chapter6 sedimentary rocks
THREE SISTERS AT GOBLIN VALLEY STATE PARK, UTAH
CHAPTER TOPICS HYPERLINKS Page 3 Origin of Sediments, Clastic Sedimentary Rocks and Erosional Processes Page 4 Modification of Sediments During Transport Page 6 Sediment Maturity and Depositional Processes Page 7 Lithification Processes and Non-Clastic Sedimentary Rocks Page 8 Case Study - Death Valley National Park, California Page 10 Evaporites Page 11 Other Non-Clastic Sedimentary Rocks - Limestone Page 12 Travertine and Dolostone Page 13 Chert and Petrified Wood Page 14 Coal Page 15 Table 6.1 - The Non-Clastic Sedimentary Rocks and Clastic Sedimentary Rocks Page 16 Table 6.2 Sediment Size Classification and Equivalent Sedimentary Rocks Page 17 Conglomerate and Breccia Page 18 Sandstone Page 19 Siltstone, Claystone and Shale Page 21 Depositional Environments and Sedimentary Structures and The Principle of Uniformitarianism Page 24 Economic Uses of Sediments and Sedimentary Rocks The red star in the lower right hand corner of the page is a hyperlink back to this page.
ORIGIN OF SEDIMENTS Sedimentary rocks are created by the weathering of pre-existing rocks and erosion of the products of that weathering. Those rocks can be igneous, metamorphic, or even other sedimentary rocks. The breakdown of rock involves both chemical and physical processes, and the products of those processes are rock fragments (sediment) and soluble ions. Once rocks are broken down, it is the work of mass wasting to move the fragments to lower elevations. Typically, sediments are then transported, modified by transport, and finally deposited — usually in horizontal layers or strata. Stratification is a primary characteristic of sedimentary rocks, as the usually wet sediments cannot support deposition at anything other than a horizontal orientation. This results in stacks of horizontal layers, with the oldest on the bottom and the strata getting younger as one moves toward the top of the stack (barring a deformation). That is the Law of Superposition. After deposition, the sediments are lithified (turned to stone) by a variety of processes. See Figures 6.1-6.2. Sedimentary rocks are divided into two general categories: clastic (or detrital, from the Greek detritus, for broken fragments) and non-clastic (or chemical). Clastic sedimentary rocks originate at the expense of other rocks that have been broken, transported, and finally deposited and lithified (hardened). The non-clastic group of sedimentary rocks were formed from chemical processes in which dissolved minerals are precipitated to form a mass of crystals that form a coherent rock body, and by the accumulation of organic debris.
CLASTIC SEDIMENTARY ROCKS Erosional Processes
FIGURE 6.1 Fractured granitic bedrock in White Tanks Regional Park, Arizona Chemical and physical weathering breaks masses of granite into smaller fragments, which
Outcrops of rocks are always under attack. The attacking agents are most commonly rain, wind, ice, or even biological processes. These agents of destruction are aided by the weakening of the rock by chemical processes. Water may work its way into cracks in the rock, or along pore space between the grains in the rock. This water, aided by weak acids, acts to break down the minerals making up the rock in a chemical way. Once weakened, the rock is more susceptible to being eroded down to smaller and smaller pieces. Even the chemical processes alone can break many minerals in a rock down to their ionic form, which can be transported away by water. The net result is that big rocks are made smaller. These smaller fragments, detached from bedrock, are what make up sediment. Sediment consists of an unconsolidated mass of fragments that are loose — that is to say, not rock.
FIGURE 6.2 The Salt River Canyon, Arizona As the basalts of the valley walls is broken into smaller fragments, rock falls and landslides move the debris to the river below, where it is transported and modified.
Modification of Sediments During Transport Sediments may be moved from their source area by gravity, wind, water, or glacial ice. As sediments are transported, from the source area to their eventual depositional environment, they change. One of the most obvious changes is that they become smaller. Over time, what might have started out as a rock the size of a car, can be eroded down to a pea-sized fragment. As an example, consider a car-sized fragment made of granite. Originally part of the mountain face (bedrock), the rock is separated along a fissure created during weathering. This fissure has allowed water to percolate into the rock mass. Remember that water often contains dissolved carbon FIGURE 6.3 A talus slope at the base of a mountain cliff dioxide, resulting in a weak carbonic acid. Although weak, given enough time (hundreds or thousands of years) this weak acid breaks down some of the minerals in the granite. A common granite may contain an abundance of feldspars, micas, and quartz. The micas and feldspars are susceptible to chemical weathering and begin to decompose. This weakens the granite along the crack. Further weathering of the micas and feldspars changes them into a clay, which can easily be washed away on an exposed surface of the mountain. Years later, that crack has grown bigger. Eventually, that car-sized fragment falls away from the mountain face. It may be quite dramatic and fall hundreds of feet at one time. In that case, the impact would be catastrophic for the boulder. Many small pieces are be created quickly. More likely, the boulder would fall away from the cliff, perhaps a few dozen feet. That shorter fall would still create fragments, and it may also create new cracks, which are more avenues for acids to begin new chemical breakdowns. Over time, this boulder, or pieces of it, will move toward the base of the cliff, where it becomes part of a pile of sediments — sediments that were created by previous similar events. Big rocks are made smaller and smaller. Given enough time, even tall mountains can be eroded down to piles of FIGURE 6.4 Streams transport sediment sediment. This process of mountain erosion can be enhanced by the A mountain stream moves rocks from the base of Mount Rainier, Washington. removal of the pile of sediments accumulating at its base. This pile of sediments is called talus (Figure 6.3). Streams are the main movers of sediments at the Earth’s surface. In certain areas, glaciers can also move a great deal of this loose material. (Wind is generally a minor mover of sediment.) However the sediment is moved, more changes take place during transport (although very little change occurs to sediments locked in glacial ice.) Most commonly, it is by stream transport and erosion (Figure 6.4). In this continuing scenario, a stream at the base of the talus slope transports the rock fragments. As the stream picks them up and moves them, the rocks bump into each other. The collision causes chips to break off. This results in rocks that are smaller and rounder. Roundness is the smoothness of the surface of the rock. It is a measure of the angularity — not sphericity — of a rock fragment. An irregularly shaped rock with sharp points and edges is called an angular rock. As the angular rock is transported, these sharp points and edges make contact with other rocks, and
are broken off. The result is smaller and rounder rocks. Thus, rounding is a relative measure of the distance sedimentary fragments have been transported (Figure 6.5). Angular fragments are found close to the source area, while rounder, smaller fragments have been carried a greater distance. The greater the distance of transport, the smaller and rounder a rock becomes.
FIGURE 6.5 Grain rounding in a rock fragment From left to right: The rock on the left is large and angular, and has had a very short transportation. With longer transport, the rock gets smaller and rounder.
Very vulnerable to this mechanical breakdown, caused by impacts, are minerals that are soft and/ or have a propensity for cleavage. The dominant mineral in an ordinary granite is feldspar. With a hardness of 6, and two good directions of cleavage, feldspar grains suffer greatly in the weathering and transportation environment. And remember, those minerals also continue to be broken down by weak carbonic acids in the environment. They are quickly (geologically speaking) reduced in size. What started off as a car-sized piece of granite on a mountain cliff, has become multiple, smaller and rounder particles. Eventually, these pieces become smaller and smaller, as well. Due to the physical and chemical weakness of the feldspars and micas, the fragments of granite deteriorate into grains of sand. The durable quartz grains survive, although made smaller and somewhat rounded; the weaker feldspar grains still persist, due to their large original number in the parent granite; while most of the mica has been chemically altered to clay and has been washed further down stream. Granite (Figure 6.6) was the starting point in this example. Petrographically speaking, a typical granite may have the following mineralogy percentages: Quartz 15% Feldspar 70% Mica 10% Others 5% After erosion and chemical weathering, the resulting sediment (Figure 6.7) has a different mineral abundance. It may FIGURE 6.6 A close-up look at a typical granite have the following: Quartz 70% Feldspar 25% Mica 1% Others 4% What happened to all of that feldspar, and from where did the quartz come? This change is attributed to the physical and chemical stability of the quartz grains and the vulnerability of feld-
spars and micas. Over time, the sediment has become compositionally mature .
SEDIMENT MATURITY Sediment maturity is the concept, that as a sediment is transported and chemically attacked, the most durable grains survive. These are the grains, quartz, that are physically durable (high hardness and lack of cleavage) and chemically stable (unaffected by acids). At one end of the maturity scale is a very mature sediment consisting entirely of durable grain, such as quartz. In addition, this very mature sediment will show the FIGURE 6.7 Effects of a long transporwear and tear of many years of transport — the grains will be tation history A beach sand derived small and rounded. See Figure 5.8. from a granitic source, reflects the At the other extreme of the maturity scale, a very immarelative increase in quartz and decrease in feldspar and mica. ture sediment will consist of an abundance of minerals that are soft, and easily cleaved or susceptible to chemical processes. Such a sediment will also have a mix of large and small particles, and would be very angular. The mixture of particle sizes (called sorting) is also an indicator of the degree of transport.
A short distance of transport, from the source area, results in a wide mix of grain sizes, and an abundance of weak minerals. As the particles are moved greater and greater distances, the grain sizes become more uniform (better sorted), and the relative abundance of durable minerals increases. With long transportation distances, both compositional maturity and sorting increase.
A
B
FIGURE 6.8 Mature and immature sands Sand A was collected very close to the rock outcrop near the Dragon Mine in Arizona. This sample is immature, in that there are many rock fragments, very poor sorting (broad mix of grain sizes), and the grains are very angular. Over time, the softer grains weather away, grain sizes become more uniform, and particles become smaller and more rounded. The end result is a fine-grained, rounded quartz sand, such as in Sand B — a very mature sediment.
DEPOSITIONAL PROCESSES Eventually, sediment grains are no longer transported. The grains accumulate in a depositional environment. Examples of depositional environments are floodplains, beaches, sand and
gravel bars, the deep ocean floor, lakes, swamps, alluvial fans, and many others. Depositional environments are ephemeral; they may be stable for millions of years, like the deep ocean floor, or they be stable for only a few years, like a sand bar in an active river. Whatever the duration of time is that the sediment spends in the depositional environment, changes can still take place. One common change is compaction. As more and more sediments
accumulate, the increase in weight cause the pores between the grains to decrease in size. This compaction reduces the overall volume of the sediment. In many cases, the environment of deposition leaves clues in the sedimentary rock as to where it was formed. This is very useful for geologists looking at rocks that are millions of years old and in assessing ancient depositional environments. More about these clues to the environment of deposition, later in this chapter.
LITHIFICATION PROCESSES
A more significant change than compaction (described above) is the change that transforms loose sediment to sedimentary rock. That change is accomplished by three methods. These methods can work independently or in combination with each other. They are as follows:
Cementation — The process by which individual grains are bound together by mineral growth within the pore spaces of the sediment. This is a common lithification process in coarse-grained sediments, like gravel and sand, to produce conglomerate and sandstone. Crystallization — The process by which individual grains (minerals) grow in the environment of deposition. This is accomplished by aqueous solutions allowing minerals to grow resulting in an interlocking network of crystal faces. This is common in calcareous muds, and produces limestones. Compaction — The process by which fine-grained particles of clay are compressed to the point where chemical bonds develop between adjacent clay particles. This is a common process in thick deposits of silt and clay. Compaction produces claystones and shale.
NON-CLASTIC SEDIMENTARY ROCKS During the weathering processes, many minerals dissolve or partially dissolve, releasing ions in solution. These solutions are carried with stream waters or groundwater, ultimately to the oceans. However, as this aqueous solution moves along, changes in temperature, pressure, or chemistry can cause the ions to crystallize from the solution. These crystals can accumulate as chemical sedimentary rocks. The saline deposits of Death Valley serve as a useful example of these non-clastic sedimentary rocks (see Case Study — Death Valley, California). Also included in this class of sedimentary rocks are rocks formed by the accumulation of organic debris — abundant shells or plants.
CASE STUDY — DEATH VALLEY NATIONAL PARK, CALIFORNIA Death Valley has been accumulating sediments for only a few million years. During that time, the mountains have been eroding down and streams have been depositing sediments within the valley. Figure 6.9 shows the coarsegrained, poorly-sorted sediments in the valley, which were derived from the mountains in the background. These clastic sediments are only part of the story for sedimentation in Death Valley. Beyond this apron of sediments, at the base of the mountains, is a more distant valley floor FIGUFRE 6.9 Clastic sediments of Death Valley This is a view looking (see Figure 6.10). In this area, north at the sedimentary deposits on the floor of Death Valley. only very fine-grained silt and clay can be carried by the slowly moving water. However, deposition may be punctuated by occasional flash flood deposits, which carry larger loads of sediments. This is the area of evaporite deposits. This is a classification of non-clastic sedimentary rocks, and they originate as water evaporites. The water originates as rain falling in the mountains. Like the streams that created the clastic sediments, this water also dissolves some of the minerals in the rocks of the mountains. This combination of clastic particles and minerals, dissolved as ions in the water, flow down the valley and onto the valley floor. The abrupt change in the steepness from the mountains to the valley floor causes the clastic sediments to be deposited quickly. The dissolved minerals, however, are carried on, to create ephemeral lakes (playa lakes) in the valley. In FIGURE 6.10 Evaporites in Death Valley This is a view looking south the arid environment of Death Valover an area called Bad Water. The fan of sediment at the base of the ley, this pool of water quickly mountain consists of unconsolidated sediments, derived from the mounevaporates. The process of evapotain. Beyond the base of the fan is a deposit of white salt. ration removes only the water. All of the dissolved minerals (as ions) in the water are left behind. As more and more water evaporates, the concentration of these ions increases to the point where they begin to precipitate out of the solution. Generally speaking, there is a sequence in which various minerals begin to appear in the evaporating pool of water. The least soluble is calcium carbonate (CaCO3), or calcite.
FIGURE 6.11 Death Valley sediments As the distant mountains erode down, they create an abundance of fragments (clastic grains) and dissolved mineral ions. This material is carried down the slopes to be deposited on the valley floor. In this image, looking across Devil’s Golf Course, the mountain source of both the clastic and non--clastic sediments can easily be seen.
FIGURE 6.12 Devil’s Golf Course near Bad Water Here is the lowest elevation in the Park, at -282 feet (below sea level). The rocks here consist of salts and fine-grained clastic sediments (sand, silt, and clay). The scant rainfall has eroded the deposit into sharp, irregular pinnacles.
EVAPORITES As calcite begins to precipitate and form small crystals, a deposit begins to form. This deposit consists of a multitude of tiny interlocking crystals that make up a rock called limestone — the first of the non-clastic sedimentary rocks, in a the sequence of evaporite deposits. It is important to note that these small crystals of calcite, being deposited in the pool, were not transported as calcite crystals, from the mountains. (If that were the case, they would have been abraded to a powder, by the time they reached the evaporating pool.) Instead, the crystals of calcite were growing, in place, in the pool. This is the difference between the clastic sediments (those that were transported) and the nonclastic sediments (those that grew in the depositional environment). As evaporation continues, the concentration of ions increases. This leads to the precipitation of CaSO4 . 2H2O, the mineral gypsum. Further evaporation allows the deposition NaCl, the mineral halite (common salt). See Figures 5.11-5.12. OTHER EVAPORITES
In the sequence of evaporites, gypsum is typically the second mineral to crystallize from the evaporating pool. Millions of years ago in what now is the Verde Valley of Arizona, the Verde River was dammed by a lava flow. Lakes were created that slowly evaporated. Beds of limestone, gypsum, and salt were formed. Fig- FIGURE 6.13 Rock gypsum from Camp Verde, Arizona ure 6.13 is an example of rock gypsum (commonly referred to as just gypsum) from the Verde Valley near Camp Verde, Arizona. Figure 6.14 is rock salt, the mineral halite. It represents the last stage of evaporite mineral precipitation in this area. This cleavage fragment of salt has pieces of limestone and a plant fragment encased. Figure 6.15 is another example of salt. This specimen is from Searles Lake, near Trona, California. Here, huge evaporating pools, on a playa lake (Figure 6.16), produce conditions ideal for the formation of evaporite minerals. The salt has a slightly pinkish cast due to the color of bacFIGURE 6.14 Rock salt from Camp Verde, Arizona teria, that thrive in the brine and are encased in the salt as it crystallizes. The dark pink base of the specimen is another evaporite mineral called burkeite, Na6(CO3)(SO4)2.
FIGURE 6.16 Evaporites and playa lakes Not far from Death Valley is Trona, California. There, salts are being mined out of the ephemeral playa lakes. This is a view of the evaporating pools of brine.
FIGURE 6.15 Halite crystals from Searles Lake, California
OTHER NON-CLASTIC SEDIMENTARY ROCKS The evaporite sequence clearly demonstrates one way in which non-clastic sedimentary rocks are created. However, there are others. Evaporite mineral deposits in caves and at hot springs, geysers, and hot pools are also classified as non-clastic sedimentary rocks. The most common type of non-clastic sedimentary rock is limestone. Although limestone is created by the evaporation process just described, the vast majority of limestone is created by biological activity in the oceans. Many marine organisms are capable of extracting calcium carbonate from sea water, to secrete a skeletal structure or shell. When these plants and animals die, their hard parts accumulate on the ocean floor. These calcium carbonate deposits lithify (generally by crystallization) to produce the chemical sedimentary rock, limestone.
LIMESTONE Figure 6.17 is an example of a limestone. The abundance of fossils obvious in the rock provides for the specific type of limestone. This is a fossiliferous limestone. These fossils are Pennsylvanian in age, and consist of warm, shallow water invertebrates like brachiopods, crinoids, snails, and clams. This assemblage of fossils provides paleontologists with valuable information that allows them to recreate the paleoenvironment (their environment of deposition) of over 200 million years ago. Figure 6.18 is another limestone. It is a crystalline limestone. This is part of the Mississippianaged Redwall Limestone. Here the remains of the organisms, which accumulated on the ocean floor, have been recrystallized during the lithification process. The reflective surfaces (especially along the right side of the specimen) are cleavage faces FIGURE 6.17 Fossiliferous limestone from near of calcite. Payson, Arizona
Figure 6.19 is a travertine. It is a special type of crystalline limestone, which is generally not associated with marine organisms. Instead, this limestone represents precipitation and crystallization from mineralized springs or as cave formations. This particular travertine is from an ancient hot spring near Mayer, Arizona. The color is due to iron as an impurity. The browns and reds provide FIGURE 6.18 5.18 Crystalline limestone Limestone from from Jerome, Arizona
an interesting color combination, and this travertine has been used as an ornamental rock. Figure 6.20 shows another travertine. This travertine is forming at New Minerva Terrace, in Yellowstone National Park, Wyoming. Here, hot acidic waters have dissolved limestone beneath the surface. When this calcium carbonate-laden water FIGURE 6.19 5.19 Travertine, a type of banded limereaches the surface, it cools and loses carstone This specimen is from Mayer, Arizona. bon dioxide (the source of its acidity). This results in the precipitation and deposition of tons of calcium carbonate in the form of travertine beds. Figure 6.21 is an example of dolostone (alternatively dolomite). Dolostone is a term often used to distinguish the rock from the mineral, dolomite. The difference between dolostone and limestone is the composition. Although some dolostone is created directly at the surface of the Earth, the vast majority is created by magnesium-rich solutions percolating through limestone. FIGURE 6.20. 5.20. New Minerva Terrace in Yellowstone National Park, Wyoming Within the limestone
(CaCO3), some of the Ca+ are replaced by Mg+ ions, to create dolomite (Ca,MgCO3)2 This change in composition varies throughout the limestone, creating rocks that are partially limestone and partially dolostone. The dolomitization process also increases the porosity of the rock. This is an important change, as it increases the capacity of the rock to hold fluids, such as hydrocarbons. This example is nearly all dolostone. The exposed pocket shows dolomite crystals. Chert is a general term used to deFIGURE 6.21 Dolomite crystals in dolostone scribe very fine-grained sedimentary quartz. from Herkimer County, New York The quartz crystals are so small that the collective term used is cryptocrystalline (crypto for hidden). Chert occurs in a wide variety of sedimentary environments, but is most commonly associated with limestones. Chert, being composed of quartz, is generally colorless, but can easily be colored by impurities. Chert can also act as a cementing agent in sedimentary rocks. Petrified wood (Figure 6.22) is most commonly chert, and displays a wide range of colors. The coloring agents are commonly iron, producing browns, tans, reds, yellows; uranium, producing a bright yellow orange color; and manganese that causes a bluish-gray to black coloration.
FIGURE 6.22 Petrified wood from Wyoming
Flint (Figure 6.23) is another common variety of chert. It is a very fine-grained, compact rock that often forms in irregular nodules. Its compact nature creates a britFIGURE 6.23 Flint from Texas tle rock. that breaks with very good conchoidal fracture, producing a razor sharp cutting edge.
Coal is an organically-formed non-clastic sedimentary rock. Coal is actually a term that refers to a series of carbonaceous rocks. Figure 6.24 shows the evolution of plant material to anthracite coal.
FIGURE 6.24
Evolution of plant debris into coal
Peat is the first step in the formation of coal. Peat consists of partially decayed plant material. Within swampy environments, the plant material (grasses, shrubs, trees, mosses, and other plants) accumulates. This begins to decay, below the surface. Peat is very porous and can contain as much as 90% water. As the mass of vegetable matter increases, the peat becomes compressed. The individual leaves and twigs become blurred, as the pressure deforms them. The amount of water and other impurities become less and less, as more pressure is applied. Lignite, brown coal, is the result. As more pressure is exerted on lignite, by the addition of other sedimentary layers, it becomes more compressed. More water and other volatiles are forced out, and a low-grade coal is created. Bituminous Coal (Figure 6.25): Bituminous coal is also called soft coal and appears shiny in some areas and dull in others. Microscopically, the bituminous coal shows only the very hardest parts of the plants and the rest has been reduced to carbon. Bituminous coal is used in industry, and has been used for centuries as a source of energy. The sedimentary transformation of coal ends with bituminous coal. Greater pressures and increasing temperatures continue to act on the carbonaceous material, but the changes that follow place the evolving rock FIGURE 6.25 into the metamorphic rock category, as anthracite Carolina coal, also called hard coal.
Bituminous coal from North
Table 6.1 summarizes the non-clastic sedimentary rocks.
CLASTIC SEDIMENTARY ROCKS The clastic sedimentary rocks make up the majority of all of the sedimentary rocks. The grains in these rocks usually show evidence of transport. Most commonly, this is seen by the rounding and sorting of grain sizes. Clastic sedimentary rocks are classified on the basis of their grain size as follows (Table 6.2).
TABLE 6.2
Grain size and nomenclature for sediment name (unconsolidated) and sedimentary rock name (lithified sediment)
Table 6.2 identifies the classic boundaries between various types of sedimentary grains. This is strictly a size classification, and does not impart any compositional values to the sediments. For example, a sand is any particle size between 1/16 mm and 2 mm. However, a sand can have a variety of compositions. Most commonly, it is quartz; however other sand deposits can consist of gypsum, olivine, rock fragments, and even fossils. Figures 6.26-6.29 are a few examples of the different compositions of sand. Table 6.2 also shows the relative amount of energy needed to transport grains of various sizes. Larger grains require more energy to move than smaller grains.
FIGURE 6.26 Gypsum sand from White Sands National Park, New Mexico
FIGURE 6.27 Quartz sand from Clearwater Beach, Florida
FIGURE 6.28 Olivine sand from South Point, Hawaii
FIGURE 6.29 Black sand concentrate from Lynx Creek, Arizona The black sand consist primarily of magnetite with a few grains of native gold.
A Conglomerate is a coarse-grained (particle size greater that 2 mm) clastic sedimentary rock. The grains within a conglomerate are often durable grains like quartz or quartzites (a metamorphic rock). Conglomerates are generally lithified by cementation. The cementing agent can be a wide range of materials, including quartz, calcite, iron oxides, or mixtures of other materials. The grains within the conglomerate show signs of rounding due to transport. Conglomerates are often poorly sorted and represent a highFIGURE 6.30 Conglomerate This conglomerate, from central Utah, shows the rounding of quartz and quartzite grains, as well as the poor sorting, commonly associated with the rock.
FIGURE 6.31 Breccia with cemented grains of gray to black limestone
FIGURE 6.32 This is a closer view of the above image. It shows how the calcium carbonate cement coats the limestone and fills the pore spaces to create a durable rock.
energy environment. “High energy” refers to the transporting medium, generally water. High energy is required to transport these larger particles. Examples of high-energy environments are rivers or stream near mountains, and beaches exposed to high waves. Because of their large particle size and normally poor sorting, conglomerates are found relatively close to their source area. See Figure 6.30. Breccias are similar to conglomerates in grain size, but differ in the angularity of the grains. Breccias show very little (if any) rounding of the clastic grains of which they are made. This angularity and very poor sorting indicates that breccias are deposited very close to their source. Figure 6.31 is an excellent example of a breccia. This breccia shows the great angularity of the individual grains, as well as the very poor sorting of the grain sizes. Grains range from the largest (~3 inches across) to the smallest sand-sized particles. This breccia consists of angular fragments of limestone located very close to an exposed fault zone in southern Nevada. A closer look, Figure 6.32, shows the process of cementation in the breccia, but can also be a representation of the process of cementation in other clastic sedimentary rocks. It is infilling of pore space in the rock by mineral growth that creates a rock from sediment. In this example, the filling mineral is calcium carbonate (CaCO3). Notice how the rock fragments are coated with the cementing material and how the cement continues to grow and fill in the pore spaces. As the cement continues to fill in the pore spaces, the rock becomes more durable and less porous.
Many other types of cementing materials can be used to create rocks from sediments. Figure 6.33 is an example of another breccia. In this case, the cementing agent is a blue mineral, chrysocolla. Sandstones consists of grains of sand! Think of where large quantities of sand collect… beaches, sand dunes, rivers, and sand bars may come to mind. Sandstones are the lithified products of these various environments. Because of the great physical and chemical stability of quartz, the vast majority of sandstones are quartz sandstones. In this textbook, when a sandstone is mentioned, it is assumed that it is a sandstone dominated by quartz grains. (Any other type of sandstone would be unusual enough that it would require an adjective to describe it, as for example, a garnet sandstone.) FIGURE 6.33 Breccia with chrysocolla cement from the Ray Mine, Arizona
FIGURE 6.34
A very pure quartz sandstone
FIGURE 6.35
A close-up view of Figure 6.34
Sandstones come in a wide variety of colors, primarily due to impurities. Figure 6.34 is a fairly pure, light-colored sandstone because it consists almost entirely of quartz grains. Figure 6.35 is a close up view and shows the sugary texture and fine-grained nature of this sand. Notice also that the sand grains are very well sorted. This is an indication that the sorting mechanism was very selective in transporting only sand-sized grains. In this instance, the transporting mechanism was wind. Figures 6.346.35 are samples of the Coconino Sandstone of Arizona. At one time (~260 mya) the sandstone was part of an extensive sand dune deposit across northern Arizona, Utah, Colorado, and into Nevada. Impurities, notably iron, can dramatically change the appearance of even a pure quartz sandstone. Figure 6.36 is a sandstone that is locally (southern Utah) FIGURE 6.36 Sandstone “Kanab Wonderstone” called “Wonderstone”. It is a quartz sandstone that has
been cemented by iron oxides. The various hues of pinks, browns, and yellows were created by iron oxides that bind the quartz grains together. In the Grand Canyon, the CoIsis Temple Isis conino Sandstone forms a cliff. Temple Figure 6.37 shows the cliffs and a prominent erosional remnant of the Coconino Sandstone called Isis Temple. Below this is the Moenkopi Formation, a deltaic deposit that is dominated by fine silty sandstones and Moenkopi Formation sandy siltstones. The reddish color of this formation is due to iron oxFIGURE 6.37 A view of the cliff-forming Coconino Sand- ides. stone in the Grand Canyon Within the nomenclature for sediments and sedimentary rocks, there are transitions. As mentioned for the Moenkopi Formation, there are silty sandstones and sandy siltstones. These terms represent a mixture of sediment sizes. The modifier, as in the examples above, refer to the less abundant grain size. It is also possible to have sandy conglomerates ( a rock that is a conglomerate, but has a significant sand component) or pebbly sandstones, and so on. Siltstones and claystones (Figure 6.38) are important types of sedimentary rocks because of their abundance at the surface of the Earth. The difference between the two is primarily their grain size. Siltstones are dominated by particles that are 1/256 mm. A claystone is dominated by particles that are very small, < 1/256 mm. Additionally, siltstone is composed of silt, which is generally equi-dimensional quartz grains, while claystone is composed of platy clay minerals. This is not always an easy distinction to make in the field, and as a result, a collective FIGURE 6.38 Shale from central Utah term — shale — is used. In some cases, after careful laboratory analysis, the classification of a shale may be refined to a siltstone or claystone. But in many cases, the term shale is sufficient. Shale is the most abundant type of all of the sedimentary rocks. Referring back to Table 5.2 (Sediment Size Classification), it can be seen that the grains in shale (silt and clay sized particles) require only low energy (weak currents) to be transported. If weak currents move these very fine-grained particles, then Coconino Sandstone
FIGURE 6.39 Shale and fossil fern from the Pennsylvanian -age Naco Formation at Promontory Butte, Arizona
FIGURE 6.40 Delicate Pennsylvanianage Asterophyllites equisiformis fossil from Promontory Butte, Arizona
they can only be deposited in areas of very low energy. Such an environment of deposition would be the deep ocean floor, floodplains, lakes, lagoons, or swamps. Because of the low energy at the site of accumulation, combined with the very fine-grained nature of the particles (especially clay), shales are the hosts of many delicate types of fossils. Figure 6.39 is an example of a delicate fern fossil that has been well preserved in shale. The fossil has been very well preserved, including the overall shape of the fern frond, as well as veining on individual leafs. Such preservation would be difficult in the higher energy environment of a sandstone and impossible in the much higher energy environment of a conglomerate. A similar fossil is in Figure 6.40.
DEPOSITIONAL ENVIRONMENTS AND SEDIMENTARY STRUCTURES The shale in Figures 6.39-6.40 provides an introduction to the concept of sedimentary environments. In other words, what where the conditions under which the sediments were deposited? As stated earlier, the very fine-grained nature of the sediments indicates a low-energy environment. The presence of the fossil plants implies a nearby land source. With more clues, a better picture of the area becomes apparent. Aside from the actual sedimentary rock, there are abundant clues in structures found in the rock. Some of the best clues to the environment of deposition are the structures known as fossils. In the shale from Promontory Butte, fern fossils are common, along with horsetail rushes and occasional coprolites, from very small sharks. With this information, paleontologists can cobble together an image of what the area may have looked like, nearly 250 million years ago. An image of a coastal, brackish-water swamp, or estuary is implied. With more fossils and more sedimentary structures, the picture becomes clearer. These plant remains suggest a temperate to sub-tropical climate. In general, fossils provide the best evidence of the environment of deposition. These plants and animals may have lived and died within this sedimentary environment. Critical to our understanding of ancient life forms is the assumption that similar organisms alive today share the same type of life style of those found in the rock record. Called the Principle of Uniformitarianism, it suggests that the processes working on the Earth today (physical, chemical, biological) are similar to those that have operated in the past. So, we make observations of organisms’ habits and habitats, physical transformation of the Earth’s surface, and nuclear interactions, and then compare those observations to geological evidence, in order to interpret the geological past by analogy. Biologically speaking, similar organisms shared similar ecological niches. Figure 6.41 provides an example. Corals today live primarily in shallow, warm, clear, well-oxygenated marine waters. It can be inferred then that this coral lived in a similar type of environment. This is good evidence for an environment of deposition, although often it is not conclusive. In using fossils, paleontologists use an entire assemblage of fossils to accurately define a rock’s environment of deposition. The horn coral was found with numerous types FIGURE 6.41 Horn coral from the of brachiopods, crinoids, as well as other types of corals. Taken as a Devonian Martin Formation (limestone) of Central Arizona group, the conclusion becomes stronger. The Principle of Uniformitarianism applies to the interpretation of clues based on rock type, the nature of the grains making up the rock, fossils, and the types of sedimentary structures found in the rocks. Even the color of the sediment provides clues. Generally, reddish clastic sedimentary rocks were deposited in well-oxygenated environments that are typical of terrestrial sediments. Greenish rocks indicate a more reducing environment, like stagnant waters or very deep waters. Very dark gray to black sedimentary rocks have a high amount of carbon. This is the result of an abundance of plant and/or animal remains. In addition to fossils, here are some of the primary sedimentary structures used by geologists (Figures 6.42-6.65):
FIGURE 6.42 Stratification is the horizontal layering, typical of most sedimentary rocks. This layering is easily seen in the Goosenecks of the San Juan River, Southern Utah. The most universal characteristic of sedimentary rocks is horizontal stratification. FIGURE 6.43 Cross-bedding is common in sand dune deposits. Dune sand builds up and sloughs down the steep slip face, depositing beds that form at an angle. The shifting winds erode and then deposit layer upon layer of sand. This crossbedded sandstone is in Zion National Park, Utah. Cross-bedding can also be preserved in deltaic deposits and stream bars. Cross-beds can be used to determine ancient wind directions — and knowing the prevailing wind direction can lead one to determine the latitude that they occupied at the time the cross-beds were forming. FIGURE 6.44 Ripple Marks are an indication of moving water. They can be associated with streams, shallow waters, and sand dunes. At the very least, one can assume that the water was moving in a direction perpendicular to the lines of ripples. Asymmetric ripple marks have their crests slanted over in the direction that a current was flowing, while symmetric ripple marks are created by water that is gently sloshing to and fro, giving no indication of dominant current direction (but a good indicator of standing water). Ripple marks may be used to determine original UP, as the apex of the ripple crest points up.
FIGURE 6.45 Salt casts represent a very arid condition. Salt is created when a saline body of water evaporates. The salt crystals subsequently dissolve, leaving voids that fill in with mud. The presence of these casts in this fine-grained, red sandstone (Triassic, Moenkopi Formation of Northern Arizona) implies the environment of deposition was a river delta, in an arid, occasionally wet environment.
FIGURE 6.46 Mudcracks are produced by desiccation of sediments rich in clay. Their presence in the rock record indicate an arid climate, and an environment that is alternately wet and dry. The reddish color of this rock further implies that the sediment was terrestrial, that is exposed to air that oxidized the included iron. Mudcracks are V-shaped, wider at the top and narrowing toward the bottom. Therefore, they may be used to determine original UP of a rock unit.
ECONONOMIC USES OF SEDIMENTS AND SEDIMENTARY ROCKS Sediments and sedimentary rocks play a vital role in our modern society. Their diversity and abundance at Earth’s surface make them vital to many modern activities and products.
Building material (limestone, sandstone) Decorative stone (limestone, conglomerate, breccia, sandstone) Agricultural amendments (limestone/lime, gypsum, dolostone) Energy source (coal, oil shale, tar sand) Glass making (limestone/lime, sandstone/quartz sand, shale/clay) Marking implement (chalk) Cement and concrete (limestone, gravel, sandstone/sand) Hardening retarder in cement/concete (gypsum) Filler (in paint, plastic, rubber, carpet backing, roofing cement) (shale, limestone) Brick-making, ceramics (shale/clay) Road aggregate (conglomerate/aggregate, shale, limestone) Play medium (sand) Food additive (halite) Abrasive paper, grinding stones (sandstone/sand) Roofing granules (limestone) Animal feed filler, chicken grits (limestone) Mine safety dust (limestone) Sorbant — to absorb pollutants in coal mines (limestone) Flue gas desulfurization (limestone) Steel-making (limestone/lime) Sugar-making (limestone/lime) Pharmaceuticals — fillers, acid-reducer (limestone) Wallboard aka Gypboard or sheet rock, plaster of paris (gypsum) Sculpture (gypsum variety alabaster) De-icer (halite/rock salt) Water softener (halite/rock salt) Sodium ore (halite/rock salt) Chlorine source for chemical industry (halite/rock salt) Rubber manufacture (halite/rock salt) Coke in steel-making (coal) Soap, aspirin, dyes, solvents, plastics, rayon, and nylon (coal and coal by-products) Activated carbon filters for water and air (coal) Carbon fibers (coal) Fly ash for high-strength concrete (coal) Gemstone minerals associated with sedimentary rocks: malachite, azurite, opal, zircon, jasper, and alluvial gems (diamond, topaz, garnet, sapphire, ruby, emerald, gold, platinum)
CHAPTER TOPICS HYPERLINKS Page 3 Origin of Sediments, Clastic Sedimentary Rocks and Erosional Processes Page 4 Modification of Sediments During Transport Page 6 Sediment Maturity and Depositional Processes Page 7 Lithification Processes and Non-Clastic Sedimentary Rocks Page 8 Case Study - Death Valley National Park, California Page 10 Evaporites Page 11 Other Non-Clastic Sedimentary Rocks - Limestone Page 12 Travertine and Dolostone Page 13 Chert and Petrified Wood Page 14 Coal Page 15 Table 6.1 - The Non-Clastic Sedimentary Rocks and Clastic Sedimentary Rocks Page 16 Table 6.2 Sediment Size Classification and Equivalent Sedimentary Rocks Page 17 Conglomerate and Breccia Page 18 Sandstone Page 19 Siltstone, Claystone and Shale Page 21 Depositional Environments and Sedimentary Structures and The Principle of Uniformitarianism Page 24 Economic Uses of Sediments and Sedimentary Rocks The red star in the lower right hand corner of the page is a hyperlink back to this page.
ORIGIN OF SEDIMENTS Sedimentary rocks are created by the weathering of pre-existing rocks and erosion of the products of that weathering. Those rocks can be igneous, metamorphic, or even other sedimentary rocks. The breakdown of rock involves both chemical and physical processes, and the products of those processes are rock fragments (sediment) and soluble ions. Once rocks are broken down, it is the work of mass wasting to move the fragments to lower elevations. Typically, sediments are then transported, modified by transport, and finally deposited — usually in horizontal layers or strata. Stratification is a primary characteristic of sedimentary rocks, as the usually wet sediments cannot support deposition at anything other than a horizontal orientation. This results in stacks of horizontal layers, with the oldest on the bottom and the strata getting younger as one moves toward the top of the stack (barring a deformation). That is the Law of Superposition. After deposition, the sediments are lithified (turned to stone) by a variety of processes. See Figures 6.1-6.2. Sedimentary rocks are divided into two general categories: clastic (or detrital, from the Greek detritus, for broken fragments) and non-clastic (or chemical). Clastic sedimentary rocks originate at the expense of other rocks that have been broken, transported, and finally deposited and lithified (hardened). The non-clastic group of sedimentary rocks were formed from chemical processes in which dissolved minerals are precipitated to form a mass of crystals that form a coherent rock body, and by the accumulation of organic debris.
CLASTIC SEDIMENTARY ROCKS Erosional Processes
FIGURE 6.1 Fractured granitic bedrock in White Tanks Regional Park, Arizona Chemical and physical weathering breaks masses of granite into smaller fragments, which
Outcrops of rocks are always under attack. The attacking agents are most commonly rain, wind, ice, or even biological processes. These agents of destruction are aided by the weakening of the rock by chemical processes. Water may work its way into cracks in the rock, or along pore space between the grains in the rock. This water, aided by weak acids, acts to break down the minerals making up the rock in a chemical way. Once weakened, the rock is more susceptible to being eroded down to smaller and smaller pieces. Even the chemical processes alone can break many minerals in a rock down to their ionic form, which can be transported away by water. The net result is that big rocks are made smaller. These smaller fragments, detached from bedrock, are what make up sediment. Sediment consists of an unconsolidated mass of fragments that are loose — that is to say, not rock.
FIGURE 6.2 The Salt River Canyon, Arizona As the basalts of the valley walls is broken into smaller fragments, rock falls and landslides move the debris to the river below, where it is transported and modified.
Modification of Sediments During Transport Sediments may be moved from their source area by gravity, wind, water, or glacial ice. As sediments are transported, from the source area to their eventual depositional environment, they change. One of the most obvious changes is that they become smaller. Over time, what might have started out as a rock the size of a car, can be eroded down to a pea-sized fragment. As an example, consider a car-sized fragment made of granite. Originally part of the mountain face (bedrock), the rock is separated along a fissure created during weathering. This fissure has allowed water to percolate into the rock mass. Remember that water often contains dissolved carbon FIGURE 6.3 A talus slope at the base of a mountain cliff dioxide, resulting in a weak carbonic acid. Although weak, given enough time (hundreds or thousands of years) this weak acid breaks down some of the minerals in the granite. A common granite may contain an abundance of feldspars, micas, and quartz. The micas and feldspars are susceptible to chemical weathering and begin to decompose. This weakens the granite along the crack. Further weathering of the micas and feldspars changes them into a clay, which can easily be washed away on an exposed surface of the mountain. Years later, that crack has grown bigger. Eventually, that car-sized fragment falls away from the mountain face. It may be quite dramatic and fall hundreds of feet at one time. In that case, the impact would be catastrophic for the boulder. Many small pieces are be created quickly. More likely, the boulder would fall away from the cliff, perhaps a few dozen feet. That shorter fall would still create fragments, and it may also create new cracks, which are more avenues for acids to begin new chemical breakdowns. Over time, this boulder, or pieces of it, will move toward the base of the cliff, where it becomes part of a pile of sediments — sediments that were created by previous similar events. Big rocks are made smaller and smaller. Given enough time, even tall mountains can be eroded down to piles of FIGURE 6.4 Streams transport sediment sediment. This process of mountain erosion can be enhanced by the A mountain stream moves rocks from the base of Mount Rainier, Washington. removal of the pile of sediments accumulating at its base. This pile of sediments is called talus (Figure 6.3). Streams are the main movers of sediments at the Earth’s surface. In certain areas, glaciers can also move a great deal of this loose material. (Wind is generally a minor mover of sediment.) However the sediment is moved, more changes take place during transport (although very little change occurs to sediments locked in glacial ice.) Most commonly, it is by stream transport and erosion (Figure 6.4). In this continuing scenario, a stream at the base of the talus slope transports the rock fragments. As the stream picks them up and moves them, the rocks bump into each other. The collision causes chips to break off. This results in rocks that are smaller and rounder. Roundness is the smoothness of the surface of the rock. It is a measure of the angularity — not sphericity — of a rock fragment. An irregularly shaped rock with sharp points and edges is called an angular rock. As the angular rock is transported, these sharp points and edges make contact with other rocks, and
are broken off. The result is smaller and rounder rocks. Thus, rounding is a relative measure of the distance sedimentary fragments have been transported (Figure 6.5). Angular fragments are found close to the source area, while rounder, smaller fragments have been carried a greater distance. The greater the distance of transport, the smaller and rounder a rock becomes.
FIGURE 6.5 Grain rounding in a rock fragment From left to right: The rock on the left is large and angular, and has had a very short transportation. With longer transport, the rock gets smaller and rounder.
Very vulnerable to this mechanical breakdown, caused by impacts, are minerals that are soft and/ or have a propensity for cleavage. The dominant mineral in an ordinary granite is feldspar. With a hardness of 6, and two good directions of cleavage, feldspar grains suffer greatly in the weathering and transportation environment. And remember, those minerals also continue to be broken down by weak carbonic acids in the environment. They are quickly (geologically speaking) reduced in size. What started off as a car-sized piece of granite on a mountain cliff, has become multiple, smaller and rounder particles. Eventually, these pieces become smaller and smaller, as well. Due to the physical and chemical weakness of the feldspars and micas, the fragments of granite deteriorate into grains of sand. The durable quartz grains survive, although made smaller and somewhat rounded; the weaker feldspar grains still persist, due to their large original number in the parent granite; while most of the mica has been chemically altered to clay and has been washed further down stream. Granite (Figure 6.6) was the starting point in this example. Petrographically speaking, a typical granite may have the following mineralogy percentages: Quartz 15% Feldspar 70% Mica 10% Others 5% After erosion and chemical weathering, the resulting sediment (Figure 6.7) has a different mineral abundance. It may FIGURE 6.6 A close-up look at a typical granite have the following: Quartz 70% Feldspar 25% Mica 1% Others 4% What happened to all of that feldspar, and from where did the quartz come? This change is attributed to the physical and chemical stability of the quartz grains and the vulnerability of feld-
spars and micas. Over time, the sediment has become compositionally mature .
SEDIMENT MATURITY Sediment maturity is the concept, that as a sediment is transported and chemically attacked, the most durable grains survive. These are the grains, quartz, that are physically durable (high hardness and lack of cleavage) and chemically stable (unaffected by acids). At one end of the maturity scale is a very mature sediment consisting entirely of durable grain, such as quartz. In addition, this very mature sediment will show the FIGURE 6.7 Effects of a long transporwear and tear of many years of transport — the grains will be tation history A beach sand derived small and rounded. See Figure 5.8. from a granitic source, reflects the At the other extreme of the maturity scale, a very immarelative increase in quartz and decrease in feldspar and mica. ture sediment will consist of an abundance of minerals that are soft, and easily cleaved or susceptible to chemical processes. Such a sediment will also have a mix of large and small particles, and would be very angular. The mixture of particle sizes (called sorting) is also an indicator of the degree of transport.
A short distance of transport, from the source area, results in a wide mix of grain sizes, and an abundance of weak minerals. As the particles are moved greater and greater distances, the grain sizes become more uniform (better sorted), and the relative abundance of durable minerals increases. With long transportation distances, both compositional maturity and sorting increase.
A
B
FIGURE 6.8 Mature and immature sands Sand A was collected very close to the rock outcrop near the Dragon Mine in Arizona. This sample is immature, in that there are many rock fragments, very poor sorting (broad mix of grain sizes), and the grains are very angular. Over time, the softer grains weather away, grain sizes become more uniform, and particles become smaller and more rounded. The end result is a fine-grained, rounded quartz sand, such as in Sand B — a very mature sediment.
DEPOSITIONAL PROCESSES Eventually, sediment grains are no longer transported. The grains accumulate in a depositional environment. Examples of depositional environments are floodplains, beaches, sand and
gravel bars, the deep ocean floor, lakes, swamps, alluvial fans, and many others. Depositional environments are ephemeral; they may be stable for millions of years, like the deep ocean floor, or they be stable for only a few years, like a sand bar in an active river. Whatever the duration of time is that the sediment spends in the depositional environment, changes can still take place. One common change is compaction. As more and more sediments
accumulate, the increase in weight cause the pores between the grains to decrease in size. This compaction reduces the overall volume of the sediment. In many cases, the environment of deposition leaves clues in the sedimentary rock as to where it was formed. This is very useful for geologists looking at rocks that are millions of years old and in assessing ancient depositional environments. More about these clues to the environment of deposition, later in this chapter.
LITHIFICATION PROCESSES
A more significant change than compaction (described above) is the change that transforms loose sediment to sedimentary rock. That change is accomplished by three methods. These methods can work independently or in combination with each other. They are as follows:
Cementation — The process by which individual grains are bound together by mineral growth within the pore spaces of the sediment. This is a common lithification process in coarse-grained sediments, like gravel and sand, to produce conglomerate and sandstone. Crystallization — The process by which individual grains (minerals) grow in the environment of deposition. This is accomplished by aqueous solutions allowing minerals to grow resulting in an interlocking network of crystal faces. This is common in calcareous muds, and produces limestones. Compaction — The process by which fine-grained particles of clay are compressed to the point where chemical bonds develop between adjacent clay particles. This is a common process in thick deposits of silt and clay. Compaction produces claystones and shale.
NON-CLASTIC SEDIMENTARY ROCKS During the weathering processes, many minerals dissolve or partially dissolve, releasing ions in solution. These solutions are carried with stream waters or groundwater, ultimately to the oceans. However, as this aqueous solution moves along, changes in temperature, pressure, or chemistry can cause the ions to crystallize from the solution. These crystals can accumulate as chemical sedimentary rocks. The saline deposits of Death Valley serve as a useful example of these non-clastic sedimentary rocks (see Case Study — Death Valley, California). Also included in this class of sedimentary rocks are rocks formed by the accumulation of organic debris — abundant shells or plants.
CASE STUDY — DEATH VALLEY NATIONAL PARK, CALIFORNIA Death Valley has been accumulating sediments for only a few million years. During that time, the mountains have been eroding down and streams have been depositing sediments within the valley. Figure 6.9 shows the coarsegrained, poorly-sorted sediments in the valley, which were derived from the mountains in the background. These clastic sediments are only part of the story for sedimentation in Death Valley. Beyond this apron of sediments, at the base of the mountains, is a more distant valley floor FIGUFRE 6.9 Clastic sediments of Death Valley This is a view looking (see Figure 6.10). In this area, north at the sedimentary deposits on the floor of Death Valley. only very fine-grained silt and clay can be carried by the slowly moving water. However, deposition may be punctuated by occasional flash flood deposits, which carry larger loads of sediments. This is the area of evaporite deposits. This is a classification of non-clastic sedimentary rocks, and they originate as water evaporites. The water originates as rain falling in the mountains. Like the streams that created the clastic sediments, this water also dissolves some of the minerals in the rocks of the mountains. This combination of clastic particles and minerals, dissolved as ions in the water, flow down the valley and onto the valley floor. The abrupt change in the steepness from the mountains to the valley floor causes the clastic sediments to be deposited quickly. The dissolved minerals, however, are carried on, to create ephemeral lakes (playa lakes) in the valley. In FIGURE 6.10 Evaporites in Death Valley This is a view looking south the arid environment of Death Valover an area called Bad Water. The fan of sediment at the base of the ley, this pool of water quickly mountain consists of unconsolidated sediments, derived from the mounevaporates. The process of evapotain. Beyond the base of the fan is a deposit of white salt. ration removes only the water. All of the dissolved minerals (as ions) in the water are left behind. As more and more water evaporates, the concentration of these ions increases to the point where they begin to precipitate out of the solution. Generally speaking, there is a sequence in which various minerals begin to appear in the evaporating pool of water. The least soluble is calcium carbonate (CaCO3), or calcite.
FIGURE 6.11 Death Valley sediments As the distant mountains erode down, they create an abundance of fragments (clastic grains) and dissolved mineral ions. This material is carried down the slopes to be deposited on the valley floor. In this image, looking across Devil’s Golf Course, the mountain source of both the clastic and non--clastic sediments can easily be seen.
FIGURE 6.12 Devil’s Golf Course near Bad Water Here is the lowest elevation in the Park, at -282 feet (below sea level). The rocks here consist of salts and fine-grained clastic sediments (sand, silt, and clay). The scant rainfall has eroded the deposit into sharp, irregular pinnacles.
EVAPORITES As calcite begins to precipitate and form small crystals, a deposit begins to form. This deposit consists of a multitude of tiny interlocking crystals that make up a rock called limestone — the first of the non-clastic sedimentary rocks, in a the sequence of evaporite deposits. It is important to note that these small crystals of calcite, being deposited in the pool, were not transported as calcite crystals, from the mountains. (If that were the case, they would have been abraded to a powder, by the time they reached the evaporating pool.) Instead, the crystals of calcite were growing, in place, in the pool. This is the difference between the clastic sediments (those that were transported) and the nonclastic sediments (those that grew in the depositional environment). As evaporation continues, the concentration of ions increases. This leads to the precipitation of CaSO4 . 2H2O, the mineral gypsum. Further evaporation allows the deposition NaCl, the mineral halite (common salt). See Figures 5.11-5.12. OTHER EVAPORITES
In the sequence of evaporites, gypsum is typically the second mineral to crystallize from the evaporating pool. Millions of years ago in what now is the Verde Valley of Arizona, the Verde River was dammed by a lava flow. Lakes were created that slowly evaporated. Beds of limestone, gypsum, and salt were formed. Fig- FIGURE 6.13 Rock gypsum from Camp Verde, Arizona ure 6.13 is an example of rock gypsum (commonly referred to as just gypsum) from the Verde Valley near Camp Verde, Arizona. Figure 6.14 is rock salt, the mineral halite. It represents the last stage of evaporite mineral precipitation in this area. This cleavage fragment of salt has pieces of limestone and a plant fragment encased. Figure 6.15 is another example of salt. This specimen is from Searles Lake, near Trona, California. Here, huge evaporating pools, on a playa lake (Figure 6.16), produce conditions ideal for the formation of evaporite minerals. The salt has a slightly pinkish cast due to the color of bacFIGURE 6.14 Rock salt from Camp Verde, Arizona teria, that thrive in the brine and are encased in the salt as it crystallizes. The dark pink base of the specimen is another evaporite mineral called burkeite, Na6(CO3)(SO4)2.
FIGURE 6.16 Evaporites and playa lakes Not far from Death Valley is Trona, California. There, salts are being mined out of the ephemeral playa lakes. This is a view of the evaporating pools of brine.
FIGURE 6.15 Halite crystals from Searles Lake, California
OTHER NON-CLASTIC SEDIMENTARY ROCKS The evaporite sequence clearly demonstrates one way in which non-clastic sedimentary rocks are created. However, there are others. Evaporite mineral deposits in caves and at hot springs, geysers, and hot pools are also classified as non-clastic sedimentary rocks. The most common type of non-clastic sedimentary rock is limestone. Although limestone is created by the evaporation process just described, the vast majority of limestone is created by biological activity in the oceans. Many marine organisms are capable of extracting calcium carbonate from sea water, to secrete a skeletal structure or shell. When these plants and animals die, their hard parts accumulate on the ocean floor. These calcium carbonate deposits lithify (generally by crystallization) to produce the chemical sedimentary rock, limestone.
LIMESTONE Figure 6.17 is an example of a limestone. The abundance of fossils obvious in the rock provides for the specific type of limestone. This is a fossiliferous limestone. These fossils are Pennsylvanian in age, and consist of warm, shallow water invertebrates like brachiopods, crinoids, snails, and clams. This assemblage of fossils provides paleontologists with valuable information that allows them to recreate the paleoenvironment (their environment of deposition) of over 200 million years ago. Figure 6.18 is another limestone. It is a crystalline limestone. This is part of the Mississippianaged Redwall Limestone. Here the remains of the organisms, which accumulated on the ocean floor, have been recrystallized during the lithification process. The reflective surfaces (especially along the right side of the specimen) are cleavage faces FIGURE 6.17 Fossiliferous limestone from near of calcite. Payson, Arizona
Figure 6.19 is a travertine. It is a special type of crystalline limestone, which is generally not associated with marine organisms. Instead, this limestone represents precipitation and crystallization from mineralized springs or as cave formations. This particular travertine is from an ancient hot spring near Mayer, Arizona. The color is due to iron as an impurity. The browns and reds provide FIGURE 6.18 5.18 Crystalline limestone Limestone from from Jerome, Arizona
an interesting color combination, and this travertine has been used as an ornamental rock. Figure 6.20 shows another travertine. This travertine is forming at New Minerva Terrace, in Yellowstone National Park, Wyoming. Here, hot acidic waters have dissolved limestone beneath the surface. When this calcium carbonate-laden water FIGURE 6.19 5.19 Travertine, a type of banded limereaches the surface, it cools and loses carstone This specimen is from Mayer, Arizona. bon dioxide (the source of its acidity). This results in the precipitation and deposition of tons of calcium carbonate in the form of travertine beds. Figure 6.21 is an example of dolostone (alternatively dolomite). Dolostone is a term often used to distinguish the rock from the mineral, dolomite. The difference between dolostone and limestone is the composition. Although some dolostone is created directly at the surface of the Earth, the vast majority is created by magnesium-rich solutions percolating through limestone. FIGURE 6.20. 5.20. New Minerva Terrace in Yellowstone National Park, Wyoming Within the limestone
(CaCO3), some of the Ca+ are replaced by Mg+ ions, to create dolomite (Ca,MgCO3)2 This change in composition varies throughout the limestone, creating rocks that are partially limestone and partially dolostone. The dolomitization process also increases the porosity of the rock. This is an important change, as it increases the capacity of the rock to hold fluids, such as hydrocarbons. This example is nearly all dolostone. The exposed pocket shows dolomite crystals. Chert is a general term used to deFIGURE 6.21 Dolomite crystals in dolostone scribe very fine-grained sedimentary quartz. from Herkimer County, New York The quartz crystals are so small that the collective term used is cryptocrystalline (crypto for hidden). Chert occurs in a wide variety of sedimentary environments, but is most commonly associated with limestones. Chert, being composed of quartz, is generally colorless, but can easily be colored by impurities. Chert can also act as a cementing agent in sedimentary rocks. Petrified wood (Figure 6.22) is most commonly chert, and displays a wide range of colors. The coloring agents are commonly iron, producing browns, tans, reds, yellows; uranium, producing a bright yellow orange color; and manganese that causes a bluish-gray to black coloration.
FIGURE 6.22 Petrified wood from Wyoming
Flint (Figure 6.23) is another common variety of chert. It is a very fine-grained, compact rock that often forms in irregular nodules. Its compact nature creates a britFIGURE 6.23 Flint from Texas tle rock. that breaks with very good conchoidal fracture, producing a razor sharp cutting edge.
Coal is an organically-formed non-clastic sedimentary rock. Coal is actually a term that refers to a series of carbonaceous rocks. Figure 6.24 shows the evolution of plant material to anthracite coal.
FIGURE 6.24
Evolution of plant debris into coal
Peat is the first step in the formation of coal. Peat consists of partially decayed plant material. Within swampy environments, the plant material (grasses, shrubs, trees, mosses, and other plants) accumulates. This begins to decay, below the surface. Peat is very porous and can contain as much as 90% water. As the mass of vegetable matter increases, the peat becomes compressed. The individual leaves and twigs become blurred, as the pressure deforms them. The amount of water and other impurities become less and less, as more pressure is applied. Lignite, brown coal, is the result. As more pressure is exerted on lignite, by the addition of other sedimentary layers, it becomes more compressed. More water and other volatiles are forced out, and a low-grade coal is created. Bituminous Coal (Figure 6.25): Bituminous coal is also called soft coal and appears shiny in some areas and dull in others. Microscopically, the bituminous coal shows only the very hardest parts of the plants and the rest has been reduced to carbon. Bituminous coal is used in industry, and has been used for centuries as a source of energy. The sedimentary transformation of coal ends with bituminous coal. Greater pressures and increasing temperatures continue to act on the carbonaceous material, but the changes that follow place the evolving rock FIGURE 6.25 into the metamorphic rock category, as anthracite Carolina coal, also called hard coal.
Bituminous coal from North
Table 6.1 summarizes the non-clastic sedimentary rocks.
CLASTIC SEDIMENTARY ROCKS The clastic sedimentary rocks make up the majority of all of the sedimentary rocks. The grains in these rocks usually show evidence of transport. Most commonly, this is seen by the rounding and sorting of grain sizes. Clastic sedimentary rocks are classified on the basis of their grain size as follows (Table 6.2).
TABLE 6.2
Grain size and nomenclature for sediment name (unconsolidated) and sedimentary rock name (lithified sediment)
Table 6.2 identifies the classic boundaries between various types of sedimentary grains. This is strictly a size classification, and does not impart any compositional values to the sediments. For example, a sand is any particle size between 1/16 mm and 2 mm. However, a sand can have a variety of compositions. Most commonly, it is quartz; however other sand deposits can consist of gypsum, olivine, rock fragments, and even fossils. Figures 6.26-6.29 are a few examples of the different compositions of sand. Table 6.2 also shows the relative amount of energy needed to transport grains of various sizes. Larger grains require more energy to move than smaller grains.
FIGURE 6.26 Gypsum sand from White Sands National Park, New Mexico
FIGURE 6.27 Quartz sand from Clearwater Beach, Florida
FIGURE 6.28 Olivine sand from South Point, Hawaii
FIGURE 6.29 Black sand concentrate from Lynx Creek, Arizona The black sand consist primarily of magnetite with a few grains of native gold.
A Conglomerate is a coarse-grained (particle size greater that 2 mm) clastic sedimentary rock. The grains within a conglomerate are often durable grains like quartz or quartzites (a metamorphic rock). Conglomerates are generally lithified by cementation. The cementing agent can be a wide range of materials, including quartz, calcite, iron oxides, or mixtures of other materials. The grains within the conglomerate show signs of rounding due to transport. Conglomerates are often poorly sorted and represent a highFIGURE 6.30 Conglomerate This conglomerate, from central Utah, shows the rounding of quartz and quartzite grains, as well as the poor sorting, commonly associated with the rock.
FIGURE 6.31 Breccia with cemented grains of gray to black limestone
FIGURE 6.32 This is a closer view of the above image. It shows how the calcium carbonate cement coats the limestone and fills the pore spaces to create a durable rock.
energy environment. “High energy” refers to the transporting medium, generally water. High energy is required to transport these larger particles. Examples of high-energy environments are rivers or stream near mountains, and beaches exposed to high waves. Because of their large particle size and normally poor sorting, conglomerates are found relatively close to their source area. See Figure 6.30. Breccias are similar to conglomerates in grain size, but differ in the angularity of the grains. Breccias show very little (if any) rounding of the clastic grains of which they are made. This angularity and very poor sorting indicates that breccias are deposited very close to their source. Figure 6.31 is an excellent example of a breccia. This breccia shows the great angularity of the individual grains, as well as the very poor sorting of the grain sizes. Grains range from the largest (~3 inches across) to the smallest sand-sized particles. This breccia consists of angular fragments of limestone located very close to an exposed fault zone in southern Nevada. A closer look, Figure 6.32, shows the process of cementation in the breccia, but can also be a representation of the process of cementation in other clastic sedimentary rocks. It is infilling of pore space in the rock by mineral growth that creates a rock from sediment. In this example, the filling mineral is calcium carbonate (CaCO3). Notice how the rock fragments are coated with the cementing material and how the cement continues to grow and fill in the pore spaces. As the cement continues to fill in the pore spaces, the rock becomes more durable and less porous.
Many other types of cementing materials can be used to create rocks from sediments. Figure 6.33 is an example of another breccia. In this case, the cementing agent is a blue mineral, chrysocolla. Sandstones consists of grains of sand! Think of where large quantities of sand collect… beaches, sand dunes, rivers, and sand bars may come to mind. Sandstones are the lithified products of these various environments. Because of the great physical and chemical stability of quartz, the vast majority of sandstones are quartz sandstones. In this textbook, when a sandstone is mentioned, it is assumed that it is a sandstone dominated by quartz grains. (Any other type of sandstone would be unusual enough that it would require an adjective to describe it, as for example, a garnet sandstone.) FIGURE 6.33 Breccia with chrysocolla cement from the Ray Mine, Arizona
FIGURE 6.34
A very pure quartz sandstone
FIGURE 6.35
A close-up view of Figure 6.34
Sandstones come in a wide variety of colors, primarily due to impurities. Figure 6.34 is a fairly pure, light-colored sandstone because it consists almost entirely of quartz grains. Figure 6.35 is a close up view and shows the sugary texture and fine-grained nature of this sand. Notice also that the sand grains are very well sorted. This is an indication that the sorting mechanism was very selective in transporting only sand-sized grains. In this instance, the transporting mechanism was wind. Figures 6.346.35 are samples of the Coconino Sandstone of Arizona. At one time (~260 mya) the sandstone was part of an extensive sand dune deposit across northern Arizona, Utah, Colorado, and into Nevada. Impurities, notably iron, can dramatically change the appearance of even a pure quartz sandstone. Figure 6.36 is a sandstone that is locally (southern Utah) FIGURE 6.36 Sandstone “Kanab Wonderstone” called “Wonderstone”. It is a quartz sandstone that has
been cemented by iron oxides. The various hues of pinks, browns, and yellows were created by iron oxides that bind the quartz grains together. In the Grand Canyon, the CoIsis Temple Isis conino Sandstone forms a cliff. Temple Figure 6.37 shows the cliffs and a prominent erosional remnant of the Coconino Sandstone called Isis Temple. Below this is the Moenkopi Formation, a deltaic deposit that is dominated by fine silty sandstones and Moenkopi Formation sandy siltstones. The reddish color of this formation is due to iron oxFIGURE 6.37 A view of the cliff-forming Coconino Sand- ides. stone in the Grand Canyon Within the nomenclature for sediments and sedimentary rocks, there are transitions. As mentioned for the Moenkopi Formation, there are silty sandstones and sandy siltstones. These terms represent a mixture of sediment sizes. The modifier, as in the examples above, refer to the less abundant grain size. It is also possible to have sandy conglomerates ( a rock that is a conglomerate, but has a significant sand component) or pebbly sandstones, and so on. Siltstones and claystones (Figure 6.38) are important types of sedimentary rocks because of their abundance at the surface of the Earth. The difference between the two is primarily their grain size. Siltstones are dominated by particles that are 1/256 mm. A claystone is dominated by particles that are very small, < 1/256 mm. Additionally, siltstone is composed of silt, which is generally equi-dimensional quartz grains, while claystone is composed of platy clay minerals. This is not always an easy distinction to make in the field, and as a result, a collective FIGURE 6.38 Shale from central Utah term — shale — is used. In some cases, after careful laboratory analysis, the classification of a shale may be refined to a siltstone or claystone. But in many cases, the term shale is sufficient. Shale is the most abundant type of all of the sedimentary rocks. Referring back to Table 5.2 (Sediment Size Classification), it can be seen that the grains in shale (silt and clay sized particles) require only low energy (weak currents) to be transported. If weak currents move these very fine-grained particles, then Coconino Sandstone
FIGURE 6.39 Shale and fossil fern from the Pennsylvanian -age Naco Formation at Promontory Butte, Arizona
FIGURE 6.40 Delicate Pennsylvanianage Asterophyllites equisiformis fossil from Promontory Butte, Arizona
they can only be deposited in areas of very low energy. Such an environment of deposition would be the deep ocean floor, floodplains, lakes, lagoons, or swamps. Because of the low energy at the site of accumulation, combined with the very fine-grained nature of the particles (especially clay), shales are the hosts of many delicate types of fossils. Figure 6.39 is an example of a delicate fern fossil that has been well preserved in shale. The fossil has been very well preserved, including the overall shape of the fern frond, as well as veining on individual leafs. Such preservation would be difficult in the higher energy environment of a sandstone and impossible in the much higher energy environment of a conglomerate. A similar fossil is in Figure 6.40.
DEPOSITIONAL ENVIRONMENTS AND SEDIMENTARY STRUCTURES The shale in Figures 6.39-6.40 provides an introduction to the concept of sedimentary environments. In other words, what where the conditions under which the sediments were deposited? As stated earlier, the very fine-grained nature of the sediments indicates a low-energy environment. The presence of the fossil plants implies a nearby land source. With more clues, a better picture of the area becomes apparent. Aside from the actual sedimentary rock, there are abundant clues in structures found in the rock. Some of the best clues to the environment of deposition are the structures known as fossils. In the shale from Promontory Butte, fern fossils are common, along with horsetail rushes and occasional coprolites, from very small sharks. With this information, paleontologists can cobble together an image of what the area may have looked like, nearly 250 million years ago. An image of a coastal, brackish-water swamp, or estuary is implied. With more fossils and more sedimentary structures, the picture becomes clearer. These plant remains suggest a temperate to sub-tropical climate. In general, fossils provide the best evidence of the environment of deposition. These plants and animals may have lived and died within this sedimentary environment. Critical to our understanding of ancient life forms is the assumption that similar organisms alive today share the same type of life style of those found in the rock record. Called the Principle of Uniformitarianism, it suggests that the processes working on the Earth today (physical, chemical, biological) are similar to those that have operated in the past. So, we make observations of organisms’ habits and habitats, physical transformation of the Earth’s surface, and nuclear interactions, and then compare those observations to geological evidence, in order to interpret the geological past by analogy. Biologically speaking, similar organisms shared similar ecological niches. Figure 6.41 provides an example. Corals today live primarily in shallow, warm, clear, well-oxygenated marine waters. It can be inferred then that this coral lived in a similar type of environment. This is good evidence for an environment of deposition, although often it is not conclusive. In using fossils, paleontologists use an entire assemblage of fossils to accurately define a rock’s environment of deposition. The horn coral was found with numerous types FIGURE 6.41 Horn coral from the of brachiopods, crinoids, as well as other types of corals. Taken as a Devonian Martin Formation (limestone) of Central Arizona group, the conclusion becomes stronger. The Principle of Uniformitarianism applies to the interpretation of clues based on rock type, the nature of the grains making up the rock, fossils, and the types of sedimentary structures found in the rocks. Even the color of the sediment provides clues. Generally, reddish clastic sedimentary rocks were deposited in well-oxygenated environments that are typical of terrestrial sediments. Greenish rocks indicate a more reducing environment, like stagnant waters or very deep waters. Very dark gray to black sedimentary rocks have a high amount of carbon. This is the result of an abundance of plant and/or animal remains. In addition to fossils, here are some of the primary sedimentary structures used by geologists (Figures 6.42-6.65):
FIGURE 6.42 Stratification is the horizontal layering, typical of most sedimentary rocks. This layering is easily seen in the Goosenecks of the San Juan River, Southern Utah. The most universal characteristic of sedimentary rocks is horizontal stratification. FIGURE 6.43 Cross-bedding is common in sand dune deposits. Dune sand builds up and sloughs down the steep slip face, depositing beds that form at an angle. The shifting winds erode and then deposit layer upon layer of sand. This crossbedded sandstone is in Zion National Park, Utah. Cross-bedding can also be preserved in deltaic deposits and stream bars. Cross-beds can be used to determine ancient wind directions — and knowing the prevailing wind direction can lead one to determine the latitude that they occupied at the time the cross-beds were forming. FIGURE 6.44 Ripple Marks are an indication of moving water. They can be associated with streams, shallow waters, and sand dunes. At the very least, one can assume that the water was moving in a direction perpendicular to the lines of ripples. Asymmetric ripple marks have their crests slanted over in the direction that a current was flowing, while symmetric ripple marks are created by water that is gently sloshing to and fro, giving no indication of dominant current direction (but a good indicator of standing water). Ripple marks may be used to determine original UP, as the apex of the ripple crest points up.
FIGURE 6.45 Salt casts represent a very arid condition. Salt is created when a saline body of water evaporates. The salt crystals subsequently dissolve, leaving voids that fill in with mud. The presence of these casts in this fine-grained, red sandstone (Triassic, Moenkopi Formation of Northern Arizona) implies the environment of deposition was a river delta, in an arid, occasionally wet environment.
FIGURE 6.46 Mudcracks are produced by desiccation of sediments rich in clay. Their presence in the rock record indicate an arid climate, and an environment that is alternately wet and dry. The reddish color of this rock further implies that the sediment was terrestrial, that is exposed to air that oxidized the included iron. Mudcracks are V-shaped, wider at the top and narrowing toward the bottom. Therefore, they may be used to determine original UP of a rock unit.
ECONONOMIC USES OF SEDIMENTS AND SEDIMENTARY ROCKS Sediments and sedimentary rocks play a vital role in our modern society. Their diversity and abundance at Earth’s surface make them vital to many modern activities and products.
Building material (limestone, sandstone) Decorative stone (limestone, conglomerate, breccia, sandstone) Agricultural amendments (limestone/lime, gypsum, dolostone) Energy source (coal, oil shale, tar sand) Glass making (limestone/lime, sandstone/quartz sand, shale/clay) Marking implement (chalk) Cement and concrete (limestone, gravel, sandstone/sand) Hardening retarder in cement/concete (gypsum) Filler (in paint, plastic, rubber, carpet backing, roofing cement) (shale, limestone) Brick-making, ceramics (shale/clay) Road aggregate (conglomerate/aggregate, shale, limestone) Play medium (sand) Food additive (halite) Abrasive paper, grinding stones (sandstone/sand) Roofing granules (limestone) Animal feed filler, chicken grits (limestone) Mine safety dust (limestone) Sorbant — to absorb pollutants in coal mines (limestone) Flue gas desulfurization (limestone) Steel-making (limestone/lime) Sugar-making (limestone/lime) Pharmaceuticals — fillers, acid-reducer (limestone) Wallboard aka Gypboard or sheet rock, plaster of paris (gypsum) Sculpture (gypsum variety alabaster) De-icer (halite/rock salt) Water softener (halite/rock salt) Sodium ore (halite/rock salt) Chlorine source for chemical industry (halite/rock salt) Rubber manufacture (halite/rock salt) Coke in steel-making (coal) Soap, aspirin, dyes, solvents, plastics, rayon, and nylon (coal and coal by-products) Activated carbon filters for water and air (coal) Carbon fibers (coal) Fly ash for high-strength concrete (coal) Gemstone minerals associated with sedimentary rocks: malachite, azurite, opal, zircon, jasper, and alluvial gems (diamond, topaz, garnet, sapphire, ruby, emerald, gold, platinum)
