Date of this Version
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., editors, Utah Geosites: Utah Geological Association Publication 48, 6 p.,
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 (CaCO3), 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 (Figure 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 (CO2) 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 flushed 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 (Figure 1), rhizoliths in ancient rocks are good indicators of semi-arid paleoclimates. The Early Permian (245-286 million year old) root systems preserved the Cedar Mesa Sandstone at Moki Dugway (Figure 2) grew on low-relief land surfaces that formed when dune fields were flattened 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.