Classify Sedimentary Rocks

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Three groups of sedimentary rocks

Most sedimentary rocks are formed from clasts produced as a result of weathering and erosion. Clasts can range in size from boulders to clay and include solid fragments called pyroclasts that have been ejected from volcanoes. The erosion, transport and deposition of clasts are by mechanical processes and they are described as mechanically formed sedimentary rocks.

The products of chemical weathering always include ions in solution. They are transported in solution in rivers and the sea. A change in the chemical environment can lead to their deposition, for example, where sea water is evaporated and CaCo3 crystals are precipitated. The weathering transport and deposition of these rocks is by chemical processes and they are called chemically formed sedimentary rocks.

Biologically formed sedimentary rocks result when organisms, such as sea creatures or trees, extract ions that are in solution in sea water or groundwater and turn them into organic tissue, such as shells or wood. When the organism dies, its remains will be deposited and may be buried to form sedimentary rocks. Often the remains are transported and become broken up. Shell fragments are an example of this. Rocks composed of these fragments are described as bioclastic.

Classification

Classification of sedimentary rocks involves examining them in the field, in hand specimen, through photos and thin sections. The three main groups are classified more precisely by using grain size, grain shape, mineral composition and fossil content.

Grain size is used to divide up the mechanically formed sedimentary rocks.

Grain shape is used to distinguish between conglomerate, which has rounded clasts and breccia which has angular clasts.

Grain size is not so useful in classifying chemically and biologically forming sedimentary rocks, but mineral composition and fossil content are, A rock containning more than 50% calcium carbonate is limestone. This is usually in the form of the mineral calcite. It is easy to recognise as it reacts strongly with dilute HCl. There are several different types of limestone, based on their fossil content and type of grain.

Mechanically formed:

transported terrestrial sediment formed by weathering, erosion and transport. Transported volcanic material.

clastic (subdivided by grain size and composition). Pyroclastic

Boulders, cobbles, pebbles, gravel, sand, silt, clay. Ash and lapilli. Blocks and bombs.

>2mm - conglomerates and breccias

2 - 0.0625mm - sandstones

<0.0625mm - mudstone, shale, clay, siltstone

Tuff. Agglomerate

Biologically formed:

formed by activity of living organisms

biogenic and bioclastic

reef organisms, shell fragments of calcareous organisms, terrestrial plant matter, calcareous algae (coccoliths), microorganism oozes

crinoidal, reef, bioclastic limestone, coal, chalk, chert

Chemically formed:

formed by evaporation of water and precipitation of minerals

chemical

evaporite minerals (subdivided by composition), ooliths, calcareous (lime) mud, iron minerals

evaporites dolomite, gypsum, halite, K salts, oolitic limestone, micritic limestone, ironstones

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