Introduction to the Elements of Sedimentary Rocks

Two views of an unconformity in NW Scotland. Torridonian (late Proterozoic) sandstones overlie Archaean high- grade metamorphic rocks known as the Lewisian Gneisses. Uplift and erosion occurred after formation of the metamorphic rocks but prior to deposition of the Torridonian mainly by ancient rivers. Thus, the Torridonian sits on an erosion surface produced by sub-aerial erosion in the Precambrian. Erosion has left isolated hills of the Torridonian surrounded by gneisses.

Sedimentation is the final stage of the process which begins with weathering, proceeds through erosion and transportation of eroded materials to their site of deposition. Physical sedimentation is the deposition of fragments of eroded material physically carried by water, wind, ice and to some degree gravity.

https://www.britannica.com/science/sedimentary-rock

Sedimentary rocks form through erosion and sedimentation of pre-existing igneous, metamorphic or even sedimentary rocks. The bases of sedimentary sequences lie above unconformities separating them from older rocks.

Chemical sedimentation is the precipitation of minerals from water, and the subsequent accumulation of this precipitate. Thus, material is transported in solution.
Precipitation may be influenced by:-

(1) biological processes (such as the extraction of calcium carbonate from water by invertebrates to make shells or coral reefs, or silica by diatoms, or sulphides by bacteria)

(2) changes in the chemistry of the water due to evaporation, loss of CO2 or mixing of waters with different chemistries. An example of this would be the so-called speleothems found in caves, such as stalactites and stalagmites.

Accumulation of chemical sediments may take the form of in situ precipitation or the physical sedimentation of precipitated mineral phases.

Some rocks have a polyphase sedimentation history which may involve physical and chemical sedimentation, such as a physically sedimented rock whose source was a chemically sedimented accumulation. e.g. pebbles in a river where the pebbles have been derived from erosion of a coral-rich limestone.

It is possible to study the mineralogical and textural relationships within rocks by examining thin-sections and hand-specimens and such a study is known as petrography. The petrographic characteristics of a sedimentary rock such as grain-sizes, grain shapes, and packing etc. help us to determine their mode of sedimentation.

A final phase in the formation of sedimentary rocks involves the chemical and physical changes which occur during the transformation of a soft sediment into a hard sedimentary rock. A lump term for these processes is diagenesis. Diagenesis may produce significant changes in the petrography of the rock, such as the dissolution of grains, re-packing of grains, crushing of grains and the precipitation of new minerals within pore spaces which hold the grains together (known as cement).

The study of sedimentary rocks including their petrography, thicknesses, lateral continuity, internal geometries of sedimentary units and their relationships with other units, allows us to infer their depositional environment. The goal of sedimentology (the study of sedimentary rocks) is to reconstruct the sedimentary environments which existed when the sediments were deposited. So, to understand sedimentary rocks we must be able to:-

Photomicrograph of a volcanic sand grain; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.()

1. Describe the petrography of the rocks, that is, the mineralogical and textural relationships within rocks revealed by observations of hand specimens and thin sections.

• Primary petrographic features will tell us about their mode of sedimentation and some information on their depositional environment.
• Secondary petrographic features will tell us about their diagenetic history.

1. Describe the field characteristics (thicknesses, lateral continuity, internal geometries of sedimentary units and their relationships with other units) of sedimentary units and use this information to further refine our assessment of depositional environments.

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Next up I will discuss the primary and secondary petrographic characteristics of sedimentary rocks.

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Sources and References:

https://pubs.usgs.gov/bul/1107f/report.pdf
http://file.scirp.org/Html/2-1110071_41785.htm
https://en.wikipedia.org/wiki/Petrography

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