Dolostones from Grand Cayman, British West Indies

Abstract

ABSTRACT Pervasively dolomitized Tertiary strata on Grand Cayman are dominated by heterogeneous dolostones that are composed of both low-Ca calcium dolomite (LCD 55 mol % CaCO3). Homogeneous dolostones, formed of either LCD or HCD, are rare. Some dolostones contain dolomite crystals that are characterized by oscillatory zones (1-10 µm thick) of LCD and HCD, whereas others have rims of LCD surrounding cores of HCD. In some dolostones, bioclasts formed of LCD are embedded in a matrix of HCD. An 1‰ 18O difference between the LCD and HCD can be attributed to the natural covariance that exists between the 18O and mol % CaCO3 in dolomite. There are no significant differences in the 87Sr/86Sr ratios of the LCD and HCD. There is no consistent vertical or lateral pattern to the distribution of LCD and HCD in the Cayman dolostones even in closely spaced sampling sites. Such variability is more consistent with growth of individual dolomite crystals being controlled by processes that operated around the surfaces of individual crystals rather than by large-scale changes in the dolomitizing fluids. This interpretation is consistent with the development of oscillatory zoning, which is attributed to precipitation of different phases or minerals from the same parent fluid because of competition between interface-controlled and diffusion-controlled growth kinetics. Dolomitization of the Tertiary strata on Grand Cayman was probably mediated by seawater or modified seawater. Nevertheless, small-scale intrinsic factors controlled the incorporation of Ca in the dolomite lattice and, accordingly, the geochemical parameters that are intimately linked to the mol % CaCO3. These small-scale intrinsic controls must be factored into any model to explain the formation of Tertiary island dolostones.