BRIDGING THE LIMESTONE TO SHALE TRANSITION: UPPER ORDOVICIAN (KENTUCKY AND OHIO)

The abstract I submitted to the North Central Geological Society of America meeting this coming May was accepted. I hope to see a few of you at the meeting!

Correlation of shallow marine limestones into deeper shale-dominated settings remains an area of great uncertainty in many sedimentary basins impairing the accuracy of depositional models. The Lexington Platform-Sebree Trough transition (Kentucky-Ohio) represents a well preserved Upper Ordovician example of this problem. In this study, outcrops and cores in central Kentucky and new drill cores from Cincinnati, Ohio were logged using litho-, bio-, and chemo-stratigraphy to develop detailed correlation of chrono-stratigraphic packages across a major facies transition from a the carbonate-dominated Lexington Limestone platform succession to the dark, mudrock-dominated deposits at the transition to the Sebree trough. A number of facies-crossing markers provide useful, first-order controls on stratigraphy: these include abrupt facies offsets, K-bentonites, fossil epiboles (e.g. Prasopora bryozoans), and deformed beds, probably representing regional seismites. These correlations are corroborated by gamma ray, carbon isotope curves, and elemental abundance profiles. This research expands the regional correlation of the Lexington Formation and its members into more distal settings and provides strong evidence that depositional sequences, and distinctive faunal epiboles, and chemo-stratigraphic signatures are of regional extent. Thus, despite local abrupt facies variations, associated with far field tectonics, there is strong evidence for allocyclic, probably eustatic control of large and small-scale cycles. However, the degree of environmental change across this gradient varies strongly with stratigraphic levels. Thus, lower units (Curdsville, Logana members) persist across the profile with relatively little change. In contrast, the upper Lexington units display much more abrupt northward change to shaly facies, suggesting increasing rates of subsidence in the Sebree trough through deposition of the Lexington sediments.

YOUNG, Allison L., Geology, University of Cincinnati, 500 GeoPhys, Cincinnati, OH 45221, younga9@mail.uc.edu, BRETT, C.E., Department of Geology, University of Cincinnati, 500 Geology/Physics Building, Cincinnati, OH 45221-0013, and MCLAUGHLIN, Patrick I., Wisconsin Geological Survey, University of Wisconsin - Extension, 3817 Mineral Point Rd, Madison, WI 53705

Ohio Geological Survey Visit

Yesterday I returned from the Ohio Geological Survey (OGS) after 2 days of drilling samples and photographing a few of the sections. I had a great time and collected a lot of information that will help answer some questions we are working on in my research group for my thesis.

OGS has a whole facility dedicated to storage and analysis of core at their office just north of Columbus, OH the Horace R. Collins Laboratory.

If you have questions about the local geology when traveling to a new state or even exploring your local rocks the state geological survey is a great place to start for information. Surveys are very different state-to-state but one thing they all have in common is a group of people who know their local rocks. In addition to the informed and helpful staff they hold a wealth of information, including records that go further back then you would think. Most of the records are becoming available on their websites including maps, publications, and well logs.

Core is very expensive to drill so we rely on core available to us at geologic surveys to collect data on these packages of rock not exposed at the surface. The advantages of core is it provides access to a large continuous section of rock while being easy to sample. Thin beds that may be unrecognizable in outcrop are easily seen in core. Additionally, its size allows for instantaneous comparison of multiple sections side by side and within a single day.

While you are restricted to the diameter of the core, often a few inches or less, when analyzing bedding planes fossils are still found and can be crucial to constraining intervals with the help of biostratigraphy.

Graptolites

Trilobites! ( Triarthrus?)

Triarthrus cephalon

Fossil finds are not restricted to the bedding planes. It is very common to see cross sections of
many fossils such as the possible bryozoan pictured here. 

Big shout out to OGS especially facility coordinator Aaron Evelsizor for all of their help!

Danville, KY: Dix River

50 and partly cloudy! Maybe Kentucky missed the memo that it is still February, but Christopher and I took advantage of it for some field work. This past Saturday we took a drive down to Danville, KY to check out my favorite Upper Ordovician deposits, the Lexington Formation with our good friend Kyle of the Dry Dredgers.

At Danville, Boyle County, KY we studied ~60 meters of exposed section. Measuring bed-by-bed we collected data to draft a stratigraphic column and made some quick fossil counts.

One of the more interesting finds was how heavily faulted the entire region was. Resulting in an offset of more than a meter to just a few centimeters.

One of the MANY faults at this exposure. 

Working in heavily faulted areas adds complexity to the otherwise layer cake stratigraphy we are spoiled with in the central Kentucky outcrop belts.


Another unique feature of this particular outcrop belt is the great abundance of preserved bentonites. The literature has noted the presence of as many as 20 bentonites in the Tyrone Formation, the main member of the lower micritic section of the exposure. Including the famed Deicke K-Bentonite.

Sticky yellow clay. K-bentonite?, a sample was collected for further analysis.

Finally we found another good chronostratigraphic tie line with a deformed (ball and pillow structure) zone believed to be the expression of the capitol.

Deformed beds of the "capitol", helpful as a marker horizon when correlating
across faults.

Versailles State Park

This past weekend Christopher and I, with our friend Kyle of the Dry Dredgers, took advantage of the nice weather and head out into the field. We made our way to Indiana, lead by Kyle, with the objective of viewing some late Ordovician exposures. 

Waynesville/Liberty (Upper Ordovician) outcrop along the cut bank of the stream

A lot of ice on the stream and outcrops, but still some nice exposures. #JanuaryFieldWork

Notice the distinct blue color of the shale, common to shales of Waynesville and Liberty Formations

View from the top of the hill we hiked up after following streams in search of outcrops

Gumdrops

insitu Prasopora Lexington Formation, KY

Bryozoans are some of the most common fossils found in the Upper Ordovician deposits of the Kentucky Bluegrass. Prasopora falesi?,  closely related to P. simulatrix Ulrich, of the Monticuliporidae family is found only in Kentucky in rocks of  Kirkfieldian and Shermanian. Specimins have been found in the Grier Member (which spans the Kirkfieldian and Shermanian stage boundries), Brannon Member, Sulphur Well Member, Tanglewood Member, and through the Shermanian Stage portion of the Clays Ferry Formation.

Specimen has a 7cm diameter base

The hemispherical growth habit of P. falesi has lead to its more colloquial name the “gumdrop bryozoan” OR "chocolate drop" to local fossil collectors.

Another fun aspect of the growth habit of these bryozoans is when examining their underside you can often tell what they were living on... a bivalve perhaps? a silty muck? reworked clasts?

Specimens from left to right have a 5cm and 7cm 

diameter base.

In reading through Karklins, 2010 I am also entertaining the possibility that the specimens photographed above could also be Mesotrypa angularis Ulrich, 1893. I will be doing thin sections or acetate peel to investigate this further. If you have any thoughts or suggestions on the genus please comment below. Thanks for checking out Ordovician Weekly.

Reference: O. L. Karklins, U.S. Geological Survey professional paper, Issue 475, G.P.O, 1984, University of Michigan, Digitized Feb. 22, 2010.