Sedimentary rocks: layers, fossils and fuels

Detail

What a sedimentary rock is

• Sedimentary rock (from the Latin sedere, "to settle"): rock made from material that settles down in layers, mostly in water. That material can be:
• bits broken off any older rock (igneous, metamorphic or sedimentary);
• shells and the remains of plants and animals;

• minerals that come out of water as solids (chemical precipitates).

• Sediment means loose material such as gravel, sand, silt, clay or shell bits that wind, water or ice has carried and dropped.

• The main processes behind sedimentary rocks are weathering, erosion, dissolution, precipitation and lithification [4].

Lithification: how loose sediment becomes rock

• Lithification means turning loose sediment into solid rock. Clay, sand and other sediment on the floor of a sea or lake are slowly pressed into rock by the weight of the sediment above them [4].
• Step 1, compaction:
• new layers pile up on top of older ones;
• their weight squeezes the lower layers;

• water is pushed out and the grains pack closer together.

• Step 2, cementation:

• groundwater moves through the gaps between grains;
• minerals dissolved in it, mainly silica (SiO₂) or calcite (CaCO₃), come out of the water as solids;

• these minerals act like glue and bind the grains into hard rock.

• Result: a stratified rock that keeps a record of where and how it formed.

Key features: area, layers, fossils, fuels

• Area against volume: sedimentary rocks cover about three-quarters (about 75%) of the Earth's land surface. By volume, though, they are only a thin skin on top of a crust that is mostly igneous and metamorphic rock.
• Stratification (layering): each layer, called a stratum (plural strata), stands for one period of deposition. Older layers are usually at the bottom. This lets geologists read geological time from the rocks.
• Fossils: plants and animals buried in soft sediment are kept as fossils. Fossils are found mostly in sedimentary rocks, because the heat of magma destroys them in igneous rock and heat and pressure destroy them in metamorphic rock.
• Minerals in beds or layers: in sedimentary rocks, minerals occur in beds or layers. They form when material builds up and becomes concentrated in horizontal strata.
• Coal: made when plant matter is buried, then pressed and heated over a long time.

• Evaporites: gypsum, potash salt and sodium salt form when water evaporates, mostly in dry regions.

• Petroleum traps:

• oil and natural gas form from buried marine organic matter;
• they move up through porous rock, meaning rock with open spaces (limestone or sandstone);
• they stop and collect under a layer of rock that does not let them through, such as shale. This can be an anticline (an upward fold) or a fault trap.

Three kinds by origin

Type	How it forms	Examples
Clastic (mechanically formed)	Pieces of older rock are carried, dropped, compacted and cemented	Sandstone, conglomerate, shale, loess; tillite (glacial)
Chemically formed	Dissolved minerals come out of water as solids, often when the water evaporates	Rock salt, gypsum, potash, some limestone, chert
Organically formed	Remains of living things pile up	Coal (plants), chalk and much limestone (shells), geyserite

• Clastic rocks are graded by grain size [S2... see note below] — from largest to smallest: conglomerate and breccia → sandstone → siltstone → mudrock (shale, claystone, mudstone) [4].
• Conglomerate has rounded pebbles. Breccia has sharp, angular pieces.
• Loess is fine silt carried and dropped by wind. One example is the Loess Plateau of China.

• Tillite is glacial till (the mixed rubble dropped by a glacier) that has turned into rock. Gondwana tillites in India are evidence of ancient glaciation.

• Chemical rocks other than carbonates are rare in the rock record but important for the economy. They include cherts (silica rocks), phosphorites (phosphate rocks), evaporites, iron formations and ironstones, and carbon-rich deposits [4].

• Organic rocks: coal forms from plants pressed together over millions of years [5].
• Stages from peat to anthracite: peat → lignite → bituminous → anthracite. Carbon content and heating value rise at each stage.

Sandstone

• Sandstone: a clastic rock made of sand-sized grains (about 0.06–2 mm), mostly quartz, cemented by silica, calcite or iron oxide. Iron oxide gives it a red colour.
• It is porous, so it can hold water in an aquifer (rock that stores and passes groundwater) and can also hold oil.
• Red Vindhyan sandstone comes from the Vindhyan Supergroup (Proterozoic). This group of rocks stretches across Madhya Pradesh, Rajasthan and Uttar Pradesh. The sandstone was used to build:
• the Red Fort (Delhi);
• Fatehpur Sikri (Uttar Pradesh);
• the Sanchi stupas (Madhya Pradesh).

Shale and limestone

• Shale: clay and mud pressed into very fine, thin layers. It is the most common sedimentary rock.
• Shale lets almost no fluid pass through, so it often seals oil traps.

• When heated and pressed, shale turns into slate.

• Limestone: made mostly of calcium carbonate (CaCO₃). It can be chemical (precipitated) or organic (from shells and coral).

• It is the raw material for cement and is used as a flux in blast furnaces. A flux is a substance that removes impurities from molten iron as slag.

• When heated and pressed, limestone turns into marble.

• Karst topography: landforms made where rainwater, which is a weak carbonic acid, dissolves limestone or dolomite. Examples are sinkholes, caves, stalactites, stalagmites and underground streams.

• India: Guptadham (Bihar), Borra Caves (Andhra Pradesh), Meghalaya plateau caves.
• World: Karst region (Slovenia), Guilin (China).

Banded iron formation (BIF)

• Banded iron formation (BIF): a very old chemical sedimentary rock. It has thin bands of iron oxide (haematite, magnetite) that take turns with bands of silica (chert or jasper).
• When it formed: BIFs first appeared about 3.85 billion years ago in the Archaean. They were most common around 2.6–2.4 billion years ago. They formed between 3.8 and 1.8 billion years ago in the Precambrian and are no longer forming today [5]. (NCERT: mostly 3.5–1.8 billion years ago.)
• How it formed:
• early oceans had no oxygen, so large amounts of iron (Fe²⁺) stayed dissolved in seawater;
• photosynthesis by early microbes changed this dissolved iron into iron oxide (Fe³⁺) that does not dissolve;
• the iron oxide sank to the sea floor. It was laid down in layers that took turns with layers of silica;

• the result was the banded rock. The bands record a seasonal or cyclic rhythm.

• Type of photosynthesis: isotope evidence suggests that some early BIF iron was oxidised by anoxygenic photosynthesis. This kind of photosynthesis releases no oxygen and is older than the oxygen-releasing kind [5].

• Global importance: BIFs hold most of the world's iron ore. Examples are the Hamersley Range (Australia), Carajás (Brazil) and Lake Superior (USA and Canada).

BIFs and iron ore in India

• Geological setting:
• India's haematite belongs to the Precambrian Iron Ore Series. The ore lies inside banded iron formations [2].

• Magnetite occurs in igneous rock or in metamorphosed banded magnetite–silica formations [2].

• Where BIFs occur: Precambrian BIF lies in the Archaean schist belts (old belts of metamorphosed rock) of Jharkhand, Odisha, Karnataka, Chhattisgarh and Goa, which have high-grade deposits. It also occurs in the granulite terrain of Tamil Nadu and Kerala [3].

• Dharwar Craton (Karnataka):
• the lower Bababudan Group of the Dharwar Supergroup has shelf sediments that are rich in iron and manganese precipitates;
• the upper Chitradurga Group is mostly greywacke and argillite (types of hardened mud and sand);

• the greenstone belts (belts of old volcanic rock turned green by metamorphism) of the craton hold gold and iron ore [3].

• Main BIF ore belts:

• Bailadila (Dantewada, Chhattisgarh);
• Kudremukh and Bababudan (Karnataka);
• the Odisha–Jharkhand belt: Keonjhar–Sundargarh–Mayurbhanj in Odisha and Noamundi–Gua in Singhbhum, Jharkhand;

• Goa.

• Resources as on 1.4.2015 (NMI, UNFC system) [2]:

• total: over 33.276 billion tonnes of haematite and magnetite;
• haematite: 22,487 million tonnes, of which 24% are "Reserves". About 79% lies in the Eastern Sector. By state: Odisha 34% > Jharkhand 23% > Chhattisgarh 22% > Karnataka 11% > Goa 5%;

• magnetite: 10,789 million tonnes, of which only 53 million tonnes are Reserves. About 93% lies in the Southern Sector, and Karnataka alone has 72%. By state: Karnataka 72% > Andhra Pradesh 13% > Rajasthan 6% > Tamil Nadu 5% > Goa 2%.

• Production, 2017-18 [2]:

• total: 200.95 million tonnes from 294 reporting mines;
• by state: Odisha 50.84% > Chhattisgarh 17% > Karnataka 14.29% > Jharkhand 10.87%;
• by form: fines about 67.5%, lumps about 32.3%.

Sedimentary rocks in India's rock systems (geological time)

Rock system	Age	Sedimentary link
Dharwar	Archaean–Palaeoproterozoic	First metamorphosed sedimentary rocks; BIF iron ore, manganese, gold
Cuddapah	Proterozoic	Unmetamorphosed sandstone, shale, limestone, quartzite
Vindhyan	Proterozoic	Red sandstone, limestone (used for cement), Panna diamond-bearing conglomerate
Gondwana	Late Carboniferous–Jurassic	River and lake sediments; most of India's coal (Damodar, Mahanadi, Godavari valleys); tillite
Tertiary	Cenozoic	Petroleum (Assam, Gujarat, Mumbai High); lignite (Neyveli in Tamil Nadu, Rajasthan, Gujarat)

Prelims Hooks

• Sedimentary rocks: about 75% of land area but only a thin skin by volume. A statement claiming they make up most of the crust's volume is false.
• Lithification has two steps: compaction, then cementation. The cement is mainly silica or calcite carried by groundwater.
• Loess (wind-blown silt) and tillite (glacial deposit) are clastic. Chert and gypsum are chemical. Coal and chalk are organic.
• Grain size, coarsest to finest: conglomerate/breccia → sandstone → siltstone → shale.
• Red Vindhyan sandstone → Red Fort (Delhi), Fatehpur Sikri (Uttar Pradesh), Sanchi (Madhya Pradesh).
• What each rock becomes under heat and pressure: limestone → marble; shale → slate; sandstone → quartzite.
• BIFs: formed about 3.8–1.8 billion years ago and are no longer forming today [5]. They hold most of the world's iron ore.
• Haematite resources (1.4.2015), largest first: Odisha > Jharkhand > Chhattisgarh > Karnataka > Goa. Magnetite: Karnataka holds 72% [2].
• Iron ore production 2017-18, largest first: Odisha > Chhattisgarh > Karnataka > Jharkhand [2].
• Fuel–rock system pairs: Gondwana → coal; Tertiary → petroleum and lignite; Dharwar → iron ore.

Mains Points

• Why sedimentary rocks matter for resources (GS-III):
• they hold almost all fossil fuels (Gondwana coal, Tertiary oil and gas), evaporites, cement limestone and BIF iron ore;

• so India's energy and steel security depends on the geology of sedimentary basins.

• Uneven spread of iron ore:

• haematite is in the east (Odisha, Jharkhand, Chhattisgarh) and magnetite is in the south (Karnataka) [2];
• this shapes where steel plants are built (Jamshedpur, Rourkela, Bhilai) and how ore moves for export (Paradip, Visakhapatnam, Mormugao);

• it also causes problems in Adivasi forest areas (Bailadila, Keonjhar): displacement and conflicts under the Forest Rights Act and PESA.

• Few proven reserves and too many fines:

• only 24% of haematite resources are proven "Reserves", and fines were about 67% of output in 2017-18 [2];

• the answer is to explore for more ore, and to beneficiate (upgrade low-grade ore) and pelletise (roll fines into small balls) so that low-grade ore can be used.

• Value for science and heritage (GS-I):

• strata and fossils record geological time, and BIFs record how Earth's air and oceans gained oxygen;
• fossil sites and Vindhyan sandstone monuments need geoheritage protection.