Abstract 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 δ 13 C mode at+2%o,δ 18 O 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.

Abstract A readily accessible 440-ft (132 m) section of the upper Jackfork Sandstone crops out along 1-30 near Arkadelphia, Arkansas. The outcrop is on the west side of 1-30 between Exits 83 (Friendship) and 78 (Arkadelphia, Hot Springs) at the Mile Post 81 marker (Fig. 1).

Abstract Electron Spin Resonance (ESR) spectroscopy can determine the absolute amounts of Mn(II) in the Ca and Mg sites in dolomite and in associated calcite. The ESR spectra of Mn(II) in dolomite can be qualitatively divided into three types that have little overlap. Type 1 spectra have sharp peaks, and the partitioning of Mn into cation sites can be determined. Such spectra are common in stoichiometric and nonstoichiometric dolomites from both lithified and unconsolidated deposits (14 of 20 modern dolomites; 13 of 27 deep-marine dolomites). There is no apparent relation between Mn partitioning ratios and the absolute amount of Mn, the presence of a free radical center peak, or the total amount of dolomite in the sample. Modern dolomite, deep-marine dolomite, and nonstoichiometric Phanerozoic dolomites have average Mn partitioning ratios of 2, 5, and 6, respectively, suggesting that the ratios are not age dependent. Stoichiometric dolomites have an average partitioning ratio of approximately 30; thus, ratios and stoichiometry may be related. Type 2 spectra were observed in six of 20 modern dolomites and in 10 of 27 deep-marine dolomites. These spectra have broad peaks, and the Mg and Ca sites cannot be individually resolved. Because they are not found in older lithifield Phanerozoic dolomites, type 2 spectra may be related to lattice disorder. Type 3 spectra, observed in four deep-marine dolomites, do not have interpretable Mn peaks. A center peak assignable to radiation damage and/or free radicals may be present, independent of the Mn spectra. Age and thermal history data can be obtained from this peak.

Abstract Two hundred ninety carbonate rock samples from nearly all pre-Cenozoic periods, from widely scattered North American sites, representing all of Dunham's carbonate rock types, and containing 5%–100% ordered dolomite in the carbonate fraction were analyzed for a possible relationship between dolomite nonstoichiometry (expressed as mole percent CaCO 3 ) and pétrographie parameters and stratigraphie position. Dolomite composition ranged from 49.3 to 57.3 mole percent CaCO 3 , with modes from 50.0% to 52.0% and 54.0% to 56.0%. Data indicate that dolomite nonstoichiometry is not related to insoluble residue, porosity, rock type, percentage of crinoid fragments, recrystallization (dolomite crystal size), and percentage of dolomite in the carbonate fraction in dolomitized limestones. A general trend toward more nearly stoichiometric composition with age is obscured by local effects. Crystalline dolomites tend to be more nearly stoichiometric and less variable in composition (mode 50.0%–51.0% CaCO 3 ) relative to dolomicrites (modes 51.0%–52.0% evaporitic; 54.0%–55.0% nonevaporitic) and dolomitized lime mudstones (modes 51.0%–52.0% evaporitic; 55.0%–56.0% nonevaporitic). Evaporite-related dolomite is almost invariable near stoichiometric or calcium depleted. Open-marine and stoichiometric, evaporite-related dolomites are products of syndepositional dolomitization; nonstoichiometric nonevaporite dolomite is syndepositional to early diagenetic; and crystalline dolomite is middle to late diagenetic.