Dolomite occurrence in ancient shallow-water carbonates: mineralogical associations and potential relevance of facies transitions

When dealing with ancient neritic carbonate deposits, variable continental and marine influences, as well as post-depositional processes result in complex geochemical records. An expedient technique for mineralogical... [ view full abstract ]

When dealing with ancient neritic carbonate deposits, variable continental and marine influences, as well as post-depositional processes result in complex geochemical records. An expedient technique for mineralogical characterization of carbonate samples is X-ray powder diffraction (XRD). Here, an innovative approach to diffractogram analysis is presented, by which X-ray intensity spectra are directly compared. Samples retrieved from the São Julião section (Ericeira, Portugal), a succession dated from Early Albian to Early Cenomanian, were chosen for this approach due to high lithofacies variability. These lower Cretaceous coastal deposits comprise alternating horizons of sandstone/siltstone, limestones and clayey/marly horizons.
A set of 81 diffractograms representing bulk XRD analysis was compiled, adding to a total dataset of approximately 250.000 data points. A similarity correlation matrix was performed, limiting statistic significance to >0.9. Eliminating the values that fail this premise provides a clear pattern that differentiates four groups of samples. The respective diffractograms of each group show the predominant mineralogy, as an expression of changes in lithology and sedimentary environments. In the case of the shallow-water carbonate samples under scope, these can be dominated by the presence of 1) quartz, denoting intervals of higher siliciclastic input; 2) calcite, which prevails along with fading terrigenous contribution; 3 and 4) a combination of quartz+calcite (+dolomite sub-group), most likely reflecting transitional lithologies between 1 and 2. More relevantly, this analysis allows detecting the presence of dolomite only in samples also presenting quartz and calcite. Such combination points to a possible relation between depositional environment, mineralogical assemblage and potential for dolomite development.
In order to test the effects of lithology transition and the potential relation to the presence of dolomite along the studied section, Markov chain analysis was performed on the stratigraphic distribution of the four identified mineralogical associations. The highest transition frequencies as well as most significant transition probabilities were obtained for the association of quartz+calcite+dolomite and calcite dominated horizons. This suggests that dolomitization occurs preferentially when quartz+calcite levels are encased by calcite dominated horizons. Fluid circulation during diagenesis is most likely a key factor for explaining this relationship, a hypothesis currently under evaluation by means of porosity/permeability tests to all mentioned facies.