Abstract

Six limestone classes occur in the Ordovician strata from Lexington, Kentucky, to Nashville, Tennessee. Sediments that make up classes 1 through 4 were deposited in “open” epicontinental marine environments, whereas classes 5 and 6 reflect more “restricted” environments. These classes were used to calculate relative mechanical energy and depth indices for a defined time-stratigraphic interval. The base of this interval is a bentonite bed that occurs near the base of the Brannon Member of the Lexington Limestone, and the top is defined by a major change in the relative abundance of platform conodonts. Relative mechanical energy and depth contours intersect the present axis of the Cincinnati arch at a high angle, which suggests that a continuous arch was not present. Shoal environments existed in the Lexington area and northeast of the present Nashville dome. These shoals correspond to the positions of the Lexington and Nashville domes or precursors to these domes which were present during the defined time interval. Paleobathymetric contours parallel east- and south-trending normal faults that are present in and south of the Lexington, Kentucky, area. This suggests that these faults were active during late Middle Ordovician time and were partly responsible for creating the bathymetric relief necessary for higher-energy carbonate sediments to accumulate. Deeper-water environments between the two shoal areas reflect the presence of the Rome trough. The general bathymetric trend for these strata is one of submergence.

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