al., 2015]. Veins are oriented subparallel to the margins of Cape York [Crumpler and Athena Science Team, 2012; Squyres et al., 2012;
Crumpler et al., 2015]. Some veins abruptly change orientation (Fig. 24b). Shear fabrics are not recognized in Endeavour veins.
Figure 25. SG gypsum vein exhibiting a shear fabric in deformed satin spar fibers. 70
Mars, Gale Crater
Sulfate veins are ubiquitous in sedimentary deposits at Gale Crater but are more dense in mudstones. Gale veins have a mean width of 2.3 mm (are usually <2 cm wide), range in exposed length from 1 to 60 cm, and have highly diverse orientations, ranging from subhorizontal to subvertical [Grotzinger et al., 2014; Nachon et al., 2014; Kronyak et al., 2015]. Fracture orientation is complex, including composite veins and elbows
(Figs. 24c; 26) [Grotzinger et al., 2014; Nachon et al., 2014; Kronyak et al., 2015]. In the
Pahrump hills, there appears to be three distinct vein orientations, with the most dominant oriented parallel to the margin of Mount Sharp (n = 108; Fig. 26). Other locations show at least two distinct vein orientations, but their relationship to Mount Sharp is usually unclear [Grotzinger et al., 2014]. Vertical exposures are uncommon at Gale Crater, but
Gale horizontal and vertical veins appear to have a mean H/V ratio of 1.2
Figure 26. Complex vein geometries, including composite veins and bends, in Gale Crater veins at the Pahrump Hills (Curiosity, Mastcam, sol 942, courtesy NASA/JPL-Caltech). The rose diagram is of vein strikes and is oriented to the image, which is oblique to the ground surface. Three orientations appear to be dominant, indicated with red lines on the rose diagram. Scale estimate of figure is ~3 m across. 71 for thickness and 1.4 for exposed length (n = 180). Horizontal veins are more dominant at intersections, with an H/V intersection dominance ratio of 1.3 (n = 125). Shear fabrics are not recognized in Gale veins.
Other Diagenetic Features
Earth, Moenkopi Formation
Locally present gray-green to yellow-green bedding-parallel horizons color sections of the upper Moenkopi Formation [Blakey, 1974; Morales, 1987; Sorber, 2007;
Biek et al., 2009; Morris et al., 2010]. Color horizons are laterally continuous for 10’s to
100’s of meters. The contact between red and green mudrock is commonly irregular and spotty. Green horizons are commonly associated with higher silt content. Sandstones tend to be orange to yellow-tan and are rarely colored green. Color contacts are rarely identical to the lithologic contact. Mudstone adjacent to primary gypsum bed nodules is sometimes colored green.
Reduction spots are also present in many places of the upper Moenkopi and their distribution appears to be stratigraphically controlled. Moenkopi reduction spots are ellipsoidal and exhibit mean 14% bedding-perpendicular shortening (n = 46).
Mars, Endeavour Crater
Dark veneers in the Matijevic formation are associated with Fe3+ smectites and are interpreted as surface varnishes [Arvidson et al., 2014b].
Ferric oxide spherules, <0.5 cm in diameter (mean 4.2 mm, n = 454) are common in the eolian and water-lain sandstones of the Matijevik, Shoemaker, and Burns formations [Chan et al., 2005; McLennan et al., 2005; Squyres and Knoll, 2005; Chan et 72 al., 2012; Crumpler et al., 2015]. These previous studies note that ferric spherules are uniformly distributed, suggesting that they are diagenetic concretions. The ferric spherules probably formed during early diagenesis or burial diagenesis at low temperatures and near-surface conditions [Squyres and Knoll, 2005; Squyres et al., 2012].
Calcium sulfate veins cross-cut spherule-bearing units.
The mineralogy of Meridiani rock cements is not constrained, but appears to involve sulfates such as jarosite, Mg-sulfate, Ca-sulfate, Fe-sulfate, as well as chlorides, ferric oxides, and amorphous silica [McLennan et al., 2005]. Tabular crystal molds <1 cm in length in diverse orientations appear to be diagenetic [McLennan et al., 2005; Squyres and Knoll, 2005; Crumpler et al., 2015].
Mars, Gale Crater
Calcium sulfate nodules, empty nodules, and raised ridges occur in Sheepbed mudstones [Grotzinger et al., 2014; McLennan et al., 2014; Nachon et al., 2014]. Nodule mean diameter is 1.2 mm (n = 4501 filled nodules, n = 1248 hollow nodules), and filled nodules are harder than the surrounding mudstone [Grotzinger et al., 2014; Stack et al.,
2014]. Nodules occur in dispersed clusters, do not form beds, and do not exhibit grading, suggesting that nodules are concretionary and represent a later phase of diagenesis
[Grotzinger et al., 2014; Stack et al., 2014]. Some nodular textures exhibit “chicken- wire” texture that is generated on Earth either by primary displacive growth of sulfates in calcium-sulfate-saturated interstitial fluid during early diagenesis or by dehydration/rehydration during burial/exhumation, and are a product of late diagenesis
[Nachon et al., 2014] (Fig. 22b). Nodules are sometimes intersected by thin sulfate veins
[Grotzinger et al., 2014; Vaniman et al., 2014; Nachon et al., 2014]. Nodule rims may 73 involve iron-bearing compounds [Grotzinger et al., 2014; Stack et al., 2014].
Raised ridges are “narrow, curvilinear ridges that weather in raised relief,” and are restricted to Sheepbed mudstones [Grotzinger et al., 2014]. Ridges are several cm in length, have pointed terminations, are generally subvertical, strike in all directions, commonly intersect at 90°, and have a mean width of 2.7 mm (n = 1619) [Grotzinger et al., 2014]. Most ridges show internal banding [Grotzinger et al., 2014]. Ridges are elevated in Fe and both Fe and Mg are correlated to Cl, whereas the cement banding is primarily Si by weight, followed by Fe and Mg [McLennan et al., 2014]. Raised ridges are interpreted as early diagenetic cracks that formed in response to shaking in consolidated or partially lithified mudstone, then filled with one or more generations of cement that encrust at the margins of the cracks and make the ridges more resistant to weathering than the surrounding mudrock [Grotzinger et al., 2014; McLennan et al.,
2014].
Discussion
The sources of calcium sulfate (hypothesis 1) for veins on Earth and Mars are compared, followed by interpretations of the timing and mode of vein formation
(hypotheses 2 and 3). Finally, the usefulness of Moenkopi veins as analogs to Mars calcium sulfate veins and potential for future directions of study is discussed.
Calcium Sulfate Source
Earth, Moenkopi Formation
Moenkopi samples are similar to each other and distinct from comparative gypsum samples (modern pedogenic gypsum and Jurassic Carmel Formation gypsum 74 from the TO area) in terms of Sr and S isotopes (Fig. 23). The Moenkopi vein network is also stratigraphically rather than lithologically distributed (Fig. 18). There are no gypsum veins in the Sinbad Limestone and Torrey members in the Torrey area, and vein density decreases to 0 in the uppermost ~20 m of the Moenkopi Formation in both field sites.
This lack of gypsum in the uppermost Moenkopi may be related to either distance from source or chemical mixing with fluids in the overlying Chinle Formation. A vein near the contact with the Chinle Formation has similar Sr and S ratios to the Shinarump
Conglomerate sandstone, suggesting that chemical mixing was a factor and that Chinle fluids generally prohibited gypsum formation.
The similarity of Sr and S isotopic ratios between veins, mudrock, and primary calcium sulfate beds, as well as the stratigraphic distribution of the veins, suggests that
Moenkopi calcium sulfate was sourced from primary calcium sulfate beds within the
Moenkopi Formation.
Mars, Endeavour Crater
The Endeavour vein network seems to be densest in mudstones, so perhaps vein distribution is lithologically controlled rather than controlled simply by stratigraphic position. There are no primary sulfate beds yet found by Opportunity. The sulfate cement present in the mudrocks is consistent with acidic alteration of the basaltic host rock
[McLennan et al., 2005; Crumpler et al., 2015], but may have been emplaced at the same time as the veins [Squyres et al., 2012].
The origin and emplacement of the veins remains largely unknown but has been hypothesized to be from sulfate-rich groundwater that resulted in light cementation
[Crumpler et al., 2015] or secondary cementation [McLennan et al., 2005], circulation of 75 hydrothermal fluids [Crumpler et al., 2015], circulation of low-temperature fluids
[Squyres et al., 2012], and postimpact circulation of hydrothermal flow [Arvidson et al.,
2014a]. Because veins are hydrous calcium sulfate, gypsum, rather than anhydrite, they were emplaced at low temperatures, so, if hydrothermal, the source was distant [Squyres et al., 2012]. Alteration from anhydrite to gypsum results in a 30-70% volume increase, which would have distorted or destroyed the fibrous texture of Endeavour veins [Heard and Rubey, 1966; Raugh and Thuro, 2007; Klein and Dutrow, 2008; Jaworska, 2012].
Meridiani Planum and Endeavour Crater sulfate may have originated during acidic fluid interactions with olivine-bearing basalts [McLennan et al., 2005]. Vein fill may have originated from a remobilization of early sulfate cement in the host rock.
Mars, Gale Crater
Similar to Endeavour Crater, the Curiosity rover has not detected any primary sulfate beds. Gale veins are ubiquitous in Curiosity rover observations, so vein distribution may be more related to stratigraphic position than lithology. Calcium sulfate veins cross-cut other, Fe-, Mg-, and Cl-rich diagenetic features, including concretions and raised ridges, so are younger than those features [McLennan et al., 2014]. Proposed sources of calcium sulfate include the dissolution of an early, diagenetic sulfate-rich layer produced from the alteration of basalts (unlikely, because there is no evidence of elevated sulfate in any particular layer) [Nachon et al., 2015], mobilization of sulfates from a stratigraphically higher unit [Nachon et al., 2015], mobilization of sulfates from a stratigraphically lower unit [Grotzinger et al., 2014; McLennan et al., 2014; Nachon et al., 2015], or mobilization of sulfates present laterally [McLennan et al., 2014].
Sulfates seem to have been added during a late diagenetic event rather than a 76 redistribution of elements during an earlier stage of diagenesis because (1) Gale veins cross-cut Fe-, Mg-, and Cl-rich and sulfate-poor raised ridges; (2) thin calcium-sulfate veins seem to intersect and fill nodules; and (3) veins have a fibrous texture that would have been destroyed or altered during calcium sulfate phase transitions [Grotzinger et al.,
2014; McLennan et al., 2014; Nachon et al., 2015]. At present, calcium sulfates in the
Yellowknife Bay formation do not appear to have originated from within the unit.
Comparison
Moenkopi veins are sourced from within the Moenkopi Formation, evidenced by their stratigraphic distribution and Sr and S isotopes. The source of calcium sulfate remains speculative for Endeavour and Gale craters. Sulfate cements may have been emplaced early in Endeavour Crater via acidic fluid alteration of basalt, so remobilization of depositional or early diagenetic sulfate could produce Endeavour vein fill. The lithologic distribution of Endeavour veins may be evidence for this. Gale crater veins cross-cut sulfate-poor diagenetic features, so veins were probably sourced from elsewhere, possibly higher or lower in the stratigraphic column and stratigraphically controlled.
Future isotope studies on Mars may constrain sulfate sources, but Sr isotopes are not likely to be useful on Mars. 87Sr/86Sr differentiation tends to occur where 87Rb is concentrated, resulting in a relatively high 87Sr/86Sr ratio [White, 2013]. Rb tends to be concentrated in continental rocks (i.e., potassium feldspar), but Mars does not have a highly evolved or differentiated crust. 77
Vein Genesis
Veins record kinematic information related to their formation that can be used to infer the dynamics involved in fracture formation. Namely, veins are filled open-mode fractures, such that dilation is experienced in the direction perpendicular to the vein face.
Therefore, the orientation of the least compressive principal stress (03) can be inferred as the “opening direction” of the veins, or perpendicular to the vein wall.
Antitaxial fabric and complex fracture patterns, especially composite veins, inconsistent cross-cutting relationships, changes in vein orientation, and wall rock inclusions in veins are indicative of complex dilation and multiple crack-and-seal events, characteristic of hydraulic fracturing [Ramsay, 1980; Machel, 1985; Engelder, 1990;
Phillip, 2008].
Earth, Moenkopi Formation
When anhydrite hydrates to gypsum, it can experience a ~30-70% volume increase [HeardandRubey, 1966; Rauh and Thuro, 2007; Kelin andDutrow, 2008;
Jaworska, 2012]. This phase transition would have deformed the wall rock as well as the vein fill, but no wall-rock deformation features are associated with the veins, and veins have a pristine, fibrous texture. Veins must have formed at relatively low temperatures
(below 60°C) and thus at shallow depths (<2 km depth). Veins also cross-cut reduction spots that record bedding-perpendicular shortening, probably associated with burial.
Several thousand meters of Jurassic and Cretaceous strata would have buried the
Moenkopi Formation until the uplift of the Colorado Plateau ~30-35 Ma [Dumitru et al.,
1994; Sprinkel, 1994; Mathis, 2000; Pederson et al., 2002; Davis and Bump, 2009;
Moucha et al., 2009; Lavendar et al., 2011; Karlstrom et al., 2014]. The Moenkopi 78
Formation would not have been exhumed to <2 km depth until ~8 Ma (Fig. 27).
Moenkopi veins must have been under enough pressure to produce shear fabrics.
Experiments find that the yield strength of gypsum is as high as 15 MPa when dry and as low as 2 MPa when wet [Castellanza, 2008; Plachy, 2009], or between ~75-560 m depth.
Horizontal veins in the Moenkopi Formation tend to be both thicker and longer than vertical veins and most intersections show horizontal vein dominance, suggesting that 03 was vertically oriented at the time of vein formation. Vertical 03 is consistent with fracture generation during unroofing. However, some vertical veins are very dominant and horizontal veins either terminate against them or join them at an elbow. The dominant vertical veins may be earlier fractures that formed during burial and filled later.
However, cross-cutting relationships are inconsistent and dominant vertical veins are cross-cut by horizontal veins, and vice-versa.
The antitaxial vein fabric, changes in vein orientation, and inconsistent cross cutting relationships suggest complex dilation and multiple crack-and-seal events, indicating hydraulic fracturing [Ramsay, 1980; Bons andMontenari, 2005; Bons et al.,
Figure 27. Modeled burial history of general southern Utah Colorado Plateau stratigraphy since Early Triassic time, applicable to burial in the Torrey area. The red hashed area indicates the time that gypsum veins mostly have been emplaced (<8 Ma). 79
2012].
Mars, Endeavour Crater
Veins at Endeavour Crater are fibrous and lack substantial deformation, suggesting minimal calcium sulfate phase transitions since vein emplacement, suggesting that the rocks that the veins cross-cut have remained at relatively low temperatures since vein emplacement [Squyres et al., 2012]. Because there also appears to be a lack of shear fabrics in the veins, they do not appear to have been buried deeply since emplacement.
Evidence of hydrothermal fluid alteration of postcrater sediments includes the development of phyllosilicates, emplacement of Fe-oxide coatings, acidic leaching, and mineralized veneers in postimpact sediments [Wray et al., 2009; Squyres et al., 2012;
Arvidson et al., 2014a, 2014b; Farrand et al., 2014; Crumpler et al., 2015]. The veins seem to have been emplaced after this earlier phase of alteration, as the mineralizing fluids must have been at relatively low temperatures and, if hydrothermal, distant from the thermal source [Crumpler and the Athena Science Team, 2012; Squyres et al., 2012;
Arvidson et al., 2014a, 2014b; Crumpler et al., 2015].
Veins are orientated subparallel to the margins of Cape York [Crumpler and the
Athena Science Team, 2012], inferring that 03 is oriented radial to Cape York at the time of fracture formation. The shape of Cape York, then, controlled the orientation of tensile strain during fracture formation. Possible mechanisms include the compaction of a thick clastic unit over the preexisting relief of Endeavour Crater rim and the corresponding development of bedding stresses [Crumpler et al., 2015] and compaction and settling of sediments on Meridiani Planum [Crumpler et al., 2015]. However, it seems unlikely that the compaction of Meridiani Planum sediments would generate sufficient horizontal 80 tensile stresses to produce the ~2 cm dilation required for each of the veins around Cape
York, unless the sediments somehow experienced horizontal compaction in additional to vertical (gravitational) compaction. Another mechanism may be gravitational collapse and gravity sliding into the adjacent basin [Young, 2011]. This gravitational collapse could exaggerate the small lateral stresses resulting from the compaction of Meridiani sandstones and open joints parallel to the margin of Cape York. It seems unlikely that
Endeavour veins were emplaced in direct response to the Endeavour Crater impact because veins are oriented along the Cape York gravitationally influenced structure.
Endeavour veins have been interpreted as antitaxial, or outward-growing veins, inferring that they grew simultaneous with, and kept pace with, fracture growth [Ramsay,
1980; Bons andMontenari, 2005; Bons et al., 2012; Squyres et al., 2012]. The evidence for this interpretation is sparse, especially at the resolution of images from Opportunity.
Mars veins generally appear featureless, probably because of wind erosion and their antiquity, but they have a subtle fibrous fabric that is continuous across the center of the vein. Fibrous texture alone does not describe antitaxial veins. However, abrupt changes in vein orientation (Fig. 24b), composite veins, and complex cross-cutting relationships are indicators of multiple crack-and-seal events, suggesting hydraulic fracturing. The potential antitaxial fabric of the veins and the complex vein network support vein growth by hydraulic fracturing, for which compaction during dewatering (simultaneous with migration of mineralizing fluids) is a possible mechanism [Crumpler et al., 2015].
The timing of vein emplacement is difficult to determine because of the lack of absolute dates for host sedimentary rocks. The burial history of Endeavour Crater is unknown, and the time since the emplacement of the host rocks is on a timescale 102-4 81 times greater than for most Earth sedimentary rocks. The veins were emplaced after the deposition of the Burns formation in the Noachian, and probably during late Noachian or early Hesperian time.
Mars, Gale Crater
The complex fracture networks and changing orientation of veins may have developed due to hydraulic fracture that exploited natural weaknesses in the rock (i.e., horizontal veins and between nodules) [Grotzinger et al., 2014; Nachon et al., 2014]. The
Yellowknife Bay formation may be a small part of a thick succession of fluvio-lacustrine sediments that may have buried this part of the Yellowknife Bay formation under hundreds of meters of rock [Grotzinger et al., 2014]. The hydration state and minor elements of the veins suggest that veins were emplaced at <3 km depth under 60°C in relatively nonacidic conditions [Nachon et al., 2014].
Gale vein 03 appears to be vertically oriented, given greater horizontal dominance in vein length, vein thickness, and intersection dominance, perhaps indicating vein emplacement during exhumation. The K/Ar and 36Ar, 21Ne, and 3He cosmogenic dating in Curiosity’s Sample Analysis at Mars (SAM) instrument, in addition to crater counting, produces ages for Sheepbed mudstone basalts of 4.21±0.35 Ga, exposure age of
Sheepbed mudstones of 78±30 Ma, and estimates for Sheepbed mudstone deposition in the 3.6-1.6 Ga [Farley et al., 2014]. Gale Crater veins are late-stage diagenetic features, so have probably been emplaced <1.6 Ga. Estimated exhumation by wind-driven scarp retreat is estimated at ~0.75 m/Myr [Farley et al., 2014]. If this rate of exhumation is representative of the Amazonian history of the region, it would have taken ~1.3 Ga to exhume Yellowknife Bay rocks from a depth of 1 km. 82
Veins and nodules may have been emplaced in four episodes, in a scenario presented by Nachon et al. [2014]: (1) Sulfate-saturated pore fluids from an evaporitic layer supply the fluid overpressure and fracture the rock, then precipitate gypsum into the resultant voids; (2) Nodular “chicken-wire” texture calcium sulfates deposit during an early, restricted stage in fluid circulation; (3) The main emplacement of gypsum and bassanite occurs during late-stage diagenesis; (4) Exhumation slightly modifies and dehydrates the gypsum and bassanite in the veins.
Comparison
Moenkopi, Endeavour, and Gale veins all appear to have been emplaced via hydraulic fracturing at low temperatures and remained at low temperatures since emplacement, evidenced by the hydrous state of veins, fibrous (antitaxial?) texture of veins, and complex vein network geometry (Figs. 21, 24, 26).
The depth at which Mars veins were formed is <3 km. The mechanism that triggered vein formation is probably similar between Moenkopi and Gale veins
(exhumation) but dissimilar to Endeavour veins (gravity collapse?). The timing of Mars veins is difficult to constrain. Moenkopi veins formed <8 Ma, compared to the older
Mars cases of Endeavour <3 Ga, and Gale <1.6 Ga. Overall, vein network geometry is simplest at Endeavour Crater, more complex in the Moenkopi Formation, and most complex at Gale Crater.
Analog Assessment
Three factors pose the greatest challenge to using Moenkopi veins as an analog to
Mars veins: (1) The resolution of Mars vein investigations remains poor and probably 83 will remain weak until thin sections can be made of Mars veins and host rock, (2)
Moenkopi and Mars host rock composition varies greatly and involves different fluid- rock interactions, and (3) the age difference between Moenkopi and Mars veins is large
(<8 Ma vs. <3 Ga).
However, even with limited data many similarities exist, including host rock grain size, vein mineralogy, vein texture, and aspects of vein geometry (Table 5). A comparative study of Moenkopi and Mars vein geometries supports the conclusion that
Mars veins are hydraulically fractured at shallow depths. The Gale Crater vein network seems to have been emplaced during exhumation like the Moenkopi vein network, evidenced by horizontal vein dominance. The lithologic distribution of the Endeavour vein network supports the concept of remobilization of early or primary sulfates in
Endeavour Crater, and the stratigraphic distribution of the Gale vein network supports injection of sulfate from another stratigraphic unit.
Potential for Future Study
Isotopic studies have been performed on Mars sediments with Curiosity’s Sample
Analysis at Mars Tunable Laster Spectrometer (SAM TLS) [Leshin et al., 2013].
Application of this technology for calcium sulfate veins and sulfate-rich deposits may, as in the Moenkopi Formation, constrain the source of vein fill.
Hydrous calcium sulfate (gypsum) veins on Mars seem to be of the fibrous satin spar variety, yet early elongate crystal casts exist in some Mars sedimentary rocks.
Crystal casts may have once hosted selenite [Kah et al., 2015]. The controls on the formation and alteration between gypsum varieties are unknown, and future experiments to this end would prove valuable to interpreting the diagenetic environments under which 84
Table 5. Moenkopi-Mars vein comparison. Fields similar to Earth are shaded. H/V = mean ratio of a metric between horizontal and vertical veins. Large vertical surfaces are not commonly encountered by Mars rovers, so the sample size for H/V is small.______Earth: Moenkopi Mars: Endeavour Mars: Gale Location St. George, UT Meridiani Planum Edge of Terra (NAD27 37.045 N, (-2.1 N, 354.8 E) Cimmeria (-5.37 N, 113.519 W), 137.81 E) Torrey, UT (NAD27 38.28 N, 111.44 W) Units Moenkopi Fm (Black Matijevik, Shoemaker, Yellowknife Bay Dragon, Moody Grasberg, Burns fms formation, CRISM Canyon mbrs) lower formation Host Rock Composition Siliceous, primary Basaltic, Basaltic, sulfates phyllosilicates phyllosilicates Host Rock Grain Size Mudstone, siltstone, Mudstone, sandstone Mudstone, sandstone sandstone Depositional Environment Tidal flat, sabkha Impact (breccia), Fluvial, lacustrine fluvial, lacustrine Age of Host Rock Early Triassic Late (?) Noachian Late Noachain-Early (~245 Ma) (~3800 Ga?) Hesperian (~3700 Ga) Diagenetic Features Ca-sulfate veins and Ca-sulfate veins, Fe Ca-sulfate veins and nodules, green concretions nodules, Fe banding and reduction concretions, raised spots ridges Vein Stage of Diagenesis Late (Telogenesis) Late (Telogenesis) Late (Telogenesis) Vein Density (% 2D High (5-15%) Very Low (0-1%) High (5-15%) coverage) Vein Thickness (H/V) 3.1 (mean 0.86 cm) 1 (mean 2 cm) ? (mean 2.3 mm) Vein Exposed Length 3.1 (mean 45 cm) 1.2 (mean 33 cm) ? (range 1-60 cm) (H/V) Intersection Dominance 1.2 ? 1.2 (H/V) Vein Geometry Notes Elbows, oriented with Oriented parallel to Elbows, random (?) modern stress field margin of Cape York orientations Mineral Gypsum Gypsum, bassanite, Gypsum (?), bassanite, anhydrite anhydrite Vein Textures Fibrous, antitaxial Fibrous Fibrous CaSO4 Source Primary sulfates Alteration of basalt, Alteration of basalt or within the unit remobilization of mobilization of sulfate cements, and/or sulfates outside the mobilization of unit primary sulfates outside the unit Mode of Vein Generation Hydraulic fracture Hydraulic fracture Hydraulic fracture Triggering Event Exhumation Compaction of Exhumation (?) Meridiani sediments, gravitational collapse of crater rim (?) Age of Veins <8 Ma <3.8 Ga <1.6 Ga 85 cast and vein minerals were emplaced, both for the Moenkopi Formation and for Mars.
Further rover explorations will continue to uncover valuable information regarding the stratigraphy of Endeavour and Gale craters, which may yield some constraints on burial history as well as expose the depositional and diagenetic history on
Mars, especially as Opportunity continues to descend into Endeavour Crater and
Curiosity ascends Mount Sharp. Detailed studies on the composition, geometry, and variety of calcium sulfate veins will provide insight on the diagenetic environment and structural history of the regions in which they are emplaced. Rover image orientation is presently unfeasible to determine in the current data format—published workflows on the process of rover image orientation or including true image orientation in data files is necessary in order to reliably measure the orientations of geologic features on Mars.
Conclusions
Antitaxial hydrous calcium sulfate veins are common in the Moenkopi Formation.
Endeavour Crater and Gale Crater veins are also hydrous calcium sulfate and fibrous
(probably antitaxial, but this requires more convincing evidence to determine). Moenkopi veins are stratigraphically distributed and have similar isotopic ratios to primary sulfate beds, so are most likely sourced from within the unit. A calcium sulfate source is difficult to constrain for the Mars localities, but the distribution of Endeavour veins appears to be controlled by lithology and may be sourced from remobilization of early diagenetic cement, and Gale veins appear to be stratigraphically distributed and sourced from an as- yet unobserved stratigraphic unit. Isotope analyses may constrain calcium sulfate sources for Mars veins as they do for Moenkopi veins.
Moenkopi and Mars veins were emplaced at low temperatures via hydraulic 86 fracturing during late-stage diagenesis. The Moenkopi Formation was buried several kilometers before being exhumed to <1 km, where veins formed in an environment where gypsum is stable. Endeavour Crater and Gale Crater veins also appear to have formed via hydraulic fracturing, as indicated by fibrous vein texture and complex vein geometry that is similar to Moenkopi vein geometry. The depth to which Endeavour Crater deposits were buried is unknown, but Gale Crater deposits appear to have formed during exhumation, as indicated by horizontal vein and intersection dominance, similar to
Moenkopi veins.
Moenkopi veins are useful analogs to Mars veins so long as they are used to better understand crustal strain on Mars (i.e., gravity collapse at the rim of Endeavour Crater and exhumation at Gale Crater) and to identify a source of calcium sulfate. However, caution should be taken when using Moenkopi veins to understand Mars fluid flow history because of different host rock compositions, varying calcium and sulfate sources, and vastly different timescales since vein emplacement.
Acknowledgments
Thanks go to the United States Geological Survey, Geological Society of
America, and University of Utah Department of Geology and Geophysics for their generous financial support. Thanks to Drs. Brenda Bowen, Thure Cerling, Peter Lippert,
John Bartley, Lauren Birgenheier, Carol Schreiber, and Kathleen Benison for advice and discussion. Thanks to Alex Fackrell and Mark Young for field assistance.
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WAYNE COUNTY, UTAH
Citations and references are in the style of the Geological Society of America
Abstract
An updated map of the Torrey 7.5’ quadrangle, Wayne County, Utah includes mapping and descriptions of extensive Triassic through mid-Jurassic sedimentary,
Noegene igneous, and covering Quaternary units. Field observations and geochemical methods were used to evaluate gypsum veins and diagenesis in Triassic Moenkopi redbed deposits, with the conclusion that veins in the Moenkopi Formation are late-stage diagenetic features sourced from primary calcium sulfate deposits within the unit.
Mapping and cosmogenic 3He dating of alluvial surfaces provide a more nuanced picture of geomorphic evolution in the Pleistocene and add to hazards identification. Alluvial surfaces were emplaced ~100-550 ka and represent episodes of mass wasting and fluvial activity. The age of a basaltic dike was determined to be 5.29±0.04 Ma.
Introduction
The Torrey 7.5’ quadrangle is located in Wayne County, southern Utah, west of
Capitol Reef National Park, and on the westernmost edge of the Colorado Plateau (Fig.
28). It contains the towns of Torrey and Teasdale in a valley through which the Fremont
River flows. The most recent geologic map of the quadrangle was published in 1957 at 94
111°30'0"W 111°15'0"W
111°30'0"W 111°15'0"W Figure 28. Locality map of the Torrey Quadrangle (GIS data from AGRC, 2011; AGRC, 2013; RITA/BTS, 2006; and USGS/AGRC, 2003). the 1:24,000 scale, when the quadrangle was then named the Notom 2 SW quadrangle
(Smith et al., 1957). The adjacent quadrangle to the east, the Twin Rocks quadrangle, was published in 2007 at the 1:24,000 (Sorber et al., 2007). The area was mapped at 1:62,500 scale in the geologic map of Capitol Reef National Park (Billingsley et al., 1987) and at
1:62,500 scale as the Loa 30’ X 60’ quadrangle (Doelling and Kuehne, 2007).
The Torrey quadrangle contains exposures of lower Triassic through middle
Jurassic sedimentary strata (Fig. 29), which underlie Quaternary units. The quadrangle straddles the transition from Basin and Range province to the west and the Colorado
Plateau to the east. Outcrop exposures are primarily Jurassic and Triassic sedimentary rocks (Figs. 29-31) and Quaternary deposits, with some igneous intrusive rocks also exposed (Smith et al., 1967; Billingsley et al., 1987). The Thousand Lakes fault zone is 95
Figure 29. Mesozoic stratigraphy, looking south from the north side of the Fremont River with the Cockscomb indicated in yellow (a) and looking north from atop a hill of Navajo Sandstone next to Black Ridge, south of Highway 24 (b). White lines = lithologic contacts. Black lines = ridges that separate the foreground from the background. Moenkopi Fm: Trmb = Black Dragon Mbr, Trms = Sinbad Limestone Mbr, Trmt = Torrey Mbr, Trmm = Moody Canyon Mbr; Chinle Fm: Trcs = Shinarump Conglomerate Mbr, Trcm = Monitor Butte Mbr, Trcp = Petrified Forest Mbr, Trco = Owl Rock Mbr; JTrw = Wingate SS; Jk = Kayenta Fm; Jn = Navajo SS; Temple Cap Fm: Jtm = Manganese Wash Mbr; Carmel Fm: Jcp = Co-op Creek, Crystal Creek, and Paria River mbrs. 96
Figure 30. Middle to Lower to Jurassic stratigraphy expressed in the Torrey quadrangle, with paleoenvironment interpretations at the far right. 97
Figure 31. Middle Jurassic to Lower Triassic stratigraphy expressed in the Torrey quadrangle, with paleoenvironment interpretations at the far right. 98 immediately to the west of the mapping area, and that area exhibits a number of NE-SW- trending normal faults. The Teasdale fault strikes WNW-ESE and passes through the town of Teasdale, Utah (Smith et al., 1957; Billingsley et al., 1987). A number of other
NW-SE-striking faults cut the Torrey area.
Description of Map Units
The Triassic units are mainly exposed in the central parts of the quadrangle, and are principally exposures of the tidal to deltaic-flood plain Moenkopi Formation (Fig. 32) with significant exposure of the fluvial-lacustrine Chinle Formation and eolian Wingate
Sandstone in the northern parts of the quadrangle (Smith et al., 1957; Billingsley et al.,
1987; Dubiel, 1989; Mathis, 2000; Morris et al., 2010; Doelling and Kuehne, 2007).
Jurassic units are principally exposed in the northern and southern parts of the quadrangle
Figure 32. The Moody Canyon Member of the Moenkopi Formation, showing typical features of gypsum veins, bedding-parallel green banding, and “columnar” cliffs. A small section of veins is annotated in the left portion of the image—this dense network of veins cross-cuts the entire cliff. 99 and include the fluvial Kayenta Formation, eolian Navajo Sandstone, eolian to marginal marine Temple Cap Formation, and marine Carmel Formation (Fig 29; Smith et al., 1957;
Mathis, 2000; Morris et al., 2010; Doelling and Kuehne, 2007).
Quaternary units include alluvial deposits related to the Fremont River and its tributaries, eolian sand and silt, and alluvial fan deposits originating from mountain ranges to the north (Boulder Mountain) and south (Thousand Lakes Mountain), which have been influenced by the Fremont and other rivers (Smith et al., 1957; Morris et al.,
2010; Doelling and Kuehne, 2007). The alluvial fans are primarily composed of debris flows, which have “run-outs” of up to 10 km and are some of the most extensive in North
America. Most inhabited portions of the quadrangle are on debris flows (Marchetti and
Cerling, 2005). Quaternary units are largely upper Pleistocene (by 3He dates on pyroxenes), but some may be Holocene in age (Doelling and Kuehne, 2007).
Quaternary
The most dramatic Quaternary features are the alluvial fan-building basaltic- andesite boulder diamicts (Fig. 33), which originate from both Thousand Lakes and
Boulder mountains. The influence of these debris flows is extensive, as evidenced in the ubiquitous presence of large (>1 m), rounded basaltic-andesite boulders in the quadrangle. One stunning example of this is the Anthill, a conical hill of Navajo
Sandstone in the east-northeastern part of the quadrangle that is draped by a mantle of volcanic boulders and Navajo Sandstone debris. Some boulders, being more resistant than the bedrock on which they come to rest, form “mushroom” pedestals, especially in the Kayenta Formation.
The unusual boulder surfaces were noted by early geologists (Gilbert, 1877; 100
Figure 33. View looking southwest from a boulder surface (Qafyn) near Torrey, Utah. The large boulder on this surface could not have been transported here by fluvial processes, indicating a mass-movement component for the deposit.
Dutton, 1890) and were originally thought to have glacial origins (Gould, 1939; Flint and
Denny, 1958) in line with the clear evidence for glaciation on Boulder Mountain (i.e.,
Marchetti, 2002). Subsequent research reinterpreted the boulder surfaces as mass movement deposits through stratigraphy (e.g., Williams, 1984; Waitt, 1997) and cosmogenic 3He ages showing that several major boulder surfaces do not correlate with known Pleistocene glacial advances (Marchetti and Cerling, 2005; Marchetti, 2006; this study). Specifically, the surfaces are poorly-sorted, generally unstratified, matrix- supported, coarse boulder diamicts dominated by angular to rounded basaltic-andesite clasts with secondary rounded chert pebbles and minor conglomerate clasts. The boulders are basaltic trachyandesites to trachyandesites, and are similar to the volcanic deposits on
Boulder and Thousand Lakes mountains. Many boulders are larger than 1 m (a-axis) and all but the most extensively eroded boulder surfaces in the quadrangle contain clasts in 101 the range of 2-5 m (a-axis). Deposits are 0-15 m thick depending on preservation
(associated with age and elevation), but weather more slowly than the surrounding sedimentary bedrock due to the armoring presence of the large boulders. Episodic erosion and building of new boulder surfaces results in inverted-age stratigraphy, where the highest boulder surfaces are the oldest.
Due to the long-lived stability of boulder surfaces, moderately cohesive desert pavements are observed, with their continuity broken up by vegetation. Thin section and
QEMScan analyses of the top 5 cm of modern soil samples show that they are predominantly composed of fine-grained, rounded quartz, independent of the underlying bedrock except at the most eroded sites. We interpret this as evidence for soil inflation and importance of eolian input for these surfaces. Near-surface soils contain very little to no calcium carbonate, even though soil carbonate is common deeper in regional soils.
The soil distribution of calcium carbonate is likely due to leaching of calcium carbonate in the near surface and precipitation deeper in the soil (precipitation depth near Torrey is
-20 cm). The stability and large clast size of the boulder surfaces are also conducive to the formation of layered soil carbonates, which can be observed within the soil and growing on the bottoms of large clasts as “pendants” (Fig. 34). While cross-sections of the boulder deposits occasionally present locally stratified material associated with nearby riverbeds, the ubiquitous nature of extremely large boulders speaks to the relative importance of mass movement over fluvial processes in the formation of alluvial fans within the quadrangle.
We add four cosmogenic 3He ages from two of the debris flow surfaces to previous established dates in the region (Marchetti and Cerling, 2005; Marchetti, 2006; 102
high energy cosmic rays
pendant development accumulation Figure 34. Relationships and processes in the Quaternary to modern boulder surface setting. The top ~5 cm of soil is predominantly carbonate-poor eolian sand and silt. The debris flow input to these surfaces armors them and makes them relatively long-lived. Calcium-carbonate is input through eolian dust, dissolves at the surface, and re precipitates at ~20 cm depth. This process produces a K- horizon in the soil and calcium carbonate “pendant” on boulders. The accumulation of cosmogenic 3He is used to determine the surface exposure date on pyroxenes in volcanic boulders, providing a minimum age of boulder alluvial fan abandonment. Because water, vegetation, and topography can block the high-energy cosmic rays that produce 3He, boulders selected for dating were in exposed areas with minimal cover or vegetation.
Marchetti et al., 2007), which represent the age at which a surface was abandoned.
Cosmogenic dating relies on the interaction between rocks and high-energy cosmic rays that produces measureable nonprimary isotopes. Because the amount of an isotope (in this case 3He) produced is dependent on the magnitude of cosmic rays reaching a target rock, it is important to consider effects that could “shield” a rock from receiving its theoretical maximum amount of rock-ray interaction (Fig. 34). Therefore, we collected samples from large (>1 m diameter) boulders with significant exposure (about 1 m above the soil) to minimize shielding effects from snow and vegetation. Additionally, as the samples were collected from the middle of the valley, they do not have a significant 103 shielding effect from the surrounding mountain ranges. Two samples are from the
Teasdale Bench, the most extensive surface on which the town of Teasdale sits, which has one prior date of 258 ± 72 ka. Our ages, 265 ± 33 ka and 300 ± 100 ka, agree with the previous date within error and give a best estimate for the surface of 265 ± 32 ka (Table
6). The other two ages are from a previously undated surface in the southeast portion of the quadrangle and give a best estimate of 313 ± 10 ka.
Alluvial deposits
Qal Stream deposits (upper Holocene) - Stratified, moderately to well-sorted gravel,
sand, silt, and clay deposited in stream channels, floodplains, and man-made flood
conveyance channels. Includes small alluvial-fan and colluvial deposits, and
minor terraces less than 10 feet (3 m) above modern base level. 0-30 feet (0-10
m) thick.
Qaly Young alluvial deposits (Holocene to upper Pleistocene) - Moderately sorted
Table 6. Inverted topography age order for boulder terraces in the Torrey quadrangle. k H Surface Elevation near Local Fremont Elevation above g e Fremont elevation Fremont (m) (m asl) (m asl) Qafy1 2093 2075 18 94 ± 5 Qafy2 2100 2075 25 265 ± 33 Qafy3 2040 2014 26 - Qafy4 2078 2040 38 313 ± 10 Qafy5 2128 2075 53 >400 Qafy6 2095 2035 60 - Qafy7 2104 2035 69 - Qafy8 2170 2095 75 - Qafy9 2163 2063 100 - Qafy10 2183 2063 120 - Qafyn 2195 2063 132 551 ± 23 Holt Draw surface - - - 487 ± 20 m asl = meters above sea level 104
sand, silt, clay, and pebble to boulder gravel deposited in low-gradient stream
channels and floodplains. Locally includes colluvium from adjacent slopes.
Incised by active stream channels. Probably less than 20 feet (6 m) thick.
Qat Youngest stream-terrace deposits (middle Holocene to upper Pleistocene) -
Stratified, moderately to well-sorted gravel, sand, silt, and clay that forms
dissected, level to gently sloping terraces as much as 10 feet (3 m) above modern
drainages. Deposited in stream-channel and floodplain environments and may
include colluvium and alluvial fans too small to map separately. 0-10 feet (0-3 m)
thick.
Qat1 Stream-terrace deposits (middle Holocene to upper Pleistocene) - Stratified,
moderately to well-sorted gravel, sand, silt, and clay that forms dissected, level to
gently sloping terraces as much as 25 feet (8 m) above modern drainages.
Deposited in stream-channel and floodplain environments and may include
colluvium and alluvial fans too small to map separately. 0-25 feet (0-8 m) thick.
Qaf Alluvial fan deposits (Holocene to upper Pleistocene) - Poorly to moderately
sorted, subangular to subrounded, clay- to boulder-size sediment deposited
principally small streams generally entering the Fremont River; deposits form
active depositional surfaces, although locally the master stream may be deeply
entrenched. Less than 30 feet (<9 m) thick.
Qaf1 Young alluvial fan deposits (Holocene to upper Pleistocene) - Similar to Qaf,
but forms inactive, incised surfaces cut by younger stream and fan deposits.
Probably less than 30 feet (<9 m) thick.
Qaf2 Old alluvial fan deposits (upper Pleistocene) - Similar to Qaf, but forms 105
inactive, incised surfaces cut by Qaf1 and younger stream deposits as fans enter
the main valley. The boundary between Qaf1 and Qaf2 is defined by a distinct
change in slope between the two deposits; probably less than 30 feet (<9 m) thick.
Qafy1-11 , Qafo
Boulder Alluvial Fans (mid- to late Pleistocene) - Very poorly sorted volcanic
boulder deposits deposited mainly by debris flows, composed of clay- to boulder-
size debris as well as layers of moderately sorted sand, silt, clay, and pebble to
boulder gravel deposited in low-gradient stream channels and floodplains.
Mapped as alluvial fans due to fan-shaped morphology. Boulders are up to 4 m in
largest dimension and commonly >1 m in size. Boulders originate from Thousand
Lakes (to N) and Boulder (to S) mountains and are composed of the Johnson
Valley Reservoir andesite. Cobbles are composed of Johnson Valley Reservoir
andesite and well-rounded chert. The large boulders “armor” the landslides,
allowing them to persist as terraces 20-775 ft (6-260 m) above the current
catchment. Deposits are grouped by continuity (similar elevation profiles) where
possible (Qmb1-n), with lower numbers indicating younger surfaces. Sixteen 3He
cosmogenic dates on five of the surfaces support this chronology. Based on dates
from a boulder terrace in the center of the quadrangle, north of Torrey (Holt Draw
Surface, HDS), isolated terraces that do not reach the valley, and thus do not have
relative ages by inverted topography, are >500 ka. These mid-Pleistocene
landslide deposits are assigned the designation Qms and are undifferentiated
(although some are likely genetically related). Cosmogenic 3He dates are from
Marchetti (2006) and this study (Table 6). The dates include a radiogenic 3He 106
correction (Marchetti 2006) and were calculated via the CRONUS Web
Calculator; they do not include erosional or shielding corrections. Eroded surfaces
higher in the catchment are undivided (Qafo) and appear to be >400 ka. 0-30 ft (0
10m) thick.
Colluvial deposits
Qc Colluvium (Holocene to upper Pleistocene) - Poorly to moderately sorted,
angular, clay- to boulder-size, locally derived sediment deposited principally by
slope wash and soil creep on moderate slopes and in shallow depressions. Locally
includes talus and alluvium deposits too small to map separately. Includes older
colluvium now incised by adjacent drainages. Typically less than 20 feet (<6 m)
thick.
Qcb Boulder colluvium (Holocene to upper Pleistocene) - Poorly to moderately
sorted, angular, clay- to boulder-size, locally derived sediment derived principally
of material from a nearby boulder terrace (Qafy, Qafo), traced to its origin where
possible. Deposited principally by slope wash and soil creep on moderate slopes
and in shallow depressions. Locally includes talus and alluvium deposits too small
to map separately. Includes older colluvium now incised by adjacent drainages.
Typically less than 20 feet (<6 m) thick.
Eolian deposits
Qe Eolian sand (Holocene to upper Pleistocene) - Very well sorted, well-rounded,
very fine to fine wind-blown sand. Often occurs as small, localized dunes on flat-
flying surfaces near sandstone formations and between domes of the Navajo 107
Sandstone. 0-10 feet (0-3 m) thick.
Mass-movement deposits
Qmt Talus (Holocene to upper Pleistocene) - Poorly sorted, angular cobbles and
boulders and finer grained interstitial sediment deposited by rock fall on or at the
base of steep slopes or as slumps. Slumps are generally pieces of the Moody
Canyon Member of the Moenkopi Formation or Shinarump Member of the Chinle
Formation. Typically grades downslope into colluvium where impractical to
differentiate the two, and may include alluvium in the bottom of washes.
Typically less than 30 feet (<9 m) thick.
Qms, Qls
Boulder landslides (Historical to middle Pleistocene) - Very poorly sorted,
locally derived material deposited by rotational and translational movement.
Composed of clay- to boulder-size debris. Characterized by hummocky
topography, numerous internal scarps, chaotic bedding attitudes, and small ponds,
marshy depressions, and meadows. Boulders are up to 4 m in largest dimension
and commonly >1 m in size. Boulders originate from Thousand Lakes (to N) and
Boulder (to S) mountains and are composed of the Johnson Valley Reservoir
andesite. Deposits are undifferentiated (Qms), with the exception of two clearly
distinct landslide deposits (Qls). Undivided as to inferred age following Knudsen
(2014) because landslides may continue to exhibit slow creep or renew movement
if stability thresholds are exceeded, even if they appear old and weathered (e.g.,
subdued morphology) (Ashland, 2003). Thickness highly variable. 108
Mixed-environment deposits
Qea Eolian sand and alluvium (Holocene to upper Pleistocene) - Light-orangish-red,
moderately to well-sorted, fine- to medium-grained eolian sand locally reworked
by alluvial processes, and poorly to moderately sorted gravel, sand, and silt
deposited in small stream channels. Generally less than 20 feet (<6 m) thick.
Neogene
Igneous Dikes
Ni Several basaltic dikes, 1-2 m thick, are identified in the northern third of the quad
(Fig. 35). Weathering and erosion preferentially removes the contacts with the
country rock, which creates crevices that fill with debris, making it difficult to
examine the exact nature of the contact. The basalt is composed of a matrix of
plagioclase (60-70%) and augite with phenocrysts of augite (25-30%), iron oxides
(5-10%), and few biotite minerals (1-5%). The age of one dike was determined to
Figure 35. Basaltic dike cross-cutting the Jurassic Kayenta Formation. This dike was sampled with resulting measurements of 5.29 ±0.04 Ma by Ar/Ar dating. 109
be 5.29±0.04 Ma by Ar/Ar radiometric dating.
Jurassic
Entrada Formation
Je Entrada Formation (Middle Jurassic) - Grayish-red to moderate-red mudstone
interbedded with moderate-reddish-orange fine-grained sandstone. Grayish-
orange cross-bedded eolian sandstone of varying thickness is locally present. The
Entrada is poorly exposed in this quadrangle due to covering Quaternary
landslides and is identified from a local exposure of sandstone above the Carmel
Formation. 430-490 feet (130-150 m) thick.
Carmel Formation
Jcw Winsor Member (Middle Jurassic) - The upper Winsor Member has been
divided into two informal submembers: the lower gypsiferous member and the
upper banded member (McLelland et al, 2007). The exposure in this area did not
permit such a division, but here is referred to as a single unit. Meter-thick beds of
gypsum interbedded with mudstone and siltstone and with gypsum stringers
throughout. The Winsor Member likely represents a very shallow marine setting
with intermittent marine flooding and evaporation characteristic of sabkha
environments. The contact with the underlying Paria River Member is at the base
of reddish mudstone beds overlying thick succession of white to yellow Paria
River Member rocks. Measured >300 ft (>100 m) thick.
Jcp Paria River Member (Middle Jurassic) - White to yellow siltstone, mudstone,
and gypsum beds interbedded with fossiliferous limestones containing marine 110
bivalves and ammonoids (Morris et al., 2010). Some mudstone and siltstone
layers have small (1-2 mm) gypsum nodules, mudcracks, and ripple marks.
Fossiliferous limestones and gypsum beds suggest deposition in a sabkha (arid
supratidal) environment flooded at times by marine waters. Basal Paria River
gypsum bed has 87Sr/86Sr values ~0.7072-0.7073, consistent with values for
Middle Jurassic seawater (McArthur et al., 2001). The contact with the underlying
Crystal Creek Member is at the base of the lowermost ~10 m-thick gypsum bed
(Fig. 36). Measured 195-425 ft (60-130 m) thick.
Jcx Crystal Creek Member (Middle Jurassic) - Reddish siltstone and mudstone with
small (1-2 mm) gypsum nodules, rare salt casts, mudcracks, and ripple marks. The
contact with the underlying Co-op Creek Member is at the base of the reddish
siltstones (Fig. 36). 30-40 ft (~10 m) thick (Sprinkel and Doelling, 2003, personal
communication).
Jcco Co-op Creek Member (Middle Jurassic) - White to yellowish fossiliferous
Figure 36. Middle Jurassic stratigraphy in the map area. Temple Cap Fm: Jtm = Manganese Wash Mbr; Carmel Fm: Jcco = Co-op Creek Mbr, Jcx = Crystal Creek Mbr, Jcp = Paria River Mbr. 111
packstones, fine-grained limestones, calcite-cemented ripple-laminated
sandstones, siltstones, and mudstones, and thin (<10 cm) alabaster gypsum beds.
The lower part of the Co-op Creek Member is sand- and mudstone-rich and the
upper part is limestone-rich. The contact with the underlying Temple Cap
Formation is at the base of white to yellowish mudstones (Fig. 36). 100-140 ft
(30-45 m) thick (Sprinkel and Doelling, personal communication). unconformity
Temple Cap Formation
Jtm Manganese Wash Member (Middle to Lower Jurassic) - The only portion of the
Temple Cap Formation present in the field area is the Manganese Wash Member
in the southeast part of the quadrangle. The lower part of this unit is very similar
to the Navajo Sandstone, except that it has a slight tendency to weather in ledges
instead of in domes, sometimes has sheets of roughly horizontally-bedded
sandstone rather than large cross-beds, tends toward medium to medium-fine
sand, and contains thin (mm-scale) horizontal burrows along the bedding planes.
The upper part of the unit is reddish sandstone and siltstone. These features
suggest an eolian environment interfingering with marine beds related to a marine
incursion during the transition from the Navajo erg to the shallow seas of the
Carmel Formation (Morris et al., 2010; Doelling et al., 2013). Measured 0-65 ft
(0-20 m) thick.
Previous work and mapping in adjacent quadrangles call this the Thousand
Pockets Member of the Page Sandstone because of its stratigraphic position above
the Navajo Sandstone and its dominance of eolian cross bedding (McLelland et al., 2007; Sorber, 2007; Morris et al., 2010). The Page Sandstone includes the
fine-grained Judd Hollow Tongue Member of the Carmel Formation, which
separates the lower Harris Wash Tongue and the upper Thousand Pockets
Member. However, the sandstone at the top of the Navajo Sandstone in this
quadrangle lacks a fine-grained unit that typically separates the two sandstone
members of the Page Sandstone. The terminology of the Manganese Wash
Member of the Temple Cap Formation is used instead of Page Sandstone or
Harris Wash Tongue (Doelling et al., 2013). unconformity
Navajo Sandstone
Jn Navajo Sandstone (Lower Jurassic) - Fine-grained, well-sorted cross-bedded
sandstone that internally contains wind ripple laminae and grainflow/avalanche
tongues (Fig. 37). Cross-bed sets are commonly 10’s of meters thick. About in the
middle of the unit, there is a ~0.5 m-thick bed of well-sorted, very fine, cross
bedded sand to silt, possibly indicating a brief wet period and preserved interdune
deposit or paleosol in an otherwise arid eolian paleoenvironment. The Navajo
Sandstone typically forms smooth domes. The contact with the underlying
Kayenta Formation is gradational but selected at the base of continuous sandstone
capping the uppermost mudstone or siltstone bed of the Kayenta Formation.
Measured 1115-1310 ft (340-400 m) thick. 113
Figure 37. Large-scale (10-15 m) cross stratification in the lower part of the Jurassic Navajo Sandstone.
Kayenta Formation
Jk Kayenta Formation (Lower Jurassic) - Fine, well to moderately sorted
sandstone interbedded with mudstone and siltstone. Sandstone beds contain high-
angle cross stratification, fining-upward sequences, asymmetric ripple marks, few
pebble lags, and channel forms. Mudstone and siltstone beds are horizontally or
ripple-laminated. Some cross-stratified beds are up to about 1.5 m thick and are
very well sorted; these beds become more common and thicker in the upper
portions of the unit. Mudstone and siltstone beds are probably fluvial flood plain
and/or abandoned channel deposits. Some tabular sand bodies show eolian cross
bedding, internal grainflow and wind ripple laminae. Eolian beds are thicker and
more numerous in the upper parts of the unit. The contact with the underlying
Wingate Sandstone is gradational but selected at the top of the continuous, cliff-
forming Wingate eolian sandstone. Measured 230-330 ft (70-100 m) thick. 114
Jurassic/Triassic
Wingate Sandstone
JTrw Wingate Sandstone (Lower Jurassic to Upper Triassic) - Fine, well-sorted
sandstone with large (1-3 m) sets of high-angle cross-stratification. Contains
grainflow and wind ripple laminae within the cross bedding, indicating an eolian
environment. Contains rare, discontinuous beds of limestone. The contact with the
underlying Chinle Formation is typically sharp (Fig. 38), but is in places
gradational, with the sandstone containing some thin beds of pebble
conglomerate, half-meter tall cross-beds, and soft-sediment deformation,
suggesting a gradual transition from a lacustrine to eolian environment. Measured
130-260 ft (40-80 m) thick.
Figure 38. Typical contact between the Chinle Formation (Owl Rock Member, Trco) and the overlying Wingate Sandstone (JTrw). This contact is sharp to gradational, with siltstone or limestone interbedded with the lowermost few meters of the eolian Wingate Sandstone. 115
Triassic
Chinle Formation (Late Triassic)
Trco - Owl Rock Member (Upper Triassic) - Purplish to greenish lacustrine siltstone
and claystone with discontinuous “candy-striped” pink and tan sandstone with
high-angle cross stratification (Fig. 39). Abundant calcium carbonate horizons
and nodules are more common toward the top of the unit, and sandstones thicker
and more common in the lower part of the unit. Carbonate nodule horizons are
interpreted as paleosols. The uppermost portion contains lungfish burrows up to
11 cm in diameter and is interpreted as lacustrine deposits (Dubiel et al., 1987).
The contact with the Petrified Forest Member is at the base of the lowermost
“candy-striped” sandstone layer. Measured 230-360 ft (70-110 m) thick.
Trcp Petrified Forest Member (Upper Triassic) - Greenish-gray, purplish-gray,
Figure 39. The contact between the Petrified Forest Member (Trcp) with the overlying Owl Rock Member (Trco) of the Triassic Chinle Formation is defined at the base of these striped pink-tan, cross-stratified sandstone bodies. 116 lavender, and red mudstone, reddish to brown siltstone and fine cross-bedded sandstone, and abundant purple and green carbonate nodules in the upper portion of the unit. Petrified wood occurs as abundant fragments and as in situ stumps in the northeastern portion of the quadrangle. These stumps are up to 1 m in diameter and tend to be highly fragmented. There are also large, vertical conglomeratic columns, pillars, or pipes in the lower part of this unit (Fig. 40).
The conglomerate columns are 1-3 m in diameter, protruding up to 4 m from the ground surface and cross-cutting the host rock bedding. Internally, columns are generally massive and are composed primarily of locally derived silt and fine
Figure 40. Vertical conglomerate pipes in the Petrified Forest Member of the Triassic Chinle Formation. These pipes are 0.6-3 m (2-9 ft) in diameter and are composed of locally derived silt, sand, and pebbles that include fragments of petrified wood and calcite nodules; they are coarser than the surrounding host rock. These pipe structures may indicate fluidization of the sediments prior to full lithification induced by seismicity. This pipe protrudes about 4 m from the ground surface due to preferential cementation (determined to be non-carbonate for lack of effervescence under HCl) and is thus more resistant to weathering. 117
sand mixed with fragments of petrified wood and carbonate nodules. Column
structures are interpreted as injectite pipes, which may be the result of fluidization
of shallowly buried, unconsolidated sediments, possibly during a seismic event
(McCallum, 1985). However, a thorough study and discussion on the nature of
conglomerate pipes is beyond the scope of this project. The Petrified Forest
Member represents a shift from the lacustrine environment of the Monitor Butte
Member to a marsh environment in the lower portion of the Petrified Forest
Member and a transition to an increasingly lacustrine environment toward the
upper parts of the unit, with the development of carbonate paleosols. The contact
with the Monitor Butte Member is selected at the top of predominantly gray,
bentonitic claystones, overlain by siltier, reddish to brown siltstone. Measured
260-395 ft (80-120 m) thick.
Trcm Monitor Butte Member (Upper Triassic) - Gray to light purplish-gray and red
bentonitic claystones with beds of siltstone, fine sandstone, and carbonate
nodules, and appears to represent a transition of the fluvial system to a marsh or
lacustrine system. The bentonite was altered from water-lain volcanic ash
probably sourced from a volcanic arc on the western and southwestern edge of the
Triassic continent (Dubiel, 1989). The contact with the Shinarump Conglomerate
Member is gradational and chosen at the top of the uppermost cross-bedded
sandstone bed of the Shinarump Conglomerate Member. Measured 30-100 ft (10
30 m) thick.
Trcs Shinarump Conglomerate Member (Upper Triassic) - Coarse, moderately
sorted lithic sand with basal scours and pebble lags, channel forms, fining-upward 118
sequences, and scattered fragments of petrified wood. The Shinarump
Conglomerate Member fines upward overall. The Shinarump Conglomerate
Member represents a fluvial environment with increasing distance from the
headwaters from the base to the top of the unit. The basal contact of this unit with
the Moenkopi Formation is sharp and channelized (Fig. 41). Measured 30-130 ft
(10-40 m) thick. unconformity
Figure 41. The contact between the Moenkopi Formation (Moody Canyon Member, TRmm) with the overlying Chinle Formation (Shinarump Conglomerate Member, TRcs). Note both the lack of gypsum veins in the uppermost Moenkopi Formation and its yellow color in the ~1 m below the contact. 119
Moenkopi Formation
The Moenkopi Formation was mostly deposited in tidal flat environments with a brief marine incursion (Blakey, 1974). Most of the Formation present in this area is
Olenekian in age, although the Moody Canyon Member persists into the Anisian
(Morales, 1987). Gypsum and mudrocks collected from the Moenkopi Formation have bulk 87Sr/86Sr values ~0.7084, higher than contemporaneous seawater, with 87Sr/86Sr
~0.7077-0.7080 (Fig. 42). The Moenkopi Sr isotope ratio indicates terrestrial influence on deposition and/or diagenesis in marginal marine settings (Veizer, 1989; Denison et al.,
1998; Blakey and Ranney, 2008). However, Moenkopi S34S plots within the Early
Triassic seawater range, and S has been demonstrated to be more sensitive to diagenesis than Sr, so the Sr is probably principally derived from terrestrial input (Strauss, 1997;
+40_ i_i_i_i_L i_i_i_i_L i_i_i_i_L i_i_i_i_L i_i_i_i__
+35
+5 Moenkopi other gyp. vein + pedogenic gyp. — gyp. bed O J Carmel gyp. O mudstone □ Tr Chinle SS U
-10 — i— i— i— |—|—i— i— i i— | i— i— i— i— |—i— i— i— i— | i— i— i i—
0^ ° 0^ ^ ^ 87Sr/86Sr Figure 42. Sr and S isotopic data of Triassic Moenkopi Formation mudrocks and gypsum, and comparative samples. The Moenkopi gypsum vein adjacent to Triassic Chinle sandstone is a selenite vein that is adjacent to the Moenkopi-Chinle contact. 120
Strauss, 1999; Kamschulte and Strauss, 2004; Strauss, 2004).
Trmm Moody Canyon Member (Middle to Lower Triassic) - Orange to reddish-brown
mudstone, siltstone, and sandstone with scattered gray-green bedding-parallel
horizons. Mudstones and siltstones contain horizontally and ripple-laminated
beds, soft-sediment deformation structures, and appear massive in some places.
Siltstones and sandstones often contain cm- to m-scale high- to low-angle bi
directional cross-stratification, climbing ripples, and symmetric ripples. This
member also has abundant antitaxial gypsum veins (Fig. 32) and beds of <1 m
thick alabaster gypsum. The lithology, sedimentary structures, and abundance of
gypsum in the Moody Canyon Member suggest intertidal zone conditions in a
tidal flat to sabkha environment. Gypsum veins are absent in the upper ~10 m of
the unit. The ~1 m below some major sand bodies within the Moody Canyon
Member, as well as the ~1 m of the Moenkopi Formation adjacent to the
Moenkopi-Chinle contact, has a yellowish color. The contact with the Torrey
Member is gradational but chosen at the lowest occurrence of gypsum veins or
primary gypsum beds. Measured 330-460 ft (100-140 m) thick.
Trmt Torrey Member (Lower Triassic) - Tan, orange, and reddish-brown interbedded
mudstone, siltstone, and calcite- and dolomite-cemented, fine-grained sandstone
containing symmetric ripples, mudcracks, horizontal bedding, low- to high-angle
bi-directional cross bedding, climbing ripples, soft-sediment deformation, and
reptilian trackways (Thomson, 2014). The Torrey Member represents a near-shore
tidal depositional environment, including a tidally influenced beach environment.
The contact with the Sinbad Limestone is gradational but chosen at the top of the 121
highest oolitic grainstone bed of the Sinbad Limestone. Measured 200-295 ft (60
90 m) thick.
Trms Sinad Limestone (Lower Triassic) - A transgression of the Early Triassic sea
resulted in the deposition of the shallow marine deposits of the Sinbad Limestone
Member, which includes calcareous mudstones, algal boundstones, and gastropod
and brachiopod packstones which include meekoceras and lingula fossils (Blakey,
1974; Morales, 1987; Morris et al., 2010). The Sinbad Limestone transitions
gradually to the Torrey Member, with oolitic grainstones at the top of the Sinbad
Limestone marking the transition to shallower wave currents related to the
shoreface. The contact with the Black Dragon Member is sharp. Measured 100
200 ft (30-60 m) thick.
Trmb Black Dragon Member (Lower Triassic) - Orange to reddish-brown mudstone
with rare gypsum beds and abundant cross-cutting gypsum veins. It is difficult to
discern sedimentary structures within weathered Black Dragon exposures, but
ripple lamina, soft-sediment deformation structures, and mudcracks are abundant.
Only the upper ~20 m of the member are exposed in the Torrey quadrangle,
although thin <20 cm interbeds of limestone and dolostone, quartz and chert
pebble conglomerates, and fining-upward sediments are exposed in Capitol Reef
National Park (Blakey, 1974; Morris et al., 2010). The Black Dragon Member
was deposited in coastal plain (lower portion) and tidal flat environments,
probably in the intertidal zone of the tidal flat. Measured >65 ft (>20 m) thick. 122
Structure
The Teasdale Fault strikes roughly N60W and dips >70° to the north. In places, it has a wide damage zone with gouge up to 1 m thick, abundant deformation bands in the
Wingate, Kayenta, and Navajo formations, and several fault splays. In the eastern part of the Torrey quadrangle, the Teasdale Fault places Jurassic Kayenta Formation over
Jurassic Navajo Sandstone. In the central part of the quadrangle, the Teasdale Fault places the Triassic Moenkopi and Chinle formations over the Jurassic Navajo Sandstone.
In the western part of the quadrangle, the Teasdale Fault places the Lower Triassic
Moenkopi Formation over the Upper Triassic Chinle Formation. The thickness of the
Chinle Formation is much thinner near the fault. The Teasedale Fault is placed in a reverse fault zone that also accommodates significant left-slip (Davis, 1999). Slickenlines on one of the fault splays rake 8-16° W.
All other faults in the area are subvertical normal faults that strike west-northwest and tend to be found only in the northern half of the quadrangle, likely due to the significant Quaternary cover in the southern half of the quadrangle. Smith (1957) mapped many of these faults, but several, including those mapped in the Navajo Sandstone, merely proved to be vertical east-west joints with no observable offset. The abundance of normal faults in the southeast corner of the quadrangle was also apparently exaggerated.
West of the Torrey quadrangle, the Thousand Lakes Fault cuts a Quaternary boulder surface, but no evidence of Quaternary faulting exists within the Torrey Quadrangle.
The Teasdale Fault cuts the south-facing Teasdale monocline (Bump and Davis,
2003), and they appear to be related structures. Bedding steepens near the fault, dipping
~70° in the footwall strata in a south-facing monocline at the easternmost portion of the 123 quadrangle. The steep dip of rocks folded over the Teasdale Fault is especially well exposed in the Cockscomb, a ~2 km-long ridge that trends N60W and parallels the
Teasdale Fault. Bedding is less tilted (20-30°) in relation to the fault on the western part of the quadrangle than on the eastern part. Another tight monocline parallels the Teasdale monocline (Fig. 43), located in the Moenkopi Formation in the southeastern corner of the quadrangle. The only other major folding present in the area is a broad, low-amplitude anticline that trends northwest and whose hinge is buried in the center of the valley. This
Figure 43. A small, tight monocline in the Moenkopi Formation, near the Cockscomb and parallel to the N60W-trending Teasdale monocline. It is highlighted by a yellow horizon underneath a sand body within the Moody Canyon Member. Solid lines trace the shape of the anticline. Dashed lines represent hinge lines. 124 low-amplitude fold is probably associated with the Miners Mountain anticline, a
Laramide structure, which is bounded on the southwest by the Teasdale monocline
(Bump and Davis, 2003). Bump and Davis (2003) determined that the maximum compression direction for the Miners Mountain anticline during the Laramide was N60E, which is consistent with the geometry of the Teasdale Fault, Teasdale monocline, and other folds in the area. It is approximately parallel to the expected maximum dilation direction for the normal faults in the area, which may be attributed to post-Laramide extension.
Diagenesis
Antitaxial (outward-growth) gypsum “stringer” veins cross-cut the Black Dragon and Moody Canyon members of the Moenkopi Formation. They range in thickness from
~2 mm to 5 cm (mean 8-9 mm). Rare, discontinuous beds of nodular gypsum up to ~15 cm thick with a pinkish color are observed in the Black Dragon and Moody Canyon members as well as some <1 m-thick white alabaster beds in the upper Moody Canyon
Member. Gypsum veins are absent in the Sinbad Limestone and Torrey members. There are no gypsum veins in the uppermost ~10 m of the Moody Canyon Member, and gypsum appears to be absent in the Chinle Formation, except as cement in the lowermost
Shinarump Conglomerate sandstones. Bulk mudstone, gypsum vein, and gypsum bed samples taken from the Moody Canyon and Black Dragon members of the Moenkopi
Formation have similar Sr and S isotope ratios. Because Moenkopi gypsum veins are stratigraphically distributed and have similar Sr and S isotope values to primary gypsum beds in the Moenkopi Formation, Moenkopi veins are interpreted as sourced from primary gypsum beds within the unit. 125
The lack of gypsum veins in the uppermost Moody Canyon Member probably relates to the lithologic differences and differences in fluid chemistry between the Moody
Canyon Member of the Moenkopi Formation and the Shinarump Conglomerate Member of the Chinle Formation. The Shinarump Conglomerate is much coarser grained than the
Moenkopi mudrocks, which makes it both more permeable to fluids and mechanically less favorable for the propagation of a dense vein network. Formation fluids in the
Shinarump Conglomerate seem to have been unfavorable for gypsum precipitation, which may have mixed with upper Moenkopi fluids to prevent the precipitation of gypsum in the uppermost ~10 m of the Moody Canyon Member. The Shinarump
Conglomerate is distinct from Moenkopi mudrocks in S and Sr isotopes, having lower
S34S and higher 87Sr/86Sr values relative to the Moenkopi Formation (Fig. 42). A solitary selenite gypsum vein undulates in the Moenkopi Formation at the Moenkopi-Chinle contact and has similar Sr and S isotope values to the cement in the Shinarump
Conglomerate. This supports chemical mixture between Moenkopi and Chinle waters.
The presence of a gypsum vein at the contact may be due to a high pressure gradient between the lower mudrock and upper sandstone, responsible for a higher energy environment that retarded crystal growth (Freyer and Voigt, 2003).
The Moody Canyon Member of the Moenkopi Formation also has distinct green bedding-parallel banding in otherwise red mudrock (Fig. 32), as well as rare ~1 m-thick yellowish coloration under significant sandstone bodies (including the Moenkopi-Chinle contact) (Figs. 41 and 43). These features seem to be unique to the Moody Canyon
Member. The upper and lower red-green contacts in mudstone are mottled, and green horizons are sometimes discontinuous. Green horizons tend to be silty mudstone or 126 siltstone. Green reduction spots <2 cm in diameter indicate 10-20% bedding- perpendicular shortening, which is consistent with burial-stage diagenesis. Green coloration is probably related to thermal maturation of organic matter buried in Moenkopi sediment.
The only other gypsiferous unit in the study area is the Carmel Formation. The basal Paria River Member of the Carmel Formation contains a thick (up to about 10 m) gypsum bed as well as small (1-2 mm) nodules of gypsum that weather out such that they resemble raindrop impressions. The Winsor Member contains ~1 m-thick gypsum beds that form ledges, with thin (1-2 cm) gypsum veins threading between the beds and elsewhere in the unit. However, because of the slope-forming nature of the Winsor
Member, it is difficult to trace the continuity of these beds, although some outcrops of siltstone in the upper Winsor Member do not contain gypsum beds. Carmel Formation gypsum beds are known to have inconsistent thickness and form diapirs due to salt flow
(McLelland, 2007), although these phenomena are not observed in the study area. One significant diapir forms Glass Mountain, a landform composed of Carmel Formation selenite crystals in Cathedral Valley of the northern section of Capitol Reef National Park
(~18 km north of the Torrey quadrangle). Carmel Formation gypsum is isotopically distinct from the Moenkopi and Chinle formations.
The Wingate Sandstone, Kayenta Formation, and Navajo Sandstone all contain fine, well-sorted sandstone with eolian and/or fluvial cross stratification. The Wingate
Sandstone and Kayenta Formation both have an overall reddish-orange color, as does the basal portion of the Navajo Sandstone. However, the upper portion of the Navajo
Sandstone and overlying Temple Cap Formation is white. The color contact is not 127 restricted to the Kayenta-Navajo formation contact, but is always at or above the
Kayenta-Navajo contact. The coloration contact is commonly sharp. Some small (mm- scale) iron-oxide concretions are occasionally scattered in the red-orange portions of the
Navajo Sandstone. This coloration difference, as well as the presence of iron-oxide concretions, may be explained by similar processes that produced color variations in the
Jurassic Navajo Sandstone throughout other portions of Southern Utah (Beitler et al.,
2005; Chan et al., 2012). These studies propose three stages of diagenesis: (1) early oxidation, coloring the formation red from iron oxide grain coatings, then (2) passage of reducing fluids (i.e., hydrocarbons) that bleach the sandstone, and then (3) passage of oxidizing fluids, possibly in multiple phases, that result in the precipitation of iron-oxide cements and masses called concretions. Precursor reduced iron phase concretions could also form, with later oxidation with open interaction with meteoric waters.
Other diagenetic events are evident within other formations. Examples of early diagenesis are preserved as carbonate concretions and banded yellow, red, and purple variegation in the Chinle and Carmel Formations. These colored zones suggest these are paleosols that developed as result of fluctuating water tables in the Chinle (and Carmel) sediments, probably in response to seasonal flooding (Dubiel, 1989).
Geologic History
The geologic history interpreted from the rock and sediment exposures in the
Torrey quadrangle commences with the deposition of the fine, rippled gypsiferous mudstones of the Black Dragon Member of the Moenkopi Formation in the Early
Triassic. The Early Triassic epeiric seaway, the “Sinbad Sea,” transgressed, favoring the deposition of limestone in a shallow sea, recorded in the Sinbad Limestone Member of 128 the Moenkopi Formation. A regression of the Sinbad Sea is recorded as the Sinbad
Limestone Member transitions from algal boundstones at the base to oolitic grainstones at the top of the unit. The tidal flat part of the system migrated over the Torrey area, with early phases of beach, upper shoreface, and intertidal zone channelized deposits represented in the Torrey Member of the Moenkopi Formation. The system then transitioned back to a tidal flat, resulting in the rippled, gypsiferous mudstones and siltstones of the Moody Canyon Member of the Moenkopi Formation.
The final regression of the Early Triassic Sea was a period of relative nondeposition, followed by the incision of fluvial channels in the Late Triassic (Tr-3
Unconformity; Pipiringos and O’Sullivan, 1978). This fluvial system, represented in the
Shinarump Conglomerate Member of the Chinle Formation, evolved into a marsh and lacustrine environment, preserved respectively as the Monitor Butte and Petrified Forest members of the Chinle Formation (Dubiel, 1987, 1989; Morris et al., 2010). Volcanism on the west and southern edge of the Triassic continent produced ash that was deposited in the marshy environment, resulting in the bentonite in the Monitor Butte and Petrified
Forest members (Dubiel, 1989). The Owl Rock Member of the Chinle Formation was then deposited in a brief fluvial episode that was followed by a lacustrine environment that produced the carbonates in the uppermost Owl Rock Member. Sometime during the middle to later stages of deposition of the Chinle Formation, seismic events (source unknown) may have produced the conglomeratic pipes in the lower Petrified Forest
Member of the unit.
Increasing aridity resulted in the cessation of wetland deposition and eolian dune field development preserved in the Wingate Sandstone. These environmental conditions 129 persisted into the Early Jurassic. Interdune deposits became better preserved, likely the result of a higher water table, and, for some periods of time, the system was dominated by fluvial activity, recorded as the Kayenta Formation. Increasing aridity permitted the development of a large-scale sand erg that is preserved as the Navajo Sandstone (Morris et al., 2010).
A marine transgression then followed where coastal eolian sands and marine muds were emplaced atop the Navajo Sandstone, probably after a brief period of nondeposition (J-1 Unconformity). This relationship is recorded as the contact of the
Manganese Wash Member of the Temple Cap Formation that overlies the Navajo
Sandstone. Another depositional hiatus followed (J-2 Unconformity). The Middle
Jurassic sea deposited the mudstones, sandstones, and fossiliferous limestones of the Co op Creek and Crystal Creek members of the Carmel Formation. The shorelines of this sea were frequently dominated by evaporation, resulting in the gypsum beds deposited in the
Crystal Creek, Paria River and Winsor members of the Carmel Formation. Further
Mesozoic deposition is not recorded in the Torrey quadrangle rock record.
Northeast-southwest Laramide compression (Late Cretaceous-early Paleogene) caused northwest-southeast-trending folds and reverse and left-slip motion across the major Teasdale Fault (Davis, 1999; Bump and Davis, 2003). After compression ceased, regional northeast-southwest dilation occurred, resulting in the generation of northwest- southeast-striking normal faults.
Oligocene-Miocene volcanism resulted in the creation of the Marysville Volcanic center and the Henry Mountain laccoliths to the north of the Torrey quadrangle, which may be the sources of the Torrey area’s ubiquitous volcanic boulders (Davis, 1999). The 130
Henry Mountain laccoliths are reported to have been emplaced during the late Oligocene,
31.2-23.3 Ma, and the Marysvale volcanic field was emplaced at ca. 20 Ma (Davis, 1999;
Morris et al., 2010). Basaltic dikes (age 5.29 Ma) were possibly generated from the same system as another volcanic field that was emplaced in the northern quarter of the Capitol
Reef National Park area about 4.6-3.7 Ma (Morris et al., 2010).
Between about 20 and 5 Ma, the entire area was broadly uplifted, probably with much or all of the Colorado Plateau, relative to the surrounding North American continent (Morris et al., 2010). The uplift of the Colorado Plateau appears to be continuing today (Levander et al., 2011). The mechanism responsible for this uplift, as well as volcanism prior to and during this period, remains enigmatic. One model involves lithospheric weakening during flat-slab subduction of the Farallon plate during Laramide time (85-50 Ma) and later delamination and foundering of that slab (30-20 Ma) which hydrated, heated, and thinned the weak overlying lithosphere (McQuarrie and Chase,
2000; Levander et al., 2011). Another model makes mantle convection the principal mechanism, with upper mantle convective currents driving the lithosphere upward
(Braun, 2010; Karlstrom et al., 2012).
Colorado Plateau uplift is likely responsible for two sets of fractures observed in the Torrey area. The first is gypsum veins in the Moenkopi Formation, which likely formed in response to exhumation. The second fracture set is vertical joints most prominent in Jurassic sandstones. Bleaching in the Navajo Sandstone is cross-cut by these joints, so diagenetic bleaching likely occurred prior to Colorado Plateau uplift.
Plio-Pleistocene climate change resulted in cooler, wetter conditions consistent with glaciation of nearby peaks. Numerous mass movement events also occurred during 131 this time, including the deposition of the several boulder alluvial fans within the Torrey quadrangle. The Fremont River and its tributaries eroded much of this material, leaving the inverted age-topography relationship visible in the valley today. Modern eolian deposits accumulated near the base of sand-rich formations, especially where the rock formations are cliff forming (e.g., the Wingate Formation). Eolian input is a significant part of the boulder alluvial fan deposits, resulting in inflation in the top ~5 cm of soil and, in places, accumulation significant enough to form dunes on top of the boulder deposits.
The long-lived stability of these surfaces also allowed for significant soil carbonate development, including the formation of layered “pendants” on the bottoms of boulders.
Hazards
The Torrey quadrangle contains several geologic hazards operating on different spatial and temporal scales. Upper Pleistocene alluvial fan and debris flow surfaces are common and widespread in Torrey and were likely caused by the interaction of steep slopes and high-intensity water discharge. Modern snowmelt, spring rainfall and high intensity summer rainfall (when storms likely related to the North American Monsoon pass through) are likely candidates for triggering mass movement events. Wildfires can also prime a slope for a mass movement event. Although some landslide areas have more subdued morphology, which may indicate age and recent inactivity, these surfaces may continue to exhibit slow creep or reactivate (Ashland, 2003; Knudsen, 2014). A section of the southern portion of the quadrangle with an access road to campsites on Boulder
Mountain recently had a wildfire; heavy rains events could most easily destabilize this slope and endanger tourists and residents of Teasdale.
We have not determined the velocity of debris flows during emplacement; 132 however, the mapped extent of older flows shows that they at a minimum present a hazard to current societal infrastructure. The >1 m size of the boulders suggests high- energy flows. If flows are sufficiently fast-moving, they may also present a hazard to human life. However, it is unlikely for debris flows to move over topographic highs except in cases of extreme concentration and speed, so infrastructure built on higher ground in the valley is unlikely to be affected during debris flow events. Buildings near the mountains and by creek outlets do run the risk of future landslides capable of flowing over ridges (for example, the recent landslide on Marvin Mountain, which is northeast of
Fish Lake on Thousand Lakes Mountain northwest of the Torrey quadrangle).
Future anthropogenic construction on or near sandstone and mudstone cliffs is strongly discouraged. Large, boulder-sized debris are located at the base of cliffs of
Shinarump Conglomerate, Wingate Sandstone, Kayenta Formation, and Navajo
Sandstone. Even cliffs as low as a few meters have shed large debris in their immediate vicinity. Slumping and falling of large blocks also occurs in mudstone and sandstone cliffs, such as those bordering the Fremont River (Fig. 44). Cliffs of the Moenkopi
Formation capped by the Shinarump Conglomerate are especially treacherous, as they form large debris fields with large boulders that can potentially roll >100 m from the base of the cliff.
Most residential and commercial structures built on bedrock or shallow
Quaternary cover in the Torrey area are built on the Torrey Member of the Moenkopi
Formation. Sandstones and shales in this unit are well cemented, mostly with calcite and some gypsum. Excepting significant clay content, this unit appears to be relatively safe to build on, but may become unstable with persistent rain or flooding, owing to the 133
Figure 44. Slumping (shown by arrows) in the Torrey Member of the Moenkopi Formation along a cliff on the north side of the Fremont River, south of the town of Torrey. dissolution of gypsum in the cement and wetting of clays with shrink and swell properties. The Black Dragon and Moody Canyon members of the Moenkopi Formation contain gypsum veins and are principally cemented with gypsum, so these units may experience a few centimeters of soil collapse as veins and cement dissolve in persistent wet weather or irrigation. The Carmel Formation contains thick gypsum beds which locally pose a threat of a meter or more of collapse due to gypsum dissolution. These units have high clay content and threaten landslides under persistent wet conditions. The
Chinle Formation contains significant amounts of bentonite clays, which swell when hydrated. This makes the unit more susceptible to landslides, soil creep, and uneven settling.
All faults in the area appear to be relatively inactive, although it is impossible to tell how young the normal faults are by observation in this area. They are likely to be related to extension and uplift that occurred in this area approximately 20-5 Ma (Morris 134 et al., 2010), but do not cross-cut Quaternary deposits in the area, which include boulder deposits with minimum ages of 400 ka. The Teasdale fault is a Laramide structure and shows no signs of movement since about 60 Ma.
Conclusions
The new Torrey 7.5’ quadrangle map updates prior mapping, geologic history interpretations, and geological hazards in the area through in-depth mapping of stratigraphic contacts, identification of unmapped Quaternary mass movement and eolian deposits, the recognition and verification of normal faults in the northern part of the quad, and more extensive mapping of the Teasdale fault. Additionally, we provide chemistry and interpretations on the deposition of diagenetic gypsum veins and several age dates for younger debris flows in the area.
Acknowledgements
This study was performed in collaboration with Tyler Huth (PhD candidate,
University of Utah). We acknowledge support of the USGS and EDMAP program for their generous funding. We extend thanks to Bob Biek, Grant Willis, and Zachary
Anderson for their professional and technical support. Finally, we thank Michael Jury for his energetic field assistance.
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KEY LOCALITIES
Appendix A summarizes important locality information from the study areas and supports the information presented in this thesis.
Two main areas in southern Utah are evaluated in this study, which are divided into key localities and addressed from west to east (Table 7).
Table 7. Details of key localities, with abbreviations, relevant figures, and directional view azimuth of figure images.______Field Lat N Long W Vein Description, Area Locality Fig. Facing (NAD27°) (NAD27°) Units Density Notable Features St. George SG-N 45a 158 37.04588 113.51790 Trmu Medium Stratigraphic position (~5%) of SG-S. St. George SG-N 45b 157 37.04585 113.51753 Trmu Medium - (~5%) St. George SG-S 45c 169 37.04350 113.51993 Trmu High Dominant vertical (~10%) veins, veins cross lithologic boundaries, gray-green banding. Torrey TO-VR 46a 006 38.31345 111.48399 Trmm, High Upper ~10 m of Trmm Trcs (~10%) has no gypsum veins. Torrey TO-W 46b 285 38.30021 111.48828 Trmm V. High Dominant horizontal (~15%) veins, veins cross lithologic boundaries, gray-green banding. Torrey TO-S 46c 014 38.25544 111.40504 Trmm High 1-2 m thick yellow- (~10%) green band beneath sandstone. Torrey TO-N 46d 331 38.26298 111.40567 Trmb, Medium Poor exposure; Upper Trms ? (~5%) ~1 m has no veins. 140
St. George (SG)
SG-N (St. George-North)
SG-N is a roadcut of ~100 m of exposed Triassic section, capped with the
Shinarump Conglomerate Member of the Chinle Formation. Moenkopi rocks include a thick sandstone section, but are otherwise bedded sandstones and mudstones (Fig. 45a-b).
Trough cross-stratification is uncommon in sandstone beds, although climbing ripples are common. Siltstones are typically rippled, locally include flute casts, and fill downward into underlying mud cracks. Mudstones commonly appear massive, but are finely ripple- or plane-laminated. A series of reverse faults cut the upper portion of the roadcut; these faults strike ~N40E, dip ~34 SE, have ~90° rake on slicken surfaces, and accommodate
<2 m net slip (<1 m throw). Mudstones and sandstones are densely fractured with networks of gypsum veins throughout, except in the upper several 10’s of meters of the roadcut. Few primary gypsum beds are present but tend to be centimeters thick, nodular, and mixed with mudstone.
Figure 45. Localities SG-N (a-b) and SG-S (c) in the upper red member of the Moenkopi Formation. These are located along a roadcut which was exploited to measure ~100 m of stratigraphic section (Appendix E). Yellow-boxed areas show where 2D vein measurements were made (Appendix F). 141
Location: Banded Hills Drive / Airport Road, St. George, Utah, USA.
Coordinates (NAD27): 37.045 N, 113.519 W to 37.045 N, 113.515 W.
Units Represented: Triassic Moenkopi Formation (upper red member), Triassic
Chinle Formation (Shinarump Conglomerate Member).
Notable Features: Thick sandstone section, selenite, composite veins, polyphase veins, gypsum nodules, primary gypsum beds, gray-green banding, reverse faults.
Significance: Measured section, sample collection, structural measurements, image analysis for 2D vein measurements.
SG-S (St. George-South)
SG-S is a cliff-face of Triassic mudstone and siltstone (Fig. 45c). Siltstones are rippled, include flute casts, and fill mud cracks. Mudstones appear massive but are finely ripple- or planar-laminated. The mudstone is densely fractured with a network of gypsum veins that consist of at least two dominant vertical veins and many horizontal veins that appear to branch off of the vertical vein “trunks.”
Location: Banded Hills Drive / Airport Road, St. George, Utah, USA.
Coordinates (NAD27): 37.044 N, 113.520 W.
Units Represented: Triassic Moenkopi Formation (upper red member).
Notable features: Composite veins, gray-green banding.
Significance: Measured section, sample collection, structural measurements.
Torrey (TO)
The Torrey (TO) field area was selected for near-complete exposure of the
Moenkopi Formation and exposure of an overlying Jurassic gypsiferous unit (the Carmel 142
Formation). Key localities are not selected in the Sinbad Limestone or Torrey members of the Moenkopi Formation because they are not gypsiferous.
TO-VR (Torrey-Velvet Ridge)
TO-VR is a south-facing, 100+ m tall Triassic mudstones and siltstones capped by the Shinarump Conglomerate Member of the Chinle Formation (Fig. 46a). Due to inaccessibility, this cliff was only closely examined with digital photographs, and samples were collected near the base of the cliff or on some of the outcrop’s steep slopes.
Siltstones and mudstones appear to be ripple- and planar-laminated. The cliff is cross-cut
Figure 46. Localities TO-VR (a), TO-W (b), TO-S (c) in the Moody Canyon Member of the Moenkopi Formation, and TO-N (d) in the Black Dragon Member. These collectively represent the gypsiferous Moenkopi redbeds in the TO area. Yello-boxed areas show where 2D measurements were made (Appendix F). 143 by a dense network of gypsum veins except for the upper ~10 m.
Location: Velvet Ridge, Near Torrey, Utah, USA. Velvet Ridge parallels the
Fremont River and Highway 24, about 0.9 km north of Highway 24 and 2.5-5 km east of
Torrey.
Coordinates (NAD27): 38.313 N, 111.484 W.
Units Represented: Triassic Moenkopi Formation (Moody Canyon Member).
Notable features: Composite veins, gray-green banding.
Significance: Sample collection, image analysis for 2D vein measurements.
TO-W (Torrey-West)
Description: TO-W is a 10-12 m tall (~30 m wide) natural cliff of Triassic mudstones and siltstones (Fig. 46b). Both mudstones and siltstones appear massive, but mudstones are ripple- and plane-laminated and siltstones have ripple cross-stratification.
A dense network of gypsum veins cross-cuts the entire outcrop, including a dominant horizontal set and less dominant and less obvious vertical set, and many other horizontal gypsum veins. Small cm-scale reduction spots are also visible in the lower part of this outcrop. South of this outcrop and up-section are <1 m-thick gypsum beds poorly exposed in ledgy slopes.
Location: Near Teasdale, Utah, USA. 0.4 km south of Highway 24, 2 km N30W of Teasdale.
Coordinates (NAD27): 38.300 N, 111.488 W.
Units Represented: Triassic Moenkopi Formation (Moody Canyon Member).
Notable features: Composite veins, gray-green banding, reduction spots.
Significance: Measured section, sample collection, structural measurements, 144 composite veins, gray-green banding, reduction spots, image analysis for 2D vein measurements.
TO-S (Torrey-South)
Description: TO-S is a generally south-facing, ~6 m-tall natural cliff with ~50 m of lateral exposure of Triassic mudstones and sandstones (Fig. 46c). Sandstones show bidirectional cross-stratification (10-100 cm thick beds). Mudstones appear massive, but are ripple- and planar-laminated. Sandstones are yellowish in color, as are adjacent mudstones (within ~1 m of the sandstone-mudstone contact); the remainder of the mudstone is red-brown with gray-green banding. A dense network of gypsum veins cross-cuts the mudstones, though few veins cross-cut the sandstone.
Location: 4.9 km S15E of Torrey, 7 km S50E of Teasdale, and 0.4 km north of
Teasdale Road, which connects Highways 12 and 24 through Teasdale.
Coordinates (NAD27): 38.256 N, 111.405 W.
Units Represented: Triassic Moenkopi Formation (Moody Canyon Member).
Notable features: Composite veins, gray-green banding, yellowish banding.
Significance: Sample collection, structural measurements, image analysis for 2D vein measurements.
TO-N (Torrey-North)
Description: TO-N is red, slope-forming Triassic mudstone and claystone underneath a cliff of the Sinbad Limestone Member of the Moenkopi Formation (Fig.
46d). Jagged gypsum veins cross-cut the slope, and some thin (cm-scale), nodular gypsum beds are also present. The red mudstone becomes yellowish, nongypsiferous 145 mudstone within ~1 m of the mudstone-limestone contact.
Location: ~4 km south of Torrey, 7 km east of Tesdale, and 1.2 km north of
Teasdale Road, which connects Highways 12 and 24 through Teasdale. South of 0.8 km north of TO-S.
Coordinates (NAD27): 38.263 N, 111.406 W.
Units Represented: Triassic Moenkopi Formation (Black Dragon and Sinbad
Limestone members).
Notable features: gypsum veins, gypsum beds.
Significance: Measured section, sample collection. APPENDIX B
GYPSUM TEXTURES
Appendix B summarizes Moenkopi gypsum vein and gypsum bed textures observed as part of this study.
Gypsum textures observed in this study include the fibrous texture of aligned satin spar crystals within a vein, unorganized satin spar fibers (only at TO-N), glassy sheets of selenite crystals, blocky selenite crystals (rarely at SG-N), polygonal texture on some selenite vein surfaces, radial fibers of satin spar nodules, massive or granular texture of alabaster nodules and primary beds, and the “chicken-wire” texture of nodular gypsum beds mixed with clay and silt (Fig. 47). Veins generally appear to be antitaxial, with fibers oriented subperpendicular to the vein walls and crystal coarsening from near the center of the vein toward the vein margins (Bons and Montenari, 2005; Bons et al.,
2012). Texture Fibrous Radial Fibrous Block Sheets
Fabric linear radial linear planar microcrystalline microcrystalline
Gypsum Variety satin spar, selenite satin spar selenite selenite selenite alabaster alabaster antitaxial antitaxial Growth Pattern antitaxial antitaxial (center to edge) (radial) SG, gyp. beds, Occurrence ubiquitous nodules (SG) SG SG gyp. beds Trm-Trc contact (TO) nodules crystal growth crystal formation and Significance kept pace with cavity fill (slow crystal (de)hydration fracture widening growth?) in arid soil Figure 47. Gypsum textures observed in the field. APPENDIX C
FRACTURE TYPES
Appendix C summarizes terminology used to describe fractures in this study. This study uses terminology to describe fractures that is consistent with published work (Table
8; Fig. 48; Sorkhabi, 2014a and 2014b).
Table 8. Fracture terminology relevant to this study (compare Fig. 48). Term Description Antitaxial Fibrous texture continuous across the width of the vein, coarsening crystals toward the vein margin, and common fragments of wall rock. This texture indicates multiple crack- and-seal events where the vein grew incrementally outward with the widening fracture aperture—the fracture opening was never as wide as the vein (Bons et al., 2012). Aperture The space between the walls of a fracture or walls of a vein. Cavity A void space preserved or generated within the rock. Fault A closed fracture with displacement across it. Fracture Any planar of sub-planar separation in a rock. Joint An open-mode fracture without appreciable displacement across it or mineral filling. Microfault A fault with no more than ~0.5 m of measurable offset. Nodule A mineral-filled cavity. Shear Vein A vein with shear fabrics, such as sigmoidal fabrics or displacement across them. Syntaxial Fibrous texture that results from the inward growth of a vein. The vein fibers grow in an open vein from the fracture wall and crystals meet at the center of the vein or grow from one vein wall to the other (Bons et al., 2012). Tension An en echelon vein array that develops in response to shear. Individual gashes are open Gash mode fractures filled with minerals. An enveloping surface that encompasses the array is parallel to the shear plane. Vein A mineral-filled open-mode fracture. 149
Figure 48. Graphical description of the terminology used to describe fracture types with increasing complexity going down. APPENDIX D
SAMPLES
Appendix D summarizes sample information important to this study, their localities (see Appendix A), and the analyses used to interrogate them.
A total of 56 samples (24 SG, 30 TO, 2 SB) were analyzed for this study (Tables
9, 10). X-ray powder diffraction was used to analyze 14 samples with analytical parameters 29 = 5-55°, 2°/min (10 SG, 4 TO). A total of 21 thin sections were prepared for petrographic and QEMScan (Quantitative electron microprobe scan) analyses by the
University of Utah Thin Section Lab and Spectrum Petrographics (Fig. 49). Whole-rock and trace-element geochemistry was performed on 35 samples (12 SG, 22 TO, 1 SB) and
Sulfur and/or Strontium isotope geochemistry on 37 samples (13 SG, 22 TO, 2 SG)
(Appendix G). 151
Table 9. SG samples with summary descriptions and corresponding analyses. Sample Locality Unit Description XRD Thin Sec. Geochem S130730 01 SG-S Trmu vein, spar X X WTSSr S141003 01 SG-N Trmu vein, spar X S141004 03 SG-N Trmu vein, spar WTSSr S141004 10f SG-N Trmu vein, spar WTSSr S131016 05 SG-N Trmu vein, spar WTSSr S131016 04v SG-N Trmu vein, spar X S131016 08 SG-N Trmu vein, sel X S141004 04 SG-N Trmu vein, sel WTSSr S141004 10s SG-N Trmu vein, spar/sel X WTSSr S131016 02 SG-N Trmu bed, al/spar XX WTSSr S131017 12 near SG-S Trmu bed, al XX WTSSr S131016 04n SG-N Trmu nodule, spar X S131016 10 SG-N Trmu nodule, al X S141003 04 SG-N Trmu nodule, spar WTSSr S131016 04r SG-N Trmu Ms, red X S131016 05r SG-N Trmu Ms, red WTSSr S131016 06r SG-N Trmu Ms, red X S131017 14 SG-S Trmu Ms, red X S131016 05g SG-N Trmu Ms, green X WTSSr S131016 06g SG-N Trmu Ms, green XX S131016 07 SG-N Trmu Ms, green X S141005 02 SG-N Trmu Ms, green X S141005 01 SG-N Trmu Ms, yellow/green WTSSr S131015 01 SG-N Trcs Ss X SSr Unit: Trmu = upper red member of Moenkopi Fm. Trcs = Shinarump Conglomerate Mbr of Chinle Fm, Jcp = Paria River Mbr of Carmel Fm, Q = Holocene surficial deposits. Description: gypsum: spar = satin spar, sel = selenite, al = alabaster. Ms = mudstone, Ss = sandstone. Geochemistry: W = whole rock, T = trace element, S = sulfur isotope, Sr = strontium isotope. 152
Table 10. TO and SB samples with summary descriptions and corresponding analyses. Sample Locality Unit Description XRD Thin Sec. Geochem T140518 01 TO-S Trmm vein, spar WTSSr T140605 07 TO-S Trmm vein, spar X+ T140829 01 TO-W Trmm vein, spar WTSSr T140523 01v TO-VR Trmm vein, spar WTSSr T141107 03 TO-N Trmb vein, spar WTSSr T140605 08 TO-S Trmm vein, spar X+ T140829 03 TO-S Trmm vein, spar X+ T140831 01 TO-S Trmm vein, spar X+ T140831 02 TO-S Trmm vein, spar WTSSr T140830 01 TO-W Trmm vein, spar WTSSr T140523 05 TO-VR Trmm vein, spar WTSSr T141107 02 TO-N Trmb vein, spar WTSSr T150821_03 near TO-W Trmm vein, sel, near X Trcs T140830 04 TO-VR Trmm bed, al X+ WTSSr T141107 01 TO-N Trmb bed, al WTSSr T150821 02 near TO-W Trmm bed, al X WTSSr T140531 01 TO (Gilbert Bench) Q pedogenic spar WTSSr T140522 01 TO (Black Ridge) Jcp bed, al WTSSr T140901 A TO (Black Ridge) Jcp bed, al WTSSr T140518 03 TO-S Trmm Ms, red WTSSr+ T140605 03 TO-W Trmm Ms, red WTSSr T140605 05 TO-S Trmm Ms, red X+ T140523 01r TO-VR Trmm Ms, red WTSSr+ T140518 02 TO-S Trmm Ms, green WTSSr T140604 02 TO-S Trmm Ms, green X+ T140605 02 TO-W Trmm Ms, green WTSSr+ T140605 04 TO-W Trmm Ms, green X+ T140523 02 TO-VR Trmm Ms, green WTSSr T140517_01 TO-N Trms algal X WTSr boundstone T150821 04 near TO-W Trcs Ss X SSr U141125 01L SB Trm? Ms, green WTSSr U141125 01p SB Trm? pyrite S Unit: Moenkopi Fm: Trmb = Black Dragon Mbr, Trms = Sinbad Limestone, Trmm = Moody Canyon Mbr. Trcs = Shinarump Conglomerate Mbr of Chinle Fm, Jcp = Paria River Mbr of Carmel Fm, Q = Holocene surficial deposits. Description: gypsum: spar = satin spar, sel = selenite, al = alabaster. Ms = mudstone, Ss = sandstone. Thin Section: + = analyzed with QEMScan. Geochemistry: W = whole rock, T = trace element, S = sulfur isotope, Sr = strontium isotope, + = Sr samples were leached again after 1 day (all Sr samples were leached after 1 hour). 153
St George Torrey
Vein Vein Vein Vein Vein Vein Vein Selenite Spar/Selenite Spar/Selenite Satin Spar Satin Spar Satin Spar Satin Spar SG-S SG-N SG-N SG-N TO-S TO-S TO-S
Gypsum Bed Gypsum Bed Nodule Vein and Mudstone Vein Vein Gypsum Bed Alabaster Alabaster Alabaster/Spar Spar/Red Mst Satin Spar Satin Spar Alabaster near SG-S SG-N SG-N near SG-S TO-W TO-W TO-VR
Vein and Mudstone Siltstone Siltstone Siltstone Mudstone Mudstone Mudstone Spar/Green Mst Green/some Red Green to Red Yellow Green/Red Green Red SG-N SG-N SG-N SG-N TO-W TO-W TO-W Figure 49. Thin sections in cross-polarized light. Vein thin sections are oriented perpendicular to the vein. Samples that have been scanned with QEMScan are shown with QEMScan result overlays (peach = sulfates, purple = Fe- and Mg-bearing species, gray/blue = quartz and feldspar). APPENDIX E
2D VEIN MEASUREMENTS
Appendix E expands upon methods and results of 2D vein measurements using high- resolution panoramic imagery.
Panoramic images were captured by mounting a digital camera on a GigaPan EPIC 100 robotic arm, stitched using GigaPan Stitch 2.3.x software, and analyzed using Adobe Illustrator software and a protractor and ruler. Several scale references were placed in images to allow for an accurate scaled image analysis and oblique-face angle correction. The cameras that were used for these photomosaics lacked exposure locking, so some gigapans have nonuniform coloration and were manually corrected for color prior to stitching individual images into panoramic images. Coloration in this study is less important than relative changes in color and other features which are generally independent of hue.
Gigapans and other images were scaled and overlain with a 1 m grid, such that each meter of stratigraphic height was measured with 2 adjacent grid cells, except where exposure was too poor to capture a 2 x 1-m area (Figs. 45, 46). These grids were then used to measure vein length, thickness, general orientation (horizontal or vertical), intersection types, and intersection angles (Figs. 50, 51). Resolution of measurements was restricted to the size and resolution of the images, but measurements are accurate to 0.5 cm for most measured sections and potential error does not exceed 1 cm. This error results in a resolution bias toward thicker veins when veins are a Vein Thickness (cm) thickness (b) and exposed length (c), from 2D analysis of high-resolution high-resolution of analysis 2D from (c), length exposed and (b) thickness panoramic images of key outcrops. Resolution permitted measurements to to measurements permitted accuracy. ~0.5-cm Resolution outcrops. key of images panoramic 50. Figure 0
1
enTikes(m Vei (cm) nLength (cm) Thickness Vein Vein exposed length vs. thickness (a) and histograms of vein vein of histograms and (a) thickness vs. length exposed Vein 2
3
4
5
0
40
80
120
160
200 155 156
Figure 51. Vein measurements as measured in 2D analyses of high- resolution panoramic images at the SG-N (top) and TO (various, bottom) localities. In the SG measurements, the interval from 7 to 25 m was covered and resolution was not sufficient to reliably discern gypsum veins above 50 m (compare Fig. 5). 157 thinner than 1 cm.
873 veins and 784 intersections were measured in the SG field area and 1477 veins and
1845 intersections were measured in the TO field area. 2D vein measurements were made of cliff faces with a variety of orientations and are believed to be representative of the 3D system.
Vein density was calculated from 2D scaled Gigapan measurements, such that
2(L XW)+N D =—------A where D is vein density, L is the length of a vein within the gridded area, W is the width of a vein in the gridded area, N is area occupied by nodules in the gridded area, and A is the total gridded area.
Tension gashes are useful for rock geomechanics because they record both a dilation direction, inferring the least principal stress (0 3), and are parallel to the maximum compressive stress (0 1 ). The surface that envelopes the gashes is a shear plane. The angle between the shear plane and the principal stresses indicates the angle of internal friction, or cohesion, of the rocks
(Fig. 52). The angle between 0 1 and the shear plane tends to be ~20°, which correlates to an angle of internal friction of ~50°. 158
Figure 52. Tension gashes and “elbowing” at the TO-S outcrop, with a dip of the tension gash package dipping about 30° E and each gash subhorizontal. Also note the elbow bend (133°) at the top of the image and elbow split (79°) at the left of the image. APPENDIX F
GEOCHEMICAL DATA
Appendix F summarizes geochemical data from samples collected as part of this study (Appendix D) and expands on analytical methods. Four geochemical techniques were used: bulk-rock geochemistry, trace-element whole-rock geochemistry, strontium stable isotope geochemistry, and sulfur stable isotope geochemistry.
Bulk-rock and trace-element geochemistry were performed on 35 samples (12
SG, 22 TO, 1 SB) by ALS Minerals using the instruments WST-SEQ (LOI, sulfur in sulfate) and ICP-AES (whole rock and trace element) (Tables 11, 12, 13, 14). SO3 was not analyzed by ICP-AES, so was estimated using the difference between total fractional weight of analytes and 100% as well as S in sulfate, such that 5/2 the weight of S would be roughly equivalent to the weight of the SO3 compound released by the plasma torch of the ICP-AES. A selenite vein was analyzed to have relative low S (T150821_03; Table
13) and Sinbad Limestone to have high S (T14057_01, Table 14)—it is possible that the sample numbers between these two were accidentally swapped.
Sr isotope geochemistry of 37 samples (13 SG, 23 TO, 1 SB) was analyzed by the
Strontium Isotope Geochemistry Laboratory at the University of Utah using the
“Neptune” multicollector ICP-MS instrument with a standard of 87Sr/86Sr 0.71028, after having dissolved samples in 5% HNO3. All samples were leached after one hour. Three mudstone samples were leached again after one day to test for dependence on clay Table 11. Whole-rock geochemistry for gypsum samples collected in the SG and TO areas, presented in percent weight. Sample Si02 A1203 Fe203 CaO MgO Na20 K20 Cr203 Ti02 MnO P205 SrO BaO *S03 LOl **51 Detection Limit 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Veins SI30730 01 0.51 0.05 0.06 33.7 0.03 0.01 0.03 ---- 0.02 - 44.7 20.9 18.0 S 131016 05 0.79 0.14 0.05 33.0 0.06 0.01 0.05 - 0.01 -- 0.03 - 44.9 21.0 17.6 S 141004 03 0.33 0.06 0.04 33.2 0.03 - 0.04 ---- 0.02 - 45.3 21.0 17.7 S 141004 04 1.78 0.41 0.21 32.6 0.12 0.03 0.13 - 0.02 - 0.01 0.03 - 43.9 20.8 17.2 S 141004 lOf 1.05 0.07 0.31 32.8 0.02 0.01 0.04 - 0.01 -- 0.03 - 44.8 20.9 17.7
S 141004 10s 0.64 0.10 0.04 32.8 0.03 0.01 0.01 - 0.01 0.03 - 0.04 - 45.3 21.0 17.9
T 140518 01 7.23 1.21 0.69 31.1 0.16 0.11 0.31 - 0.05 0.01 0.01 0.02 0.01 38.8 20.3 16.9
T140523 01v 1.33 0.19 0.18 34.0 0.04 0.02 0.07 0.01 -- 0.02 - 43.1 21.0 18.8 T 140523 05 2.47 0.55 0.22 33.1 0.07 0.03 0.14 - 0.03 -- 0.02 - 42.5 20.9 18.4 T 140829 01 4.02 0.78 0.29 32.6 0.10 0.05 0.2 - 0.03 - 0.01 0.01 - 40.9 21.0 17.9 T140830 01 2.50 0.37 0.33 32.6 0.04 0.01 0.12 - 0.03 - 0.01 0.01 - 64.0 18.4 T140831 02 2.29 0.37 0.23 33.3 0.06 0.02 0.11 - 0.02 -- 0.02 - 42.6 21.0 18.6 T141107 02 2.43 0.42 0.40 33.5 0.17 0.01 0.14 - 0.02 - 0.01 0.06 - 62.8 18.1 T141107 03 3.95 0.79 0.34 32.7 0.14 0.03 0.20 0.04 - 0.01 0.03 - 41.0 20.8 18.0 T150821 03 4.56 1.31 0.55 50.2 0.88 0.05 0.37 - 0.05 0.02 0.03 0.07 - 0.64 41.3 0.03 Gypsum beds
S131016 02 0.73 0.10 0.16 33.5 0.04 - 0.03 - 0.01 -- 0.2 - 44.7 20.5 18.0 SI 31017 12 0.88 0.15 0.19 32.6 0.05 - 0.05 - 0.01 -- 0.21 - 45.1 20.8 17.8 T140830 04 2.32 0.49 0.21 32.7 0.17 0.01 0.18 - 0.02 -- 0.11 - 43.0 20.8 18.2
T141107 01 6.23 1.40 0.74 31.1 0.52 0.04 0.41 - 0.07 0.01 0.02 0.09 0.01 59.4 16.8
T 1 50821 02 1.08 0.23 0.18 31.7 0.09 - 0.04 - 0.01 -- 0.09 45.3 21.3 17.9 Gypsum nodules
S 141003 04 0.70 0.10 0.05 32.8 0.03 0.02 0.01 - 0.01 -- 0.14 - 45.1 21.0 17.9 Modem pedogenic gypsum
T140531 01 3.00 0.39 0.22 32.9 0.16 0.03 0.12 - 0.03 - 0.01 0.20 - 41.7 21.2 17.8 Carmel Formation (Paria River Member) gypsum beds
T140522 01 0.70 0.12 0.07 33.4 0.03 - 0.04 - 0.01 -- 0.12 - 44.5 21 18.6
T 140901 A 3.68 0.59 0.30 33.0 0.21 0.02 0.25 - 0.03 - 0.01 0.10 - 40.9 20.9 17.6 S .., = SG sample, T ... = TO sample, blank = non-sufficient sample, - = concentration below detection limits. * - S03 was calculated from sulfate S from a separate analysis using the same device (ICP-AES) ** S from sulfate. Table 12. Whole-rock geochemistry for host rock samples collected in the SG and TO areas, presented in percent weight. Sample Si02 A1203 Fe203 CaO MgO N a20 K 20 Cr203 Ti02 MnO P205 SrO BaO *S03 LOl Detection Limit 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Red mudstone S131016 05r 59.8 11.2 4.12 7.24 4.14 1.41 2.78 0.01 0.64 0.09 0.16 0.04 0.07 - 10.1 0.26 T 140518 03 47.7 14.2 6.40 7.04 5.02 0.31 4.60 0.01 0.56 0.07 0.12 0.04 0.05 0.30 13.6 0.81 TI40605 03 46.0 12.6 5.17 6.68 7.00 0.36 4.58 0.01 0.51 0.07 0.13 0.08 0.04 3.10 13.7 1.27 TI40523 Olr 31.7 9.39 4.10 14.8 4.11 0.23 3.49 0.01 0.37 0.05 0.10 0.07 0.03 15.3 16.4 6.8 Green mudstone S131016 05g 56.0 10.45 2.70 8.86 4.00 1.37 2.62 0.01 0.64 0.09 0.16 0.11 0.07 1.70 11.3 0.86 S 141005 01 79.5 11.25 3.07 0.17 0.58 0.13 2.05 0.01 0.68 0.01 0.12 0.01 0.03 - 4.3 0.01 T 140518 02 59.7 10.95 2.62 5.73 4.51 0.40 3.5 0.01 0.37 0.07 0.08 0.04 0.08 0.20 11.8 0.50 T 140523 02 48.4 9.81 2.59 8.87 7.52 0.37 3.58 0.01 0.48 0.08 0.12 0.08 0.05 1.80 16.3 1.01 T 140605 02 50.4 13.65 3.42 5.45 7.31 0.40 4.86 0.01 0.55 0.07 0.13 0.05 0.05 0.40 13.3 0.39 Sinbad Limestone algal boundstone
TI40517 01 2.13 0.54 0.21 31.5 0.05 - 0.05 - 0.02 - 0.01 0.01 - 43.8 56.2 n/a SB mudstone U 141125 OIL 61.2 17.7 3.84 1.44 1.56 0.25 5.12 0.01 0.76 0.02 0.16 0.03 0.05 1.42 6.5 0.01 S... = SG sample, T ... = TO sample, blank = non-sufficient sample, - = concentration below detection limits, n/a = not analyzed. * = S03 was calculated from sulfate S from a separate analysis using the same device (ICP-AES) ** = S from sulfate. Table 13. Trace-element geochemistry for gypsum samples collected in the SG and TO areas, presented in ppm or percent weight (specified with %). Ag, As, B, Be, Bi, Cd, Ga, Hg, La, Mo, Sb, Sc, Th, Tl, U, and W are omitted because concentrations are consistently near or below detection level. Sample Al% Ba Ca% Co Cr Cu Fe% K% Mg% Mn Na% Ni P Pb S% Sr Ti% V Zn Detection Limit 0.01 10 0.01 1 1 1 0.01 0 .0 1 0.01 5 0.01 1 10 2 0.01 1 0.01 1 2
SI30730 01 0.01 - 18.9 -- 3 0.04 - 0.01 ---- 4 >10 168 ---
S 131016 05 0.03 10 16.6 0 1 1 0.03 0.01 0.03 10 -- 10 2 >10 275 - 1 -
SI41004 03 0.01 - 20.3 -- 1 0.03 - 0.01 0 0 0 10 2 >10 205 - 1 - SI41004 04 0.07 - 17.4 - 1 2 0.13 0.02 0.05 14 0.01 1 20 3 >10 280 - 3 S H I004 lOf 0.01 - 18.4 - 1 2 0.15 - 0.01 19 --- 3 >10 229 - 1 - S 141004 10s 0.02 10 16.6 2 - 2 0.03 - 0.01 216 - 1 10 - >10 305 - 1 2
T 140518 01 0.08 10 19.0 - 5 9 0.34 0.05 0.04 47 - 6 50 7 >10 161 - 11
Tl40523 01v 0.02 - 20.4 - 1 1 0.07 0.01 0.01 9 - 1 - 2 >10 191 - 1 -
T140523 05 0.04 - 18.5 - 2 1 0.11 0.02 0.01 14 - 2 10 2 >10 153 - 1 2 Tl40829 01 0.06 10 19.8 - 3 4 0.17 0.03 0.02 25 - 3 20 4 >10 130 - 1 15 T l40830 01 0.05 10 20.1 - 4 8 0.22 0.02 0.01 26 - 4 10 4 >10 133 - 1 41 Tl40831 02 0.04 10 20.0 - 2 2 0.13 0.02 0.02 18 - 3 10 4 >10 161 - I 14 T141107 02 0.05 20 19.5 - 3 14 0.21 0.02 0.07 32 - 4 20 4 >10 583 - 2 27 T141107 03 0.09 10 19.8 - 3 6 0.21 0.04 0.05 32 - 3 20 4 >10 286 - 2 53
T 150821 03 0.22 10 >25 - 3 5 0.31 0.10 0.44 116 0.02 0 120 2 0.11 630 - 4 12 Gypsum beds S 131016 02 0.03 10 19.6 1 2 0.11 0.01 0.02 13 0.01 -- 2 >10 1810 - 1 - S131017 12 0.03 10 15.5 1 1 0.12 0.01 0.02 15 0.01 1 10 2 >10 1710 - 1 - Tl40830 04 0.04 10 19.7 2 1 0.12 0.02 0.08 21 - 2 10 3 >10 943 2 3 T141107 01 0.15 20 19.0 1 5 6 0.44 0.07 0.24 68 0.01 5 70 5 >10 807 4 29 T 150821 02 0.03 - 20.1 1 11 0.07 0.01 0.04 10 - 3 10 2 >10 812 15 Gypsum nodules
SI41003 04 0.02 10 16.4 - 1 0.04 - 0.01 5 0.01 -- 2 >10 1210 - 1 - Modem pedogenic gypsum
T 140531 01 0.05 10 19.4 - 3 3 0.12 0.02 0.06 23 0.01 3 40 3 >10 1915 - 3 - Carmel Formation (Paria River Member) gypsum beds
T 140522 01 0.01 10 19.8 1 - 0.04 0.01 0.01 6 - 2 -- >10 1045 - - 4 T l40901 A 0.07 10 19.7 2 1 0.18 0.04 0.09 32 0.01 2 30 2 >10 896 2 2 S... — SG sample, T ... - TO sample, blank - non-sufficient sample, - = concentration below detection limits. Table 14. Trace-element geochemistry for host rock samples collected in the SG and TO areas, presented in ppm or percent weight (specified with %). Ag, As, B, Be, Bi, Cd, Ga, Hg, La, Mo, Sb, Sc, Th, Tl, U, and W are omitted because concentrations are consistently near or below detection level. Sample Al% Ba Ca% Co Cr Cu Fe% K% Mg% Mn Na% Ni P Pb S% Sr Ti% V Zn Dctcction Limit 0.01 10 0.01 1 1 1 0.01 0.01 0.01 5 0.01 1 10 2 0.01 1 0.01 1 2 Red mudstone S131016 05r 1.58 290 4.84 10 25 11 2.36 0.43 2.12 615 0.04 24 650 12 0.29 334 0.03 55 70 T1405I8 03 1.25 100 4.44 10 31 11 3.80 0.73 2.37 503 0.14 27 320 9 0.98 137 0.04 38 53 T140605 03 1.99 100 4.29 11 23 16 2.85 0.69 3.46 475 0.19 27 520 17 1.38 655 0.03 42 76 Tl40523 Olr 1.36 40 9.8 8 18 12 2.30 0.54 1.92 358 0.11 20 370 12 7.31 552 0.02 22 53 Green mudstone S131016 05g 1.52 270 6.1 10 22 12 1.44 0.42 2.13 626 0.05 23 700 2 0.93 987 0.02 47 68 SI41005 01 0.46 10 0.12 9 12 69 1.70 0.17 0.14 47 0.04 18 440 2 0.01 39 0.01 19 30 T1405I8 02 0.75 350 3.75 15 14 28 1.48 0.43 2.29 484 0.21 37 240 3 0.60 239 0.01 33 60 T 140523 02 2.04 80 3.46 11 22 88 1.69 0.71 3.49 461 0.19 28 500 4 0.49 368 0.01 490 99 T 140605 02 1.42 130 5.57 8 17 29 1.28 0.56 3.78 570 0.17 21 450 4 1.06 642 0.01 44 54 Sinbad Limestone algal boundstone
T140517 01 0.09 10 19.1 - 2 234 0.13 0.03 0.02 9 -- 10 6 >10 108 - 1 11 SB mudstone U141125 OIL 0.46 10 0.12 9 12 69 1.70 0.17 0.14 47 0.04 18 440 2 0.01 39 0.01 19 30 S... — SG sample, T ... — TO sample, blank - non-sufficient sample, - = concentration below detection limits. 164 mineral dissolution—the difference in Sr isotope ratio due to digestion time was negligible (Table 15).
S isotope geochemistry of 37 samples (13 SG, 22 TO, 2 SB) was analyzed by the
Stable Isotope Ratio Facility for Environmental Research (SIRFER) at the University of
Utah, with standards of S34S of -32%o and about +18%o. The ratio of 34S/32S is standardized to Diablo Canyon Troilite. One of the SB samples was macroscopic secondary pyrite crystals, which were analyzed only for S isotopes (Table 15). Table 15. Stable-isotope geochemistry for samples collected in the SG and TO areas. Sample 87Sr/86Sr S (wt%) 534SCDT Sample 87Sr/86Sr S (wt%) 534SCDT Veins Carmel Fm gypsum S I 30730 01 0.7082 4.4 +13.4 T140522 01 0.7072 20.7 +17.5 SI31016 05 0.7083 10.7 +15.3 T 140901 A 0.7073 19.8 +19.5 SH I 004 03 0.7086 18.8 +18.4 Red mudstone S141004 04 0.7087 9.2 + 16.2 SI3I0I6 05r 0.7083 0.6 + 14.4 S141004 lOf 0.7085 2.4 +11.5 T 140518 03 0.7088 -- S141004 10s 0.7087 13.4 +17.7 T140518 03* 0.7089 T 140518 01 0.7085 27.1 +18.2 T 140605 03 0.7084 0.6 +13.4
IT40523 01v 0.7083 22.6 +24.3 T 140523 Olr 0.7084 -- IT 40523 05 0.7084 23.0 +24.4 T 140523 Olr* 0.7084 n/a n/a T140829 01 0.7084 23.8 +22.6 Green mudstone T140830 01 0.7083 26.4 +23.0 SI3I0I6 05g 0.7086 0.7 +14.8 T 140831 02 0.7084 25.4 +18.4 S141005 01 0.7110 0.1 +15.7 T141107 02 0.7082 19.0 +16.9 TI40518 02 0.7095 -- T141107 03 0.7082 23.0 +18.3 T 140523 02 0.7085 -- T 150821 03 0.7092 6.8 -6.0 T 140605 02 0.7084 0.9 +16.7 Gypsum beds T 140605 02* 0.7084 n/a n/a SI31016 02 0.7083 7.7 +14.5 Sinbad Limestone S131017 12 0.7083 9.4 +14.5 T 140517 01 0.7079 n/a n/a T140830 04 0.7085 16.9 +28.7 SB mudstone T 141107 01 0.7080 18.2 +20.0 U141125 OIL 0.7104 0.8 +0.0 T 150821 02 0.7086 25.5 +20.0 U 141125 01 pyrite n/a 69.2 +6.7 Gypsum nodules Shinarump sandstone
SMI003 04 0.7086 12.1 +15.9 S131015 01 0.7103 -- Pedogenic gypsum T15082104 0.7091 2.6 -6.2 T140531 01 0.7087 15.0 +36.2 S... = SG sample, T .. . = TO sample, - = concentration below detection limits, n/a = not analyzed. * = Sr leached again after 1 day (all Sr samples leached after 1 hour). APPENDIX G
QEMSCAN RESULTS
Appendix G summarizes results from QEMScan (Quantitative Electron
Microprobe Scan) analyses of select samples (Appendix D).
Thin sections were polished in preparation for QEMScan (Quantitative Electron
Microprobe Scan) analysis. The electron microprobe produces an electron beam, which scatters when it strikes a surface. The signal that returns to the sensor is roughly correlative to the size of atoms where the beam struck, which can be correlated to specific minerals. QEMScan is not sensitive to Hydrogen, however, so hydration state could not be determined with this technique.
The scan produces a 2D map of minerals (Fig. 49; Table 16). Because this study’s sample grain sizes are so small, “edge effects” are commonly encountered, or where the
QEMScan instrument detects elements from multiple grains, leading to a misinterpretation of that pixel. Red mudstone samples are rich in iron oxides, so there is a tendency to report micas as biotite, calcite as dolomite, etc. Hence, the chemistry represented in each pixel is more important than the assigned mineral name. Table 16. QEMScan results presented in percent normalized area covered by top-pick species. ------% normalized area------(excluding blank area, if different) Sample Type Blank Sulfates Quartz Mica + Plag Feldspar Biotite Fe-sulfide Dolomite Chlorite Other Oxides 49.59 19.01 9.48 3.35 1.29 0.96 0.22 0.25 0.43 T 140604 02 R/GMs 15.42 (58.63) (22.47) (11.21) (3.96) (1.52) (1.13) (0.26) (0.29) (0.51) T140605 04A GMs 0.00 4.86 75.03 5.74 8.94 2.27 0.45 0.61 1.02 1.08 42.64 26.73 6.44 4.30 3.00 2.05 0.45 0.34 0.83 T14060504B R/GMs 13.23 (49.14) (30.80) (7.42) (4.96) (3.46) (2.36) (0.52) (0.39) (0.95) 15.39 7.47 15.51 4.02 18.50 3.30 0.11 T 140605 05 RMs 35.70 (23.93) (11.61) (24.13) (6.25) (28.78) (5.13) (0.17) 93.13 T 14060507 vein 6.870 ------(100.00) 81.48 0.62 0.35 0.18 0.49 0.04 T 140605 08A vein 16.83 - 0.01 - (97.97) (0.74) (0.42) (0.22) (0.59) (0.05) 95.01 0.28 0.05 0.11 T140605 08B vein 4.49 0.03 0.01 --- (99.48) (0.29) (0.06) (0.12) 96.10 1.43 T 140605_08uA vein 2.08 0.08 0.07 0.22 0.02 - 0.01 (98.14) (1.46) 67.74 3.68 0.02 TI40605_08uB vein 28.55 --- (94.81) (5.14) (0.01) (0.01) (0.01) (0.03) 96.55 1.86 T 140829 03 vein 1.42 0.03 0.14 ----- (97.94) (1.89) 99.89 T I4083004C bed 0.06 0.04 --- 0.01 --- (99.95) 99.47 T140830 08B bed 0.31 0.12 0.01 0.01 - 0.07 --- (99.78) 95.59 1.54 0.56 T I4083101 vein 2.17 0.10 - 0.01 -- 0.03 (97.71) (1.58) (0.57) R/GMs — red/green mudstone, vein = gypsum vein, bed = gypsum bed nodule, - - <0.01%. APPENDIX H
REDUCTION COLORATION OF THE MOENKOPI FORMATION
Appendix H is a discussion of the origin of gray-green banding and reduction spots in otherwise red mudstones. It was not included in the main body of the text because it was determined to be a derivative of the text, which focuses on the gypsum veins that cross-cut the Moenkopi Formation. The red coloration was hypothesized to be a product of early or syn-depositional diagenesis and green coloration also early-stage diagenesis, possibly related to groundwater or paleosols. Furthermore, the green coloration was hypothesized to be unrelated to the presence of gypsum. This is a discussion expanded from material presented in chapter 1: Triassic Moenkopi Formation gypsum veins: Diagenesis and structure, southern Utah.
Red coloration is characterized by the presence of iron oxides around subrounded to rounded quartz and feldspar grains, whereas gray-green areas are primarily quartz and feldspar with little to no iron oxides, and both changes in coloration sometimes gradually pinch out laterally within 90 to 120 m (Van Deventer, 1974). Coloration parallels stratification but often crosses lithologic boundaries. Green coloration seems to have occurred later or synchronous with red coloration, suggested by its convex-out spotty character and the presence of reduction spots in otherwise red mudstone, where the reduction spots have ellipticity consistent with modeled vertical shortening during compaction during the second of two burial events (Figs. 14b, 15). 169
Most mudstones are oxic by the Ni/Co and V/Cr proxies, though green mudstones have a greater tendency toward dysoxia (Fig. 12). Some gypsum beds and veins plot toward anoxia. Some green coloration is coincident with gypsum bed nodules, coloring nodule walls (Fig. 14c), but green coloration is not coincident with more discrete gypsum nodules. The green coloration tends to be associated with siltier layers so may be due to locally present organics deposited with Moenkopi sediments which, when buried to 3-4 km depth (Fig. 15), released hydrocarbons that produced local reduction during or after the emplacement of iron oxidation in the remainder of the unit. The green coloration along gypsum bed nodules may be due to migration of some of these primary hydrocarbons, retarded at the interface with the relatively impermeable calcium sulfate, and/or reduction adjacent to primary sulfates, possibly due to bacterial sulfate metabolism shortly after deposition or during early burial. The reason that the SB core is not red is probably related to hydrocarbons, as intervals in the SB core are oily (Fig. 5).
Green coloration follows vertical fractures in uppermost SG redbeds, and the uppermost few meters of the Moenkopi in both areas tends to be coarser grained and colored pale gray-green to yellow. Chemical mixing with Chinle waters, evidenced by a color change and lack of gypsum veins in the uppermost Moenkopi and the similarity in
S and Sr isotopes between a selenite vein near the Moenkopi-Chinle contact and the
Shinarump Conglomerate, may be responsible for prohibiting the precipitation of gypsum to the uppermost Moenkopi as well as mobilizing reducing and/or bleaching agents that traveled along vertical fractures instead of the emplacement of gypsum in those fractures.
In summary, red coloration is early or burial-stage diagenetic, followed by or synchronous with one or more phases of green coloration (in situ reduction) between 160 170 and 95 Ma. Green coloration was followed by the emplacement of the veins, which was suppressed in the uppermost Moenkopi because of chemical mixture with Chinle waters and simultaneous remobilization of reducing agents along fractures in the uppermost
Moenkopi Formation. APPENDIX I
IGNEOUS DIKE AR-AR AGE DATA
Appendix I presents information concerning the analysis of an igneous dike sampled in the Torrey 7.5’ quadrangle.
An age for a basaltic dike, vertically oriented with a N-S strike, 2-3 m thick, and located at 38.350476 N, 111.451149 W (NAD27) was determined by Ar-Ar age dating by
ALS Minerals in 2015 with the ALS code Ar-ISTP01. The sample was processed at the
New Mexico Geochronology Research Laboratory (NMGRL) Argon Lab, Socorro, New
Mexico. The sample delivered was 0.15 kg. Amphiboles/hornblende in the sample were analyzed for Ar. The result of the analysis models dike crystallization at 5.290±0.040 Ma.