Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Geology of Mineral Resources

The geology of mineral resources is intimately related to the entire geologic cycle, and nearly all aspects and processes of the cycle are involved to a lesser or greater extent in producing local concentrations of useful materials.

Local Concentrations of Metals 

The term ore is sometimes used for those useful metallic minerals that can be mined at a profit, and locations where ore is found have anomalously high concentrations of these minerals. The concentration of metal necessary for a particular mineral to be classified as an ore varies with technology, economics, and politics. Before smelting (extraction of metal by heating) was invented, the only metal ores were those in which the metals appeared in their pure form; gold, for example, was originally obtained as a pure, or native, metal. Now gold mines extend deep beneath the surface, and the recovery process involves reducing tons of rock to ounces of gold. Although the rock contains only a minute amount of gold, we consider it a gold ore because we can extract the gold profitably. The concentration factor of a metal is the ratio of its necessary concentration for profitable mining (that is, of its concentration in ore) to its average concentration in the earth's crust. Aluminum has an average concentration of about 8 percent in the Earth's crust and needs to be found at concentrations of about 35 percent to be mined economically, giving it a concentration factor of about 4. Mercury, on the other hand, has an average concentration of only a tiny fraction of 1 percent and must have a concentration factor of about 10,000 to be mined economically. Nevertheless, mercury ores are common in certain regions, where they and other metallic ores are deposited. The percentage of a metal in ore (and thus the concentration factor) is subject to change as the demand for the metal changes.

Igneous Processes 

Most ore deposits, caused by igneous processes, result from an enrichment process that concentrates an economically desirable ore of metals, such as copper, nickel, or gold. In some cases, however, an entire igneous rock mass contains disseminated crystals that can be recovered economically. Perhaps the best-known example is the occurrence of diamond crystals, found in a coarse-grained igneous rock called kimberlite, which characteristically occurs as a pipe shaped body of rock that decreases in diameter with depth. Almost the entire kimberlite pipe is the ore deposit, and the diamond crystals are disseminated throughout the rock. Diamonds, which are composed of carbon, form at very high temperatures and pressures, perhaps at depths as great as 150 km well below the crust of the earth and into the mantle. Some kimberlite pipes in South Africa are believed to be as old as 2 billion years. Near the surface, diamonds are not stable over geologic time and will eventually change to graphite (the mineral in lead pencils). The transformation will not happen at surface temperature and pressure, and, as a result, diamonds are metastable, remaining beautiful and mysterious for periods of time of interest to humans. The fact that the kimberlite pipes are so old suggests that they must be intruded (moved upward) from deep diamond forming depth to near the surface relatively quickly. If this were not the case, the diamonds would have been transformed to graphite.

Crystal Settling 

More concentrated ore deposits can result from igneous processes called crystal settling that segregate crystals formed earlier from those formed later. For example, as magma cools, heavy minerals that crystallize early may slowly sink or settle toward the lower part of the magma chamber, where they form concentrated layers. Deposits of chromite (ore of chromium) have formed by this process.

Late Magmatic Processes and Hydrothermal Replacement 

Late magmatic processes occur after most of the magma has crystallized, and rare and heavy metalliferous materials in water- and gas-rich solutions remain. This late-stage metallic solution may be squeezed into fractures or settle into interstices (empty spaces) between earlier-formed crystals. Other late-stage solutions form coarse-grained igneous rock known as pegmatite, which is rich in feldspar, mica, and quartz, as well as certain rare minerals. Pegmatites have been extensively mined for feldspar, mica, spodumene (lithium mineral), and clay that forms from weathered feldspar. Hydrothermal (hot-water) mineral deposits are a common type of ore deposit. They originate from late-stage magmatic processes and give rise to a variety of mineralization, including gold, silver, copper, mercury, lead, zinc, and other metals, as well as many non-metallic minerals. The hydrothermal solutions that form ore deposits are mineralizing fluids that migrate through a host rock, crystallizing as veins or small dikes. The mineral material is either produced directly from the igneous parent rock or altered by metamorphic processes, as magmatic solutions intrude into the surrounding rock (alteration by metamorphic processes, called contact metamorphism, is discussed under Metamorphic Processes). Many hydrothermal deposits cannot be traced to a parent igneous rock mass, however, and their origin remains unknown. It is speculated that circulating groundwater, heated and enriched with minerals after contact with deeply buried magma, might be responsible for some of these deposits.
Two types of hydrothermal deposits can be recognized: cavity-filling and replacement. Cavity-filling deposits are formed when hydrothermal solutions migrate along openings in rocks (such as fracture systems, pore spaces, or bedding planes) and precipitate (crystallize) ore minerals. Replacement deposits form as hydrothermal solutions react with the host rock, forming a zone in which ore minerals precipitate from the mineralizing fluids and replace part of the host rock. Although replacement deposits are believed to dominate at higher temperatures and pressures than cavity-filling deposits, both may be found in close association as one grades into the other; that is, the filling of an open fracture by precipitation from hydrothermal solutions may occur simultaneously with replacement of the rock that lines the fracture.
Hydrothermal replacement processes are significant because, excluding some iron and non-metallic deposits, they have produced some of the worlds largest and most important mineral deposits. Some of these deposits result from a massive, nearly complete replacement of host rock with ore minerals that terminate abruptly; others form thin replacement zones along fissures; and still others form disseminated replacement deposits that may involve huge amounts of relatively low-grade ore.
The actual sequence of geologic events leading to the development of a hydrothermal ore deposit is usually complex. Consider, for example, the tremendous, disseminated copper deposits of northern Chile. The actual mineralization is thought to be related to igneous activity, faulting, and folding that occurred 60 million to 70 million years ago. The ore deposit is an elongated, tabular mass along a highly sheared (fractured) zone separating two types of granitic rock.
The concentration of copper results from a number of factors:

  • A source igneous rock supplied the copper. 
  • The fissure zone tapped the copper supply and facilitated movement of the mineralizing fluids. 
  • The host rock was altered and fractured, preparing it for deposition and replacement processes that produced the ore. 
  • The copper was leached and redeposited again by meteoric water, which further concentrated the ore.
Metamorphic Processes 

Contact Metamorphism 

Ore deposits are often found along the contact between igneous rocks and the surrounding rocks they intrude. This area is characterized by contact metamorphism, caused by the heat, pressure, and chemically active fluids of the cooling magma interacting with the surrounding rock, called country, or host rock. The width of the contact metamorphic zone varies with the type of country rock. The zone is usually thickest in limestone because limestone is more reactive: The release of carbon dioxide increases the mobility of reactants. The zone is generally thinnest for shale because the fine-grained texture retards the movement of hot, chemically active solutions, and the zone is intermediate for sandstone. As we have already mentioned, some of the mineral deposits that form in contact areas originate from the magmatic fluids and some from reactions of these fluids with the country rock.

Regional Metamorphism 

Metamorphism can also result from regional increase of temperature and pressure associated with deep burial of rocks or tectonic activity. This regional metamorphism can change the mineralogy and texture of the pre-existing rocks, producing ore deposits of asbestos, talc, graphite, and other valuable non-metallic deposits. Metamorphism has been suggested as a possible origin of some hydrothermal fluids. It is a particularly likely cause in high-temperature, high-pressure zones, where fluids might be produced and forced out into the surrounding rocks to form replacement or cavity-filling deposits. For example, the native copper found along the top of ancient basalt flows in the Michigan copper district was apparently produced by metamorphism and alteration of the basalt, which released the copper and other materials that produced the deposits.
Our discussion of igneous and metamorphic processes has focused primarily on ore deposits. However, igneous and metamorphic processes are also responsible for producing a good deal of stone used in the construction industry. Granite, basalt, marble (metamorphosed limestone), slate (metamorphosed shale), and quartzite (metamorphosed sandstone), along with other rocks, are quarried to produce crushed rock and dimension stone in the United States. Stone is used in many aspects of construction work; but many people are surprised to learn that, in total value, with the exception of iron and steel, the stone industry is one of the largest non-fuel mineral industries in the United States.

Sedimentary Processes 

Sedimentary processes are often significant in concentrating economically valuable materials in sufficient amounts for extraction. As sediments are transported, wind and running water help segregate the sediment by size, shape, and density. Thus, the best sand or sand and gravel deposits for construction purposes are those in which the finer materials have been removed by water or wind. Sand dunes, beach deposits, and deposits in stream channels are good examples.

Sand and Gravel 

The U.S. sand and gravel industry amounts to about $8.5 billion per year, and, by volume mined (about 1300 million tons in 2006), it is one of the largest non-fuel mineral industries in the United States. Currently, most sand and gravel are obtained from river channels and water-worked glacial deposits. The United States now produces more sand and gravel than it needs, but demand is increasing. Environmental restrictions on extraction are causing sand and gravel operations to move away from areas with high population density, and shortages of sand and gravel are expected to increase as zoning and land development restrict locations where they may be extracted. Extraction from river channels and active floodplains can cause degradation to the river environment, and objections to river extraction operations are becoming more common.

Placer Deposits 

Stream processes transport and sort all types of materials according to size and density. Therefore, if the bedrock in a river basin contains heavy metals such as gold, streams draining the basin may concentrate heavy metals to form placer deposits (ore formed by deposit of sediments) in areas where there is reduced turbulence or velocity of flow, such as between particles on riffles, in open crevices
or fractures at the bottoms of pools, or at the inside curves of bends. Placer mining of gold known as a poor man method because a miner needed only a shovel, a pan, and a strong back to work the stream side claim helped to stimulate settlement of California, Alaska, and other areas of the United States. Furthermore, the gold in California attracted miners who acquired the expertise necessary to locate and develop other resources in the western conterminous United States and Alaska. Placer deposits of gold and diamonds have also been concentrated by coastal processes, primarily wave action. Beach sands and near-shore deposits are mined in Africa and other places.

Evaporite Deposits 

Rivers and streams that empty into the oceans and lakes carry tremendous quantities of dissolved material derived from the weathering of rocks. From time to time, geologically speaking, a shallow marine basin may be isolated by tectonic activity (uplift) that restricts circulation and facilitates evaporation. In other cases, climatic variations during the ice ages produced large inland lakes with no outlets, which eventually dried up. In either case, as evaporation progresses, the dissolved materials precipitate, forming a wide variety of compounds, minerals, and rocks called evaporite deposits that have important commercial value.
Most evaporite deposits can be grouped into one of three types:
Marine evaporites (solids) potassium and sodium salts, calcium carbonate, gypsum, and anhydrite; non-marine evaporites (solids) sodium and calcium carbonate, sulphate, borate, nitrate, and limited iodine and strontium compounds; and brines (liquids derived from wells, thermal springs, inland salt lakes, and seawaters) bromine, iodine, calcium chloride, and magnesium. Heavy metals (such as copper, lead, and zinc) associated with brines and sediments in the Red Sea, Salton Sea, and other areas are important resources that may be exploited in the future. Extensive marine evaporite deposits exist in the United States. The major deposits are halite (common salt, NaCl), gypsum anhydrite , and inter-bedded limestone Limestone, gypsum, and anhydrite are present in nearly all marine evaporite basins, and halite and potassium minerals are found in a few. Evaporite materials are widely used in industry and agriculture. Marine evaporites can form stratified deposits that may extend for hundreds of kilometres, with a thickness of several thousand meters. The evaporites represent the product of evaporation of seawater in isolated shallow basins with restricted circulation. Within many marine evaporite basins, the different deposits are arranged in broad zones that reflect changes in salinity and other factors controlling the precipitation of evaporites; that is, different materials may be precipitated at the same time in different parts of the evaporite basin. Halite, for example, is precipitated in areas where the brine is more saline, and gypsum where it is less saline. Economic deposits of potassium evaporite minerals are relatively rare but may form from highly concentrated brines. Non-marine evaporite deposits form by evaporation of lakes in a closed basin. Tectonic activity, such as faulting, can produce an isolated basin with internal drainage and no outlet. However, to maintain a favourable environment for evaporite mineral precipitation, the tectonic activity must continue to uplift barriers across possible outlets or lower the basin floor faster than sediment can raise it. Even under these conditions, economic deposits of evaporites will not form unless sufficient dissolved salts have washed into the basin by surface run off from surrounding highlands. Finally, even if all favourable environmental criteria are present, including an isolated basin with sufficient run off and dissolved salts, valuable non-marine evaporates, such as sodium carbonate or borate, will not form unless the geology of the highlands surrounding the basin is also favourable and yields run off with sufficient quantities of the desired material in solution. Some evaporite beds are compressed by overlying rocks and mobilized, then pierce or intrude the overlying rocks. Intrusions of salt, called salt domes, are quite common in the Gulf Coast of the United States and are also found in north western Germany, Iran, and other areas. Salt domes in the Gulf Coast are economically important because:

  • They are a good source for nearly pure salt. Some have extensive deposits of elemental sulphur. 
  • Some have oil reserves on their flanks. 

Salt domes are also environmentally important as possible permanent disposal sites for radioactive waste, although, because salt domes tend to be mobile, their suitability as disposal sites for hazardous wastes must be seriously questioned. Evaporites from brine resources of the United States are substantial, assuring that no shortage is likely for a considerable period of time. But many evaporites will continue to have a place value because transportation of these mineral commodities increases their price, so continued discoveries of high-grade deposits closer to where they will be consumed remains an important goal.

Biological Processes 

Organisms are able to form many kinds of minerals, such as the various calcium and magnesium carbonate minerals in shells and calcium phosphate in bones. Some of these minerals cannot be formed inorganically in the biosphere. Thirty-one different biologically produced minerals have been identified. Minerals of biological origin contribute significantly to sedimentary deposits. 
An interesting example of mineral deposits produced by biological processes are phosphates associated with sedimentary marine deposition. Phosphorus-rich sedimentary rocks are fairly common in some of the western states, as well as in Tennessee, North Carolina, and Florida. The common phosphorus-bearing mineral in these rocks is apatite, a calcium phosphate associated with bones and teeth. Fish and other marine organisms extract the phosphate from seawater to form apatite, and the mineral deposit results from sedimentary accumulations of the phosphate-rich fish bones and teeth. The richest phosphate mine in the world, known as Bone Valley, is located about 40 km east of Tampa, Florida. The deposit is marine sedimentary rocks composed in part of fossils of marine animals that lived 10 million to 15 million years ago, when Bone Valley was the bottom of a shallow sea. That deposit has supplied as much as one-third of the worlds phosphate production. Another important source of phosphorus is guano (bird faeces), which accumulates where there are large colonies of nesting sea birds and a climate arid enough for the guano to dry to a rock like mass. Thus, the formation of one of the major sources of phosphorus depends upon unique biological and geographical conditions.

Weathering Processes 

Weathering is responsible for concentrating some materials to the point that they can be extracted at a profit. Weathering processes can produce residual ore deposits in the weathered material and provide secondary enrichment of low-grade ore.

Residual Ore Deposits 

Intensive weathering of rocks and soils can produce residual deposits of the less soluble materials, which may have economic value. For example, intensive weathering of some rocks forms a type of soil known as laterite (a residual soil derived from aluminium- and iron-rich igneous rocks). The weathering processes concentrate relatively insoluble hydrated oxides of aluminium and iron, while more soluble elements, such as silica, calcium, and sodium, are selectively removed by soil and biological processes. If sufficiently concentrated, residual aluminium oxide forms an aluminium ore known as bauxite. Important nickel and cobalt deposits are also found in laterite soils, developed from ferromagnesian-rich igneous rocks. Insoluble ore deposits, such as native gold, are generally residual, meaning that, unless they are removed by erosion, they accumulate in weathered rock and soil. Accumulation of the insoluble ore minerals is favoured where the parent rock is a relatively soluble material, such as limestone. Care must be taken in evaluating a residual weathered rock or soil deposit because the near-surface concentration may be a much higher grade than ore in the parent, un-weathered rocks.

Secondary Enrichment 

Weathering is also involved in secondary enrichment processes to produce sulphide ore deposits from low-grade primary ore. Near the surface, primary ore containing such minerals as iron, copper, and silver sulphides is in contact with slightly acid soil water in an oxygen-rich environment. As the sulphides are oxidized, they are dissolved, forming solutions rich in sulphuric acid and in silver and copper sulphate; these solutions migrate downward, producing a leached zone devoid of ore minerals. Below the leached zone, oxidation continues, as the sulphate solutions continue to move toward the groundwater table. Below the water table, if oxygen is no longer available, the solutions are deposited as sulphides, increasing the metal content of the primary ore as much as tenfold. In this way, low-grade primary ore is rendered more valuable, and high-grade primary ore is made even more attractive.
The presence of a residual iron oxide cap at the surface indicates the possibility of an enriched ore below, but it is not always conclusive. Of particular importance to the formation of a zone of secondary enrichment is the presence in the primary ore of iron sulphide (for example, pyrite). Without it, secondary enrichment seldom takes place, because iron sulphide in the presence of oxygen and water forms sulphuric acid, which is a necessary solvent. Another factor favouring development of a secondary-enrichment ore deposit is the primary ore being sufficiently permeable to allow water and solutions to migrate freely downward. Given a primary ore that meets these criteria, the reddish iron oxide cap probably does indicate that secondary enrichment has taken place.
Several disseminated copper deposits have become economically successful because of secondary enrichment, which concentrates dispersed metals. For example, secondary enrichment of a disseminated copper deposit at Miami, Arizona, increased the grade of the ore from less than 1 percent copper in the primary ore to as much as 5 percent in some localized zones of enrichment.

Other Minerals from the Sea 

Mineral resources in seawater or on the bottom of the ocean are vast and, in some cases, such as magnesium, nearly unlimited. In the United States, magnesium was first extracted from seawater in 1940. By 1972, one company in Texas produced 80 percent of our domestic magnesium, using seawater as its raw material source. In 1992, three companies in Texas, Utah, and Washington extracted magnesium, respectively, from seawater, lake brines, and dolomite (mineral composed of calcium and magnesium carbonate). The deep-ocean floor may eventually be the site of a next mineral rush. Identified deposits include massive sulphide deposits associated with hydrothermal vents, manganese oxide nodules, and cobalt-enriched manganese crusts.

Sulfide Deposits 

Massive sulphide deposits containing zinc, copper, iron, and trace amounts of silver are produced at divergent plate boundaries (oceanic ridges) by the forces of plate tectonics. Pressure created by several thousand meters of water at ridges forces cold seawater deep into numerous rock fractures, where it is heated by up-welling magma to temperatures as high as The pressure of the heated water produces vents known as black smokers, from which the hot, dark-coloured, mineral-rich water emerges as hot springs. Circulating seawater leaches the surrounding rocks, removing metals that are deposited when the mineral-rich water is ejected into the cold sea. Sulphide minerals precipitate near the vents, forming massive tower like formations, rich in metals. The hot vents are of particular biologic significance because they support a unique assemblage of animals, including giant clams, tube worms, and white crabs. Ecosystems including these animals base their existence on sulphide compounds extruded from black smokers, existing through a process called chemosynthesis, as opposed to photosynthesis, which supports all other known ecosystems on earth. The extent of sulphide mineral deposits along oceanic ridges is poorly known, and, although leases to some possible deposits are being considered, it seems unlikely that such deposits will be extracted at a profit in the near future. Certainly, potential environmental degradation, such as decreased water quality and sediment pollution, will have to be carefully evaluated prior to any mining activity. Study of the formation of massive sulphide deposits at oceanic ridges is helping geologists understand some of the mineral deposits on land. For example, massive sulphide deposits being mined in Cyprus are believed to have formed at an oceanic ridge and to have been later uplifted to the surface.

Manganese Oxide Nodules 

Manganese oxide nodules cover vast areas of the deep-ocean floor. They contain manganese (24 percent) and iron (14 percent), with secondary copper (1 percent), nickel (1 percent), and cobalt (0.25 percent). Nodules are found in the Atlantic Ocean off Florida, but the richest and most extensive accumulations occur in large areas of the north eastern, central, and southern Pacific, where they cover 20 to 50 percent of the ocean floor.
Manganese oxide nodules are usually discrete, but are welded together locally to form a continuous pavement. Although they are occasionally found buried in sediment, nodules are usually surficial deposits on the seabed. Their size varies from a few millimetres to a few tens of centimetres in diameter (many are marble to baseball sized). Composed primarily of concentric layers of manganese and iron oxides, mixed with a variety of other materials, each nodule formed around a nucleus of a broken nodule, a fragment of volcanic rock, or, sometimes, a fossil. The estimated rate of nodular growth is 1 to 4 mm per million years. The nodules are most abundant in those parts of the ocean where sediment accumulation is at a minimum, generally at depths of 5 to 7 km. 
The origin of the nodules is not well understood; presumably, they might form in several ways. The most probable theory is that they form from material weathered from the continents and transported by rivers to the oceans, where ocean currents carry the material to the deposition site in the deep-ocean basins. The minerals from which the nodules form may also derive from submarine volcanism, or may be released during physical and biochemical processes and reactions that occur near the water sediment interface during and after deposition of the sediments. Mining of manganese oxide nodules involves lifting the nodules off the bottom and up to the mining ship; this may be done by using suction or scraper equipment. Although mining of the nodules appears to be technologically feasible, production would be expensive compared to mining manganese on land. In addition, there are uncertainties concerning ownership of the nodules, and nodule mining would cause significant damage to the sea-floor and local water quality, raising environmental concerns.

Cobalt-enriched Manganese Crusts 
Oceanic crusts rich in cobalt and manganese are present in the mid- and south-west Pacific, on flanks of sea-mounts, volcanic ridges, and islands. Cobalt content varies with water depth; the maximum concentration of about 2.5 percent is found at water depths of 1 to 2.5 km. Thickness of the crust averages about 2 cm. The processes of formation are not well understood. Both the nature and the extent of the crusts, which also contain nickel, platinum, copper, and molybdenum, are being studied by U.S. Geological Survey scientists.

Water Pollutants

Many different materials may pollute surface water or groundwater. We will focus on oxygen-demanding waste, pathogenic organisms, nutrients, oil, hazardous chemicals, heavy metals, radioactive materials, and sediment. 

Oxygen-Demanding Waste 

Dead organic matter in streams decays; that is, it is consumed by bacteria, which require oxygen. If there is enough bacterial activity, the oxygen in the water can be reduced to levels so low that fish and other organisms die. A stream without oxygen is a dead stream, devoid of fish and many organisms we value. The amount of oxygen used for bacterial decomposition is the biochemical oxygen demand (BOD), a commonly used measure in water quality management. The BOD is measured as milligrams per litre of oxygen consumed over five days at 20° C. A high BOD indicates a high level of decaying organic matter in the water.
Dead organic matter in streams and rivers comes from natural sources (for example, dead leaves from a forest), as well as from agriculture and urban sewage. Approximately 33 percent of all BOD in streams results from agricultural activities, but urban areas, particularly those with sewer systems that combine sewage and storm-water run off, may add considerable BOD to streams during floods, when sewers entering treatment plants can be overloaded and overflow into streams, producing pollution events.
Relationship between dissolved oxygen and biochemical oxygen demand (BOD) for a stream, following the input of sewage.
The threshold for water pollution is a dissolved oxygen content of less than 5 mg per litre (mg/l) of water. The diagram in Figure above illustrates the effect of BOD on dissolved oxygen content in a stream when raw sewage is introduced as a result of an accidental spill. Three zones are recognised. The pollution zone has a high BOD and a reduced dissolved oxygen content as initial decomposition of the waste begins. In the active decomposition zone, the dissolved oxygen content is at a minimum, owing to biochemical decomposition as the organic waste is transported downstream. In the recovery zone, the dissolved oxygen increases, and the BOD is reduced because most oxygen demanding organic waste from the input of sewage has decomposed and natural stream processes are replenishing the water with dissolved oxygen. All streams have some capability to degrade organic waste after it enters the stream. Problems result when the stream is overloaded with biochemical oxygen-demanding waste, overpowering the streams natural cleansing function.

Pathogenic Organisms 

Pathogenic (disease-causing) micro-organisms are important biological pollutants. Among the major water borne human diseases are cholera, typhoid infections, hepatitis, and dysentery. Because it is often difficult to monitor the pathogens directly, we use the count of human faecal coliform bacteria as a common measure of biological pollution and a standard measure of microbial pollution. These common and, usually, harmless bacteria are normal constituents of human intestines and are found in all human waste.
However, not all forms of faecal coliform bacteria are harmless. Escherichia coli (also known as E. coli 0157), a strain of E. coli bacteria, has been responsible for many human illnesses and deaths. E. coli 0157 produces strong toxins in humans that may lead to bloody diarrhea, dehydration, kidney failure, and death. In 1993, outbreaks of disease, apparently caused by E. coli 0157, occurred as a result of peoples eating contaminated meat at a popular fast-food restaurant. In 1998, E. coli apparently contaminated the water in a Georgia water park and a Wyoming towns water supply, causing illness and one death.
One of the worst outbreaks of E. coli bacterial infection in Canadian history unfolded in May 2000 in Walkerton, Ontario. It is believed that the likely cause of the contamination in Walkerton was the result of E. coli bacteria in cow manure that washed into the public water supply during heavy rains and flooding that occurred on May 12, 2000. The local Public Utility Commission was aware as early as May 18 that water from wells serving the town was contaminated, but they did not report this contamination immediately to health authorities. As a result, people were not advised to boil water until it was too late to avoid the outbreak of disease. By May 26, 5 people had died, over 20 were in the intensive care unit of the local hospital, and approximately 700 were ill with severe symptoms, including cramps, vomiting, and diarrhoea. The old and very young are most vulnerable to the ravages of the disease, which can damage the kidneys, and two of the first victims were a 2-year-old baby and an 82-year-old woman. Government officials finally took over management of the water supply, and bottled water was distributed. Tragically, before the outbreak was over, at least 7 people had died and over 1000 had been infected.
Authorities launched an investigation, focusing on why there was such a long delay between identifying the potential problem and warning people. Had there not been such a long delay, illnesses might have been avoided. We must remain vigilant in testing our waters and immediately report problems to public health authorities if any problems arise.
In the fall of 2006, E. coli 0157 was traced to farms in northern California. Contaminated spinach was shipped to 23 states. About 150 people became sick and one person died. In 2009, peanut butter was responsible for several hundred E. coli illnesses, with several deaths across the United States.
In the past, epidemics of water borne diseases have killed thousands of people in U.S. cities. Such epidemics have been largely eliminated by separating sewage water and drinking water and treating drinking water before consumption. Unfortunately, this is not the case worldwide, and, every year, several billion people, particularly in poor countries, are exposed to water borne diseases. For example, an epidemic of cholera occurred in South America in the 1990s. Although developing nations are more vulnerable, the risk of water borne diseases is a potential threat in all countries.
The threat of an outbreak of a water borne disease is exacerbated by disasters such as earthquakes, floods, and hurricanes; these events can damage sewer lines or cause them to overflow, resulting in contamination of water supplies. For example, after the 1994 North ridge earthquake, people in the San Fernando Valley of the Los Angeles Basin were advised to purify municipal water by boiling because of the threat of bacterial contamination.


Relationship between land use and average nitrogen and phosphorus concentration in streams (in milligrams per liter).
Nutrients released by human activity may lead to water pollution. Two important nutrients that can cause problems are phosphorus and nitrogen, both of which are released from a variety of materials, including fertilizers, detergents, and the products of sewage-treatment plants. The concentration of phosphorus and nitrogen in streams is related to land use. Forested land has the lowest concentrations of phosphorus and nitrogen, while the highest concentrations are found in agricultural areas, such as fertilized farm fields and feed lots. Urban areas can also add phosphorus and nitrogen to local waters, particularly where waste water treatment plants discharge treated waters into rivers, lakes, or the ocean. These plants are effective in reducing organic pollutants and pathogens, but, without advanced treatment, nutrients pass through the system.
High human-caused concentrations of nitrogen and phosphorus in water often result in the process known as cultural eutrophication. Eutrophication (from the Greek for well fed ), a natural process, is characterized by a rapid increase in the abundance of plant life, particularly algae. Blooms of algae form thick mats that sometimes nearly cover the surface of the water in freshwater ponds and lakes. The algae block sunlight to plants below, and the plants eventually die. In addition, the algae consume oxygen as they decompose, thereby lowering the oxygen content of the water, and fish and aquatic animals may die as well.
Algae blooms from blue-green algae may produce toxins as part of their life cycle. Lakes in Wisconsin, Minnesota, Oregon, and other areas with blooms of blue-green algae turn pea green, and toxins that are produced have been responsible for deaths of dogs and other animals that drink the water. People who live near the lake have reported nauseating smells from the water, along with rashes, headaches, and sore throats. People have not been killed by the toxins because they generally avoid contact with the water.
Algae contaminated beaches in Hawaii (a) Ocean-front condominium on the island Maui, Hawaii. The brown line along the edge of the beach is an accumulation of marine algae (locally called seaweed). (b) On the beach itself, the algae pile up, sometimes to a depth of about 0.5 m (1.7 ft), and people using the beach avoid the areas of algae piles. (c) Condominium complexes often have small wastewater-treatment plants, such as the one shown here, inside the plant-covered fence, that provide primary and secondary treatment. After this treatment, the water is injected underground at a relatively shallow depth. The treatment does not remove nutrients such as phosphorus and nitrogen that apparently encourage the accelerated growth of marine algae in the nearshore environment.
In the marine environment, nutrients in near shore waters may cause blooms of seaweed, referred to as marine algae, to flourish. The marine algae become a nuisance when they are torn loose and accumulate on beaches. Algae may also damage or kill coral in tropical areas. For example, the island of Maui in the Hawaiian Islands has a cultural eutrophication problem resulting from nutrients entering the near shore environment from waste-disposal practices and agricultural run off. Beaches in some areas become fouled with algae that washes up on the shore, where it rots and creates a stench, providing a home for irritating insects that eventually drive away tourists.
Dead zone in Gulf of Mexico Area in the Gulf of Mexico in July 2001 with bottom water with less than 2 mg/L dissolved oxygen.
A serious and ongoing cultural eutrophication problem is occurring in the Gulf of Mexico, offshore of Louisiana. A so-called dead zone develops in the summer, over a large area about the size of New Jersey. Water in the zone has low concentrations of oxygen, killing shellfish and crabs, and blooms of algae occur. The cause of the cultural eutrophication is believed to be the Mississippi River. The Mississippi drains about 40 percent of the lower 48 states, and much of the land use in the drainage basin is agricultural. The nutrient believed to cause the problem is nitrogen, which is used in great amounts to fertilize fields. The problem will not be easy to solve, as long as agriculture continues to use tremendous amounts of fertilizer. Part of the solution will be modification of agricultural practices to use less nitrogen by using it more efficiently, so that less of the nutrient runs off the land into the river.


Oil spill from the Exxon Valdez in Alaska, 1989 (a) Aerial view of oil being offloaded from the leaking tanker Exxon Valdezon theleft to the smaller Exxon Baton Rougeon the right. Floating oil is clearly visible on the water.
(b) Attempting toclean oil from the coastal environment by scrubbing and spraying with hot water.

Oil discharged into surface water (rivers, lakes, and the ocean) has caused major pollution problems. The largest oil discharges have usually involved oil-tanker accidents at sea. For example, just after midnight on March 24, 1989, the oil tanker Exxon Valdez ran aground on Bligh Reef, 40 km (25 mi) south of Valdez, Alaska, in Prince William Sound. Crude oil poured out of the ruptured tanks of the vessel at a rate of approximately 20,000 barrels per hour. The Exxon Valdez was loaded with 1.2 million barrels of crude oil, and, of this, more than 250,000 barrels (11 million gallons) gushed from the hold of the 300-m (984-ft) tanker. The oil remaining in the Exxon Valdez was loaded into another tanker.
Mercury in the environment Input and changes of mercury in aquatic ecosystems. 
The oil spilled into what was considered one of the most pristine and ecologically rich marine environments of the world, and the accident is now known as the worst oil spill in the history of the United States. Short term impacts were very significant; commercial fisheries, sport fisheries, and tourism were disrupted. In addition, many sea birds and mammals were lost. Lessons learned from the Exxon Valdez spill have resulted in better management strategies for both the shipment of crude oil and emergency plans to minimize environmental degradation.
A large oil spill in 2006 was caused by the war in Lebanon, when a coastal power plant was bombed and over 100,000 barrels of fuel oil entered the Mediterranean Sea. Over half of Lebanon's tourist beaches were polluted, including a popular public beach visited by people from the capital city Beirut.

Toxic Substances

Many substances that enter surface water and groundwater are toxic to organisms. Three general categories of toxic substances synthetic organic chemicals, heavy metals, and radioactive waste will be discussed.

Synthetic Organic Chemicals 

Organic compounds are compounds of carbon that are produced naturally by living organisms or synthetically by industrial processes. Up to 100,000 new chemicals are now being used or have been used in the past. It is difficult to generalize concerning the environmental and health effects of synthetic organic compounds because there are so many of them and they have so many uses and produce so many different effects.
Selected Persistent Organic Pollutants (POPs).
Synthetic organic compounds have many uses in industrial processes, including pest control, pharmaceuticals, and food additives. Some of these compounds are called persistent organic pollutants, also known as POPs. Many of these chemicals were produced decades ago, before their harm to the environment was known, and a number have now been banned or restricted. Table above lists some of the common persistent organic pollutants and their uses. POPs have several general properties useful in defining them.First, they have a carbon-based structure and often contain reactive chlorine. Second, most are produced by human processes and, thus, are synthetic chemicals. Third, they persist in the environment, do not break down easily, are polluting and toxic, and tend to accumulate in living tissue. Fourth, they occur in a number of forms that allow them to be easily transported by water and wind, with sediment, for long distances.
A significant example of water polluter is the chemical MTBE (methyl tertbutyl ether). The Clean Air Act Amendments that were passed in 1990 required cities with air pollution problems to use what are known as oxygen additives in gasoline. MTBE is added to gasoline with the objective of increasing the oxygen level of the gasoline and decreasing emissions of carbon monoxide from gasoline-burning cars. It is used because MTBE is more economical than other additives, including alcohol. MTBE is very soluble in water and is a commonly detected volatile organic compound (VOC) in urban groundwater. It is hypothesized that the MTBE detected in shallow groundwater originates from three sources: urban storm-water runoff, leaking underground gasoline tanks, and leakage occurring at service stations when car tanks are being filled.
Pathways for chemical pollutants within the hydrologic cycle of the environment.
It is ironic that a gasoline additive intended to improve air quality contaminated the groundwater that was used as a source of drinking water for approximately 15 million people in California. In 1997, MTBE-polluted groundwater in Santa Monica, California, forced the city to stop pumping groundwater, eliminating approximately 50 percent of the total drinking water supply for the city. Concentrations of MTBE in Santa Monicas groundwater ranged from about 8 to 600 micrograms per litre. The Environmental Protection Agency has stated that concentrations of 20 to 40 of MTBE per litre of water are sufficient to cause objectionable taste and odour. MTBE in that concentration smells like turpentine or fresh paint and is nauseating to some people. Studies are under way concerning the toxicity of MTBE, and some researchers fear it is a carcinogenic chemical. As a result of the contamination, some states, such as California, have terminated the use of MTBE. Many other states followed and, by 2006, MTBE was all but phased out in the United States. However, MTBE remains a groundwater pollution problem that can contaminate surface water (see Putting Some Numbers on Water Pollution). Figure above illustrates some of the pathways of MTBE, as well as other volatile organic compounds in the hydrologic cycle of an urban area.

Heavy Metals 

Heavy metals, such as lead, mercury, zinc, cadmium, and arsenic, are dangerous pollutants that are often deposited with natural sediment in the bottoms of stream channels. If these metals are deposited on floodplains, they may become incorporated into plants, including food crops, and animals. Once the metal has dissolved in water used for agricultural or domestic use, heavy-metal poisoning can result.
As an example, consider mercury contamination of aquatic ecosystems. It has been known for decades that mercury is a significant pollutant of aquatic ecosystems, including ponds, lakes, rivers, and the ocean.
Perhaps the best-known case history of mercury toxicity comes from Minamata, Japan. Minamata is a coastal town on the island of Kyushu and was the site of a serious illness that was first recognized in the middle of the twentieth century.
It was first called the disease of the dancing cats because the illness was first observed in cats that seemingly went mad and ran in circles, foaming at the mouth. It was also noticed that birds flew into buildings or fell to the ground. People were subsequently affected, most being families of fishermen. Some of the first symptoms were fatigue, irritability, numbness in arms and legs, and headaches, as well as difficulty in swallowing. Some of the more severe symptoms included blurred vision, loss of hearing, and loss of muscular coordination. Some people complained of a metallic taste in their mouths and suffered from diarrhoea. By the time the disease ran its course, over 40 people died and over 100 were severely disabled. The people affected by the disease lived in a relatively small area, and their diet mostly came from fish harvested from Minamata Bay.
The disease was eventually traced to a vinyl chloride factory on Minamata Bay that used mercury in its production processes. Inorganic mercury was released as waste into the bay, and it was believed that the mercury would not get into the food chain. However, the inorganic mercury was converted by bacterial activity in Minamata Bay to methyl mercury. Methyl mercury readily passes through cell membranes and is transported throughout the body by red blood cells. It can enter and damage brain cells. The harmful effects of methyl mercury depend on a number of factors that include the amount of exposure and intake, the duration of the exposure, and the species affected. The effects of the mercury are often delayed from several weeks to months in people from the time of ingestion. Furthermore, if the intake of mercury ceases, some of the symptoms may disappear, but others are difficult to reverse.
The disease of the dancing cats eventually became known as Minamata disease, and nearly 800 people were officially recognized as having the disease but as many as several thousand may have been involved. The mercury pollution in the bay ceased in 1968. As recently as the 1990s, some of the people afflicted by the disease were still being compensated for damages.
 Fish may contain toxic metals People cooking and eating fish, here in the Fiji Islands, are taking in chemicals, including metals that the fish have in their tissue. Mercury is a potential problem with fish, such as tuna and swordfish. 
There are several natural sources of mercury, including input from volcanoes and erosion of natural mercury deposits. In most cases, however, we are most concerned with the input of mercury into the environment through processes such as burning coal, incinerating waste, and processing metals. Although the rates of mercury input into the environment by humans are poorly understood, it is believed that human activities have doubled or tripled the amount of mercury in the atmosphere, and it is increasing at about 1.5 percent per year. Deposition from the atmosphere through rainfall is the primary source of mercury in most aquatic ecosystems. Once ionic mercury is in surface water, it enters into complex cycles, during which a process known as methylation may occur. Bacterial activity changes the inorganic mercury to methyl mercury This process is important from an environmental viewpoint because methyl mercury is much more toxic than is ionic mercury. Furthermore, living things require longer periods of time to eliminate methyl mercury from their systems than they do to eliminate inorganic mercury. As the methyl mercury works its way through food chains, a process known as bio-magnification occurs, in which concentrations of methyl mercury increase in higher levels of the food chain. Thus, big fish in a pond contain higher concentrations of mercury than do the smaller fish and aquatic insects that the large fish feed upon. The input side of the mercury cycle shows the deposition of inorganic mercury through the formation of methyl mercury. On the output side of the cycle, mercury entering fish may be taken up by the organism that eats the fish (figure above). Sediment may also release mercury by a variety of processes, including re-suspension in the water; this can eventually result in the mercury entering the food chain or being released back into the atmosphere through volatilization, the process of converting a liquid or solid to a vapour.
Arsenic is an example of a highly toxic natural metal that is found in soil, rock, and water. There are many industrial and commercial uses of arsenic compounds, including the processing of glass, pesticides, and wood preservatives. Arsenic may enter our water supplies through a number of processes, including natural rain, snow melt, or groundwater flow. It may also be released with industrial waste water and agricultural processes. Finally, it may be released through the production of pesticides, the burning of fossil fuels, and as a by-product of mining.
Arsenic has been known as a deadly poison since ancient times, and, more recently, it has been recognized that elevated levels of arsenic in drinking water may cause a variety of health problems that affect organs such as the bladder, lung, and kidney. It may also cause disease to the central nervous system. Finally, arsenic is known to be a carcinogen (capable of causing or promoting cancer).
The occurrence of arsenic in drinking water is now recognized as a global problem. It certainly is not found in all water supplies, but it is found in many around the world. For example, arsenic in groundwater in Bangladesh has affected many millions of some of the poorest people on Earth. Ongoing research has the objective of identifying those locations where arsenic pollution occurs and of developing appropriate technology or methods to avoid or reduce the hazard of exposure to arsenic.

Radioactive Waste 

Radioactive waste in water may be a dangerous pollutant. Environmentalists are concerned about the possible effects of long-term exposure to low doses of radioactivity to people, other animals, and plants.


Sediment consists of unconsolidated rock and mineral fragments, the smallest of which range in size from sand particles to very small silt- and clay-sized particles. It is these small particles that cause most sediment pollution problems. Sediment is our greatest water pollutant by volume; it is clearly a resource out of place. It depletes soil, a land resource; can reduce the quality of the water resource it enters; and may deposit undesired materials on productive crop lands or on other useful land.

Thermal Pollution 

Thermal pollution is the artificial heating of waters, primarily by hot-water emission from industrial operations and power plants. Heated water causes several problems. First, heated water contains less oxygen than cooler water; even water only several degrees warmer than the surrounding water holds less oxygen. Second, warmer water favours different species than does cooler water and may increase the growth rates of undesirable organisms, including certain water plants and fish. In some cases, however, the warm water may attract and allow better survival of certain desirable fish species, particularly during the winter.

Prospect exploration stages for minerals

Once a prospect has been identified, and the right to explore it acquired, assessing it involves advancing through a progressive series of definable exploration stages. Positive results in any stage will lead to advance to the next stage and an escalation of the exploration effort. Negative results mean that the prospect will be discarded, sold or joint ventured to another party, or simply put on hold until the acquisition of fresh information/ideas/technology leads to its being reactivated. Although the great variety of possible prospect types mean that there will be some differences in the exploration process for individual cases, prospect exploration will generally go through the stages listed below.

Target Generation

This includes all exploration on the prospect undertaken prior to the drilling of holes directly targeted on potential ore.The aim of the exploration is to define such targets. The procedures carried out in this stage could include some or all of the following:
  • a review of all available information on the prospect, such as government geological mapping and geophysical surveys, the results of previous exploration and the known occurrence of minerals.
  • preliminary geological interpretations of air photographs and remote sensed imagery. 
  • regional and detailed geological mapping.
  • detailed rock-chip and soil sampling for geochemistry. 
  • regional and detailed geophysical surveys.
  • shallow pattern drilling for regolith or bedrock geochemistry.
  • drilling aimed at increasing geological knowledge.

Target Drilling

This stage is aimed at achieving an intersection of ore, or potential ore. The testing will usually be by means of carefully targeted diamond or rotary-percussion drill holes, but more rarely trenching, pitting, sinking a shaft or driving an adit may be employed. This is probably the most critical stage of exploration since, depending on its results, decisions involving high costs and potential costs have to be made. If a decision is made that a potential ore body has been located, the costs of exploration will then dramatically escalate, often at the expense of other prospects. If it is decided to write a prospect off after this stage, there is always the possibility that an ore body has been missed.

Resource Evaluation Drilling

This stage provides answers to economic questions relating to the grade, tonnes and mining/metallurgical characteristics of the potential ore body. A good understanding of the nature of the mineralization should already have been achieved that understanding was probably a big factor in the confidence needed to move to this stage. Providing the data to answer the economic questions requires detailed pattern drilling and sampling. Because this can be such an expensive and time-consuming process, this drilling will often be carried out in two sub-stages with a minor decision point in between: an initial evaluation drilling and a later definition drilling stage. Evaluation and definition drilling provide the detail and confidence levels required to proceed to the final feasibility study.

Feasibility Study

This, the final stage in the process, is a desk-top due-diligence study that assesses all factors: geological, mining, environmental, political, economic relevant to the decision to mine. With very large projects, the costs involved in evaluation are such that a preliminary feasibility study is often carried out during the preceding resource evaluation stage. The preliminary feasibility study will identify whether the costs involved in exploration are appropriate to the returns that can be expected, as well as identify the nature of the data that must be acquired in order to bring the project to the final feasibility stage.

Development of modern chronostratigraphy

One of the central themes of the development of modern chronostratigraphy has been the work to establish an accurate geological times scale. Here are nine reasons:

Some of these geological problems and questions include: 
  1. Rates of tectonic processes.
  2. Rates of sedimentation and accurate basin history. 
  3. Correlation of geophysical and geological events.
  4. Correlation of tectonic and eustatic events.
  5. Are epeirogenic movements worldwide.
  6. Have there been simultaneous extinctions of unrelated animal and plant groups.
  7. What happened at era boundaries.
  8. Have there been catastrophes in earth history which have left a simultaneous record over a wide region or worldwide.
  9. Are there different kinds of boundaries in the geologic succession (That is, “natural” boundaries marked by a worldwide simultaneous event versus “quiet” boundaries, man-made by definition). 
It is, in fact, fundamental to the understanding of the history of Earth that events be meticulously correlated in time. For example, current work to investigate the history of climate change on Earth during the last few tens to hundreds of thousands of years has demonstrated how important this is, because of the rapidity of climate change and because different geographical regions and climatic belts may have had histories of climate change that were not in phase. If we are to understand Earth’s climate system thoroughly enough to determine what we might expect from human influences, such as the burning of fossil fuels, a detailed record of past climate change will be of fundamental importance. That we do not now have such a record is in part because of the difficulty in establishing a time scale precise enough and practical enough to be applicable in deposits formed everywhere on Earth in every possible environmental setting. Until the early twentieth century, the geologic time scale in use by geologists was a relative time scale dependent entirely on biostratigraphy. The standard systems had nearly all been named. Estimates about the duration of geologic events, including that of chronostratigraphic units, varied widely, because they depended on diverse estimation methods, such as attempts to quantify rates of erosion and sedimentation. The discovery of the principle of radioactivity was fundamental, providing a universal clock for direct dating of certain rock types, and the calibration of the results of other dating methods, especially the relative scale of biostratigraphy. Radiometric dating methods may be used directly on rocks containing the appropriate radioactive materials. For example, volcanic ash beds intercalated with a sedimentary succession provide an ideal basis for precise dating and correlation. Volcanic ash contains several minerals that include radioactive isotopes of elements such as potassium and rubidium. Modern methods can date such beds to an accuracy typically in the ±2% range, that is, ±2 million years at an age of 100 Ma, although locally, under ideal conditions, accuracy and precision are now considerably better than this (±104–105 years). Where a sedimentary unit of interest (such as a unit with a biostratigraphically significant fauna or flora) is overlain and underlain by ash beds it is a simple matter to estimate the age of the sedimentary unit. The difference in age between the ash beds corresponds to the elapsed time represented by the succssion of strata between the ash beds. Assuming the sediments accumulated at a constantrate, the rate of sedimentation can be determined by dividing the thickness of the section between the ash beds by the elapsed time. The amount by which the sediment bed of interest is younger than the lowest ash bed is then equal to its stratigraphic height above the lowest ash bed divided by the rate of sedimentation, thereby yielding an “absolute” age, in years, for that bed. This procedure is typical of the methods used to provide the relative biostratigraphic age scale with a quantitative basis. The method is, of course, not that simple, because sedimentation rates tend not to be constant, and most stratigraphic successions contain numerous sedimentary breaks that result in underestimation of sedimentation rates. Numerous calibration exercises are required in order to stabilize the assigned ages of any particular biostratigraphic unit of importance. Initially, the use of radiometric dating methods was relatively haphazard, but gradually geologists developed the technique of systematically working to cross calibrate the results of different dating methods, reconciling radiometric and relative biostratigraphic ages in different geological sections and using different fossils groups. In the 1960s the discovery of preserved (“remanent”) magnetism in the rock record led to the development of an independent time scale based on the recognition of the repeated reversals in magnetic polarity over geologic time. Cross-calibration of radiometric and biostratigraphic data with the magnetostratigraphic record provided a further means of refinement and improvement of precision. These modern developments rendered irrelevant the debate about the value and meaning of hypothetical chronostratigraphic units. The new techniques of radiometric dating and magnetostratigraphy, where they are precise enough to challenge the supremacy of biostratigraphy, could have led to the case being made for a separate set of chronostratigraphic units, as Hedberg proposed. However, instead of a new set of chronostratigraphic units, this correlation research is being used to refine the definitions of the existing, biostratigraphically based stages. Different assemblages of zones generated from different types of organism may be used to define the stages in different ecological settings (e.g., marine versus nonmarine) and in different biogeographic provinces, and the entire data base is cross-correlated and refined with the use of radiometric, magnetostratigraphic and other types of data. The stage has now effectively evolved into a chronostratigraphic entity of the type visualized by Hedberg. For most of Mesozoic and Cenozoic time the standard stages, and in many cases, biozones, are now calibrated using many different data sets, and the global time scale, based on correlations among the three main dating methods, is attaining a high degree of accuracy. The Geological Society of London time scale (GSL, 1964) was an important milestone, representing the first attempt to develop a comprehensive record of these calibration and cross-correlation exercises. Formal methods of accounting for “time in stratigraphy”, including the use of “Wheeler plots” for showing the time relationships of stratigraphic units, provided much needed clarity in the progress of this work. Timescales for the Cenozoic and the Jurassic and Cretaceous are particularly noteworthy for their comprehensive data syntheses, although all have now been superseded. More recent detailed summation and reconciliation of the global data provided a comprehensive treatment of the subject. In the 1960s, several different kinds of problems with stratigraphic methods and practice had begun to be generally recognized. There are two main problems. Firstly, stratigraphic boundaries had traditionally been drawn at horizons of sudden change, such as the facies change between marine Silurian strata and the overlying nonmarine Devonian succession in Britain. Changes such as this are obvious in outcrop, and would seem to be logical places to define boundaries. Commonly such boundaries are unconformities. However, it had long been recognized that unconformities pass laterally into conformable contacts. This raised the question of how to classify the rocks that formed during the interval represented by the unconformity. When it was determined that rocks being classified as Cambrian and Silurian overlapped in time, a new chronostratigraphic unit the Ordovician, as a compromise unit straddling the Cambrian-Silurian interval. The same solution could be used to define a new unit corresponding to the unconformable interval between the Silurian and the Devonian. In fact, rocks of this age began to be described in central Europe after WWII, and this was one reason why the Silurian-Devonian boundary became an issue requiring resolution. A new unit could be erected, but it seemed likely that with additional detailed work around the world many such chronostratigraphic problems would arise, and at some point it might be deemed desirable to stabilize the suite of chronostratigraphic units. For this reason, the development of some standardized procedure seemed to be desirable. A second problem is that to draw a significant stratigraphic boundary at an unconformity or at some other significant stratigraphic change is to imply the hypothesis that the change or break has a significance relative to the stratigraphic classification, that is, that unconformities have precise temporal significance. This was specifically hypothesized by Chamberlin who was one of many individuals who generated ideas about a supposed “pulse of the earth”. In the case of lithostratigraphic units, which are descriptive, and are defined by the occurrence and mappability of a lithologically distinctive succession, a boundary of such a unit coinciding with an unconformity is of no consequence. However, in the case of an interpretive classification, in which a boundary is assigned time significance (such as a stage boundary), the use of an unconformity as the boundary is to make the assumption that the unconformity has time significance; that is, it is of the same age everywhere. This places primary importance on the model of unconformity formation, be this diastrophism, eustatic sea-level change or some other cause. From the methodological point of view this is most undesirable, because it negates the empirical or inductive nature of the classification. It is for this reason that it is inappropriate to use sequence boundaries as if they are chronostratigraphic markers. A time scale is concerned with the continuum of time. Given our ability to assign “absolute” ages to stratigraphic units, albeit not always with much accuracy and precision, one solution would be to assign numerical ages to all stratigraphic units and events. However, this would commonly be misleading or clumsy. In many instances stratigraphic units cannot be dated more precisely than, say, “late Cenomanian” based on a limited record of a few types of organisms (e.g., microfossils in subsurface well cuttings). Named units are not only traditional, but also highly convenient, just as it is convenient to categorize human history using such terms as the “Elizabethan” or the “Napoleonic” or the “Civil War” period. The familiar terms for periods (e.g., Cretaceous) and for ages/stages (e.g., Aptian) offer such a subdivision and categorization, provided that they can be made precise enough and designed to encompass all of time’s continuum. The problem solution was explained in this way:

There is another approach to boundaries, however, which maintains that they should be defined wherever possible in an area where “nothing happened.” The International Subcommission on Stratigraphic Classification has recommended that “Boundary-stratotypes should always be chosen within sequences of continuous sedimentation. The boundary of achronostratigraphic unit should never be placed at an unconformity. Abrupt and drastic changes in lithology or fossil content should be looked at with suspicion as possibly indicating gaps in the sequence which would impair the value of the boundary as a chronostratigraphic marker and should be used only if there is adequate evidence of essential continuity of deposition. The marker for a boundary stratotype may often best be placed within a certain bed to minimize the possibility that it may fall at a time gap.” This marker is becoming known as “the Golden Spike.” By “nothing happens” is meant a stratigraphic succession that is apparently continuous. The choice of boundary is then purely arbitrary, and depends simply on our ability to select a horizon that can be the most efficiently and most completely documented and defined. This is the epitome of an empirical approach to stratigraphy. Choosing to place a boundary where “nothing happened” is to deliberately avoid having to deal with some “event” that would require interpretation. This recommendation was accepted in the first International Stratigraphic Guide, although noted the desirability of selecting boundary stratotypes “at or near markers favourable for long-distance time-correlation”, by which he meant prominent biomarkers, radiometrically-datable horizons, or magnetic reversal events. Boundary stratotypes were to be established to define the base and top of each chronostratigraphic units, with a formal marker (a“golden spike”) driven into a specific point in a specific outcrop to mark the designated stratigraphic horizon. Such boundary-stratotypes be used to define both the top of one unit and the base of the next overlying unit. However, further consideration indicates an additional problem, which was noted in the North American Stratigraphic Code:

Designation of point boundaries for both base and top of chronostratigraphic units is not recommended, because subsequent information on relations between successive units may identify overlaps or gaps. One means of minimizing or eliminating problems of duplication or gaps in chronostratigraphic successions is to define formally as a point-boundary stratotype only the base of the unit. Thus, a chronostratigraphic unit with its base defined at one locality will have its top defined by the base of an overlying unit at the same, but more commonly, another locality.

Even beds selected for their apparently continuous nature may be discovered at a later date to hide a significant break in time. Detailed work on the British Jurassic section using what is probably the most refined biostratigraphic classification scheme available for any pre-Neogene section has demonstrated how common such breaks are. The procedure recommended by NACSN is that, if it is discovered that a boundary stratotype does encompass a gap in the temporal record, the rocks (and the time they represent) are assigned to the unit below the stratotype. In this way, a time scale can be constructed that can readily accommodate all of time’s continuum, as our description and definition of it continue to be perfected by additional field work. This procedure means that, once designated, boundary stratotypes do not have to be revised or changed. This has come to be termed the concept of the “topless stage.” The modern definition of the term “stage” indicates how the concept of the stage has evolved. The Guide states that “The stage has been called the basic working unit of chronostratigraphy. The stage includes all rocks formed during an age. A stage is normally the lowest ranking unit in the chronostratigraphic hierarchy that can be recognized on a global scale. A stage is defined by its boundary stratotypes, sections that contain a designated point in a stratigraphic sequence of essentially continuous deposition, preferably marine, chosen for its correlation potential”. The first application of the new concepts for defining chronostratigraphic units was to the Silurian Devonian boundary, the definition of which had begun to cause major stratigraphic problems as international correlation work became routine in post-WWII years. Aboundary stratotypewas selected atalocation called Klonk, in what is now the Czech Republic, following extensive work by an international Silurian-Devonian Boundary Committee on the fossil assemblages in numerous well-exposed sections in Europe and elsewhere. The establishment of the new procedures led to a flood of new work to standardize and formalize the geological time scale, one boundary at a time. This is extremely labour-intensive work, requiring the collation of data of all types (biostratigraphic, radiometric and, where appropriate, chemostratigraphic and magnetostratigraphic) for well-exposed sections around the world. In many instances, once such detailed correlation work is undertaken, it is discovered that definitions for particular boundaries being used in different parts of the world, or definitions established by different workers using different criteria, do not in fact define contemporaneous horizons. This may be because the original definitions were inadequate or incomplete, and have been subject to interpretation as practical correlation work has spread out across the globe. Resolution of such issues should simply require international agreement; the important point being that there is nothing significant about, say, the Aptian-Albian boundary, just that we should all be able to agree on where it is. Boundaries be places where “nothing happens”, the sole criterion for boundary definition is that such definitions be as practical as possible. The first “golden spike” location was chosen because it represents an area where deepwater graptolite-bearing beds are interbedded with shallow-water brachiopod-trilobite beds, permitting detailed cross-correlation among the faunas, thereby permitting the application of the boundary criteria to a wide array of different facies. In other cases, the presence of radiometrically datable units or a well defined magneto stratigraphic record may be helpful. In all cases, accessibility and stability of the location are considered desirable features of a boundary stratotype, because the intent is that it serves as a standard. Perfect correlation with such a standard can never be achieved, but careful selection of the appropriate stratotype is intended to facilitate future refinement in the form of additional data collection. Despite the apparent inductive simplicity of this approach to the refinement of the time scale, further work has been slow, in part because of the inability of some working groups to arrive at agreement. In addition, two contrasting approaches to the definition of chronostratigraphic units and unit boundaries have now evolved, each emphasizing different characteristics of the rock record and the accumulated data that describe it. The first approach, which Castradori described as the historical and conceptual approach, emphasizes the historical continuity of the erection and definition of units and their boundaries, the data base for which has continued to grow since the nineteenth century by a process of inductive accretion. The alternative method focuses on the search for and recognition of significant “events” as providing the most suitable basis for rock-time markers, from which correlation and unit definition can then proceed. The choice of the term “pragmatic” is unfortunate in this context, because the suggested method is certainly not empirical. The followers of this method suggest that in some instances historical definitions of units and their boundaries should be modified or set aside in favour of globally recognizable event markers, such as a prominent biomarker, a magnetic reversal event, an isotopic excursion, or, eventually, events based on cyclostratigraphy. Boundaries be defined in places where “nothing happened,” although it is in accord with suggestions in the first stratigraphic guide that “natural breaks” in the stratigraphy could be used or boundaries defined “at or near markers favorable for long-distance time-correlation”. The virtue of this method is that where appropriately applied it may make boundary definition easier to recognize. The potential disadvantage is that is places prime emphasis on a single criterion for definition. From the perspective of this book, which has attempted to clarify methodological differences, it is important to note that the hyperpragmatic approach relies on assumptions about the superior time-significance of the selected boundary event. The deductive flavour of hypothesis is therefore added to the method. In this sense the method is not strictly empirical. As has been demonstrated elsewhere, assumptions about global synchroneity of stratigraphic events may in some cases be misguided. The hyper-pragmatic approach builds assumptions into what has otherwise been an inductive method free of all but the most basic of hypotheses about the time-significance of the rock record. The strength of the historical and conceptual approach is that it emphasizes multiple criteria, and makes use of long established practices for reconciling different data bases, and for carrying correlations into areas where any given criterion may not be recognizable. For this reason to eliminate the distinction between time-rock units (chronostratigraphy) and the measurement of geologic time (geochronology). History has repeatedly demonstrated the difficulties that have arisen from the reliance on single criteria for stratigraphic definitions, and the incompleteness of the rock record, which is why “time” and the “rocks” are so rarely synonymous in practice. The latter article provides several case studies of how each approach has worked in practice. For our purposes, the importance of this history of stratigraphy is that the work of building and refining the geological time scale has been largely an empirical, inductive process. Note that each step in the development of chronostratigraphic techniques, including the multidisciplinary cross-correlation method, the golden spike concept, and the concept of the topless unit, are designed to enhance the empirical nature of the process. Techniques of data collection, calibration and crosscomparison evolved gradually and, with that development came many decisions about the nature of the time scale and how it should be measured, documented, and codified. These decisions typically were taken at international geological congresses by large multinational committees established for such purposes. For our purposes, the incremental nature of this method of work is significant because it is completely different from the basing of stratigraphic history on the broad, sweeping models of pulsation or cyclicity that have so frequently arisen during the evolution of the science of geology.

Sediment Supply and the Importance of Big Rivers

Sediment supply is controlled primarily by tectonics and climate. In geologically simple areas, where the basin is fed directly from the adjacent margins and source-area uplift is related to basin subsidence, supply considerations are likely to be directly correlated to basin subsidence and eustasy as the major controls of basin architecture. Such is the case where subsidence is yoked to peripheral upwarps, or in proximal regions of foreland basins adjacent to fold-thrust belts. However, where the basin is supplied by long-distance fluvial transportation, complications are likely to arise. Where the rate of sediment supply is high, it may overwhelm other influences to become a dominant control on sequence architecture. Many sedimentary basins were filled by river systems whose drainage area has been subsequently remodeled by tectonism, and it may take considerable geological investigation to reconstruct their possible past positions. For example, stratigraphic successions may occur that cannot be related to the evolution of adjacent orogens. In North America, dynamic topographic processes have generated regional uplifts and continental tilts that have resulted in deep erosion and large-scale continental fluxes of detrital sediment. For example, much of the detritus derived by uplift and erosion of the Grenville orogen of eastern North America during the late Precambrian may have ended up contributing to the thick Neoproterozoic sedimentary wedges on the western continental margin. Detailed study of detrital zircons from sedimentary rocks of this age in the western Canadian Arctic indicated that 50% of them are of Grenville age. A major west-flowing river system was established during the late Proterozoic which transported this detritus some 3,000 km across the continental interior. Much of the thick accumulations of late Paleozoic and Mesozoic fluvial and eolian strata in the southwestern United States had been derived from Appalachian sources, and this was confirmed by the detrital-zircon. Tertiary river system draining from the continental interior of North America into Hudson Bay, ultimately delivering sediment to the Labrador Shelf. This has been supported by the studies of Cenozoic landforms and sediments. 

Major river systems may cross major tectonic boundaries, feeding sediment of a petrographic type unrelated to the receiving basin, into the basin at a rate unconnected in any way with the subsidence history of the basin itself. The modern Amazon river is a good example. It derives from the Andean Mountains, flows across and between, and is fed from several Precambrian shields, and debouches onto a major extensional continental margin. From the point of view of sequence stratigraphy, the important point is that large sediment supplies delivered to a shoreline may overwhelm the stratigraphic effects of variations in sea level. A region undergoing a relative or eustatic rise in sea level may still experience stratigraphic regression if large delta complexes are being built by major sediment-laden rivers. Effects of upstream controls on the development of fluvial graded profiles, fluvial style and the development of nonmarine sequences downstream. Upstream controls may also be significant in the case of deep-marine deposits. Major episodes of submarine-fan sedimentation in the North Sea and Shetland-Faeroes basins correlate with pulses of Iceland plume activity, which caused magmatic underplating of the continental margin, and uplift, erosion, and enhanced sediment delivery to offshore sedimentary basins. A significant example of this long-distance sedimentary control is the Cenozoic stratigraphic evolution of the Texas-Louisiana coast of the Gulf of Mexico. This continental margin is fed with sediment by rivers that have occupied essentially the same position since the early Tertiary. The rivers feed into the Gulf Coast from huge drainage basins occupying large areas of the North American Interior. Progradation has extended the continental margin of the Gulf by up to 350 km. This has taken place episodically in both time and space, developing a series of major clastic wedges, some hundreds of metres in thickness. The major changes along strike of the thickness of these clastic wedges is also evidence against a control by passive sea-level change. Highly suggestive are the correlations with the tectonic events of the North American Interior; for example, the timing of the Lower and Upper Wilcox Group wedges relative to the timing of the Laramide orogenic pulses along the Cordillera. It seems likely that sediment supply, driven by source-area tectonism, is the major control on the location, timing and thickness of the Gulf Coast clastic wedges. A secondary control is the nature of local tectonism on the continental margin itself, including growth faulting, evaporite diapirism and gravity sliding. Variations in deep-marine sediment dispersal in the Gulf of Mexico show very similar patterns to the coastal and fluvial variations. Large-scale submarine-fan systems are therefore dependent, also, on considerations of long-term sediment supply variation, which may be controlled by plate-margin tectonism, in-plane stress regime and dynamic topography.

In arc-related basins volcanic control of the sediment supply may overprint the effects of sea-level change. Sediment supply and tectonic activity overprinted the eustatic effects and enhanced or lessened them. If large supplies of clastics or uplift overcame the eustatic effects, deep marine sands were also deposited during highstand of sea level, whereas under conditions of low sediment input, thin-bedded turbidites were deposited even during lowstands of sea level.

Other examples of the tectonic control of major sedimentary units are provided by the basins within and adjacent to the Alpine and Himalayan orogens. Sediments shed by the rising mountains drain into foreland basins, remnant ocean basins, strike-slip basins, and other internal basins. But the sediment supply is controlled entirely by uplift and by the tectonic control of dispersal routes. For example, the Oligocene Molasse of the Swiss proforeland basin was deposited by rivers flowing axially along the basin, and that these underwent reversal in transport directions as a result of changes in the configuration of the basin and the collision zone during orogenesis. The shifting of dispersal routes through basins and fault valleys within the Himalayan orogen of central and southeast Asia. Some of the major rivers in the area (Tsangpo, Salween, Mekong) are known to have entirely switched to different basins during the evolution of the orogen. Much work remains to be done to relate the details of the stratigraphy in these various basins to the different controls of tectonic subsidence, tectonic control of sediment supply, and eustatic sea-level changes.