THE PALEONTOLOGICAL AGE OF THE ERIN SLATE, NORTHEAST ALABAMA
Samantha D. Harrell
Columbus State University, Department of Earth and Space Sciences
Figure 6. Athyris spiriferoides
Methods
Abstract
Results
A number of slate specimens from various locations in the Erin Slate were collected, brought back
to Columbus State University, and processed for fossil inclusions. One method for examining the
slate for fossils involved the use of a refractive oil to accentuate potential fossils under a binocular
microscope. The slate samples were inundated with mineral oil and placed under a precisely
angled light source. After an initial examination was completed, select samples were mounted in
epoxy, allowed to harden, and systematically polished, revealing new layers of the slate and
potential fossil material. No definitive fossils were found using either of these techniques. Samples
from the New Albany Shale were collected from Periastron discovery sites in central Kentucky in
an attempt to determine whether or not Periastron might have a wider age range that previously
reported. The rocks where initial Periastron fossils were recovered in the late 1970's are also of
debatable age. I attempted to relocate the fossil sites based on Charles Beck’s original descriptions
and directions from his paper “Periastron Reticulatum Unger and Aerocortex Kentuckiensis, n.
gen. et sp., from the New Albany Shale of Kentucky”. By collecting samples from the original fossil
localities, we hope to resolve the exact stratigraphy and age of the Periastron bearing rocks, further
constraining the age of the Erin Slate and providing a better understanding of the history of the
southern Appalachian Mountains.
The ability to identify fossils that are preserved in rock can aid in
determining not only the environment of said rock, but, the
conditions in which they were preserved, forces that may have
altered them, and perhaps most importantly the age in which they
were deposited. Since organisms evolve over time and on occasion
go extinct, only certain species are found in rocks of different ages;
making some fossils good indicators of age. The age of the Erin
Slate in northeastern Alabama has been and continues to be a
controversial issue in the geologic history of the southern
Appalachian Mountains. An anomously young fossil, Periastron,
found in the Erin Slate suggests that these rocks are of early
Mississippian age (340- 360 million years ago), although abundant
fossils in stratigraphically lower rocks suggest a slightly older age
(>370 million years). Extracting fossils from the Erin Slate, and its
equivalents, will help in resolving the age of the Erin as well as aid
in understanding the geologic history of this portion of North
America.
After surficial examination of the collected slate samples under refracted light was
completed, the samples were mounted in a two part epoxy resin and prepared for polishing.
No definitive fossils were discovered using either technique. The samples collected from the
Erin chronostratigraphic equivalent, the New Albany Shale, in central Kentucky produced a
probable Athyris spiriferoides fossil from the Middle Devonian period ( 380 -395 million
years ago). Athyris sp. (figure 6.) is an extinct species of brachiopod that dwelled in warm,
shallow marine environments and survived through filter feeding(paleobiology database).
Since the stratigraphic position of phosphate nodules from which Beck (1978) collected some
his samples have come into question, it is possible that some of the nodules may not have
been part of the Falling Run. If so, then Beck may have collected samples from
stratigraphically older portions of the New Albany Shale - suggesting that Periastron has a
wider age range that that previously thought .
Area of research
Overview
Figure 7. Possible Athyris sp. fossil in phosphate nodule
collected from New Albany Shale, Erin Slate equivalent, in
Kentucky
The Erin Slate (figure 2.) , a variably deformed black slate, is a member
within the larger Talladega Group which is a sequence within the
Talladega slate belt, the southwestern extension of the western Blue Ridge
belt, in northeast Alabama. The Talladega belt consists of four distinct
lithologic units, including: a lower clastic sequence, the Kahatchee
Moutain Group, which is conformably overlain by a middle carbonate
sequence, called the Sylacauga Marble Group, that is unconformably
overlain by an upper clastic sequence, the Talladega Group (figure 4.),
(Tull et al, 1988) which is overlain by the Hillabee Greenstone. The
Ordovician age Hillabee is in faulted (or tectonic) contact with the
stratigraphically lower and younger Talladega Group (Tull et al, 2007).
The Talladega belt is the westernmost crystalline thrust sheet in the
southernmost exposed Appalachians and lies between the foreland foldthrust belt to the northwest and the eastern Blue Ridge (Gastaldo et al,
1993). The Talladega belt is bounded both on top and on bottom by
southeast dipping, post-metamorphic fault systems. This suite of lowgrade metasedimentary and metavolcanic rocks are interpreted to have an
age ranging from the latest Neoproterozoic to earliest Mississippian
(Tournaisian) (figure 4A). This determination of age is based on
radiometric data as well as lithostratigraphic and biostratigraphic
correlations with fossiliferous units within the foreland fold-thrust belt.
These fossil containing units include the Cambrian age Jumbo Dolomite,
the Silurian-Early Devonian Lay Dam Formation, the Early Devonian, and
Erin Slate facies equivalent, Jemison Chert, and the controversial Erin
Slate argued as ranging into the Late Devonian or Mississippian
(Gastaldo et al, 1993).
Figure 3. Stratigraphy of
Talladega Group, including the
Erin Slate.
Figure 5. Stratigraphic column of New
Albany Shale, Kentucky.
Figure 1. Regional geologic
map. Hatcher et.al, 2007
Falling Run
Clegg Creek Member
Further Research
Samples collected from the New Albany Shale in central Kentucky need to be further
analyzed for fossil inclusions. Positive identification of recovered fossils need to be
carried out. Samples collected from the Erin Slate need to be processed further using the
polishing method for fossil inclusions. However, a working lap wheel used to polish
specimens would greatly benefit this research, reducing hand polishing time and
providing a more consistent surface for examination. In addition, a wider sample area
in both the Erin Slate and New Albany Shale, should be collected to provide a greater
probability for finding fossils in order to more precisely constrain the age of the Erin
Slate. More definitive Devonian fossils are needed to asses a true age of the New Albany,
which requires a more lengthy study of the area. Analysis of the phosphate nodules
recovered from the Falling Run bed will be carried out to assess if the nodules are from
the correct strata to determine if the age range of Periastron should be adjusted
accordingly. Also, an updated map of the original Periastron localities in central
Kentucky should be produced to aid in further research.
Camp Run Member
Morgan Trail Member
Charles Beck, a noted botanist from the University of Michigan,
discovered the fossil Periastron reticulatum within the New Albany Shale
in central Kentucky in the 1970’s. Dr. Robert Gastaldo, professor of
geology, located Periastron in the Erin Slate in northern Alabama in the
early 1990’s. The constrained age of Periastron is known from the Early
Mississippian Unterculm at Saalfeld, Germany; the “Lydinnes ”
Formation of the Montagne Noire, France; and the New Albany Shale in
Kentucky. Thus an Early Missippian age has been assigned to the Erin
Slate in northeastern Alabama based on the presence of this fossil
(Gastaldo et al, 1993).
An Erin Slate chronostratigraphic equivalent, the New Albany Shale, part
of the larger Cincinnati Arch (figure 4.) in central Kentucky, is made up of
a five-member subdivision, that alternates between gray and black shales.
The New Albany Shale consists of the Blocher, Slemier, Morgan Trail,
Camp Run and Clegg Creek members respectively (figure 5.). Within the
Clegg Creek member is the Falling Run bed (Ettensohn et al, 1988) . It is
in this Falling Run bed where Periastron fossils have been documented,
located in phosphate nodules (Beck, 1978) . However, it has been debated
whether the Falling Run exists at all in the sites where Periastron have
been discovered by Charles Beck. In Beck’s description of Periastron
localities, he uses generalized geographic locations rather than specific
geographic data, adding to the debate over the existence of the Falling
Run bed in the areas where the prized phosphate nodules and Periastron
fossils are found.
Figure 4. Regional map of the
Cincinnati Arch. Ettonsohn et al,
1988
Selmier Member
Blocher Member
New Albany Shale, Nelson County, KY
Birmingham
References
Beck, Charles. B., Periastron Reticulatum Unger and Aerocortex Kentuckiensis, n. gen. et sp., from the New Albany Shale of
Kentucky. American Journal of Botany, Vol. 65, No. 2 (Feb., 1978), pp. 221-235
Ettensohn, Frank R., Peter T. Goodman, Rodney D. Norby, and Thomas H. Shaw., Stratigraphy and Biostratigraphy of the
Devonian-Mississippian Black Shales in West-Central Kentucky and Adjacent Parts of Indiana and Tennessee. 1988 Eastern
Oil Shale Symposium. Institute for Mining and Minerals Research. November 30 – December 2, 1988.
Mississippian
(359-315 Mya)
Tournasian (359-345 Mya)
Devonian
Gastaldo, Robert A., Gregory M. Guthrie, and Mark G. Steltenpohl., Mississippian Fossils from Southern Appalachian
Metamorphic Rocks and Their Implications for Late Paleozoic Tectonic Evolution. Science, New Series, Vol. 262, No. 5134.
(Oct. 29, 1993), pp. 732-734.
(416-359 Mya)
Silurian
(444-416 Mya)
Ordovician
Collection Site
Hatcher Jr, Robert D., Brendan R. Bream, and Arthur J. Merschat, Tectonic map of the southern and central
Appalachians: A tale of three orogens and a complete Wilson Cycle. Geological Society of America
Memoir 200, 2007
(488-444 Mya)
Paleobiology Database: http://paleobio.org
Cambrian
Peterson, Warren., 1966 Geologic Map of the New Haven Quadrangle, Nelson and Larue Counties, Kentucky. Geologic
Quadrangle Map New Haven Quadrangle, Kentucky. Kentucky Geologic al Survey.
(542-488 Mya)
Neoproterozoic
(1000–542 Mya)
Figure 4A. Timescale
indicating age range of
Periastron.
Latest
Neoproterozoic
Tull, J.F., Barineau, C.I., Mueller, P.A., and Wooden, J.L., 2007, Volcanic Arc Emplacement Onto The Southernmost
Appalachian Laurentian Shelf: Characteristics and Constraints: Geological Society of America Bulletin, v. 119, p. 261-274.
(630-542 Mya)
Coastal Plain
Figure 2. Location of Erin Slate. Modified from Barineau and
Pearce, 2010
Tull, James F., Anita .G Harris, John E. Repetski, Frank K. McKinney, Connie B. Garrett, and Denny N. Bearce., New
Paleontologic Evidence Constraining the Age and Paleotectonic Setting of the Talladega Slate Belt, Southern Appalachians.
Geological Society of America Bulletin; August 1988; v. 100; no. 8
This project was funded in part by an Undergraduate Research Grant provided by the Office of the Provost, Columbus State University