WETLAND TRACE FOSSILS IN THE PALEOGENE BRIAN HEAD FORMATION, SOUTHWEST UTAH

The largely unstudied Brian Head Formation records the first post-Laramide volcanism in the high plateau region of southwestern Utah. The Brian Head erosionally overlies the fluvio-lacustrine Claron Formation and is overlain by younger volcanic units. Preliminary study of the Brian Head in the Casto Canyon area indicates deposition mostly in fluvial environments. Study of a thin limestone bed containing abundant root traces, crayfish burrows, and a lack of desiccation features, suggests a localized, stable wetland environment.

In the study area the Brian Head is about 400 m thick; it consists of a basal conglomerate (~30 m); a white, grey, green and red variegated sandstone and mudstone unit (~120 m); and a white volcaniclastic sandstone and bentonitic mudstone unit (~250 m). Throughout the variegated unit sandstone beds contain trace fossils, the red units are locally intensely bioturbated. In the volcaniclastic unit bioturbation is less common; however the limestone bed, located a few meters above the variegated unit, is heavily bioturbated.

The limestone bed is about 20 cm thick and contains four distinct trace fossils. Root traces are pervasive, 1-2 mm dia., and filled with spar calcite. The three other traces are burrows that typically extend through the entire bed. Each of the burrow types are unlined, straight or bent at acute angles, subvertical, with limited subhorizontal orientation and occur individually or in complexes. Burrows may contain fill: types 1 and 2 passive pelletal muds, type 3 active and passive fill.

Type 1 burrows, previously undescribed, are approximately 1-2 cm dia. and 7 to >20 cm in length. Burrow walls contain 1-2 mm deep and wide annular grooves with distinct sub-mm cusp-shaped scratches. One specimen has a blunt termination, one a flared end, and one with a Y-branch. Type 1 burrows are similar to ichnogenera Lunulichnus and Loloichnus, but unique with positive relief in the wall instead of fill, and are tentatively interpreted as crayfish burrows. Type 2 burrows are 3-4 cm dia., >10 cm in length with 2-3 mm convolute hummocky knobs oriented parallel or transverse to burrow axis, and are similar to Camborygma, another ichnogenus of crayfish burrows. Type 3 burrows are 1-3 mm dia., 5-8 cm long, with straight and convoluted geometries, generally grouped, and are interpreted as insect or larval traces.