Black Shale from the Basin to the Shoreface: Mapping the Type Area of the Global Taghanic Onlap (New York State Appalachian Basin)

James J. Zambito IV
University of Cincinnati Department of Geology Cincinnati, OH 45221-0013
[email protected]

The Middle-Upper Devonian transition is marked by widespread black shale deposition coincident with eustatic sea-level rise (known as the Taghanic Onlap) and associated with a pulsed extinction event termed the Global Taghanic Biocrisis. Although the northern Appalachian Basin Tully and Genesee group deposits are the type-strata for the Taghanic Onlap and Biocrisis, relatively little attention has been placed on documenting the stratigraphy and paleoecology of depositional settings above storm wave base. Recent fieldwork shows that low-oxygen conditions conducive to organic matter preservation are found throughout the basin from offshore to shoreface settings during the initial pulse of the Taghanic Onlap. In basinal settings, widespread fossil-poor black shale deposits are observed. Slope depositional settings, resulting from foreland basin crustal flexure and increased progradation associated with the third tectophase of the Acadian Orogeny, interbedded black silty shale and turbiditic silts were deposited. Shelf settings were dominated by dark gray to black shale and sandy siltstone deposits containing storm-deposited low-diversity shell beds; these deposits contain also early diagenetic pyrite, indicating the presence of low-oxygen conditions above storm wave base. In shoreface settings, thin- to medium-bedded sandstones with swaley and hummocky cross-stratification are seen interbedded with organic-rich dark gray silty shales containing a low-diversity marine fauna. These observations suggest that the Taghanic Onlap elevated the oxycline to unprecedented levels. Such widespread low-oxygen conditions, coupled with drastically increased sediment influx into the basin and the formation of a shelf-slope depositional setting, were apparently the ultimate driving mechanism for the Taghanic Biocrisis in the Appalachian Basin.

AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid