Ichnology of a restricted epicontinental sea, Arapien Shale, Middle Jurassic, Utah, USA

Three trace fossil assemblages associated with different lithofacies have been recognized in the Arapien Shale (Middle Jurassic) near Nephi, UT, USA. The Planolites montanus assemblage occurs in thinly laminated micrites, and it is characterized by low ichnotaxonomic diversity, shallow burrow penetration depth, small size of the traces, and low intensity of bioturbation. The Lockeia siliquaria assemblage is restricted to few-millimeters-thick grainstone layers, and it exhibits low ichnodiversity, small size and shallow burrow penetration of the traces as well. The Gyrochorte comosa assemblage is associated with 1-5-cm-thick grainstone storm beds, and it shows higher ichnodiversity and higher intensity of bioturbation. The trace fossil assemblages suggest unfavorable environmental conditions for benthic life during fair-weather periods, which partly improved after storm disturbances. The possible stress factors are related to the paleogeographic restriction and arid paleoclimate of the epicontinental sea where the Arapien Shale was deposited. In this particular setting, salinities would have reached values above normal marine producing thermohaline stratification of the body of water and, consequently, low oxygen levels in the bottom. Hence, hypersalinity and poor bottom oxygenation are proposed here as the main controls on the trace fossil assemblages of the Arapien Shale.