REGIONAL CARBON ISOTOPE (d13C) STRATIGRAPHY OF CHATFIELDIAN (UPPER MIDDLE ORDOVICIAN) CARBONATES IN CENTRAL AND EASTERN NORTH AMERICA

Carbon isotope (d¹³C) analyses of Chatfieldian marine carbonates are compared from two well-studied sections from Central and Eastern North America. The Guttenberg d¹³C excursion (GICE), which was first recognized in Iowa, and later found in Pennsylvania, Illinois, Kentucky, New York, Tennessee, and Baltoscandia we now recognize the GICE in two sections in Oklahoma and Virginia in the Viola Springs Fm. in Oklahoma, and in the Trenton Ls. in Virginia. The excursion begins in the Midcontinent P. undatus Conodont Zone and reaches its peak values in the P. tenuis Conodont Zone, and in the North American C. americanus Graptolite Zone. An ~3‰ positive shift in d13C is recorded from these two sections: the shift in Oklahoma begins with values of -1.6‰ and reaches its peak values as heavy as +1.5‰, and in Virginia the shift begins with values of ~-0.1‰ and reaches its peak values as heavy as +2.8‰.

It has been shown that in Mohawkian seas there was a wide variance in d¹³C values (4‰ difference) indicating the presence of different temperature-salinity-defined water masses (aquafacies) (Holmden and others, 1998). Based on lithologies and marine faunas present the Viola Springs Fm. (Oklahoma) is interpreted to have been deposited in a deeper water/basinal-type setting and the Trenton Ls. (Virginia) in a shelf-type setting. The beginning of the GICE in North America was previously reported occurring a few meters above the Millbrig K-bentonite bed in Kentucky, however, in Virginia it occurs ~35 m above the Millbrig. The occurrence of the GICE higher in this section is consistent with previous ideas of relatively high subsidence and depositional rates for this area of the Taconic Foreland Basin. The differences in the d¹³C values, marine faunas, and lithologies shows that the Oklahoma and Virginia sections were deposited in two different aquafacies. The (Chatfieldian) Guttenberg d¹³C excursion, apart from the larger end Ordovician excursion, is in terms of magnitude, the most prominent d¹³C excursion currently identified in the Ordovician and it is of major significance for local and regional correlation.