Biostratigraphy and paleoecology of the Floyd Shale, Upper Mississippian, northwest Georgia




Broadhead, Thomas W.

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The Floyd Shale is a thick sequence of shale and siltstone containing minor units of sandstone and limestone that crops out in Georgia in the Valley and Ridge physiographic province. The formation represents the prodelta and embayment shale facies of high constructive deltas that were developed in the area during Late Mississippian time. Important facies associated with the Floyd Shale are the Hartselle delta front sandstone and the Tuscumbia, Monteagle, and Bangor shelf and carbonate bank limestones. Paleontologic evidence suggests that the Floyd ranges in age from Middle Meramecan through Middle Chesteran, but the lowermost Floyd may possibly be Early Meramecan. Floyd deltas, fed from source areas to the south and east, prograded northwestward during the Middle and Late Meramecan over the Tuscumbia Limestone shelf. Delta abandonment during the Late Meramecan resulted in marine reworking of terrigenous facies and transgression by carbonate sediments of the Monteagle Limestone. In the Early and Middle Chesteran, deltas prograded to the north and west over the Monteagle Limestone shelf. Final delta destructive phases occurred in the Middle Chesteran culminating with the Bangor Limestone transgressive sequence. During the period of Floyd deposition, communities of benthic marine organisms became established in sedimentary environments where favorable conditions prevailed. Five communities identified from the Floyd Shale may be distinguished on the basis of numerically dominant organisms; distribution can be related to sedimentary facies. Community 1 is dominated by linguloid brachiopods that probably inhabited areas of moderate to high sedimentation such as prodelta environments. Community 2 is dominated by mollusks and spiriferid and productid brachiopods and may have lived in shallow embayments established along strike from delta lobes. Community 3 includes assemblages dominated by fenestellid bryozoans that probably lived in shallow embayments similar to Community 2. Community 4 is dominated by pelmatozoan echinoderms, spiriferid brachiopods, and bryozoans and probably lived on the open shelf. Community 5 is characterized by dominant rugose or tabulate corals and pelmatozoan echinoderms that may have thrived on carbonate banks. Previous studies of marine benthic communities by such authors as Bretsky and Anderson have suggested a rigid onshore to offshore shelf sequence to explain the distribution of communities. Lack of work on a larger scale has resulted in a gap in the "offshore" community of the Mississippian in Bretsky's (1969) chart. Community distributions proposed by these previous workers are an oversimplification that ignores the control of local sedimentary processes on the distribution of marine organisms. The distribution of communities in the Floyd Shale explained in terms of depositional framework suggests that communities vary both perpendicular and parallel to the paleoshoreline depending upon sedimentary environment.