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

Ordovician Atlas

You have probably noticed that in past posts about the local paleontology, I have often linked specific fossil genera to an 'Ordovician Atlas' page. I thought I would share a little bit about this resource because I think it is a wonderful way to learn about the paleontology of the Cincinnati Arch.

This Ordovician Atlas is part of a collaborative NSF-funded initiative to produce digital atlases for multiple intervals of Earth history.

The Ordovician Atlas website is a series of interactive pages which allow you to search different fossil taxa that can be found in the strata along the Cincinnati Arch. Whether it be arthropods, brachiopods, echinoderms, cnidaria, mollusks or bryozoans, the Ordovician Atlas is a great resource for identifying taxa, identifying that strata you might find them in, ecological niches, descriptions or publication relevant to the taxa in question.


I know personally when dealing with fossils genera that I may be unfamiliar with, I always check the Ordovician Atlas to see if they have a listing. Below I have posted a few screenshots from the website to show some of the features present for individual genera, in this case Isotelus.

Example of the Isotelus page showing the Linnaean classification, geologic range and location map. 

Some of the pages shows these nice paleoecological diagrams depicting the range of habitats and organism may have lived. This image is for Isotelus maximus

These two images show the stratigraphic distribution of the general in question (Isotelus maximus) along with basic fossil descriptions.

Lastly, many of these pages will have a list of references and descriptions from those references, of the fossil in question.

The site contains more than just information about the various Cincinnatian genera. There is a page about the local geologic setting, the Richmondian Invasion (a person interest of mine) and various educational resources for the teachers among you. For those of you who like to go collecting, they have a page dedicated to listing good sites for doing just that!

I want to thank Dr. Alycia Stigall and the students in her lab for the great work they have put in and continue to put into this great resource.


I also would like to point you to this recent publication which discusses the program further.

Stigall, Alycia L., Jennifer E. Bauer, and Hannah-Maria R. Brame. "The Digital Atlas of Ordovician Life: digitizing and mobilizing data for paleontologists and the public." Estonian Journal of Earth Sciences 63.4 (2014): 312œ316.

Keep checking back for more additions to the Ordovician Atlas. If you are Twitter savvy, @PaleoDigAtlas is one source for updates to not just the Ordovician Atlas but the entirety of the 'Digital Atlas of Ancient Life' project.

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!

A Bundle of Brachiopods - Zygospira

The other day when Allison and I were out in the field, we made a quick stop near Dillsboro, IN to look at an outcrop of what was probably Mount Auburn Member (of the Grant Lake Fm) or the Arnheim Formation. We knew we were not any younger than that because we came across some rather large Platystrophia ponderosa (Vinlandostrophia ponderosa) which are known the make through the Arnheim, yet disappearing before the Waynesville Formation. However we didn't see any of the true diagnostic fossils like Lapteana  or Retrosirostra carleyi. 

Outcrop of Grant Lake or Arnheim Formation near Dillsboro, Indiana

We did come across this rather interesting pair of rocks in the float that showed some sort of groove or gutter carved into what must have been a semi-harden marine floor and in this groove was dozens of small Zygospira brachiopods.

Groove or gutter filled with the small brachiopod Zygospira

Another accumulation of Zygospira from the same locality. The large brachiopod on the bottom right is a Rafinesquina.

Zygospira are known for adhering themselves to whatever hard substrate they can find. In a paper by Micheal Sandy (1996), the author describes a specimen from the Waynesville Formation of Ohio where an articulated crinoid stem is thoroughly encrusted by Zygospira. There have also been reports by local trilobite collectors that they have found Zygospira concentrated around fragments of Isotelus in mudstone deposits colloquially referred to as butter shales (more on these next week).

A closer look at one of the accumulations of Zygospira

Lastly, an interesting note is that on at least one of the blocks we see the dumbbell shaped trace fossil Diplocraterion generally thought to be the dwelling trace of a crustacean or worm.

Arrow is pointing to the Diplocraterion trace fossils associated with one of these Zygospira accumulations.

Versailles State Park Pt 2 - Fun with Fossils

So as Allison mentioned in her last post, we spent a recent Saturday hiking around Versailles State Park looking for outcrops of the Waynesville and Liberty Formations (Katian; Richmondian). Many of the outcrops as you probably noticed and would have suspected, were covered in varying degrees of ice. It was also about 5 degrees C out, so we didn't feel like hiking through ice-cold streams was a wise idea. #JanuaryFieldWork As a result, most of our fossil hunting was restricted to the various cobble deposits along the stream's banks. There were plenty of goodies to be had there though!

This first image is of a rather sizable fossil that was seemingly randomly located along the trail approaching the stream. Not sure where it came from exactly BUT, it is rather neat! Here we have a large coral, likely a Tetradium (click the link for additional images of Tetradium from the Cincinnati Arch region).  

This image I am rather fond of. Here we have two different Ordovician bivalves, an Ambonychia  (link to the Ordovician Atlas page) on the right, and a Caritodens on the left.  Bivalved mollusks are typically aragonitic in composition which is why these clams are moldic (aragontie does not preserve as well as calcite) but what is notable about these critters is they are impressively large! My guess is these guys came from either the Wayensville or Liberty Formations.

This next image is from one of the outcrops Allison shared previously. To the immediate right of the scale bar is a jumble of brachiopods called Rafinesquina, very common to the Ordovician of the KY-IN-OH tristate. We often refer to these beds of jumbled Rafinesquina as 'cornflakes beds'. I imagine the name is self-explanatory!

This is another set of brachiopods from that same outcrop as the Rafinesquina, There are two primary brachs that you can see in this image, the one just right of center is an orthid brachiopod, likely a type of Hebertella.

Lastly a nice view of the iced lake as we were hiking back to our car.

All in all, it was a fun day hunting for outcrops and fossils and I look forward to getting out in the field more and sharing with you our discoveries!

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.