6-4 Freshwater Ichnofacies of Basal Pennsylvanian Channel Sandstones in Southwest Missouri

Session: Advances in ichnology: walking in Memphis and other records of behavior

Presenting Author:

Author:

Abstract:

Pennsylvanian channel-sandstone bodies are preserved in southwest Missouri. These are incised into Mississippian bedrock, oriented north-south, and preserved over lengths >200 km. Paleoflow indicators consistently record southward flow toward the late Paleozoic continental margin. Cross-bedding and anastomosing patterns indicate an initial braided system followed by a meandering phase. Body fossils are absent in these deposits, but trace fossils are present locally, and most indicate freshwater environments.

The braided system preserves local abundances of vertical cylindrical traces visible mostly on upper bedding planes and sometimes weathered into full relief.  Many of these are Skolithos, but some shafts appear to be paired, so U and Y forms may also be present. This ichnofacies is clearly Skolithos. At one site, endogenic meniscate traces are preserved. These have convex outer lobes with arcuate striae on either side of the axial meniscate tunnels; these features are defining characteristics of the Nereites ichnogenus. The meandering system, which includes lateral-accretion and crevasse-splay sandstones along with floodplain shale, has slightly more diverse assemblages, generally preserved at the base of sandstone beds in positive hyporelief. The most common ichnogenera are Cochlichnus, Gordia, and Helminthopsis, all important members of the fresh-water Mermia ichnofacies. Bi-lobate and poorly preserved meniscate traces are less common.

The meniscate traces raise questions. Are some actually Nereites? If so, does Nereites occur in fresh-water deposits, or did saline water occupy these channels intermittently? Alternatively, have younger marine deposits sometimes been mapped incorrectly as channel sandstones?

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.