THE PALEOECOLOGY OF A MISSISSIPPIAN REEF FROM NORTHWESTERN GEORGIA

Mississippian reefs in North America are rare. They were recovering from the Frasnian-Famennian extinction at the end of the Devonian. Since the Devonian, bioherms are manifest as smaller reefs and mud mounds with limited fossil assemblages. It is therefore important to study the paleoecology of Mississippian bioherms. Our work focuses on an autochthonous Mississippian reef deposit in northwest Georgia. It appears to be the largest known bioherm within the Bangor Limestone. The Bangor Limestone, which stretches across portions of Alabama, Georgia, and Tennessee, consists mainly of crinoidal grainstone. The Ft. Payne underlies this limestone and is predominately a chert unit, but carbonate mud mounds do occasionally occur, however these are dominated by microbial attributes in contrast to the bioherm discussed here. There are only a few mud mound-type bioherms located within Alabama and Tennessee. Most of the described Bangor mudmounds are small in areal extent, but the largest one described from Alabama is 1.6m thick and 25m wide, and is characterized by rugose corals, blastoids, crinoids, and bryozoans. The reef complex in Georgia is 3m thick, and 400m wide composed of chaetetid sponges, rugose corals, microbialites, bryozoans, foraminiferans, crinoids and Pentremites blastoids. Petrographic analysis and fieldwork indicate a framestone-bindstone reef association with chaetetid sponges and rugose corals building the principal structure. Microbial cement binds the reef. Some baffling does occur chiefly by fenestrate bryozoans. The chaetetid sponges range in size from just a few centimeters to 45cm in diameter and are the first successional stage in this reef complex. They are overlain by large rugose coral colonies that can grow to over 1m across in width. Some colonies are overturned suggesting a storm deposit. In the chaetetid portion of the reef, the porous space is filled with micrite and small skeletal fragments composed of crinoid and foraminifera elements. In the rugose framework, there is extensive dolomitization in the surrounding sediments. The significance of this reef is it is larger in size than other documented bioherms in this region, and it bears a more complex faunal assemblage with extensive chaetetidae sponge development not seen in other Mississippian reefs of this age.