TRACE FOSSILS IN MIDDLE PENNSYLVANIAN COASTAL-ESTUARINE FACIES, TRADEWATER FORMATION, WESTERN KENTUCKY

The Tradewater Formation is a lower Middle Pennsylvanian (Atokan) unit of the Illinois Basin. The lower part of the formation typically contains gray silty shales, fine-to coarse-grained, rhythmically laminated- to crossbedded sandstones, and discontinuous coals and limestones. Coarsening-upward sequences are common, although correlation of specific stratigraphic intervals is complicated by a lack of key beds and thickness variation above underlying Lower Pennsylvanian paleovalley fills. Trace fossils are common and are described from a series of outcrops along the Western Kentucky Parkway.

Silty, generally coarsening-upward, coastal bay or outer estuarine shales contain flaser to rhythmically laminated bedding, abundant Teichichnus and Planolites, and less common (although abundant in monotypic layers) Asterosoma, Rosselia, Conostichus, Lockeia, and unidentified surficial markings. Ripple-bedded sandstones at the tops of coarsening-upward intervals contain low-diversity, vertical to sub-horizontal (.25-.75mm) Skolithos, or U-shaped Arenicolites/ Diplocraterion. Float material contains abundant small, structureless, semi-circular markings, as well as paired holes interpreted as U-shaped burrows, although it is often difficult to see the U-shaped connection in outcrop. Grazing traces, such as Olivelittes and Scalarituba are less common and identified from float material. Numerous horizontal, tubular to ovate traces with constrictions similar to, but less well-defined than Scalarituba are also common. A scour-based, inner estuarine channel facies with inclined heterolithic bedding and laminar siderites is mostly devoid of trace fossils.

Nodular sideritic “ironstone” layers occur at the base of several coarsening-upwards intervals. One bed contained Zoophycos burrows and likely represents a ravinement or transgressive flooding surface that can be used for correlation. In another, however, tubular structures are slightly carbonaceous and occur at the stratigraphic horizon of a lateral coalbed, so represents a rooted paleosol. In other ironstones, abundant horizontal tubes lacking internal structure are common and it is uncertain if these structures represent burrows or roots, which illustrates the complexity of using these beds for correlation.