GRAINS: Skeletal Fragments: Mollusks
Phylum Mollusca, Subphylum Cyrtosoma
Class Gastropoda — Early Cambrian-Recent
Order Thecosomata (pteropods) — Cretaceous-Recent (possible precursors Cambrian?-Permian?)
Gastropods are the largest class of both living and fossil mollusks (with nearly 8,000 genera), although they are rarely major rock-forming organisms.
Gastropods (snails) are a remarkably wide-ranging group of organisms. They are found at all latitudes and in normal marine, brackish, hypersaline, and fresh water as well as subaerial environments. They rarely are major sediment formers, however, except in stressed (especially hypersaline or freshwater) settings.
Warm-water forms generally are thicker shelled than cold-water forms.
Pteropods are open-marine, predominantly warm-water, nektic organisms that contribute mainly to deep-sea oozes on seafloors shallower than about 3,000 m (because of aragonite dissolution effects).
Gastropod shells have a thin outer coating of organic material (conchiolin) plus a thick carbonate layer generally consisting of only aragonite. Some families, however, have shells with separate layers of calcite and aragonite. Where present, the calcite layer normally is thicker than the aragonite layer. Gastropod calcite has a low Mg content (typically less than 0.3 mole% Mg; rarely exceeding 1 mole% Mg). Pteropods have aragonite shells.
Both shell-bearing and non-shell-bearing gastropods exist. The shelled forms are univalves that have an unchambered cone, most commonly coiled about a central axis. Some forms are able to withdraw fully into their shell and have a plate (an operculum) that they can draw behind themselves to close the shell opening; opercula can be composed entirely of conchiolin (proteinaceous organic material that is rarely preserved) or aragonite.
Diverse coiling patterns exist: high-spired, conical, and planispiral forms are common; some groups (such as the vermetids) have very open spirals and form shells that resemble serpulid worm tubes.
Adult gastropods typically are about 2-3 cm in length (modern forms of up to 60 cm length are known, however). Fragments typically mm- to cm-sized.
Pteropods are nektic gastropods and although the majority are shell-less, some have slender, conical, generally uncoiled, thin-walled shells, typically less than 1-2 cm in length.
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