Recent Intertidal Cements--Their Mineralogy, Texture, and Significance, Grand Cayman, British West Indies: ABSTRACT

Abstract

Recent intertidal rocks of Grand Cayman are cemented by metastable carbonates. Both aragonite and high magnesium calcite are present in various morphologic forms, as shown by X-ray, stains, and chemical analysis. Aragonite is present either as clear acicular crusts or as a fine-grained (<5µ) crust. These 2 modes of aragonite are present in the same specimen and within the same pore as alternating layers. High magnesium calcite cement is present only as a fine-grained (<5µ) crust. Both the acicular and aragonite cements appear to be a chemical precipitate from normal-marine water. Chemical analysis of a sample of the aragonite cement shows a high sodium content (4,580 ppm) which would support the hypothesis of marine rigin of this cement. The magnesium calcite cement data are incomplete and its origin is more uncertain. Both the aragonite and magnesium calcite cements contain trapped detritus such as tiny foraminifers and silt-size shell debris and many areas appear to be distinctly pelleted. Once the pore is completely filled, the cement mimics the fine-grained carbonate-mud matrix characteristic of quiet water conditions. The presence of significant amounts of fine-grained metastable cements as described from Holocene high-energy environments raises the question of whether these cements can survive in ancient rock sequences, and be confused with carbonate-mud matrix material characteristic of much lower energy situations. The presence of high magnesium cement in this s tuation and its known propensity for stabilization without significant change in form leads me to believe that such cements can be preserved and incorporated into the rock record, and could be confused with true micrite matrix. End_of_Article - Last_Page 861------------