Origin of 'island dolostones': A case study from the Cayman Formation (Miocene), Cayman Brae, British West Indies

摘要

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 calrian dolomite (LCD -<55 mol% CaCO_3) and lesser amounts of high-Ca Calcian dolomite (HCD - >55 mol% CaCO_3). Geochemically, they are characterised by δ~(13)C values of 1.6 to 3.5‰, δ~(18)O values of 2.3-4.0‰., 80-279 ppm Sr, 52-340 ppm Fe, and 9-82 ppm Mn. The inverse correlation between δ~(18)O and mol% CaCO_3 and positive correlation between Sr and mol% CaCO_3 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 δ~(18)O values, corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation, indicate formation from normal seawater-like fluids under near surface conditions. The ~(87)Sr/~(86)Sr 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 Formatioa 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.