The Lexington Limestone

by Rick Schrantz

The Lexington Limestone is the major rock unit exposed in the Inner Bluegrass region of east-central Kentucky surrounding Lexington. It is mainly Middle Ordovician in age, but locally the uppermost beds are considered Upper Ordovician in age. It is underlain by the Tyrone Limestone (Middle Ordovician ) and overlain by the Clays Ferry Formation (mainly Upper Ordovician). A bentonite layer in the upper Tyrone is dated at 454 million years old. The Lexington's lower contact with the Tyrone Limestone is fairly sharp, as the Tyrone is a very pure limestone with few fossils (see figures left and right), and is one of the premier quarry rocks in the area. The Lexington Limestone is mostly fossiliferous limestone with minor amounts of shale. The upper contact with the overlying Clays Ferry is fairly obvious, as the Clays Ferry has much more shale (alternating with limestone layers) [See the figure to the lower left. In this image, the Clays Ferry is above the Tanglewood at the red line. Note the contorted bedding in the lower part of the image; this is a seismite (which results from an ancient earthquake) located north of Frankfort.]

An excellent publication that details the Lexington Limestone and all its members is "Lithostratigraphy and Depositional Environments of the Lexington Limestone (Ordovician) of Central Kentucky" by Earle Cressman. It is a Geological Survey Professional Paper (768) published in 1973.

The Lexington Limestone was named in 1898 by M. R. Campbell, and ranges from about 200 to 320 feet thick. There are fossils to be found in it, but it is generally not as fossiliferous as the Upper Ordovician Cincinnatian rocks overlying it. It is important to be able to recognize certain members of the Lexington Limestone, as that will enhance your chance of finding nice fossils. Here is a summary of those members from the viewpoint of a fossil collector.

Curdsville Member

This member is the basal unit of the Lexington Limestone. It is disconformably underlain by the Tyrone, and overlain by the Grier or Logana members. It is about 20 - 40 feet thick, and is composed of mostly thick layers of calcarenite limestone deposited in a shallow marine setting above wave base. However, there are some shale partings indicative of quiet water (perhaps in topographic low spots or protected areas), and that is where the fossil action is! Within these shale partings, and on the limestone surface covered by a shale parting, are excellent echinoderm fossils along with other rare fossils. Crinoids, edrioasteroids, paracrinoids, and Brachiospongia sponges can be found here. Collecting these fossils is tough for several reasons: there are not many exposures of this member; the shale partings are thin; and there is usually no talus pile of rocks to look through at the base of a cut. In addition, although the fossils can be spectacular, they are very sparse.

Logana Member

This member, while not always present, is underlain by the Curdsville and overlain by the Grier. It ranges from 0 to 50 ft. thick, and exhibits repeating and fairly regular cycles of alternating limestone and shale. The limestone and shale accumulated in deeper water below wave base, and the grain size is finer than most of the Curdsville. There are not many exposures of this member to be found, but there are some around the Frankfort area. Fossils include dense brachiopod-shell horizons (Dalmanella), and nice clam fossils. Also, a few Cryptolithus trilobite fragments, and flattened Conularia can be found.

Grier Member

This member is the thickest member of the Lexington Limestone. It ranges from 100 to 180 feet thick, and was deposited in shallow, moderately agitated water. It is underlain by the Curdsville or Logana, and overlain by the Tanglewood or Brannon. It is fossiliferous, but the fossils to be found are usually brachiopods, bryozoans, and sometimes gastropods. It can be recognized by its thin, irregular or nodular beds of limestone. Although it is composed largely of broken fossil debris, fossil hunting in this member is generally unrewarding .

Tanglewood Member

This member is one of the thicker members of the Lexington Limestone, ranging from 60 up to 100 ft. thick, and forms much of the upper part of the Lexington Limestone in the Bluegrass region. It is the surface rock in much of the Bluegrass. Its lower and upper contacts with other members is difficult to briefly describe because of complex intertonguing. It is generally massive looking to bedded (0.2 to 1 ft thck) calcarenite limestone with a larger grain size than most other units of the Lexington. The Tanglewood is cross bedded in some places. It was deposited in a turbulent, very shallow marine enviroment. Farm ponds overlying this member have trouble holding water. It is fossilliferous, but the fossils are almost always broken and abraded small hash fragments. Therefore, the fossil collecting in the Tanglewood is mostly unrewarding.

Perryville Member

This member can range from 0 to about 50 feet thick. It is underlain by the Tanglewood, and overlain by the Brannon Member. There are three subdivisions of this member (Faulconer, Salvisa, and Cornishville beds). It is exposed best in Boyle county, and the limestone at these exposures contains silicified mollusks and brachiopods. Fossil collecting can be good in this member. Don't forget to check the weathered limestone surfaces for silicified fossils poking out in relief. Check the soil above the rock cut for weathered-free silicified fossils.

Brannon Member

The Brannon Member is easily recognized by its alternating layers of shale and limestone. It ranges from 0 to 30 feet thick, and was deposited below wave base. It is usually both underlain and overlain by the Tanglewood Member, but can be underlain and overlain by other members. Sometimes the limestone exhibits a ball-and-pillow structure and contorted bedding. It weathers more than most other members of the Lexington Limestone, and is very obvious on the high roadcut at the Kentucky River on the Bluegrass Parkway. The fossil collecting in the Brannon is generally poor. A few brachiopods and bryozoans, and rarely, a trilobite such as Isotelus or Proetus, can be found in it. Many years ago, some spectacular sponges (Brachiospongia) were found in it, but none since.

Millersburg Member

This member ranges from 0 to 90 feet thick, and was deposited in water of moderate turbulence. It complexly intertongues with the Tanglewood and the Clays Ferry. A Millersburg exposure is very obvious due to its shaley, nodular appearance and rubbly weathering. A roadcut of this layer will weather quickly. This is an excellent member for fossil hunting. Gastropods, brachiopods, bryozoans, corals, stromatoporoids, and rare echinoderms and trilobite pieces can be found in this member.

Strodes Creek Member

This member is considered a lens within the Millersburg. It is exposed in only a few locations, generally near Winchester. It is largely limestone and shale. Fossils found here include beautiful red algae and stromatoporoids.

Sulphur Well Member

This is locally the top member of the Lexington Limestone. It ranges from 0 to 35 feet thick, and was deposited in moderately turbulent water. It is usually present in the southern area of the Lexington Limestone around Danville. Fossil collecting in it can be good. A spectacular layer of edrioasteroids has been found in Danville. Also, bored bryozoans and brachiopods can be found in it. It is alternating layers of limestone and shale.

Devils Hollow Member

This member can range from 0 to 30 feet thick, and is underlain by the Tanglewood, and overlain by the Tanglewood or Millersburg. It was deposited in very shallow, but quiet, protected water with a higher salinity. A spectacular series of large ripple marks (see figure to the right) was exposed in a field where the soil was removed. Bryozoans, crinoid holdfasts, and gastropods can be found here.

Stamping Ground Member

This member is 0 to 15 feet thick and is nodular limestone and shale. Fossils can be found here. It is underlain and overlain by the Tanglewood.

For a more detailed geologic description of the Lexington Limestone and other Ordovician units in Kentucky see the US Geological Survey "Ordovician System" [Contributions to the Geology of Kentucky], by Earle Cressman and Warren Peterson.