The role and impact of freshwater–seawater mixing zones in the maturation of regional dolomite bodies within the proto Floridan Aquifer, USA

Abstract

Palaeogene dolostones from the sub-surface of Florida are ideal for the study of dolomite maturation because they record the early stages of a secondary dolomite overprint without destruction by later diagenetic overprints. Two distinct dolomite textures occur in the dolostones of the Upper Eocene Ocala and Lower Oligocene Suwannee limestones in west-central Florida: a porous and permeable sucrosic dolomite and a less porous and relatively impermeable indurated non-sucrosic dolomite. In both textures, the initial matrix dolomite is dully luminescent, whereas the secondary overprint is dominantly luminescent cement in the Suwannee and only neomorphic luminescent dolomite in the Ocala. The abundance of luminescent dolomite ranges from 2% to 38%, which translates to 1AE6 km 3 of material in the Suwannee and 13AE5 km 3 in the Ocala. Extrapolated trace-element contents (Sr and Na) and d 18 O values for the matrix and luminescent end-members indicate a marine origin for the matrix dolomite in both units, and a freshwater-seawater mixingzone origin for the secondary luminescent dolomites. The d 18 O values indicate that a saline, middle mixing-zone environment overprinted the Suwannee but a more dilute mixing zone affected the Ocala. Fluid-fluid mixing models constrained by modern Floridan aquifer hydrochemistry and extrapolated 87 Sr/ 86 Sr values of the luminescent phases indicate that the mixing zones operated during the Late Miocene to Pliocene in the Ocala and affected the Suwannee in the Pliocene. The luminescent Suwannee mixing-zone cement reduced porosity up to threefold and permeability up to 100-fold, which converted many sucrosic dolomites to indurated dolomites. By contrast, the neomorphic luminescent Ocala dolomite did not have an appreciable impact on the maturations. Although freshwater-seawater mixing zones were not the sites of the initial dolomitization, the mixing-zone environment did dramatically overprint and mature the regionally widespread dolomites of the Ocala and Suwannee limestones. This maturation occurred shortly after formation of the proto-Floridan aquifer; the timing suggests the matrix dolomites were 'ripe' for alteration and that the only prerequisite for mixingzone dolomite is pre-existing dolomite substrates to reduce kinetic barriers. In contrast to recent claims, the results of this study demonstrate that mixing zones can be effective in forming regionally significant amounts of secondary dolomite and influencing the petrophysical maturation of dolomite bodies.

Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk, Switzerland

“…Sharp CL and UV‐fluorescence zonations indicate multiple dolomite generations and repeated crystallization in reducing fluids with different geochemistries and organic compounds. The overprinting mottled CL texture suggests that the dolomites were recrystallized (Kupecz et al ., ) or formed in mixed fluids (Gaswirth et al ., ). In contrast, the distinct fluorescent–non‐fluorescent zonations suggest that dolomites were indeed not recrystallized.…”

Shallow‐burial dolomite cement: a major component of many ancient sucrosic dolomites

“…(1993). Recently, Gaswirth (2004) (see Gaswirth et al. , 2007) used petrographic digital‐image analysis (PIA) and digital backstripping to evaluate the roles of dolomite cements in low‐temperature dolomites in the Ocala and Suwannee formations of the Upper Floridan aquifer (UFA) in west‐central Florida.…”

“…, 1994). Dolomite cement in the Avon Park dolomites has been reported to date in a single interval of one well (Cander, 1994), but could be more widespread according to Gaswirth (2004) and Gaswirth et al. (2007).…”

“…The UFA is reported to be the largest in the United States (Miller, 1986), and involves several Palaeogene and Neogene stratigraphic units totalling hundreds of metres in thickness that underlie most of Florida and southern Alabama, Georgia and South Carolina (Gaswirth, 2004, after Miller, 1986; Gaswirth et al. , 2007).…”

“…, 1994a,b; Choquette & Hiatt, 1995; Jones & Luth, 2002; Kyser et al. , 2002; Gaswirth, 2004; Jones, 2004, 2005; Gaswirth et al. , 2007).…”