cementation

The fusion of rock sediments or the remains of microscopic marine organisms into sedimentary rocks. As the material builds up it is compacted together. Cementation occurs as chemical precipitation deposits micro crystals in the spaces between the mineral fragments and grains, holding them together. The ‘cement’ formed may be silica, iron oxide, or carbonate. Cementation contributes to the processes involved in the rock cycle, the formation, change, and reformation of rocks.

Clastic sedimentary rocks, such as sandstone, siltstone, and claystone, are made from the sediments of other rocks. Non-clastic sedimentary rocks, such as limestone and chalk, are of organic origin. Silica cementation takes place below the water table surface; its source is groundwater. Carbonate cementation, also sourced from groundwater, takes place either at above or below ground level. Iron-oxide cementation is less frequent.

Clastic sedimentary rocks The process that turns sediments into rocks is known as lithification. Sediments are small broken pieces of rocks and mud grains obtained from the breakdown of other igneous, metamorphic, and sedimentary rocks by a combination of chemical weathering and physical weathering. The sediments are transported by various means, including gravity, wind, rain, rivers, and ice, and are eventually deposited in a lake or sea. The larger rock pieces settle at the bottom and smaller rocks and grains are deposited on top. Large and small pieces of rock and mud debris are compacted together by the weight of overlying rock material pressing on the lower layers. Water is squeezed out, and precipitation deposits minerals between the grains, cementing the rock pieces together.

Conglomerate sedimentary rocks are formed by cementation process. Sandstone is formed by the compaction of sand grains. There are many different types of sandstone rock, such as quartz sandstone and arkoses sandstone.

Different types of clastic sedimentary rocks have different grain sizes. Conglomerate rock can contain pieces over 250 mm/10 in in diameter; sandstone up to 2 mm/0.08 in; siltstone up to 0.02 mm/0.0008 in; and claystone, less than 0.02 mm/0.0008 in.

Non-clastic sedimentary rocks Chalk and limestone are examples of non-clastic sedimentary rocks. They are formed at the bottom of the oceans. Chalk is a soft white, fine grain rock made chiefly from the remains of calcium carbonate-secreting algae, along with the minute remains of other marine organisms. Smaller grains of lime mud particles have settled on top and mixed with the calcium carbonate sediments. Silica mineral cemented together the different size particles to create chalk.

Limestone is made from the remains of sea shells containing calcium carbonate, or precipitated from solution. As layers of sediments of seashells and silt particles deposit, the weight of the top layers have compacted and cemented the sediments together into a solid rock; the process has taken over 60 million years.