Dolomite Nonstoichiometry; Its Relation to Carbonate-Rock Fabric: ABSTRACT

Abstract

Nearly 300 samples of carbonate rocks representing all pre-Tertiary periods (except Cambrian) were analyzed for their dolomite stoichiometry by X-ray peak displacement. These samples represent a wide variety of fabric types (0 to 100% micrite, and boundstone), and a wide variety of depositional environments (shelf edge, subtidal shelf interior, shore zone, and deep marine). The percent calcium carbonate in the dolomite lattice ranged from 48.67 to 57.93, and was fairly uniformly distributed over the range 50.0 to 56.0%. There does not appear to be any clear relation through time and over wide geographic areas between the percent calcium carbonate in the dolomite lattice and (1) facies; (2) total percent dolomite; (3) degree of recrystallization; (4) spar-crystal size; (5) total fossil content; (6) percentages of gastropods, brachiopods, bryozoans, and echinoids; (7) percent insoluble residue; and (8) percent visible porosity (in thin section). There does appear to be a trend to more nearly stoichiometric dolomite (50% calcium carbonate) with increasing age, but this generalization has many exceptions. Results from samples of geologically related suites do show systematic facies-related patterns, but the trends vary and even reverse from suite to suite. We conclude that dolomite nonstoichiometry may be a useful parameter in facies analysis of geologically related (time and space) sets of samples, but it shows no unambiguous facies-related trends over the geologic record. End_of_Article - Last_Page 431------------