9.2 CARBONATES
413
A second type of vadose diagenesis occurs near the shore in the intertidal and supratidal spray zone, where sediment pores are intermittently flushed, not by freshwater,
but by seawater. As this evaporates in the pores the salinity increases and generates a
characteristic type of cement. Because the pore fluids are only filled intermittently the
cement tends to be irregular. Uniform isopachous rim coats are unusual. Cement is often restricted to throat passages (meniscus fill), or depends from the upper surface of
pores like stalactites. Cement fabrics are generally fibrous or micritic and are of aragonitic or high magnesium calcite co~.position. This is referred to as beachrock cementation (Plate 7A).
Below the water table, in the phreatic zone, the pores are soaked in water. For a carbonate bank or island a biconvex lens of meteoric freshwater overlies denser seawater
(near the coast) which merges inland to connate water of modified marine and meteoric
origin (Fig. 9.8). In the freshwater phreatic zone percolating water becomes carbonate
saturated, so an even cement of dog-tooth calcite may grow out into the pores. At the
same time, however, aragonite becomes unstable, and thus goes into solution. Because
some shells are aragonitic, and some calcitic, a selective biomoldic porosity develops.
These biomolds also develop a drusy cement along their internal surfaces. This can end
up as a curious fabric in which drusy calcite crystals are aligned back to back along the
rim of a biomold. They grow out from an organic pellicle or from an algally formed micrite rim.
The overall effect of diagenesis at this stage has thus been to increase the total porosity of the rock. Because of the isolated nature of the biomoldic pores, little of this new
porosity is effective. In fact, the drusy calcite has destroyed some of the intergranular
porosity, and by bridging pore throats, has also diminished permeability (Plate 7B).
Dedolomitization may also occur in the freshwater phreatic zone, leaving dolomoldic
pores which may later be infilled by calcite pseudomorphs (Evamy, 1967). Immediately
beneath the sea floor, where the pores are full of seawater, a second phreatic zone occurs with its own distinctive diagenetic fabric. Here an aragonitic rim cement develops
around the grains. The aragonite develops either as micrite (i.e., cryptocrystalline aragonite, not a depositional matrix) or as a radial acicular fabric. In either case the cement
is isopachous, rimming the grains evenly (Fig. 9.8). This hardground type of cement
generally develops as a layer only a meter or so deep below the sea floor. In modern examples the carbonate sand beneath is unconsolidated.
Below the sea-floor "hardground" and beneath the freshwater phreatic zone is the
deep connate environment. Here sediment that has escaped the various surface diagenetic processes will still be unconsolidated and highly porous to begin with. Because
these sands have not been lithified already they will be very susceptible to porosity loss
by compaction, whereas those sediments which have already lost some porosity during
early shallow diagenesis will not undergo compaction to the same degree. The effects of
compaction are manifest by signs of pressure solution at the point of contact of grains,
and by stylolites due to wholesale solution of rock (Park and Schot, 1968). Simultaneously the pores become infilled with a coarse sparite mosaic. Unlike the spar fill of freshwater cementation, this does not grow inward from dog-tooth crystals, but develops
crystals of uniform size, though these may sometimes be seen cross-cutting "ghosts" of
grains and earlier shallow diagenetic fabrics (Plate 7C).
413
A second type of vadose diagenesis occurs near the shore in the intertidal and supratidal spray zone, where sediment pores are intermittently flushed, not by freshwater,
but by seawater. As this evaporates in the pores the salinity increases and generates a
characteristic type of cement. Because the pore fluids are only filled intermittently the
cement tends to be irregular. Uniform isopachous rim coats are unusual. Cement is often restricted to throat passages (meniscus fill), or depends from the upper surface of
pores like stalactites. Cement fabrics are generally fibrous or micritic and are of aragonitic or high magnesium calcite co~.position. This is referred to as beachrock cementation (Plate 7A).
Below the water table, in the phreatic zone, the pores are soaked in water. For a carbonate bank or island a biconvex lens of meteoric freshwater overlies denser seawater
(near the coast) which merges inland to connate water of modified marine and meteoric
origin (Fig. 9.8). In the freshwater phreatic zone percolating water becomes carbonate
saturated, so an even cement of dog-tooth calcite may grow out into the pores. At the
same time, however, aragonite becomes unstable, and thus goes into solution. Because
some shells are aragonitic, and some calcitic, a selective biomoldic porosity develops.
These biomolds also develop a drusy cement along their internal surfaces. This can end
up as a curious fabric in which drusy calcite crystals are aligned back to back along the
rim of a biomold. They grow out from an organic pellicle or from an algally formed micrite rim.
The overall effect of diagenesis at this stage has thus been to increase the total porosity of the rock. Because of the isolated nature of the biomoldic pores, little of this new
porosity is effective. In fact, the drusy calcite has destroyed some of the intergranular
porosity, and by bridging pore throats, has also diminished permeability (Plate 7B).
Dedolomitization may also occur in the freshwater phreatic zone, leaving dolomoldic
pores which may later be infilled by calcite pseudomorphs (Evamy, 1967). Immediately
beneath the sea floor, where the pores are full of seawater, a second phreatic zone occurs with its own distinctive diagenetic fabric. Here an aragonitic rim cement develops
around the grains. The aragonite develops either as micrite (i.e., cryptocrystalline aragonite, not a depositional matrix) or as a radial acicular fabric. In either case the cement
is isopachous, rimming the grains evenly (Fig. 9.8). This hardground type of cement
generally develops as a layer only a meter or so deep below the sea floor. In modern examples the carbonate sand beneath is unconsolidated.
Below the sea-floor "hardground" and beneath the freshwater phreatic zone is the
deep connate environment. Here sediment that has escaped the various surface diagenetic processes will still be unconsolidated and highly porous to begin with. Because
these sands have not been lithified already they will be very susceptible to porosity loss
by compaction, whereas those sediments which have already lost some porosity during
early shallow diagenesis will not undergo compaction to the same degree. The effects of
compaction are manifest by signs of pressure solution at the point of contact of grains,
and by stylolites due to wholesale solution of rock (Park and Schot, 1968). Simultaneously the pores become infilled with a coarse sparite mosaic. Unlike the spar fill of freshwater cementation, this does not grow inward from dog-tooth crystals, but develops
crystals of uniform size, though these may sometimes be seen cross-cutting "ghosts" of
grains and earlier shallow diagenetic fabrics (Plate 7C).