Enigmatic Trace Fossils and Depositional Setting of the Clifty Formation (Devonian), Northwest Arkansas

The Clifty formation (Devonian) is a very mature, medium- to fine-grained quartz arenite with sporadic outcrops across northwest Arkansas. Contrary to early research on the Clifty formation, reconnaissance mapping across northwest Arkansas indicated that the Clifty formation displays a variety of depositional geometries and trace fossil assemblages indicative of a range of depositional environments. Depositional geometries and trace fossil associations include 1) massive sandstone deposits with abundant horizontal burrows or trails, 2) festoon cross-bedded sandstone monoliths without trace fossils, 3) trough cross-bedded, thoroughly bioturbated small channels, 4) cross-bedded deeply incised channels with a variety of isolated, enigmatic burrows, 5) planar tabular cross-bedded layers with enigmatic circular structures up to 1-m diameter, 6) thin planar-tabular layers with Skolithos, and 7) tabular layers containing carbonized plant remains (probably cycads) and Zoophycos. Interpreted depositional environments of these facies are (respectively) 1) shallow, offshore shelf, 2) eolian dunes developed as part of a barrier island complex, 3) interdune tidally influenced channels, 4) tidal passes (inlets) separating barrier islands, 5) shallow tidally influenced (but subtidal), near-shore shelf, 6) shoreface and beach, and 7) back-barrier lagoon. Spatial associations of facies are also indicative of coastal lagoon-barrier island-offshore shallow shelf settings. Analog environments are observed today along portions of the western shore of Baja California.