RELATIONSHIP BETWEEN DOLOMITE NONSTOICHIOMETRY AND CARBONATE FACIES PARAMETERS

Two hundred ninety carbonate rock samples from nearly all pre Cenozoic periods from widely scattered North American sites representing all of Dunham s carbonate rock types and containing 5 100 ordered dolomite in the carbonate fraction were analyzed for a possible relationship between dolomite nonstoichiometry expressed as mole percent CaC03 and petrographic parameters and stratigraphic position Dolomite composition ranged from 49 3 to 57 3 mole percent CaC03 with modes from 50 0 to 52 0 and 54 0 to 56 0 Data indicate that dolomite nonstoichiometry is not related to insoluble residue porosity rock type percentage of crinoid fragments recrystallization dolomite crystal size and percentage of dolomite in the carbonate fraction in dolomitized limestones A general trend toward more nearly stoichiometric composition with age is obscured by local effects Crystalline dolomites tend to be more nearly stoichiometric and less variable in composition mode 50 0 51 0 CaC03 relative to dolomicrites modes 51 0 52 0 evaporitic 54 0 55 0 nonevaporitic and dolomitized lime mudstones modes 51 0 52 0 evaporitic 55 0 56 0 nonevaporitic Evaporite related dolomite is almost invariable near stoichiometric or calcium depleted Open marine and stoichiometric evaporite related dolomites are products of syndepositional dolomitization nonstoichiometric nonevaporite dolomite is syndepositional to early diagenetic and crystalline dolomite is middle to late diagenetic