Monday, 6 October 2008
George R. Brown Convention Center, Exhibit Hall E
The Eifelian-Givetian (E-G) boundary is associated with the globally recognized Kacák-otamari events. The boundary is defined by the first occurrence of the conodont Polygnathus hemiansatus Bultynck; this horizon at the global section and stratigraphic point at Jebel Mech Irdane, Morocco, is above a black shale interval (otamari-shale) and the Late Eifelian Magnetic Susceptibility Event. In western New York three widespread black shales occur in the Marcellus subgroup close to the E-G boundary. In the absence of Polygnathus hemiansatus the boundary is resolved to be between the Cherry Valley Member, which is assigned to the kockelianus Zone based on the occurrence of the nominative taxon, and the Halihan Hill Bed, 2 m higher in the Genesee River Valley, which has yielded the conodonts Icriodus angustus Stewart and Sweet, I. stephensoni Sparling, Polygnathus aff. P. robusticostatus Bischoff and Ziegler, and a species of Tortodus? - a fauna similar to that known from the Delaware Limestone and the upper Dundee Limestone in Ohio, Michigan, and Ontario - that are associated with a Hamilton Group brachiopod fauna and goniatites that are considered Givetian. An abrupt and significant rise in magnetic susceptibility (MS) values from 1.5E-8 to 5E-8 m3/kg 40 cm above the base of the Berne Member (top of the Cherry Valley) is similar to the MS shift measured in Morocco at the GSSP and other localities in Morocco and Europe. The E-G boundary is provisionally placed where this shift stabilizes above the Cherry Valley Member within the black shale of the lower Berne Member. The dacryoconarids Nowakia (Dmitriella) sulcata postsulcata? Alberti and Viriatellina fortistriata Lütke recovered from above the Halihan Hill Bed in the lower Chittenango Shale Member at Oatka Creek do not serve to constrain the boundary as both taxa are known from Eifelian strata.