Competitive Exams: Sedimentary rocks

Sedimentary rocks are produced by the weathering of pre-existing rocks and the subsequent transportation and deposition of the weathering

Important characteristics

  1. It contains strata or layers.

  2. The layers are rarely horizontal and generally tilted due to lateral compressive and tensile forces.

  3. It is formed of sediments derived from the older rocks, plants and animals remains.

  4. It covers the 75 per cent of the surface area of the globe.

  5. Most of the sedimentary rocks are permeable and porous.

  6. 1t is characterised by different sizes of joints. These are generally perpendicular to the bedding plains.

  7. The riverine sedimentary rocks develop cracks when exposed to the sun. These cracks are generally of polygonal shape.

  8. The most favourable sites of their formation is shallow sea floor hording continents.

  9. The connecting plane between two consecutive beds or layers of sedimentary rocks is called ‘bedding plane’ The uniformity of two beds along a bedding plane is called conformity (i.e.when beds are similar in all respect). When two consecutive beds are not uniform or conformal, the structure is called unconformity. In fact, ‘an unconformity is a break in a stratigraphic sequence resulting from a change in conditions that caused deposition to cease for a considerable time’ There are several types of unconformity e-g

    1. non-conformity (where sedimentary rocks succeed igneous or metamorphic rocks)

    2. angular unconformity (where horizontal sedimentary beds are deposited over previously folded or tilted strata)

    3. disconformity (where two conformable beds are seperated by mere changes of sediment type)

    4. paraconformity (where two sets of conformable beds are separted by same types of sediments) etc.

  10. Sedimentation units in the sedimentary rocks having a thickness of greater than 1cm are called beds. The upper and lower surfaces of a bed are called bedding planes or bounding planes. Sometimes the lower surface of a bed is called sole while the upper surface is called upper bedding surface. There are further sedimentary units within a bed. The units having a thickness of more than 1 cm are called as layers or strata whereas the units below 1 cm thickness are known as laminae. The several strata and laminae makeup a bed. When the beds are deposited at an angle to the depositional surface, they are called cross-beds and the general phenomena of inclined layers are called cross-lamination or cross-bedding.

  11. Soft muds and alluviam deposited by the rivers during flood period develop cracks when baked in the sun. These cracks are generally of polygonal shapes. Such cracks are called mud cracks or sun cracks.

  12. Most of the sedimentary rocks are permeable and porous but a few of them are also non-porous and impermeable. The porosity of the rocks depends upon the ratio between the voids and the volume of a given rock mass. l, Clastic (Composed of rock and mineral fragments)

Rock Type

Sandstone Cemented sand grains, Conglomerate (Sand-stone with pebbles of hard rock), Mudstone (Silt and clay with some sand), Clay-stone (Clay), Shale (Clay, broken into flat flakes and plates, with thin laminite; rich in organic material; found in lagoons, shallow seas and tidal flats), Siltstones (Fine grained clastic rock; carried by rivers). 2. Chemically Precipitated (From sea water or salty inland lakes)

Limestone (Calcium Carbonate; formed by sea or lake), Dolomite (Magnesium and calcium carbonate), Chert (Silica, a non-crystalline form of quartz), Evaporites (Minerals formed by evaporation of salty solutions in shallow inland lakes or coastal lagoons). 3. Organic (Formation due to organic material). Rock Type-Coal (It is formed from peat), Petroleum (It is a mineral fuel; found in liquid hydrocarbon), Natural Gas (It is a mineral fuel; a gaseous hydrocarbon).

There are three major categories in which sedimentary rocks are recognized:

  1. terrigenous clastic sedimentary rocks

  2. carbonates (lime-stone and dolomite)

  3. non-carbonate chemical sedimentary rocks. Terrigenous clastic sedimentary rocks are composed of the detrital fragments of preexisting rocks and minerals and are conventionally considered to be equivalent to clastic sedimentary rocks in general. Because most of the clasts are rich in silica, they are also referred to as siliciclastic sedimentary rocks. Silicic-lastics are further subdivided on the basis of clast diameter into conglomerate and breccia, sandstone, siltstone, and finer-than-silt-sized mudrock (shale, claystone, and mudstone). The carbonates, limestones arid dolomites, consist of the minerals aragonite, calcite, and dolomite.

Limestones

Limestones and dolostones (dolomites) make up the bulk of the nonterrigenous sedimentary rocks. Limestones are for the most part primary carbonate rocks. They consist of 50 percent or more calcite and aragonite. Dolomites are mainly produced by the secondary alteration or replacement of limestones; i.e.. the mineral dolomite replaces the calcite and aragonite minerals in limestones during diagenesis.

  • The colour of a sandstone depends on its detrital grains and bonding material. Mudrocks: It includes all siliciclastic sedimentary rocks composed of silt and clay-size particles: Siltstone (1/16 millimetre to 1/256 millimetre diameters), claystone (less than 1/256 millimetre), and mudstone (a mix of silt and clay).

  • Shale refers specifically to mudrocks that regularly exhibit lamination or fissility or both. Mudrocks are also loosely referred to as both lutites and pelites and as argillaceous sedimentary rocks.

  • Coal: Coals are the most abundant organic-rich sedimentary rock.

  • With increasing compaction and carbon content, peat can be transformed into the various kinds of coal: Initially brown coal or lignite, then soft or bituminous coal, and finally, with metamorphism, hard or anthracite coal. In the geologic record, coal occurs in beds, called seams, which are blanketlike coal deposits a few centimetres to metres or hundreds of metres thick.

  • Many coal seams occur within cyclothems, rhythmic successions of sandstone, mudrock, and limestone in which nonmarine units are regularly and systematically overlain by an underclay, the coal seam itself, and then various marine lithologies.

  • Oil and natural gas: Major natural gas varieties, include methane, ethane, propane, and butane.

  • These natural gases are commonly, though not invariably, intimately associated with the various liquid hydrocarbons-mainly liquid paraffins, napthenes, and aromatics that collectively constitute oil.

Sandstone Components

Sandstones are siliciclastic sedimentary rocks. There are three basic components of sandstones:

  1. Detrital grains, mainly transported, sand-size minerals such as quartz and feldspar

  2. A detrital matrix of clay or mud, which is absent in “clean” sandstones

  3. a cement that is chemically precipitated in crystalline form from solution and that serves to fill up original pore spaces.