Recently, in an email exchange with another Paleozoic geek, we discussed the relationship of the stratigraphy of Ontario with that of Kentucky. The discussion centered around the Veralum Formation up north(Canada), and which units of the Lexington Limestone(Kentucky) share a temporal equivalence, i.e., which beds of the Lexington were deposited at the same time as the Veralum.
It has been know for some time that the Bobcaygeon Fm. of Ontario was deposited at about the same time as the Curdsville mb. of the Lexington Lm. Further, the ecology and the environment was very similar; many of the same rare echinoderm/trilobite assemblages are found in both beds.
Outside of the carbonate build-ups of Central Kentucky, that time in the Ordovician represented a transgressive sequence, so one would assume that the relationship could be established by looking at the stratigraphic record for both areas.
Mitchell, et. al. in 2004 did just that. Proceeding from the works of previous authors, they studied a prominent meta-bentonite(the Millbrig K). Since, ash falls are distributed over a large region, could they be correlated?
- 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 Kbentonite 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 d13C 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 Kbentonite 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.
One of the real gems of this paper is the correlation chart. Using the ash falls, they correlated the rocks from Illinois through Kentucky, and then up through the foreland basin of New York.
So, pertinent to the email exchanges, it appears that the Veralum’s equivalent in Central Kentucky is the Tanglewood and Millersburg members, and some basal units of the Clays Ferry and Kope Formations in the Outer Bluegrass region.
Further reading and references:
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
Charles E. Mitchell, et. al.(2004)PDF
USING GIS TO ASSESS THE BIOGEOGRAPHIC IMPACT OF SPECIES INVASIONS
ON NATIVE BRACHIOPODS DURING THE RICHMONDIAN INVASION IN THE
TYPE-CINCINNATIAN (LATE ORDOVICIAN, CINCINNATI REGION)
Alycia L. Stigall(2010), et. al.
Ordovician K-bentonites of eastern North America
Dennis R. Kolata, et. al.(1996)