GRAINS: Skeletal Fragments: Bryozoans
Subphylum Entoprocta — Middle Cambrian?, Late Jurassic-Recent
Class Phylactolaemata — Middle Jurassic-Recent
Class Gymnolaemata — Early Ordovician-Recent (dominates Mesozoic-Recent)
Order Ctenostomida — Early Ordovician-Recent
Order Cheilostomida — Late Jurassic-Recent
Class Stenolaemata — Early Ordovician-Recent (dominates Ordovician-Permian)
Order Cyclostomida — Early Ordovician-Recent
Order Cystoporida — Early Ordovician-Late Triassic
Order Trepostomida — Early Ordovician-Late Triassic
Order Cryptostomida — Early Ordovician-Late Triassic
Bryozoans are sessile, filter-feeding organisms with a wide salinity tolerance — most are marine, but a few species (from the Entoprocta, Phylactolaemata, and Ctenostomida) inhabit fresh water and a few others (from the Cheilostomida) are found in brackish-water environments.
Bryozoans have wide latitudinal (tropical to polar), temperature, and depth ranges (0 to 8.5 km). They can be the main constituents in Mesozoic and Cenozoic temperate- and cold-water shelf carbonates, as well as in deeper shelf and slope settings; in the Paleozoic, they were more conspicuous in tropical to subtropical habitats.
Many bryozoans require a firm substrate on which to encrust; some are free living, and others have roots extending into sandy substrates. Massive and encrusting varieties are found in high-energy environments; delicate, erect varieties are indicative of low-energy environments.
Entoproct bryozoans are soft bodied and, therefore, are rarely preserved.
Most ectoproct bryozoan zooecial walls are composed of calcite (usually low-Mg calcite; a few consist of high-Mg calcite and others are partially aragonitic). Some species have chitinous or gelatinous walls.
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