RELATIONSHIPS BETWEEN BOUNDING SURFACES, ICHNOCOENOSES, AND ICHNOFACIES IN ERG SYSTEM STRATA: EXAMPLES FROM THE JURASSIC NAVAJO SANDSTONE

Ichnofacies must be constructed within the context of the environments in which they were produced. In erg deposits––which include eolian dunes and associated non-eolian facies––the context must include recognition of: (1) bounding surfaces, associated stratal architecture, and pedogenic processes; (2) patterns and recurrence of sedimentary facies produced by eolian and/or non-eolian depositional systems; and (3) well-defined hydrogeological and hydroclimatic parameters. Our proposed new hierarchical classification of eolian and non-eolian deposits––developed within the Navajo Sandstone of southeastern Utah––provides a framework to capture the spatial and temporal variability of eolian and non-eolian depositional systems within the larger erg system while capturing the most accurate association between bioturbation, pedogenesis, bounding surfaces, and sedimentary units of the landscape. More objective, accurate, and informative ichnocoenoses and ichnofacies can, therefore, be constructed that reflect the autogenic and allogenic processes that produced the eolian and non-eolian surfaces and sedimentary facies in which they are preserved. Previously defined ichnofacies have purported to represent arid animal communities controlled by the presence or absence of surface water and climate. However, these ichnofacies lack the environmental and ichnological context that our new classification can provide. Thus, previous defined ichnofacies are merely lists of ichnotaxa found in “eolian deposits”.

Trace fossils of plants and animals and the communities (i.e., ichnocoenoses) they form are intimately associated with sedimentary deposits and bounding surfaces on and in which they occur in conterminous, contemporaneous, and interstratified eolian and non-eolian deposits. Traces associated with lamina-deviation, bed-deviation, bedset-deviation, and simple surfaces reflect bounding surfaces in eolian cross-strata where eolian processes dominate, which are mostly autogenic in nature. Traces associated with composite, complex, and amalgamated surfaces that bound conterminous and contemporaneous eolian and non-eolian strata reflect processes related to one or more changes in the groundwater profile, sedimentation, tectonics, and climate, which are mostly allogenic in nature. The trace fossils, degree of bioturbation, and their association with pedogenesis are ultimately controlled by water: the local, regional, and basinal groundwater hydrology and hydroclimate.