PALEOECOLOGY OF THE LOWER JURASSIC NAVAJO SANDSTONE (COLORADO PLATEAU, USA)

Abstract Continental biofacies are useful indicators of the environment, groundwater profile, and hydroclimate, as well as paleobiodiversity, organismal interrelationships, and paleoecology in the Navajo Sandstone (Ss) (Lower Jurassic, Utah, USA). Six biofacies are identified based on their associations of lithofacies and trace fossils and body fossils of microbes, plants, and animals. Biofacies I consists of limited traces within active eolian sedimentary environments. Biofacies II and III consist mostly of traces within two different types of interdune environments characterized by differing sediment stability, soil formation, and water availability. Biofacies IV occurs in environments with plentiful water, slow sedimentation, and moderate to strong pedogenesis. Biofacies V occurs at the tops of carbonate-precipitating and clastic drying ponds. Biofacies VI is aquatic, occurring in carbonate-precipitating ponds and springs and clastic ponds. The terrestrial and aquatic biofacies food webs were supported by living (green food web) and dead (brown food web) plant material, which interconnect the above- and belowground food webs and their consumers through their interactions and life histories of animals at all levels. The biofacies and their sedimentary environments indicate that the primary control on the biota was hydroclimate in the form of effective precipitation, which dictates: (1) the stability of the landscape; (2) the type of depositional environment present; (3) the amount of effective precipitation received; (4) the degree of postdepositional modification of sediments; and (5) the composition and complexity of the trophic dynamics. Increasingly greater amounts of effective precipitation resulted in greater landscape stability via increasing amounts of vegetative cover, above- and below-ground biodiversity, and soil formation.