The Carmel Formation (Middle Jurassic) consists of carbonates, siliciclastics and evaporites deposited in a shallow, restricted, epicontinental sea. Two main depositional sequences are recognized in the San Rafael Swell (Emery County, central Utah). These are composed of subtidal carbonates and sandstones and intertidal and supratidal clays, siltstones and evaporites.

Trace fossils are common in the subtidal deposits and their stratigraphic occurrence is controlled by paleoenvironmental setting. Ichnotaxa include Arenicolites isp., Aulichnites, Chondrites isp., Gyrochorte comosa, Lockeia silliquaria, Planolites beverleyensis, Protovirgularia dicotoma, Rosselia socialis, Scalarituba missouriensis, Skolithos isp., Taenidium isp., Teichichnus rectus and Teichichnus flexuosus. The highest ichnologic diversity is recorded in thin grainstone-mudstone intercalations interpreted as the transition between shoreface and offshore, where Chondrites and Teichichnus are dominant. Coarser-grained storm beds exhibit very few trace fossils, mostly Planolites, which are very abundant on the top of some layers. Unfilled Skolithos and Arenicolites occur in oolitic limestone beds that were deposited as small subtidal shoals.

Overall, these ichnologic assemblages are characterized by: (1) dominance of shallow-tier, constructionally simple, facies-crossing ichnotaxa, (2) absence of “typical” crustacean burrows, such as Rhizocorallium or Thalassinoides, that typically dominate Jurassic shallow and marginal marine carbonates elsewhere, (3) small size of all the ichnotaxa, especially remarkable in Teichichnus, Chondrites and Lockeia, (4) low to occasionally moderate ichnodiversity, and (5) low degree of bioturbation. All these characteristics seem to indicate stressed, restricted paleoenvironments.

The abundance of shallow-tier Chondrites in the quieter settings suggests oxygen-deficient conditions in the sediment. The higher energy settings inhibited a higher degree of bioturbation. In addition the paleogeographic and paleoclimatic setting, which resulted in the deposition of evaporites in supratidal areas, could have played an important role in mantaining the salinity of the water above normal marine levels.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas