Ichnology of a late Pleistocene ichnofabric, Nebraska, USA

The “Citellus zone” is a regionally extensive biohorizon with abundant burrows and remains of ground squirrels (Spermophilus=“Citellus”) and other rodents. One hundred and four outcrops were evaluated to determine the stratigraphic relationship of the zone and to confirm its geographic extent. “Citellus zone” bioturbation exists over the entire 1700-km2 study area in south-central Nebraska and was synchronous with mid-Wisconsinan Gilman Canyon loess deposition (Altonian and Farmdalian Substages), since burrow abundances are constant throughout its thickness. Calculations based upon maximum burrow abundance and the known digging rates of the burrowers confirm that the zone could have been produced during Gilman Canyon deposition. In the lower part of the zone, decline in burrow abundance with increasing depth in the Loveland Formation (Sangamonian and Illinoian Stages) suggests that this unit predates burrowing. Thomomys-sized geomyid trace fossils become proportionally less common with increasing depth, consistent with the transition from fodinichnia to domichnia, as Spermophilus richardsonii-sized spermophiline and prairie dog domichnia become more common.