Taphonomic and paleoecologic aspects of the Early Devonian (Pragian) Tarrantine Formation near Jackman, Maine

The Early Devonian (Pragian) Tarrantine Formation crops out in northwestern Maine within The Moose River Synclinorium. The present study focuses on a series of pavement outcrops exposed along a logging road south of Jackman, Maine (N 45º 33' 33.4” W 70º 05' 29.8”), and is the first paleontological study of the formation since the 1960s. Lithologies were characterized sedimentologically, taphonomically, and paleontologically, and collections from both siltstone and sandstone units were identified systematically. These data are used for paleoenvironment reconstruction prior to collision of the Avalonia.

The study area is characterized by interbedded sandstone, siltstone, and mustone facies, with fine-grained clastics predominating this part of the section. Fossil assemblages in fine clastic facies are autochthonous or parautochthonous. Isolated trilobites (Symphoroides) and bivalves (cf. Grammysia) are preserved within a Cruziana ichnofacies assemblage, wherein Chondrites is common and allochthonous terrestrial plant debris is rare. Allochthonous faunal assemblages characterize all sandstone facies, representing both event and lag concentration deposits. Atrypid (Leptocoelia), strophomenid (Chronostrophiella), rhynconellid (Beachia, Rensselaeria), and spirifid (Acrospirifer, Costispirifer) brachiopods, along with archaeogastropods and tentaculitids, are preserved as casts and molds in size-sorted, fining upwards shell beds generally <0.1 m in thickness.

The outcrops preserve both in situ and transported faunal assemblages deposited in the sublittoral zone of a muddy marine shelf above storm-wave base. Coarser sandstone beds represent event concentrations of more proximal brachiopod communities. These were eroded, size-sorted, and deposited within storm-generated beds in a distal offshore setting. The occasional terrestrial plant fragments indicate proximity to a coastal environment. The influx of plant matter may have been related to high freshwater discharge events associated with storm activity that may have been responsible for shell concentrations.