ORIENTATION OF GASTROPODS 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 gastropods, cephalopods, 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 high spired gastropod fossils in an outcrop of the Green Vedder Member near Catskill, NY. Gastropod orientations 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 gastropod orientations were plotted on rose diagrams.

Orientations of 73 gastropods were measured and indicate a generally eastward unidirectional current with a slight southeast trend. 82% of the gastropod fossils cluster between azimuths of 60° and 120° (mode at 81-100°), although the total range of directions fell between 36° and 157°. Using data bins of different sizes on the rose diagrams revealed different trends in the data: 20° bins show that the gastropod fossils pointed almost directly east. With bins of 5°, the data appeared slightly more bimodal, with one mode between 75° and 85°, and another between 95° and 115°. Cephalopod conches measured using the same procedure also indicated an eastward paleocurrent but with a secondary mode to the northeast, which is not present in the gastropod data.

The eastward paleocurrent mode recorded by the gastropods likely reflects exhumation and reorientation of shells 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.