Bird's-eye Structures in Carbonate Rocks and Their Environmental Significance
Previous workers have usually considered bird's-eye structures as typical indicators of tidal flats (supratidal to intertidal) environments. However, recently collected data from the Middle Devonian Jipao Formation in Dushan, Guizhou, China, suggest alternative environmental interpretations. The results from the combined analyses of rock texture, sedimentary structure, fossils and paleoecology, and sedimentary sequence, as well as the paleogeographic background of bird's-eye-bearing formations indicate the bird's-eye structures must have formed in a restricted shallow subtidal lagoon environment. Therefore, bird's-eye structures can be distributed not only in tidal flats but also in some confined environments within subtidal zones, such as in a semiclosed lagoon (low-ener y environment).
Bird's-eye structures are also described from supratidal and intertidal limestones in the Upper Devonian Yaosuo Formation (Feifengjing, Guizhou) and in the Upper Devonian Rongjiang Formation (Buzhai, Guizhou). The correlation of bird's-eye structures from tidal flat limestones with those from the subtidal zone demonstrates that bird's-eye structures from different environments can, to some degree, be distinguished on the basis of their form, origin, and rock association. But the writer must emphasize that when bird's-eye structures are used to determine sedimentary environments, all available facies indicators (e.g., other sedimentary structures, rock textures, fossils and their assemblages, mineralogic facies, and especially sedimentary sequences) must be synthesized.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.