ABSTRACT: Water/Rock Interaction Efficiency and Seawater Dolomitization in the Eocene Avon Park Formation, Floridan Aquifer

Harris S. Cander

The Floridan aquifer has often been proposed as a system of extensive meteoric carbonate diagenesis and mixing zone dolomitization. However, the dominance of marine isotope (C, O, ⁸⁷Sr/⁸⁶Sr) and trace element (Sr, Fe, Mn) compositions in dolomites and limestones in the Eocene Avon Park Formation, Floridan aquifer, suggests that the very active low temperature meteoric groundwater system has, over the past 40 m.y., been an inefficient mechanism of diagenesis.

^dgr¹⁸O values of all but two replacement dolomites sampled range from +2.0 to +5.1 (PDB) with high Sr concentrations (90-325 ppm), indicating dolomitization by near-normal marine water involving no significant interaction with meteoric groundwater. The two ^dgr¹⁸O-depleted (0.0 ± 1) dolomites have low Sr concentrations (^sim100 ppm) suggesting limited recrystallization in meteoric water. Several dolomite samples have radiogenic ⁸⁷Sr/⁸⁶Sr compositions (0.70810-0.70883 ± 2), but have heavy

oxygen isotope compositions (> +2.0) and high Sr concentrations (>200 ppm) suggesting precipitation from cold Miocene age or younger seawater that circulated through the Florida platform.

Most limestone stable isotope compositions cluster around marine values (^dgr¹⁸O = -1 to +1, PDB; ^dgr¹³C = +0.5 to +2.5) and have Eocene seawater ⁸⁷Sr/⁸⁶Sr compositions (0.70775 ± 2 to 0.70779 ± 2) with 400 to 500 ppm Sr. Isotopic compositions of limestones from the east coast of Florida are all within these ranges. Only some limestones from central Florida and the west coast contain depleted stable isotopic compositions and low Sr concentrations. The sample with the most depleted stable isotope values has a radiogenic ⁸⁷Sr/⁸⁶Sr composition (0.70870±2), suggesting that diagenetic meteoric water migrated through Miocene or post-Miocene strata. The higher degrees of meteoric water interaction in central an western Florida correlate with higher present-day transmissivities in the western half of the aquifer.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990