An Example of Mixing-zone Dolomite, Middle Eocene Avon Park Formation, Floridan Aquifer System

Abstract

ABSTRACT A late-formed dolomite cement in a core of the Middle Eocene Avon Park Formation, peninsular Florida, provides an example of dolomite cement from a mixing zone and illustrates how dolomite textural alteration and stabilization can occur at earth-surface conditions. The Avon Park Formation is a pervasively dolomitized peritidal platform carbonate 400 m thick in the Floridan aquifer system. Typical Avon Park dolomite is inclusion-rich, fine-grained (< 40 mm), noncathodoluminescent, highly porous (average, 20%), and formed during the Eocene by normal to hypersaline seawater (18O = + 3.7 PDB; 13C = + 2.0; 87Sr/86Sr = 0.70778; Sr = 167 ppm). In a 20 m interval in a core from southwest Florida, inclusion-free, cathodoluminescent dolomite overgrows the early-formed noncathodoluminescent marine dolomite. The cathodoluminescent dolomite cement profoundly alters the texture of Avon Park dolomite from typical Cenozoic-like porous, poorly crystalline dolomite to hard, dense, low-porosity, highly crystalline Paleozoic-like dolomite. The dolomite cement is not a replacement of limestone but an overgrowth of early-formed marine dolomite and pore-occluding cement. From analyses of samples containing both the early marine dolomite and the late dolomite cement, quantitative estimates of pure cathodoluminescent dolomite cement are: 13C = -0.5 and 18O = +1.7; 87Sr/86Sr = 0.7085; Sr = 225 ppm. The stable-isotope compositions are intermediate between Avon Park marine dolomite and dolomite predicted to be in equilibrium with Floridan aquifer freshwater (estimated 18O = 0 to + 0.5, P B), indicating the dolomite precipitated from waters intermediate in composition between seawater and freshwater. Due to the rise in seawater 87Sr/86Sr since the Middle Eocene, the 87Sr/86Sr composition of the cathodoluminescent dolomite cement demonstrates that it must have formed after the Middle Miocene (incorporating radiogenic Sr from seawater or rocks of Middle Miocene age or younger). A water sample was taken from this same core from an interval in the Avon Park Formation in the dilute part of the modern mixing zone, approximately 60 m above the first occurrence of altered dolomite. Fluids from this interval have 3400 ppm TDS and are a mixture of 94% Floridan aquifer freshwater and 6% normal seawater. These pore fluids are Sr-rich (24 ppm) and SO4-rich (755 ppm) due to extensive Avon Park gypsum dissolution and have a 87Sr/86Sr composition (0.7078) in equilibrium with Avon Park host rock. The salinity of the fluid that precipitated the cathodoluminescent dolomite cement is quantitatively calculated with a fluid-fluid mixing model using (1) the Sr concentration (24 ppm) and 87Sr/86Sr composition (0.7078) of the von Park pore fluid sample from the core, (2) the Sr concentration (8 ppm) and 87Sr/86Sr compositions of Modern (0.7092) and Late Miocene (0.7089) seawater, and (3) the 87Sr/86Sr composition (0.7085) of the cathodoluminescent dolomite cement. These calculations indicate that the dolomite cement precipitated from a mixed marine-meteoric fluid with salinity approximately 75% seawater. This same fluid would be both calcite supersaturated and contain about 12 ppm Sr. Such a fluid could precipitate dolomite cement with 225 ppm Sr using a reasonable Kd = 0.05; this accounts for the higher Sr concentrations in the mixing-zone dolomite cement (225 ppm) relative to Avon Park marne dolomite (167 ppm). This study demonstrates that: (1) dolomite precipitated from a 75% seawater mixing-zone fluid that was both calcite saturated and sulfate-rich, and (2) dramatic textural maturation and stabilization in dolomite can occur in the near surface environment, without elevated temperature and burial conditions.