Ichnofossils of a Wet Desert Depositional System, Lower Jurassic Navajo Sandstone, Southeastern Utah

Abstract

Ichnofossils in the Lower Jurassic Navajo Sandstone (Ss) represent the activity of microbes, plants, invertebrates, and vertebrates in an eolian depositional system that record the environmental, ecological, pedogenic (soil formation), and hydroclimatic settings in southeastern Utah near the Moab, Utah. Spatial and temporal parameters of ichnopedological facies in dune, dry interdune, and wet interdune environments are used to help identify supersurfaces, 1st, 2nd, and 3rd order bounding surfaces in the Navajo Ss, which also incorporates fluvial and lacustrine deposits produced by elevated groundwater levels, fluvial input, and springs. Ichnofossil assemblages record short- to long-term hiatuses in deposition and record weak to strong soil formation associated with particular lithofacies and bounding surfaces. In general, the longer term and more extensive the bounding surface, the greater the degree and depth of bioturbation and soil formation record in these continental deposits. Microbial activity of prokaryotes, eukaryotes, and protozoans in lacustrine environments formed biolaminates and stromatolites in predominantly limestone. Karst-forming processes also modified many of these biogenically mediated limestones shortly after burial. Rhizoliths, rhizocretions, rhizohaloes, and rhizolmats of herbaceous plants, shrubs, and trees were mostly formed in wet interdune and water-margin environments, followed by dry interdune and to-of-slope dune environments. Rhizolith-lithofacies associations form at least four rhizofacies that reflect landscape stability in terms of sedimentation rate, depositional energy, groundwater profile, and speed of dune movement. Ichnofossils interpreted as invertebrate activity include: Ancorichnus, Arenicolites, Beaconites, Cylindrichum, Entradaichnus, Fuersichnus, Lockeia, Macanopsis, Naktodemasis, Palaeophycus, Planolites, Scolicia, Scoyenia, Skolithos, Steinichnus, Termitichnus, and Treptichnus. Large-diameter (4 to >50 cm) burrows likely represent the activity of mammals, therapsids, and reptiles in mostly wet interdune, followed by dry interdune settings. Ichnofossils on bounding surfaces and super-surfaces range from low (< 2 ichnogenera) to high diversity (> 10 ichnogenera), depending on the time in between depositional events. Ichnodiversity of the Navajo Ss in this area was associated with periods of greater rainfall that supported the primary productivity and detritivore-dominated nutrient cycling system.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016