REVISITING THE MOHAWKIAN BLACK RIVER – TRENTON GROUP BOUNDARY ISSUE FROM THE PERSPECTIVE OF SEQUENCE STRATIGRAPHY

The late Middle Ordovician (Mohawkian) Black River and Trenton groups, originally defined in central New York State and adjacent Ontario, Canada, are among the most classic units in North America. It is surprising, therefore, that details of the upper Black River to lower Trenton stratigraphy are still poorly understood on a regional basis. For example, the precise position of the Black River-Trenton boundary and of the North American stage boundaries are still poorly defined in the type region, despite almost a century of debate. A well-developed sequence framework has been established by Holland and Patzkowsky for the southern Appalachians, the Cincinnati Arch, and the Nashville Dome, but this scheme, too, has not previously been extended into New York. In this talk, we discuss important highlights of recent work in the NY/ Ontario region, and relate these new observations with the work of these previous authors. Specifically, the recognition of the Millbrig (=Hounsfield) K-bentonite within Kay’s "Glenburnie Shale" in northern NY, permits calibration of the Turinian-Chatfieldian stage boundary in the Turinian type area and correlation with the sequence stratigraphic framework of Holland and Patzkowsky. An erosion surface at the base of the Lowville Fm slightly below the MX K-bentonite (Deicke?) probably represents the M3-M4 sequence boundary. While a disconformity at the base of the Watertown Fm, above the "Glenburnie Shale", correlates with the M4-M5 boundary, while the Selby-Napanee contact is reinterpreted as the M5 maximum flooding surface. On this basis we suggest that: a) the Black River Group comprises most or all of M3 and M4; b) the Watertown Fm. plus the lower Trenton Group (Selby-Napanee fms.) comprises M5; c) the base of the Watertown should be used as the Black River-Trenton group boundary; d) the Turinian – Chatfieldian boundary lies slightly (<1m) below the sequence boundary and thus, e) M4 lies almost entirely within the Turinian and not the Rocklandian (lower Chatfieldian), as previously assumed.