Abstract
ABSTRACT Two kinds of birdseye structures are recognized in Recent and ancient carbonate rocks: (1) planar isolated vugs 1 to 3 mm high by several millimeters in width and (2) isolated more or less bubblelike vugs 1 to 3 mm in diameter. Both types are generally filled with calcite or anhydrite. Study of more than 100 cores and samples of Recent carbonate sediments showed that birdseye voids are preserved in supratidal sediments (sediments deposited above normal high tide level), sometimes in intertidal sediments (sediments deposited between normal high and normal low tide), and never in subtidal sediments (sediments permanently below water). Both kinds of birdseyes are particularly abundant in supratidal dolomitic sediments. Laboratory experiments and observations in limestone quarry tailings suggest that bubblelike vugs were made by gas bubbles and that planar vugs were made by shrinkage resulting from desiccation of exposed sediments. Birdseye vugs may be valuable indicators of depositional environment, especially when they occur in carbonate rocks containing no depositional structures.
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