Petrography and Significance of Zoned Dolomite Cements from the Cayman Formation (Miocene) of Cayman Brac, British West Indies

Abstract

ABSTRACT Cavities in dolostones from the Cayman Formation (Miocene) from Cayman Brac commonly contain complex cement successions that record three main phases of diagenesis. Phase I cements, formed of low-Ca calcian dolomite (LCD) and high-Ca calcian dolomite (HCD), are divided into monomineralic LCD crystals, oscillatory-zoned LCD-HCD crystals, and step-zoned HCD-LCD crystals. Dissolution during phase II produced dolomite crystals with hollow cores, hollow growth zones, and hollow crosscutting zones. Calcite precipitated during phase III filled many of the cavities that had not been filled by the dolomite cements. Etching shows that the LCD is formed of blocky subcrystals that have their long axes perpendicular to the growth surfaces whereas HCD is characterized by a modulated structure that is evident as growth-parallel, slightly sinuous strands, 10-20 nm wide. The contrast in crystallographic microstructures supports the notion that LCD and HCD are distinct types of dolomite. Two levels of variability that operated on different time scales controlled diagenesis in the Cayman Formation of Cayman Brac. Long-term variability controlled the evolution from dolomite precipitation to dolomite dissolution to calcite precipitation. Short-term variability controlled variations in the physiochemical environments that characterized each phase of diagenesis. Thus, during Phase I diagenesis, short-term fluctuations in water chemistry and/or crystal growth patterns controlled the mol % CaCO3 in the dolomite cement crystals. Similarly, short-term variations in pore-water compositions controlled the style of dolomite dissolution during the second phase of diagenesis.