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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Petrography and cement stratigraphy of four regionally extensive cement zones in Mississippian crinoidal limestones indicate that these cements precipitated in meteoric phreatic environments. Each major zone has a distinct isotopic and trace element composition.
Marine cements and lime muds in bioherms associated with the crinoidal sands have marine ^dgr13CPDB (+4.0 ppm). Phreatic cements become lighter in ^dgr13C in progressively younger zones 1, 2, and 3, representing a trend toward more contribution of organically derived carbon to precipitating waters. Since zones 1, 2, and 3 were all precipitated at shallow burial depths, their trend toward lighter ^dgr18O with decreasing age suggests increasingly light waters isotopically. At 25°C, the waters responsible for zone 1 and 3 cements are estimated as ^dgr18OSMOW= +1.5 and -0.9 ppm, respectively. The ^dgr13C of zone 5 cement is interpreted as a combination of rock-derived and organic-derived carbon, some of whic probably came from overlying Pennsylvanian strata. The distinctively light ^dgr18O of zone 5 is attributed to precipitation at somewhat elevated temperatures, averaging about 45°C, a value in agreement with estimated burial depths.
Zone 1 cements appear to have formed in seaward parts of a freshwater phreatic system at shallow burial depths (relatively heavy ^dgr180). Zone 1 magnesium and carbon were derived from dissolution of skeletal high-Mg calcites (high Mg and heavy ^dgr13C). The driving force for zone 1 cementation was thus the solubility difference between high-Mg crinoidal calcite and low-Mg zone 1 calcite. Zone 2 also precipitated at shallow depths but in a more widespread groundwater system that contained some organic carbon (light ^dgr13C) and had a more landward recharge area (lighter ^dgr18O). Likewise, zone 3 is formed in a shallow-phreatic lens but with the most landward (freshest) recharge (lightest ^dgr18O). The inferred importance of organ c-derived carbon in zone 3 (light ^dgr13C) suggests degassing of CO2 as a driving force for precipitation. The light ^dgr18O of zone 5 reflects precipitation at elevated temperatures of deeper burial (750); its element composition (table) suggests a variety of intraformational and extraformational sources.
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