PROCESS ICHNOLOGY INDICATES DELTAIC ENVIRONMENTS FOR THE NORTHVIEW FORMATION, SOUTHWEST MISSOURI

The Early Mississippian Northview Formation in southwest Missouri is a siliciclastic sequence reaching 25m in thickness. It coarsens upward from shale to coarse siltstone and then to interbedded siltstone and thin shale. The siltstones have high concentrations of trace fossils, mostly just two ichnogenera: meniscate burrows of Nereites missouriensis and spreiten of Zoophycos. Thus, the ichnofauna is dominated by namesakes of the two deepest marine ichnofacies. Nevertheless, the Northview interbeds with, and grades into, shallow carbonate-platform sediment, so it is not a bathyal or abyssal deposit.

Recent exposures reveal a succession of trace fossils typical of deltaic sequences. Most of the lower shale has sparse populations of Zoophycos, Planolites and Phycosiphon, three of the most common elements of the Phycosiphon ichnofacies for a prodelta environment. The overlying interval is dominated by Phycosiphon at the base, grading upward to sparse Thalassinoides, Teichichnus and assorted asterosomids, and then to variable concentrations of N. missouriensis and Zoophycos at the top of the lowest siltstone. The low diversity and concentrations point to brackish water, and the asterosomids to the delta front Rosselia ichnofacies. The overlying siltstones have high concentrations of N. missouriensis with other less common meniscate traces. Zoophycos covers bedding planes near the top of each siltstone. The shaley interbeds in the upper Northview are quite variable but otherwise not well characterized. The low diversity and high concentration of meniscate burrows, including freshwater-friendly elements, point to the Scoyenia ichnofacies and an intermittently emergent environment with brackish to fresh water i.e., the lower delta plain.