Page 32 - Carbonate Facies in Geologic History

Diagenetic Susceptibility 19

shallow water carbonate known to have been produced in the geologic record.
Presumably, the production mechanism is sensitive and easily shut off by building
to sea level or by turbidity induced by change of climate or source area. This is an
important consideration in interpreting time correlation on stratigraphic surfaces
in the geologic record.

Diagenetic Susceptibility

11. Carbonate sediments and rocks are very sensitive to diagenesis. Changes
begin in the loose grains before and during deposition and continue on the sea
floor. Drastic solution and mineralogical changes occur when carbonate sediment
is taken from the marine and placed in a subaerial, fresh water environment.
Other changes occur when evaporative brines and other types of connate waters
migrate through limy strata both before and after deep burial. The changes occur
through various stages of lithification. They include altering of marine carbonate
minerals to low Mg calcite, organic rotting of particles, solution, cementation of
pore space in lime sands, crystal rearrangement (neomorphism), metasomatic
replacements (dolomitization) which rearrange and enlarge pore space, and anhy-
drite replacement which makes the sediment more susceptible to later dissolution.