EIFELIAN (EARLY MIDDLE DEVONIAN) DEPOSITIONAL SEQUENCES AND BIOEVENTS IN EASTERN LAURENTIA: APPALACHIAN BASIN-WABASH PLATFORM COMPARISONS

The late-Eifelian (australis to lower ensensis conodont zones) Marcellus subgroup (Hamilton Group) of New York comprises two 3rd-order composite depositional sequences (Seneca-Union Springs and Cherry Valley-Oatka Creek formations) and the highstands of these, in turn, are subdivided into four 4th-order subsequences. Each sequence comprises a thin (0.2-2 m), widespread transgressive limestone (TST), overlain by a thicker succession of black/dark gray shale (HST). Three discrete faunas: Onondaga, Stony Hollow/Cherry Valley, and Hamilton, are recognized within this succession. Boundaries show evidence of abrupt, widespread extinction, immigration and ecological restructuring. In western-central New York new faunas appear abruptly within widespread condensed beds at the bases (TSTs) of each 4th-order sequence. However, correlations with thicker/shallower sections in eastern New York indicate that faunal overturns actually occurred during periods of widespread dysoxia associated with preceding 4th-order highstands.

Equivalent late Eifelian strata of Ohio also appear to comprise two 3rd-order sequences. The Venice Mbr. in northern Ohio and lower Delaware Fm. in central Ohio are approximately age equivalent (australis to lower kockelianus zones) to the Union Springs (NY sequence 1). Dysoxic (Dublin) black shaly facies in central Ohio carry low diversity leiorhynchid brachiopod-dominated faunas very similar to those of NY sequence 1. More oxic argillaceous limestone facies contain long-ranging Eifelian-Givetian taxa (e.g. Stropheodonta demissa, Leptaena) and new elements (e.g. Martinia maia, Schizophoria spp., Brevispirifer lucasensis); showing some similarities to the distinct Stony Hollow fauna of NY. These beds are abruptly overlain by 1-2 m of grainstones rich in the small coral Hadrophyllum, corresponding to the base of a second sequence comparable to the Cherry Valley limestone (NY sequence 2 TST). The upper Delaware comprises dark, bituminous limestones with dysoxic biotas comparable to those of the Oatka Creek (NY sequence 2 HST). Thus, although preliminary, present sequence stratigraphic and K-bentonite data suggest that physical and biotic events in Ohio are coeval with those of New York and perhaps the European Kacák-otomari bioevents.