Foraminiferal evidence for the age of the Mississippian Pella Formation (Southeastern Iowa, USA)

摘要

The Pella Formation of southeastern Iowa has been correlated variously with the St. Louis Limestone and the Ste. Genevieve Limestone of the upper Meramecian and lower Chesterian stages, respectively, of the type Mississippian succession. The age of the Pella Formation is of interest because in Keokuk County the Pella immediately overlies a fossil tetrapod deposit that is the oldest of its kind in North America. It sits above Romer's Gap in the fossil record of early tetrapod evolution. Occurrences of the foraminifer Asteroarchaediscus rugosus indicate that the Pella Formation can be no older than the upper part of the Ste. Genevieve Limestone. Foraminifers are not useful in constraining the upper age limit of the Pella, but previously reported conodonts assignable to the Gnathodus bifineatus-Cavusgnathus charactus Assemblage Zone (=Lower bilineatus Zone; Faunal Unit 9) suggest that the unit can be no younger than mid-Renault Limestone of the Chesterian reference area. The foraminifer Holkeria avonensis occurs abundantly in the Pella Formation. This is the youngest occurrence of the genus and species anywhere in the world, as Holkeria spp. previously were known only from the Holkerian-lower Asbian stages (lower Upper Visean) in Britain, and from the lower St. Louis Limestone (=Salem Limestone equivalent) in the upper Meramecian of midcontinental North America.