Abstract

Although some skepticism has been expressed regarding the occurrence of displacive calcite, this type of growth occurs in the calcrete profiles in Lower Old Red Sandstone of Carnoustie, eastern Scotland. The calcretes are restricted both laterally and vertically, implying they formed on the tops of bars and other bed forms. Petrographic evidence that the calcite crystals are displacive in origin includes the following: 1) sparry calcite crystals occur between textural components (i.e., micas, rock fragments, fragments of a quartz grain) that have an "exploded" texture, and many of the fragments can be visually fitted back together; and 2) cathodoluminescence reveals that the calcite crystals are compositionally zoned, implying that the crystals are a cement and definitely are not due to replacement. The displacive calcite has variable crystal morphology and is restricted vertically to a maximum depth of I m below the subaerial exposure surface. In addition, the precipitation of displacive calcite concurrently with subaerial expansion and brecciation of detrital fragments indicates that displacive calcite was generated within the vadose zone. In the vadose zone, crystallization was promoted by rapid surface evaporation, leading to supersaturation. The recognition of displacive calcite provides evidence of supersaturated precipitating solutions.

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