--> ABSTRACT: Geochemistry of Calcite Cements from the Middle Ordovician of S. Appalachians: A Record of Fluid-Rock Interaction, by Suvrat Kher, W. C. Parker; #91020 (1995).

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Geochemistry of Calcite Cements from the Middle Ordovician of S. Appalachians: A Record of Fluid-Rock Interaction

Suvrat Kher, W. C. Parker

Sparry calcite cements are the major void-filling phase in the Middle Ordovician (foreland basin) Chickamauga limestone of NW Georgia. The cement paragenesis, combined with regional stratigraphic data and hydrologic considerations imply basin-scale fluid-flow early in the burial history of these strata. Here we explore the applications and limitations of commonly used geochemical tracers (^dgr18O, ^dgr13C, Mg+2) in studying fluid-rock interaction and interpret the paleohydrology of the basin at the end of the Ordovician period.

Values of ^dgr18O (-5.02 to -8.42^pmil) are depleted relative to Ordovician marine brachiopods (-4 to -5^pmil) indicating at least a component of meteoric waters in cementation. Although ^dgr18O shows no vertical or geographic trends, pore central cements are systematically depleted (2^pmil) relative to pore marginal crystals. Values of ^dgr13C (.08 to -1.47^pmil) are relatively invariant. This probably represents a state of dynamic equilibrium with marine carbonate, which was quickly reached in the absence of any significant (negative) soil-derived carbon due to absence of terrestrial vegetation in the Ordovician. The distribution of Mg+2 is apparently erratic. Irregular complex zoning is seen in stained samples, and analysis of microprobe raverses suggests large-scale disequilibrium fractionation of Mg+2. Intra-sample variations are comparable with variations between samples, and the utility of this element in interpreting progressive interactions along fluid-flow pathways appears to be limited in this case. Cross plots of ^dgr18O vs Mg+2 show large scatter and cannot be used to interpret trends in fluid-rock interaction. The range of ^dgr18O however suggests extensive fluid-rock interaction and the more enriched values, which approach those of marine carbonate, suggest buffering of fluid by the host rock. Depletion of pore central cements is probably a result of progressive stabilization of metastable carbonate and isolation of pore fluids from the host rock by earlier deposite crystals.

We interpret the ^dgr18O values and a lack of non-luminescent zoning in these cements to indicate precipitation in the distal portions of a subsurface aquifer. Conditions for generating large-scale fluid flow were met during the Late Ordovician basin-margin uplift to the southeast, with orogenic elevation providing the drive for recharging meteoric waters to circulate into the basin and cause widespread cementation.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995