Preliminary report on the Oldenburg “butter shale” in the Upper Ordovician (Katian; Richmondian) Waynesville Formation, USA

Abstract

The Cincinnatian Series (Upper Ordovician; upper Katian) of the Cincinnati Arch region, Ohio, Indiana and Kentucky) contains several bed packages informally referred to as "butter shales" or "trilobite shales". These packages are typically 1-2 m of relatively pure, homogeneous claystone with isolated, lenticular limestone beds. These claystones are most widely known for their excellent preservation of abundant trilobites, especially Isotelus and Flexicalymene, as well as diverse and commonly articulated bivalves, and nautiloids. A newly recognized butter shale interval in the Clarksville Member of the Waynesville Formation contains a typical butter-shale fossil assemblage, dominated by bivalves, orthoconic cephalopods and trilobites. To better study the fabric of this claystone, a large, epoxy-coated block of the claystone was dry-cut. Polished surfaces show a variety of otherwise cryptic features, including pervasive bioturbation and the presence of probable lingulid escape burrows (Lingulichnus), as well as abundant fodinichnia (Chondrites, Planolites, Teichichnus). Preservation of articulated trilobites and closed bivalves in approximate living position, as well as escape burrows, indicates deposition as a series of mud burial events or obrution deposits. We suggest that the butter shales resulted from net accumulation of multiple episodes of re-suspended mud deposition, which rapidly smothered organisms and resulted in exceptional preservation. Between events the seafloor was colonized by abundant deposit-feeding infaunal organisms, which destabilized the substrate and generated turbidity near the sediment-water interface, thus inhibiting sessile suspension feeders. Rapid net deposition was also interrupted by more prolonged periods (tens to hundreds of years) of low sedimentation that permitted colonization by epifaunal brachiopod-dominated communities. While most butter shale units are regionally extensive, the Oldenburg is confined to a few outcrops, apparently because it has been removed in most areas beneath a subtle but significant disconformity.

General models of ecological diversification. II. Simulations and empirical applications

“…Lithologically, these units are composed of shales interbedded with thin limestone tempestites, representing offshore, soft-substrate conditions below storm-wave base in the Cincinnati Arch epeiric sea (Holland 1993). Taphonomic conditions are excellent, with preservation by obrution deposits, and most assemblages include evidence of fossils preserved in life position, articulated specimens, and autochthonous and parautochthonous communities (Frey 1987a,b; Schumacher and Shrake 1997; Hughes and Cooper 1999; Aucoin et al 2015). The shelly and trace-fossil biotas are also extensively well studied, with much attention given to their life habits (Pojeta 1971; Richards 1972; Frey 1987a,b; 1989; Brandt et al 1995; Lescinsky 1995; Brandt 1996; Feldmann 1996; Sandy 1996; Schumacher and Shrake 1997; Leighton 1998; Gaines et al 1999; Meyer et al 2002; Morris and Felton 2003; Novack-Gottshall and Miller 2003; English and Babcock 2007; Freeman et al 2013).…”