The Lexington Limestone is a prominent geologic formation that constitutes a large part of the late Ordovician bedrock of the inner Bluegrass region in Kentucky. Named after the city of Lexington, the geologic formation has heavily influenced both the surface topography and economy of the region.
It overlies the approximately Middle to Late Ordovician High Bridge Group, which is only exposed in some valleys of the Kentucky River, 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. Due to this, it is divided into 11 sub-units which sometimes complexly grade into and intertongue with each other. The formation is largely composed of limestone, however shale is also usually present in varying amounts, and in some sub-units is dominant.
Geological setting
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 since the craton was rotated around 45 degrees clockwise compared to its modern position. As a result of the ongoing Taconic orogeny, the region was cut off from the mainland by the deep Sebree trough. The trough served as a passage for cold, nutrient-rich oceanic water from the Iapetus ocean, which would mix with the warm surface waters of the surrounding carbonate platforms, enabling a large number of bryozoan-brachiopod-echinoderm patch reefs to develop throughout the area. Hence, several parts of the Lexington Limestone are characteristically rich in fossil content.
In preceding periods, particularly during the deposition of the Tyrone Limestone, most of the region was covered in tidal mudflats due to low sea levels. 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.
Eventually, however, water levels began to fall, and a plethora of ecological environments emerged, ranging from extremely shallow and turbulent sandbars, through mid-depth waters highly suitable for patch reef development, to dark, anoxic depressions in the terrain. Each of these environments facilitated the deposition of distinct facies of varying lithological characters. Due to the lack of significant sources of terrigenous sediments, most sedimentary material was supplied through carbonate production of the local ecosystem.
Subdivisions
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.