MUSGROVE, MARYLYNN, and JAY L. BANNER, Department of Geological Sciences, University of Texas, Austin, TX

ABSTRACT: Mixing and Evolution of Saline Groundwaters in the Midcontinent, USA: Implications for Carbonate Diagenesis

Regionally continuous carbonate strata in southeastern Kansas, southwestern Missouri and northern Oklahoma are predominantly marine carbonates and comprise portions of three adjacent large-scale flow systems. Groundwaters collected over the 40,000 sq km study area exhibit an extreme range in geochemical characteristics with salinities from 200 to 250,000 mg/l. (delta)D and (isotope){18}O values range from -106 to -5 o/oo and -14 to 3 o/oo (SMOW), respectively. {87}Sr/{86}Sr values vary from 0.7088 to 0.7166. Each flow system contains waters of markedly different origins, with distinct geochemical signatures. Fluid mixing processes between these three end members exert a fundamental control on regional groundwater geochemistry. This allows compositions of end member groundwaters associ ted with each flow system to be tightly constrained and independently evaluates hydrogeologic models. These pore fluids have significant implications for diagenetic studies of ancient carbonate aquifers.

Both end-member and intermediate (i.e., mixed) groundwaters are carbonate-saturated and mixing processes do not create a driving mechanism for water/rock interaction. Based on the carbonate saturation states of the groundwaters and evidence for limited extents of interaction with the carbonate aquifer rocks, modeled carbonate cements precipitated from these fluids will reflect both the extreme range of isotopic parameters exhibited by the waters and their gross differences from the aquifer rocks (e.g. measured {87}Sr/{86}Sr values for carbonate rocks range from 0.708-0.712).

The relationship between tectonic and hydrologic processes indicates that regional gravity-driven flow systematics may provide an effective mechanism for hydrocarbon migration and MVT ore deposition. The lowest (isotope){18}O values of saline groundwaters in this study reflect far traveled meteoric recharge from high altitude areas to the west of the study area in a gravity driven flow system. Modeled modem carbonate cements associated with these groundwaters may provide an analog for ancient carbonate cements with low (isotope){18}O values, which are common to carbonate sequences. Such low values (-19 to -14 o/oo PDB) are frequently attributed to cement precipitation during high temperature burial diagenesis or meteoric diagenesis during periods of cold climate. Alternatively, such l w (isotope){18}O cement values may reflect meteoric recharge from high altitude areas associated with a gravity driven flow system.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.