ABSTRACT: Late-Stage Diagenesis in the Floridan Aquifer, Middle Eocene Avon Park Formation

CANDER, HARRIS, Amoco Production Company, Houston, TX

The middle Eocene Avon Park Formation is a partly dolomitized peritidal limestone whose gypsiferous middle portions separate the upper and lower Floridan aquifers. Geologic and geochemical data indicate that pervasive dolomitization was caused by normal to hypersaline Eocene seawater (dolomite: Oxygen 18 = +3.7o/oo, PDB; Carbon 13 = +2.0o/oo; Strontium 87/Strontium 86 = 0.70778). Avon Park limestones have retained most primary porosity as well as near original marine isotopic compositions (Oxygen 18 = -0.4o/oo, PDB; Carbon 13 = +2.0o/oo; Strontium 87/Strontium 86 = 0.7078). Frequent meteoric water saturation over the past 40 Ma has resulted in localized carbonate dissolution in the Avon Park, yet precipitation of late-stage diagenetic phases has involved little interaction between Avo Park pore fluids and limestone or dolomite host rock. Gypsum dissolution continues to be an important porosity-modifying reaction.

Near-recharge meteoric Avon Park pore fluids are Oxygen 18-depleted, Sr-poor (<20 ppm) and in Sr disequilibrium with the host rock (fluids: Strontium 87/Strontium 86 = 0.7085). These meteoric fluids precipitated late-stage (<10 Ma) calcite cement whose composition reflects limited water-rock interaction (Calcite: Oxygen 18 = -3.3o/oo, PDB; Carbon 13 = -7.2o/oo; Strontium 87/Strontium 86 = 0.7087; Sr <20 ppm).

Downflow coastal mixing-zone pore fluids are in Strontium 87/Strontium 86 equilibrium with the host rock (0.7078), but excess Ca and SO4 in these waters indicate that the dominant reaction is dissolution of gypsum, not limestone or dolomite. Late-stage gypsum dissolution is enhancing porosity and improving communication between the upper and lower Floridan aquifers. More saline mixing-zone pore fluids (75% seawater) precipitated late-stage (<10 Ma) dolomite cement as an overgrowth of early-formed dolomite but did not cause dolomitization of limestone. This late dolomite cement results in dramatic textural alteration that is not the result of recrystallization of host carbonate rock.

This study illustrates that late-stage calcite and dolomite precipitation can involve very limited interaction with the host rock carbonate. The host carbonate serves as a nucleation template but the isotopic and elemental compositions of the late phases are either fluid dominated or can be determined by accessory minerals, such as soluble evaporites, in the carbonate formation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)