FRESHWATER TRACE FOSSILS ASSOCIATED WITH A FLUVIALLY REWORKED HUCKLEBERRY RIDGE ASH DEPOSIT

Trace fossils associated with a deposit of Huckleberry Ridge Ash (HRA) at Borchers Badlands, southwest Kansas, provide insight into freshwater-organism response to a Yellowstone hotspot eruption (2.01 Ma). The HRA occurs in the study area as a white, vitric ash that is fluvially-reworked into a lenticular, flat-bottomed paleochannel. This paleochannel overlies a pedogenically modified clayey sandstone and grades upward into eolian, tuffaceous siltstone. A monospecific ichnoassemblage of Camborygma originates within the fluvially reworked HRA deposit and cross-cuts the underlying paleosol. Vertical to subvertical shafts range in diameter from 2.6-8.5 cm, and vary by having a helical and/or sinuous morphology to being relatively straight. Shafts typically transition downward into subhorizontal tunnels that are 4.9-13.3 cm in diameter. Shafts penetrate up to 25 cm into the underlying paleosol in places where the paleosol has a silty texture. However, where the paleosol is relatively coarser grained, shafts abruptly transition to horizontal tunnels that remain at the HRA-paleosol contact. Shaft and tunnel walls are smooth to knobby and occasionally contain linear to arcuate scratches. The burrows are filled with graded ash and clay-capped, convex-upward ash shards that together indicate passive infilling by an ash-laden, sediment-water mixture. Elliptical pellets are abundant along the base of shafts and tunnels. Thin sections of pellets indicate that they are composed of paleosol matrix, therefore are interpreted to have been actively excavated and molded by the tracemaker. These pellets likely served as either ornamentation along upper shaft walls within the poorly consolidated ash matrix and/or were components of pelleted chimney structures that were passively reworked into burrows. Because these traces are morphologically analogous to modern crayfish burrows, we interpret this as a demonstration of the resiliency of freshwater arthropods to extreme environmental perturbations.