OLIGOCENE ICHNOFOSSILS IN NON-MARINE LIMESTONE OF THE BRIAN HEAD FORMATION, UTAH

The Late Eocene to Oligocene Brian Head Formation records the onset of post-Laramide volcanism in southern Utah. Deposited in fluvial environments, it erosionally overlies the lacustrine Claron Formation and is overlain by Tertiary volcanics. In the Sevier Plateau (Casto Canyon area), it consists largely of sandstone and bentonitic mudstone with minor conglomerate and limestone, and is informally divided into a basal conglomerate, variegated sandstone and upper volcaniclastic sandstone units. A thin marly limestone near the base of the volcaniclastic unit is extensively bioturbated. The purpose of this paper is to describe the unique, previously undescribed trace fossil assemblage found in the limestone bed, deposited in a wetland environment within a volcaniclastic fluvial system.

The limestone bed is 5-25 cm thick and extends laterally for more than 100 m. The most intense bioturbation is concentrated within a few square meters, where the bed is thickest. Although many of the traces extend through the full bed thickness, continuation of the traces could not be found in the overlying and underlying fine-grained units. Four distinct ichnofossil types are recognized.

Type 1 burrows are a probable new ichnogenus that shares characteristics of the ichnogenera Lunulichnus and Loloichnus. They are 1-2 cm diameter, nearly vertical tubes, up to 20 cm in length, and lack any lining, branching or chambers. Typically, the upper 5-10 cm of the burrows are bent 40° relative to bedding. The burrow wall contains mm-wide and -deep annular grooves that may exhibit either a cross-hatch or chevron pattern of scratches. The burrows commonly contain pelletal fill. Burrows are often clustered with others of the same type.

Type 2 burrows are identified as Camborygma. The burrows are 3-4 cm in diameter, and up to 20 cm in length. Grooves in the wall form convolute hummocky knobs that run either parallel or transverse to the long axis of the burrow. Type 3 burrows are interpreted to be either larval or insect traces. They are 1-3 mm in diameter and 5-10 cm long with pelletal fill, convolute geometries, and commonly found in clusters. Type 4 traces are rhizoliths, 1-2 mm in diameter with spar calcite fill. They are generally linear, sub-vertical or sub-horizontal, and often branched.