Last winter, after recently returning to the Central Kentucky area, I decided to visit an area that is well known, locally, for its Ordovician fauna. It is also one of the few places around here where structure can be observed.
Though out the Central Kentucky region, enigmatic structures in beds of Ordovician limestones have been observed for years. Originally, the structures were thought to form from density differences during lithification of the sediments. The structures are now interpreted as de-watering events associated with seismic activity related to the Taconic Orogeny.
Seismites are deformation features formed as seismic waves pass through the soft sediment. They occur before lithification, and result from liquefaction of the sediments. Research into the origin, and interpretation of seismites is a relatively new field of study in geology. Their significance is that they can help locate paleo-epicenters of long past earthquakes.
Primary structural features are also seen in the area, but they are related to a Neoproterozoic-Early Cambrian failed rift system(aulacogen), and/or, the formation of the Rome Trough in the eastern part of the state. The aulacogen has been reactivated at various times throughout the geologic past, and most recently, sometime, within the Pliocence or Pliestocene.
On this trip, the exposed rocks are of late Mid. Ordovician-early Late Ordovician in age. They are comprised of the High Bridge Group and Lexington Lm in Fayette Co. Ky, and the Clays Ferry Lm and Garrard Siltstone in Madison Co. Ky.
Generalized geologic map showing location of outcrops(from KGS)
Generalized strat column for the region(from where???)
Some minor faulting on the way to the outcrop
Before the descent into the Kentucky River valley, part of the Elk Lick Creek graben is seen to the side of the highway.
A view across the river of one of the two main faults in the area(there are many smaller en-echelon faults). The Tyrone Lm. of the High Bridge Group on the up side. Lexington Lm on the down side. The High Bridge Group are Mid Ordovician. The Lex Lm is late Mid Ord.
A photo of the Camp Nelson Lm of the High Bridge Group. The High Bridge Gr. was deposited on a shallow water carbonate bank. Local unconformities are common, and several significant meta-bentonites have been mapped in the unit.
Moving on to the south side of the river one can find the contact of the Lexington Lm(
Curdsville Mb. ooops, that is at the bottom of the Lex Lm, this is the Tanglewood mb) and the Clays Ferry Formation. (Stick boy for scale-about 1.5m)
Ball and pillow seismite.
A view down the river on the south side.
My next stop was farther up the hill on the south side of the river. About a couple of km from the river, some nice outcrops of the Garrard Siltstone can be view. The Garrard is the unit that over lies the Clays Ferry Lm.
The deposition of the Garrard represents a regressive sequence. It marks a change from the relatively shallow water off-shore carbonates and shales to near shore clastics as a result of eustatic sea level changes due to the Late Ordovician glaciation covering Gondwana.
Cross bedding in the Garrard. (beds about 10 or 12 cm thick)
Herring bone cross beds.
More seismites! Ball and pillow structure.
Close up of ball and pillow structure.
Garrard Siltstone. Notice the oil seeps even though there hasn’t been a successful well drilled within 80 or 100 km of here.
Further reading and references:
Drs. Rast and Ettonsohn’s paper can be read here(no paywall)