(1) Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, AB
ABSTRACT: Significance and Development of Heterogeneous Dolostones in the Tertiary Succession on Grand Cayman and Cayman Brac
Most of the Tertiary Bluff Group is formed of fabric-retentive, finely crystalline dolostones. Cavities in these dolostones commonly contain dolomite cement, calcite cement, and various types of internal sediment. The dolostones are formed of low-calcium calcian dolomite (LCD – 50-55 mol %
CaCO3) and high-calcium calcian dolomite (HCD – > 55 mol % CaCO3). Potentially, their heterogeneous composition could be attributed to (1) partial recrystallization of HCD to LCD, (2) different phases of dolomitization, (3) differences between the replacive matrix dolomite and dolomite cement, (4) natural stratigraphic and/or geographic variations, and/or (5) crystal growth zoning.
Many dolomite crystals in the matrices (< 50 µm long) and cements are characterized by oscillatory growth zoning and step-zoning with the alternating zones of LCD and HCD. The development of syntaxial overgrowths around many matrix dolomite crystals led to their enlargement and transformation from euhedral to anhedral crystals. The zoning shows that they must have formed through crystal growth.
The distribution of LCD and HCD is stratigraphically and geographically variable at all scales. For example, some sections through the Cayman Formation are formed mainly of HCD whereas LCD dominates other sections. Such contrasts suggest that variations in the calcium content of the dolostones reflect natural variances in the dolomitization process. Any model invoked to explain the origin of “island dolostones”, like those on the Cayman Islands, must explain the development of compositionally heterogeneous dolostones, the growth of compositionally zoned dolomite crystals, and the stratigraphic and geographic variances in the compositions of the dolostones.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.