Clastic rock
Clastic rocksare composed of fragments, or clasts, of pre-existingmineralsand rock. Aclastis a fragment ofgeological detritus,[1]chunks, and smaller grains of rock broken off other rocks byphysical weathering.[2]Geologists use the termclasticto refer tosedimentary rocksand particles insediment transport,whether insuspensionor asbed load,and insedimentdeposits.
Sedimentary clastic rocks[edit]
Clasticsedimentary rocksare rocks composed predominantly of broken pieces orclastsof olderweatheredanderodedrocks. Clastic sediments or sedimentary rocks are classified based ongrain size,clast and cementing material (matrix) composition, and texture. The classification factors are often useful in determining a sample'senvironment of deposition.An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieceserodedfrom solid rock upstream.
Grain size varies fromclayinshalesandclaystones;throughsiltinsiltstones;sandinsandstones;andgravel,cobble,tobouldersized fragments inconglomeratesandbreccias.TheKrumbein phi (φ) scalenumerically orders these terms in a logarithmic size scale.
Siliciclastic sedimentary rocks[edit]
Siliciclasticrocks are clastic noncarbonate rocks that are composed almost exclusively of silicon, either as forms ofquartzor as silicates.
Composition[edit]
The composition ofsiliciclasticsedimentary rocks includes the chemical and mineralogical components of the framework as well as the cementing material that make up these rocks. Boggs divides them into four categories; major minerals, accessory minerals, rock fragments, and chemical sediments.[3]
Major minerals can be categorized into subdivisions based on their resistance to chemical decomposition. Those that possess a great resistance to decomposition are categorized as stable, while those that do not are considered less stable. The most common stable mineral in siliciclastic sedimentary rocks is quartz (SiO2).[3]Quartz makes up approximately 65 percent of framework grains present in sandstones and about 30 percent of minerals in the average shale. Less stable minerals present in this type of rocks arefeldspars,including both potassium and plagioclase feldspars.[3]Feldspars comprise a considerably lesser portion of framework grains and minerals. They only make up about 15 percent of framework grains in sandstones and 5% of minerals in shales. Clay mineral groups are mostly present in mudrocks (comprising more than 60% of the minerals) but can be found in other siliciclastic sedimentary rocks at considerably lower levels.[3]
Accessory minerals are associated with those whose presence in the rock are not directly important to the classification of the specimen. These generally occur in smaller amounts in comparison to the quartz, and feldspars. Furthermore, those that do occur are generally heavy minerals or coarse grained micas (bothmuscoviteandbiotite).[3]
Rock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10–15 percent of the composition of sandstone. They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition ofmudrocks.Though they sometimes are, rock fragments are not always sedimentary in origin. They can also bemetamorphicorigneous.[3]
Chemical cements vary in abundance but are predominantly found in sandstones. The two major types are silicate based and carbonate based. The majority of silica cements are composed of quartz, but can includechert,opal,feldspars andzeolites.[3]
Composition includes thechemicalandmineralogicmake-up of the single or varied fragments and the cementing material (matrix) holding the clasts together as a rock. These differences are most commonly used in the framework grains of sandstones. Sandstones rich in quartz are calledquartz arenites,those rich in feldspar are calledarkoses,and those rich inlithicsare calledlithic sandstones.
Classification[edit]
Siliciclastic sedimentary rocks are composed of mainly silicate particles derived from the weathering of older rocks and pyroclastic volcanism. While grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories:conglomerates,sandstones,andmudrocks.The termclayis used to classify particles smaller than.0039 millimeters. However, the term can also be used to refer to a family of sheet silicate minerals.[3]Siltrefers to particles that have a diameter between.062 and.0039 millimeters. The termmudis used when clay and silt particles are mixed in the sediment;mudrockis the name of the rock created with these sediments. Furthermore, particles that reach diameters between.062 and 2 millimeters fall into the category of sand. When sand is cemented together and lithified it becomes known as sandstone. Any particle that is larger than two millimeters is considered gravel. This category includespebbles,cobbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates.[3]
Conglomerates and breccias[edit]
Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a finer grained matrix.[4]These rocks are often subdivided into conglomerates and breccias. The major characteristic that divides these two categories is the amount of rounding. The gravel sized particles that make up conglomerates are well rounded while in breccias they are angular. Conglomerates are common in stratigraphic successions of most, if not all, ages but only make up one percent or less, by weight, of the total sedimentary rock mass.[3]In terms of origin and depositional mechanisms they are very similar to sandstones. As a result, the two categories often contain the same sedimentary structures.[3]
Sandstones[edit]
Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together. These sand-size particles are oftenquartzbut there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition. Classification schemes vary widely, but most geologists have adopted theDott scheme,[5][better source needed]which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains.
Mudrocks[edit]
Rocks that are classified as mudrocks are very fine grained. Silt and clay represent at least 50% of the material that mudrocks are composed of. Classification schemes for mudrocks tend to vary, but most are based on the grain size of the major constituents. In mudrocks, these are generally silt, and clay.[6]
According to Blatt, Middleton and Murray[7]mudrocks that are composed mainly of silt particles are classified as siltstones. In turn, rocks that possess clay as the majority particle are called claystones. In geology, a mixture of both silt and clay is called mud. Rocks that possess large amounts of both clay and silt are called mudstones. In some cases the term shale is also used to refer to mudrocks and is still widely accepted by most. However, others have used the term shale to further divide mudrocks based on the percentage of clay constituents. The plate-like shape of clay allows its particles to stack up one on top of another, creating laminae or beds. The more clay present in a given specimen, the more laminated a rock is. Shale, in this case, is reserved for mudrocks that are laminated, while mudstone refers those that are not.
-
Red mudrock -
Black Shale
Diagenesis of siliciclastic sedimentary rocks[edit]
Siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds. The process of turning loose sediment into hard sedimentary rocks is calledlithification.During the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock. The primary physical process in lithification is compaction. As sediment transport and deposition continues, new sediments are deposited atop previously deposited beds, burying them. Burial continues and the weight of overlying sediments causes an increase in temperature and pressure. This increase in temperature and pressure causes loose grained sediments become tightly packed, reducing porosity, essentially squeezing water out of the sediment. Porosity is further reduced by the precipitation of minerals into the remaining pore spaces.[3]The final stage in the process isdiagenesisand will be discussed in detail below.
Cementation[edit]
Cementation is the diagenetic process by which coarse clastic sediments become lithified or consolidated into hard, compact rocks, usually through the deposition or precipitation of minerals in the spaces between the individual grains of sediment.[4]Cementation can occur simultaneously with deposition or at another time. Furthermore, once a sediment is deposited, it becomes subject to cementation through the various stages of diagenesis discussed below.
Shallow burial (eogenesis)[edit]
Eogenesis refers to the early stages of diagenesis. This can take place at very shallow depths, ranging from a few meters to tens of meters below the surface. The changes that occur during this diagenetic phase mainly relate to the reworking of the sediments. Compaction and grain repacking,bioturbation,as well as mineralogical changes all occur at varying degrees.[3]Due to the shallow depths, sediments undergo only minor compaction and grain rearrangement during this stage. Organisms rework sediment near the depositional interface by burrowing, crawling, and in some cases sediment ingestion. This process can destroy sedimentary structures that were present upon deposition of the sediment. Structures such as lamination will give way to new structures associated with the activity of organisms. Despite being close to the surface, eogenesis does provide conditions for important mineralogical changes to occur. This mainly involves the precipitation of new minerals.
Mineralogical changes during eogenesis[edit]
Mineralogical changes that occur during eogenesis are dependent on the environment in which that sediment has been deposited. For example, the formation ofpyriteis characteristic of reducing conditions in marine environments.[3]Pyrite can form as cement, or replace organic materials, such as wood fragments. Other important reactions include the formation ofchlorite,glauconite,illiteandiron oxide(if oxygenated pore water is present). The precipitation of potassium feldspar, quartz overgrowths, and carbonate cements also occurs under marine conditions. In non marine environments oxidizing conditions are almost always prevalent, meaning iron oxides are commonly produced along withkaolingroup clay minerals. The precipitation of quartz and calcite cements may also occur in non marine conditions.
Deep burial (mesogenesis)[edit]
Compaction[edit]
As sediments are buried deeper, load pressures become greater resulting in tight grain packing and bed thinning. This causes increased pressure between grains thus increasing the solubility of grains. As a result, the partial dissolution of silicate grains occurs. This is called pressure solutions. Chemically speaking, increases in temperature can also cause chemical reaction rates to increase. This increases the solubility of most common minerals (aside from evaporites).[3]Furthermore, beds thin and porosity decreases allowing cementation to occur by the precipitation of silica or carbonate cements into remaining pore space.
In this process minerals crystallize from watery solutions that percolate through the pores between grain of sediment. The cement that is produced may or may not have the same chemical composition as the sediment. In sandstones, framework grains are often cemented by silica or carbonate. The extent of cementation is dependent on the composition of the sediment. For example, in lithic sandstones, cementation is less extensive because pore space between framework grains is filled with a muddy matrix that leaves little space for precipitation to occur. This is often the case for mudrocks as well. As a result of compaction, the clayey sediments comprising mudrocks are relatively impermeable.
Dissolution[edit]
Dissolution of framework silicate grains and previously formed carbonate cement may occur during deep burial. Conditions that encourage this are essentially opposite of those required for cementation. Rock fragments and silicate minerals of low stability, such asplagioclasefeldspar,pyroxenes,andamphiboles,may dissolve as a result of increasing burial temperatures and the presence of organic acids in pore waters. The dissolution of frame work grains and cements increases porosity particularly in sandstones.[3]
Mineral replacement[edit]
This refers to the process whereby one mineral is dissolved and a new mineral fills the space via precipitation. Replacement can be partial or complete. Complete replacement destroys the identity of the original minerals or rock fragments giving a biased view of the original mineralogy of the rock.[3]Porosity can also be affected by this process. For example, clay minerals tend to fill up pore space and thereby reducing porosity.
Telogenesis[edit]
In the process of burial, it is possible that siliciclastic deposits may subsequently be uplifted as a result of a mountain building event orerosion.[3]When uplift occurs, it exposes buried deposits to a radically new environment. Because the process brings material to or closer to the surface, sediments that undergo uplift are subjected to lower temperatures and pressures as well as slightly acidic rain water. Under these conditions, framework grains and cement are again subjected to dissolution and in turn increasing porosity. On the other hand, telogenesis can also change framework grains to clays, thus reducing porosity. These changes are dependent on the specific conditions that the rock is exposed as well as the composition of the rock and pore waters. Specific pore waters, can cause the further precipitation of carbonate or silica cements. This process can also encourage the process of oxidation on a variety of iron bearing minerals.
Sedimentary breccias[edit]
Sedimentarybrecciasare a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They may form either:
- In submarinedebris flows,avalanches,mud flow or mass flow in an aqueous medium. Technically,turbiditesare a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow.
- As angular, poorly sorted, very immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithifiedcolluvium.Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps ingrabens.
In the field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely fromdrillinginformation. Sedimentary breccias are an integral host rock for manysedimentary exhalative deposits.
Igneous clastic rocks[edit]
Clasticigneous rocksincludepyroclasticvolcanic rockssuch astuff,agglomerateand intrusivebreccias,as well as some marginaleutaxiticandtaxiticintrusive morphologies. Igneous clastic rocks are broken by flow, injection or explosive disruption of solid or semi-solid igneous rocks orlavas.
Igneous clastic rocks can be divided into two classes:
- Broken, fragmental rocks produced byintrusiveprocesses, usually associated withplutonsor porphyry stocks
- Broken, fragmental rocks associated with volcanic eruptions, both of lava and pyroclastic type
Metamorphic clastic rocks[edit]
Clasticmetamorphic rocksincludebrecciasformed infaults,as well as someprotomyloniteandpseudotachylite.Occasionally, metamorphic rocks can be brecciated viahydrothermalfluids, forming ahydrofracturebreccia.
Hydrothermal clastic rocks[edit]
Hydrothermal clastic rocks are generally restricted to those formed byhydrofracture,the process by whichhydrothermalcirculation cracks and brecciates the wall rocks and fills them in with veins. This is particularly prominent inepithermaloredeposits and is associated withalteration zonesaround many intrusive rocks, especiallygranites.Manyskarnandgreisendeposits are associated with hydrothermal breccias.
Impact breccias[edit]
A fairly rare form of clastic rock may form duringmeteoriteimpact. This is composed primarily of ejecta; clasts ofcountry rock,melted rock fragments,tektites(glass ejected from the impact crater) and exotic fragments, including fragments derived from the impactor itself.
Identifying a clastic rock as an impact breccia requires recognisingshatter cones,tektites,spherulites,and the morphology of an impactcrater,as well as potentially recognizing particular chemical and trace element signatures, especiallyosmiridium.
References[edit]
- ^Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3
- ^Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-5
- ^abcdefghijklmnopqrBoggs, Jr., Sam. Principles of Sedimentology and Stratigraphy. Pearson Prentice Hall: Upper Saddle River, New Jersey, 2006
- ^abNeuendorf, Klaus; Mehl, James; Jackson, Julia Glossary of Geology, Fifth Edition. American Geological Institute: Alexandria, VA; 2005.
- ^Dott, R. H., Wacke, graywacke and matrix – What Approach to Immature Sandstone Classification: Journal of Sedimentary Petrology, v. 34, pp. 625–32., 1996.
- ^Spears, D.A., Sam. Towards a classification of Shales. J. geol. soc., London, 137, 1990.
- ^Blatt, h., Middleton, G. V. & Murray, R. C. 1972. Origin of Sedimentary Rocks. Prentice Hall Inc., Englewood Cliffs, 634 pp.
External links[edit]
The dictionary definition ofclastat Wiktionary
