DISTINGUISHING BETWEEN AUTHIGENIC AND PRIMARY DOLOMITE SEDIMENT IN STRATIGRAPHIC DOLOMITES BASED ON GRAIN MORPHOLOGY

Field sedimentologists have long fretted that much of the dolomite appears to be a primary accumulation (i.e., as pre-existing dolomite grains), but that dolomite is only formed as a diagenetic mineral. The lack of available theories for the formation of dolomite either by direct organic action or on the surficial sea bed has led to extremely convoluted models for the replacement of existing primary carbonate sediments by in-situ diagenesis that are inconsistent with the field and petrographic relations. Furthermore, when one looks at the kinematics of dolomite crystal growth in the context of the grain shapes seen in most of the dolomite sedimentary record, it is obvious that these dolomite grains could not have formed in situ and must have been deposited as existing dolomite crystals.

Because of the alternating Ca and Mg (001) layers in ordered dolomite, and of the impossibility of forming disordered dolomite at surface temperatures and pressures, ordered dolomite grains must grow by alternative accretion parallel to the c-axis. They cannot grow by accretion on standard rhomb surfaces (111) because that would require simultaneous precipitation of alternating Ca and Mg stripes on those surfaces. Only needles or hexagonal platelets of dolomite can be formed by crystal growth. So, transport is strongly indicated by the common presence of rhomb faces because meaning these faces must have been formed by breakage on the (111) faces ( on the the primary cleavage surfaces). Similarly, rounded and pitted grains must have been formed by mechanical or chemical alteration of original grains.

Therefore, we propose that most stratigraphic dolomites are formed by remobilization of carbonate sediments following initial dolomitization when diagenesis has altered the original carbonate mineralogy. This requires dolomitization to be be primarily occurring in the surficialsediments mmediately after inital deposition. These dolomites then are redeposited (at least once) before being incorporated into the stratigraphic record. Any arguments against this model must account for the grain shapes observed in stratigraphic dolomites.