Origin of island dolostones: case study based on Tertiary dolostones from Cayman Brac, British West Indies
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Abstract
Cayman Brac (19 km long, 1.5 to 3 km wide), which is the easternmost of the Cayman Islands, is characterized by a thick Tertiary carbonate succession that has been pervasively dolomitized. The finely crystalline dolostones in the Cayman Formation (Miocene) are fabric-retentive whereas the coarse, sucrosic dolostones in the Brac Formation (Lower Oligocene) are mostly fabric-destructive.
Analyses of these dolostones indicate that (1) the dolostones are formed of various mixtures of low-Ca calcian dolomite (LCD – < 55 mol% CaCO3) and high-Ca Calcian dolomite (HCD – > 55 mol% CaCO3), (2) their geochemical signatures (e.g., δ18O and Sr) are heavily influenced by the dolomite stoichiometry, (3) the 87Sr/86Sr ratios point to two phases of dolomitization with Phase I (Late Miocene, 6-8 Ma) that caused partial dolomitization of the upper part of the Brac Formation and basal part of the Cayman Formation and Phase II (Pliocene to Early Pleistocene, 1-5 Ma), that completed dolomitization of the Cayman Formation. Interpretation of these data indicates that dolomitization was probably linked to sea level fluctuations whereas the distribution of the dolomite and evolution of their textures was controlled largely by permeability pathways that governed circulation patterns of the dolomitizing fluids. The large sucrosic dolomite crystals in the Brac Formation probably developed as a result of repeated cycles of limestone matrix dissolution and dolomite precipitation.
Rare earth elements (REE) and yttrium (Y) concentrations of carbonates from Cayman Brac are characterized by (1) LREE depletion relative to HREE, (2) positive La anomalies, (3) negative Ce anomalies, and (4) superchondritic Y/Ho molar ratios. Dolomitization did not have a major impact on their REE +Y signatures, which indicates that the dolomitization was probably mediated by seawater-like fluids. The variations in DyN/SmN, LaN/NdN, Ce/Ce*, and Y/Ho with depth reflect the influences of the diagenetic processes (e.g., DyN/SmN ) and possibly, secular changes in the REE+Y composition of seawater (e.g., LaN/NdN, Y/Ho, and Sm/Nd) on the REE+Y signature of carbonates.