Origin of “island dolostones”: A case study from the Cayman Formation (Miocene), Cayman Brac, British West Indies

Cayman Brac, the easternmost of the Cayman Islands, is 19 km long, 1.5 to 3 km wide, and rises ~ 40 m above sea level at its eastern end. Geographically isolated by the deep oceanic waters of the Caribbean Sea, this island, with its thick succession of pervasively dolomitized Tertiary rocks, is a natural laboratory for assessing the origin of “island dolostones”. Dolostones in the Miocene Cayman Formation (~ 100 m thick), with most crystals < 20 μm long, are formed of low-Ca calcian dolomite (LCD — < 55 mol% CaCO3) and lesser amounts of high-Ca Calcian dolomite (HCD — > 55 mol% CaCO3). Geochemically, they are characterised by δ13C values of 1.6 to 3.5‰, δ18O values of 2.3–4.0‰, 80–279 ppm Sr, 52–340 ppm Fe, and 9–82 ppm Mn. The inverse correlation between δ18O and mol% CaCO3 and positive correlation between Sr and mol% CaCO3 largely reflect kinetic effects on oxygen isotope fractionation and the Sr partition behaviour between dolomite and water. Dolomite stoichiometry indicates formation from waters with a high Mg/Ca ratio and δ18O values, corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation, indicate formation from normal seawater-like fluids under near surface conditions.The 87Sr/86Sr ratios point to two phases of dolomitization. Phase I, in the Late Miocene (6–8 Ma), caused partial dolomitization of the basal part of the Cayman Formation. Phase II, during the Pliocene to Early Pleistocene (1–5 Ma), completed dolomitization of the Cayman Formation. Both phases of dolomitization were mediated by similar fluids. Although available information suggests that dolomitization was probably linked to sea level fluctuations, the timing of dolomitization relative to the transgressive–regressive cycle remains open to debate.

► Oxygen isotope and Sr contents of dolomites covary with their stoichiometry.
► Oxygen isotope of dolostones must be corrected before the fractionation calculation.
► Dolomite can form in the normal seawater.
► Two-episode dolomitization was responsible for the formation of Cayman dolostones.