GRAINS: Skeletal Fragments: Echinoderms
2003. "GRAINS: Skeletal Fragments: Echinoderms", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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Class Echinoidea — Late Ordovician-Recent
Echinoids (sea urchins) live in normal marine environments because they with a very limited range of salinity tolerance (generally only a few ppm).
They occur mainly as grazers or burrowers in sandy shelf areas or as grazers and bioeroders along rocky shorelines. They occur in deeper waters as well, extending to abyssal depths. Fossil forms are most common in normal marine, open shelf or platform deposits.
Echinoids are common in both warm- and cold-water settings, although they rarely are major rock-forming organisms (i.e., they rarely exceed 10-15% of the total sediment).
Modern and ancient echinoids are/were composed of moderate- to high-Mg calcite. Modern forms contain between 2 and 17 mole% Mg; the Mg content varies with generic group and increases with increasing water temperature (see Milliman, 1974, p. 130-134, for details and citations).
Echinoids, like all echinoderms at some stage in their life cycle, show pentameral (five-fold) symmetry. They have heavily calcified, globular to discoidal, hollow, endoskeletal tests (coronas) that are composed of individual sutured, interlocking or imbricated calcite plates. The calcitic coronal plates are porous and sponge-like; echinoids with rapid growth rates have spongier plates (with more holes and less calcification) than slowgrowing counterparts. Thus, slow growing, cold-water forms can be more heavily calcified than those from warmer waters (Raup. 1958)
In life, echinoid tests are covered with elongate, moveable spines (in some species extremely short, but in others longer than 8 cm). The spines normally detach after death and can themselves be significant sediment contributors.
Generally, each plate of an echinoid behaves optically as a single, extensively perforated, calcite crystal (see comments below). Echinoid teeth, however, are polycrystalline.
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