About This Item
Share This Item
The Brassfield limestone and the overlying Crab Orchard formation of Madison and Estill Counties, central Kentucky, represent a nearly continuous sequence of strata of Llandovery and early Wenlock age. The conodont sequence very closely parallels that of Europe, and zones I and II (Llandovery in age) and zone III (early Wenlock(?) in age), as recognized by Otto Walliser, are found here.
The Brassfield limestone and the Plum Creek clay, Oldham limestone, Lulbegrud clay, and Waco limestone members of the Crab Orchard contain conodonts of zone I for which Icriodina is the most important guide. Zone II conodonts are found in a thin zone transitional between the Waco and the Estill clay member, and the lower part of the Estill contains new genus A and Bryantodus? sp., which are characteristic of European zone III. Zone III conodonts have been considered to be early Wenlock in age, but exact correspondence with the graptolite zones is uncertain.
Northward in Ohio and west of the Cincinnati arch in Indiana and Kentucky, the Brassfield also contains a zone I fauna. As the Crab Orchard formation is traced
northward, the part below the position of zone II conodonts becomes progressively thinner, and only a thin wedge of the lower part extends into southernmost Ohio. Here this lower unit is overlain by the Dayton limestone, which underlies a shale sequence called the Alger formation in Ohio. The Alger is a northern extension of the Estill clay.
Zone I is represented in the Kankakee limestone of northern Illinois, and zone III conodonts have been obtained from the Joliet limestone of northern Illinois and the basal few inches of the Osgood formation of southern Indiana. Insufficient work in other areas of the United States precludes precise correlation. The close resemblance of the European and central Kentucky conodont faunas, together with the other known occurrences, shows the potential value of these fossils for stratigraphic work in this part of the column.
End_of_Article - Last_Page 544------------