Discovery of the Ordovician Millbrig K-bentonite Bed in the Trenton Group of New York State: implications for regional correlation and sequence stratigraphy in eastern North America

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.     

High-resolution sequence stratigraphy of a mixed carbonate-siliciclastic, cratonic ramp (Upper Ordovician; Kentucky–Ohio, USA): insights into the relative influence of eustasy and tectonics through analysis of facies gradients

Detailed facies analysis and event stratigraphy of an Upper Ordovician (Rocklandian–Edenian) cratonic ramp succession in eastern North America yields insights into eustatically driven sequence architecture and localized tectonic instability. Seven, predominantly subtidal, mixed carbonate-siliciclastic depositional sequences (3rd order) are identified and correlated across the length of a 275-km ramp–to–basin profile. Within the larger depositional sequences (3rd order) at least two smaller orders (4th and 5th) of cyclicity are recognizable. Three systems tracts occur within each sequence (transgressive, TST; highstand, HST; regressive, RST) and are considered in terms of their component parasequences (5th order). Generally, TSTs are composed of skeletal grainstone–rudstone facies, HSTs are dominated by shaly nodular wacke-packstone facies, and RSTs are mostly calcarenite facies. Systems tracts, sequence boundaries and their correlative conformities, maximum flooding surfaces, and forced regression surfaces were traced from shallow shelf to basinal settings. This high-resolution framework also provides insight into the timing of tectonic fluctuations on this cratonic ramp during the Taconic Orogeny and documents the relative influence of tectonism on lateral facies distributions and eustatically derived cyclicity.