Primary Constituents: Skeletal Grains/Bioclasts
2003. "Primary Constituents: Skeletal Grains/Bioclasts", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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Cyanobacterial stromatolites usually are grouped in the Phylum Cyanophyta — Precambrian (Archean)-Recent Classification of other microbes is complex, uncertain, and ever changing (generally placed under the Prokaryotes, but most of these organisms are really best considered as “microproblematica”). Organisms once termed blue-green algae are now generally termed cyanobacteria.
Ranges of some specific calcimicrobes depicted in this section:
Girvanella— Cambrian-mid. Cretaceous (Eocene?)
Renalcis — Cambrian-Devonian
Frutexites — Latest Cambrian-Devonian
Many are photosynthetic and therefore require light; non-photosynthetic microbes also are important, especially in cryptic settings. Recognition of photosynthetic forms is especially critical in paleoenvironmental studies.
Wide salinity tolerance from strongly hypersaline to freshwater; rare as dominant sediment formers in modern, normal-salinity marine environments.
Wide temperature tolerance: sub-glacial to hot springs settings; most common in temperate- to warm-water marine settings.
Marine stromatolites range from subtidal to intertidal settings — intertidal forms predominate today.
A progressive shift occurred from normal-salinity environments in the Precambrian to highly stressed environments today, possibly due to the Phanerozoic increase in grazing organisms or interspecific competition. Cenozoic microbial carbonate deposits are predominantly peritidal.
Marine forms are mainly aragonitic; incorporated detrital components can have any carbonate or terrigenous mineralogy; lacustrine forms are mostly calcitic.
Most are uncalcified and the remainder have “nonskeletal” or “extraskeletal” calcification largely incidental to their growth. Calcification results from biochemical processes (removal of CO2), but generally is not necessary for, or beneficial to, the organism’s survival.
Calcimicrobial deposits, thus, have no clearly defined and consistent skeletal morphologies (hence the difficulty of classifying these microproblematica). Calcimicrobial deposits are recognized by overall sediment structure, by externally calcified filaments or spherical bodies, and by trapped sediment. Flat-lying laminated sediment, domal stromatolites, or clotted, finger-like thrombolite structures are characteristic — shapes vary with environmental conditions (water depth, current strength, and others).
Lamination in stromatolites reflects microbial growth through day-night cycles and tidal cycles; those organic laminae commonly are interspersed with micritic or peloidal carbonate or terrigenous detritus that was deposited during episodic storms.
Non-stromatolitic calcimicrobes typically form lumpy encrustations or small upright “shrubs”.
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.