Dolomitization of the Pedro Castle Formation (Pliocene), Cayman Brac, British West Indies

Abstract

The Pedro Castle Formation (Pliocene) on Cayman Brac is variably dolomitized by texture preserving but non-mimetic and texture destructive replacive dolomite. Mimetic replacement of skeletal grains is limited to echinoderm plates, and with few rare exceptions, there is no mimetic replacement of red algae, foraminifera, green algae, or any other type of skeletal grain. The lack of mimetic dolomite is atypical of “island dolostones” found in the Caribbean Sea and the Pacific Ocean. Dolostones in the Pedro Castle Formation are formed entirely of high-Ca calcian dolomite (average of 57.4 mol% CaCO 3). Oxygen isotopes (mean 1.25‰ PDB) from the dolomite indicate that dolomitization was mediated by seawater or modified seawater. Carbon isotopes in the dolomite, which range from −1.81‰ to 1.42‰ PDB, were probably inherited from the precursor limestone. The average Sr content in the dolomite (360 ppm) is higher than that found in most other island dolomites. The sediments that now form the Pedro Castle Formation were deposited in shallow water on an open bank during the early Pliocene. Pre-dolomitization diagenesis of those sediments included syntaxial overgrowths around echinoderm fragments, dissolution of aragonitic bioclasts, stabilization to low-magnesium calcite, and local precipitation of vadose cements. Thus, the limestones had been extensively stabilized by the time that dolomitization took place during the late Pliocene. The general paucity of mimetic replacement in these dolostones can probably be attributed to the calcite stabilization that took place before dolomitization.