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Abstract

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).

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