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The Making and Unmaking of Limestones or the Downs and Ups of Porosity
Gerald M. Friedman
In modern unconsolidated carbonate sediments the volume of pore space may exceed that of sedimentary particles. During lithification calcium carbonate is introduced from the outside. Surprisingly compaction accounts for only little reduction in pore space. In cementation the waters may be (1) percolating freshwaters on surface or in subsurface, (2) seawaters, and (3) formation waters.
Percolating freshwaters may affect carbonate sediments as the generally subsiding sea floor at times rises ("yo-yo tectonics") or as sea level falls ("yo-yo oscillations") or as freshwaters circulate in the subsurface through carbonate sediments. In the phreatic zone (below the groundwater table), especially in moist climatic belts, the original pore space between particles as well as any secondary moldic pore space fill with cements of various fabrics. As carbonate sediments in the phreatic zone are converted to limestone, they not only lose their pore space and become tight, but they may acquire, as their depositional texture vanishes, a xenotopic and poikilotopic fabric of recrystallization. Phreatic diagenesis, however, may also follow a different sequence: reefs micritize beyond ecognition; geologists mistake such reefs for "lime mud." Phreatic diagenesis, however, does not always lead to a lack of porosity; carbonates, including reefs, may "chalkify" into porous micron-size, soft, friable calcite.
In the vadose zone, (above the groundwater table) which in arid regions may be thousands of feet thick, the pores remain largely open. In fact secondary pores may develop, and stay open, as the aragonite of ooids or of "critters" is leached to form oo- or "crittermolds." Porous strata as products of vadose diagenesis may occur interbedded, as cyclic sequences, with tight strata of phreatic diagenesis.
Cements in limestones may also be derived from the effects of dissolution which stylolitization provides.
On the sea bottom cementation may be related to bacterial activity. By such activity organic matter can yield methane or other gases that oxidize on the sea floor and are effective in generating bicarbonate ions which then combine with calcium to deposit a a cemented calcium-carbonate deposit. Sulfate-reducing bacteria use sulfate from seawater as an oxidant for that part of organic matter which they oxidize for energy production. Calcium carbonate as a cement is formed when CO2 produced in the bacterial oxidation of organic matter combines with calcium ions. In the subsurface this reaction involving formation waters is probably even more important than in seawater and may be the missing link in our explanation of the source of calcite cement. In reefs photosynthesis and re piration of the biomass cause a shift in the bicarbonate buffer system of seawater with the uptake of CO2. In this process micro-levels of pH 10 and even 10.5 may be reached and maintained in thin jell-like or monomolecular layers. Such high pH levels trigger the precipitation of carbonate cements. In response to increasing salinities during periods of falling sea level carbonate cements likewise are precipitated.
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