Ichnofacies of an ancient erg: A climatically influenced trace fossil association in the Jurassic Navajo Sandstone, Southern Utah, USA

Arid eolian environments usually exhibit a paucity of organism traces, but some eolianite facies in the geologic record contain a great abundance of trace fossils, which characterize a distinctive ichnofacies, herein termed the Entradichnus Ichnofacies. This arid landscape ichnofacies is exemplified by a locally dense and diverse invertebrate trace fossil assemblage, which is preserved in the Navajo Sandstone, a Jurassic eolianite exposed in the Paria Canyons Primitive Area in southern Utah. The trace fossils (Planolites, Palaeophycus, Skolithos, Arenicolites, Entradichnus, Taenidium and Digitichnus) all appear to be the products of shallow burrowing by desert-dwelling arthropods, such as beetles and other insects, that kept pace with the dune migration. The paleoclimate was monsoonal, characterized by rainy summers and windy (but relatively dry) winters. The burrowed beds were produced during long-lived pluvial intervals that brought higher than usual amounts of moisture to the Navajo dune fields. Most of the sand in the Navajo at the study site was deposited as dry grain flows during the winter months, and the only possibility of rainfall or dew precipitation came during the summer months. Nevertheless, the burrowers apparently were active year-round and exploited resources within both dry and damp sand.