Two kinds of birdseye structure, both generally filled with calcite or anhydrite, are found 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. Laboratory experiments and observations in limestone quarry tailings suggest that the first kind results from shrinkage due to desiccation of exposed sediments and that the second kind is formed by gas bubbles. Study of many samples of Recent carbonate sediment shows that birdseye voids are preserved in supratidal and intertidal sediment, but never occur in subtidal sediments. Birdseye structure thus may be an indicator of depositional environment.

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