COMPARISON OF TWO UPPER DEVONIAN ARCHAEOPTERIS FORESTS IN PENNSYLVANIA: PALEOECOLOGICAL AND TECTONIC IMPLICATIONS

Exposures of the Catskill Formation (Upper Devonian) located in Selinsgrove (central) and Tunkhannock (northeastern) PA contain Archaeopteris tree stump casts that appear to have been deformed by compressive stresses related to tectonic activity. Cross sections of the tree stumps at both localities are elliptical, with long axes oriented in an east-west direction. The density of tree stump casts at these two localities provides an opportunity to study the paleoecology of early forests in coastal to near shore settings.

The elliptical cross sections of in situ tree stump casts, located north of Selinsgrove, are up to 43 cm wide, and on average, the long axes of the cross-sections are oriented N84E. They occur on a bedding plane that also contains Archaeopteris branches, small asymmetrical ripples, large mudcracks with smaller superimposed mudcracks, and scattered dark reddish-gray clay-lined plant material. Some of the branches appear to be oriented toward the northwest (N52W), while other plant material (root trace molds?), and some small clams (Glossites?) are scattered along the bedding plane. In Tunkhannock, a greater abundance of smaller in situ tree stump casts, up to 16 cm wide (long axis), exhibit an average orientation of N88E. These tree casts, also identified as Archaeopteris, are situated in a reddish-gray shaley siltstone; their root systems are located in the underlying cross-stratified multistoried channel sandstone succession. Some small, scattered and broken fish fragments are located in the sediments around the tree stump casts.

Preliminary work suggests that each of these Upper Devonian forests (Famennian) formed in a coastal to nearshore environment. Though the populations differed in regard to tree abundance and density, the tectonic events that influenced the deposits may have been similar. The strained tree stump casts exhibit orientations similar to the folds, faults, and fractures produced during the Alleghanian Orogeny.