File:M98Ch3Fig2.JPG

Diagram showing the chronostratigraphic units in the uppermost (Lower Ordovician) part of the Sauk megasequence and the Middle and the Upper Ordovician part of the Tippecanoe megasequence and their relationship to modern North American mid-continent conodont zones, as well as the original alphanumeric conodont ldquofaunal unitsrdquo of Ethington and Clark (1971) and Sweet et al. (1971). Global series and stage names and boundaries are from Webby et al. (2004) as modified by Bergstrom et al. (2008). Positions and approximate range in ages of the original conodont ldquofaunal unitrdquo boundaries relative to modern zones are based on a chart compiled by J. E. Repetski and A. G. Harris for use in Poole et al. (1995). Abbreviations for species whose first appearance datums (FADs) define the bases of the global stages (shown in rectangles on right margin) include one conodont (Bt = Baltoniodus triangularis) and five graptolites (Ta = Tetragraptus approximatus; Ua = Undulograptus austrodentatus; Ng = Nemagraptus gracilis; Dc = Diplacanthograptus caudatus; and Ne = Normalograptus extraordinarius). Hirn. = Hirnantian Stage; Tremadoc. = Tremadocian Stage. Estimated position of peak of drawdown between Sauk and Tippecanoe megasequences in Histiodella holodentata Zone adopted from Derby et al. (1991).

From Taylor, John F., John E. Repetski, James D. Loch, and Stephen A. Leslie, 2012, Biostratigraphy and chronostratigraphy of the Cambrian–Ordovician great American carbonate bank, in J. R. Derby, R. D. Fritz, S. A. Longacre, W. A. Morgan, and C. A. Sternbach, eds., The great American carbonate bank: The geology and economic resources of the Cambrian–Ordovician Sauk megasequence of Laurentia: AAPG Memoir 98, p. 15–35.