Classification of sedimentary rocks

By 

Wajiha Almas

Chemistry Department 

University Of Karachi 

Rudacious rock

This corresponds to sedimentary rocks with a particle size of more than 4mm size (ie pebble) and essentially is equivalent to terrigenous conglomerates. These rocks are subdivided depending upon their texture and composition.

Texture:

Orthoconglomerates = grain supported
Paraconglomerates (syn. diamictites) = mud supported

Composition:

Polymictic = pebbles of several rock types, indicate sources exposed several different rock types, for instance in areas of recent tectonic uplift.

Oligomictic = pebbles of one type which is often quartz due to its resistance to abrasion.

Carbonated Rocks

Carbonate rocks are "Chemical or Biochemical" in origin
They form within the basin of deposition, and are called Intrabasinal Rocks.

Composition

1. Limestones
   • Calcite
   • Aragonite
2. Dolostones
   • Dolomite

Depositional Conditions

• Warm, shallow seas - South Florida, Bahamas, etc.
• Tropical climate
• 30 ° N - 30 ° S of equator
• Low to no terrigenous input
• Light required
  • Algae are photosynthetic
  • Corals are animals with symbiotic algae (dinoflagellates) living in their tissues, called zooxanthellae. This benefits both parties. It protects the dinoflagellates from being eaten, and they have access to the by-products of coral metabolism (waste products containing phosphorus, nitrogen, and carbon dioxide). The coral benefits from the removal of its wastes, and from the oxygen and organic nutrients that are produced by the photosynthetic algae.

     

  • Much lime mud forms from the disintegration of calcareous algae (such as Halimeda and Penicillus). When the calcareous algae die, their skeletons break down and disintegrate producing aragonite needle muds. The lime mud lithifies to form micrite or calcilutite (fine-grained limestone).

     

    Halimeda

    Penicillus

Under-water view of calcareous algae in Florida Bay. Several specimens of Halimeda are shown.

Under-water view of calcareous algae in Florida Bay. Several specimens of Penicillus are shown.

• Blue-green algae are involved in the formation of oolites (or ooids). Oolites form in warm shallow seas with constant wave agitation.

Parts of the reef

• Back-reef (lagoon) - low energy, lime muds; bordered by tidal flat on landward side
• Reef - high energy, "boundstone"
• Fore-reef (deep water) - turbidites, breccias, grading seaward into organic-rich lime mud

Types of plankton

1. Foraminifera - CaCO3, animal-like (such as Globigerina ooze)
2. Coccolithophores - CaCO3, plant-like (forms chalk)
3. Diatoms - SiO2, plant-like (forms diatomaceous earth - diatomite)
4. Radiolarians - SiO2, animal-like

Carbonate compensation depth or CCD

• Above this depth, water is warmer, and precipitation of CaCO3 is greater than dissolution. Calcarous plankton can be found in the water column, and on the bottom, if water depth is shallower than the CCD.
• Above the CCD, bottom sediments can consist of calcareous sediments forming chalk or limestone.
• Below this depth, water is colder, and CaCO3 tends to dissolve (dissolution is greater than precipitation). Tiny shells of CaCO3 dissolve, and do not accumulate on the bottom if water is deeper than the CCD.
• Below the CCD, the bottom sediments consist of:
  1. Clay
  2. Silica shells of plankton (diatoms, radiolarians)
• At the CCD, the rate of precipitation of CaCO3 = the rate of dissolution of CaCO3
• Approximately 4000 - 5000 m deep in Pacific

Chemical rocks

These are rocks formed in situ.

The major types of chemical rocks include:
Carbonates
Evaporites
Coal
Ironstones
Phosphates

Carbonates 

high in CO₃, but low in SiO₂

These rocks have a complex genesis, diagenesis and petrography.

They are often intrabasinal in origin and are easily weathered and dissolved.

Common minerals in these rocks are calcite (CaCO₃), aragonite (CaCO₃), dolomite CaMg(CO₃)₂ and siderite (FeCO₃).

Aragonite has a greater density and hardness and a less distinct cleavage compared with calcite. Dolomite is found in extensive beds as a compact limestone or dolomite rock. It is also precipitated directly from seawater, possibly under warm shallow conditions. Siderite is often found in impure form in beds and nodules (of clay ironstone) in clays and shales and as a directly precipitated deposit partly altered into iron oxides. Siderite is a valuable ore of iron.

Carbonate rock components in sand rocks include skeletal fragments, ooliths, pellets, grapestone, oncolites, intraclasts, lithoclasts and in mud rocks micrite.

ooliths = spherical, accretionary bodies about 0.25 to 2mm diameter formed in shallow, wave-agitated water; often showing a an internal radiating fibrous structure indicating outward growth.

grapestone = cluster of small calcareous pellets or other grains commonly of sand size and stuck together by incipient cementation shortly after deposition; occur in modern carbonate environments eg Bahama Banks.

oncolites = small, variously shaped (often spheroidal), concentrically laminated, calcareous sedimentary structure, resembling an oolith, and formed by accretion of successive layered masses of gelatinous sheaths of blue-green algae.

intraclasts = component of a limestone, representing a torn-up and reworked fragment of a penecontemporaneous sediment and redeposited.

lithoclast = mechanically formed and deposited fragment of a carbonate rock, normally larger than 2mm in diameter, derived from an older, lithified limestone or dolomite within, adjacent to, or outside the depositional site.

micrite = descriptive term for the semiopaque, crystalline, interstitial component (matrix) of limestones, consisting of chemically precipitated carbonate (calcite) mud with crystals which are less than 4 microns diameter and interpreted as a lithified ooze.

Evaporites 

We will briefly examine the conditions underwhich evaporites are developed. Evaporite deposits include the following minerals: - anhydrite (CaSO₄), gypsum (CaSO₄.2H₂O), polyhalite (CaSO₄.MgSO₄.K₂SO₄.2H₂O), halite (NaCl), sylvite (KCl) and carnalite (KMgC_l3.6 H₂O).

A common method by which brines are developed, through the isolation of a body of salty water and its concentration by evaporation.

Coral reef in Hanauma Bay, Oahu, Hawaii

Salt structures as revealed by the progressive stripping back of the cover rocks to
reveal the true shape of the salt. Modified from Guglielmo et al.

Ironstone 

These include magnetite, haematite and greenalite (Fe⁺², Fe⁺³)₅-6Si₄O₁₀(OH). Greenalite occurs as small ellipsoidal granules in cherty rock associated with iron ores of the Mesabi district, Minnesota.

The occurrences of these ironstone include:

Banded magnetite and haematite (BIF) eg Hammersley

Hematite ooliths and matrix replacement eg "Clinton-type" = UK mine. Seem to have formed in shallow water along the margins of continents, on continental shelves or in shallow parts of basins.

Minette ores = another UK mine. These are composed of an intimate mixture of siderite, chamosite and limonite as oolites, matrix and shells. These were important in Mesozoic Europe such as the ironstones of the English Midlands plus Lorraine, Luxembourg and Saxony in Europe.

chamosite = (Mg,Fe)₃ Fe₃⁺³(Al,Si₃)O₁₀(OH)₈

An example of a silica-rich chemical sediment (jasper). The red-brown colour reflects iron-oxidation. Early Ordovician Brangan Volcanics, Grenfell, NSW, Australia.

Phosphates 

Phosphate minerals are extensively used as agricultural fertilizers and as such are a valuable natural resource. The main modes of occurrence include primary igneous (apatite, Ca₅(PO₄.CO₃)₃(F,OH,Cl), sedimentary (bedded or placer which are composed of collophane and apatite) and guano which is a complex of phosphates and nitrates 

collophane = massive cryptocrystalline variety of apatite, the chemical formula Ca₃P₂O₈.2H₂O is probably not a true mineral.

Fig 1. Aerial view of Nauru Island which was mined in the past for phosphate derived from guano (bird-droppings). Nauru is 42km from the equator and 4000km from Melbourne.	Fig 2. Aerial view of the coral pinnacles within the centre of the island.
Fig 3. Close up view of the coral pinnacles left on Nauru Island after the mining of guano (phosphate). The pinnacles are 3-15 metres high and occur about 60m above sea level.	Fig 4. View from the sea of Nauru Island with the fringing vegetation obscuring the bald plateau in the interior.

Development of phosphate deposits above an upwelling, cold ocean current which moves onto the continental shelf where the water warms and precipitates phosphate.

Arenaceous Rocks

Raw material is sand, mainly quartz but feldspar, mica may be present in smaller amounts;  angularity, roundedness may also be classified.  Typically deposited in waters with moderate velocity.  May exhibit current bedding.

Are those which are derived from pre-existing rocks. They are composed of rock fragments and mineral grains which have been weathered, eroded, transported, deposited, and cemented together to form a sedimentary rock. They are sometimes referred to as extrabasinal, because they are derived from rocks outside of the basin of deposition. The individual grains (or clasts) in these rocks are mechanically durable (to withstand abrasion during transport), and chemically stable. Typical clasts are made of quartz, feldspar, muscovite, clay minerals, or rock fragments.

Texture

There are three "textural components" to most clastic sedimentary rocks:

1. Clasts (gravel, sand, silt)

2. Matrix (fine-grained material surrounding clasts)

3. Cement (silica, calcite, or iron oxide - the "glue" that holds the rocks together).

Clasts and matrix (labelled), and iron oxide cement (reddish brown color surrounding clasts)

Clast size

Arenaceous sedimentary rocks are classified by the size of the clasts they contain. The size ranges of Arenaceous  are given below:

1/16 to 1/256 mm

 Arenaceous sedimentary rocks or arenites are those with sand-sized grains. Arenite means "sand". 

Clast shape

Shape of clasts is important in naming the coarser-grained sedimentary rocks (those with gravel-sized clasts). Gravel may be rounded or angular (based on the sharpness of the corners of the clasts). Gravel rapidly becomes rounded in the first few miles of transport.

Sorting

Sorting refers to the distribution of grain sizes in a rock. If all of the grains are the same size, the rock is "well sorted". If there is a mixture of grain sizes, such as sand and clay, or gravel and sand, the rock is "poorly sorted".

        Cementation

due to enlargement of mineral particles from quartz saturated percolating solutions - cementing agent same as particle.

  percolation of pore fluid deposits cementing solute, typically: silica, iron oxides, carbonates which then form matrix. 

Sandstones may be named according to cementation material,

Siliceous sandstone - quartz cement - very hard rock with 90% silica; (Craigleith)

Ferruginous sandstone - iron oxide coats grains thus entire rock has red/brown appearance.

e.g. Old Red Sandstone (Devon, Pentlands), New Red Sandstone (Cheshire, Merseyside - Bunter)

Calcareous sandstone - fairly weak, easily weathered by acid rain

Example of Arenite

Sandstones contain sand-sized clasts. Sand grains may be either rounded or angular, and they are generally more or less the same size (this is called well sorted). The sand grains are held together by cement, which may be silica (quartz), calcite, or iron oxide. (Calcite will fizz in hydrochloric acid; iron oxide makes the rock red, brown, or orange). (Arenite is another word for sandstone; the word is derived from the material that covered the floor of the Roman arenas where the gladiators fought.)

Sandstones are classified according to the composition of the sand grains into three main groups:

1. Quartz sandstone or quartz arenite is composed mainly of quartz sand grains.

    

   Quartz sandstone or quartz arenite

    

2. Arkose is composed mainly of pink or white feldspar grains, with quartz, and generally some muscovite mica or sand-sized rock fragments.

    

    

   Arkose

    

3. Litharenite (meaning rock-sand) or lithic sandstone or graywacke is predominantly composed of dark sand-sized rock fragments, with some mica, quartz, and feldspar grains in a clay-rich matrix. A wacke is defined as a "dirty" sand. The term "graywacke" is best used loosely; there is no strict definition of the term with which all geologists agree. A litharenite is more strictly defined as a rock primarily composed of sand-sized rock fragments.

    

    

   Litharenite or lithic sandstone sometimes called graywacke

    

   Thin Section of sandstone

Argillaceous Rocks

The raw materials of silt & clay usually settle out under water (marine) and possess some inherent cohesive strength.  Under the weight of overburden, the initially soft sediment consolidates - water is expelled, closer packing leads to stronger interparticle bonding and produces a stiffer deposit of much greater strength known as siltstone or mudstone.

MUDSTONE / SILTSTONE - lithified sediments comprising appropriate particle sizes

SHALES -are compacted muds with a laminated structure - the clay particles take on a preferred orientation.

MARL     -  essentially a calcareous mudstone,  "Keuper Marl".  calcium carbonate is precipitated from solution, typically 5%-20% of total.

Argillaceous sedimentary rocks or argillites are those with mud. (Mud is defined as a mixture of silt and clay.) Argillite means "mud". In general, it takes higher energy (higher water velocity) to transport larger grains.

Argillaceous sedimentary rocks are fine Grained Rocks.A sedimentary rock composed of detrital sediment particles less than 1/256 millimeter in diameter. Argelacious Rocks tend to be red, brown, black, or gray, and usually originate in relatively still waters.

Clay-dominated rocks

Shale or claystone is a fine-grained rock composed of tiny (less than 1/256 mm) clay minerals, mica, and quartz grains. The individual grains are too small to see with the naked eye or a hand lens, and the rock feels smooth to the touch (not gritty).

Shale and claystone differ in the way that they break.

• Shale is fissile; this means that it splits readily into thin, flat layers.
• Claystone, on the other hand, is not fissile, and breaks irregularly.

   

Shale is fissile	Claystone is not fissile (variety = kaolin or kaolinite)

The color of shale or claystone may reveal something about its composition.

• Black shales contain organic matter (they are sometimes called bituminous shales).
  
• Red shales contain iron oxide.
  
• Kaolin, a type of white claystone mined in Georgia, is composed of the mineral kaolinite (used in the manufacture of china, coatings for glossy paper, rubber, etc.).

   

Mud is defined as a mixture of silt and clay. Rocks with both silt and clay are referred to as mudstones or mudshales, depending on whether or not they are fissile.

 

Shale

SiltStone

Mud Stone Thin beds of mudstone

Oil Shale

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