Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Mississippian (Serpukhovian) framework reef in Northwestern Georgia,
USA
100
Edwin K. LORD 1 , Sally E. WALKER 1 & Markus ARETZ 2
1 Department of Geology, University of Georgia, Athens, GA, 30602, U.S.A.; geolord04@yahoo.com,
swalker@gly.uga.edu
2 Université de Touloluse (UPS), GET (OMP), 14, Avenue Edouard Belin, 31400 Toulouse, France;
markus.aretz@get.obs-mip.fr
This study documents the occurrence of a lower Carboniferous (Serpukhovian, 326-318 mya), shallow
water, microbially-bound metazoan framework reef in the Bangor Formation of Walker County in
Northwestern Georgia, USA. The reef measures 2.7 m at its thickest point, and extends laterally
approximately 400 m. The reef is exposed inside two neighboring caves on Pigeon Mountain as well as
where it crops out on the surface. This unique location allows for a 360-degree view of the reef complex.
Inside the caves, the reef can be seen from underneath as well as a vertical section that encompasses the
reef framework with underlying and overlying strata. Outside the caves, where the reef intersects the
topography, the reef is seen from above as evidenced by in situ coral colonies.
The reef framework is dominated by a chaetetid sponge-coral association. Microbialites extensively coat
much of the metazoan framework adding secondary stabilization. Individual chaetetids range from a few
centimeters to 45 cm in diameter; although extensive cloning occurs, producing masses of chaetetids that
may reach sizes of up to 1 meter. Caninostrotion colonies (rugose corals) may grow up to 1.5 m in width.
Other reef-dwelling organisms include Pentremites, crinoids, regular echinoids (based on spines),
Archimedes and other fenestrate bryozoans, fistuliporid bryozoans, gastropods, brachiopods, and
foraminifera.
The 2.7 meters of reef facies is comprised of 3 repeating growth cycles. Biological succession appeared to
occur within each of these 3 cycles. The base of each cycle is colonized by a monoculture of chaetetids.
These chaetetid layers can be observed from underneath within the cave (Fig. 1). Contemporaneous to, or
shortly after chaetetid growth, microbialites began encrusting the tops and interstitial space between the
chaetetids. This initial framework created hard surfaces, suitable for coral nucleation, and colonies of
Caninostrotion soon after began to take root, greatly adding to the framework (Fig. 2). During the main
phase of reef growth, all reefal organisms, including skeletal metazoans, encrusters, bafflers, and mobile
organisms, continued to appear throughout the section until the subsequent cycle begins with another
chaetetid monoculture.
1
Fig. 1: Chaetetid monoculture at the base of reef
framework, seen from underneath (chaetetids outlined
to show contrast).
2
Fig. 2: Colony of Caninostrotion (rugose corals).