Genesis of fabric-destructive dolostones: A case study of the Brac Formation (Oligocene), Cayman Brac, British West Indies

Abstract

The origin of fabric-destructive and fabric-retentive textures in dolostones of all ages remains open to debate. Evolution of the fabric-destructive textures, which effectively destroys all traces of the original limestones, remains particularly controversial. On Cayman Brac, a small island located in the Caribbean Sea, the Brac Formation (Lower Oligocene) is formed of limestones and fabric-destructive dolostones. The top of this formation is defined by the Brac Unconformity, which developed as a karst surface during the Late Oligocene and Early Miocene. The overlying Cayman Formation (Lower to Middle Miocene) is pervasively dolomitized and characterized by fabric-retentive textures that have preserved the depositional textures of the original limestones. The stratigraphic juxtaposition of these two formations provides an ideal opportunity to compare the factors that controlled the development of fabric-destructive and fabric-retentive fabrics in dolostones.The dolostones in the Brac Formation are formed of dolomite that replaced the limestone and dolomite cements (typically zoned) that were precipitated in pores throughout the dolostones. Interpretations based on the composition of the dolomite, δ13C values, δ18O values, and Sr, Fe, and Mn contents indicate that dolomitization of both the Brac Formation and the Cayman Formation was probably controlled by similar, slightly modified seawater with the distribution of the dolomite in the limestones and the evolution of dolostone textures being controlled largely by permeability pathways that governed circulation patterns of the dolomitizing fluids. The large sucrosic dolomite crystals, which are commonly zoned, probably developed as a result of repeated cycles of limestone matrix dissolution and dolomite precipitation.