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Selective dolomitization, where certain carbonate components are preferentially dolomitized over others, can be significant in the overall dynamic context of the global magnesium cycle. Thus, the abundance of these components can modify the Mg balance between the ocean and sediments, thereby disrupting the Mg cycle in certain geological times. Selective dolomitization may be connected to the apparent correlation between global dolomitization events in the Neogene and the synchronous rise in species abundance of coralline red algae (CRA), but the underlying issue remains unclear. In the Caribbean islands, excellent examples of Neogene partially dolomitized carbonates containing coralline red algal facies are described to understand selective dolomitization of different components (grains and matrix) by examining the well-preserved outcrop of partially dolomitized Mio–Pliocene carbonates at the Seru Grandi locality on Bonaire Island, in the Caribbean. The degree and timing of selective dolomitization of various carbonate components are assessed using petrographical and geochemical methods. The micrite matrix is dolomitized first, followed by coralline red algal bioclasts, and subsequently all other grains. Dolomite crystals appear to originate from within and immediately around coralline algal fragments, suggesting that dolomite could have initiated from internally sourced Mg of the CRA’s high-magnesium calcite skeleton. Collectively, these observations suggest that selective dolomitization is controlled primarily by reactive surface area of the carbonate components, but it is less clear as to whether there is a dependency on original mineralogy.

The Early Cretaceous Fort Terrett Formation of Mason County, central Texas, is a succession of subtidal to peritidal mud-dominated facies with minor intervals of bioclastic packstone–grainstone, rudist floatstone, and interbedded chert nodules. The strata conformably overlie the Hensel Formation, which was deposited unconformably on Precambrian basement. The Hensel Formation also contains a significant percentage of dolomite, precipitated within a fine-grained clayey matrix. The Hensel and Fort Terrett Formations were deposited during a transgressive episode, which provided the conditions for the extensive shallow-water Comanche carbonate platform. Siliciclastic and carbonate sediments were deposited along the coastal margin in subtidal, intertidal to supratidal areas. Previous dolomitization models have suggested that high permeability layers are required for dolomitizing brines to flow through a carbonate succession. Although, interparticle porosity in muddy tidal-flat successions can be significant, it has a limited flow capacity. However, interconnected fenestral porosity can allow sufficient fluid flow to move dolomitizing fluids more efficiently through the succession. Thus, it is hypothesized that interconnected fenestral porosity could have had a significant impact on permeability within this muddy succession and provided the pathways and conduits for Mg-rich brines. Four types of dolomite are recognized in the Fort Terrett succession. Three of these dolomite types formed largely by replacement and they occur throughout the succession. Features such as crystal size, crystal face geometry and zonation reflect the progressive development and recrystallization of the dolomite types. Only type 4 dolomite formed as a cement in void spaces during a late diagenetic stage. The direction of the dolomitizing fluid movement is difficult to determine, but it was likely downward in this case, controlled by a density-head driving-mechanism generated by dense hypersaline fluids from an evaporating lagoon.