Maturation of regional dolomite bodies in the Late Eocene Ocala limestone and Early Oligocene Suwannee limestone, west-central Florida: Processes and effects.

摘要

Comparisons of Neogene and Paleozoic dolomites suggest that dolomites in general undergo a maturation process as they undergo diagenetic alteration during both near-surface and burial processes. They evolve from a porous initial dolomite ("immature") to an indurated, nonporous and impermeable dolomite ("mature"). This maturation also typically involves a coarsening of crystal size and the addition of dolomite cements and recrystallization products.; Most studies of dolomite maturation focus on either the Neogene or Mesozoic/Paleozoic end members of the maturation process. The study of Paleogene dolomites that are intermediate between the younger and older end members provides a more thorough understanding of the maturation process(es). Dolomite in the Late Eocene Ocala and Early Oligocene Suwannee Limestones of west-central Florida are one such example of dolomite bodies that are in the process of maturing. Two types of dolomite are present in the Suwannee and Ocala dolostones. Initial, matrix dolomite, which replaced the limestone precursor, is dully luminescent. Secondary, luminescent dolomite is present as cement in the Suwannee dolostones, and as recrystallized cores of matrix crystals in the Ocala. In the Suwannee, this cement lines fractures and moldic pores, and in some cases, fills matrix pore space.; Trace element (Na, Sr, Fe, and Mn) and isotopic data (delta 18O and 87Sr/86Sr) suggest that a freshwater-seawater mixing zone associated with an early Late Miocene sea-level fluctuation was the origin of the secondary luminescent dolomite, and near-normal seawater in the Late Oligocene was the origin of the initial matrix dolomites. The luminescent dolomite, when present as cement, evolves the preexisting dolomite to a more mature rock by reducing porosity and permeability in both indurated and sucrosic dolomites. The abundance of dolomite cement is a function of the preexisting porosity, which is a direct function of the precursor limestone facies.; Results of triaxial strength tests on the indurated and sucrosic dolomites indicate elevated cohesion magnitude and tensile strength as the degree of induration (maturation) and moldic porosity increase. Sucrosic dolomites are likely to fracture due to their low cohesion strength, as are indurated and moderately indurated dolomites with high moldic porosity.