Effect Of Pore-Water Composition

Abstract Introduction Geologic Setting Stratigraphic Setting Porosity Determination Data Set Porosity vs. Depth Depth vs. Age Effect of Pore- Water Composition Limestone & Dolomite Porosity Summary References

A widespread change in depositional conditions in south Florida occurred between Eocene and Paleocene time. Evaporites, common in pre-Eocene strata (except chalks of Gulfian age), are very limited in Eocene and younger rocks (Puri and Winston, 1974). To the extent that evaporates reflect saline conditions, depositional environments became less saline in the Eocene and remained less saline. Today, Eocene and younger carbonate rocks contain fresh- and brackish-water aquifers in much of south Florida, and may have contained similar waters during periods of emergence since the Eocene.

Theoretical and experimental evidence indicates that resistance to pressure solution and, thus, to loss of porosity with depth increases as the magnesium content of the pore water increases (Neugebauer, 1973, 1974; Folk, 1974; Scholle, 1977).

Thus, rocks of Eocene age and younger in south Florida might be expected to show an accelerated loss of porosity with depth if they contained, on the average, fresher pore waters with less magnesium than older carbonate rocks. It appears from Figure 8 that the rate of porosity decrease with depth of Eocene and younger rocks is indeed faster than expected when compared to Paleocene and older rocks. Thus, the general line of evidence is internally consistent and supports the idea that pressure solution in shallow-water carbonates is inversely related to the magnesium content of pore waters.

An alternate hypothesis is that Eocene-age and younger rocks of south Florida form a closed system, sealed at the base by Paleocene anhydrites, in which carbonate is leached near the surface by fresh waters and precipitated at depth. The strength of this argument depends in part on the assumption that a near-surface porosity of 45% (Fig. 8) is exceptionally high because of leaching. However, we favor the idea that the near-surface porosity of 45% is representative of the normal transition from unconsolidated to consolidated sediments (Halley and Beach, 1979; Halley and Harris, 1979) and does not support the concept of mass transport from the surface to depth by freshwater circulation.

Fig. 8 - Exponential porosity-depth curves for carbonate strata of Eocene age and younger and Paleocene age and older, showing a faster than expected porosity decrease with depth for Eocene age and younger rocks. [larger image]

Go back to Depth vs. Age | Go ahead to Limestone & Dolomite Porosity