This article is semi-protected until January 29, 2019. Sedimentation may also occur as minerals precipitate from water non clastic sedimentary rocks pdf or shells of aquatic creatures settle out of suspension. Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic sedimentary rocks, are subdivided according to the dominant particle size.
The relative abundance of sand-sized framework grains determines the first word in a sandstone name. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Most authors presently use the term “mudrock” to refer to all rocks composed dominantly of mud. While material dissolves at places where grains are in contact, that material may recrystallize from the solution and act as cement in open pore spaces. As a result, there is a net flow of material from areas under high stress to those under low stress, producing a sedimentary rock becomes more compact and harder. Loose sand can become sandstone in this way.
In this way, loose clasts in a sedimentary rock can become “glued” together. When sedimentation continues, an older rock layer becomes buried deeper as a result. The lithostatic pressure in the rock increases due to the weight of the overlying sediment. This causes compaction, a process in which grains mechanically reorganize. During compaction, this interstitial water is pressed out of pore spaces. The dissolved material precipitates again in open pore spaces, which means there is a net flow of material into the pores.
However, in some cases, a certain mineral dissolves and does not precipitate again. Burial of rocks due to ongoing sedimentation leads to increased pressure and temperature, which stimulates certain chemical reactions. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange colour. However, a red colour does not necessarily mean the rock formed in a continental environment or arid climate.
The presence of organic material can colour a rock black or grey. Organic material is formed from dead organisms, mostly plants. Under anoxic circumstances, however, organic material cannot decay and leaves a dark sediment, rich in organic material. This can, for example, occur at the bottom of deep seas and lakes. There is little water mixing in such environments, as a result oxygen from surface water is not brought down, and the deposited sediment is normally a fine dark clay. When all clasts are more or less of the same size, the rock is called ‘well-sorted’, and when there is a large spread in grain size, the rock is called ‘poorly sorted’.