GRAINS: Skeletal Fragments: Corals, Octocorals, and Hydrozoans
2003. "GRAINS: Skeletal Fragments: Corals, Octocorals, and Hydrozoans", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
Download citation file:
Phylum Cnidaria, Class Anthozoa
Subclass Zoantharia — Ordovician (Cambrian ?)- Recent
Order Tabulata — Early Ordovician-Late Permian
Possible tabulate corals have been reported from the Early Cambrian (Sorauf and Savarese, 1995). The group was widespread and diverse from Late Ordovician to Middle Devonian, but declined in Late Devonian and into the Carboniferous. The group became extinct during the great end-Permian faunal crisis.
Tabulate corals were fully marine, sessile organisms and were contributors to stromatoporoid and microbial reefs and bioherms of Ordovician to Carboniferous age. Although substantial contributors to some reefs, tabulates rarely were dominant reef framework formers. Tabulate corals did build smaller, isolated bioherms that are widely distributed in muddy, open shelf carbonate rocks.
Many tabulate corals were attached to their substrates, others were unattached, rolling free on the sea floor.
Whether tabulate corals had a symbiotic relationship with zooxanthellate analogs, and therefore were restricted to living in the photic zone, is an open question because such soft tissues are not preserved. Environmental reconstructions, however, indicate that tabulates lived at shallow marine depths within the photic zone, so an analogous symbiotic relationship is possible, but has neither been proven nor refuted.
Virtually all tabulate corals probably were originally calcitic; very few (probably on the Tetradiidae, a group that is not universally classed with the tabulates) may have been aragonitic. Determination of original mineralogy is based mainly on the quality of structural preservation.
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks