BIVALVE BURROWS AND ASSOCIATED TRACE FOSSILS IN THE UPPER PENNSYLVANIAN (VIRGILIAN) HALGAITO FORMATION NEAR MEXICAN HAT, UTAH

Preliminary fieldwork in the Upper Pennsylvanian (Virgilian) Halgaito Formation near Mexican Hat, Utah, discovered an interesting association of trace fossils in shoaling-upward cycles of limestone, shale, and sandstone deposited in a shallow marine setting. The Halgaito Formation overlies the Upper Pennsylvanian (Missourian) Honaker Trail Formation, and is overlain by the Lower Permian Cedar Mesa Sandstone. Vertical-to-subvertical, non-branching burrows occurred in great abundance in calcareous sandstones ~50–70 cm thick. In some cases, the burrows contained bivalves in life position. The morphology of the bivalves was distinguished by an elongate shell ~2.5 tall and ~7 cm long with widely spaced ridges that parallel the commissure. Burrow densities ranged from 1 to 15 per 20 cm square, and individual burrows were circular to elliptical in cross section, ~3–4 cm and ~3–6 cm in diameter and ~40–50 cm long. These trace fossils represent beds of bivalves that likely moved upward as the sediments accumulated through time, suggesting they represent equilibrichnia. These clams are similar to the present-day burrowing razor clams that inhabit shallow marine environments. Bivalve-bearing calcareous sandstone beds are laterally continuous and can be traced over several kilometers. Sandstones also contained large-diameter burrow networks, comprised of branches in three dimensions in T- and Y-shaped intersections. Vertically oriented elements are referred to as shafts, whereas horizontal-to-subhorizontal elements are referred to tunnels. Individual burrows ranged in diameter from ~3–5 cm, and 20–50 cm long before intersecting with another shaft or tunnel. In some cases these intersections were slightly wider in diameter, ~4–6 cm, than the intersecting burrows. Burrow walls are well defined and mostly smooth. The burrow fill is similar to the surrounding matrix. These burrows are assigned to Thalassinoides and interpreted as deposit-feeding crustacean burrows.