MOENKOPI FORMATION EARLY-MIDDLE TRIASSIC LITHOLOGY: FOSSILS: SEDIMENTARY DEPOSITIONAL PALEOGEOGRAPHY/ MISCELLANEOUS: (Spathian to Anisian) Mudstone, sand- Amphibians, STRUCTURES: ENVIRONMENT: TECTONIC SETTING: The Moenkopi Formation 245 to 240 Million Years Ago stone, shale, dark reptiles, fresh- Mudcracks, salt River channels, delta, Westward-sloping thickens to the west, where red to brown; water clams, crystal casts, ripple tidal flat, arid coastal coastal plain of North it was partly deposited in gypsum land plants marks, cross-beds plain where evaporite America shallow marine environ- minerals (salts) are ments in southwestern deposited Utah and Nevada During the early Triassic, the supercontinent Pangaea showed signs of break- ing apart, as the North American continent began to separate from Africa and Europe and move westward. The Colorado Plateau was located just above the equator and formed part of a large, flat coastal plain near the edge of the continent. Sediments were carried to this plain by rivers that started as far away as the Appalachians, which at that time, formed the continental divide. The sea also contributed sedimentary deposits through tidal flows and by overrunning the flat-lying plain during periods of sea-level rise. The Moenkopi Formation contains many ripple marks and mudcracks, as well as tracks of reptiles and amphibians, however, few bones have been found. Both marine and terrestrial life forms were challenged during the early Triassic by the depleted biosphere that followed the Permian extinction. Outcrop of Moenkopi Formation near Holbrook, Arizona Tracks of Evidence of an Aqueous Environment Cheirotherium, or in the Moenkopi Formation “hand-beast,” have been found in North America, Casts of mudcracks Europe, and Africa, but formed by drying of no bones for this creature mud on river flood have ever been discov- plains in the ered. The name refers to Moenkopi Formation the outermost toe, which extended to the side like a Moenkopi thumb, and may have provided a firmer grip in mud. Ripple marks formed by Reptiles replaced waves on tidal flats in amphibians as the Moenkopi the dominant Formation land-dwelling animal following the Permian extinction. Reptiles produce Salt crystal casts in Generalized map of environments an egg contain- Moenkopi mudstone, across Arizona during the early ing nutrients the result of evapora- Triassic period, the time of deposi- within a protec- tion of sea water on tion of the Moenkopi Formation. tive shell, which keeps the embryo inside from drying out. tidal flats (modified from Blakey, This adaptation allowed reptiles to move away from waterside Basham, and Cook, 1993) habitats and colonize drier regions, because unlike amphib- ians, they do not need to return to water to lay their eggs. (Image courtesy of the Lunar and Planetary Institute) All images courtesy of Dale Nations, (Based on Geological Society of America timescale, 2009) unless otherwise noted. Information provided by WESTCARB at www.westcarb.org.