Pore networks in Holocene carbonate sediments

Permeabilities of Holocene carbonate sediments from Florida and the Bahamas, determined for 74 samples by water flow rates in a "variable-head" permeameter, range from 0.6 to 57,000 millidarcies. Porosities range from 40 to 78 percent. These parameters are related to depositional texture as follows: Table The muddiest sediments (i.e., the finest-grained) have the highest porosities but lowest permeabilities; this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks, even as young as Pleistocene. High porosity and low permeability in sediments show a strong correlation with percentage of fines (<62 mu m). From capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 mu m in diameter, at least after drying. Measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation; the calculated rates would require excessive time to produce the degree of cementation seen in Late Pleistocene rocks of Florida and the Bahamas. The amounts of rainfall and evaporation are the most important factors in the degree of cementation.