STRANDED IN UPSTATE NEW YORK: CAMBRIAN SCYPHOMEDUSAE FROM THE POTSDAM SANDSTONE

Soft-tissue preservation in sandstones is rare in Phanerozoic strata, except in Laurentian intertidal epicratonic facies, where microbial binding is common and metazoan bioturbation is minimal. For example, abundant soft-bodied scyphomedusae, arthropod and mollusk impressions occur in intertidal arenites of the Upper Cambrian Elk Mound Group in Wisconsin, but are absent from subtidal portions of these exposures. Well preserved Elk Mound equivalents occur in the Ozarks, MO (Middle-Upper Cambrian Gunter and Lamotte Sandstones) and in the St. Lawrence Lowlands, NY-QUE-ONT (Middle Cambrian-Lower Ordovician Potsdam Formation and equivalents), and represent fluvial to subtidal lithofacies dominated by quartz arenites and orthoquartzites. Microbial structures and well-preserved fossils are common in intertidal facies of these units.

The Potsdam Formation exemplifies how Neoproterozoic-style taphonomic processes dominate epicratonic intertidal settings prior to the advent of vegetated landscapes. In the Adirondacks, NY, scyphomedusae impressions occur in fine- medium-grained quartz arenites of the Potsdam. Medusae are preserved in convex epirelief and are 10-41 cm in diameter, mirroring size distributions from the Elk Mound. Most specimens are simple circular mounds, but some are twisted helically, folded laterally, or bear concave ring structures surrounding mounds; the latter are characteristic excavation structures produced by prestranding or synstranding pumping of scyphomedusae bells. The upper part of the 200+ m succession is characterized by horizontally-oriented trace fossils and medusae; much lower in the section obolellid and lingulid brachipods, Arenicolites, hyolithids, and trilobites occur, constraining these occurrences to be no older than late Middle Cambrian.

Vertical stacking patterns, paleocurrent data, and primary structures in a section measured through the upper Potsdam at Au Sable Chasm suggest that most of the succession represents shallow marine wave-influenced settings. Intervals characterized by flat-topped ripples, ladder ripples, polygonal mudcracks, suspect-microbial structures and adhesion-marked surfaces likely represent episodically emergent intertidal facies within the Potsdam, and it is in these settings that soft-bodied fossils occur.