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 (delta 18 O = + 3.7 per thousand PDB; delta 13 C = + 2.0 per thousand ; 87 Sr/ 86 Sr = 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: delta 13 C = -0.5 per thousand and delta 18 O = +1.7 per thousand ; 87 Sr/ 86 Sr = 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 delta 18 O = 0 to + 0.5 per thousand , PDB), indicating the dolomite precipitated from waters intermediate in composition between seawater and freshwater. Due to the rise in seawater 87 Sr/ 86 Sr since the Middle Eocene, the 87 Sr/ 86 Sr 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 SO 4 -rich (755 ppm) due to extensive Avon Park gypsum dissolution and have a 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr composition (0.7078) of the Avon Park pore fluid sample from the core, (2) the Sr concentration (8 ppm) and 87 Sr/ 86 Sr compositions of Modern (0.7092) and Late Miocene (0.7089) seawater, and (3) the 87 Sr/ 86 Sr 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 K d = 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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.