UGA-Geosite-Loope-Moki.indd Utah Geosites 2019 Utah Geological Association Publication 48 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors David B. Loope Earth & Atmospheric Sciences, University of Nebraska Lincoln, NE 68588-0340 dloope1@unl.edu Cover Image: Permian rhizoliths in the Cedar Mesa Sandstone at Moki Dugway. Plant Root Systems Preserved in the Permian Cedar Mesa Sandstone at Moki Dugway, Southeastern Utah 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. � e geosites re� ect the interests of the many volunteers who wrote to share some of their favorite geologic sites. � e list is eclectic and far from complete, and we hope that additional geosites will be added in the coming years. � 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. � is is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and 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., 2019, Plant root systems preserved in the Permian Cedar Mesa Sandstone at Moki Dugway, southeastern Utah, in Milligan, M., Biek, R.F., Inkenbrandt, P., and Nielsen, P., ed- itors, Utah Geosites: Utah Geological Association Publication 48, 6 p., https://doi.org/10.31711/geosites.v1i1.60. Presidents Message I have had the pleasure of working with many di� 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 di� erent geological provinces. We have the Basin and Range to the west and the Central Utah Hingeline and � rust Belt down the middle. � 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 � ows, and ancient laccoliths, stratovolcanoes, and plutonic rocks. � 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. � 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. � 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. � 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, � ank you to the American Association of Petroleum Geologists, Rocky Mountain Section Foundation for their 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 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 Plant Root Systems at Moki Dugway 3 INTRODUCTION Rooted green plants represent the base of the food chain for most terrestrial ecosystems, but, compared to animal burrows, root sys- tems are relatively rarely recognized in ancient sedimentary rocks. Plant roots that penetrate unconsolidated sand dunes, especially those containing not only quartz grains, but also abundant grains of calcite (CaCO3), are commonly replaced by ne crystals of calcite (Klappa, 1980). � ese structures (known by geologists as rhizoliths from the Greek for “root rock”) are one form of calcite cemented soil and sediment called caliche ( gure 1). Caliche crystallizes well above the water table and its calcite crystals are tiny because of rapid evaporation of soil water. One source of the calcium (Ca) and carbonate (CO3) ions necessary for making the calcite of caliche is falling dust, and another source is the dissolu- tion of calcite grains already in the soil. Caliche is widespread in semi-arid regions. In regions with abundant rainfall, available calcium and carbonate ions are rapidly � ushed downward, out of the soil, preventing calcite crystals from growing in the root zone. In arid regions there is too little available soil water for crystal growth. Because plant roots in modern semi-arid settings are commonly preserved by caliche ( gure 1), rhizoliths in ancient rocks are good indicators of semi-arid paleoclimates. � e Early Permian (245-286 million year old) root systems preserved the Cedar Mesa Sandstone at Moki Dugway ( gure 2) grew on low-relief land surfaces that formed when dune elds were � attened by wind erosion. A near-surface water table may have prevented further erosion of the Permian dune sand and allowed the land surface to be colonized by woody plants. DRIVING AND WALKING DIRECTIONS Perhaps the best place in the U.S. to see ancient rhizoliths is in a Utah 261 roadcut along the upper portion of Moki Dugway and on the outcrop just above the highest switchback ( gure 3). If you are approaching this geosite from the north, stop rst at the viewpoint on the south edge of Cedar Mesa, at the top of Moki Dugway ( gure 3). � is viewpoint provides a dramatic view of the Cedar Mesa Sandstone, and in the distance, the Goosenecks of the San Juan River and Monument Valley ( gure 4). � e Moki Dugway is a steep (11% grade), well-maintained, dirt road with Figure 1. Rhizolith collected from loose, bioturbated dune sand north of Moab, UT. Arrow points to the boundary between lightly cemented quartz sand and sand-free caliche composed tiny calcite crystals. Void spaces were made by small- er roots that penetrated the so� , chalky caliche that � lls the cylindrical space produced by growth of a large root. Figure 2. Permian rhizoliths in the Cedar Mesa Sandstone at Moki Dugway. � ese rhizoliths have a darker color than those found in recently deposited, uncemented sand (� gure 1) because all the pore space originally in them has been � lled by calcite cement. � e lighter patches of sandstone are reduction halos—iron oxide was removed from surrounding quartz grains by organic acid from the root. to Utah 95 and Natural Bridges to Utah 163 and Mexican Hat to M ul ey P oi nt Park on turn 37°16'30.72" N; 109°56'11.61"W 1.0 km N to V al le y of th e G od s Uta h 2 61 Utah Moki Dugway GeoSight Viewpoint Figure 3. Topographic map showing the roads that provide access to this geosite and the recommended stopping places. Moki Dugway Geosite 4 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 a series of switchbacks. Unless it is wet, neither high-clearance nor 4WD are needed. � e State of Utah recommends it only for vehicles less than 28 feet long and less than 10,000 pounds. For viewing the rhizoliths up close, the best place to park is at the uppermost sharp switchback on the highly sinuous road ( gure 3). It is possible to climb up the outcrop at the outer bend of this switchback; you can also see rhizoliths exposed along both sides of the roadcut as you walk back up the road a few hundred meters. If you approach this geosite from the south, use gures 3 and 5 to help you recognize the correct switchback for parking. GPS Location: N37.2752; W109.9365 (WGS84) THE CEDAR MESA SANDSTONE � e Permian Cedar Mesa Sandstone is one of more than a dozen thick sedimentary rock formations on the Colorado Plateau that were deposited by large, wind-blown sand dunes ( gure 5). All of these sandstones have large-scale crossbedding that formed as the dunes migrated downwind and accumulated vertically in subsid- ing sedimentary basins. Because Mesozoic rock formations have been eroded from the broad crest of the Monument Upli� ( gure 6) in southeastern Utah (named for Monument Valley) older rocks including the Cedar Mesa Sandstone crop out widely there, forming Cedar Mesa (type locality of the formation), as well as � e Needles and � e Maze (Canyonlands National Park), and the natural bridges at Natural Bridges National Monument. Halgaito Shale Cedar Mesa Sandstone Muley Point Monument Valley Figure 4. View southward from the parking area at the top of Moki Dugway. Park here * siltstone paleosol roadcut climb here Figure 5. Cedar Mesa Sandstone outcrops along upper part of Moki Dugway where the rhizoliths in one of the Permian paleosols are best exposed. Navajo Ss Lukachukai Mbr Cedar Mesa Ss Coconino Ss Moenkopi Fm Chinle Fm Kaibab Ls Organ Rock Fm Halgaito Fm Wingate Ss Moenave Fm Kayenta Fm Wingate Ss, Rock Point Mbr White Rim Ss DeChelly Ss Guadalupean Leonardian Wolfcampian lower mid. upper Triassic Jurassic Hettangian Sinemurian Pliensbachian Toarcian Permian 296 my 187 my Figure 6. Eolian sandstones of the Colorado Plateau (orange). Vertical scale is geologic time. � e black bar shows the strata that crop out at Moki Dugway. Modi� ed from Blakey and others (1988). Most of the sand grains that accumulated to form the Cedar Mesa Sandstone are abraded crystals of quartz and feldspar (ultimately derived from granite or gneiss). But calcite (calcium carbonate) grains are also present, and some of these can (using a micro- scope) be recognized as fragments of the skeletons of marine in- vertebrates—brachiopods, bryozoans, and crinoids. � ese animals D.B. Loope Plant Root Systems at Moki Dugway 5 sand to the dune eld diminished), the dunes were eroded and silt (probably delivered as dust fall and from small streams) started to accumulate on � at surfaces. Abundant rhizoliths provide evidence that these � at surfaces were colonized by terrestrial plants. Be- cause vegetation slows the wind, the plants likely caused the falling silt to accumulate instead of continuing its downwind migration. � e rock layers with rhizoliths are thus paleosols (ancient soils) preserved in the stratigraphic record. � e diameters of the rhizo- liths ( gure 2) indicate that many of the plants were medium to large trees. � e roots of trees and the burrows of small inverte- brates (probably insects) penetrated the uncemented sand, oblit- erating the bedding in the upper portions of each tabular accumu- lation. Beyond the reach of the roots and burrows, undisturbed crossbedding was preserved in the lower portions of each tabular sand accumulation. Crustaceans such as shrimp and cray sh build complex, branching burrow systems, but, unlike root systems, the burrows are lled by loose sand (never by sand-free, ne-grained calcite). � e rod-like form and coloration of the Cedar Mesa structures are produced by: 1) elongation and radial expansion of a cylindrical plant root that pushed sand grains aside; 2) a living root and its rootlets that lived in a shallow seaway that lay to the north and west, and were incorporated into a broad coastal dune eld by onshore winds. A� er the dunes were eroded � at (see below), the presence of these calcite grains in the dune sand became important to the excellent preservation of root systems in the Cedar Mesa Sandstone. STACKING OF SEDIMENT SLABS AND PRESERVATION OF ROOT SYSTEMS Although some of the eolian sandstones of the Colorado Plateau form massive, uninterrupted cli� s (for example, the Jurassic Nava- jo and Wingate Sandstones, gures 6 and 7), the cli� s exposing the Cedar Mesa Sandstone are discontinuous, giving the Cedar Mesa a tabular, “layer-cake” appearance caused by alternation of thin, red siltstones with up to 40 distinct 6 to 65 feet thick (2 to 20 meters) slabs of sandstone ( gures 4, 5, 8; Loope, 1985; Mountney, 2006). During the Permian Period, massive glaciers were dynamically forming and melting on the supercontinent Gondwana. Clearly, global climate was � uctuating widely. When sand was abundant and winds were relatively gentle, sand built up as dunes climbed over one another. Onshore winds led to the incorporation of calcite grains (fragments of marine fossils) into the desert dunes (Loope, 1984). But as the wind strengthened (or the supply of # # Tr1 J1 Qao Qa J1 J2 Tr2 K1 Qe J2 Qe Qe Qe Qa Qe K1 K1 Jg Jg Jg Qe Qe K1 K1 J1 Qao Blanding Montezuma Creek Pcm PP PI I JTr Qa Qe Cedar Mesa Monument Valley Co m b Ri dg e Goosenecks N 15 mi 24 km Trc Trc Trm Pcm UT-95 UT-163 UT-261 Pennsylvanian Hermosa Fm Qa Quaternary alluvium Jm Jurassic Morrison J1 Jurassic Carmel-Entrada- Summerville Fm Trc Triassic Chinle Gp Trm Triassic Moenkopi Fm Pcm Permian Cedar Mesa SS- P P Permian-Pennsylvanian Halgaito Fm Jg Jurassic Glen Canyon Gp. I PI Qe Quaternary eolian sand Organ Rock Shale UT-AZ border Moki Dugway Navajo Sandstone Kayenta Fm. Wingate Sandstone Chinle Fm. Moenkopi Fm. DeChelly SS Cedar Mesa Sandstone Halgaito Formation Hermosa Fm Paradox Fm. salt and gypsum limestone siltstone and sandstone eolian sandstone eolian sandstone eolian sandstone eolian sandstone mudstone and sandstone siltstone siltstone and sandstone Ju ra ss ic Tr ia ss ic Pe rm ia n Pe nn sy l- va ni an Organ Rock Fm. sandstone Pa le oz oi c M es oz oi c Figure 7. Geologic map of the area surrounding Moki Dugway (le� ). � e older rocks (blue and pur- ple) are exposed because of erosion of younger rocks (tan and green) from the Monument Upli� . � e column on the right shows strata exposed in the area. � e black bar shows the strata that crop out at Moki Dugway. Map based on Interactive Geologic Map of Utah (Utah Geological Survey); column based on Hintze and Kowallis (2009). # # Tr1 J1 Qao Qa J1 J2 Tr2 K1 Qe J2 Qe Qe Qe Qa Qe K1 K1 Jg Jg Jg Qe Qe K1 K1 J1 Qao Blanding Montezuma Creek Pcm PP PI I JTr Qa Qe Cedar Mesa Monument Valley Co m b Ri dg e Goosenecks N 15 mi 24 km Trc Trc Trm Pcm UT-95 UT-163 UT-261 Pennsylvanian Hermosa Fm Qa Quaternary alluvium Jm Jurassic Morrison J1 Jurassic Carmel-Entrada- Summerville Fm Trc Triassic Chinle Gp Trm Triassic Moenkopi Fm Pcm Permian Cedar Mesa SS- P P Permian-Pennsylvanian Halgaito Fm Jg Jurassic Glen Canyon Gp. I PI Qe Quaternary eolian sand Organ Rock Shale UT-AZ border Moki Dugway Navajo Sandstone Kayenta Fm. Wingate Sandstone Chinle Fm. Moenkopi Fm. DeChelly SS Cedar Mesa Sandstone Halgaito Formation Hermosa Fm Paradox Fm. salt and gypsum limestone siltstone and sandstone eolian sandstone eolian sandstone eolian sandstone eolian sandstone mudstone and sandstone siltstone siltstone and sandstone Ju ra ss ic Tr ia ss ic Pe rm ia n Pe nn sy l- va ni an Organ Rock Fm. sandstone Pa le oz oi c M es oz oi c 6 M. Milligan, R.F. Biek, P. Inkenbrandt, and P. Nielsen, editors 2019 Utah Geological Association Publication 48 produced acid that dissolved iron oxide from the surrounding sediment, producing a “reduction halo” ( gures 2 and 8). Much of the dead organic tissue is eventually replaced by ne-grained calcite ( gures 1 and 2; Loope, 1988). A sandstone like the Cedar Mesa, with abundant sand-size, ma- rine fossils could have been interpreted as forming below sea level. And the obliteration of bedding (called “bioturbation” by many geologists) could have been interpreted as evidence for burrowing by marine invertebrates such as crustaceans. But the distinctive, large-scale crossbedding shows that onshore winds blew the marine fossils (along with abundant quartz and feldsper) onto an emergent land surface covered by sand dunes, and the sand- free calcite rods representing ancient caliche show that the desert dunes were periodically � attened and colonized by land plants growing in a semi-arid climate. ACKNOWLEDGMENTS Nigel Mountney, Jon Mason, and Jodi Norris provided very help- ful reviews of this paper. REFERENCES Blakey, R.C., Peterson, F., and Kocurek, G., 1988, Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interi- or of the United States: Sedimentary Geology, v. 56, p. 3-125. Hintze, L.F., and Kowallis, B.J., 2009, Geologic History of Utah: Brigham Young University Geology Studies, Special Publica- tion 9, 225 p. Klappa, C.F., 1980, Rhizoliths in terrestrial carbonates: classi ca- tion, recognition, genesis and signi cance: Sedimentology, v.27, p. 613-629. Loope, D.B, 1984, Eolian origin of upper Paleozoic sandstones, southeastern Utah: Journal of Sedimentary Research, v. 54, p. 563-580. Loope, D.B., 1985, Episodic deposition and preservation of eolian sands: A late Paleozoic example from southeastern Utah: Geology, v. 13, p. 73-76. Loope, D.B., 1988, Rhizoliths in ancient eolianites: Sedimentary Geology, v. 56, p. 301-314. Mountney, N.P., 2006, Periodic accumulation and destruction of aeolian erg sequences in the Permian Cedar Mesa Sandstone, White Canyon, southern Utah: Sedimentology, v. 53, p. 789-823. calcitic rhizoliths with reduction halos in Permian paleosols siltstone 5 m crossbedded eolian sandstone 16.4 ft Figure 8. Relationships of thin siltstones and rhizolith-bearing paleosols to tabu- lar sheets of crossbedded sandstone in the Permian Cedar Mesa Sandstone.