Curdsville Limestone The Curdsville Limestone Member is the oldest and lowermost member of the Lexington formation. It was deposited as the sea level rose and transgressed across the tidal flats that dominated the region during the deposition of the preceding
Tyrone Limestone. It is predominately composed of
calcarenite initially deposited in turbulent, shallow water, but as the water column deepened, grain sizes became increasingly smaller. 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. However, due to calmer conditions, fossils become more common in the upper parts of the member, when the water was not deep enough to be anoxic, but too deep for most wave-action. Whole and broken silicified fossils of
brachiopods and
gastropods are known, along with the occasional
bivalve or
trilobite.
Logana Member The Logana Member was deposited during the time when the earlier deepening trend of the sea level reached its apex. It consists of an alternation of fine limestone and shale, with winnowed organic debris periodically arriving from shallower waters and covering a terrigenous, muddy substrate that continuously accumulated in the deep waters. 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). However, periodic improvements to the oxygen supply of the area resulted in the formation of shell beds when
brachiopods of the species
Dalmanella sulcata were able to gain a foothold. 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. 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. On the other hand, the member continuously thickens northward and westward as it approaches the deep waters of the former Sebree trough.
Grier Member As sea levels began to fall, oxygen levels normalized throughout the platform, leading to a boom in biodiversity. Much of the Grier Member was deposited in waters with little wave action, so unabraded fossils of
bryozoans and
brachiopods are exceedingly abundant.
Crinoids were also very common during the time of deposition, but few are preserved as complete fossils. 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. Rocks of the Grier Member also have an unusually high
phosphate concentration - this was most likely due to the influx of nutrient-rich cold waters from the
Sebree trough northwest, which was also a reason for the abundance of fauna throughout the member.
Tanglewood Member 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. The area of deposition of the Tanglewood has undergone several changes since the initial deposition of the member. 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. Later, an apparent reversal in the polarity of tectonic subduction at the Laurentia-Taconic boundary caused the entire horst-graben structure to invert - the formerly deep graben between the two Tanglewood horsts was raised, while the two horsts subsided, causing the area of deposition of the Tanglewood member to shift to the area previously associated with the Brannon member.
Millersburg Member In the areas surrounding the Tanglewood shoals, conditions remained optimal for the existence of a relatively rich ecosystem. The Millersburg Member was deposited in these areas. In terms of fossil content, it is similar to the stratigraphically lower Grier Member, but is lithologically distinct due to having a much higher shale content. Shale is abundant due to a general increase in terrigenous sediment influx towards the end of Lexington time, rather than depth like in the Logana and Brannon members.
Strodes Creek Member This member is rather bouldery, owing to its pinch-and-swell bedding, ball-and-pillow structure, and abundant stromatoporoids. Fossils of other fauna are also present, but only sporadically and in lesser numbers. == References ==