Abstract

Dolostones in the Cayman Formation on Grand Cayman and Cayman Brac, which are the product of two and possibly three phases of dolomitization, are formed of very small (< 100 μm) but architecturally complex dolomite crystals that are characterized by planar and non-planar boundaries, corrosional internal discontinuities, oscillatory low-Ca calcian dolomite (LCD) and high-Ca calcian dolomite (HCD) zoning, and syntaxial (?) overgrowths. The dolomite overgrowths, which grew on earlier formed but corroded rhombs, increased crystal size and modified crystal shape and, hence, caused significant textural modifications. The size and shape of the overgrowths that developed preferentially on those crystal faces that formed the walls of micropores was controlled by the competition for growth space with neighboring crystals. Interpretation of this textural information suggests that the development of planar and non-planar dolomite boundaries crystals is probably related to critical supersaturation levels rather than critical roughening temperatures.

The recognition that individual dolomite crystals, irrespective of their size, may be architecturally heterogeneous carries important implications for dolomite genesis. Many interpretations of finely crystalline dolostones, for example, have tacitly assumed that they formed through one phase of dolomitization. In the Cayman Formation, however, dolomite crystals as small as 10 μm are characterized by internal heterogeneities that record their development through multiple, time-separated growth phases. Any model developed to explain dolomitization must take this into account. The interpretation of chemical data, for example, must consider the multiple-stage development of the dolostones that is evident from their internal zoning and discontinuities.