Sedimentary Rocks and Processes

Sedimentary rocks are those rocks formed from sediment- material consisting of sand, gravel, mud, ions in solution derived from preexisting rocks or organic debris derived from living organisms.

How does the sedimentary rock are formed?

There are five basic steps involved in the formation of sedimentary rocks:

Weathering: making the sediment by breaking down or dissolving preexisting rocks or living organisms.
Erosion: picking up the sediment by water, wind, or glaciers
Transportation: moving the sediment by water, wind, or glaciers.
Deposition: depositing the sediment.
Lithification: turning the sediment to rock.

Weathering

All rocks (igneous, metamorphic, and sedimentary) exposed at the Earth's surface are subjected to the relentless effects of weathering. Physical weathering acts to break up rocks into smaller pieces while chemical weathering acts to change the composition of various minerals into other minerals or forms which are stable at the temperature and pressure conditions found at the Earth's surface.

For example, when a plagioclase is exposed at the surface it begins to react with the water in the atmosphere to break down into clay minerals and ions that dissolve in the water. In fact, all of the common rock-forming silicate minerals EXCEPT quartz will react (with rain etc.) to form clay minerals and ions that remain dissolved in the water (solution). The weathering process, then, vastly reduces the possible minerals likely to be found in sedimentary rocks.

Sedimentary Rocks and Processes

Quartz, clay minerals and rock fragments are sediments that remain as solid particles during the process of becoming sedimentary rocks. These particles are called clasts- meaning loose material from the disintegration of rocks.

Sedimentary rocks formed from the accumulation and lithification of clasts are called clastic sedimentary rocks. The ions dissolved in solution (water) remain dissolved throughout the erosion and transportation phases of the sedimentary rock forming process. They do not become solid again until they are deposited by chemical precipitation at the deposition site. Sedimentary rocks formed entirely by chemical (or biochemical) precipitation of the dissolved ions are called chemical sedimentary rocks.

Other sedimentary rocks form from the accumulation of organic material or remnants. These rocks are called either biochemical or bioclastic sedimentary rocks. This difference in the mechanism of sedimentary deposition forms the basis for the major division in the classification of sedimentary rocks.

Erosion and Transportation

The major agents of erosion and transportation are water, wind, and ice (in the form of glaciers). Erosion is the physical "pickup" of weathered material from the source area. Transportation is the movement of that material away from the source. These processes are most important in the characteristics and history of clastic sediments. The chief effects that erosion and transportation produce on clastic sediments are to change the grain size, grain sorting and grain rounding.

Grain size

Grain size is dependent on a variety of factors, the most important of which are transport distance & rate of transportation. The farther the material is transported, the smaller the grains tend to be. The faster the velocity (higher energy) of transport (wind or water), the larger the clasts may be because the higher the energy, the larger are the particles that can be moved. Water has the capacity to transport much larger clasts than wind, and glaciers have the capacity to carry any size and mixture of materials.

Geologists have developed a system for the classification of grain sizes. These are shown under the classification of Clastic rocks and illustrated in the Rock Classification chart.

Grain Roundness

Grain Roundness is a measure of the angularity of fragments in clastic sedimentary rocks and ranges from very angular (with sharp edges and corners) to well rounded (considerably more spherical). The degree of grain rounding is related to the distance the grain has been transported, with more roundness indicating a greater distance of transport. Harder clasts also tend to stay angular for longer transport distances.

Sorting

Sorting refers to the variation of grain size in clastic rocks. Rocks made of grains of uniform size are described as well sorted. If the constituent grains are of various sizes, the clastic rock is referred to as poorly sorted. The degree of sediment sorting is controlled by the rate of deposition of the clasts (how quickly they are deposited) and the viscosity of the medium that carried the particles (i.e. grains deposited by wind – a low-viscosity medium – are considerably more well sorted than those deposited from a glacier – a high-viscosity medium).

Deposition

The area in which sediments are deposited (in the case of clasts/sediment) or chemically precipitated (in the case of ions in solution) is called the environment of deposition Environments of deposition can be as varied as the landscapes of the Earth today, but are usually simplified as belonging to the continent (subaerial or terrestrial), the ocean (marine), or a little of both (transitional).

Continental environments include alluvial fans, playa lakes, lakes, rivers, swamps, deserts, and glacial. Marine environments include the shallow and deep varieties, whereas the transitional environments include deltas, the beach, and tidal flat areas. Like the agents of erosion and transportation, depositional systems affect certain characteristics to the resulting sedimentary rocks- including grain sorting, rounding and size as described above.

Lithification

In the process of going from unconsolidated clasts/ sediment or crystals to sedimentary rocks, sediments are compacted and cemented. During compaction, clasts are forced together and some of the water in the sediments is driven out. Compaction usually results from the weight of overlying sediments pressing down on the sediment beneath them. Most sediments need not only compaction, but also require cementation to become sedimentary rock.

Cementation

Cementation describes the process where dissolved mineral components seep between the sediment clasts and crystallize between the clasts to form a sort of mineral “glue” that holds the rock together. The most common cementing materials are calcite, quartz, hematite, and limonite, all of which are usually provided by solution activity (chemical weathering).

These cements may impart certain characteristics to the resulting sedimentary rock. For example, rocks cemented with calcite will react to hydrochloric acid. Rocks cemented by hematite have a characteristic red color (the sandstones around Sedona, for example). Rocks cemented by limonite have a characteristic yellow, yellow-brown color. The degree of cementation can range from minor (poorly cemented) to substantial (well cemented).