[5] It overlies the approximately Middle to Late Ordovician High Bridge Group, which is only exposed in some valleys of the Kentucky River,[6] and underlies the Clays Ferry Formation.
Since the time of its deposition spans several million years which were accompanied by constant sea level and topographical changes, the lithology of the Lexington Limestone varies significantly with geographic location and stratigraphic position within the rock column.
[3] Around 450 million years ago, during the Late Ordovician period, much of what is now eastern Kentucky was a shallow, tropical carbonate platform located on the southern fringes of Laurentia[7] since the craton was rotated around 45 degrees clockwise compared to its modern position.
The lower parts of the Lexington Limestone, the Curdsville and Logana members, are characterized by the continuous encroachment of the sea onto the vast tidal flats, culminating in a period where much of the platform was too deep for an adequate supply of oxygen to be retained or for photosynthesis to be performed effectively.
Due to heavy wave action, most organic debris in the lower part of the formation was broken up and heavily altered, making fossils rare and of poor quality.
The water level was normally too deep to provide an adequate supply of oxygen, hence fossils are rare, and mostly restricted to Cryptolithus trilobites (which are characteristic of deep-water environments).
During longer periods of nondeposition, more complex ecological communities could form, as evidenced by the presence of limestones that progress upwards from low-diversity, whole fossil packstones to high-diversity grainstones.
[11] Due to the region's topographical height compared to the surrounding landscape at the time, the Logana Member is absent around and southeast of Winchester since the area did not reach depths required for the shale's deposition.
The presence of gastropods with lime mud indicates that the environment was also highly rich in algae, while the lenticular and nodular structure of some beds point to an abundance of soft-bodied burrowing organisms which have not been fossilized.
The relative topographical uniformness of the region was disrupted after two linear horst-like blocks were uplifted around a central graben structure south of Versailles, Kentucky as a result of Taconic tectonism.
Owing to their shallower depth compared to the surrounding landscape, tidal currents began to cause accretions of bioclastic debris to form on top of them, giving rise to two major shoal complexes.
Most of the debris that makes up the Tanglewood member represents the broken and abraded fragments of locally abundant calcified animals, mainly crinoids, brachiopods, bryozoans and ostracodes.
Over time, due to rising sea levels or subsidence, the southwestern block came under deeper waters, as evidenced by the expansion of the shaley Brannon Member into the area.
However, the Brannon Member was not limited purely to this graben structure - it gradually pinches out northeastward as well, and overlies the southwestern Tanglewood area, apparently as a result of its horst subsiding.