Sedimentary rocks

Saturday, March 1, 2003

Alan V. Morgan

In the last issue of Wat On Earth I covered the classification of igneous (or fire-formed) rocks. In this issue I would like to explore the topic of sedimentary rocks. This is the second great family of rocks and they are of interest to humans because they have provided many tools in the past and today give us most aggregate resources as well as providing (or housing) many of our energy minerals, and certain essential minerals of major economic importance. Sedimentary rocks are also of significance since they house the fossil record of life on Planet Earth.

As their name suggests, sedimentary rocks are derived from pre-existing sediments. Two issues back in WAT ON EARTH I described the Rock Cycle; how rocks are broken down from still older rocks by weathering and erosion. The liberated grains are carried or transported by various mechanisms to a place where they accumulate in sequences of sediments. There are many transitional resting places, but ultimately most sediments end up in a marine depositional environment. Sedimentary rocks can therefore be composed of grains of various sizes, shapes and compositions that have been cemented together or compressed and recrystallised. These are the clastic sedimentary rocks. Other sedimentary rocks can be formed from deposits secreted by chemical solutions (chemical precipitates), or from deposits made up from the remains of dead organisms (both animals and plants). This last group forms sedimentary rocks of largely biological origin. Post-depositional change from sediment to sedimentary rock is called diagenesis, and the end result is usually lithification, where the former unconsolidated sediment is turned to rock. Two processes are common; compaction (where the sediments are compressed and water is driven from the pore spaces between grains), and cementation. Cementation is where certain minerals (for example, calcite, iron oxides and silica) get carried by percolating groundwater into the pore spaces. Here they precipitate and eventually cement the grains together.

As in the last copy of Wat On Earth, the centre page of this issue is devoted to an illustration of many of these different rock types. The page opposite illustrates several features associated with sedimentary rocks. 

Clockwise around the page: top left - photograph of a group of Waterloo students examining a section of Carboniferous beds at Mullaghmore Head in northern Ireland. The section consists of alternating sandstones, siltstones and shales. The softer shales have been more easily eroded whereas the sandstones stand out as resistant beds. Top right -diagram illustrates the position of the sedimentary rocks (2) in the rock cycle. Note that there is an input from space that adds, particularly, to marine sediments. Immediately below the rock cycle diagram is a chart illustrating the Wentworth classification in terms of grain size and how the sediments form different sedimentary clastic rocks. Compare this chart with the first two columns (Sediments and Rocks) that make up the "Clastic Rock" portion of the centrefold Chart. The third and lowest diagram illustrates some of the more important sedimentary rocks that are either of chemical or biological origin. Most of these are also illustrated on the centrefold page. The bottom photograph illustrates Delicate Arch in Arches National Monument in Utah. This 26m high by 20m wide structure is made up of Entrada Sandstone of Upper Jurassic age. The feature has been produced by the erosion of "fins" of local bedrock that have been cut away by wind and frost action. Immediately above is a view looking down the Niagara Gorge where the water from Lake Erie falls over the edge of the Silurian Lockport Dolomite on the Niagara Escarpment. Each of the photographs illustrates geomorphology associated with sedimentary formations of various ages.

Centre fold page: Understanding sedimentary rocks is fairly simple since they relate to materials that we see on a daily basis. At the top is a panoramic view across one of the most spectacular exposures of sedimentary rock in the world, in the upper levels of the Grand Canyon. Practically all of the rocks that you can see are horizontal sandstones, limestones and shales deposited over about 250 million years in the seas of the Paleozoic Era. These same seas were occupied elsewhere by the giant trilobites illustrated in another article in this issue. 

Moving lower on the centrefold page, I have tried to illustrate most of the common rock types. A few are more "exotic" but they are also important. The chart is divided into three sections that relate to clastic, chemically precipitated and biological sedimentary rocks. The left hand block consists of a double column. The first column illustrates examples of the sediment type (refer back to diagram 2 on the previous page), and the second column to their lithified equivalent. Angular clasts (rock fragments) are usually derived from frost shattering. The fragments have not traveled far and are usually moved under gravity, falling off mountains to accumulate as scree piles or talus cones at the bottom of slopes. Their counterparts are found in the rock type known as breccia. When the agents of erosion become involved and transportation takes place, particularly by water, the angular clasts become abraded, and form rounded boulders or cobbles. The rock equivalent is known as a conglomerate. Long distance transport gradually reduces the size of the clasts through granules to sand-size particles. The rock equivalent is sandstone. These are frequently cemented by different minerals, calcite, iron oxides or silica. The sandstone then acquires a secondary descriptor, such as "calcareous sandstone", or "ferruginous sandstone". A sandstone is normally made up of silica grains. When these grains are cemented by silica a different name is given - quartzite (see image to the right of sandstone). One rock type that is not illustrated is an arkose - a sandstone with more that 25 percent of the rock made up of feldspar. Such rocks are usually formed in montane, relatively dry, environments. Dry, because the clasts have not deteriorated too much (under hot, wet conditions feldspar is often easily weathered). The angular nature of the grains indicates that transport has not been that important. 

However, if the sediment continues to be transported the grain size is still further reduced and silt is formed. Particle sizes are now reaching the point where they can easily be transported by wind as well as water. The wind-transported sediment that accumulates as loess falls into this category. The rock type known as a siltstone, is formed of very fine quartz, mica and other miscellaneous minerals, and is the lithified equivalent of silt. Finally the finest fraction in the clastic size range is clay. These fragments are so tiny they remain in suspension for a long time, usually only settling after they have coalesced to form larger particles. When they do accumulate the lithified rock type is known as shale. Highly organic equivalents can form very black shales with distinctive "oily" or "petroliferous" odours, since they are very rich in carbon and aromatic compounds. They form fossiliferous and "oil shales", that might be future sources of petroleum.

The second double column block illustrates rocks that are formed by chemical deposits. Such rocks are formed where material is carried in solution to the place, most usually a marine basin or a lake in a desert area, where water is evaporating. Solutions of certain ions are precipitated and form rocks rich in calcium, magnesium, silica, sodium, and (more in the past) iron. Some of these are illustrated, starting with the calcium-rich precipitates. Limestones are an important example. All limestone fizzes easily when dilute (10% solution) hydrochloric acid is applied since the acid reacts with the calcium carbonate. The example shown is a grey, fine-grained limestone with white veins of calcite running through it. In warm tropical waters abundant calcium in solution in seawater subject to strong current movements may be precipitated around minute shell fragments. These can form oolitic limestones, when small grains of calcium carbonate have layers of calcite deposited in a concentric fashion around a nucleus. 

Dolostone, a rock made up of calcium and magnesium, is closely related to limestone usually with the magnesium from the seawater replacing much of the original calcium content of a pre-existing limestone. Acid reaction is slower. Dolostones can be seen close to Waterloo in certain rock sequences along the outcrop of the Niagara Escarpment. Travertine and tufa (strong acid reaction) are chemically precipitated rocks formed from calcium carbonate. Travertine is usually dense and banded, while tufa is more spongy. Travertine (in the form of speleothems) is most frequently seen in cave deposits, particularly in the spectacular formations known as dripstone, flowstone, stalactites, stalagmites, helictites and columns. 

Within limestones and dolostones you can frequently see layers or nodules composed of silica. These are layers of chert, or, in the case of the chalk, flint. Flints have been used since the Paleolithic with well known mass production sites for spear points, scrapers, arrowheads and tool kits described in the archeological literature from eastern England. They were also used until recently in firearms (flint-lock rifles) and in the construction of buildings where this rock type is common. Cherts from limestone and dolostone beds along the Niagara escarpment were used by Paleo-Indian hunters of caribou, mastodons and mammoth in southwest Ontario. Both flints and cherts are usually grey in colour (although they might range from almost white through buff to almost black or even red) and split with an even to slightly conchoidal fracture. The one thing they have in common is that they can be shaped by skilled tool makers and will retain an incredibly sharp edge for a long period of time.

Gypsum is a chemical precipitate (hydrous calcium sulphate) frequently found in beds of marl, a calcium-rich, clay-dominated, rock type. Beds of marls containing gypsum and rock salt were originally deposited in areas with high evaporation that were flooded by marine waters. Such areas today can be seen along parts of the sub-tropical Persian Gulf. Rock salt (made up almost entirely of sodium chloride) is an important economic mineral. In Ontario it is found around Windsor and northward to Goderich (see WAT ON EARTH 14 (2) Spring 2001). Salt deposits are also found in the Maritime provinces and also in western Canada. The western Canadian salt sequences are dominated by another economically important, red- and white-coloured salt rich in potassium, known as Sylvite. The last rock type illustrated in this group is the red- and grey-banded sedimentary ironstone, common in the region around upper Lake Superior. There is a vast literature associated with these deposits (Blatt et al., 1980), but generally they are of Precambrian age and usually between 2600 and 1800 million years old. Some are considerably older and a few small deposits are younger. They are characterized by thin and thick bands of alternating jasper (red chert) and magnetite- and haematite-rich layers of iron. Other forms of iron may also alternate with the cherty horizons. These are economically important rocks and are the source of major iron deposits on both sides of the Canada/USA border.

The third block of images comprises common sedimentary rocks that have a strong biological component. As I mentioned when discussing igneous rocks, nature abhors being "compartmentalized" and some of these "boxes" transgress boundaries. However, seawater contains large quantities of calcium carbonate that not only precipitates to form the chemical sedimentary rocks but is also utilized by organisms that build carbonate shells. When they die the shells can make up huge deposits of shell-detritus that often gets preserved as "shelly limestone". This can be seen today in the form of coquina (made up almost entirely of complete or broken valves of the marine shallow water pelecypod, Coquina). In the past clams were not as common as they are today. The "Shelly Limestone" illustration shows brachiopods in a Devonian carbonate mud deposit from the Arkona region of southwest Ontario. Crinoidal limestones (not illustrated), are made up of fragments of the stalks of crinoids (Echinodermata) and form shelly-detritus bands in certain rocks along the Niagara Escarpment. Coral, made up of the remains of countless trillions of carbonate-secreting polyps, form massive deposits today as in the Great Barrier Reef, and also in the more distant geological past. Many of the drilling targets for oil and natural gas in southern Ontario (and elsewhere) are aimed at small patch reefs of corals and algal communities, where the porosity within and between the fossil organisms have allowed these energy minerals to accumulate. Chalk is a particularly pure limestone, and dilute hydrochloric acid applied to this rock type produces an extremely vigorous reaction. Chalk was originally formed as seafloor ooze, and made up with the remains of trillions of foraminifera organisms known as coccoliths. Electron scanning images are provided next to the chalk image, with the central, cream-coloured organism, providing an example of the modern calcareous foraminifera called Globigerina. 

The next block of four images (peat to anthracite) illustrates another energy mineral - coal. Coal results from the accumulation of vegetation under anaerobic conditions. The detritus forms in swamps or lagoons and creates a water-saturated organic bed of peat. With time and compaction from overlying sediments, the peat looses water and other volatiles. The moisture content falls, the carbon content rises and peat changes to brown coal or lignite. These are low quality coals, often high in sulphur, not suited to long-distance transport and subject to spontaneous combustion. They form the types of coals found in southeastern Saskatchewan and many of the coals of eastern Germany. With further compaction and stress the carbon content continues to rise, more volatiles are lost, the potential energy output increases and a dull to shiny coal known as bituminous coal, forms. This is an excellent steam-raising coal and was the dominant coal that powered the first century of the Industrial Revolution in Britain. The western Canadian provinces and the former mining areas of New Brunswick and Nova Scotia mined bituminous coal. It is normally used for metallurgical feed and is used in electrical power production. The ultimate stage of coal as an energy mineral is Anthracite, a hard shiny coal that is difficult to burn unless crushed. This coal is mined in Pennsylvania and is used in electrical power production as a feed when mixed with bituminous coal. The changes that accompany the loss of volatiles, the increase of carbon and the increase of heat output is described as a change in the rank of the coal.

The last two images illustrate shale rocks that have high organic contents but are not defined as coals. Fossiliferous shales can contain abundant fossils (particularly plant materials, but they may contain others, such as ammonites). Oil shales (such as those found in a belt from Bowmanville, east of Toronto to the Collingwood region near Owen Sound, Ontario) contain abundant fossils of trilobites and other Ordovician animals. Such deposits, although not necessarily of the same age, are found in many parts of the world and may provide future sources of energy or petrochemical feeds.

The page following the centrefold contains illustrations of several of the topics mentioned in the sedimentary rock descriptions above.

Top Row, left to right. All are examples of chemically-deposited silica: Hand axe made from flint, Swanscombe, Kent, UK; Flint nodule (knobbly white concretion) weathering from softer chalk matrix; Three Clovis-type chert points from the Brophey Site near Parkhill, Ontario. The largest is 10cm in length.

Centre Row, left to right: A large stalagmite (note "g" Grows up from the ground) composed of dripstone (chemically precipitated calcium carbonate deposits); Stalactite (note "c" grows down from the ceiling of a cave), with light-catching calcium-charged water about to drip from the tip; a peculiar, contorted, stalactitic form of dripstone known as a helictite, also made from calcium carbonate. Note that in parts it even grows upward! 

Bottom Row, left to right; vug (cavity) with quartz lining centre of void in limestone; some very large chemically-precipitated concretions at Moeraki, South Island, NZ; moss-like dendrites. These are manganese precipitates on a limestone, and have no connection with organic growths. 

Alan V. Morgan

 


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