UGA-Geosite-Loope-Calf-Creek.indd M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors David B. Loope and Richard M. Kettler Earth & Atmospheric Sciences, University of Nebraska Lincoln, NE 68588-0340 dloope1@unl.edu Cover Image: View of a concretion along a trail in Upper Calf Creek Falls. Rinded, Iron-Oxide Concretions in Navajo Sandstone Along the Trail to Upper Calf Creek Falls, Garfield County 2 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors Utah Geosites showcases some of Utah’s spectacular geology, both little-known localities and sites seen by visitors to Utah’s many national and state parks and monuments. Th e geosites refl ect the interests of the many volunteers who wrote to share some of their favorite geologic sites. Th e list is eclectic and far from complete, and we hope that additional geosites will be added in the coming years. Th e Utah Geological Survey also maintains a list of geosites https://geology.utah.gov/apps/geosights/index.htm. We thank the many authors for their geosite contributions, Utah Geological Association members who make annual UGA publications possible, and the American Association of Petroleum Geologists—Rocky Mountain Section Foundation for a generous grant for desktop publishing of these geosite papers. Design and desktop publishing by Jenny Erickson, Graphic Designer, dutchiedesign.com, Salt Lake City, Utah. Th is is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and fi gures that are not noted as copyrighted, provided the original author and source are credited. See the Utah Geological Association website, www.utahgeology.org, and Creative Commons https://creativecommons.org/licenses/by/4.0/ for details. Suggested citation for this geosite: Loope, D.B., and Kettler, R.M., 2019, Rinded iron-oxide concretions in Navajo Sandstone along the trail to Upper Calf Creek Falls, Garfi eld County, in Milligan, M., Biek, R.F., Inkenbrandt, P., and Nielsen, P., editors, Utah Geosites: Utah Geological Association Publication 48, 7 p., https://doi. org/10.31711/geosites.v1i1.59. Presidents Message I have had the pleasure of working with many diff erent geologists from all around the world. As I have traveled around Utah for work and pleasure, many times I have observed vehicles parked alongside the road with many people climbing around an outcrop or walking up a trail in a canyon. Whether these people are from Utah or from another state or country, they all are quick to mention to me how wonderful our geology is here in Utah. Utah is at the junction of several diff erent geological provinces. We have the Basin and Range to the west and the Central Utah Hingeline and Th rust Belt down the middle. Th e Uinta Mountains have outcrops of some of the oldest sedimentary rock in Utah. Utah also has its share of young cinder cones and basaltic lava fl ows, and ancient laccoliths, stratovolcanoes, and plutonic rocks. Th e general public comes to Utah to experience our wonderful scenic geology throughout our state and national parks. Driving between our national and state parks is a breathtaking experience. Th e “Utah Geosites” has been a great undertaking by many people. I wanted to involve as many people as we could in preparing this guidebook. We have had great response from authors that visit or work here in the state. Several authors have more than one site that they consider unique and want to share with the rest of us. I wanted to make the guidebook usable by geologists wanting to see outcrops and to the informed general public. Th e articles are well written and the editorial work on this guidebook has been top quality. I would like to personally thank Mark Milligan, Bob Biek, and Paul Inkenbrandt for their editorial work on this guidebook. Th is guidebook could not have happened without their support. I would like to thank Jenny Erickson for doing the great desktop publishing and the many authors and reviewers that helped prepare the articles. Your work has been outstanding and will certainly showcase the many great places and geology of Utah. Last, but not least, Th ank you to the American Association of Petroleum Geologists, Rocky Mountain Section Foundation for their fi nancial support for this publication. Guidebook 48 will hopefully be a dynamic document with the potential to add additional “geosites” in the future. I hope more authors will volunteer articles on their favorite sites. I would like to fi ll the map with locations so that a person or family looking at the map or articles will see a great location to read about and visit. Enjoy Guidebook 48 and enjoy the geology of Utah. Peter J. Nielsen 2019 UGA President D.B. Loope and R.M. Kettler Giant Concretions of Upper Calf Creek Falls 3 INTRODUCTION Concretions are hard rock masses, usually spheroidal, but com- monly oblate or discoidal, that are formed by strongly localized precipitation of minerals in the pores of an otherwise weaker sedimentary rock (see Bates and Jackson, 1980, for a more extensive definition). The iron-oxide-rich concretions in the Jurassic Navajo Sandstone in southern Utah are unusual in two fundamental ways. First, they are cemented by iron oxide (Fe2O3, or Fe(OH)3); most other concretions are cemented by silica (SiO2), calcite (CaCO3), dolomite (CaMg(CO3)2), or siderite (FeCO3). Second, unlike other concretions, they are not strongly cemented throughout, but instead, the iron oxide is concentrated in a very strongly cemented, sharply defined, exterior rind or shell. In the smaller concretions, the entire interior lacks iron-oxide cement, and is similar to the rock outside the concretion; in the larger concretions, there is a cen- tral zone that is strongly cemented by iron oxide, but the sandstone between the central core and the rind has no iron-oxide cement. The processes involved in the formation of the Navajo concretions have been extensively studied by geologists ever since the Martian rover Opportunity imaged small, spheroidal, iron-rich concretions on the surface of Mars (Moore, 2004). The Navajo concretions were quickly recognized as possible terrestrial analogs to the Martian “blueberries” (Chan and others, 2004). At present there are three main explanations for how these concretions formed. The first inter- pretation of the Navajo concretions (Chan and others, 2004; Beitler and others, 2005) views the iron-oxide cement and the “rinded” as- pect of today’s concretions as primary features—just a thin, three-di- mensional band of sandstone (circular when viewed in two dimen- sions) that had been cemented by iron oxide (Chan and others, 2004). A complication of this interpretation is that, because iron is not transported by water containing oxygen, mixing of an iron-bear- ing water (devoid of oxygen) with a second, oxygenated water mass is required (Chan and others, 2004). Mixing is typically not required for growth of silica, calcite, dolomite, or siderite concretions. The second interpretation (Loope and others, 2010; Weber and others, 2012) is that solid, spheroidal concretions were originally well-cemented throughout by siderite (FeCO3). The rinds formed secondarily, during much later alteration of the older, primary siderite. A weakness of this interpretation is that unaltered siderite has not been found in any Navajo concretions. The third interpretation (Yoshida and others, 2018) holds that the original spheroidal concretions were solid spheres cemented by calcite (CaCO3). The rinds formed later when acidic, iron-bearing waters invaded the Navajo. The calcite of the concretions buffered the acidic waters, forcing iron oxide to precipitate. A weakness of this interpretation is that it does not explain the localization of iron into a distinct, strongly cemented rind nor the presence of abundant iron in a spatially isolated core zone. All the research cited above was stimulated by the discovery, on Mars, of small, iron-rich, spheroidal concretions (the aforemen- tioned blueberries), but the concretions exposed at this GeoSite are neither small nor spheroidal. They are, however, iron-rich, calcite-free, rinded, and very interesting. We think that the spatial arrangement of their internal features aid interpretation of the small, rinded spheroids. In this article, we briefly present our interpretation (Loope and others, 2010; Loope and others, 2011; i.e., the second interpretation above) of the very large, non-sphe- roidal, rinded concretions that are common on the beautiful trail to Upper Calf Creek Falls. Driving Instructions and Hiking Suggestions The trailhead is 7.4 miles (11.8 km) southwest of Boulder, UT and 20.5 miles (32.8 km) northeast of Escalante along a portion of Utah Highway 12 that follows the narrow, flat surface of New Home Bench, above the canyons of Calf Creek (to the west) and Dry Hollow (to the east; figure 1). The turnoff for the trailhead is on the west side of the highway, between mile 80 and mile 81. The parking area is ≈ 500 feet (150 m) from the highway, but getting there can be rough for a low-clearance vehicle (you can instead park along the highway). At its start, the trail to Upper Calf Creek Falls drops about 500 feet (150 m) down a steep outcrop of Navajo Parking to Boulder to Escalante Figure 1. Topographic map showing location of parking area and trail head for Upper Calf Creek Falls. 4 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 Sandstone, the most difficult part of the short 0.9 mile (1500 meter) one-way hike. Take care to step only on bare sandstone, avoiding the ball-bearing-like gravel. In summer, you can depend on Calf Creek as a great way to cool off; soak your shirt and hat (at least) before hiking out of the canyon. If you wander off the trail, avoid stepping on the dark soil crust that stabilizes the loose sand. GPS Location of Trailhead: 37°51'34" N, 111°26'18" W (WGS84) GEOLOGIC CONTEXT Boulder-strewn New Home Bench is an example of inverted topography. About one million years ago, major flood events de- posited large boulders (derived from the Aquarius Plateau to the west) in a stream valley cut into the Navajo Sandstone (Marchetti and others, 2012). The boulders effectively armored the stream- bed, preventing further erosion of the bedrock below the boulders. Soon thereafter, erosion of the bedrock commenced on both the east and west sides of the boulder accumulation; one million years of erosion produced the two deep canyons of Calf Creek and Dry Hollow, and stranded the (inverted) flat surface between them (figures 1 and 2). The Navajo Sandstone, well-exposed in the canyons below New Home Bench, was deposited in the Early Jurassic (200 mil- lion years ago) by very large, southward migrating sand dunes (Kocurek and Dott, 1983). Even though the Navajo has not been tilted, most of the bedding in the Navajo slopes southward at about 20-25°. These layers of sand were not deposited parallel to the floor of the giant dune field. Instead they were deposited on the dunes’ downwind slopes at angles as steep as 32° (the angle of repose of dry sand); compaction of the sandstone formation during burial under thousands of feet of younger rock diminished the slopes we see today by about 20% (Hunter, 1981). Iron-Oxide-Cemented Concretions Shortly after you reach the base of the steep descent of the trail from the parking area, you will start to see black, iron-oxide-ce- mented slabs (some displaying bedding, and some with connected corners like boxes; figures 3 and 4). Also present along the trail are to Boulder to Escalante Parking New Hom e Bench 12 Jn Jn Qa Qb Qb Dry Hollow 1 mile Quaternary alluvium Quaternary boulder deposits Jurassic Navajo Sandstone unconsolidated bedrock Qa Qb Jn Calf Creek Upper Calf Creek falls N Figure 2. Geologic map showing distribution of unconsolidated deposits and bedrock in the vicinity of Upper Calf Creek Falls. Figure 3. Slabs of rinds from broken concretions. Note that some slabs connect form- ing “boxworks” (see figure 9, part 3 for origin). Figure 4. Dense piece of concretion rind that shows bedding (black arrow) and a central fracture (white arrow; See figure 9, part 3). D.B. Loope and R.M. Kettler Giant Concretions of Upper Calf Creek Falls 5 gray, discoidal masses of bedded sandstone also cemented by iron oxide (figure 5). These are texturally distinct from the dark gray (also iron-rich), non-bedded volcanic boulders that have rolled down from New Home Bench; you should be able to see layer- ing and feel the evenly-sorted grains of relatively fine sand in the remains of the concretions (but not in the volcanic boulders). The slabs are fragments of the dense rinds and the discoids were the central core zones of concretions. Along the trail, there are several examples of concretions that are in place and sufficiently eroded to show their interior structure (figures 6 and 7). Note that some iron-rich rinds follow fractures in the concretions. Recall that all three interpretations of the iron- rich Navajo concretions (see Introduction) have them forming in bedded (explaining the texture of the rinds and discoids) rock, not loose sediment (explaining why the iron accumulations followed fractures). Figure 8a shows a large, nearly intact concretion, and figure 8b shows a microscopic view of a core stone with siderite-shaped iron-oxide crystals (pseudomorphs) that provide strong evidence for the former presence of siderite. The iron that remains in the core of large concretions provides a major clue to the origin and evolution of all rinded concretions (figure 9). Figures 10 and 11 explain the role that iron-oxidizing microbes played in initiation and thickening of the densely cemented rinds. Many of the minerals that form in the deep, oxygen-deprived subsurface are unstable near Earth’s land surface and atmosphere. Before uplift of the Colorado Plateau, the Navajo Sandstone was buried by thousands of feet of younger sedimentary rocks. John Wesley Powell’s research team first realized that many thousands of feet of sedimentary rocks had been stripped away during uplift of the Colorado Plateau (Dutton, 1880). We now know that canyon cutting and uplift in this area accelerated about six million years ago (Marchetti and others, 2012). If siderite crystals formed the concretions while the Navajo Sandstone was deeply Figure 5. Core stone representing the central portion of a broken concretion. White arrows show bedding planes. Inset shows scattered pseudomorphs (see text). Figure 6. Horizontal view of an in-place concretion that is composed of a boxwork of dense rinds; core has been completely removed by weathering. Site is along the trail, 250 feet (75 m) from Upper Calf Creek Falls. 37°51’17” N, 111°27’04 W. Figure 7. Downward view of weathered, in place concretion, showing boxwork of rinded sandstone slabs. Same site as Figure 6. 6 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 A B CS CS CS CS r r r Figure 8. A) Large, nearly intact, rinded concretion on the rim of Dry Gulch (N37°50’44.9”; W111°25’29.4). Rinds (r) plate the concretion’s perimeter, but some have fallen away, exposing cores (CS). Note that rinds are also developed along joints (arrows). Hat is 14.5 inches (37 cm) wide. B) Photo taken of a thin section through a microscope showing the fine-scale composition of the core of a large Calf Creek con- cretion. White is quartz, black is iron oxide, blue is open pore space. Arrows point to rhombic (diamond-shaped), iron-oxide in the shape of (pseudomorphs after) siderite crystal. Width of field of view is 0.015 inches (0.37 mm; see Loope and Kettler, 2015, figure 2) and Loope et al. (2011, figure 5). Figure 10. Microbes in the iron-oxide-cemented rind of a concretion. A) Arrows point to fuzz in pore space between sand grains at the inner edge of a rind. White is quartz, black is iron-oxide cement. Field of view ~500 microns. B) Scanning Electron Micros- copy (SEM) image of microbes (about 4 microns in diameter) on the surface of a sand grain; the surface of a second sand grain (in the background) is also covered by microbes. 1 2 3 up lif t Iron oxide, many joints no siderite Siderite, no joints no iron oxide dense rind ox yg en at io njoints moat core stones with rhombic pseudomorphs after siderite rhombic crystals of siderite form concretion joints cut bedded sandstone matrix and concretion bedding Figure 9. Interpretation of the evolution of a large, rinded concretion: 1) in deep, oxy- gen-free pore water, siderite crystals grow, forming a concretion in bedded, unfractured Navajo Sandstone (size of crystals greatly exaggerated). 2) with uplift and erosion of the Colorado Plateau, joints break the surrounding sandstone and the concretion; 3) in shallow, oxygenated water near the land surface, siderite starts to dissolve into the only remaining oxygen-free water (inside the concretion). Dissolved iron diffuses outward to the perimeter and to the joints. At these locations where oxygen-free water meets oxygenated water, rinds form and thicken. Moats represent space occupied by pseudo- morphs that were dissolved, providing iron for rind growth. If the concretion remains water saturated, rinds continue to thicken, the moats to widen, and the cores to shrink. When the water table dropped below large concretions, iron could no longer migrate, so siderite crystals were oxidized in place, forming pseudomorphs. In smaller concretions, all the siderite dissolved before the water table dropped below the concretion, so no pseudomorphs remain in them (they are just rind and moat; see figure 11D). Modified from Loope et al. (2016, figure 23), see also Loope and Kettler (2011, figure 3). buried, those crystals would have been oxidized about one million years ago, when they encountered oxygen-bearing groundwater (beneath the stream that carried the boulders that now cap New Home Bench; Marchetti and others, 2012). The iron oxide in the concretions along Calf Creek do not bear sufficient uranium for U-Th/He radiometric dating, but those along the canyon of Russell Gulch in western Zion National Park do have enough uranium (P.W. Reiners, personal communication, 2015). Analy- sis of samples from both the rinds and cores of Zion concretions showed that the iron oxide crystallized about half a million years ago (Loope and others, 2016). That timing fits well with can- yon-cutting rates that are based on one-million-year-old lavas on the rim of Russell Gulch, and puts siderite alteration, rind growth and, core oxidation at shallow depth. The presence of rhombic, iron-oxide pseudomorphs in concretion cores (figure 8b) is a strong indication that iron oxide was not a primary cement: rhombic crystals of siderite that formed in the relatively deep subsurface were the initial cement of these con- cretions. Siderite crystals were unstable in shallow groundwater and many (but not all) were dissolved to form the rind; those that didn’t contribute to the rind were oxidized in place, above the water table. Further, the overall distribution of iron in the large concretions is inconsistent with alteration of an iron-free (calcite) precursor (see third interpretation in the Introduction). Entry of iron from outside a pre-existing concretion cannot explain the rind-moat-core structure of the concretions. In many of the largest concretions most of the iron lies in their cores (figure 8a), and each iron-rich core is surrounded by an iron-free moat, which, in turn, is surrounded by very-strongly cemented rind (figure 9). Admittedly, siderite crystals have not been found in the concre- tions. We attribute this to their (former) location within a relatively porous and permeable matrix—this guaranteed full exposure of the crystals to oxidizing water. Like all science, our interpretation (and all three hypotheses for the Navajo concretions) could be wrong. D.B. Loope and R.M. Kettler Giant Concretions of Upper Calf Creek Falls 7 ACKNOWLEDGMENTS We thank Peter Mozley and Edward Simpson for providing help- ful reviews of this paper. REFERENCES Bates, R.L., and Jackson, J.A., 1980, Glossary of Geology, second edition: American Geological Institute, 749 p. Beitler, B., Parry, W.T., and Chan, M.A., 2005, Fingerprints of fluid flow: chemical diagenetic history of the Jurassic Navajo Sandstone, southern Utah, U.S.A: Journal of Sedimentary Research, v. 75, p. 547–561. Chan, M.A., Beitler, B., Parry, W.T., Ormo, J., and Komatsu, G., 2004, A possible terrestrial analogue for haematite concre- tions on Mars: Nature, v. 429, p. 731-734. Dutton, C.E., 1880, Report on the geology of the High Plateaus of Utah: Geographical and Geological Survey of the Rocky Mountain Region (U.S.): Government Printing Office, 307 p. Hunter, R.E., 1981, Stratification styles in eolian sandstones: some Pennsylvanian to Jurassic examples from the western interior U.S.A., in Ethridge, F.G., and Flores, R.M., editors., Recent and ancient nonmarine depositional environments: models for exploration: Society of Economic Paleontologists and Mineralogists Special Publication 31, p. 315–329. Kocurek, G., and Dott, R.H., 1983, Jurassic paleogeography and paleoclimate of the central and southern Rocky Mountains regions, in Reynolds, M.W., and Dolly, E.D., editors., Meso- zoic paleogeography of the west-central United States: Rocky Mountain Section, SEPM (Society for Sedimentary Geology), p.101–116. Loope, D.B., Kettler, R.M., and Weber, K.A., 2010, Follow the water: connecting a CO2 reservoir and bleached sandstone to iron-rich concretions in the Navajo Sandstone of south-cen- tral Utah, USA: Geology, v. 38, p. 999-1002 Loope, D.B., Kettler, R.M., and Weber, K.A., 2011, Morphologic clues to the origins of iron oxide-cemented spheroids, boxworks, and pipelike concretions, Navajo Sandstone of south-central Utah, U.S.A.: Journal of Geology, v. 119, p. 505-520.  Loope, D.B., and Kettler, R.M., 2015, The footprints of ancient CO2-driven flow systems: ferrous carbonate concretions be- low bleached sandstone: Geosphere, v. 11, p. 943-957. Loope, D., Kettler, R., Murray, K., Pederson, J., and Reiners, P., 2016, Sandstones and Utah’s canyon country: deposition, dia- genesis, exhumation, and landscape evolution, in Keller, S.M., and Morgan, M.L., editors., Unfolding the geology of the West: Geological Society of America Field Guide 44, p. 41–71. Marchetti, D.W., Hynek, S.A., and Cerling, T.E., 2012, Grav- el-capped benches above northern tributaries of the Escalante River, south-central Utah: Geosphere, v. 8, p. 835-853. Moore, J.M., 2004, Blueberry fields forever: Nature, v. 428, p. 711-712. Weber, K.A., Spanbauer, T.L., Wacey, D., Kilburn, M.R., Loope, D.B., and Kettler, R.M., 2012, Biosignatures link microorgan- isms to iron mineralization in a paleoaquifer: Geology, v. 40, no. 8, p. 747-750. Yoshida, H., Hasegawa, H., Katsuta, N., Maruyama, I., Sirono, S., Minami, M., Asahara,Y., Nishimoto, S., Yamaguchi, Y., Ichinnorov, N., and Metcalfe, R., 2018, Fe-oxide concretions formed by interacting carbonate and acidic waters on Earth and Mars: Science Advances, vol. 4, no. 12, eaau0872. O2 O2 O2 O2 O2 FeCO3 Fe(III)s Organic carbon Time O2 H2O H+ + Fe(III)s Organic carbon FeCO3 + H+ O2 H2O H+ + Fe(III)s Organic carbon FeCO3 + H+ CO2 - + Fe(II) n c n CO2 - + Fe(II) A B C D Figure 11. (A) spheroidal siderite (FeCO3) concretion that grew in oxygen-free water (and therefore contains no iron oxide) is exposed to oxygenated groundwater; B) microbial colony starts to metabolize the siderite (gaining energy and carbon); iron-oxide rind (the waste of the microbial colony) is starting to form as the siderite recedes; C) rind further thickens as siderite recedes; D) siderite entirely dissolved, rind growth is complete, and colony dies in fully oxygenated water. In this case, no core stone (or iron-oxide pseudomorphs after siderite) remains (compare to figure 9).