Miocene invertebrate trace fossils from a braided river environment, western Nebraska, U.S.A

Abstract

Lower Miocene cross-stratified sands of the Gering and Monroe Creek Formations exposed on Scotts Bluff National Monument in western Nebraska, U.S.A., were deposited by migrating sand bars in a braided river system similar to the modern Platte River in eastern Nebraska and, like the Platte, contain local lenses of parallel stratified sediment that accumulated in ponded areas of abandoned channels. During times of low discharge, broad areas of river bar sands and abandoned channel sediments were subaerially exposed on the Miocene river plain. These sediments, like those exposed in the Platte River today, were subjected to burrowing by insects and other animals. Trace fossils in Lower Miocene braided river deposits are: vertical shelter burrows, horizontal deposit-feeding burrows, bioturbated layers, and vertical passageways between bioturbated layers. The burrows are cylindrical to sub-cylindrical in cross-section, internally meniscate or massive, generally non-branching, and smooth walled. Shelter burrows are similar in shape and size to recent burrows dug by beetles in river sediment for protection from day-time temperatures, to pass the night, and to hibernate. The shelter burrows, deposit-feeding burrows, and vertical passageways in the Lower Miocene sediments occur in four distinct “populations” with modal diameters of 1–2, 3–4, 7–8, and 10–12 mm. The occurrence of both vertical and horizontal burrows in all four “populations” suggests that they could have been made by the same insect. “Populations” with modal diameters of 1–2, 3–4, and 7–8 mm also occur in modern Platte River sediment and are made by tiger-beetle larvae (3–4 mm) and heterocerid (1–2 mm) beetles. Miocene shelter burrows, deposit-feeding burrows, bioturbated layers, and vertical passageways, therefore, could have been formed by different types of beetles, and/or larval instars and adults of the same beetle species.