Chemical Diagenesis of Pennsylvanian Brush Creek (Pennsylvania) Carbonate Components: Trace Elements: ABSTRACT
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The various (low-Mg calcite, intermediate-Mg calcite, high-Mg calcite, and aragonite) carbonate components from the Pennsylvanian Brush Creek Formation of Pennsylvania are preserved in different stages of diagenetic alteration. In general, these components follow the predicted diagenetic changes in structure, mineralogy, and chemistry deduced from theoretical considerations for progressively altered carbonates. The low-Mg calcite brachiopods show no signs of either structural or chemical alteration. The shell material is preserved as low-Mg calcite fibers with no apparent dissolution and/or infilling by diagenetic calcite. Also, the average Sr2+ content of the brachiopods is 820 ppm, which is in agreement with the chemical content of their Holocene counterparts. In contrast, the intermediate-Mg calcite rugose corals show signs of structural aggrading neomorphism. The trabecular fibers are in part recrystallized to small mosaic calcite grains. This structural alteration is concomitant with chemical changes in the Brush Creek rugose corals. The least-altered components contain about 1,770 ppm Sr2+, whereas the most altered components contain only about 1,030 ppm Sr2+ /SUP>. For the high-Mg calcite crinoids, diagenetic alteration is mostly a cementation process with minor mineralogical alteration. The open meshwork structure typical of the Echinodermata is infilled in the Brush Creek crinoids by diagenetic cement. This infilling cement has decreased the average Sr2+ content of 2,140 ppm of unaltered crinoids to that of 1,090 ppm Sr2+ for the most-altered Brush Creek crinoids. Scanning electron microscope analysis of the originally aragonitic mollusks (gastropods, pelecypods, and cephalopods) shows a complete structural diagenetic transition series. The original and least-altered mollusk material is preserved as nacre, which is always aragonite. The second phase of the transition series in the mollusks is represented by the aggrad ng neomorphism of the nacreous tablets into small, coarse mosaic calcite crystals. The structural transition is completed by the replacement of the mosaic calcite by coarse calcite spar. Mineralogically, the mollusk material changes from aragonite to aragonite-calcite to calcite, relative to the least- and to the most-altered specimens, respectively. The structural and mineralogical changes of the originally aragonitic mollusks are also confirmed by changes in their overall chemical composition. Average Sr2+ values measured for the least-altered Brush Creek mollusk material is 4,470 ppm. Mollusk material of the second phase of the diagenetic transition series contains on average about 2,170 ppm Sr2+. The most-altered mollusks, which are calcite, contain on average a out 1,110 ppm Sr2+. Similarily, the Na+ values follow the diagenetic trend of strontium. The least-altered material contains 750 ppm Na+, the intermediate-altered material contains 420 ppm Na+, and the most-altered material contains 250 ppm Na+. Thus the diagenetic alteration process and rate proceed in accordance with mineralogical stability. This sequence is aragonite, high-Mg calcite, intermediate-Mg calcite, and low-Mg calcite relative to fastest to slowest reaction, respectively. The diagenetic alteration and preservation process of the Brush Creek carbonate components is probably a two-stage event. The first stage occurs in the marine phreatic zone, and the second stage occurs in the meteoric phreatic zone. End_of_Article - Last_Page 520------------Carbonate diagenesis is an important branch in carbonate sedimentology,especially in carbonate reservoir sedimentology.Recently,there have been remarkable advances on carbonate diagenesis in the following aspects: 1) the geometry and petrogenesis of dolomite hydrocarbon reservoir,particularly referring to the review and reappraisal on dolomitization models cited by many sedimentologists for a long time;2) the mechanism of structurally controlled hydrothermal alteration of carbonate reservoirs,concerning the origin of hydrothermal dolomite and its effects on hydrocarbon reservoir,especially the influences of hydrothermal fluids on carbonate diagenesis in deep-burial setting;3) renewals of classic diagenesis theories of carbonate and challenges to past diagenesis theories that people have been using until now,some new sights of diagenesis in marine-meteoric mixing-zone and marine-burial environments should make us to reappraisal the understanding of carbonate diagenesis;4) wide application of strontium isotope composition to marine carbonate sedimentology,including isochronous correlation of sedimentary sequences,directly dating marine sediments,and interaction of water-rock during diagenesis.However,some following questions existing in the carbonate diagenesis need us to pay more attentions to: 1) the most important dissolving fluids to carbonates are probably H2S and CO2 as byproducts of sulfate reduction in deep-buried setting with sulfate minerals,but carbonates are more soluble in relatively low temperature,which is socalled retrograde solubility;2) a temperature difference(ΔT) for cooling water to dissolve carbonates can be implemented by following two geological processes: ① fluids flowing upward along faults,showing the importance of faults in carbonate diagenesis;② tectonic uplift,suggesting the significance of burial history related to tectonics in carbonate diagenesis;3) the first thing controlling Sr content in dolomite is the crystalline chemistry habits,lower distribution coefficient of Sr into dolomite results in pervasively lower Sr content in dolomite,thus,people can not only depend upon contents of Sr in dolomite to evaluate the characteristics of dolomitization fluids,and a lower content of Sr in dolomite can not indicate that dolomitization fluid is irrespective to seawater;4) remove of SO4~(2-) related to sulphate reduction may place a premium on the dolomitiztion or precipitation of dolomite,and the presence of SO4~(2-) in fluid may increase the rate of dolomite dissolution.
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This research is to assess the influence of regional versus global controls on the diagenesis and reservoir quality of Tertiary carbonate platforms. The three main objectives are: (1) to evaluate controlling influences on diagenesis, fracturing and potential reservoir quality of a Cenozoic syntectonic carbonate platform (Tonasa Formation, Indonesia). (2) To contribute towards understanding diagenetic alteration of volcanogenic covered and/or influenced carbonate platforms. (3) To evaluate controlling influences on diagenetic variability of slope and basinal deposits.
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This chapter contains sections titled: Introduction Carbonate diagenesis Sequence stratigraphy Carbonate diagenesis and relative sea-level falls (LST) Carbonate diagenesis during relative sea-level rises (TST) Carbonate diagenesis during relative sea-level highstands (HST) Carbonate diagenesis on the parasequence scale Carbonate diagenesis and sequence stacking patterns Carbonate diagenesis and role of associated siliciclastics Carbonate diagenesis through the Phanerozoic: broad trends and the first-order sea-level curve Carbonate diagenesis and sequences through the Phanerozoic Summary Acknowledgements References
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Fusulinacean foraminifera have been described from several localities and horizons within the Upper Pennsylvanian and Lower Permian of the North American midcontinent (Beede, 1916; Dunbar and Condra, 1927; Skinner, 1931; Newell, 1934; Newell and Keroher, 1937; Merchant and Keroher, 1939; Dunbar and Henbest, 1942; Burma, 1942; Thompson, 1954, 1957; Thompson et al., 1956; Douglass, 1962; King, 1988; Sanderson and Verville, 1988), but surprisingly, a comprehensive account of fusulinacean biostratigraphy of the stratotype of the Upper Pennsylvanian Virgilian Stage has never been published. This report summarizes fusulinacean biostratigraphic data from the Virgilian section in the stratotype region of southeastern Kansas. The publication of this dataset is particularly important because of the current efforts to standardize global Pennsylvanian chronostratigraphy (Metcalfe, 1997), and related efforts and debates concerning the definition and correlation of stages in the Upper Pennsylvanian and Lower Permian succession of North America.
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