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The Ordovician (Chatfieldian) Millbrig K-bentonite Bed is a key stratigraphic marker horizon that is regionally synchronous over much of eastern and central North America. This prominent marker is an independent source of correlation among the major chronostratigraphic and sequence stratigraphic units in this region. The general stratigraphic position of the Millbrig K-bentonite has suggested to some authors that it is identical with the Hounsfield K-bentonite at Dexter, New York (the traditional type area of the Middle Ordovician, or Mohawkian Series in North America), but previously available geochemical and biostratigraphical evidence has been insufficient to confirm this correlation. Analyses of apatites and melt inclusions in quartz phenocrysts from the Millbrig K-bentonite at eight localities in Kentucky, Missouri, Virginia, and Pennsylvania and the Hounsfield K-bentonite at its type locality at Dexter, New York, using high-precision electron microprobe analysis techniques shows that the Millbrig and the Hounsfield have identical apatite and melt inclusion chemistry indicating a geochemical correlation. This correlation is supported by conodont biostratigraphy, and δ13C isotope chemostratigraphy. The new data demonstrate that the Millbrig K-bentonite, and therefore the base of the Chatfieldian Stage of the North American Mohawkian Series (by definition), lies very close to the base of the traditional Rocklandian Stage of New York. Furthermore, the Millbrig K-bentonite Bed lies in close proximity to the base of the Taconic supersequence over much of the Midcontinent region and in particular lies just below the M5 sequence boundary recognized in Kentucky and Tennessee. Our results permit extension of the Chatfieldian sequences into New York State and southern Ontario, and contributes to the resolution of the long-standing uncertainty about the position of the base of the Trenton Group in Ontario. Furthermore, viewed in a broad context of regional stratigraphic relations, we conclude that our results suggest that the persistent correlation difficulties reflect diachronous effects of widespread changes in oceanographic circulation patterns that emerged during Taconic Orogeny. Finally, regional differences in the timing and character of sequence bounding surfaces and facies similarities summarized here suggest that the causes of relative sea level change during the Chatfieldian may have been primarily tectonoeustatic mechanisms. [Copyright &y& Elsevier]