SPIRORBIS AS AN INDICATOR OF MARINE INFLUENCE IN PENNSYLVANIAN CYCLOTHEMS

Marine conditions in the Pennsylvanian-Permian rocks of the northern Appalachian basin supposedly occurred for the last time during the deposition of the mid-Conemaugh Group of the lower Upper Pennsylvanian (earliest Virgilian). However, evidence for marine influence extends up into the Permian Dunkard Group. The cyclothemic rocks in this basin are interpreted as sediments from upper to lower deltaic plains, estuaries, and nearshore marine situated along an extensive lowland coast. The transition between marine and freshwater environments is blurred in some cyclothems because of long-term sea level changes as well as short-term marine incursions of a daily, seasonal, or catastrophic nature along a coastal setting.

The fossil Spirorbis can aid in identifying marine influence along a coastal transition at outcrop scale. Spirorbis is a serpulid worm tube which is calcareous and mostly spiral with dimensions of a few mm in cross-section and up to several cm in length. The polychaete worms making these tubes were strictly marine throughout the Phanerozoic and are known to occur today along coastal transition zones, including coastal lakes with connections to the sea and even in subterranean caves linked to the marine. The distribution and preservation mode of Spirorbis within cyclothemic facies can give paleoecologic information on the extent of marine influence across a freshwater to marine coastal gradient. For example, when unattached small specimens of Spirorbis are restricted to cm-scale units within m-thick freshwater limestones or attached to freshwater plants in coal sequences, this indicates episodic marine incursions along a paleocoast on a time scale much less than that of a cyclothemic cycle. Spirorbis is an excellent tool in refining the sedimentology and sequence stratigraphy of Pennsylvanian cyclothems.