ORIENTATION OF CEPHALOPODS AS PALEOCURRENT INDICATORS IN THE GREEN VEDDER MEMBER, MANLIUS FORMATION, NEW YORK STATE

In eastern New York State, the Green Vedder Member of the Manlius Formation is a coarse skeletal packstone and grainstone with numerous hardgrounds (Matteson and Ebert 2011) and a diverse faunal assemblage that includes cephalopods, gastropods, stromatoporoids, bryozoa, bivalves, and ostracods. Some cephalopods and gastropods are filled with dolomicrite, which differs markedly from the encasing skeletal grainstone, indicating exhumation and reworking. The Silurian-Devonian boundary occurs within the Green Vedder Member, as indicated by the presence of scyphocrinitid loboliths (Matteson and Ebert, 2011) and a positive δ¹³C excursion (Kleffner et al. 2009; Wilson, Ebert and Matteson 2011; Ebert and Matteson 2013).

Students enrolled in Sedimentary Geology at SUNY Oneonta (Fall 2018) studied abundant cephalopods in an outcrop of the Green Vedder Member near Catskill, NY. Orientations of orthocone conches were obtained by measuring the strike and dip of bedding with Brunton compasses and angle of rake of each fossil with protractors. The bedding plane was rotated to horizontal via stereonet and cephalopod orientations were plotted on rose diagrams.

Orientations of 75 cephalopods were measured and record a generally eastward unidirectional current with 57.3% of cephalopods oriented from 75-125° (mode at 91-100°). A second paleocurrent mode to the north-east is present as well with 42.7% of the cephalopods plotting between 11-75° (mode at 36-40°). Gastropod shells measured at the same outcrop also record a dominantly eastward paleocurrent direction, but lack the secondary mode recorded by the cephalopods.

The eastward and northeastward paleocurrent modes recorded by the cephalopods likely reflect exhumation and reorientation of conches by strong unidirectional currents associated with storm-driven combined flow, though indicators of the oscillatory component are absent. An alternate interpretation is that the eastward mode reflects tidal outflow from the Appalachian Basin into the Early Devonian Flysch Basin of New England.