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In Dark Canyon, the transition from Capitan reef facies to Tansill back-reef facies occurs across a distance of 150 m (500 ft). Detailed 3-dimensional sampling of the transition reveals facies based on biota content and textural changes. Also revealed are postdepositional modifications owing to diagenesis.
Three microfacies (A, B, and C), defined by the presence of a few diagnostic biota, occur between the Capitan reef and Tansill back-reef deposits. Several constituent biota occur throughout the zone, including calcareous sponges, gastropods, ostracods, problematic Tubiphytes, and foraminifera. Grading from Tansill shelf deposits to the Capitan reef, the facies are: (1) Tansill sensu stricto: dasyclad alga Mizzia, large gastropods, and brachiopods; (2) facies A; massive colonies of Collenia, bivalves, and red alga Parachaetetes; (3) facies B: large gastropods, bivalves, brachiopods, and bryozoans; (4) facies C: heads of Archaeolithoporella, and Mizzia; (5) Capitan reef: Archaeolithoporella, crinoids, and Mizzia. the reef proper is an algal boundstone, while backreef facies are packston s-grainstones.
Submarine cements that have been modified diagenetically are dominant. The majority of the biota has been micritized. Large voids are filled by fibrous aragonite which has been replaced by botryoidal radial fibrous calcite. Many reef and near-reef limestones are extremely recrystallized. Back-reef areas have undergone several stages of dolomitization, whereas the actual reef is limestone. Some anhydrite moldic porosity is occluded
by sparry calcite. More recent episodes of cementation have left some gravitational voids.
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