Siliciclastic rocks are the lithified accumulations of clastic grains of silicate minerals and rocks that have typically been deposited by water. The classification of siliciclastic rocks is based on grain size: Conglomerates > 30% gravel (>2 mm) and larger clastic Grains & Breccias: (< 5 %) Sandstones: > 50% sand-sized (0.06 - 2 mm) clastic grains (~30%) Mudstones: > 50% silt (0.06 - 004 mm) and ( 65 %) / or clay (< 0.004 mm)
Grain-Size versus Transport Mechanism There is a rough correspondence between the major grain size divisions and the transport mechanism, which is in return responsible for their physical separation during the fluid transport process, and the final distinction between sedimentary rocks. pebbles and larger are carried in the ‘bed load’ by traction - conglomerates sands are carried in ‘bed load’ by intermittent saltation and suspension - sandstones silts and clays are carried in suspension in the ‘wash load’ - shales
Conglomerates and Breccias (> 30% gravel (>2 mm) and larger clastic grains): Conglomerates and breccias can be distinguished by the sphericity of the clasts in the rock: if the clasts are rounded the rock is referred to as a conglomerate, if they are angular it is a breccia. With both conglomerates and breccias, grain-size, shape and orientation can be measured accurately in the field and may provide valuable information about the depositional environment. It is also important to note the ‘maximum clast size’, since it is often a reflection of the competency of the flow (i.e. a measure of the hydraulic energy of the transport process).
Sandstones ( > 50% sand-sized (0.062 - 2 mm) clastic grains ): Sandstones are classified according to the types of clastic grains present (quartz, feldspar, & lithic fragments) and the presence (wackes) or absence (arenites) of significant fine-grained matrix material (< 0.03 mm). After this subdivision, they are described in terms of the types of preserved sedimentary structures, using terms like cross-bedded sandstone and relative maturity using criteria such as degree of sorting, roundness of the clasts, diversity of clast types, etc. greywacke
Arenites:fine-grained matrix not visible to naked eye (<10%). quartz arenite: quartz grains 90 %. (~ 35 %) Rare in modern environment, but quite common in late Precambrian and Paleozoic. Tend to be relatively mature, and may represent end-product of several cycles of erosion, transport, and deposition. Abundance of quartz arenites in Archean may reflect more intensive early Earth weathering conditions. Commonly silica cement predominates. synonym = orthoquartzite feldspathic arenite: visible feldspar / (felds + rock frag.) 50 %. (~ 15 %) C ommonly developed in granitic terranes and therefore restricted to local basins, but may also develop in cold or arid climates where feldspar is relatively resistant to decomposition, or in areas of high erosion rates. Typically cemented by calcite. synonym = arkose, if felds is K-spar lithic arenite:visible rock fragments / (felds + rock frag.) 50% (~ 20 %) The most abundant sandstone, as the sand-sized sediment loads of most modern rivers are dominated by lithic clasts. Furthermore, if greywackes are derived from the decomposition of lithic and feldspar clasts, then lithic arenites comprise 50 % of all arenites. Tend to be immature, poorly sorted. Typically cemented by calcite. synonym = sub-greywacke
Greywacke: fine-grained ( 10-15%). (35%) The presence of the matrix gives the rock a dark grey colour. The clastic grains are polymictic and commonly relatively angular. The matrix is composed finely crystalline chlorite and sericite developed during diagenesis, along with silt-size quartz and albite. This fine-grained matrix has reacted with and obliterated the original outline of the clastic grains, acting as the cementing agent. There are two hypotheses for the origin of the matrix: 1. diagenetically altered interstitial silt and clay. 2. diagentically altered lithic and feldspar grains of a former lithic arenite. Most true greywackes are Paleozoic, or older in age, occurring as ‘flysch’ sequences of marine turbidites in response to orogenic events. Greywackes are not found in fluviatile or any other continental environment. Few modern sediments or sandstones, including marine turbidites, contain significant fine-grained matrix. The question is thus are all greywackes produced by diagenesis of lithic arenite protoliths, or was there something different about the transport and depositional mechanism of greywackes in the past which is not operative today. Relative abundance: lithic wacketypical feldspathic wackeless common quartz wacke rare
Mud Rocks: ( > 50% silt (0.062 - 004 mm) and / or clay (< 0.004 mm)) Mud rocks are composed of silt-sized quartz and feldspar grains and much smaller clay mineral particles. Depending of the relative proportions of these two types of grains, mud rocks range from siltstones to shales, mudstones, and claystones. Siltstones can be distinguish from shales and mudstones by biting a piece between your teeth. If it feels "gritty" then it is a siltstone, if it feels smooth or slick, then it is a shale or claystone. One of the most important features of mud rocks is their colour, an indication of their oxidation state and the paleo-environment of their deposition: Red shales are oxidized and typically represent sub-aerial detritus derived from the continents. They may represent in actual sub-aerial deposits, but also are formed by continental dust settling into organic-poor deep marine environments. Green shales are relatively reduced, and common in the shallow submarine environments depleted in oxygen by the decay of organic matter. Black shales are rich in organic matter and highly reduced, typically deposited in anoxic environments. They sometimes act as source rocks from which oil and gas are released during burial and diagenesis.