The Detrital Zircon Record of Back-Arc Basin Deposits, Triassic Buckskin Formation

Research Paper

GEOSPHERE Evolution of the early Mesozoic Cordilleran arc: The detrital zircon record of back-arc basin deposits, Triassic Buckskin Formation,

GEOSPHERE, v. 16, no. 4 western Arizona and southeastern California, USA https://doi.org/10.1130/GES02193.1 N.R. Riggs1, T.B. Sanchez1,*, and S.J. Reynolds2 1Division of Geosciences, School of Earth and Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA 8 figures; 1 table; 1 set of external supplemental 2School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA files (hosted by EarthChem Library)

During deposition of the lower and quartzite members, the Cordilleran arc CORRESPONDENCE: [email protected] ABSTRACT was offshore and likely dominantly marine. Sedimentation patterns were CITATION: Riggs, N.R., Sanchez, T.B., and Reynolds, A shift in the depositional systems and tectonic regime along the western most strongly influenced by the Sonoma orogeny in northern Nevada and S.J., 2020, Evolution of the early Mesozoic Cordilleran margin of Laurentia marked the end of the Paleozoic Era. The record of this Utah (USA). The Tr-3 unconformity corresponds to both a lull in magmatism arc: The detrital zircon record of back-arc basin depos- transition and the inception and tectonic development of the Permo-Trias- and the “shoaling” of the arc. The phyllite and upper members were deposited its, Triassic Buckskin Formation, western Arizona and southeastern California, USA: Geosphere, v. 16, no. 4, sic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern in a sedimentary system that was still influenced by a strong contribution of p. 1042–1057, https://doi.org/10.1130/GES02193.1. North America, in successions in the distal back-arc region on the Colorado detritus from headwaters far to the southeast, but more locally by a develop- Plateau, and in the more proximal back-arc region in the rocks of the Buckskin ing arc that had a far stronger effect on sedimentation than the initial phases Science Editor: David E. Fastovsky Formation of southeastern California and west-central Arizona (southwestern of magmatism during deposition of the basal members. Associate Editor: Todd LaMaskin North America). The Buckskin Formation is correlated to the Lower–Middle Triassic Moen- Received 26 August 2019 Revision received 27 April 2020 kopi and Upper Triassic Chinle Formations of the Colorado Plateau based on ■■ INTRODUCTION Accepted 2 June 2020 stratigraphic facies and position and new detrital zircon data. Calcareous, fine- to medium-grained and locally gypsiferous quartzites (quartz siltstone) of the Early Mesozoic sedimentary successions in southwestern North America Published online 30 June 2020 lower and quartzite members of the Buckskin Formation were deposited in a record a shift from a primarily carbonate- and clastic-dominated platform to marginal-marine environment between ca. 250 and 245 Ma, based on detrital continental-fluvial depositional systems (Lawton, 1994), coinciding with a zircon U-Pb data analysis, matching a detrital-zircon maximum depositional change in the tectonic setting of the southwestern margin of Laurentia from age of 250 Ma from the Holbrook Member of the Moenkopi Formation. An passive to convergent (Lawton, 1994; Dickinson and Lawton, 2001). Permian unconformity that separates the quartzite and phyllite members is inferred strata from the forearc or arc-proximal areas, exposed in northwestern Sonora to be the Tr-3 unconformity that is documented across the Colorado Plateau, (Mexico) and south-central California (USA), record nascent subduction along and marks a transition in depositional environments. Rocks of the phyllite and the western margin of Laurentia (Rains et al., 2012; Dobbs et al., 2016). On the upper members were deposited in wholly continental depositional environ- Colorado Plateau, Triassic marginal-marine and fluvial successions are the ments beginning at ca. 220 Ma. Lenticular bodies of pebble to cobble (meta) earliest retro-arc sedimentary record of the development of the early Mesozoic conglomerate and medium- to coarse-grained phyllite (subfeldspathic or quartz Cordilleran arc (Cadigan, 1971; Stewart et al., 1972b; Riggs et al., 2012, 2016). wacke) in the phyllite member indicate deposition in fluvial systems, whereas The Lower–Middle Triassic Moenkopi Formation (Fig. 1) has rare arc-derived the fine- to medium-grained beds of quartzite (quartz arenite) in the upper detritus (Dickinson and Gehrels, 2008; Riggs et al., 2017), whereas the Upper member indicate deposition in fluvial and shallow-lacustrine environments. Triassic Chinle Formation contains abundant volcanic detritus and clasts that The lower and phyllite members show very strong age and Th/U overlap record arc magmatism, most plausibly from the Mojave Desert segment of with grains derived from Cordilleran arc plutons. A normalized-distribution the Cordilleran arc (Riggs et al., 2012, 2016). Variably metamorphosed upper plot of Triassic ages across southwestern North America shows peak mag- Paleozoic and lower Mesozoic sedimentary successions in western Arizona, matism at ca. 260–250 Ma and 230–210 Ma, with relatively less activity at ca. southeastern California, and northern Sonora are exposed closer to arc-related 240 Ma, when a land bridge between the arc and the continent was established. plutonic rocks and should therefore provide a record of arc initiation and the Ages and facies of the Buckskin Formation provide insight into the tec- earliest sedimentary transport of arc-derived material. The Buckskin Formation tono-magmatic evolution of early Mesozoic southwestern North America. (Fig. 1) of west-central Arizona and southeastern California has been tentatively This paper is published under the terms of the correlated with the Lower–Middle Triassic Moenkopi and the Upper Triassic CC‑BY-NC license. *Now at Pioneer Natural Resources, 5205 N. O’Connor Blvd., Suite 200, Irving, Texas 75039, USA Chinle Formations based on stratigraphic position and facies (Reynolds and

GEOSPHERE | Volume 16 | Number 4 Riggs et al. | Early Mesozoic Buckskin Formation, southwestern Laurentia Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/4/1042/5144881/1042.pdf 1042 by guest on 28 September 2021 Research Paper

118oW 114oW A overlying Vampire Formation. The Buckskin Formation therefore provides an excellent opportunity to investigate the influence of the incipient Cordilleran magmatic arc on proximal and regional depositional systems and to enhance Boundary of Colorado Plateau our understanding of the paleogeographic region between the arc and the B A S I N basin system in the back-arc region. This study provides additional strati- A N D graphic detail and uses detrital zircon geochronology to (1) confirm regional Golconda thrust R A N G E 0 150 km correlation of the Buckskin Formation to the Triassic successions of the Col- orado Plateau, (2) determine the maximum depositional age of the Buckskin S I E R R A N E V A D A N Formation and thus constrain the earliest retro-arc detrital record of early Las UTAH Mesozoic Cordilleran arc magmatism, and (3) relate the detrital zircon record of Vegas ARIZONA the Buckskin Formation to early Mesozoic regional tectonics and the influence CALIFORNIANEVADA COLORADO 36oN of the nascent magmatic arc on regional depositional systems. El Paso PLATEAU Mtns Barstow Kingman ■■ BACKGROUND Mojave Desert Winslow Los XPermo-Triassic Permo-Triassic Geologic Setting and Paleogeography Angeles plutonic rocks Blythe The tectonic development of the Permian–Triassic Cordilleran margin is 114o 30’W recorded by the sedimentary successions of basins that developed in response B USA to the Sonoma orogeny and to the incipient Cordilleran magmatic arc (Blakey, BUCKSKIN MEXICO MTNS 1989; Dickinson, 1992; Miller et al., 1992; Saleeby et al., 1992; Lawton, 1994). o CALIFORNIA Prior to development of the arc, a transcurrent fault (California-Coahuila fault 34 N MIDLAND Sonoran SYNCLINE PLOMOSA of Dickinson [2000]; Fig. 1) brought dominantly passive-margin tectonism to a ARIZONA PASS Permian o PALEN 32 N close beginning in Pennsylvanian time (see summary in Stevens et al. [2005]) MTNS plutonic rocks N and translated crustal slivers southeastward along the margin (Dickinson, 2000; 0 25 km SONORA Blythe Saleeby, 2011). A record of earliest arc magmatism is preserved in ca. 275 Ma plutonic suites in the Mojave and Sonoran Deserts (Walker, 1987; Barth et al., Figure 1. (A) Southwestern North America (not restored for Mesozoic or Cenozoic 1997; Barth and Wooden, 2006; Arvizu et al., 2009; Riggs et al., 2009; Arvizu deformation) showing approximate outcrop limit of Triassic sedimentary strata and Iriondo, 2015; Cecil et al., 2018). For the most part, these granitic suites related to the Moenkopi-Chinle deposystems in Nevada, Arizona, and Utah (USA) intrude Proterozoic crust of the Mojave and Yavapai provinces and overlying in gray; lighter shading indicates more fragmented outcrop pattern toward the west. Winslow, Arizona, marks sample sites of the Moenkopi Formation (see Paleozoic miogeoclinal shelfal deposits (Barth et al., 1997; Arvizu et al.; 2009; Table 1). Dashed line is the possible trace of a Pennsylvanian–Permian left-lateral Arvizu and Iriondo, 2015). The Cordilleran magmatic arc formed above an transcurrent fault (after Dickinson, 2000). Pale pink area is the highly approximate east-dipping subduction zone, likely in response to tectonic conversion of the area of Permo-Triassic magmatism (many areas have no exposures of arc rocks); transcurrent fault that cut across the passive margin in late Paleozoic time (i.e., blue X is location of specific plutons (Liebre Mountain, Manzanita Springs, San induced nucleation of subduction; Stern, 2004). Gabriel suite) mentioned in text. Golconda thrust is the eastern edge of the Sonoma orogenic belt. (B) Locations of Buckskin Formation outcrops discussed in this study. MTNS—mountains. Lower Mesozoic Sedimentary Units in Western Arizona and Southeastern California and on the Colorado Plateau Spencer, 1989; Hargrave, 1999), and we demonstrate that it preserves a record of the initiation of Cordilleran arc magmatism. Buckskin Formation The Buckskin Formation has also been considered critical in understanding patterns of uplift within the arc. Reynolds et al. (1989) documented an The Triassic Buckskin Formation is the oldest Mesozoic stratigraphic unit in early Mesozoic uplift that they speculated was influential in supplying detritus the deserts of western Arizona and southeastern California. It is thickest and to Upper Triassic and Lower Jurassic Colorado Plateau sedimentary basins, best exposed in the Buckskin and Palen Mountains (Reynolds and Spencer, based on an unconformity that separates the Buckskin Formation from the 1989; Hargrave, 1999), but is also exposed in the Midland syncline region of the

Little Maria Mountains (Fig. 1) and in stratigraphic and structural fragments in to represent a complex of deposits formed in continental and marine environ- other ranges throughout western Arizona and southeastern California (Fig. 1). ments (McKee, 1954; Stewart et al. 1972a). The unit is bounded stratigraphically The Buckskin Formation disconformably overlies the Permian Kaibab Formation by regional unconformities that generally separate the Moenkopi Formation (Reynolds et al., 1989; Hargrave, 1999), the uppermost unit of the Paleozoic from both underlying Pennsylvanian or Permian strata and the overlying passive-margin platform succession in the region, along a contact that is well Chinle Formation on the Colorado Plateau (Stewart et al., 1972a; Pipiringos exposed in the Palen Mountains but crops out only locally elsewhere (Stone and O’Sullivan, 1978). Fluvial deposits in the east grade laterally to marine and Kelly, 1989; Hargrave, 1999). The Buckskin Formation is divided into four deposits toward the west (Dubiel, 1994; Fig. 4A). Paleocurrent indicators in the members (Figs. 2, 3A, and 3B): (1) the lower member variably consists of quart- Moenkopi Formation primarily record sediment transport toward the north- zose to quartzofeldspathic sandstone and siltstone, phyllite, sandy metapelite, west and west with a dominant source region to the east (Stewart et al., 1972a; chloritic schist, and gypsiferous rocks; (2) the quartzite member is a massive Dubiel, 1994), but also record less-prominent southern and western sources to indistinctly bedded quartzite; (3) the phyllite member variably consists of of detritus (Stewart et al., 1972a). phyllite, slate, phyllitic siltstone, micaceous sandstone, and conglomerate; and In southern Nevada, northwestern Arizona, and southwestern Utah, the (4) the upper member is massive to poorly bedded, moderately to well-sorted Moenkopi Formation is divided into six members: (1) the Timpoweap Member quartzite (Reynolds and Spencer, 1989; Reynolds et al., 1989; Hargrave, 1999). is variably composed of siltstone, limestone, and a chert-pebble conglomerate; The Buckskin Formation has a structural thickness of as much as 900 m in the (2) the lower red member comprises horizontally stratified to structureless silt- Buckskin Mountains (Reynolds et al., 1989) and >500 m in the Palen Mountains. stone; (3) the Virgin Limestone Member is interbedded limestone and siltstone; The lower two members of Buckskin Formation have been correlated to the (4) the middle red member consists of horizontally laminated siltstone with Lower–Middle Triassic Moenkopi Formation, and the upper two members are interbedded gypsum layers; (5) the Shnabkaib Member comprises thick beds considered stratigraphically equivalent to the Upper Triassic Chinle Formation of red siltstone, gypsum, and some dolomite and limestone; and (6) the upper (Reynolds et al., 1987, 1989; Reynolds and Spencer, 1989; Hargrave, 1999). The red member includes laminated siltstone with variable beds of ripple-laminated depositional environments of the lower two members of the Buckskin Forma- siltstone and cross-stratified sandstone. These members together represent tion have been interpreted as shallow marine to marginal marine, whereas the cycles of transgression and regression (Stewart et al., 1972a). upper two members represent continental fluvial to possibly shallow-lacustrine depositional systems (Hargrave, 1999). The uppermost Buckskin Formation is regionally truncated by an irregular Chinle Formation unconformity that separates it from the overlying Jurassic Vampire Forma- tion (Reynolds et al., 1989). This unconformity is best exposed in the Buckskin A widespread paleosol marks the unconformity between the Moenkopi Mountains where it cuts downsection from west to east across the upper three Formation and overlying strata (Stewart et al., 1972a; Dickinson and Gehrels, members of the Buckskin Formation. Elsewhere, the unconformity reflects 2008). Across the Colorado Plateau, the Upper Triassic Chinle Formation dis- erosional removal of all Paleozoic and early Mesozoic rocks, locally exposing conformably overlies this paleosol in places and fills deep paleovalleys cut Proterozoic basement (Reynolds et al., 1989). Subsequent to the deposition of into the Moenkopi in others (Stewart et al., 1972b). Paleocurrent indicators the Buckskin and Vampire Formations and overlying Jurassic volcanic rocks, in the Chinle Formation are highly variable but suggest that sediment was the region proximal to the arc experienced several episodes of deformation dominantly transported toward the northwest, though the abundant volcanic and metamorphism (Reynolds et al., 1988) that subjected the Buckskin units to detritus in the deposit implies some sediment transport from the magmatic lower- to middle-greenschist-facies conditions (Reynolds and Spencer, 1989). arc developing to the west and southwest (Stewart et al., 1972b; Blakey and These episodes modified the original thicknesses of the members of the Buck- Gubitosa, 1983; Lupe and Silberling, 1985; Lucas and Marzolf, 1993; Riggs et al., skin Formation and destroyed most of the primary sedimentary structures, 1996; Howell, 2010; Howell and Blakey, 2013). complicating paleogeographic reconstructions and correlations of Mesozoic The lower, volcaniclastic part of the Chinle Formation in northern Arizona units to those of the Colorado Plateau (Reynolds et al., 1989). Sections in which is broadly divided into four distinct units that reflect continental depositional sedimentary structures are preserved provide the basis for environmental environments: (1) the Shinarump Conglomerate and Mesa Redondo Member interpretations. are thin but extensive, cross-stratified, fine- to coarse-grained sandstone with lenses of conglomerate; (2) the Blue Mesa and Bluewater Creek Members are claystone to clay-rich sandstone and siltstone; (3) the Sonsela Member com- Moenkopi Formation prises siliceous conglomeratic sandstone; and (4) the Petrified Forest Member is a variable bentonitic mudstone to sandstone unit that is the thickest and The Moenkopi Formation is the oldest regional sedimentary succession of most extensive member of the Chinle (Stewart et al., 1972b; Woody, 2006; the Triassic Period in southwestern North America and has been interpreted Martz and Parker, 2010). The upper part of the formation comprises the Owl

b) Buckskin Mountains Explanation Jurassic(?) Vampire Formation Quartzite, light grey; calcareous; weakly to strongly foliated Phyllite, pale to blue green or grey to pink-grey; weakly to strongly foliated a) Palen Mountains 550 Quartzite, white to silvery blue-green or Jurassic(?) Vampire Formation greenish-white, grey, brown; may be calcareous 550 500 Quartzite, grey to green, thin to thick bedded; may be calcareous or foliated Metaconglomerate, silvery green; 500 450 pebble- to cobble-size quartzite clasts Gypsiferous phyllite, silvery blue- green; weakly to strongly foliated 3-4-m beds, 450 ne to medium grained 400 Marble, grey to light pink, weakly to Upper member strongly foliated 3-4-m beds Covered interval; underlying pattern shows likely substrate lithology 400 350 Upper member Coarsening-upward sequence Zircon sample

350 300 indistinct 3-4-m bedding throughout local quartzite-clast layers; clasts <3 cm quartzite clasts 1 - <5 cm ne to medium grained 300 250 very ne to medium grained

1-8-m beds, ne to medium grained e member quartzite clasts <5 cm Phyllite interbedded very ne to coarse grained, 2-5 m ylli t 200 member 250 basal cgl w/ 1-3-cm quartzite clasts h P distinct 10-20-cm porcellanite beds common Quartzite 0.5-1-m beds; 10-20-cm member porcellanite interbeds Quartzite interbedded quartzite and phyllite 200 member 150 0.5-2 m; interbedded very- ne- grained phyllitic quartzite and ne-grained phyllite 1-3-m interbeds of non-gypsiferous quartzite er member

w 100

150 o interbedded 2-3 m ne-grained quartzite and 1 m L oat blocks dominated by micaceous quartzite gypsiferous phyllite Buckskin-Rawhide detachment fault (missing section) Lower member 100 phyllite and quartzite vf f m c vc p co Permian 50 Kaibab Formation

Permian Kaibab vf f m c vc p co Formation

Figure 2. Composite stratigraphic logs of the Buckskin Formation in the Palen Mountains and Buckskin Mountains; scale in meters. See Table 1 for sample numbers and locations of starred zircon samples. Grain size designations: vf—very fine; f—fine; m—medium; c—coarse; vc—very coarse; p—pebble; co—cobble. See Sanchez (2015) for detailed stratigraphic logs. cgl—conglomerate.

115°W 111°W Moenkopi Formation lithofacies Limestone, dolomite, and primary gypsum: shallow to restricted marine D Siltstone with primary gypsum: quiet water: restricted marine Horizontally strati ed claystone: 39°N quiet water, tidal to shelf Horizontally strati ed siltstone: quiet NEVADA water, tidal to low-energy uvial Cross-strati ed sandstone and siltstone: uvial and deltaic deposits

UTAH COLORADO Las ARIZONA NEW MEXICO Vegas Colorado Plateau

Flagsta Albuquerque 35°N ORNIA CALI F

Estuarine with tidal- Phoenix and uvial-currentinuence TEXAS Buckskin depocenter (minimum extent) 0 200 km

Upper part of Chinle Formation: Owl Rock Member; volcanic detritus rare

Structureless Trough-cross-strati ed Wavy-strati ed siltstone and sandstone, siltstone, siltstone to sandstone limestone and conglomerate Lower part of Chinle Formation: Mesa Redondo / Shinarump Members and Blue Mesa / Sonsela / Petri ed Forest Members, volcanic detritus common Clayey siltstone and sandstone; Sandstone, conglomerate, cross-strati ed sandstone with siltstone; silty claystone local conglomerate 39°N

NEVADA

Figure 3. Buckskin Formation. (A) Panoramic view of overturned Buckskin Formation (overturned) at Palen Pass, Palen Mountains. Rounded hills with poor exposure are characteristic UTAH COLORADO of the fine-grained and gypsiferous lower member. Yellow-orange lines indicate contacts between units. Photo taken at 33.90837°N, 115.06376°W, looking east. Flat bench in foreground Las ARIZONA NEW MEXICO Vegas is ~100 m in length. (B) Phyllite, quartzite, and upper members. Photo taken at 34.21007°N, Colorado Plateau 114.00854°W, looking west-northwest. Cliff of resistant upper Buckskin Formation is ~10 m high.

Flagsta Albuquerque 35°N ORNIA CALI F Figure 4. (A) Facies patterns in the Moenkopi and lower Buckskin Formations, after Stewart et al. (1972a). The lower Buckskin Formation correlates well with quiet-water facies of the Phoenix Moenkopi Formation; note that facies follow a northeast trend. (B) Facies patterns in the Buckskin depocenter (minimum extent) Chinle Formation, after Stewart et al. (1972b); note that facies in the terrestrial Chinle Forma 0 200 km tion are more haphazard. Buckskin Formation facies have broad stratigraphic similarities to Chinle Formation members. 113°W 109°W

Rock Member, which is made up of interbedded coarse siltstone and tan to Thin-section examination shows that the quartzite member is composed of white limestone, and the Church Rock Member, an arkosic, horizontally bed- quartz, calcite, and epidote, and less commonly, muscovite, feldspars, biotite, ded to structureless sandy siltstone (Stewart et al., 1972b). The Owl Rock and chlorite, and oxide minerals. Where best exposed, in the Palen Mountains, the Church Rock Members are similar to the overlying uppermost Triassic and porcellanite unit consists of extremely fine-grained muscovite (sericite) and Lower Jurassic Glen Canyon Group (Dickinson, 2018). Chinle Formation strata chlorite in a bluish groundmass (chlorite?). were deposited in a wide variety of continental environments dominated by The protolith of the quartzite member is hypothesized to be a calcite-cemented fluvial, floodplain, and lacustrine settings (e.g., Stewart et al., 1972b; Lawton, quartz siltstone or mudstone. Coarser units within the quartzite have calcite- 1994; Dickinson and Gehrels, 2008; Howell and Blakey, 2013; Fig. 4B). cemented subfeldspathic wacke protoliths. The porcellanite beds are interpreted to be a very-fine-grained, water-lain ash-fall tuff or muddy claystone.

■■ LITHOSTRATIGRAPHY AND DEPOSITIONAL SETTING OF THE BUCKSKIN FORMATION Phyllite Member

Lower Member The structural thickness of the phyllite member is as much as 60 m; the stratigraphic thickness is 20 m. Rock types include variable fine- to coarse- Detailed descriptions and stratigraphic sections of the Buckskin Formation grained, non-calcareous phyllite, phyllitic quartzite, and pebble to cobble are available in Sanchez (2015), with additional descriptions in Reynolds and metaconglomerate. The rock has a distinctive blue-green tint in most expo- Spencer (1989), Stone and Kelly (1989), and Hargrave (1999). The structural sures. Visible grains in hand sample include quartz, chlorite, muscovite, and thickness of the lower member (Fig. 3A) varies between 100 and 400 m, and magnetite, as well as oblong-shaped “clasts” composed of muscovite and the stratigraphic thickness is 100–200 m. The lower member includes resistant, fine magnetite (identified by Hargrave [1999]). fine-grained quartzite as well as variably fine- to medium-grained phyllite, cal- Near the base, a thin metaconglomerate unit contains subrounded to careous quartzite, gypsiferous phyllite, gypsum, and phyllitic quartzite (Fig. 2). subangular quartzite pebbles and cobbles (<10 cm). The middle portion of In thin section, rocks of the lower member consist of poorly to moderately the member consists of fine-grained phyllitic quartzite that is overlain by an well-sorted detrital grains in a matrix of quartz, chlorite, white mica, and epi- upper portion of alternating phyllite and coarse-grained (locally conglomer- dote. Monocrystalline, angular to subrounded quartz makes up as much as atic) phyllite. The phyllite member commonly has beds 0.05–1 m thick and a 60% of grains. Quartz grains show some evidence of strain, with undulose millimeter-scale pervasive foliation imparted by post-depositional metamor- extinction, formation of subgrains, and locally incipient formation of quartz phism and deformation. ribbons. Feldspar (5%–35%; microcline, albite, and plagioclase), zircon (<1%), In thin section, rocks of the phyllite member show poor sorting and con- and spinel (<1%) are also present. Laminations in thin sections are defined by sist primarily of angular to subrounded monocrystalline quartz (30%–60%) variations in the size of quartz and feldspar grains. and muscovite (30%–40%) grains. Chlorite, feldspars, epidote, and magnetite The protolith of the fine-grained quartzite is interpreted as quartz siltstone are also present in the rock in smaller percentages. Zircon is present in small with calcite cement. Protoliths of the coarser units of the basal portion of the amounts (<3%), generally clustered in pods or concentrations of chlorite and lower member are calcareous subarkosic arenites and subfeldspathic wackes. muscovite. The protolith of gypsiferous phyllite beds is interpreted to be packages of The mineral assemblages of the phyllite member imply that the protoliths gypsiferous mudstone. are probably micaceous quartz wacke and subfeldspathic wacke. Hargrave (1999) interpreted “clasts” of muscovite that include groups of zircon grains to be metamorphosed pumice clasts. The sedimentary protoliths of the phyllite Quartzite Member member are volcanogenic.

The structural thickness of the quartzite member is 0–45 m, and the mea- sured stratigraphic thickness is 10–20 m. The member is highly resistant and Upper Member exposed in characteristic bands that stand out above finer-grained surround- ing units (Fig. 3B). Rock types are very fine- to fine-grained, thickly bedded, The upper member (Figs. 2 and 3B) has a structural thickness >300 m; the epidote-rich quartzite (0.5–1 m) or calc-silicate rock. A distinctive section of stratigraphic thickness is uncertain due to folding within the member (Reyn- porcellanite beds (10–20 cm) that locally interfingers with calcareous quartzite olds and Spencer, 1989). It consists of massive, variably calcareous, fine- to beds (20–40 cm) crops out ~4.5 m below the top of the member. Exposures of medium-grained quartzite and calc-silicate rock. Beds of phyllitic quartzite and this unit pinch and swell along strike. pebble to (local) cobble metaconglomerate are present in lesser proportions.

Clasts within the metaconglomerate are subrounded to rounded quartzite. During Early Triassic time, the Buckskin depositional system was probably The upper member is locally characterized by coarsening-upward successions. an estuarine environment affected by tidal and fluvial currents. Evaporative Thin sections reveal moderate to poor sorting and mineralogy that con- basins may have formed in the estuary or on shallow tidal flats that promoted sists dominantly of monocrystalline subrounded to subangular quartz. Biotite, formation of the gypsum. chlorite, muscovite, and epidote are also common grain types. The samples The phyllite member represents a distinct change in depositional setting from the Palen Mountains are relatively more arkosic (15%–30% feldspar of the Buckskin Formation based on the compositional dominance of quartz, grains) than those from the Buckskin Mountains (0%–10% feldspar grains). muscovite, and magnetite grains and the lack of pervasive calcite cement in the The protoliths of the upper member are interpreted as calcite-cemented sub- two lower members. The wide range in grain size (fine to cobble), the compo- arkosic quartz arenite and micaceous quartz arenite. The calcareous nature sition, and the lenticular bedding of the phyllite member suggest deposition by of the upper member contrasts strongly with the composition of the phyl- fluvial or alluvial processes, including gravel bars and sand sheets for coarser lite member. beds and floodplains and levees for finer beds. The coarse-grained micaceous quartz wacke and subfeldspathic wacke beds of the member are distinct from the fine-grained, quartz-rich units of the underlying members. We infer that Depositional Setting of the Buckskin Formation the phyllite member was deposited in a continental fluvial environment that tapped an active volcanic source region. The rocks of the Buckskin Formation commonly lack distinct preserved The upper member comprises calcite-cemented subarkosic quartz arenite sedimentary structures, and relict small-scale sedimentary structures in many and micaceous quartz arenite. The calcite cement, especially in contrast to cases have been overprinted by post-depositional metamorphic and defor- the underlying phyllite member, suggests deposition in a deltaic environment mational events. Depositional environments are interpreted using observed adjacent to a lake, or possibly in a shallow-marine environment. or inferred sedimentary features, particularly mineral constituents, grain size, The phyllite and upper members of the Buckskin Formation reflect deposi- degree of sorting, preserved lamination and bedding characteristics, cement tion in dominantly to wholly continental systems. Although a shallow-marine types, and stratigraphic context. environment cannot be ruled out, we prefer a lacustrine interpretation for Gypsiferous siltstone beds in the lower portion of the lower member were parts of the upper member as part of a complex fluvial system with channel deposited in restricted bodies of water (evaporative basins), such as sabkhas avulsions, crevasse splays, and shallow lakes (Stewart et al., 1972b; Lawton, or restricted marine basins (cf. Douglas and Goodman, 1957; Schreiber and 1994; Dickinson and Gehrels, 2008; Howell and Blakey, 2013). El Tabakh, 2000); low levels of precipitation and enhanced seasonality during Early Triassic time would have promoted the formation of gypsum in isolated bodies of water (Parrish, 1993). This interpretation is supported by the poor ■■ DETRITAL ZIRCON RECORD to moderate sorting, pervasive calcite cement, and the horizontal laminations and bedding in other rocks of the lower member, which suggest deposition The Buckskin Formation yielded >100 concordant zircon grains from 11 of in a marginal-marine environment or very low-profile fluvial system. The the 14 samples collected from the lower, phyllite, and upper members in the fine-grained, horizontally laminated siltstone-mudstone facies may repre- Buckskin Mountains, Palen Mountains, and Midland syncline and at Plomosa sent quiet-water deposition with little to no influence of currents, likely on a Pass (Fig. 1; Table 1); a sample of the quartzite member yielded only 25 con- shallow-marine shelf and/or in tidal flats and levees. Coarser-grained units cordant grains (Supplemental Table S11). Samples were crushed and zircon (subfeldspathic wacke) represent more fluvial-dominated, tidal-channel, or separated by standard methods (e.g., Gehrels, 2000). U-Pb geochronology deltaic depositional settings. and trace element geochemistry were conducted by laser ablation–inductively Fine-grained, thinly bedded quartz siltstone and subfeldspathic wacke of coupled plasma–mass spectrometry (LA-ICP-MS) at the laser ablation split- the quartzite member also lack sedimentary structures (except for bedding), stream (LASS) facility at the University of California, Santa Barbara (UCSB), are fine grained, and have calcite cement. These features suggest a continu- in 2013 and 2014 using a Nu Plasma high-resolution multi-collector–induc- ation of the relatively calm-water marginal-marine depositional setting. Less tively coupled plasma–mass spectrometer (HR MC-ICP-MS) and a Nu AttoM common, coarser-grained subfeldspathic wackes may have been deposited single-collector ICP-MS (Nu Instruments Ltd., Wrexham, UK), and an Analyte in deltas and tidal channels. Porcellanite near the top of the quartzite mem- 193 excimer ArF laser-ablation system equipped with a HeLex sample cell ber was likely deposited from ash fall or as a slack-water claystone, while (Photon Machines, San Diego, California) using a 24 μm beam. Analytical 1 The complete data table, which contains isotopic the interfingering relation between it and the calcareous siltstone suggests and procedural details and all analytical data are provided in Supplemental ratios, dates, and uncertainties, is available through reworking in a low-energy, nearshore environment. If the ash-fall interpretation Table S1; all age uncertainties reported are 2σ, and error assessment follows the EarthChem library, https://doi.org/10.26022/IEDA /111538. The table is referred to as Supplemental is correct, this porcellanite is the first direct evidence of arc-derived volcanic Kylander-Clark et al. (2013). Analyses were evaluated for discordance based on Table S1 herein. ash reaching the basin. a comparison of 235U/207Pb and 238U/206Pb for Phanerozoic grains, and 207Pb/206Pb

TABLE 1. LOCATIONS FOR U-Pb SAMPLES Member Location Sample no. Coordinates (°N) (°E) Buckskin Formation Top of upper member Buckskin Mountains 012414-3 34.23981 −113.97452 Palen Mountains 012514-1 33.90441 −115.06417 Basal upper member Buckskin Mountains 031713-5 34.21642 −114.01752 Palen Mountains 031813-1 33.90593 −115.06426 Phyllite member Buckskin Mountains 031713-4 34.21663 −114.01655 Buckskin Mountains 031813-2 33.90808 −115.06253 Quartzite member Palen Mountains 031813-3 33.90847 −115.06248 Top of lower member Buckskin Mountains 031713-2 34.21715 −114.01594 Palen Mountains 012514-4 33.90864 −115.06158 Buckskin Mountains 012314-2 34.21754 −114.01481 Basal lower member Buckskin Mountains 031713-1 34.16202 −114.18056 Palen Mountains 012514-7 33.90808 −115.06280 Midland syncline 012614-2 33.86315 −114.82478 Plomosa Pass 160510-1 33.80980 −114.08415 Palen Mountains 031813-5 33.91097 −115.06271 Moenkopi Formation Holbrook Member northern Arizona US99 35.07558 −110.93186 northern Arizona Sunset Mtn 34.77076 −110.89572 Note: Datum is WGS84.

and 238U/206Pb for Proterozoic grains. Grains that were >10% normally discor- Phanerozoic grains are 500–300 Ma populations. Precambrian grains have ages dant (i.e., 235U/207Pb age or 207Pb/206Pb age >10% older than 238U/206Pb age) or of 1300–1000 Ma, 1400 Ma, and 1600 Ma. No age analysis was done on the 5% reversely discordant (i.e., 238U/206Pb age or 207Pb/206Pb age >5% older than quartzite member owing to the small number of concordant grains (25) and 235U/207Pb age) were not used in interpretations; these are indicated by “dis- the wide dispersion of ages (Supplemental Table S1 [footnote 1]). cordant” in Supplemental Table S1. A distinct shift in the distribution of grains ages occurs between the lower The basal portion of the lower member of the Buckskin Formation has a and upper members of the Buckskin Formation, consistent with the change distinct distribution of ages that is dominated by late Permian to Early Triassic in lithology and the unconformity along that contact. In the phyllite member, grains. Of 280 Phanerozoic grains (68% of all grains), 80% are middle Perm- ~48% of all grains are Late Triassic in age, with a continuous range from ian–earliest Triassic in age; the weighted average of the youngest grains from 241–218 Ma and a 15-point MDA of 220 ± 2 Ma (MSWD = 1.8; Fig. 5C). Older the four localities (Fig. 1) yields a maximum depositional age (MDA) of 249 Permian–Triassic grains make up 12% of the age distribution. The remain- ± 2 Ma with a mean square of weighted deviates (MSWD) of 1.15 (Fig. 5A). ing grains are characterized by Proterozoic ages of 1200–1000 Ma, 1400 Ma, Single grains or two-grain groups are as old as 278 ± 3 Ma. The remaining and 1800–1600 Ma; 500–300 Ma grains are very rare in both the phyllite and grains within the detrital zircon suite are mostly Proterozoic and Archaean in upper members. age, with distinct populations of 1300–1100 Ma, 1400 Ma, and 1800–1600 Ma. The lower portion of the upper member shows a much greater diversity The upper portion of the lower member has a greater diversity in grain in ages than the phyllite member (Fig. 5D). The Phanerozoic age distribution ages than the basal portion of the member, as well as a greater spread of has no distinct groupings, with subequal amounts of the Early Triassic and Phanerozoic ages (Fig. 5B). Only 21% of grains yielded Permian–Triassic ages Late Triassic ranges that are common in the lower and phyllite members. that make up a broad range without a statistically viable MDA. The remaining Approximately 65% of the total age distribution is Proterozoic grains in broad

Basal lower member, Buckskin Fm Top of lower member, Buckskin Fm Phyllite member, Buckskin Fm 300 N=5; n=411 N=2; n = 124 160 N=2; n = 279 223 253 20

250 221

249 120 219 age (Ma) 15 200 age (Ma)

245 age (Ma) 217 Mean = 220 ± 2 Ma; 150 Mean = 249 ± 2 Ma; 0 of 15 rejected; MSWD = 1.8 0 of 32 rejected; MSWD = 1.15 80 215 241 10 Number Number Number 100

5 40 50

0 0 0 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Age (Ma) Age (Ma) Age (Ma)

Basal upper member, Buckskin Fm E Top of upper member, Buckskin Fm Holbrook Member, Moenkopi Fm 30 N=2; n = 213 20 N=2; n = 139 N=2; n = 263 262 100 25 256

15 80 20 248 age (Ma) 60 15 240 10 Mean = 250 ± 2 Ma; 0 of 9 rejected; MSWD = 2.3 Number Number Number 232 40 10 5 5 20

0! 0 0 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Age (Ma) Age (Ma) Age (Ma)

Figure 5. Weighted average and probability density plots (PDPs) of ages of the Buckskin Formation using combined member data from Buckskin and Palen Mountains, Midland syncline, and Plomosa Pass. (A) PDP of the basal lower member; inset is a weighted average plot. (B) PDP of the top of the lower member. (C) PDP of the phyllite member; inset is a weighted average plot. (D) PDP of the basal upper member. (E) PDP of the top of the upper member. (F) PDP of the Holbrook Member, Moenkopi Formation, sampled near Winslow, Arizona; inset is a weighted average plot. N—number of samples; n—number of grains; MSWD—mean square of weighted deviates.

ranges of 1200–1000 Ma, 1500–1300 Ma, and 1800–1600 Ma. The uppermost correlation is guided by the interpreted protoliths, sedimentary features, and portion of the upper member is dominated by Proterozoic grains, which make detrital zircon ages. up 92% of the entire age distribution (Fig. 5E). No significant age groupings The dominant lithology of the Moenkopi Formation is horizontally stratified are present in the suite of only 11 Phanerozoic grains. Of 139 Proterozoic sandstone and siltstone interpreted as shallow- to marginal-marine deposits, grains, 46% of those grains are within a continuous age range of 1500–1130 Ma. including primary gypsum deposits (Stewart et al., 1972a; Reif and Slatt, 1979). Approximately 30% of the grain ages is in the 1800–1600 Ma range, with lesser The lower two members of the Buckskin Formation are interpreted to have and subequal amounts of 1200–1000 Ma, 1500–1300 Ma, and >1800 Ma ages. been deposited at the interface of marine and fluvial depositional environments Thorium/uranium (Th/U) ratios in zircon can be used to gather information and correlate well with these dominant facies of the Moenkopi Formation. about source magmas and, in some cases, isolate areas of likely provenance The gypsiferous phyllite beds of the lower member are lateral equivalents to (Riggs et al., 2013). Th/U ratios in the Buckskin Formation samples are homo- interstratified gypsum and siltstone-to-claystone beds, such as the Shnabkaib geneous, with >95% of Permian and Triassic grains from both the Middle and Member in the middle of the Moenkopi Formation, which is exposed in the Late Triassic parts of the formation falling in a range of 0.1–1.2 (Fig. 6). southwestern Utah, northwestern Arizona, and southern Nevada (Stewart et al., 1972a). A reasonable extension of the Moenkopi depositional system to the southwest (e.g., Stewart et al., 1972a; Fig. 4A) aligns the Buckskin depocenter ■■ DISCUSSION with Moenkopi facies dominated by quiet-water deposits. The Chinle Formation is interpreted as a wholly continental deposit (Stew- Correlation of the Buckskin Formation to Colorado Plateau art et al., 1972b; Blakey and Gubitosa, 1983). The two upper members of the Successions Buckskin Formation were likely similarly deposited in fluvial depositional systems (Fig. 4B) based on grain-size and bedding characteristics. Conglomeratic Facies of the Triassic successions of the Colorado Plateau have been beds and the volcanogenic nature of the phyllite member correlate well with delineated primarily by the dominant rock types and sedimentary structures the facies in different parts of the Chinle Formation. Coarse-grained units are preserved in the rocks (Stewart et al., 1972a, 1972b; Blakey, 1989; Dubiel, present in the Shinarump Conglomerate and Mesa Redondo Member at the 1994). The paucity of sedimentary structures in the Buckskin Formation com- base of the formation (Stewart et al., 1972b; Blakey, 1989; Dubiel, 1994; Martz plicates correlation with the Moenkopi and Chinle Formations, but the regional et al., 2012), and the Sonsela Member in its middle. The fine-grained units of the phyllite member correlate well with the poorly sorted clay-rich sandstone and bentonitic siltstone facies such as the Petrified Forest Member (Stewart et al., 1972b; Dubiel, 1994). 10 Hargrave (1999) inferred an erosional unconformity between the quartzite Buckskin Mtns and phyllite members of the Buckskin Formation, based on the sharp contact Palen Mtns between those members, the abrupt change in sandstone composition, and Midland syncline a coarse-grained conglomeratic bed at the base of the phyllite member. This Plomosa Pass unconformity would correlate to the unconformity between the Moenkopi 1 Moenkopi Fm, Winslow and Chinle Formations of the Colorado Plateau (Tr-3 unconformity of Pipirin- Range of Mojave Desert plutons gos and O’Sullivan [1978]). Fine-grained structureless to horizontally bedded Range of El Paso Mtns and conglomeratic facies of the upper member may correlate with the Owl Th/U plutons Range of Los Tanques Rock Member of the Chinle Formation (Stewart et al., 1972b; Dubiel, 1994; (Sonora) pluton 0.1 Tanner, 2000). Range of Chinle Fm detrital grains To further understand the likely correlation between the lower Buckskin Formation and the Moenkopi Formation on the Colorado Plateau, we dated two samples of “tuffaceous sandstone” (Cadigan, 1971) from the Holbrook 0.01 Member of the upper Moenkopi Formation near Winslow, Arizona (Figs. 1 200 210 220 230 240 250 260 270 280 290 and 5F; Supplemental Table S1 [footnote 1]). Permian and Triassic grains are Age (Ma) acicular with little abrasion, indicating a low amount of transport. Twenty-four grains from the two samples fall within a range of 270–245 Ma (Supplemental Figure 6. Th/U ratios versus age of Buckskin Formation and Moenkopi Formation Permian– Table S1), and the maximum depositional age is ca. 250 Ma (Fig. 5F). Triassic grains. Mojave Desert pluton data from Barth and Wooden (2006), Chinle Formation data from Riggs et al. (2013), and El Paso Mountains data from Cecil et al. (2018) are shown Correlation of the Buckskin Formation and the Triassic units on the Colorado for comparison. Plateau is indicated by the MDA of the units, although use of MDAs does not

provide member-to-member correlations. The maximum depositional age of ? t ca. 249 Ma for the basal portion of the lower member of the Buckskin Forma- s ca. 255 Ma u ca. 245 Ma r h Eastern limit of tion is similar to biostratigraphic ages (e.g., McKee, 1954) and our new data t

limestone from the Holbrook Member. These comparisons, together with the correlation

of the gypsiferous lower member with the Shnabkaib Member, suggest that d

n o the lower and quartzite members of the Buckskin Formation best correlate to c l ? o

middle and upper members of the Moenkopi Formation. G

The maximum depositional age of the phyllite member is 220 ± 2 Ma, which ? Environment is comparable to the 220–230 Ma detrital zircon age distributions from the El Paso terrestrial Chinle Formation (Dickinson and Gehrels, 2008; Heckert et al., 2009; Howell, Mtns 2010; Irmis et al., 2011; Ramezani et al., 2011, 2014; Riggs et al., 2012, 2013; N N Howell and Blakey, 2013; Atchley et al., 2013). A MDA of 220 Ma for the phyllite Mojave Buckskin Desert member corresponds to single-crystal chemical abrasion–thermal ionization Mojave Buckskin depocenter mass spectrometry (CA-TIMS) dates of 220 Ma obtained by Atchley et al. (2013) Desert

depocenter elevation) (relative for the top of the Blue Mesa Member or 219 Ma for the basal Sonsela Member marine (Ramezani et al., 2011). Thus, the phyllite and upper members of the Buckskin Formation likely correlate with the Sonsela or Petrified Forest and Owl Rock Members of the Chinle Formation, rather than the basal Shinarump Conglom- ca. 225-220 Ma ca. 200 Ma erate. These assignments corroborate the interpretations of Hargrave (1999), who used stratigraphic comparisons to make the same correlations. Thorium/uranium (Th/U) ratios of the zircon crystals support a hypothesis Ash clouds of similar source regions for upper Buckskin Formation and Colorado Plateau formations. Ratios from the lower member of the Buckskin Formation and the Moenkopi Formation are very similar (Fig. 6); although this does not suggest Mojave a correlation between units, we infer that magmatic source rocks were part of N Desert N the same arc complex. Ratios from both units are dominantly within the range of values from Permian and Lower Triassic plutons in the El Paso Mountains Buckskin Buckskin depocenter depocenter in southern California (Cecil et al., 2018). Complementing observations by Riggs et al. (2013), Th/U in grains from our samples of the phyllite member are likewise very similar to those from detrital grains within the Chinle Forma- Figure 7. Paleogeography and arc activity during Buckskin Formation deposition, in present-day coordinates. Red arrows depict general paleocurrent trends (after Stewart et al., 1972a; Dickin tion (Riggs et al., 2013, 2016). Triassic plutonic rocks and presumed volcanic son and Gehrels, 2008). (A) Basal lower member at 260–255 Ma. Map is offset northward equivalents from the Mojave Desert (Barth and Wooden, 2006) are a viable from the following maps to show broader-scale tectonic features. Color gradient reflects the source for these grains based on these ratios. original Moenkopi basin (e.g., Dickinson and Gehrels, 2008). Dashed green arrows depict input of detritus from the El Paso Mountains (northern arrow) and San Bernardino suite (southern arrow). (B) Upper lower and quartzite members at ca. 245 Ma. Note likely magmatism farther north, but highly diminished in Buckskin source areas. Boundary between terrestrial Early Mesozoic Sediment-Dispersal Systems and marine rocks moves northwest. (C) Phyllite member at ca. 220 Ma. Magmatic arc has “shoaled,” causing the Tr-3 unconformity, and fluvial systems from the arc join the dominant Lower and Quartzite Members northwest-directed transport system. Coarse detritus bypasses the Buckskin depocenter, but Plinian clouds contribute ash. (D) Approximate time of development of the post-Buckskin unconformity with the overlying Vampire Formation (ca. 200 Ma; greyed-out Buckskin de- Cordilleran arc signature. Detrital zircon data includes ages that indicate pocenter label indicates post-Buckskin time); note the Cordillera-wide lull in magmatism. abundant arc-derived grains and suggest that Lower Triassic sediment in the basal portion of the lower member of the Buckskin Formation was transported in drainage systems that originated in active segments of the arc. One system of the arc, potentially in igneous rocks related to the San Bernardino suite of may have originated in or near the El Paso Mountains segment of the Cordil- Barth and Wooden (2006). Plutonic rocks in the Manzanita Springs and Liebre leran arc (Fig. 7A), where ages and Th/U ratios of late Permian–Early Triassic Mountain (Fig. 1) areas are ca. 250 Ma and have Th/U values between 0.3 and plutonic rocks are well within the range of those of Permian detrital grains 0.6, suggesting a source within older segments of that suite. We agree with (Fig. 6). The primary dispersal system originated in the Mojave Desert segment previous ideas (Howell and Blakey, 2013; Riggs et al., 2016) that the magmatic

arc lay offshore of the Laurentian continent, and suggest that it was some tens Early Mesozoic Cordilleran Arc Magmatism of kilometers from the Buckskin basin (Fig. 7A). Zircon grains were transported in tuffaceous turbidites or slurries and reworked by marine currents. We infer The Buckskin Formation records two distinct pulses of late Permian to Early that the resultant MDA of ca. 249 Ma very closely approximates the age of Triassic and Late Triassic Cordilleran arc magmatic activity. The first pulse is the lower member. recorded by the ca. 270–245 Ma zircon grains in the lower member. Distinctly Sonoran arc or El Paso Mountains grains. In addition to grains that were younger grains (ca. 220 Ma) in the phyllite member document the second pulse. derived from proximal regions of the arc, 275–265 Ma zircon grains were A plot of the distribution of arc-derived ages (ca. 280–200 Ma; Fig. 8) in sourced either in the Sonoran arc (Fig. 1), where plutons range from ca. 275 Triassic units across southwestern North America provides the basis for obser- to 260 Ma (Arvizu et al., 2009; Riggs et al., 2009; Arvizu and Iriondo, 2015) and vations about arc magmatism and sediment dispersal. (1) Two periods of detrital grains in Permian strata document middle Permian sedimentation Cordilleran magmatic arc activity supplied detritus to the Buckskin Formation (Dobbs et al., 2016), or in older plutons in the El Paso Mountains terrane (Cecil deposition (i.e., ca. 250 Ma and 220 Ma); other pulses of magmatism, however, et al., 2018). This latter area has the geographic advantage of being closer, but were sources for other retro-arc successions, particularly during and after grains of this age are relatively few in the Buckskin Formation (i.e., ~5% total) formation of the land bridge between the arc and the retro-arc region (ca. and a proximal source might have been expected to swamp the signature 240–235 Ma; e.g., Waterman formation, southern Arizona; Riggs et al., 2013). more completely if longshore currents were from that direction. These variations, especially in upper Buckskin time, may represent changes Eastern and central North American sources. Early Paleozoic (500–300 Ma) in the arc and/or integration of river systems over time. (2) The time between grains were transported by the larger Moenkopi westward-flowing stream the two pulses of more pronounced magmatism (i.e., ca. 245–235 Ma, as seen systems that originated in and near the Ouachita orogenic belt and the rem- in all units) does not reflect a complete cessation in magmatism, but a period nant uplifts of the Ancestral Rocky Mountains (Dickinson and Gehrels, 2003, of reduced activity. This interpretation takes into account plutonism within 2008; Gehrels et al., 2011; Leary et al., 2017). Permian ergs in the midcontinent this range reported by Barth and Wooden (2006). (3) Magmatic activity was may have stored sand grains (e.g., Lawton et al., 2015); ultimately, they were continuous in southwestern Laurentia from Permian (ca. 275 Ma) to Late Tri- reworked into the Buckskin basin by eolian or marine currents. Ages in the assic time (ca. 215 Ma). Age data from other retro-arc units to the north (Davis, ranges of 1800–1600 Ma (Yavapai and Mazatzal provinces), 1500–1300 Ma 2019) indicate that ca. 250–245 Ma magmatism was common at that latitude. (A-type “anorogenic” granites), and 1300–1000 Ma (Grenville province) are common in North America (Whitmeyer and Karlstrom, 2007).

Buckskin Fm Phyllite and Upper Members Shinarump Conglomerate Chinle Fm Upper Triassic sediment of the phyllite member (Chinle equivalent) was Waterman fm deposited by stream systems that likely originated in the Mojave Desert seg- Mojave Desert ment of the arc (San Gabriel suite; Barth and Wooden, 2006; Fig. 1), based plutons on zircon ages. These systems were linked to a larger drainage system that intersected the Buckskin depocenter and, together with Plinian eruptive clouds, delivered arc-derived detritus to more distal reaches of the greater Chinle depocenter (Fig. 7C; Riggs et al., 2012, 2013). The MDA of 220 ± 2 Ma is interpreted as the age of deposition of the phyllite member. Deposition of the upper member of the Buckskin Formation marked a 200 240 280 change in the primary sediment-dispersal pathways, as suggested by the significant decrease in the input of arc-derived zircon grains. A relative paucity of Age (Ma) arc-derived grains in the upper member also suggests a lull in magmatism in latest Triassic time; such a lull has been noted by Barth et al. (2013). This lack Figure 8. Normalized distribution of Permian–Triassic zircon grains in the Buckskin Formation, of magmatic detritus is also reflected in the comparatively nonvolcanic upper Chinle Formation, Waterman formation, and Shinarump Conglomerate (“Chinle Fm” indicates members of the Chinle Formation (Stewart et al., 1972b), which reinforces the beds exclusive of Shinarump Conglomerate and Mesa Redondo Member). Permian–Triassic data from the Cordilleran arc (Mojave Desert plutons) are provided for comparison. Ca. 275 Ma suggestion that the uppermost members of the Chinle, and the upper member is taken as the limit of Cordilleran magmatism. Chinle Formation and Waterman formation of the Buckskin, are more likely correlative with the basal units of the Glen data are from Riggs et al. (2013); Shinarump Conglomerate data are from Riggs et al. (2016); Canyon Group (Dickinson, 2018). data on Cordilleran arc igneous rocks are from Barth and Wooden (2006) and Cecil et al. (2018).

Early Mesozoic Regional Deformation The Tr-3 unconformity between the quartzite and phyllite members suggests passive uplift and “shoaling” of the magmatic arc in latest Middle to early Dickinson (2018) speculated that development of the Tr-3 unconformity on Late Triassic time. Atchley et al. (2013) provided a CA-TIMS date of 227.604 ± the Colorado Plateau was largely due to a transition between major tectonic 0.082 Ma for the basal Chinle Formation; grains of this age are present in the influences on basin development. In Early–Middle Triassic (i.e., Moenkopi phyllite member but are older than those used to define the MDA. Volcanic Formation) time, the Sonoma orogenic belt caused down-flexure that resulted detritus may have mostly bypassed the Buckskin depocenter as dispersal in a basin that deepened to the northwest along a gentle plain (Blakey, 1989; pathways were initially established; although conglomerate lenses are pres- Dickinson, 2006); the observation that Buckskin Formation lower and quartzite ent in the phyllite member, volcanic clasts, which are common in the Chinle member facies patterns follow a northeast trend to link with the Moenkopi Formation conglomerates, are missing. Formation in southern Nevada and southwestern Utah (Fig. 4A) suggests The strong difference in detrital zircon ages between the Moenkopi- and that this northwest deepening was not influenced by early arc development. Chinle-equivalent parts of the Buckskin Formation also points to a change in Development of the arc in later Triassic time (Chinle Formation) brought about sediment routing. Zircon grains with ages of ca. 300–500 Ma in the lower two dynamic back-arc subsidence (Lawton, 1994). In terms of arc development, members are most easily considered derived from the Appalachian orogenic however, the major tectonic change was a transition from an offshore arc belts via late Paleozoic transcontinental river systems proposed by Dickin- that was likely dominantly submarine, although built on continental crust, to son and Gehrels (2003). Grains derived from eastern North America are very a wholly terrestrial arc with fluvial pathways to the retro-arc area (Howell and sparsely represented in phyllite and upper members, however, and the dispar- Blakey, 2013; Riggs et al., 2016). As previously noted, the unconformity within ity is also reflected in grain ages from the Moenkopi Formation (Supplemental the Buckskin Formation likely marks this same transition in tectonic setting. Table S1 [footnote 1]) and data presented by Riggs et al. (2013, 2016) and Howell (2010) on the Chinle Formation. The Chinle Formation across the Colorado Plateau is characterized by variable Regional Synthesis facies distributions (Fig. 4B) influenced by the distribution of the large transcontinental drainage systems and climate patterns (e.g., Stewart et al., 1972b; Dubiel, The lower and quartzite members of the Buckskin Formation provide the 1989; Parrish, 1993; Lawton, 1994; Howell, 2010: Atchley et al., 2013; Howell and earliest para-autochthonous record of Cordilleran arc magmatism in the west- Blakey, 2013; Dickinson, 2018). Correlation of specific Chinle Formation member ernmost part of the evolving back arc. Our MDA of ca. 250 Ma for the upper facies with the phyllite and upper members of the Buckskin Formation is difficult; Moenkopi Formation corresponds well to the 249 Ma MDA for the basal portion depositional ages, however, provide the basis for correlation. These patterns, of the Buckskin Formation, although lower Buckskin Formation facies correlate which show the strong influx of arc detritus through Late Triassic time, support best with middle Moenkopi Formation members (e.g., Shnabkaib Member) in the hypothesis that the upper Buckskin–Chinle basin had a flexural origin that southwestern Utah and Nevada. Although arc-derived zircon grains in samples was influenced, in part, by the load of the developing Cordilleran magmatic arc of the Holbrook Member of the Moenkopi Formation are acicular, suggesting (cf. Lawton, 1994). Pronounced uplift that caused the post-Buckskin unconformity very little reworking and abrasion, these MDAs cannot be used to establish (Reynolds et al., 1989) may be due to deformation within the arc. member-to-member correlations. At the approximate latitude of the Buckskin basin, the evolving magmatic arc was offshore and did not strongly influence depositional patterns in the ■■ CONCLUSIONS back arc. The abundance of as much as 25% plagioclase feldspar in lower-member sandstones suggests that volcanic detritus gained access to the depocenter, The Lower and Upper Triassic Buckskin Formation in the Mojave Desert of but the lack of volcanic-lithic clasts of any size indicates that the active arc was California and Arizona represents the southwestward continuation of coeval some distance to the west or southwest. Subaqueous volcanism in the arc Triassic units on the Colorado Plateau and elsewhere. Although rocks are may have provided the sand-size detritus; the alteration and greenschist-grade greenschist-facies metamorphosed, we infer depositional environments similar metamorphism, however, precludes identification of vitroclastic textures that to those in the Moenkopi and Chinle Formations. Marginal-marine facies in would support this inference. the lower and quartzite members of the Buckskin Formation comprise locally Deposition of the upper part of the lower member and the quartzite mem- gypsiferous, horizontally laminated siltstone and mudstone to medium sand- ber may have occurred during a lull in volcanism at ca. 245–240 Ma (Fig. 8), or stone interpreted as estuarine and affected by tides and low-profile rivers; they may reflect changing topography as the arc developed. Both members these facies correspond well to units in the middle Moenkopi Formation such are quartzose, suggesting they were dominantly sourced from reworking of as the Shnabkaib Member (Fig. 4A). A MDA of 249 ± 2 Ma for the lower mem- lower-member detritus, with continued input from commonly inferred south- ber is matched by a MDA of ca. 250 ± 2 Ma for the Holbrook Member of the eastern sources. Moenkopi Formation (Figs. 5A, 5F).

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ACKNOWLEDGMENTS Dickinson, W.R., 2018, Tectonosedimentary Relations of Pennsylvanian to Jurassic Strata on the TBS acknowledges financial support from the Geological Society of America and Northern Ari- Colorado Plateau: Geological Society of America Special Paper 533, 184 p., https://doi.org zona University awards from Pioneer Natural Resources, the Ronald C. Blakey Scholarship, and /10.1130/SPE533. the Montgomery Prize. We very much appreciate conversations with Andy Barth and the ideas Dickinson, W.R., and Gehrels, G.E., 2003, U-Pb ages of detrital zircons from Permian and Jurassic he contributed, and the help of Andrew Kylander-Clark at the UCSB LASS facility. Field and lab eolian sandstones of the Colorado Plateau, USA: Paleogeographic implications: Sedimentary assistance from Jessica Savage, Tanner Sanchez, and Cortni Haislet are gratefully acknowledged. Geology, v. 163, p. 29-66, https://doi.org/10.1016/S0037-0738(03)00158-1. 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