Plant Root Systems Preserved in the Permian Cedar Mesa Sandstone at Moki Dugway, Southeastern Utah

Rooted green plants represent the base of the food chain for most terrestrial ecosystems, but, compared to animal burrows, root systems 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 (CaCO) are commonly replaced by fine crystals of calcite (Klappa, 1980). These structures (known by geologists as rhizoliths from the Greek for “root rock”) are one form of calcite cemented soil and sediment called caliche. 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 (CO) ions necessary for making the calcite of caliche is falling dust, and another source is the dissolution of calcite grains already in the soil.

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INTRODUCTION
Rooted green plants represent the base of the food chain for most terrestrial ecosystems, but, compared to animal burrows, root systems 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 (CaCO ), 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 (CO ) ions necessary for making the calcite of caliche is falling dust, and another source is the dissolution 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  Moki Dugway Geosite 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.    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 microscope) be recognized as fragments of the skeletons of marine invertebrates-brachiopods, bryozoans, and crinoids. ese animals 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. Because 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 rhizoliths ( gure 2) indicate that many of the plants were medium to large trees. e roots of trees and the burrows of small invertebrates (probably insects) penetrated the uncemented sand, obliterating the bedding in the upper portions of each tabular accumulation. 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 Navajo 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  sandstone Paleozoic Mesozoic 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, marine 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 sandfree calcite rods representing ancient caliche show that the desert dunes were periodically attened and colonized by land plants growing in a semi-arid climate.