GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 7 2020 © 2020 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. RHYOLITE IGNIMBRITE BOULDERS AND COBBLES IN THE MIDDLE JURASSIC CARMEL FORMATION OF UTAH AND ARIZONA—AGE, COMPOSITION, TRANSPORT, AND STRATIGRAPHIC SETTING Bart J. Kowallis, Douglas A. Sprinkel, Eric H Christiansen, Skylor Steed, and David F. Wheatley GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Large, rounded igneous, boulder-sized clast from near where sample WH-1 was collected in the White House area, Kane County, Utah. The boulder has broken into several large pieces having maximum diameters of 1.3, 1.0, and 0.8 m with several other 0.5 m blocks and is compositionally similar to other igneous clasts in the area. These igneous clasts are weathered out from de- ris-flow beds within the Paria River Member of the Car- mel Formation. David Wheatley for scale. Photograph by Mark Hansford. i Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. Volume 7 2020 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. 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Sprinkel Azteca Geosolutions 801.391.1977 GIW@utahgeology.org dsprinkel@gmail.com Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov John R. Foster Utah Field House of Natural History State Park Museum 435.789.3799 eutretauranosuchus@ gmail.com Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 7 2020 69 ABSTRACT A stratigraphic layer containing rhyolite cobbles and boulders in the Middle Jurassic Carmel Forma- tion of southern Utah represents a singular, unusual event in the otherwise low-energy sedimentation of this formation. A laser-fusion, single-crystal 40Ar/39Ar age of 171.73 ± 0.19 Ma obtained from sanidine in one of the clasts is about 8 m.y. older than a zircon U-Pb age obtained on a fallout tuff from the sediments surrounding the clasts (163.9 ± ~3.3 Ma). The volcanic clasts are poorly-welded rhyolite ignimbrites that may have been deposited as much as 200 km from the eruptive center, perhaps along pre-existing valleys. The tuff deposits then remained in place for several million years during which time they were subjected to weathering, alteration, and perhaps topographic inversion, creating mesas capped with tuff underlain by soft Middle Jurassic silt and mud. Triggered by unusual rainfall or earthquakes, debris flows carried the clasts a few 10s of kilometers from their outcrops to the depositional site. Earlier work proposed that the Middle Jurassic arc was a low-standing, arc-graben. If this was the case, then the tectonic setting was likely similar to the modern Central American arc in the vicinity of Nicaragua where tuffs erupted from a low-standing arc deposited onto an adjacent highland and were then eroded by streams flowing to the east onto a fluvial plain that is near the sea. Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Bart J. Kowallis1, Douglas A. Sprinkel2, Eric H Christiansen1, Skylor Steed1, and David F. Wheatley3 1Department of Geological Sciences, Brigham Young University, Provo, UT 84604 (bkowallis@byu.edu) 2Azteca Geosolutions, Pleasant View, UT 84414 and Utah Geological Survey, Salt Lake City, UT 84114; dsprinkel@gmail.com 3Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112; davidfwheatley@gmail.com Citation for this article. Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley D.F., 2020, Rhyolite ignimbrite boulders and cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—age, composition, transport, and stratigraphic setting: Geology of the Intermountain West, v. 7, p. 69–96, appendix, https://doi.org/10.31711/giw.v7.pp69-96. © 2020 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. INTRODUCTION The Middle Jurassic (163–174 Ma) of the Western Cordillera was a time of active arc magmatism, leaving behind volcanic and plutonic rocks within the arc that have been widely studied (Dunne, 1986; Karish and others, 1987; Busby-Spera, 1988; Busby-Spera and oth- ers, 1990; Dunne and Walker, 1993; Riggs and others, 1993; Schermer and Busby, 1994; Fackler-Adams and others, 1997; Sorensen and others, 1998; Fohey-Breting and others, 2010; Tosdal and Wooden, 2015; Barth and others, 2017). In addition, ash beds preserved in more distal sedimentary environments have provided further insights into the character and frequency of eruptions along the Jurassic arc (Wright and Dickey, 1963; Mar- vin and others, 1965; Nielson, 1988; Everett and others, 1989; Christiansen and others, 1994, 2015; Blakey and Parnell, 1995; Zhang, 1996; Kowallis and others, 2001; Dickinson and others, 2010; Sprinkel and others, 2011; Doelling and others, 2013. Chapman (1987, 1989, 1993), Marzolf (1990), and 70 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Blakey and Parnell (1995) reported on volcanic pebbles, cobbles, and boulders ranging in size from 2.5 cm to 2.6 m in the upper member of the Carmel Formation (equivalent to the Thousand Pockets and Paria River Members; see Doelling and others, 2013) in southern Utah and northern Arizona (figure 1). Chapman (1987, 1989) described the clasts as rhyolitic, welded tuffs (ig- nimbrites) with quartz, sanidine, biotite, plagioclase, hornblende, and magnetite, but having almost all the plagioclase replaced by calcite and the hornblende re- placed by iron oxides. Based upon directional transport indicators in the enclosing sedimentary rocks, the clasts were transported north and northeast into the Carmel Formation depositional basin (Chapman, 1989; Blakey and Parnell, 1995). But how did cobble- and boulder-sized clasts travel from somewhere near the western continental margin Jurassic volcanic arc to their current depositional loca- tion? Chapman (1993) proposed that debris flows gen- erated by huge floods carried the cobbles and boulders as much as 250 to 300 km from their source in the arc to their present depositional site (palinspastically cor- rected for later extension), but acknowledged the unre- solved problem of how large boulders could move over such large distances. Blakey and Parnell (1995) suggest- ed that now-eroded outcrops of these volcanic rocks may have only been several kilometers from the sites of boulder and cobble deposition. But Blakey and Par- nell (1995) did not explain why outcrops of ignimbrite might have been so close to the depositional sites when eruptive centers must have been 250 to 300 km away as Chapman (1993; Luscombe, 2018) recognized. A simi- lar problem of source for coarse volcanic detritus (up to 25 cm+ diameter) occurs in the Triassic Chinle Forma- tion (Stewart and others, 1972; Dodge, 1973). Stewart and others (1986) suggested that one possible solution to the problem was tectonic removal of the source by strike-slip offset along a major shear system cutting be- tween the source area and the deposits. However, this does not seem to be a reasonable solution for the Mid- dle Jurassic deposits. Chapman (1987) examined sever- al possibilities including: (1) the magmatic source was closer to the Colorado Plateau at the time of clast depo- sition than the palinspastic reconstruction estimate of 250 to 300 km; (2) a broad, now eroded, volcanic apron existed around the arc region bringing source material closer to the depositional sites of the clasts; (3) transport might have been enhanced by paleovalleys that helped to extend ash-flow and debris-flow runout distances; and (4) an undiscovered Jurassic volcanic complex ex- isted much closer to the Colorado Plateau with deposits that lapped up onto the plateau (similar to the proposal of Blakey and Parnell, 1995). In this paper we review the stratigraphic setting of the volcanic clasts, re-examine their composition, tex- ture, and age, then, using these data, re-examine pos- sible solutions to the problem of transport distance be- tween the volcanic arc and depositional basin. MIDDLE JURASSIC STRATIGRAPHY OF THE SOUTHERN KAIPAROWITS BASIN The Kaiparowits Basin of south-central Utah is one of several Laramide basins on the Colorado Plateau (figure 1). Middle Jurassic formations exposed in the basin and surrounding region include (in ascending stratigraphic order) the Temple Cap Formation, Carmel Formation, and Entrada Sandstone (figure 2). The Temple Cap For- mation unconformably overlies the Lower Jurassic Na- vajo Sandstone. It was deposited on the eroded Navajo in what was identified as the J-2 unconformity by Pipir- ingos and O’Sullivan (1978) but is now considered to be the J-1 unconformity because of regional stratigraphic work of Sprinkel and others (2011) and Doelling and others (2013). The Temple Cap Formation is well ex- posed in southwestern and south-central Utah. The formation irregularly thins eastward from about 120 m in the St. George area (about 150 km west of the study area) to about 8 m in the Lake Powell area (about 50 km east of the study area), but is locally missing across the paleotopographic high of the Kaibab uplift (Peterson and Pipiringos, 1979; Wright and others, 1979; Doelling and others, 2013). The Temple Cap Formation is seem- ingly missing within the study area but is present about 7 km to the southeast at Judd Hollow (Sprinkel and others, 2011; Doelling and others, 2013), suggesting the easternmost flank of the Kaibab uplift extends into this area. Thin fallout tuffs in the Temple Cap Formation in the region provided 40Ar/39Ar (sanidine and biotite) and U-Pb (zircon) ages that range from 172.93 ± 0.56 to 71 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Area of Paleohigh 0 20 40 60 80 100 Miles 0 20 40 60 80 100 Kilometers ¹ San Rafael Swell Monument Upwarp Kaiparowits Basin Henry Mtns Basin Uinta Basin Blanding Basin Markagunt- Paunsaugunt Basins ERODED MIDDLE JURASSIC TOTAL THICKNESS (ARAPIEN-CARMEL-TWIN CREEK) in meters 0-30 30-60 60-120 120-180 180-240 240-300 300-600 > 600 Area of Igneous Clasts Basin Boundary ERODED ERODED A R A P I E N B A S I N Twin Creek Limestone Twin Creek Limestone Carmel FormationCarmel Formation Ar ap ie n Fo rm at io n Ar ap ie n Fo rm at io n U I N T A A R C H St. George Kanab X Johnson Canyon Moab Extent of sand influxExtent of sand influx Zone of pipes and deformation Panguitch Page Escalante Salt Lake City Vernal Colo ra do R ive r Dirty Devil R. Gr ee n Ri ve r San Juan River Waterpocket Fold Teasdale anticline Circle Cliffs-Escalante Uplift Figure 1. Paleogeologic map of Utah showing relationships for the Middle Jurassic along with the location of the igneous clasts in the Paria River Member of the Carmel Formation, southwestern part of the Kaiparowits Basin (modified from Doelling and others, 2013). The map also depicts the area of sand influx that formed the Thousand Pockets Member and is responsible for the sandy nature of the other Carmel members. The Middle Jurassic Carmel Formation (and equivalent stra- ta) generally thins from west to east; however, note that the Carmel thins across a paleohigh that stretches from the western margin of the Kaiparowits Basin northeastward to the western flank of the San Rafael Swell. The paleotopography developed on the Lower Jurassic Navajo Sandstone on which the Middle Jurassic Temple Cap Formation was irregularly deposited; in some areas along the paleohigh, the Temple Cap is missing. The red box shows the area enlarged in figure 3. Also shown is a zone of pipes and deformed beds in Permian to Cretaceous strata (between dashed lines), the width approximately shown by the purple arrows (Wheatley and Chan, 2013, 2018; Wheatley and others, 2016; Wheatley, 2018. 72 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 170.5 ± 0.95 Ma, which indicate the Temple Cap is Aale- nian (Kowallis and others, 2001; Dickinson and others, 2010; Sprinkel and others, 2011; Doelling and others, 2013; Sprinkel and others, in preparation) (figure 3 and table 1). The Carmel Formation conformably overlies the Temple Cap Formation and contains the igneous clasts; thus, it is described in detail below. Conformably over- lying the Carmel Formation is the Entrada Sandstone. The Entrada is exposed about 8 km northwest of the study area and consists mostly of reddish-brown, medi- um- to large-scale, cross-bedded sandstone that weath- ers to form rounded bare rock (slickrock) and cliffs (Doelling and others, 1989; Doelling and others, 2010). The Entrada ranges from 90 to 180 m thick in the south- ern Kaiparowits Basin (Doelling and others, 1989). Carmel Formation The Carmel Formation was described and mapped by pioneering geologists investigating the geology of the Kaiparowits area (Gregory and Moore, 1931), the nearby San Rafael Swell (Gilluly and Reeside, 1928), and elsewhere in southern and eastern Utah (Baker and others, 1936). The Carmel is formally subdivided into four members throughout southern Utah with varying names for the lowest two members, which reflect sig- nificant lithofacies changes across the region (figure 2). The four members of the Carmel Formation in the Kaip- arowits Basin include (in stratigraphic ascending order) the Judd Hollow, Thousand Pockets-Crystal Creek, Paria River, and Winsor Members (figure 2; Phoenix, 1963; Thompson and Stokes, 1970; Blakey and others, 1983; Doelling and others, 1989, 2013). The Carmel 0 20 4010 30 Miles N Enterprise Cedar City Gunlock Leeds Hurricane St. George Kanab Big Water Cannonville Henrieville Escalante Boulder Alton UTAH ARIZONA 1 2 3 4 5 166.1 163.5 166.3 170.3 174.1 Winsor Member Paria River Member Crystal Creek Member Judd Hollow Mbr Paria River Member Winsor Member Thousand Pocket Member Co-op Creek Limestone Member Sinawava, White Throne, Esplin Point Members Manganese Wash Member J-1 unconformity 1-Johnson Wash 2-Lick Wash 3-Cockscomb 4-Study Area 5-Hole-in-Rock Road WEST EAST Winsor = 165–164 Ma Crystal Creek-Thousand Pockets-Paria River = 166–165 Ma Co-op Creek Limestone-Judd Hollow = 169–167 Ma Temple Cap Formation = 173–170 Ma Carmel Formation Navajo Sandstone Temple Cap Formation Entrada Sandstone Entrada Sandstone Carmel Formation Temple Cap Formation Navajo Sandstone Callovian Bathonian Bajocian Aalenian Toarcian Lo w er Ju ra ss ic M id dl e Ju ra ss ic FORMATIONTIME MaSTAGESYSTEM Igneous clasts Gypsum Figure 2. Southwest-to-northeast diagram of formation and member names recommended across the Middle Jurassic em- bayment in south-central Utah. The Co-op Creek Limestone Member should only be used in southwestern Utah where the member is greater than 25 m thick and where it is mostly limestone in composition. The Carmel Formation east of the Green and Colorado Rivers (not shown on figure) consists of a lower, mostly planar-bedded sandstone unit (for which we will pro- pose the name Rone Bailey Member in a future publication) and an upper, mostly siltstone and mudstone of the established Dewey Bridge Member. An influx of sand from the south is responsible for the sandy nature of the Carmel Formation in the Kaiparowits Basin and the deposition of the Thousand Pockets Member. 73 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Formation in this area was deposited near the southern end of a Jurassic Western Interior seaway that occupied nearly all of Utah (Blakey and others, 1983; Brenner, 1983; Kocurek and Dott, 1983). The depositional set- ting ranges from shallow marine to marginal marine, eolian, and fluvial environments. These members are identifiable by their lithology throughout southwestern and eastern Utah. The basal member, the Judd Hollow, is dominated by dark to medium reddish-brown sandstone and silt- stone beds of marine to marginal marine origin depos- ited on a tidal flat. The clastic beds of the Judd Hollow Member grade laterally westward to mostly thick ma- rine limestone strata, interbedded with thin marginal marine sandstone and mudstone beds of the correlative Co-op Creek Limestone Member (Doelling and others, 1989; Doelling and others, 2013). Palynomorphs re- covered from the mudstone beds and isotopic ages of 169.0 ± 0.62 to 168.2 ± 1.3 Ma (concordant U-Pb zir- con) from ash beds indicate the Judd Hollow is Bajocian in age (Sprinkel and others, 2011; Doelling and others, 2013; Sprinkel and others, in preparation) (figure 3 and table 1). These ages are consistent with the palynologic and isotopic ages obtained from the Co-op Creek Lime- stone Member (Kowallis and others, 2001; Sprinkel and others, 2011; Sprinkel and others, in preparation). The ### ## # # # # ## " " " " " " " " " " Alton Big Water Cannonville Escalante Henrieville Kanab Panguitch Page Fredonia Glendale FBE EB FOW TW-1 TW-2 WR-1 WR-6 JH-1 WH-1 WH-21-1 S ev i e r R ive r Escalante River Ea st Fo rk Vir g in R iv er Paria River Paria River 111°0'0"W 111°0'0"W 111°30'0"W 111°30'0"W 112°0'0"W 112°0'0"W 112°30'0"W 112°30'0"W 37°30'0"N 37°30'0"N 37°0'0"N 37°0'0"N 0 5 10 15 20 Miles 0 10 20 30 Kilometers¹ Jc Jc Jc 0 500 1,000 Feet 0 100 200 300 Meters WH-2 1-1 # # Jcw Jcw Jcp Jcp Jcw U T A H Lake Powell Figure 3. Locations and sample numbers of fallout tuffs with isotopic ages from the Temple Cap (yellow triangles) and Carm- el (red triangles) Formations. The Carmel Formation outcrop belt is shown in green (Jc). The inset map shows the locations of the igneous clast collected from the Paria River Member of the Carmel Formation. Jcp – Paria River Member and Jcw – Winsor Member of the Carmel Formation. Results of isotopic age analyses are in table 1. 74 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Judd Hollow Member thins eastward from 17 m in the study area to 8 m near Lake Powell (Peterson and Pip- iringos, 1979; Wright and others, 1979; Doelling and others, 1989). The overlying Thousand Pockets Member of the Carmel Formation (Doelling and others, 2013) is dom- inated by eolian sand. The unit generally intertongues with fine-grained, marginal marine (tidal flat) red beds of the Crystal Creek Member (Peterson and Pipirin- gos, 1979; Doelling and others, 2013). Fallout tuff beds are preserved in several sections of the Crystal Creek Member within and outside of the study area. Outside of the study area ages of 166.0 ± 0.7 to 165.0 ± 1.2 Ma (U-Pb zircon; Sprinkel and others, 2011; Doelling and others, 2013; Sprinkel and others, in preparation) have been reported (figure 3 and table 1). An age of 167.1 ± 0.7 Ma (2σ, sanidine 40Ar/39Ar) comes from a sample collected at the top of the Thousand Pockets or the base of the overlying Paria River Member. In the study area, the Thousand Pockets and intertonguing Crystal Creek Members are about 60 to 85 m thick and thin eastward near Lake Powell to about 38 m thick (Peterson and Pipiringos, 1979; Wright and others, 1979; Doelling and others, 1989). The Paria River Member overlies the Thousand Pockets-Crystal Creek Members and consists gener- ally of dark reddish-brown to light-gray sandstone of marginal marine to fluvial origin deposited mostly on a tidal flat. Some sandstone beds are mottled or band- ed with shades of light- to dark-brown, light-gray, and purplish-gray hues. The Paria River includes interbed- ded dark reddish-brown siltstone and thin beds of dark reddish-brown mudstone and thin beds of calcarenite to limestone. The unit also contains several conglomer- ate beds, which are of interest because they contain the ignimbrite clasts. The description, chemistry, and age Sample Number Field Number Type of Sample Latitude Longitude (± 2σ) Mineral Dated 1-1 PH-2015-05-15-1-1 Cobble-sized clast 37.041537 -111.824346 171.73 ± 0.19 Ar/Ar sanidine 1-2 PH-2015-05-15-1-2 Cobble-sized clast 37.041550 -111.824321 1-3 PH-2015-05-15-1-3 Cobble-sized clast 37.041687 -111.824493 1-4 PH-2015-05-15-1-4 Cobble-sized clast 37.041687 -111.824493 1-5 PH-2015-05-15-1-5 Cobble-sized clast 37.041687 -111.824493 2-1 PH-2015-05-15-2-1 Boulder-sized clast 37.041687 -111.824493 WH-1 WH-07-20-2013-1 Fallout tuff 37.087886 -111.882425 163.6 ± 3.3 U-Pb zircon WH-2 WH-07-20-2013-2 Cobble-sized clast 37.041766 -111.824458 174 ± 5 U-Pb zircon WR-1 WR-111808-1 Fallout tuff 36.950250 -111.492433 164.40 ±4.50 Ar/Ar sanidine WR-6 WR-111808-6 Fallout tuff 36.962217 -111.490983 167.10 ±0.70 Ar/Ar sanidine JH-1 JH-111708-1 Sandstone 37.004900 -111.808200 170.5 ±0.95 U-Pb zircon EB East Bay Fallout tuff 37.015167 -111.249817 171.02 ±0.92 Ar/Ar biotite FBE Face Bay East Fallout tuff 37.009567 -111.245800 169.52 ±0.99 Ar/Ar biotite FOW Face One West Fallout tuff 37.014333 -111.268450 171.90 ±1.9 U-Pb zircon TW-1 TW-111908-1 Sandstone 37.645417 -111.464393 168.20 ± 1.30 U-Pb zircon TW-2 TW-111908-2 Fallout tuff 37.645958 -111.465062 165.30 ± 1.20 U-Pb zircon NOM GR-051809-1A Fallout tuff 38.680934 -110.154717 167.68 ± 0.82 U-Pb zircon NOM RC-051909-2 Fallout tuff 38.752650 -110.038550 166.70 ± 0.52 U-Pb zircon Note: Locations are also shown on figure 3 except 2 sample marked as NOM = locations not on map. Samples 1-1, 1-2, 1-3, 1-4, and 2-1 all come from the locality and are labeled as 1-1 on figure 3. Table 1. Sample locations and radiometric ages. 75 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 of the igneous clasts are discussed in a separate section as they are the focus of this study. Regionally, the Paria River Member ranges from about 80 to 180 m thick and thins eastward to 15 m near Lake Powell (Peterson and Pipiringos, 1979; Wright and others, 1979; Doelling and others, 1989). Within the study area, the Paria River Member is 20 to 23 m thick (Wheatley, 2018) (figure 4). The capping Winsor Member of the Carmel Forma- tion consists of interbedded sandstone, siltstone, and silty mudstone of marginal marine to fluvial origin de- posited on a tidal flat. The Winsor strata are varicolored forming a colorful banded unit that ranges from dark reddish-brown, reddish-orange, pale-orange, and light- brown to grayish purple, greenish-gray, and light-gray beds. Gypsum beds are typically found in the Winsor Member but are notably absent in the study area. Also atypical in the study area is the amount of sandy mate- rial that has infiltrated the member. The sandy nature of the Winsor is likely related to the same source of sand that is responsible for deposition of the Thousand Pock- ets Member (Doelling and others, 2013). A volcanic ash bed sampled from near the base of the Winsor Member (figure 3 and table 1) in the study area provided a U-Pb zircon age of 163.6 ± 3.3 Ma (U-Pb zircon) and indi- cates a Callovian age. Palynomorphs recovered from mudstone beds outside the study area indicate that the Winsor Member is Bathonian to Callovian (Sprinkel and others, 2011; Sprinkel and others, in preparation). The Winsor Member ranges from about 95 to 110 m in the general study area but thins eastward to about 78 m near Lake Powell. METHODS Samples of volcanic cobbles and of possible volcanic ash were collected in 2013 and 2015 from the Carm- el Formation east of Kanab (table 1 and figure 3). The outcrop consists of a layer that contains mostly volcanic pebbles and cobbles in various stages of preservation (figure 5). We collected several of the best-preserved clasts (clasts 1-1, 1-2, 1-3, 1-4, and 1-5), in addition to a sample of a large, highly-weathered volcanic boulder (2-1) that was at least 0.5 m across (we could not deter- mine its full size because it was partially buried). Two of us (Sprinkel and Wheatley) earlier collected materi- al from a purplish-gray, weathered clast (WH-2) out of the same conglomeratic layer from which we collected the other clasts. In addition, we collected a biotite-rich, purple, altered, and likely reworked, fallout tuff (WH-1) from near the White House campground on the Paria River approximately 1 m above the base of the Winsor Member. The cobbles were cut to a billet from which polished thin sections were produced for petrographic and electron microprobe analysis (table 2). A split of each sample was first washed in acid to remove carbonate cement and areas where carbonate has replaced the original tuff. After drying, the split was then pulverized in a tungsten carbide shatter box or ag- ate ball mill for whole-rock chemical analysis. X-ray flu- orescence (XRF) analyses were done at Brigham Young University using a Siemens SRS-303 spectrometer. Ma- jor elements (Si, Al, Ti, Fe, Mn, Mg, Ca, Na, K, and P) were determined on glass disks formed by fusing rock powder with lithium metaborate and are reported as oxides. Trace elements (Ba, Ce, Cr, Cu, Ga, La, Nb, Nd, Ni, Pb, Rb, Sc, Sm, Sr, Th, U, V, Y, Zn, and Zr) were determined on pressed powder pellets with a cellulose backing (table 3). In both cases, natural rocks were used as calibration standards. Analytical precision and accu- racy were assessed from repeat analyses of international geochemical reference materials. Clast 1-1 contained an abundance of clear, appar- ently unaltered sanidine crystals and a piece of this clast was sent to the WiscAr Geochronology Laboratory at the University of Wisconsin-Madison for mineral sep- aration 40Ar/39Ar dating. The 40Ar/39Ar ages were cal- culated relative to the FC-201 sanidine standard age of 28.201 Ma and a total 40K decay constant of 5.643 e-10/a (Kuiper and others, 2008). Methodology for laser fusion dating of single crystals of sanidine is outlined on the WiscAr Lab website (https://geochronology.geoscience. wisc.edu/analytical-approaches/), and values used in calculations are given in table 4 along with data on in- dividual grains. Zircons were extracted by heavy-liquid separation methods from the possible fallout tuff (WH- 1) and from a cobble-sized clast (WH-2). Laser ablation ICP-MS spot analyses were collected in zircon grains by Apatite to Zircon, Inc. Their analytical methodology is available upon request to the authors. 76 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Q U AT ER N A RY Q a - St re am a llu vi um Q ag - A llu vi al g ra ve l Q ea - M ix ed e ol ia n- al lu vi um JU RA SS IC Je - En tr ad a Sa nd st on e Jc u - Ca rm el F m ., W in so r- Pa ri a M br s. Jp t - Ca rm el F m ., Th ou sa nd s Po ck et s M br . Jc j - Ca rm el F m ., Ju dd H ol lo w M br . Jn - N av aj o Sa nd st on e 1 2 3 4 5 6 2930 m et er s 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 2930 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 8 7 6 5 4 3 2 1 0 8 7 6 5 4 3 2 1 0 clay silt v.f. sand v. sand m. sand c. sand v.c. sand gravel clay silt v.f. sand v. sand m. sand c. sand v.c. sand gravel clay silt v.f. sand v. sand m. sand c. sand v.c. sand gravel clay silt v.f. sand v. sand m. sand c. sand v.c. sand gravel clay silt v.f. sand v. sand m. sand c. sand v.c. sand gravel clay silt v.f. sand v. sand m. sand c. sand v.c. sand gravel En vi ro nm en t o f D ep os iti on Sa bk ha Fl uv ia l Eo lia n D eb ris F lo w Co ve re d Pl an ar C ro ss -b ed di ng U nd ul at or y/ Co nt or te d Be dd in g Ri pp le s Sy n- Se di m en ta ry F au lti ng U nd i� er en tia te d Cr os s- be dd in g Ro ot C as ts Ri p- up C la st s Se di m en ta ry S tr uc tu re s Pl an ar L am in at io ns Tr ou gh C ro ss -b ed di ng M ud C ra ck s Ta ng en tia l C ro ss -b ed di ng Ig ne ou s Cl as t A sh B ed W H -1 ; 1 63 .6 ± 3 .3 M a N W SE Se ct io n 1 W hi te H ou se N 37 ° 0 5’ 0 9. 75 ” W 11 1° 5 3’ 0 6. 05 ” Se ct io n 2 N W P ar ia O ve rlo ok N 37 ° 0 3’ 2 0. 77 ” W 11 1° 5 1’ 1 3. 85 ” Se ct io n 3 Pa ria O ve rlo ok N 37 ° 0 3’ 1 3. 17 ” W 11 1° 5 1’ 0 8. 79 ” Se ct io n 4 Pi pe H ol lo w D S N 37 ° 0 2’ 3 8. 37 ” W 11 1° 4 9’ 5 3. 03 ” Se ct io n 5 Pi pe H ol lo w G al le ry N 37 ° 0 2’ 3 0. 67 ” W 11 1° 4 9’ 2 7. 77 ” Se ct io n 6 Pi pe H ol lo w N 37 ° 0 2’ 2 4. 52 ” W 11 1° 4 9’ 2 3. 53 ” 4. 34 k m 0. 26 k m 2. 18 k m 0. 67 k m 0. 22 k m vo lc an ic la st ic sa nd st on e 3 cm v ol ca ni c fr ag m en ts vo lc an ic la st ic sa nd st on e TW -2 ; 1 65 .3 0 ± 1. 2 M a W in so r M em be r Pa ria Ri ve r M em be r Cry sta l Cre ek Th ou sa nd P oc ke ts M em be r M em be r 1- 1; 1 71 .7 3 ± 0. 19 M a W H -2 ; 1 74 ± 5 M a Fi gu re 4 . M ea su re d se ct io ns o f t he P ar ia R iv er M em be r o f t he C ar m el F or m at io n in th e P ar ia C an yo n ar ea sh ow in g th e b ed s t ha t c on ta in th e i gn eo us cl as ts . Th es e be ds a re in te rp re te d as d eb ris -fl ow d ep os its . Sa m pl es W H -1 a nd W H -2 (s ee ta bl e 1) w er e co lle ct ed fr om n ea r m ea su re d se ct io n 4. M od ifi ed fr om W he at le y an d C ha n (2 01 8) . G eo lo gi c m ap fr om D oe lii ng a nd W ill is (2 00 6) . S tr uc tu re c on to ur s ( re d da sh ed li ne s) d ra w n at b as e of th e N at ur ita F or m at io n (fo rm er ly D ak ot a Fo rm at io n) . C on to ur in te rv al is 1 00 m . 77 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 CHARACTER OF THE CLASTS Petrography: Phenocrysts, Textures, and Welding Chapman (1987, 1989, 1993) characterized a large number of clasts from these deposits and concluded that almost all of the large clasts (>1 cm) were welded rhy- olite tuffs. In the clasts smaller than about 1 cm, some were of intermediate and mafic composition, as well as intrusive igneous rocks, and a few metamorphic rocks. Our focus was not to duplicate Chapman’s efforts, but to better characterize the rhyolite tuff clasts that comprise the majority of the clasts within the conglomerates and all of the larger cobbles and boulders. Thin section examination of the rhyolite clasts re- veals that most of them have nicely preserved relict shard textures (figures 6 and 7). Shards are not flattened and only weakly oriented preferentially in most of the samples, preserving round bubble walls and bubble tri- ple junctions (figures 6A, 6B, and 7C). Relict pumice fragments are also not significantly flattened (figure 7). Figure 7D shows several slightly flattened pumices, but in figure 7C the large relict pumice is fairly equant. Our examination of these thin sections shows that the tuffs were not significantly welded, and must have had fair- ly low rock densities at the time of transportation and deposition. Phenocrysts preserved in the clasts include sani- dine, quartz, biotite, apatite, and zircon. Sanidine grains are preserved with little or no alteration in some of these volcanic clasts. They appear as clear euhedral to subhedral grains in thin section (figures 6A, 6E, 6F, 6H, and 7B,). Quartz is frequently embayed (figures 6E and 6F), suggesting that it was being resorbed at the time of eruption. Biotite occurs as partially to completely oxidized booklets (figure 6G). Dark oxides are concen- trated on cleavage planes where fluids could alter them. Plagioclase is not preserved, but must have initially been present in the tuffs as noted for other tuffs from the Temple Cap and Carmel Formations (Kowallis and others, 2001). Relict amphibole appears as iron oxides surrounding calcite cores (figures 6C and 6D) and relict pumice fragments as fine-grained aggregates of quartz and feldspar (figure 7B). Thin sections from sample WH-2 have less abundant quartz and sanidine, more abundant relict pumice, and fewer relict amphiboles. No primary Fe-Ti oxides appear to have survived in any of the samples. Nonetheless, the mineral assemblage is consistent with a rhyolitic bulk composition. XRF analyses (figure 8 and table 2) also show the rocks were probably rhyolites and trachytes before al- teration. However, their K2O contents are anomalously high (6.5 to almost 8% in the fresher clasts and over 10% in the altered boulder), and Na2O values are low (0.8 to 1.3% in the fresher clasts and 0% in the altered boul- der), when compared to fresh rhyolite, indicating that secondary alteration has significantly modified their chemistry. CaO concentrations (0.13 to 0.02 wt%) are Figure 5. (top) Large, rounded igneous, boulder-sized clast from the White House area (37.089487, -111.885517) near sample WH-1. The boulder has broken into several large pieces having maximum diameters of 1.3, 1.0, and 0.8 m with several other 0.5 m blocks and is compositionally similar to other igneous clasts in the area. (Photo credit: Mark Hans- ford). (bottom) Layer with volcanic cobbles where samples 1-1 through 1-5 were collected (see table 1 for location). 78 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 also lower than the 0.5 wt% typically found in rhyolite. Immobile element concentrations can be used to infer the probable original volcanic rock types before alter- ation (see for example, Christiansen and others, 2015). On tectonic discrimination diagrams (figure 9) us- ing Nb versus Y, most of the samples plot in the volcanic arc field (Pearce, 1996) and are similar to analyses from the Temple Cap and Carmel Formations published pre- viously (Kowallis and others, 2001). We also collected electron microprobe analyses from biotite and feldspar in the clasts (table 3). Only sanidine feldspar grains are preserved in the tuff clasts. This preservation is like that found in altered fallout tuff beds from the Carmel and Temple Cap Formations (Kowallis and others, 2001). The composition of sani- dine grains falls into three groups (figure 10). Clast 1-5 is distinctive with the highest Or content (Or72–Or76). WH-2 sanidine is also distinctive, but has lower Or val- ues (Or64–Or69). All the other clasts clump between Or59 and Or64. The same three groups appear on a plot of K versus Ba in these sanidines (figure 11). WH-2 grains have the highest Ba content and are typically unzoned in Ba (one grain out of six that were probed had a rim with lower Ba content). Clast 1-5 is also distinctive with high K and intermediate Ba concentrations. All of the sanidine in the other clasts clump together. Biotite occurs in all of the ignimbrite clasts we ex- amined. Compositions of biotite in terms of molar Fe/ (Fe + Mg) and total Al relative to ideal end members Sample Number Analysis Number SiO2 TiO2 Al2O3 FeO MnO MgO CaO BaO Na2O K2O F Cl SUM H2O* Total Sanidine 1-1 6b 66.17 18.94 0.19 0.25 0.07 4.12 11.03 100.76 1-1 6c 66.47 18.94 0.16 0.23 0.21 4.24 11.01 101.26 1-2 10c 66.07 19.03 0.14 0.24 0.62 4.30 10.55 100.95 1-2 10d 65.88 18.97 0.17 0.22 0.52 4.39 10.82 100.97 1-3 2-8 65.72 18.79 0.17 0.27 0.50 4.16 10.88 100.48 1-3 2-9 66.00 18.42 0.18 0.28 0.34 4.05 10.96 100.23 1-4 5d 65.57 19.01 0.08 0.36 0.39 4.29 10.94 100.64 1-4 5e 65.68 19.11 0.18 0.26 0.47 4.05 11.11 100.87 1-5 3-2 65.08 18.92 0.08 0.21 0.80 3.00 12.50 100.59 1-5 3-3 65.28 18.52 0.08 0.17 0.66 3.04 12.60 100.35 WH-2 b-1c 63.41 19.25 0.17 0.26 2.41 3.60 11.47 100.56 WH-2 b-3b 63.90 19.04 0.19 0.29 1.78 3.66 11.60 100.45 Biotite 1-1 2a 37.69 4.21 13.35 15.42 0.60 15.41 0.00 0.33 0.60 9.51 1.28 0.18 97.99 3.33 101.32 1-1 2b 37.78 4.37 13.19 15.31 0.61 15.11 0.01 0.22 0.60 9.43 1.31 0.20 97.53 3.28 100.81 1-2 5a 36.81 4.19 12.38 12.74 1.63 18.56 0.03 0.17 0.57 9.35 4.48 0.16 99.14 1.87 101.01 1-2 5b 37.26 4.35 13.03 13.08 1.31 17.53 0.01 0.17 0.68 9.62 4.01 0.17 99.48 2.08 101.56 1-3 3b 36.91 4.28 12.93 15.74 0.55 15.03 0.03 0.36 0.62 9.49 1.70 0.18 97.05 3.06 100.11 1-3 3c 37.73 4.31 12.81 15.68 0.55 15.17 0.02 0.09 0.59 9.67 1.58 0.16 97.66 3.16 100.82 1-4 1c 36.61 4.21 13.11 13.63 1.16 16.93 0.03 0.49 0.57 9.38 4.07 0.18 98.62 2.00 100.62 1-4 2a 35.73 4.13 13.24 13.50 1.45 17.74 0.06 0.60 0.52 9.04 4.16 0.15 98.54 1.99 100.53 1-5 2c 36.22 4.58 13.60 19.52 0.29 12.85 0.00 0.17 0.50 9.47 1.12 0.24 98.02 3.32 101.34 1-5 3a 36.48 4.55 13.49 18.16 0.28 12.98 0.00 0.24 0.28 9.62 2.21 0.26 97.56 2.76 100.32 WH-2 2-a8 35.28 5.19 14.56 15.93 0.45 14.23 0.00 2.39 0.54 9.06 1.00 0.05 98.25 3.42 101.67 WH-2 3-a12 34.36 5.55 14.21 16.13 0.49 13.49 0.00 2.69 0.55 8.55 0.92 0.07 96.60 3.37 99.97 *Calculated value for water in biotite Table 2. Selected electron microprobe analyses of sanidine and biotite. 79 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 fall mostly within the field for calc-alkaline igneous rocks from the western United States (Christiansen and others, 1986; figure 12a), with sample 1-5 bordering on the lower end of the field for the rhyolitic Bishop Tuff, and similar to the composition of the dacitic Fish Can- yon Tuff (Hildreth, 1979; Whitney and Stormer, 1985; Christiansen and others, 1986). The low Fe/Mg ratios of the biotite indicate a high oxygen fugacity and are thus consistent with generation in a subduction-relat- ed magmatic arc. Fluorine content of biotite from these tuffs is also similar to that of the Fish Canyon Tuff but only slightly overlaps into the field for biotite from other Middle Jurassic tuff beds in the Temple Cap and Carmel Formations that we have previously examined (Kowal- lis and others, 2001; figure 12b). A number of grains from samples 1-2 and 1-4 plot at higher F and Mg con- tent than the rest of the grains, suggesting that they have been affected by secondary alteration; these anomalous grains also have low Altot (figure 12a). Based on the biotite and feldspar compositions, it appears that the cobble and boulder deposits in the Carmel Formation have clasts from a minimum of three different volcanic units. Because our sampling of clasts was fairly limited, it is likely that the actual number of different eruptive units is higher. In conclusion, the volcanic clasts in this part of the Carmel Formation appear to be rhyolite ignimbrites, in agreement with Chapman’s (1987, 1989, 1993) orig- inal assessment of the clasts. However, we disagree with Chapman on the degree of welding. Chapman (1987) stated that the clasts are “partly welded to welded rhy- olitic tuff ” and that the “boulders of welded tuff could not have been derived from the distal non-welded edges of an ignimbrite sheet” (Chapman, 1993). Smith (1960) examined welding in ash flow tuffs and stated, “The de- gree of welding may range from incipient stages marked by the sticking together or cohesion of glassy fragments at their points of contact and within the softening range of the glass to complete welding marked by the cohe- sion of the surfaces of glassy fragments accompanied by their deformation and the elimination of pore space.” Smith (1960) continued by saying that, “deformation of pumiceous fragments and shards is the only positive criterion of welding in the tuffs which have crystallized, particularly in older rocks.” Using these criteria, we pro- pose that the clasts were derived from the non-welded to incipiently welded parts of an ignimbrite sheet; none of the clasts we examined showed evidence of strong weld- ing. Welded rhyolite tuffs have rock densities of 1.8 to 2.4 g/cm3, whereas poorly to non-welded rhyolite tuffs have densities of 1.0 to 1.8 g/ cm3 (Healey, 1970; Olsson, Sample Number 1-1 1-2 1-3 1-4 1-5 2-1 2-2 2-3 Major Elements (wt%) SiO2 74.44 72.83 73.79 77.71 76.50 62.69 63.25 62.60 TiO2 0.16 0.16 0.24 0.16 0.32 0.35 0.35 0.35 Al2O3 12.80 13.76 12.49 10.90 10.39 18.23 18.55 19.01 Fe2O3 1.93 2.04 2.53 1.73 3.05 3.65 3.46 3.42 MnO 0.03 0.03 0.03 0.06 0.02 0.02 0.02 0.01 MgO 0.12 0.18 0.20 0.10 0.22 0.42 0.45 0.40 CaO 0.12 0.13 0.07 0.11 0.12 0.02 0.01 0.01 Na2O 1.25 1.31 0.84 1.14 0.88 0.00 0.00 0.00 K2O 6.45 6.65 7.87 7.21 7.06 10.11 10.31 10.47 P2O5 0.01 0.01 0.02 0.00 0.01 0.02 0.01 0.01 LOI 2.08 2.09 1.26 0.64 0.72 4.40 3.73 3.78 TOTAL 99.38 99.19 99.33 99.76 99.29 99.90 100.15 100.06 Trace Elements (ppm) Ba 383 364 589 751 591 324 275 297 Ce 18 20 141 7 23 75 68 64 Cr 0 0 1 1 16 1 1 0 Cu 5 4 3 4 7 18 4 6 Ga 12 12 10 9 8 18 18 18 La 11 12 40 5 12 29 23 24 Nb 13 13 17 12 16 21 20 21 Nd 10 12 61 3 7 34 30 29 Ni 3 2 2 1 2 2 2 3 Pb 9 11 14 9 15 8 8 8 Rb 173 175 191 168 208 146 145 151 Sc 1 2 1 0 1 4 4 4 Sm 4 3 10 2 3 6 6 5 Sr 25 23 38 40 47 20 17 15 Th 14 17 19 13 24 19 19 17 U 3 4 4 4 5 4 4 4 V 14 15 32 12 37 24 20 24 Y 26 25 34 7 21 39 36 33 Zn 20 19 10 14 12 32 19 21 Zr 131 123 190 139 181 250 244 244 Table 3. X-ray fluorescence analyses (major and trace ele- ments) for Carmel Formation clasts. 80 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Sa m pl e: P H -2 01 5- 05 -1 5- 1 J- va lu e: 0 .0 07 66 92 ± 0 .0 00 00 38 (1 σ) D /a m u: 1 .0 08 44 ± 0 .0 00 81 (1 σ) M at er ia l: sa ni di ne Fi le La se r Po w er (% ) 40 A r (m ol es ) 40 A r (V ol ts ) ± 1σ 40 (V ol ts ) 39 A r (V ol ts ) ±1 σ39 (V ol ts ) 38 A r (V ol ts ) ± 1σ 38 (V ol ts ) 37 A r (V ol ts ) ± 1σ 37 (V ol ts ) 36 A r (V ol ts ) ± 1σ 36 (V ol ts ) % 40 A r* 40 A r* /39 A rK ± 1σ A ge ± 2 σ (M a) ( M a) K /C a In cl ud ed in w td . m ea n M A A 65 59 -0 00 35 4. 96 1E -1 4 8. 40 60 77 0. 00 35 47 0. 65 35 55 0. 00 08 10 0. 00 79 48 0. 00 00 69 0. 00 79 86 0. 00 02 27 0. 00 00 63 0. 00 00 06 99 .7 8 12 .8 34 01 5 0. 01 70 5 17 1. 84 ± 0 .4 4 35 .1 88 ✓ M A A 65 60 -0 00 35 4. 36 6E -1 4 7. 39 72 86 0. 00 24 44 0. 57 44 71 0. 00 05 75 0. 00 68 84 0. 00 00 69 0. 00 67 29 0. 00 02 03 0. 00 01 57 0. 00 00 06 99 .3 7 12 .7 95 57 8 0. 01 38 4 17 1. 35 ± 0 .3 5 36 .7 11 ✓ M A A 65 62 -0 00 35 3. 98 4E -1 4 6. 75 06 25 0. 00 17 03 0. 52 33 07 0. 00 05 49 0. 00 64 43 0. 00 00 83 0. 00 61 44 0. 00 01 63 0. 00 01 38 0. 00 00 06 99 .3 9 12 .8 21 50 1 0. 01 42 6 17 1. 68 ± 0 .3 6 36 .6 23 ✓ M A A 65 63 -0 00 35 2. 55 5E -1 4 4. 32 88 82 0. 00 11 03 0. 33 65 89 0. 00 03 69 0. 00 39 21 0. 00 00 68 0. 00 35 93 0. 00 01 20 0. 00 00 47 0. 00 00 05 99 .6 7 12 .8 19 29 7 0. 01 52 2 17 1. 66 ± 0 .3 9 40 .2 80 ✓ M A A 65 65 -0 00 35 4. 84 2E -1 4 8. 20 44 02 0. 00 32 31 0. 63 91 51 0. 00 06 85 0. 00 76 47 0. 00 00 96 0. 00 71 06 0. 00 02 19 0. 00 00 45 0. 00 00 06 99 .8 4 12 .8 15 71 2 0. 01 48 8 17 1. 61 ± 0 .3 8 38 .6 74 ✓ M A A 65 66 -0 00 35 2. 85 1E -1 4 4. 83 01 54 0. 00 12 00 0. 37 22 99 0. 00 03 99 0. 00 43 46 0. 00 00 58 0. 00 40 61 0. 00 01 38 0. 00 01 78 0. 00 00 06 98 .9 0 12 .8 31 66 7 0. 01 49 3 17 1. 81 ± 0 .3 8 39 .4 21 ✓ M A A 65 68 -0 00 35 3. 57 7E -1 4 6. 06 03 12 0. 00 25 78 0. 46 82 78 0. 00 05 91 0. 00 56 11 0. 00 01 07 0. 00 61 66 0. 00 01 76 0. 00 00 88 0. 00 00 06 99 .5 7 12 .8 86 43 9 0. 01 75 7 17 2. 51 ± 0 .4 5 32 .6 54 M A A 65 69 -0 00 35 4. 38 7E -1 4 7. 43 38 03 0. 00 26 55 0. 57 55 98 0. 00 07 25 0. 00 68 95 0. 00 00 63 0. 00 74 01 0. 00 02 32 0. 00 01 49 0. 00 00 06 99 .4 1 12 .8 38 28 6 0. 01 70 9 17 1. 90 ± 0 .4 4 33 .4 42 ✓ M A A 65 72 -0 00 35 3. 36 6E -1 4 5. 70 30 67 0. 00 25 95 0. 44 39 05 0. 00 05 07 0. 00 52 03 0. 00 00 72 0. 00 50 78 0. 00 01 83 0. 00 00 41 0. 00 00 06 99 .7 9 12 .8 20 24 9 0. 01 62 0 17 1. 67 ± 0 .4 1 37 .5 93 ✓ M A A 65 74 -0 00 35 3. 84 4E -1 4 6. 51 31 83 0. 00 15 94 0. 50 67 06 0. 00 06 41 0. 00 60 28 0. 00 00 47 0. 00 61 52 0. 00 01 70 0. 00 00 75 0. 00 00 06 99 .6 6 12 .8 10 43 4 0. 01 68 6 17 1. 54 ± 0 .4 3 35 .4 17 ✓ M A A 65 75 -0 00 35 2. 98 4E -1 4 5. 05 63 47 0. 00 15 47 0. 39 36 71 0. 00 04 33 0. 00 46 11 0. 00 00 77 0. 00 43 89 0. 00 01 57 0. 00 00 35 0. 00 00 06 99 .7 9 12 .8 17 78 1 0. 01 52 2 17 1. 64 ± 0 .3 9 38 .5 67 ✓ M A A 65 77 -0 00 35 2. 87 0E -1 4 4. 86 33 05 0. 00 15 98 0. 37 77 42 0. 00 03 88 0. 00 45 19 0. 00 00 39 0. 00 44 32 0. 00 01 26 0. 00 00 27 0. 00 00 06 99 .8 4 12 .8 53 86 4 0. 01 45 6 17 2. 10 ± 0 .3 7 36 .6 53 ✓ M A A 65 78 -0 00 35 6. 10 3E -1 4 10 .3 40 82 0 0. 00 33 01 0. 80 37 46 0. 00 08 04 0. 00 95 23 0. 00 00 70 0. 00 97 83 0. 00 03 18 0. 00 00 95 0. 00 00 06 99 .7 3 12 .8 31 06 2 0. 01 36 9 17 1. 81 ± 0 .3 5 35 .3 28 ✓ M A A 65 80 -0 00 35 2. 00 7E -1 4 3. 40 11 86 0. 00 10 73 0. 26 52 14 0. 00 03 18 0. 00 32 21 0. 00 00 59 0. 00 31 87 0. 00 01 32 0. 00 00 36 0. 00 00 06 99 .6 9 12 .7 84 54 5 0. 01 70 8 17 1. 21 ± 0 .4 4 35 .7 80 ✓ M A A 65 81 -0 00 35 3. 00 3E -1 4 5. 08 78 68 0. 00 20 12 0. 39 55 52 0. 00 05 18 0. 00 47 49 0. 00 00 52 0. 00 69 15 0. 00 01 95 0. 00 00 48 0. 00 00 06 99 .7 3 12 .8 27 47 6 0. 01 80 4 17 1. 76 ± 0 .4 6 24 .5 96 ✓ M A A 65 83 -0 00 35 2. 72 2E -1 4 4. 61 13 22 0. 00 13 12 0. 35 85 35 0. 00 03 77 0. 00 42 03 0. 00 00 35 0. 00 40 62 0. 00 01 62 0. 00 00 51 0. 00 00 06 99 .6 7 12 .8 19 62 6 0. 01 47 3 17 1. 66 ± 0 .3 8 37 .9 53 ✓ M A A 65 84 -0 00 35 3. 65 5E -1 4 6. 19 23 23 0. 00 20 86 0. 48 15 34 0. 00 05 52 0. 00 58 03 0. 00 00 89 0. 02 03 70 0. 00 04 78 0. 00 02 29 0. 00 00 06 98 .9 2 12 .7 20 70 2 0. 01 57 2 17 0. 40 ± 0 .4 0 10 .1 65 M A A 65 86 -0 00 35 3. 68 2E -1 4 6. 23 80 65 0. 00 26 62 0. 48 14 07 0. 00 05 42 0. 00 57 40 0. 00 00 80 0. 00 56 66 0. 00 02 28 0. 00 00 69 0. 00 00 06 99 .6 7 12 .9 15 91 6 0. 01 59 7 17 2. 89 ± 0 .4 1 36 .5 32 M A A 65 87 -0 00 35 4. 48 8E -1 4 7. 60 38 98 0. 00 22 30 0. 58 99 59 0. 00 06 52 0. 00 71 06 0. 00 01 10 0. 00 69 37 0. 00 01 85 0. 00 00 97 0. 00 00 06 99 .6 2 12 .8 40 24 3 0. 01 49 6 17 1. 92 ± 0 .3 8 36 .5 71 ✓ M A A 65 89 -0 00 35 3. 32 6E -1 4 5. 63 53 94 0. 00 11 35 0. 43 73 54 0. 00 04 86 0. 00 53 24 0. 00 00 70 0. 00 50 70 0. 00 01 85 0. 00 00 62 0. 00 00 06 99 .6 7 12 .8 43 17 9 0. 01 50 7 17 1. 96 ± 0 .3 9 37 .0 91 ✓ M A A 65 90 -0 00 35 2. 89 9E -1 4 4. 91 13 42 0. 00 28 50 0. 38 19 40 0. 00 04 99 0. 00 45 59 0. 00 00 55 0. 00 40 48 0. 00 01 55 0. 00 00 32 0. 00 00 06 99 .8 1 12 .8 34 45 4 0. 01 88 6 17 1. 85 ± 0 .4 8 40 .5 69 ✓ M A A 65 93 -0 00 35 2. 34 5E -1 4 3. 97 23 90 0. 00 14 41 0. 30 81 18 0. 00 03 65 0. 00 36 41 0. 00 00 33 0. 00 35 28 0. 00 01 37 0. 00 00 55 0. 00 00 06 99 .5 9 12 .8 39 65 1 0. 01 68 4 17 1. 92 ± 0 .4 3 37 .5 58 ✓ M A A 65 95 -0 00 35 2. 86 9E -1 4 4. 86 02 77 0. 00 13 56 0. 37 80 92 0. 00 04 75 0. 00 44 18 0. 00 00 40 0. 00 41 28 0. 00 01 43 0. 00 00 46 0. 00 00 06 99 .7 2 12 .8 18 76 1 0. 01 70 5 17 1. 65 ± 0 .4 4 39 .3 86 ✓ Th e va lu es in th is ta bl e re pr es en t b la nk , d isc rim in at io n, a nd d ec ay (37 A r a nd 39 A r) co rr ec te d va lu es . w ei gh te d m ea n ag e (2 0 of 2 3) : 17 1. 73 ± 0 .1 9 At m os ph er ic a rg on ra tio s D ec ay co ns ta nt s In te rf er in g iso to pe p ro du ct io n ra tio s 40 A r/ 36 A r 29 8. 56 ± 0 .3 1 Le e an d ot he rs (2 00 6) λ 40 A r (0 .5 80 ± 0 .0 14 ) x 1 0- 10 a -1 M in a nd o th er s ( 20 00 ) (40 A r/ 39 A r) K ( 5. 4 ± 1. 4) x 1 0-4 38 A r/ 36 A r 0. 18 85 ± 0 .0 00 3 Le e an d ot he rs (2 00 6) λ B- (4 .8 84 ± 0 .0 99 ) x 1 0- 10 a -1 M in a nd o th er s ( 20 00 ) (38 A r/ 39 A r) K (1 .2 10 ± 0 .0 02 ) x 1 0-2 39 A r (2 .5 8 ± 0. 03 ) x 1 0- 3 a- 1 St oe nn er a nd o th er s ( 19 65 ) (39 A r/ 37 A r) C a (6 .9 5 ± 0. 09 ) x 1 0-4 37 A r (8 .2 3 ± 0. 04 2) x 1 0- 4 h- 1 St oe nn er a nd o th er s ( 19 65 ) (38 A r/ 37 A r) C a (1 .9 6 ± 0. 08 ) x 1 0-5 36 C l (2 .3 03 ± 0 .0 46 ) x 1 0- 6 a- 1 (36 A r/ 37 A r) C a (2 .6 5 ± 0. 02 2) x 1 0-4 Ta bl e 4. C om pl et e 40 A r/ 39 A r r es ul ts fo r s am pl e 1- 1 re la tiv e to 2 8. 20 1 M a fo r t he F ish C an yo n sa ni di ne st an da rd . 81 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Ap B BB B G S Q E S Q H F C Am C D S A B Figure 6. Textures and minerals in the clasts. Clast 1-1 shown in A, E, F, and G; clast 1-2 in B, C, and D; clast 1-4 in H. The wide dimension on all the photomicrographs is about 1 mm, except for photomicrograph H, which is 5 mm. S = sanidine, Q = quartz, C = calcite, B = biotite, Am = relict amphibole, Ap = apatite. Pairs C-D and E-F are plane and cross-polarized images of the same area. All other photomicrographs are in plane-polarized light. 82 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 1991; Wohletz and Heiken, 1992). Moving low-density, non-welded tuff boulders over long distances would be much easier than moving blocks of more densely weld- ed tuff, but they would also be more friable. Age Chapman (1993) reported a 40Ar/39Ar age of 169 ± 4 Ma for one clast provided only as an oral commu- nication from John Obradovich at the U.S. Geological Survey (this age would be ~171 Ma using current decay constants and standards). The age of the Carmel Formation sediments en- closing the volcanic clast layer can be estimated from the age of fallout tuff sample WH-1, which gave a Tuff- Zirc age of 163.6 +3.3/-1.4 Ma for a group of 45 coher- ent grains and a U-Pb weighted mean age of 163.9 ± ~3.3 Ma (figure 13; assuming a 2% error, 2-sigma, [see Gehrels and others, 2008]). This age fits well with other ages (~162–164 Ma) obtained from the upper part of the Carmel Formation by palynology and radiometric dating of other ash beds (table 1; Sprinkel and others, 2011). No zircons older than the Middle Jurassic were identified. On the other hand, zircons from tuff clast WH-2 gave a spectrum of ages ranging from Middle Jurassic to Archean even though it lacks any evidence of a sedimentary component. A TuffZirc analysis of the youngest grains from WH-2 produced a coherent group of 20 grains with an age of 174 ± ~5 Ma (figure 14). This age is significantly older than other ages we have ob- tained from the volcanic deposits in the upper members SQ S S Pm Pm B Pm Pm Pm Pm Pm Pm Pm Pm Pm Pm PmA C D B Figure 7. Textures and minerals in clast WH-2. The wide dimension on all the photomicrographs is about 5 mm, except for photo B, which is 1 mm. S = sanidine, Q = quartz, C = calcite, B = biotite, Pm = pumice. Photomicrographs A, C, and D are plane-polarized light. Photomicrograph B is a blow-up of part of A taken in cross-polarized light. 83 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 of the Carmel Formation (Sprinkel and others, 2011) and seems likely to represent the age of the youngest detritus in this sample. The older detrital grains in sam- ple WH-2 match up well with the signal obtained from Lower and Middle Jurassic strata of the Colorado Pla- teau by Dickinson and Gehrels (2010). Sanidine grains have been preserved with little or no alteration in some of the volcanic clasts. They appear as clear euhedral to subhedral grains in thin section (fig- ures 6A, 6E, 6H, and 7B). A laser-fusion, single-crystal 40Ar/39Ar age of 171.73 ± 0.19 Ma was obtained on san- idine from clast 1-1 (figure 15 and table 4). This age is similar to ages obtained by Kowallis and others (2001) from sanidine from fallout tuffs in the Temple Cap For- mation in southwestern Utah (ages ranging from 170.24 to 172.93 ± ~0.5 to 0.6 Ma; corrected to new Fish Can- yon age standard and decay constants following Kuiper and others [2008]). Dickinson and others (2010) dated a tuff from an eolianite equivalent to the Temple Cap Formation collected farther east in southernmost Utah along the shores of Lake Powell (see figure 1 in Dickin- son and others, 2010). Their sample gave a U-Pb zircon age of 171.5 to 171.9 ± ~2.0 Ma and an 40Ar/39Ar biotite 1 10 100 1000 1 10 100 1000 Y (ppm) Within-Plate Granite Volcanic-Arc & Collision Granite Ocean-Ridge Granite N b (p pm ) 1-5 1-1 1-2 1-3 2-1 1-4 Figure 9. Nb and Y trace element compositions of volca- nic clasts from the Carmel Formation fall in the volcanic arc field on the discriminant diagram of Pearce and others (1984). The green area compares the clast compositions to those of ash beds in the Carmel and Temple Cap Formations from Kowallis and others (2001). Dacite SiO2 - weight % K 2O + N a 2O - w ei gh t % 41 0 8 6 4 2 16 10 12 14 6661565146 7671 1-11-2 2-1 1-5 1-3 1-4 Phonotephrite Tephriphonolite Phonolite Trachyandesite Basaltic Trachy- andesiteTrachy- basalt Picro- basalt Basalt Basanite Basaltic Andesite Rhyolite Andesite Trachyte Figure 8. Total alka- li versus silica volcanic rock classification dia- gram (after Le Bas and others, 1986). Sample numbers are shown next to data points. 84 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 age of 171.02 ± 0.92 Ma. Our new ages, along with the earlier age reported in Chapman (1993), provide clear evidence that the clasts in the conglomerates found in the upper Carmel For- mation are derived from outcrops of volcanic rocks of Temple Cap Formation age that are approximately 8 to 10 million years older than the Carmel Formation de- posit in which they are found. MOVING BOULDERS ACROSS A LARGE DISTANCE Nearest Outcrops The nearest current outcrops of Middle Jurassic vol- canic rocks of the right age (~170 Ma) are located in the lower Colorado River region of southern California and southwestern Arizona (Tosdal and Wooden, 2015). Chapman (1987) proposed that the magmatic arc was closer during the Jurassic than it is today due to Ba- sin and Range extension in order to account for trans- porting coarse volcanic boulders and cobbles ~350 km (straight line, closest current distance) from the Jurassic arc to the depositional site in south-central Utah (figure 16). However, because a significant portion of the dis- tance between the arc and the deposit falls on the Colo- rado Plateau (~180 km), which was not extended, how much shortening can we reasonably propose for the remaining 170 km? Estimates of horizontal extension due to Basin and Range activity vary widely from ex- tremes of 400% to as little as 20% (Hintze and Kowallis, 2009). Extension near the latitude of Las Vegas, Nevada, in an area called the Colorado River extensional cor- ridor has been the subject of several papers. Wernicke and others (1988) proposed extension in this region of 300 to 400%. Faulds and others (1990, 2001) discuss the extension in this region but do not give a percentage, only stating that large amounts of extension occurred. Marzolf (1990), in general agreement with Wernicke and others (1988), produced a Middle Jurassic palinspastic reconstruction of the southwestern United States show- ing a significant reduction (225 km as opposed to 350 km) in the distance the boulders would need to travel from a proposed tuff apron around the arc (figure 16). Two-hundred and twenty-five km is still a substantial distance over which to move large boulders. 333 3333 333334444444412 222222222211111 555555555555555511 Ab Or An 0 10 20 30 40 50 50 60 70 80 90 100 3 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0 0.01 0.02 0.03 0.04 0.05 0.06 K ( a p f u ) Ba (apfu) Figure 10. Compositions of sanidine grains from the clasts collected at sites PH-2015-05-15 (samples 1-1 to 1-5) and WH-07-20-2013-2 (WH-2 stars). Figure 11. Barium (Ba) concentration in sanidine grains (at- oms per formula unit) plotted versus K from clasts collected at sites PH-2015-05-15 (samples 1-1 to 1-5) and WH-07-20- 2013-2 (WH-2 stars). 85 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Run-out Distances for Tuffs Chapman (1987) also suggested that a factor to con- sider was the run-out distance for ash-flow tuffs and compiled a list of 17 eruptions where the run-out was over 30 km; two of them, a member of the Bates Moun- tain Tuff (now called the Nine Hill Tuff) and the Peach Springs Tuff, perhaps exceeded 200 km of run-out. We have compiled a larger list of 48 ignimbrites from the Cenozoic of the western United States with maximum run-outs greater than 40 km (corrected for post-erup- tion extension, table 5). Of these tuffs, 22 have maxi- mum run-outs of over 100 km, and 5 have run-outs of 200 km or more. The Nine Hill Tuff provides an infor- mative comparison. Henry and Faulds (2009) showed that this tuff was channeled in river valleys that cut across the (then lower) Sierra Nevada batholith from its eruptive source in western Nevada—a distance of over 200 km when corrected for later extension. These pale- ovalleys are typically 7 to 10 km wide and as much as 1 km deep. If the Carmel Formation cobbles and boul- ders were sourced from distal run-out lobes where flow was extended because of focusing in paleovalleys, then the distance from the tuff outcrops to the conglomer- ate depositional site could be reduced by another 100 to 200 km. As discussed earlier in this paper, our analysis of the clasts shows them to be poorly welded to unweld- ed, not unusual for distal parts of an ignimbrite. If we use the palinspastically reconstructed distance proposed by Marzolf (1990) of about 225 km and the run-out distances from table 5, then distal outflow ig- nimbrites could have been deposited close to the loca- tion of the gravel and boulder beds of the Paria River Member of the Carmel Formation. Remobilizing the Outcrops as Debris Flows Based upon careful analysis of the sedimentology of the clast-bearing outcrops, Chapman (1987, 1993) concluded that these deposits were formed from debris flows or lahars that likely occurred shortly after erup- tion, with the tuffs being laid down on a softer substrate of quartz-rich sedimentary rock (Chapman, 1989). We agree that the clast-bearing deposits represent one or more debris flows but disagree that these flows occurred soon after eruption of the tuffs. The ages on the clasts range between 171 and 174 Ma, whereas the age of the enclosing Paria River Member is ~163 to 167 Ma (table 1 and figure 4; Doelling and others, 2013), 5 to 10 Ma younger. Regardless of the age of the clasts, they were incorporated into a debris-flow rich in volcanic detritus 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Fe /( Fe +M g) Al (apfu) (A) Two-Mica Granites A-Type Granites & Topaz Rhyolites Calc-alkaline Rhyolites & Granites Fish Canyon Tu� and Middle Jurassic ash beds Bishop Tu� 1111 111 1 1 1 12 2 2 2 22 2 2 2 2222 22 2 2 2 2 2 2 3 3 33 3 3 3 333 3 3 4 4 4 4 4 4 44 4 4 5 5 5 5 555 55 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 -1.2 -0.8 -0.4 0.0 0.4 1 111 1 111 1 1 1 2 2 2 2 2 2 2 2 2 2 22 2 2 2 2 2 2 2 2 2 3 3333 3 3 3 3 3 3 4 4 4 4 44 4 4 4 4 5 5 5 5 55 5 5 Fish Canyon Tu� Middle Jurassic Ash Beds I-SCR I-WC I-MC I-SC Lo g (X F / X O H ) Log (XMg /XFe) (B) Figure 12. (A) Compositions of biotite from Carmel Forma- tion clasts compared to Fish Canyon and Bishop Tuffs (Hil- dreth, 1979) in terms of molar Fe/(Fe + Mg) and total Al rela- tive to ideal end member. Fields for different types of granite are from Christiansen and others (1986). (B) Compositions of biotite from Carmel Formation clasts compared to bio- tite from the Fish Canyon Tuff and Middle Jurassic ash beds from Temple Cap and Carmel Formations (Kowallis and others, 2001). Granite fields from Ague and Brimhall (1988): I-SCR = I-type, strongly contaminated and reduced; I-SC = I-type, strongly contaminated; I-MC = I-type, moderately contaminated; I-WC = I-type, weakly contaminated. 86 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 that must have been sourced in an area where multiple volcanic tuffs had accumulated on a surface with a sig- nificant east or northeast slope. Debris flows are capable of transporting very large clasts and may travel distances in excess of 100 km (Ne- all, 1976; Siebert and others, 1987; Carrasco-Núñez and others, 1993; Mothes and others, 1998; Scott and oth- ers, 2001). However, they are typically sourced in fairly steep terrain. As an example, Mothes and others (1998) stated that, “The Chillos Valley Lahar (CVL), the largest Holocene debris flow in area and volume as yet recog- nized in the northern Andes, formed on Cotopaxi vol- cano’s north and northeast slopes and descended river systems that took it 326 km north–northwest to the Pa- cific Ocean and 130+ km east into the Amazon basin.” More recently, the eruption of Nevado del Ruiz volcano in Colombia in 1985 produced lahars that traveled 60 to 70 km from their source along the channels of the 90 110 130 150 170 190 210 230 23 8 U – 20 6 Pb A ge (M a) Tu�Zirc Age = 163.6 (+3.3, -1.4) Ma box heights are ± 2σ U-Pb Weighted Mean Age = 163.9 ± 3.3 Ma 0 5 10 15 20 25 30 35 Re la tiv e Ag e Fr eq ue nc y (r ed c ur ve ) U-Pb Age (Ma) WH-1 WH-1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 100 120 140 160 180 200 220 240 U /T h (b lu e po in ts ) (A) (B) Figure 13. 238U-206Pb ages for zircons from the basal Win- sor Member of the Carmel Formation ash bed WH-1. (A) A relative age probability diagram of individual U-Pb grain ages (red curve) (Hurford and others, 1984, and Kowallis and others, 1986). Also shown are U/Th ratios (blue dots) for each analyzed spot. (B) TuffZirc plot (Ludwig and Mundil, 2002) for zircons from sample WH-1. Blue bars and open bars are analyses not included in the overall age. The blue bars do not overlap the error range and the open bars are low resolution ages. 90 110 130 150 170 190 210 23 8 U – 20 6 Pb A ge (M a) Tu�Zirc Age = 174.3 (+4.1, -5.7) Ma box heights are ± 2σ WH-2 0 5 10 15 20 25 30 35 0 500 1000 1500 2000 2500 Re la tiv e Ag e Fr eq ue nc y (r ed c ur ve ) U-Pb Age (Ma) WH-2 Dickinson and Gehrels (2010) Lower to Middle Jurassic �uvial and marine detrital zircon, Colorado Plateau, Utah This Paper: Zircon population from clast WH-2 in Paria River Member of Carmel Formation, Colorado Plateau, Utah 169-177 Ma peak from volcanic arc (A) (B) Figure 14. (A) 238U-206Pb zircon ages from sample WH-2 (red line) plotted with the detrital zircon ages reported by Dickin- son and Gehrels (2010) from Lower to Middle Jurassic sedi- mentary rocks of the Colorado Plateau. The peaks from both curves correlate quite well neglecting the young population of grains in WH-2. (B) U-Pb TuffZirc plot for the subset of young zircons from clast WH-2. 87 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Rio Azufrado and Rio Lagunillas, burying the city of Armero and killing more than 25,000 people (Naranjo and others, 1986; Voight, 1990). To provide the necessary slope for debris flows to form and transport the clasts, we propose that the distal tuff deposits described above were deposited in a high- land area northeast of the low-standing Middle Juras- sic arc. They remained in place for several million years after their initial emplacement during which time they were subjected to weathering and alteration. Even poor- ly welded tuffs would likely be more resistant to erosion than the silt and mud of the underlying Temple Cap-age sediments. As erosion and weathering progressed, it is possible that inverted valley topography formed (Cund- ari and Ollier, 1970; Hamblin, 1987) with the tuffs rising somewhat above the surrounding country, providing additional gravitational potential and the slope need- ed for moving the clasts by debris flows over the last few tens of kilometers to reach the depositional site. In southwestern Utah, basaltic lavas have produced invert- ed topography of over 300 m relief in less than 3 million years (Hamblin, 1987; Biek and others, 2009). DISCUSSION The eruptive sources of the large ignimbrite clasts in the Jurassic Carmel Formation of southern Utah remain problematic. Emplacement of the pa- rental ignimbrites occurred in southern California and across southern Nevada and northern Arizona with distal lobes extending up to 200 km from their vents (figure 16). These lobes would have reached across the location of the modern-day Grand Can- yon. After remaining in place and weathering for some 5 to 10 million years, they were remobilized as debris flows that carried the clasts the remaining distance (±50 km) into their current depositional setting. This would account for even the large boul- der-sized clasts found in the Paria River Member of the Carmel Formation. However, the event or events that brought the clasts into the depositional basin were not common, as we do not find the volcanic clasts throughout the Carmel Formation. Their oc- currence is restricted to only one or two horizons. Chapman (1993) proposed that the boulder beds in the Carmel Formation required catastrophic floods to carry the boulders over the transport distance of 200 to 300 km to get them from their source to the depositional site. This proposed flooding could have originated in a highland arc terrain due to excessive rainfall or due to the collapse of a crater lake or natural volcanic dam. Alexander and Cook- er (2016) have shown that during flash floods—de- fined as any overland flow of water within or out- side a river channel that arrives suddenly at a fixed point, changes quickly, and lasts a short time—large boulders can be moved farther than would normally be predicted due to the inherently unstable nature of the flow. We agree with Chapman that this type of event could be a possible trigger for the deposits. However, as we have discussed above, the distance required for transport was likely much less than 200 to 300 km and more likely in the range of 50 km. This distance is not unreasonable for a debris flow to carry the low-density tuff boulders and cobbles. Near an active arc, another possible triggering fac- tor may have been earthquakes. Scott and others (2001) give examples of several earthquake-triggered debris flows, with some traveling over 100 km from their sources. For example, a 1994 earthquake in Colombia generated a debris flow that “conveyed a catastroph- ic wave of debris” along the Río Páez for over 100 km (Scott and others, 2001). It is apparent that earthquakes large enough to liquefy and deform the sedimentary de- Age -- Ma 170 0 4 3 2 1 175174173172171 Relative ProbabilityN um be r o f A ge s Weighted Mean Age = 171.73 ± 0.19 Ma Peak Age = 171.75 Ma 23 grains Figure 15. Relative probability plot for 40Ar/39Ar sanidine ages from clast 1-1. Bars are individual grain ages. 88 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 42° 34° 36° 46° 44° 40° 38° 32° 124° 120° 116° 112° 108° 0 Middle Jurassic tectonics 0 0 50 100 150 200 250 mi 200100 300 400 km SL-MS SZ Great A nc es tr al Ro ck ie s Valley Group Klamath Mtns M odern Shoreline Blue Mtns Ar ap ie n- Ca rm el Em ba ym en t Sundance Sea Mogollon Highland Paria R ive r F luvial S yste m dunes tidal �at dunes 20° N 15° N UA Middle Jurassic felsic pluton Plate motion Wind direction Volcanic clast deposits Stream �ow Ancestral Uinta arch Snow Lake-Mojave- Sonora shear zone (SL-MS SZ) Triassic Luning-Fencemaker terrane Jurassic Coast Range ophiolite and Great Valley Group Jurassic Franciscan subduction complex Paleozoic-early Mesozoic oceanic arcs (Klamath, Blue Mtns, N. Sierra Nevada) Paleozoic oceanic terranes (Roberts Mountains and Golonda allochthons) North America craton Possible source area for volcanic clasts UA Figure 16. Paleogeographic and tectonic map for the Middle Jurassic during deposition of the Paria River Member of the Carmel Formation. Information used in compiling this figure comes from Blakey and others (1983), Kocurek and Dott (1983), Chapman (1987), Busby-Spera (1988), Thorman and others (1991), Dilek and Moores (1993), Taylor and others (1993), Christiansen and others (1994), Lawton (1994), Marzolf (1994), Peterson (1994), Blakey and Parnell (1995), DeCelles and Currie (1996), Lawton and McMillan (1999), Kowallis and others (2001), and Dickinson (2006). 89 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 Table 5. Runout distances for selected ignimbrites from the western United States. Stratigraphic Unit Age (Ma) Composition Location Runout (km) Corrected Correction Factor Runout (km) Today Reference Fraction Tuff 18.6 Rhyolite Central Nevada VF 44 0.714 62 Best and others, 2013a Pahranagat Formation 22.9 Rhyolite Central Nevada VF 114 0.714 160 Best and others, 2013a Tuff of Clipper Gap 25.0 Rhyolite Central Nevada VF 214 0.714 300 Best and others, 2013a Tuff of Lunar Cuesta 25.7 Dacite-rhyolite Central Nevada VF 130 0.850 153 Best and others, 2013a Upper Tuff Member of SPF 26.4 Rhyolite Central Nevada VF 200 1.000 200 Best and others, 2013a Tikaboo Tuff Member of SPF 26.8 Rhyolite Central Nevada VF 114 0.850 134 Best and others, 2013a Hancock Tuff Member of SPF 26.8 Rhyolite Central Nevada VF 80 0.714 112 Best and others, 2013a Lower Tuff Member of SPF 27.0 Rhyolite Central Nevada VF 121 1.000 121 Best and others, 2013a Tuff of Orange Lichen Creek 27.1 Rhyolite Central Nevada VF 61 0.850 72 Best and others, 2013a Monotony Tuff 27.6 Dacite-rhyolite Central Nevada VF 136 0.850 160 Best and others, 2013a Tuff of Hot Creek Canyon 30.0 Rhyolite Central Nevada VF 43 1.000 43 Best and others, 2013a Windous Butte Formation 31.7 Dacite-rhyolite Central Nevada VF 228 0.850 268 Best and others, 2013a Pancake Summit Tuff 35.3 Rhyolite Central Nevada VF 78 0.850 92 Best and others, 2013a Stone Cabin Upper Member 35.8 Rhyolite Central Nevada VF 76 1.000 76 Best and others, 2013a Stone Cabin Middle Member 35.8 Rhyolite Central Nevada VF 76 0.850 89 Best and others, 2013a Hiko 22.6 Rhyolite Caliente caldera complex 65 0.733 89 Best and others, 2013a Racer Canyon 18.6 Dacite-rhyolite Caliente caldera complex 49 1.000 49 Best and others, 2013a Harmony Hills Tuff 22.6 Andesite Caliente caldera complex 116 0.870 133 Best and others, 2013a Bauers Tuff 23.0 Rhyolite Caliente caldera complex 120 1.000 120 Best and others, 2013a Swett Tuff 24.2 Rhyolite Caliente caldera complex 143 0.733 195 Best and others, 2013a Leach Canyon 24.0 Rhyolite Caliente caldera complex 120 0.870 138 Best and others, 2013a Hole in the Wall Member of Isom Fm 24.6 Trachydacite Indian Peak VF 80 0.666 120 Best and others, 2013a Bald Hills Member of Isom Fm 27.5 Trachydacite Indian Peak VF 107 0.666 160 Best and others, 2013a Ripgut Tuff 29.0 Rhyolite Indian Peak VF 61 0.870 70 Best and others, 2013a Petroglyph Cliff Tuff 29.1 Dacite-trachydacite Indian Peak VF 43 0.870 50 Best and others, 2013a Lund Tuff 29.2 Dacite Indian Peak VF 97 0.666 145 Best and others, 2013a Silver King Tuff 29.4 Dacite-rhyolite Indian Peak VF 42 0.666 63 Best and others, 2013a Mackleprang Tuff 30.0 Rhyolite Indian Peak VF 61 0.666 92 Best and others, 2013a Greens Cayon 30.1 Rhyolite Indian Peak VF 19 0.666 29 Best and others, 2013a Deadman Spring 30.0 Dacite Indian Peak VF 50 0.870 58 Best and others, 2013a Wash Wah Springs Tuff 30.1 Dacite Indian Peak VF 133 0.666 200 Best and others, 2013a Cottonwood Wash Tuff 31.1 Dacite Indian Peak VF 104 0.870 120 Best and others, 2013a Lamerdorf 32.0 Rhyolite Indian Peak VF 64 0.733 87 Best and others, 2013a Marsden 33.0 Rhyolite Indian Peak VF 48 0.870 55 Best and others, 2013a Sawtooth 33.5 Rhyolite Indian Peak VF 40 1.000 40 Best and others, 2013a Tunnel Spring Tuff 35.3 Rhyolite Indian Peak VF 15 0.666 22 Best and others, 2013a The Gouge Eye 36.0 Dacite-rhyolite Indian Peak VF 25 1.000 25 Best and others, 2013a Blue Sphinx/Hu Pwi 24.5 Rhyolite Western Nevada VF 76 0.870 88 Henry and John, 2013 Bates Mtn A/Rattlesnake Canyon 31.2 Rhyolite Western Nevada VF 126 0.666 189 Henry and John, 2013 Arc Dome 25.1 Rhyolite Western Nevada VF 61 0.666 91 Henry and John, 2013 Campbell Creek/Bates Mtn C 28.9 Rhyolite Western Nevada VF 257 0.733 350 Henry and John, 2013 Candelaria Hills 26.0 Rhyolite Western Nevada VF 47 0.980 48 Henry and John, 2013 Gabs Valley 25.1 Rhyolite Western Nevada VF 123 0.733 168 Henry and John, 2013 Nine Hill Tuff 25.4 Rhyolite Western Nevada VF 167 0.666 250 Henry and John, 2013 New Pass Tuff 25.3 Rhyolite Western Nevada VF 97 0.666 146 Henry and John, 2013 Singatse Tuff 26.9 Rhyolite Western Nevada VF 136 0.733 185 Henry and John, 2013 Toiyabe Tuff 23.3 Rhyolite Western Nevada VF 157 0.733 214 Henry and John, 2013 Mickey Pass Tuff 27.1 Rhyolite Western Nevada VF 210 0.733 287 Henry and John, 2013 Notes: Corrected for 50% post volcanic east-west extension in western Nevada and central Nevada volcanic fields (VF); corrected for 40% post volcanic east-west extension in Indian Peak volcanic field and Caliente caldera complex; distances in red are for runouts over 100 km; SFP = Shingle Pass Formation. 90 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 posits of the Carmel Formation did occur during depo- sition of this formation. Marzolf (1988, 1990) noted that associated with these boulder and cobble beds in the Carmel Formation are “massive silty sandstones [that] display abundant evidence of liquefaction and fluid- ization, including remnants of contorted stratification, foundered conglomerate beds, and spectacular elutria- tion pipes.” More recently, Wheatley and others (2016) also concluded that these structures and pipes were like- ly caused by one or more earthquakes. An earthquake at the right time of year when the source outcrops were saturated with seasonal rainfall or snowmelt could have provided a trigger for the debris flows. Whatever the triggering mechanism, a highland area to the southwest of the depositional site was need- ed. From the time of eruption of the tuff beds at about 172 to 174 Ma until the formation of the transporting debris flows, subduction and volcanic activity appear to have been fairly continuous along the Jurassic arc as evidenced by the numerous fallout ash beds that were deposited in the Carmel-Twin Creek (Sundance) sea- way (Marvin and others, 1965; Everett and others, 1989; Zhang, 1996; Kowallis and others, 2001; Sprinkel and others, 2011). Busby-Spera (1988) proposed that the arc during this time period was a low-standing, arc-gra- ben. Chapman (1993) countered, arguing that by the time the debris flows were carrying boulders over long distances into the Carmel basin, there must have been a significant highland to the southwest of the deposi- tional site. We agree that debris flows require a signifi- cant gravitational potential in order to travel even a few tens of kilometers and that the source area must have been at a significantly higher elevation than the depos- its, but this may not necessitate a change in the model of a low-standing arc-graben at the time these deposits were formed. The modern Central American volcanic arc formed along the Middle American trench is in part a low-standing arc-graben similar to what Busby-Spera (1988) proposed for the Middle Jurassic. Figure 17 shows the Central American arc with a low-standing, arc-graben formed between northern Costa Rica and northern Nicaragua. To the northeast of this part of the arc is a highland region with rivers flowing off of it toward the Caribbean coast. We have superimposed on the figure our modeled source area and location of the clast deposits from figure 16. The scale of the mod- ern arc and distance from the arc to the eastern fluvi- al plain are remarkably similar to what we propose for the Middle Jurassic. Additionally, studies of landslides and debris flows in the highland areas of Guatemala, El Salvador, Costa Rica, and Nicaragua have documented run-out distances of up to 50 km (Alvarado and others, 2004; Siebert and others, 2006; Devoli and others, 2007, 2009), similar to what we propose would have been needed to transport the Middle Jurassic clasts from the outcrop area to the depositional site. Triggers for landslides in Nicaragua include earthquakes, volcanic eruptions, and rainfall associated with tropical storms or hurricanes (Devoli and others, 2007). CONCLUSIONS The source for coarse volcanic detritus in the Mid- dle Jurassic Carmel Formation of southern Utah has been problematic. We propose that poorly welded rhy- olite tuffs generated during Temple Cap Formation time (~173 Ma) from the Middle Jurassic arc in southern California were likely emplaced in distal run-out lobes focused in fluvial valleys, perhaps as much as 200 km from the source vents extending out across the location of the modern Grand Canyon. These tuff outcrops were then stable for several million years as the processes of weathering and alteration proceeded perhaps produc- ing an inverted topography with the tuffs capping mesas above weaker sedimentary units. About 164 Ma, debris flows sourced in these tuff-capped mesas formed and then flowed perhaps 50 km (but possibly even farther) carrying cobbles and some boulders along channels to the depositional site in fluvial channels on a broad tid- al flat. Triggers may have been torrential rain, earth- quakes, or some combination of these factors. ACKNOWLEDGMENTS We acknowledge the assistance of Michael Dorais, David Tingey, Magnolia Serrano-Tomlinson, and Da- vid Tomlinson at Brigham Young University for their assistance with electron microprobe and geochemical analyses, Brian Jicha at the WiscAr Lab at the Univer- sity of Wisconsin for assistance with 40Ar/39Ar analyses, 91 Rhyolite Ignimbrite Boulders and Cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—Age, Composition, Transport, and Stratigraphic Setting Kowallis, B.J., Sprinkel, D.A., Christiansen, E.H, Steed, S., and Wheatley, D.F. Geology of the Intermountain West 2020 Volume 7 and Paul O’Sullivan at Apatite to Zircon Inc. for U-Pb analyses. We also thank the Utah Geological Survey and Brigham Young University for financial support for iso- topic and geochemical analyses, as well as travel to the field area. 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