Outward from the reefs, in the deeper water, muds rich in CaCO 3 accumulated on the floor of the vast shallow seas, generally producing thinly bedded dolomite that contains few fossils.

But the genial climates of the Silurian came to an end. The close of the Silurian was a long time of aridity in which the seas became so salty that living creatures swam to a more suitable home. The conditions were roughly analogous to the present-day Mediterranean Sea where water from the Atlantic flows in over the rather shallow restriction at the Straits of Gibraltar.

The arid climate in the region evaporates the sea water and concentrates the salt. The heavier, highly saline water sinks to the bottom and cannot escape and therefore tends to accumulate. Great salt beds named the Salina Salt Beds were deposited in the basin.

In the central part of the basin feet of alternating salt, shale, and limestone beds have been penetrated in the Salina, with feet of rock salt. The rock salt bowl becomes thinner towards its rim. Source: Detroit Free Press Thus, visualize a subsiding, circular basin in lower Michigan, surrounded by shallow waters and coral reefs.

At that time the Great Lakes area was near the equator, and as a result, there was extensive evaporation of sea water. To make up for this, salt water was constantly being added to the Michigan evaporation Basin through shallow passages in the reefs.

Waters in the basin were therefore saltier than in the "open ocean" outside of the basin. Since the higher salt concentration caused the water to become heavier, the highly saline water settled to the bottom of the basin rather than mixing with the inflowing fresher waters from the surrounding seas.

An aggregate thickness of m of gypsum and salt accumulated in the basin, with one bed of salt nearly m thick. It would take a column of sea water nearly km deep to form a layer of salt m thick. Great thicknesses of anhydrite and gypsum are also present. The deposition of this immense volume of salt took perhaps million years and ended about million years ago.

The map below shows the North American Silurian reef system. Marginal marine halite: sabkhas and salinas. In Melvin, J. New York: Elsevier. Developments in Sedimentology , 50, pp. Hardie, L. Evaporites: marine or non-marine. Horita, J. Chemical evolution of seawater during the Phanerozoic: implications from the record of marine evaporites.

Acta , 66 , — Kendall, A. Marine evaporites: arid shorelines and basins. In Reading, H. Oxford: Blackwell, pp. Krijgsman, W.

Chronology, causes and progression of the Messinian salinity crisis. Nature , , — Lowenstein, T. Paleoclimate record from death valley salt core. Geology , 27 , 3—6. Oscillations in Phanerozoic seawater chemistry: evidence from fluid inclusions. Science , , — Murray, R. Origin and diagenesis of gypsum and anhydrite. Nijman, W. Over long times — geological times — chemical weathering has a much greater effect than mechanical weathering.

Even apparently dry climates have enough water to promote chemical weathering on exposed surfaces, although the weathering rate may be slow. Mechanical weathering breaks large or solid material into smaller pieces.

Clastic material , also called detritus or detrital material , may be fine grains of individual minerals or it may be lithic fragments rock fragments composed of multiple minerals. The photo seen here Figure 7. Freezing, thawing, and the action of ice created large blocky pieces of what was originally solid bedrock. Even apparently solid granites or other rocks can be broken apart this way.

Talus slopes are examples of very coarse sediment. More commonly, mechanical weathering produces smaller rock fragments, or sand, or silt composed of individual mineral grains. After chemical weathering, leftover rock may have a dissolved or eroded appearance, such as the sandstone seen in Figure 7. This sandstone has weathered to obtain a honeycomb texture , typical of sandstone in which the cementation of grains is not uniform. Weathering textures are not unique to sandstone.

After chemical weathering, outcrops of many sorts often become rounded or pitted. The limestone outcrop shown in Figure 7. The surfaces are a dull chalky white and the corners are all rounded. Figure 4. Minor chemical weathering can cause minerals to alter, perhaps to oxidize rust. More intense weathering may cause some minerals to disappear. They may dissolve completely in water and be carried away in a hydrolysate water containing dissolved ions.

More often, minerals react to produce secondary minerals — minerals that were not present before weathering. Reactions that produce secondary minerals most commonly involve the reaction of water with previously existing minerals such as feldspars common in many igneous rocks , to produce clays and dissolved elements. We call such reactions hydrolysis reactions. Secondary minerals may also form by oxidations reactions when primary minerals react with oxygen in air or water.

For example, oxidation of iron-rich olivine or pyroxene commonly produces hematite Fe 2 O 3. Mineral matter remaining after chemical weathering often includes original mineral grains that did not decompose. We sometimes call these minerals the residual minerals , or the resistate , because the minerals resisted weathering. Typical resistate minerals include quartz, clay, K-feldspar, garnet, zircon, rutile, or magnetite. After the more easily decomposed minerals break down and disappear, the resistate minerals remain to become sediment.

If we examine fresh unweathered outcrop in a road cut, rock often appears hard and shiny. Examination with a hand lens reveals that minerals have well-defined boundaries and generally sharp outlines.

They may show good cleavage or crystal faces. Minerals may have their normal diagnostic colors: quartz is clear, feldspars are white or pink, muscovite is silvery and sparkly, magnetite appears metallic, and biotite and other mafic minerals appear black. The picture is not the same if we examine outcrops exposed to weathering for a long time. After weathering, rock and most minerals have a dull or drab appearance.

Grain boundaries and cleavages are obscured. Oxidation rusting and hydration may produce reddish, yellow, brown, or gray hues. Sometimes a layer of clay or other material coats all surfaces, obscuring diagnostic minerals.

The photo shown here Figure 7. The once solid crystalline granite is now a dull earthy mass. Mafic silicates weather to create secondary clay minerals and iron oxides. Feldspars of all sorts weather to become clay minerals and dissolved material.

Quartz is usually unchanged by weathering. Calcite weathers by dissolution producing dissolve ions. And aluminous minerals weather to gibbsite or other aluminum hydroxides. The table below lists weathering products for the most common minerals.

Clays and limonite a general term describing for a mix of hydrated Fe-oxides and hydroxides dominate the list. Quartz and aluminous minerals may also be produced. While creating these secondary minerals, weathering also produces dissolved cations especially alkalis and alkali earths and anions, which may have a significant impact on water chemistry and quality.

Some minerals break down more easily than others. Geologists can compare weathering rates by looking at minerals in rock outcrops, and by studying the minerals present in sediments of different ages. The series ranked the ease with which common igneous minerals break down. Goldich found that minerals that crystallize from a magma at high temperature — minerals relatively poor in silicon and oxygen — are generally less resistant to weathering than those that crystallize at low temperature.

Iron-magnesium silicates, such as olivine, pyroxene, or amphibole break down relatively easily. Calcic feldspars, and many minerals with high solubilities in water, are also quick to decompose. Quartz, some feldspars, and some nonsilicate minerals are relatively resistant to weathering because they contain more bonds, especially Si — O bonds, that do not break easily.

It should not be surprising that minerals that characterize high-temperature igneous rocks, or those most often precipitated from water, are the first to decompose under Earth surface conditions where temperature is low and water is abundant.

Sedimentologists have made comprehensive lists of the relative ease with which minerals weather. Although there is some variation, the list shown here is typical from Birkeland Like primary minerals, secondary minerals can break down and disappear, so this table compares weathering rates for both primary minerals and secondary minerals.

Weathering resistance, however, does not necessarily mean that a particular mineral is abundant in weathered materials. Some of the minerals at the top of the list in the table are uncommon compared with others. Zircon, rutile, and tourmaline, for example, are very resistant to weathering but rarely are major components of sediments because they are only minor minerals in most parent rocks. Minerals at the bottom of the list are very unstable when exposed to the elements and, consequently, are absent from all but the youngest sediments.

After chemical weathering, dissolved material is carried away. Residual minerals and secondary minerals such as clay may remain where they form. For example, prolonged weathering of bedrock can lead to thick layers of reddish soil called laterite in tropical areas see Box , below. Laterites vary but are always rich in oxide minerals and clays. Laterites are easily eroded.

Over time, erosion by water, gravity, or wind can transport laterite debris, just like any other detrital material, away from its place of origin. Consider a tropical area with warm weather and abundant rainfall. Weathering and leaching will be extreme, and even clay minerals may decompose.

Normally soluble elements, and even relatively insoluble silica, will be dissolved and removed. The remaining material, called a residual deposit , is often composed primarily of aluminum oxides and hydroxides, the least soluble of all common minerals. We term such deposits laterites if unconsolidated or bauxites if lithified into rock. Bauxites and laterites are our most important source of aluminum. But, the mineralogy of a laterite depends on the composition of rocks weathered to produce it.

Laterites can also be important sources of iron, manganese, cobalt, and nickel, all of which have low solubilities in water. Most laterites are aluminous. The most important aluminum ore bauxite , is a mixture of several minerals, including the polymorphs boehmite and diaspore , both AlO OH , and gibbsite, Al OH 3. Bauxite is mined in large amounts in Australia and Indonesia, and in smaller quantities in the Americas and in Europe.

In some places, relatively young laterites produce ore, but in Australia economical laterite deposits are more than 65 million years old. The term siliciclastic refers to sediments composed mostly of silicate minerals. The most common sedimentary rocks — including shale, sandstone, and conglomerate — form from siliciclastic sediments.

Other, less common, kinds of sedimentary rocks consist of carbonates in limestones , iron oxides and hydroxides such as hematite or goethite , or other minerals. Geologists classify siliciclastic sediments based on grain size.

The standard classification system is the Wentworth Scale see table. Depending on size, grains may be boulders, cobbles, pebbles, gravel, sand, silt, or clay. The word clay sometimes causes confusion. Sedimentary petrologists use the term to refer to clastic grains smaller than 0. In this text, however, we also use it to refer to minerals of the clay mineral group, no matter the grain size. Clast sizes vary from fine clay and silt to huge boulders.

Small clasts are usually composed of a single mineral, generally quartz or clay. Larger clasts are commonly lithic fragments composed of multiple minerals. The photos below show some examples. The mud comprises fine grains of silt and clay. Quartz dominates most common sand, but the sand seen here contains mostly rosy garnet, and also epidote, zircon, magnetite, spinel, staurolite, and only minor quartz. Most of the pebbles are lithic fragments rock fragments composed of more than one mineral.

These cobbles are all lithic fragments. The mineral grains in the Pfeiffer Beach sand are angular, but the clasts in the last two photos have been well rounded by abrasion caused by them being tumbled by flowing water. Grains are about 1 mm across.

Grains are cm across. The piece of wood is about 15 cm long. Wind, gravity, and other agents can move clastic material as well. Eventually, sediments are deposited when the forces of gravity overcome those trying to move them.

Large grains may not move far and are deposited first. As the energy of transportation decreases, smaller material is deposited. So, during transportation, sediments commonly become sorted , which means that sediment deposits often have relatively uniform grain size. Thus, for example, coarse material may be deposited near the headwaters of a stream, while only fine material makes it to a delta.

Sorting is not ubiquitous; streambed gravel, for example, may contain a mix of silt, sand, and larger clasts, and glacial deposits often contain a jumble of material of many different sizes. The photo above Figure 7.

After deposition, unconsolidated sediment may, over time, change into a clastic sedimentary rock by the process called lithification from lithos , the Greek word meaning stone.

Lithification involves compaction and cementation of clastic material. Common cementing agents include the minerals quartz, calcite, and hematite. Before, during, and after lithification, sedimentary rocks undergo textural or chemical changes due to heating, compaction, or reaction with groundwaters.

Biological agents, including small animals or bacteria, also can be important, as can chemical agents brought in by flowing water. We call these changes collectively diagenesis. Dissolution and removal of minerals leaching and the formation of clay or other minerals are both common during diagenesis. We call any new minerals that form, authigenic minerals. Zeolites, clays, feldspar, pyrite, and quartz can all be authigenic minerals. Although diagenesis creates many authigenic minerals, most are so fine grained that we cannot identify them without X-ray analysis.

Textural changes, including compaction and loss of pore space, are common and are part of diagenesis. Recrystallization , the changing of fine-grained rocks into coarser ones, is another form of diagenetic textural change.

During recrystallization, as individual mineral grains grow together, secondary minerals may precipitate in open spaces, and more mineral cements may develop. Consequently, rocks become harder. Diagenesis is equivalent to a low-temperature, low-pressure form of metamorphism , and the processes of sedimentation, lithification, diagenesis, and low-grade metamorphism form a continuum.

Lithification changes unconsolidated sediment into a rock. Cementation by quartz, calcite, or hematite may be part of the lithification process. It also may be considered a diagenetic process. Similarly, the formation of many low-temperature minerals such as zeolites , a normal part of diagenesis, overlaps with the beginnings of metamorphism.

Metamorphic petrologists often define the onset of metamorphism by the first occurrence of metamorphic minerals. This definition can be hard to apply because many diagenetic minerals are also metamorphic minerals. Furthermore, laumontite , often considered to be formed at the lowest temperature of all metamorphic minerals, is a zeolite that is hard to distinguish from minerals that form diagenetically.

Chemical weathering yields dissolved material that water transports until precipitation of chemical sediment occurs. Several things may cause precipitation; the most common causes are evaporation, changes in temperature or acidity pH , and biological activity. Hot springs deposit a form of calcite called travertine , for example, when cooling water becomes oversaturated with CaCO 3. This photo Figure 7. In freshwater streams or lakes, a pH change due to biological activity may cause precipitation of another form of calcite called marl.

In marine settings, many reef-building organisms have shells or skeletons made of organic calcite. Calcite and other chemical sedimentary minerals, then, precipitate in many ways. In contrast with clastic sediments, chemical sediments usually lithify at the same time they precipitate. Natural waters contain dissolved minerals, and all minerals are soluble in water to some extent.

Halides, many sulfates, and other salts have very high solubilities. Carbonate minerals, including calcite and dolomite, have moderate solubilities. Silicate minerals have relatively low solubilities. If water evaporates, it may become oversaturated in particular minerals and deposit chemical sediments, such as the salt deposits in the photo seen here Figure 7.

Precipitation will continue, decreasing concentrations of dissolved material, until the solution and sediments achieve equilibrium. Because of their high solubility, large amounts of evaporation may be necessary before salts such as halite, precipitate. In contrast, carbonate minerals calcite and dolomite and silica often precipitate early during evaporation. Silica SiO 2 , in the form of chert , is the only silicate mineral that commonly forms a chemical sedimentary rock. So, their chemical components are common as dissolved species.

As water evaporates, perhaps in a closed inland basin or an isolated sea, these minerals may precipitate to form thick beds of evaporite minerals. Besides these four minerals, many other less common minerals occur in evaporites, too. Evaporites are found in many parts of the world. Some of these pinnacles rise more than 40 m above the lakebed.

These pinnacles consist of trona a hydrated sodium carbonate that precipitated from briny water. Like all playas, this lake is dry most of the time. But, past flooding and subsequent evaporation produced thick layers of evaporite minerals.

More than 25 different minerals are found in the Searles Lake bottom. The list includes sodium and potassium carbonates, sulfates, borates, and halides.

Borax hydrated sodium borate , trona, and several other minerals are profitably mined from Searles Lake sediments today. In the subsurface, massive gypsum and halite beds are common, as are the salt domes found in Texas and other Gulf Coast areas of the United States. Although generally dominated by just a few minerals, many other minerals may be present. In all, petrologists have reported nearly minerals from evaporites.

Less than a dozen are common. Evaporites may be marine deposits associated with evaporation of ocean water. They may also be non-marine , associated with freshwater lakes or other continental waters. For water to become oversaturated, a water body must be somewhat isolated and the evaporation rate must be faster than any water flowing in.

This is most common in an arid environment. For example, at various times in the past, the Mediterranean Sea has been cut off from an ocean. Evaporation led to thick salt deposits that lie beneath the Mediterranean today. And, today, evaporite minerals are collecting along the shores of the Dead Sea between Jordan and Israel see Figure 7.

As ocean water evaporates, minerals precipitate in predictable order from those that are least soluble to those that are most soluble. Calcite is first, followed by gypsum, anhydrite, and then halite. Many other minerals may precipitate in lesser amounts. Continental water contains different dissolved solids than marine water, so continental evaporites contain different minerals than marine evaporites.

Water chemistry is also quite variable, so many different minerals are possible. Continental evaporite deposits may contain halite, gypsum, and anhydrite but also typically have borax, trona, and many other non-marine salts. The table lists some minerals reported from evaporite deposits in North America. It is a long list. The previous chapter discussed silicate minerals common in igneous rocks.

In principle, they could all be detrital grains in sediments and sedimentary rocks. In practice, most break down so quickly that they cannot be weathered or transported very much before completely decomposing. Quartz is the most resistant to weathering.

Many minerals weather to produce clays. It is no surprise, therefore, that quartz and clays are the main silicate minerals in most clastic rocks. Feldspars and sometimes muscovite may also be present but are usually subordinate to quartz. They are absent from rocks formed from sediments transported long distances or weathered for long times. Mafic silicate minerals are exceptionally rare in sediments or sedimentary rocks.