UNUSUAL FACIES ASSOCIATED WITH THE LATE MIDDLE DEVONIAN ( GIVETIAN ) TAGHANIC BIOEVENT IN NEW YORK STATE; PROBLEMS OF INTERPRETING RECURRENT CHAMOSITE BEDS IN THE TULLY FORMATION

The Late Middle Devonian Taghanic Bioevent represented a time of widespread community disruption and biotic incursions. In New York, the type Taghanic succession is represented by the Tully Formation; it records an initial invasion of distinctive taxa (Tully Fauna) that displaces many endemic taxa of the long-standing Hamilton Fauna. In the upper Tully, the Hamilton Fauna returns prior to onset of widespread, post-Tully anoxia.The Tully Formation in west-central New York is anomalously composed of massive to nodular limestone in marked contrast to adjacent, thick siliciclastic units. Moreover, units of the lower Tully, yielding the Tully Fauna, grade eastward into several discrete chamositic "oolitic" beds representing condensed facies and discontinuity lag deposits. The fossil-poor chamositic facies, in turn, grades shoreward into increasingly fossil-rich, non-chamositic, bioturbated, siltstone and sandstone deposits. Previous authors have interpreted the Tully Formation as a shallow, even lagoonal, unit, and the chamosite to have formed as a type of platform oolite. We view the Tully as representing mainly low energy, offshore conditions and the chamosite to be the product of poorly understood processes. The black, discoidal shape of ooids, their mud-supported texture in beds, their association with fossil-poor deposits, and the eastward (shoreward) passage of chamosite into fossil-rich facies, all suggest an origin in moderate to low energy settings and, possibly, even dysoxic conditions. The paleoenvironmental significance of the chamosite and its possible link to the biotic changes are being investigated.