GRAINS: Skeletal Fragments: Foraminifers
2003. "GRAINS: Skeletal Fragments: Foraminifers", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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Kingdom Protista, Phylum Sarcomastigophora, Subphylum Sarcodina, Superclass Rhizopoda, Class Granuloreticulosea, Order Foraminiferida — Basal Cambrian-Recent
Benthic foraminifers: Cambrian-Recent (early forms were exclusively agglutinating)
Calcareous benthic foraminifers — Ordovician-Recent; large forms from Late Carboniferous-Recent
Planktic foraminifers: Middle Jurassic-Recent
Despite being single-celled protozoans, this is a very complex group of organisms, with 12 suborders recognized by Loeblich and Tappan (1984) and some 60-80,000 species identified from Phanerozoic strata. So many shape, size, and wall-structure varieties exist, however, that this chapter can provide only the minimal information needed to identify the most important groups.
Modern foraminifers are fully marine to marginal marine organisms, extending from the intertidal zone to abyssal oceanic depths and from cold-water polar settings to warm tropical environments. Some genera live in marginalmarine hypersaline or subsaline water bodies where they are commonly found in great numbers (but low species diversity).
Most foraminifers are benthic organisms (of the roughly 4,000 modern species, only about 40 are planktic).
Some of the largest living benthic species harbor symbiotic algae in their tissues and thus live primarily in the photic zone; the vast majority, however, are not light dependent.
For reasons related mainly to food supply, most planktic foraminifers live in the upper 300 m of the water column, although after death, their tests fall to the underlying, deeper seafloor.
Foraminifers can be major rock forming elements in open- or restricted-shelf as well as deeper marine deposits. In some cases, foraminiferal abundances reach tens of thousands of individuals per m3 of sediment.
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