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PART 4

Sedimentary rocks

Sedimentary rocks are composed of two groups:

•    Terrigenous clastic rocks, which are largely composed of fragments of pre-exist-ing rocks and minerals transported from their source in a fluid (air or water) and deposited.

•    Rocks formed by precipitation from solution. either from the secretions of organ-isms, as in many limestones, or directly as in the case of salt deposits.

Sedimentary petrography usually refers to the study of sediments under the microscope. It is important sińce it is oflen the only easily available method of studying the detailed mineralogy and grain types of sediments. It can reveal the original source of the eroded fragments of terrigenous clastic rocks (prorenance) and shed light on the depositional environment of limestones. Microscopic studics are particularly important in understanding post-depositional changes which occur in sediments. These changes, known as diagenesis, include physical and Chemical modifications which occur during burial as a result of inereasing load pressure and the passage of Solutions through the sediment. Diagenesis may profoundly affect porosiły (the percentage of porę spacc in a bulk volume of rock) and permeabiUty (ability of a rock to allow fluid to flow through it). This is of great relevance to the study of aquifers and hydrocarbon reservoirs.

Ideałly petrological studies of sediments should be integrated with field data in the case of outerop studies, or well-log data in the case of subsurface studies, in order to clucidate fully the depositional and post-depositional history of sedimentary sequences.

Terrigenous clastic rocks

The primary division of terrigenous clastic rocks is according to average grain sizc. It is dominantly the sandsłones or areniles (average grain size rangę from '_mto 2mm) which are studied using the petrological microscope.

In finer grained sediments (mudstones) the particles cannot be easily studied without special techniqucs or by the use of an electron microscope. In coarser sediments (conglomerates) the grains can usually be identified in hand specimen using a lens. Furthermore, the smali arca of a typical thin section will contain rela-tively few grains of a coarse sediment which may not be rcprescntative of the rock as a whole.