Organodiagenetic Dolomite on a Deep Subtidal Shelf, Fort Payne Formation (Mississippian), Tennessee, U.S.A
David N. Lumsden, 2003. "Organodiagenetic Dolomite on a Deep Subtidal Shelf, Fort Payne Formation (Mississippian), Tennessee, U.S.A", Permo-Carboniferous Carbonate Platforms and Reefs, Wayne M. Ahr, Paul M. (Mitch) Harris, William A. Morgan, Ian D. Somerville
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This Study Analyzed Dolomite In The Lower Mississippian Fort Payne Formation And Compared It To Dolomite In Similar Deposits In Order To Substantiate Its Organodiagenetic Origin. The Fort Payne Formation In Tennessee Is ADolomitic Porcelaneous Chert That Was Deposited During Transgression Onto A Deep Subtidal Shelf. The dolomite occurs as 10-50 mm euhedral, commonly zoned rhombs enclosed in a very fine-grained spiculiferous chert. The dolomiteis extremely calcian (modes at 55% and 59% Ca), contains up to 1% iron, and has a δ13C mode at+2%o,δ18O modes at +1%o and -2%o, and a manganese partitioning ratio of 5 to 10. Three dolomite fabric types are present: Type 1, which occurs both as isolated 10-20 mm rhombs enclosed by chert and as corescontained in Type 2 dolomite; Type 2, with rhombs that are typically 30-50 mm on edge with luminescent rims thatenclose a nonluminescent core; and Type 3, volumetrically minor, ferroan, with 100-200 mm crystals that line vugs and fractures. In contrast to findings in previous studies the conclusion here is that Type 1 dolomite formed during organodiagenesis, syndepositionally, as a primary precipitate, and before chert formation. Type 2 dolomite precipitated as overgrowths on Type 1 cores, using Mg largely from ambient seawater supplemented by limited amounts from opal diagenesis. Type 3 dolomite formed after lithification. When one compares data from the Fort Payne with findings for lithologically similar coeval units, deep marine deposits of the modern ocean basins, deep-shelf deposits off California, the Monterey Formation, and deposits of the Great Australian Bight, it is apparent that Fort Payne dolomite largely formed by organodiagenesis in a deep-shelf marine setting.
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