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Water-rock interactions in a modern coastal mixing zone

Carol M. Wicks, Janet S. Herman, Anthony F. Randazzo and Jonathan L. Jee
Water-rock interactions in a modern coastal mixing zone
Geological Society of America Bulletin (September 1995) 107 (9): 1023-1032

Abstract

The chemical evolution of ground water and the diagenetic history of the rocks in a modern saline-fresh-water mixing zone were investigated to test the hypothesis that this zone is a chemically active environment in which carbonate mineral diagenesis occurs. The field site was in the unconfined section of the upper Floridan aquifer system in west-central Florida, a regionally extensive hydrostratigraphic unit composed of Eocene-Oligocene limestone and dolomite. The mixing zone, marked by a steep concentration gradient, is >90 m thick at the study site in New Port Richey, Pasco County, Florida. Interpretation of aqueous geochemical data indicates that the coastal zone is most likely a location where three waters of distinct composition mix. A fresh Ca-HCO (sub 3) water that derives its composition from calcite dissolution and CO (sub 2) influx in the up-gradient recharge area, a Na-Cl water that results from salt-water intrusion, and a Ca-SO (sub 4) -rich water that upwells subsequent to contact with underlying gypsiferous beds of the Avon Park Formation are likely all converging in this coastal setting. Poor core recovery and an enlarged core hole provide evidence of dissolution. The ground water, however, is oversaturated with respect to calcite; thus, the large-scale dissolution features observed are unrelated to modern-day mixing. The mixing of Ca-SO (sub 4) -rich ground water in the coastal region led to oversaturation with respect to calcite. At the petrographic scale, there is little evidence of diagenesis related to the modern mixing zone. Calcite crystals from the lower section of the cored interval have isotopic signatures that are different than those from the upper section, suggesting that the latter has undergone more water-rock interactions. Calcium-rich dolomite, which occurred relatively late in the paragenetic sequence, has thin cathodoluminescent zonations but shows no evidence of subsequent alteration. The paragenetic sequence includes marine micritization and cementation; meteoric mineralogic stabilization; dissolution enhancement of porosity; phreatic, equant calcite cements; and echinoderm overgrowths, all typical of the region and not unique to the mixing zone.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 107
Serial Issue: 9
Title: Water-rock interactions in a modern coastal mixing zone
Affiliation: University of Virginia, Department of Environmental Sciences, Charlottesville, VA, United States
Pages: 1023-1032
Published: 199509
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 49
Accession Number: 1995-059758
Categories: General geochemistrySedimentary petrologyHydrogeology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 1 table, sketch map
N28°10'00" - N28°19'60", W82°49'60" - W82°40'00"
Secondary Affiliation: University of Florida, Gainesville, FL, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 199521

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