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 ^dgr¹³C_PDB (+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 ^dgr¹⁸O_SMOW= +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.

Table

End_of_Article - Last_Page 750------------