SEDIMENTOLOGY OF THE GREEN VEDDER MEMBER (MANLIUS FM.) AND NEW QUESTIONS REGARDING THE STRATIGRAPHIC EVOLUTION OF THE HELDERBERG GROUP

The sedimentology and stratigraphy of the Green Vedder Member (GVM) and associated Rondout, Manlius, and Coeymans formations in New York State are examined at ten outcrops from Skaneateles Falls to Schoharie, N.Y. Throughout its extent, the GVM overlies the Thacher Mbr. of the Manlius Fm. but is overlain by a variety of units. Stratigraphic relationships between the GVM and these overlying units are currently under evaluation.

The Green Vedder Mbr. (informal, upper thin-bedded Thacher) of the Manlius Fm. is present within most sections, and is characterized by hummocky cross-stratified wackestones and packstones interbedded with mudstones and calcareous shales. Beds of the GVM typically display flat basal contacts overlain by planar to cross-laminae and less commonly ripple caps, a sequence typical of tempestitic shelf deposits. Firmgrounds to hardgrounds developed on upper surfaces of many bedding planes (mm to few cm) are strewn with ostracods, brachiopods, bryozoans and horizontal burrows of Planolites indicative of post-storm recolonization. Equant spar-filled shrinkage cracks (mudcracks of Laporte (1969), and others) are common along hardground horizons in mudstone-wackestone beds and are interpreted as having a subaqueous origin. Interbedded calcareous shales and mudstones exhibit ramifications of Medusaegraptus (a dacycladacean alga), vascular plant debris, soft tissues of annelids, microbial mats, and rare tentaculites.

Thicknesses at Oriskany Falls and Munnsville indicate maximum basinal subsidence in this area during GVM deposition. At western outcrops, hummocks are much broader (> 1m) than their counterparts to the east. Shallower facies to the east are demonstrated by hummocks with much shorter wavelengths, sometimes displaying a nodular appearance associated with large ripple/dune forms. Correlations between outcrops displaying the GVM indicate an eastward shallowing from Oriskany Falls to Schoharie. To the west, upper beds of the GVM are truncated by the overlying Olney Member of the Manlius Formation via the Terrace Mountain Unconformity. The presence of this sediment-starved, storm-dominated shelf setting in the presumed peritidal Manlius Fm. raises new questions regarding the evolution of the Appalachian Basin during Helderbergian time.