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Abstract
Florida’s eogenetic karst development is influenced by groundwater flow, sea level and water table position, sediment mineralogy and fabric, porosity, chemical reactions and geochemical saturation state of the water, residence time, and sediment surface area.
Primary porosity such as intergranular porosity, burrows and borings, intraparticle porosity, and shelter porosity can initially affect the pathways of water flow. Secondary porosity includes potentially extensive, interconnected, moldic porosity, and karstic secondary porosity includes caves and other karst conduits. Diagenetic processes, such as cementation of limestones and dolomitization, may destroy primary and secondary porosity.
Fractures or existing karst conduits provide fast flow routes along which karstification occurs. Photolineament analysis can, with ground truthing, confirm the existence of vertical fractures and faults in basement or overlying rocks. In Florida, photolineament sets occur in two main alignments that are consistent with earth tides.
Bedding planes are often poorly represented but can be dissolutionally enlarged. Epikarst consists of irregular, weathered surfaces exhibiting enhanced porosity and permeability, with cutters, pinnacles, and limestone fragments. Epikarst at unconformities may not relate to sinkhole development.
Reaction rates of groundwater with the carbonate rock are typically slow, and sudden collapse events are usually due to failure of cover materials rather than of the roofs of voids in limestone.
Florida’s epigenetic caves form near the water table when the phreatic surface is stable. In most systems, vadose caves form concurrently with and up gradient from phreatic caves. Many caves reveal evidence of water-table fluctuations driven by climate and sea-level changes.
Some Florida caves have been attributed to mixing-zone dissolution, including caves with large rooms at depth. Evidence is relatively weak for sulfate dissolution-related hypogenetic karst. The dependence of epigenic and freshwater/saltwater mixing zones on sea level creates complex layering of caves with different positions and ages.
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It is poor practice to assume that single closed depressions that are elongated have obtained their orientations by fractures since sinkholes tend to develop multiple, nearby depressions and cover materials that are washed into depressions may be affected by drainage, land slope, or other factors.
Soil tones are anomalous color or darkness variations observed on the ground or in aerial photography. Often the soil tones are darker than surrounding areas because the soils have higher moisture content. Photolineament indicators are linear strips or aligned spots of darker (or lighter) soil.
Marine terraces are the accumulations of sediment that resulted from a Plio-Pleistocene high sea stand. In Florida, the terrace deposits are typically quartz sand. Wave-cut scarps, relict aeolian dunes, and relict sand bars often mark the shorelines that formed during these high sea stands.
The ages of corals and other fossils in the Miami Limestone and equivalent strata have been shown to be 127,000 to 116,000 years before present. We generalize the age of the Miami Limestone, therefore, at about 125,000 years. This age is equivalent to marine isotopic stage 5e based on oxygen isotope data.
The age and relationship of the Silver Bluff wave-cut exposures to Pleistocene/Holocene sea levels in southern Miami and Coconut Grove (Dade County) is poorly known.
Since the age of the Miami oolite has been dated to isotopic marine stage 5e, this date appears too old for the Silver Bluff erosional episode. The “bluff” is clearly younger than the oolitic facies of the Miami Limestone and early karstification, and it must have developed after substantial lithification of the oolite had occurred.
The investigations by Missimer (1973) and Mitchell-Tapping et al. (1998) are based on sediments observed in southwestern Florida where the post-Pleistocene sea-level record is better preserved in the stratigraphic record.
Note that losses of drilling fluid circulation during drilling reflect openings in the host sediment large enough to accept a somewhat viscous fluid, but they may not be large enough to accept a volume of raveled overburden sediment or rock collapse material. Studies that report losses of drilling fluid circulation as representing voids, make no distinction as to whether the opening is a void that might participate in sinkhole development or a pore system too small to accept sediment and cause sinkhole development. Such usage is problematic because it is likely to exaggerate the importance and size of the pore or void space that represents the loss of drilling fluid circulation.