Primary Sedimentary Structures
Primary sedimentary structures are physical and/or biological features formed during the process of sediment deposition. Generally such structures are best seen in outcrop, core, or polished hand sample, but smaller features such as borings or fenestral fabrics are both readily apparent in thin section and important to recognize. Their identification can improve interpretations of depositional environments and can also help to decipher patterns and timing of diagenesis. The characteristic features for the recognition and interpretation of primary sedimentary structures are provided in the figure captions. Diagenetic sedimentary structures, such as hardgrounds, soil crusts, or stylolites, are covered in the appropriate diagenetic chapters.
Borings - Openings created in relatively rigid rock, shell, or other material by boring organisms. The rigid host substrate is the feature that distinguishes borings from soft-sediment burrows.
Burrow porosity - Feature created by organic burrowing in relatively unconsolidated sediment, in contrast to borings. Most burrows collapse, become filled with sediment, or are back-filled by the burrow-forming organism itself.
Fenestrae (fenestral fabric) - Primary or penecontemporaneous gaps in rock framework larger than grainsupported interstices. Such features may be open pores or may have been partially or completely filled with internal sediment and/or sparry cement. Fenestrae occur as somewhat rounded features of spherical, lenticular, or more irregular shapes; their large size in comparison to normal interparticle openings and their multigranular roofs, floors, and other margins are key characteristics. Fenestrae are commonly somewhat flattened parallel with the laminae. They may, however, be round or very irregular, and some are elongate in a vertical dimension. Although isolated fenestrae occur in sedimentary carbonates, it is more common to find many in close association. Fenestrae are generally associated with microbial mats and result from shrinkage, gas formation, organic decay, trapping of air through swash-zone wave action, or other synsedimentary processes (Choquette & Pray, 1970).
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