Page 14 - Carbonate Facies in Geologic History

Chapter I

Principles of Carbonate Sedimentation

The Requisite Marine Environment: Warmth, Light,
Water Movement

Most, though not all, carbonate sedimentation results basically from chemical or
biochemical processes occurring in a special marine environment: one of clear,
warm, shallow water. A world map (Fig. 1-1) showing areas of modern carbonate
deposition demonstrates clearly a positive correlation between such deposition
and the equatorial belt and areas of warm ocean currents. Fairbridge (Chilingar et
al., 1967, p.404) presented a graph showing that neritic carbonates exist chiefly
north and south of the equator below latitudes of 30 degrees. Furthermore,
whereas the bottoms of deep ocean basins between 40 degress north and south
latitudes contain much planktonic carbonate, those in higher latitudes do not,
except in the north Atlantic along the Gulf stream. Invertebrates precipitate
thicker calcite and aragonite shells in clear, warm waters, many more calcareous
algae thrive there, and algal-dependent hermatypic or reef-building corals are
restricted to such an environment. Cooler marine waters do support swarms of
invertebrates whose tests and shells may form local accumulations of shelly lime
sand (Lees, 1973; Chave, 1967), but other types of lime sediment, such as ooids,
grapestones, peloids, reefy boundstone and lime mud accumulations, are confined
to tropical and subtropical waters.
However, tropical water alone is insufficient for calcium carbonate produc-
tion. The water must be clear. Three great carbonate banks around the Gulf of
Mexico are located in areas most protected from major influx of fine clastic
sediment. These are far to the southeast of the westward-moving longshore drift
of the Mississippi River mud and are protected by deep water from the influx of
clay and silt off the large island of Cuba. A reverse situation appears across the
Sunda Shelf north of Indonesia where a vast platform, covered by shallow, equa-
torial water exists. The Sunda Shelf bears only isolated reef accumulations, lo-
cated along its northern and eastern edges, because large rivers from Sumatra,
Java, and Borneo to the south and west muddy the sea and inhibit carbonate
formation. Fairbridge (in Chilingar et al., 1967) pointed out that practically all
shallow carbonate shelves in strictly equatorial regions seem to be smothered by
fine terrigenous clastics brought by the large tropical rivers.
Given the requisite marine environment, exactly what chemical and biological
controls operate to produce abundant carbonate? The complex chemical prob-
lems of precipitation of CaC0 3 minerals from sea water are beyond the scope of
this book. The reader is referred to the discussion ofthe subject by Bathurst (1971,
Chapter 6) and Milliman (1974) who cited pertinent references to modern litera-
ture. At the present time, tropical seas are essentially saturated with regard to