Calcite Cements in the Modern Floridan Aquifer
HAMMES, URSULA, and D. A. BUDD, University of Colorado, Boulder, CO
Calcite cements in the Ocala (Eocene) and Suwannee (Oligocene) formations, southwestern Floridan aquifer have been studied to determine updip to downdip variations in cement chemistries and cathodoluminescence within a modern regional confirmed aquifer. These rocks experienced unconfined conditions throughout the Eocene and Oligocene. The current confined aquifer conditions probably developed at the end of the middle Miocene.
Interparticle, intraparticle, and fracture-fill cements comprise 5-15% of the limestones. Five different calcite cement morphologies are distinguishable and occur throughout the aquifer: (1) circumgranular microspar, (2) fine- to medium-crystalline rhombs, (3) medium-crystalline syntaxial overgrowths on echinoderms, (4) fine-cystalline pore-filling mosaics, and (5) micrite. Type 5 occurs only below former exposure surfaces. Volumetrically, type 3 is the most important and type 4 is the least.
Cathodoluminescence observations reveal only nonluminescent cements updip and an increase in luminescent zones and luminescent intensity downdip. Updip nonluminescent cements have very low Fe and Mn concentrations, but high Mg and Sr concentrations. These relations are interpreted to reflect oxidizing conditions and high rock/ water interaction. Fe and Mn concentrations increase and Sr and Mg contents decrease downdip. These trends are interpreted to reflect reducing conditions, cross-formational flow, and slower rock/water interaction. Downdip cathodoluminescence zonations consist of a broad nonluminescent zone (Fe: 90-450 ppm, Mn < 130 ppm), followed by a the bright orange zone (Mn: 90-230 ppm, Fe: 90380 ppm), and then a dull luminescence zone (Fe and Mn < 90 ppm).
These geochemical and luminescent patterns along a regional flow line in the confirmed Florid;m aquifer have many similarities to those observed in calcite cements described from ancient aquifers.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)