The Birdsong Shale of Western Tennessee

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Located throughout west central Tennessee, the Birdsong Shale is exposed in a narrow, north south trending belt situated in the Western Valley of the Tennessee River - specifically Decatur and Benton County. A dominance of Silurian deposits in the study area and flanking by Mississippian deposits to the east and Cretaceous sediments to the west causes outcrops  to be sporadic in occurrence but exposures can be found along road cuts and in quarries (although access to the latter has become greatly restricted in recent years). Part of the Ross Formation, the Birdsong Shale (like all members of the Ross) is rich in fossils and the fauna is largely consistent throughout the section.

At the base of Tennessee's Lower Devonian deposits is the Ross Formation, which is highly fossiliferous and the central focus of this study due to the remarkable preservation and diversity of the fauna. 

The Ross Formation is made up of three members; the Rockhouse Limestone, Rockhouse Shale and the Birdsong Shale: 

The Rockhouse Limestone is a coarse grained, glauconitic, skeletal packstone up to 17 feet thick with thin shale partings. The Rockhouse Shale is a grey green, calcareous shale up to 25 feet thick. Either of these members can form the base of the Ross Formation, depending on which part of the state is being sampled (Wilson 1949). The Birdsong Shale overlays the Rockhouse Shale and Rockhouse Limestone and represents the top of the Ross Formation. 

The Birdsong Shale was named by Carl Dunbar "because of the good development of the strata seen along the valley of the Birdsong Creek...in Benton County." (Dunbar, 1919, P.56). Dunbar stated the boundary of the Birdsong Shale occurred where the fossil content was similar to that of the Rockhouse. However, Clement (1989) stated that it was impossible to distinguish between the lowest sections of the Birdsong Shale and upper Rockhouse, suggesting an uninterrupted depositional sequence, and so no clear boundary has been established.

The Birdsong Shale consists of alternating layers of bluish green, clay shale and calcareous shale beds with lenses of course grained glauconitic limestone and is extremely fossiliferous. The beds vary in thickness but are usually only 2 to 3 inches reaching a maximum of 8 inches. The shale beds predominate and are up to 4 feet thick. The thickness of the Birdsong as a whole is variable, as is unconformably overlain by the Flat Gap Limestone or Harriman Formation. Thickness is usually 25 to 40 feet, but can reach 60 feet (Wilson 1949).

Dunbar (1919) divided the Birdsong shale into three zones based on fossil content:

1.) Trilobite Zone.  At least 22 feet thick.

2.) Bryozoan Zone.  6 feet thick.

3.) Brachiopod Zone  33 feet thick.

Each zone was named from the dominant fossil types seen within them. Although used by a number of authors, many papers have largely ignored the Trilobite Zone, simply using the Brachiopod and Bryozoan titles (e.g Clement 1989). This is partly due to the fact that the Trilobite Zone is seen only in the northern most part of the state (Henry County) and may only be a feature of this area (Dunbar 1919). Erosion during the Devonian may have removed the Trilobite Zone from the rest of the state but this issue has not been fully resolved.

References:

Clement, D.A. 1989. Echinoderm Faunas of the Decatur Limestone and Ross Formation (upper Silurian to Lower Devonian) of West Central Tennessee. PHD Thesis, University of Knoxville.

Dunbar, C. 1919. The Stratigraphy and Correlation of the Devonian of West Central Tennessee. State Geologic Survey

Wilson, c. 1949 Pre-Chatanooga Stratigraphy in Central Tennessee. Tennessee Division of Geology.

At the beginning of the Lower Devonian (408-387 Myr) Tennessee was a low, flat landscape only a little above sea level. Palaeogeographical reconstruction has been inconclusive as to determining the exact position of the area during this time, but work by Barret (1985 in Broadhead and Gibson, 1995) have indicated the Ross Shelf was located between 25 and 30 degrees south of the palaeoequator (below right).

During the Lower Devonian an invasion of the sea from the Gulf of Mexico covered the Western Valley region. Warping of the land followed, allowing the sea to spread extensively northwards. The presence of large numbers of well preserved brachiopods within the deposits suggests that sediments were laid down on a thriving, shallow marine shelf. Clement (1987) stated that, "the beds are graded and were deposited by storm events. The limestone lenses (within the shale) were produced by a winnowing of the substrate which concentrated the calcareous material." (Clement, 1989, p.21). Broadhead et al (1992) suggested that the storms were seasonal, accounting for the large number of alternating beds seen in the deposits. The uneven  distribution and thickness of the limestone lenses in the Birdsong suggest at times there were prolonged periods of relative calm where the storm activity was reduced, allowing colonisation of the shelf by some species that could otherwise not survive (see Favosites helbergiae and Favosites sp. below)

The muddy shelf environment of the Ross showed little change throughout the Lower Devonian and was a continuance of that seen in the Ordovician and Silurian. On the shelf, tabulate corals reached their prime becoming one of the most dominant benthos on the shelf. Rugose corals continued to evolve and increase in number and brachiopods remained the dominant faunal type. Trilobites, despite the appearance of some new forms, continued to decline and crinoids were rooted into the substrate (although there is evidence to suggest at least one form was mobile). Nautiloids and conodonts swam in the waters above and although no conclusive evidence has been found to suggest their presence it is believed fish were also present. Bryozoans like the tabulate corals were an important part of the fauna, with bryozoan dominated mud mounds providing areas of stability on the ocean floor. Bivalves, gastropods and other groups represented only minor parts of the fauna, but persisted through the period.

References:

Broadhead, T.W. and Gibson, M. 1995 - Upper Silurian and Lower Devonian Biotas of the Western Tennessee Shelf. University of Tennessee Studies in Geology No. 25

Broadhead, T.W., Gibson, M. & Dolson, C. 1992 - Silurian to Devonian Stratigraphic Succession of the Western Tennessee Shelf. GSA Abstracts with Program 24:A4

Based on the fauna present, the Birdsong Shale has repeatedly been correlated to the Helderberg of New York and the Haragan/Bois d'Arc Formation of Oklahoma (Dunbar, 1919; Amsden and Boucot, 1958 etc.). 

Because of the strong similarities between the faunas of the three regions it is believed they were deposited in a a single epicontinental sea (the Lower Devonian Epeiric Sea) with New York representing the eastern most limit and Oklahoma the western most. [Further information on the correlation between the faunas can be found on our Bois d'Arc reference page].

References: 

Amsden, T.A. & Boucot A.J. 1958. Stratigraphy and Palaeontology of the Arbuckle Regions Parts 2, 3, and 4. Oklahoma Geological Survey Bulletin 78

The following analysis of the Birdsong coral fauna is all based on the author's university thesis studies with all conclusions and data coming from materials collected during 1995 and 1996.

Corals are among the most common benthos on the Ross Shelf, second only to the brachiopods in terms of absolute numbers, although the number of species is greatly restricted. Rugosans and Favositid tabulates dominate accounting for 98% of all coral specimens (with an almost even split between the two groups). The remaining 2% is made up of the small encrusting chain coral Aulopora schoharie.

The life habits of all the corals was the same - finding a way to keep the polyps above the muddy ocean floor. 

• The chain corals grew on objects that themselves were already above the sea bottom such as brachiopods, crinoid stems and even other corals. 
• Rugose corals exhibited the typical horn shape produced by upward vertical and lateral growth. As the mud on the ocean floor rose due to sedimentation the coral's upward growth kept it out of the danger zone. Lateral growth meant that as the weight of the coral increased it did not simply sink onto the mud - the increased surface area increasing friction and holding the colony in place. Typically it was only storm events inundating or overturning the colony that lead to the colony's demise as there is no evidence of predation on any of the coral fossils found in the Birdsong.
• Tabulate corals adopted identical strategies as the Rugosans, in that lateral growth  of the base was accompanied by vertical growth that ranged from moderate to extreme (see image left for an example of the latter).

With all the coral species adopting some sort of growth strategy that kept them above the ocean floor it is evident that inundation from sediment brought in by turbulent water was a frequent problem. When one considers that rarely are any large specimens found (more than 2 cm) and often damage or evidence of overturning is seen (in the form of other organisms growing on the underside of the tabulates - something that could only have happened if the base was facing upwards) it is clear the Ross Shelf was of a water depth shallow enough to be affected by regular/seasonal storm activity.

Because of their limited size, and the evidence of storm deposits in the Birdsong Shale, Broadhead and Gibson (1989) suggested corals such as Favosites conicus had a monocarpous life habit - growing rapidly after attachment in order to reach maturity and reproduce before being killed during the next storm season. New polyps were released into the water column just prior to the inundation of the parent colony and those polyps that survived settled on the sea floor and began rapid lateral and vertical growth to provide a stable base for the colony. 

The base of F. conicus was flat and it is believed the coral simply sat of  the harder sediments of the ocean floor or periodically a host. Living for only a year meant there was no need for the coral to adopt a growth form and life habit that required little more than preventing sinking into the mud while awaiting storm season.  Philcox (1971) estimated a growth rate in Favositids of 2 cm per year, a figure that is perfectly represented on the Ross Shelf and supports the theory of a monocarpous life cycle. [The consistently small size of Streptelasma strictum is indicative that it too was likely monocarpous].

Favosites foerstei employed a different strategy to that of F. conicus, in that although there was increased lateral growth to produce a wide base, vertical growth was apparently paramount. In all the available specimens, the base is  often 4 cm wide, but vertical growth can be more than twice this - sizes that based on Philcox's study can only be accounted for if the coral was polycarpus (surviving for more than one year). 

An additional adaptation seen in F. foerstei that further supports the polycarpous theory is that the base of the coral was not flat, but cone shaped, a feature that provided stability to the colony that was completely lacking in F. conicus. As F. foerstei grew vertically and increased in weight the cone shape would have allowed the colony to sink a short distance into the sediment - something that was extremely important if the colony was to survive being toppled during multiple storm events. With the base secured in the sediment, the spire was better positioned to survive higher energy environments.

Erosion of the spire after death sometimes accentuates the taper of the spire compared to how it would have looked in life - after death the egg-like shape of the complete colony would have allowed it to be easily rolled around the sea floor and the less robust polyp chambers readily damaged compared to the more solid base.  The majority of specimens of this coral found in the Birdsong are in fact just the basal cone (see below) with the entire spire having been eroded away or broken off (additional evidence of the sometimes harsh conditions of the shelf). 

References: 

Broadhead, T.W. & Gibson, M. 1989. Species Specific Growth Rates of Favositid Corals to Soft Bottom substrates. Lethaia 22, 287-299.

Philcox, M.E. 1971. Growth Forms and the Role of Colonial Coelenterata in Reefs of the Gower Formation (Silurian) of Iowa. Journal of Palaeontology, Vol. 45, 338-346.