{"id":259,"date":"2019-06-11T14:49:30","date_gmt":"2019-06-11T14:49:30","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/6-1-clastic-sedimentary-rocks\/"},"modified":"2022-07-15T19:05:58","modified_gmt":"2022-07-15T19:05:58","slug":"6-1-clastic-sedimentary-rocks","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/6-1-clastic-sedimentary-rocks\/","title":{"raw":"6.1 Clastic Sedimentary Rocks","rendered":"6.1 Clastic Sedimentary Rocks"},"content":{"raw":"A [pb_glossary id=\"1442\"]clast[\/pb_glossary]<\/strong> is a fragment of rock or mineral, ranging in size from less than a micron[footnote]A micron is a millionth of a metre. There are 1,000 microns in a millimetre.[\/footnote]\u00a0(too small to see) to as big as an apartment block. Various types of clasts are shown in Figure 5.3.1 and in Exercise 5.3. The smaller ones tend to be composed of a single mineral crystal, and the larger ones are typically composed of pieces of rock. As we\u2019ve seen in Chapter 5, most sand-sized clasts are made of quartz because quartz is more resistant to weathering than any other common mineral. Many of the clasts that are smaller than sand size (less than 1<\/sup>\/16<\/sub>th millimetre) are made of clay minerals. Most clasts larger than sand size (greater than 2 millimetres) are actual fragments of rock, and commonly these might be fine-grained rock like basalt or andesite, or if they are bigger, coarse-grained rock like granite or gneiss. Sedimentary rocks that are made up of \"clasts\" are called clastic sedimentary rocks.\u00a0 A comparable term is \"detrital sedimentary rocks\".\r\n

Grain-Size Classification<\/h1>\r\nGeologists that study sediments and sedimentary rocks use the Udden-Wentworth grain-size scale for describing the sizes of the grains in these materials (Table 6.1).\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 6.1 The Udden-Wentworth grain-size scale for classifying sediments and the grains that make up sedimentary rocks<\/caption>\r\n
[Skip Table]<\/a><\/td>\r\n<\/tr>\r\n
Type<\/th>\r\nDescription<\/th>\r\nSize range (millimetres)<\/th>\r\nSize range (microns)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Boulder<\/td>\r\nlarge<\/td>\r\n1024 and up<\/td>\r\n<\/td>\r\n<\/tr>\r\n
medium<\/td>\r\n512 to 1024<\/td>\r\n<\/td>\r\n<\/tr>\r\n
small<\/td>\r\n256 to 512<\/td>\r\n<\/td>\r\n<\/tr>\r\n
Cobble<\/td>\r\nlarge<\/td>\r\n128 to 256<\/td>\r\n<\/td>\r\n<\/tr>\r\n
small<\/td>\r\n64 to 128<\/td>\r\n<\/td>\r\n<\/tr>\r\n
Pebble (Granule)<\/td>\r\nvery coarse<\/td>\r\n32 to 64<\/td>\r\n<\/td>\r\n<\/tr>\r\n
coarse<\/td>\r\n16 to 32<\/td>\r\n<\/td>\r\n<\/tr>\r\n
medium<\/td>\r\n8 to 16<\/td>\r\n<\/td>\r\n<\/tr>\r\n
fine<\/td>\r\n4 to 8<\/td>\r\n<\/td>\r\n<\/tr>\r\n
very fine<\/td>\r\n2 to 4<\/td>\r\n<\/td>\r\n<\/tr>\r\n
Sand<\/td>\r\nvery coarse<\/td>\r\n1 to 2<\/td>\r\n1000 to 2000<\/td>\r\n<\/tr>\r\n
coarse<\/td>\r\n0.5 to 1<\/td>\r\n500 to 1000<\/td>\r\n<\/tr>\r\n
medium<\/td>\r\n0.25 to 0.5 (1<\/sup>\/4<\/sub> to 1<\/sup>\/2<\/sub> mm)<\/td>\r\n250 to 500<\/td>\r\n<\/tr>\r\n
fine<\/td>\r\n0.125 to 0.25 (1<\/sup>\/8<\/sub>th to 1<\/sup>\/4<\/sub> mm)<\/td>\r\n125 to 250<\/td>\r\n<\/tr>\r\n
very fine<\/td>\r\n0.063 to 0.125 (or 1<\/sup>\/16<\/sub>th to 1<\/sup>\/8<\/sub>th mm)<\/td>\r\n63 to 125<\/td>\r\n<\/tr>\r\n
Silt<\/td>\r\nvery course<\/td>\r\n<\/td>\r\n32 to 63<\/td>\r\n<\/tr>\r\n
course<\/td>\r\n<\/td>\r\n16 to 32<\/td>\r\n<\/tr>\r\n
medium<\/td>\r\n<\/td>\r\n8 to 16<\/td>\r\n<\/tr>\r\n
fine<\/td>\r\n<\/td>\r\n4 to 8<\/td>\r\n<\/tr>\r\n
very fine<\/td>\r\n<\/td>\r\n2 to 4<\/td>\r\n<\/tr>\r\n
Clay<\/td>\r\nclay<\/td>\r\n<\/td>\r\n0 to 2<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

There are six main grain-size categories; five are broken down into subcategories, with [pb_glossary id=\"1444\"]clay[\/pb_glossary]<\/strong> being the exception. The diameter limits for each successive subcategory are twice as large as the one beneath it. In general, a [pb_glossary id=\"1445\"]boulder[\/pb_glossary]<\/strong> is bigger than a toaster and difficult to lift. There is no upper limit to the size of boulder.[footnote]The largest known free-standing rock (i.e., not part of bedrock) is Giant Rock in the Mojave Desert, California. It\u2019s about as big as an apartment building\u2014seven stories high![\/footnote]\u00a0A small [pb_glossary id=\"1446\"]cobble[\/pb_glossary]<\/strong> will fit in one hand, a large one in two hands. A [pb_glossary id=\"1447\"]pebble[\/pb_glossary]<\/strong> is something that you could throw quite easily. The smaller ones\u2014known as [pb_glossary id=\"1448\"]granules[\/pb_glossary]<\/strong>\u2014are gravel size, but still you could throw one. You can\u2019t really throw a single grain of [pb_glossary id=\"1449\"]sand[\/pb_glossary]<\/strong>. Sand ranges from 2 millimetres down to 0.063 millimetres, and its key characteristic is that it feels \u201csandy\u201d or gritty between your fingers\u2014even the finest sand grains feel that way. [pb_glossary id=\"1450\"]Silt[\/pb_glossary]<\/strong> is essentially too small for individual grains to be visible, and while sand feels sandy to your fingers, silt feels smooth to your fingers but gritty in your mouth. Clay is so fine that it feels smooth even in your mouth.<\/p>\r\n\r\n

\r\n

Exercise 6.1 Describe the sediment on a beach<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nProviding that your landscape isn\u2019t covered in deep snow at present, visit a beach somewhere nearby\u2014an ocean shore, a lake shore, or a river bank. Look carefully at the size and shape of the beach sediments. Are they sand, pebbles, or cobbles? If they are not too fine, you should be able to tell if they are well rounded or more angular.\r\n\r\nThe beach in Figure 6.1.1 is at Sechelt, B.C. Although there is a range of clast sizes, it\u2019s mostly made up of well-rounded cobbles interspersed with pebbles. This beach is subject to strong wave activity, especially when winds blow across the Strait of Georgia from the south. That explains why the clasts are relatively large and are well rounded.\r\n\r\n \r\n\r\n[caption id=\"attachment_248\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 6.1.1 Pebbles on an ocean beach at Sechelt, B.C.[\/caption]\r\n\r\nSee Appendix 3 for Exercise 6.1 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\nIf you drop a granule into a glass of water, it will sink quickly to the bottom (less than half a second). If you drop a grain of sand into the same glass, it will sink more slowly (a second or two depending on the size). A grain of silt will take several seconds to get to the bottom, and a particle of fine clay may never get there. The rate of settling is determined by the balance between gravity and friction, as shown in Figure 6.1.2.\u00a0\u00a0Large particles settle quickly because the gravitational force (which is proportional to the mass, and therefore to the volume of the particle) is much greater than the frictional resistance (which is proportional to the surface area of the particle). For smaller particles the difference between gravitational push and frictional resistance is less, so they settle slowly.\r\n\r\n[caption id=\"attachment_249\" align=\"alignright\" width=\"400\"]\"\"<\/a> Figure 6.1.2 The two forces operating on a grain of sand in water. Gravity is pushing it down, and the friction between the grain and the water is resisting that downward force.[\/caption]\r\n\r\n
\r\n\r\nSmall particles that settle slowly spend longer suspended in the water, and therefore tend to get moved farther than large particles if the water is moving.\r\n

Transportation<\/h1>\r\nOne of the key principles of sedimentary geology is that the ability of a moving medium (air or water) to move sedimentary particles\u2014and keep them moving\u2014is dependent on the velocity of flow. The faster the medium flows, the larger the particles it can move. This is illustrated in Figure 6.1.3. Parts of the river are moving faster than other parts, especially where the slope is greatest and the channel is narrow. Not only does the velocity of a river change from place to place, but it changes from season to season.\u00a0\u00a0During peak [pb_glossary id=\"1451\"]discharge[\/pb_glossary]<\/strong>[footnote]Discharge of a stream is the volume of flow passing a point per unit time. It\u2019s normally measured in cubic metres per second (m3<\/sup>\/s).[\/footnote]\u00a0at the location of Figure 6.1.3, the water is high enough to flow over the embankment on the right, and it flows fast enough to move the boulders that cannot be moved during low flows.\r\n
\r\n\r\n[caption id=\"attachment_250\" align=\"aligncenter\" width=\"975\"]\"\"<\/a> Figure 6.1.3 Variations in flow velocity on the Englishman River near Parksville, B.C. When the photo was taken the river was not flowing fast enough anywhere to move the boulders and cobbles visible here.\u00a0 During flood events the water flows right over the snow-covered bank on the right, and is fast enough to move boulders.[\/caption]\r\n\r\n<\/div>\r\n
\r\n\r\nClasts within streams are moved in several different ways, as illustrated in Figure 6.1.4. Large [pb_glossary id=\"1452\"]bed load[\/pb_glossary]<\/strong> clasts are pushed (by traction) or bounced along the bottom (by saltation), while smaller clasts are suspended in the water and kept there by the turbulence of the flow. As the flow velocity changes, different-sized clasts may be either incorporated into the flow or deposited on the bottom. At various places along a river, there are always some clasts being deposited, some staying where they are, and some being eroded and transported. This changes over time as the discharge of the river changes in response to changing weather conditions.\r\n\r\nOther sediment transportation media, such as waves, ocean currents, and wind, operate under similar principles, with flow velocity as the key underlying factor that controls transportation and deposition.\r\n\r\n[caption id=\"attachment_251\" align=\"aligncenter\" width=\"750\"]\"\"<\/a> Figure 6.1.4 Transportation of sediment clasts by stream flow. The larger clasts, resting on the bottom (bedload), are moved by traction (sliding) or by saltation (bouncing). Smaller clasts are kept in suspension by turbulence in the flow. Ions (depicted as + and - in the image, but invisible in real life) are dissolved in the water.[\/caption]\r\n\r\nClastic sediments are deposited in a wide range of environments, including glaciers, slope failures, rivers\u2014both fast and slow\u2014lakes, deltas, and ocean environments\u2014both shallow and deep. If the sedimentary deposits last long enough to get covered with other sediments they may eventually form into rocks ranging from fine mudstone to coarse breccia and conglomerate.\r\n\r\n[pb_glossary id=\"1453\"]Lithification[\/pb_glossary]<\/strong> is the term used to describe a number of different processes that take place within a deposit of sediment to turn it into solid rock (Figure 6.1.5). One of these processes is burial by other sediments, which leads to compaction of the material and removal of some of the intervening water and air. After this stage, the individual clasts are touching one another. [pb_glossary id=\"1454\"]Cementation[\/pb_glossary]<\/strong> is the process of crystallization of minerals within the pores between the small clasts, and especially at the points of contact between clasts. Depending on the pressure, temperature, and chemical conditions, these crystals might include a range of minerals, the common ones being calcite, hematite, quartz and clay minerals.\r\n\r\n[caption id=\"attachment_1331\" align=\"aligncenter\" width=\"1024\"]\"\"<\/a> Figure 6.1.5\u00a0 Lithification turns sediments into solid rock. Lithification involves the compaction of sediments and then the cementation of grains by minerals that precipitate from groundwater in the spaces between these grains.\u00a0Source: Karla Panchuk (2016) CC BY 4.0<\/em>[\/caption]\r\n\r\nThe characteristics and distinguishing features of clastic sedimentary rocks are summarized in Table 6.2. [pb_glossary id=\"1455\"]Mudrock[\/pb_glossary]<\/strong> is composed of at least 75% silt- and clay-sized fragments. If it is dominated by clay, it is called [pb_glossary id=\"1456\"]claystone[\/pb_glossary]<\/strong>. If it shows evidence of bedding or fine laminations, it is [pb_glossary id=\"1457\"]shale[\/pb_glossary]<\/strong>; otherwise, it is mudstone. Mudrocks form in very low energy environments, such as lakes, river backwaters, and the deep ocean.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 6.2 The main types of clastic sedimentary rocks and their characteristics.<\/caption>\r\n
[Skip Table]<\/a><\/td>\r\n<\/tr>\r\n
Group<\/th>\r\nExamples<\/th>\r\nCharacteristics<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Mudrock<\/td>\r\nmudstone<\/td>\r\nGreater than 75% silt and clay, not bedded<\/td>\r\n<\/tr>\r\n
shale<\/td>\r\nGreater than 75% silt and clay, thinly bedded<\/td>\r\n<\/tr>\r\n
Coal<\/td>\r\n<\/td>\r\nDominated by fragments of partially decayed plant matter often enclosed between beds of sandstone or mudrock.<\/td>\r\n<\/tr>\r\n
Sandstone<\/td>\r\nquartz sandstone<\/td>\r\nDominated by sand, greater than 90% quartz<\/td>\r\n<\/tr>\r\n
arkose<\/td>\r\nDominated by sand, greater than 10% feldspar<\/td>\r\n<\/tr>\r\n
lithic wacke<\/td>\r\ndominated by sand, greater than 10% rock fragments, greater than 15% silt and clay<\/td>\r\n<\/tr>\r\n
Conglomerate<\/td>\r\n<\/td>\r\nDominated by rounded clasts, granule size and larger<\/td>\r\n<\/tr>\r\n
Breccia<\/td>\r\n<\/td>\r\nDominated by angular clasts, granule size and larger<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
\r\n

Most coal forms in fluvial or delta environments where vegetation growth is vigorous and where decaying plant matter accumulates in long-lasting swamps with low oxygen levels. To avoid oxidation and breakdown, the organic matter must remain submerged for centuries or millennia, until it is covered with another layer of either muddy or sandy sediments.\u00a0It is important to note that in some textbooks coal is described as an \u201corganic sedimentary rock.\u201d In this book, coal is included with the clastic rocks for two reasons:\u00a0first, because it is made up of fragments of organic matter; and second, because coal seams (sedimentary layers) are almost always interbedded with layers of clastic rocks, such as mudrock or sandstone. In other words, coal accumulates in environments where other clastic rocks accumulate.<\/p>\r\n\r\n

\r\n\r\n[caption id=\"attachment_253\" align=\"alignright\" width=\"400\"]\"\"<\/a> Figure 6.1.6 A compositional triangle for arenite sandstones, with the three most common components of sand-sized grains: quartz, feldspar, and rock fragments. Arenites have less than 15% silt or clay. Sandstones with more than 15% silt and clay are called wackes (e.g., quartz wacke, lithic wacke).[\/caption]\r\n\r\nIt\u2019s worth taking\u00a0a closer look at the different types of sandstone because sandstone is a common and important sedimentary rock. Typical sandstone compositions are shown in Figure 6.1.6. Sandstones are mostly made up of sand grains of course, but they also include finer material\u2014both silt and clay. The term [pb_glossary id=\"1458\"]arenite[\/pb_glossary]<\/strong> applies to a so-called clean sandstone, meaning one with less than 15% silt and clay. Considering the sand-sized grains only (the grains larger than 1<\/sup>\/16<\/sub>th mm), arenites with 90% or more quartz are called quartz arenites. If they have more than 10% feldspar and more feldspar than rock fragments, they are called feldspathic arenites or [pb_glossary id=\"1459\"]arkosic arenites[\/pb_glossary]<\/strong> (or just [pb_glossary id=\"1460\"]arkose[\/pb_glossary]<\/strong>). If they have more than 10% rock fragments, and more rock fragments than feldspar, they are [pb_glossary id=\"1461\"]lithic arenites[\/pb_glossary]<\/strong>.<\/strong>[footnote]\u201cLithic\u201d means \u201crock.\u201d Lithic clasts are rock fragments, as opposed to mineral fragments.[\/footnote] A sandstone with more than 15% silt or clay is called a [pb_glossary id=\"1463\"]wacke[\/pb_glossary]<\/strong> (pronounced wackie<\/em>). The terms quartz wacke, lithic wacke<\/em>, and feldspathic wacke<\/em> are used with limits similar to those on the arenite diagram. Another name for a lithic wacke is [pb_glossary id=\"1462\"]greywacke[\/pb_glossary]<\/strong>.\r\n\r\nSome examples of sandstones, magnified in thin section are shown in Figure 6.1.7. (A thin section is rock sliced thin enough so that light can shine through.)\r\n\r\n \r\n\r\n[caption id=\"attachment_254\" align=\"aligncenter\" width=\"800\"]\"\"<\/a> Figure 6.1.7 Microscope photos of three types of sandstone in thin-section. Some of the minerals are labelled: Q=quartz, F=feldspar and L= lithic (rock fragments). The quartz arenite and arkose have relatively little silt-clay matrix, while the lithic wacke has abundant matrix.[\/caption]\r\n\r\nClastic sedimentary rocks in which a significant proportion of the clasts are larger than 2 millimetres are known as [pb_glossary id=\"1464\"]conglomerate[\/pb_glossary]<\/strong> if the clasts are well rounded, and [pb_glossary id=\"1465\"]breccia[\/pb_glossary]<\/strong> if they are angular. Conglomerates form in high-energy environments such as fast-flowing rivers, where the particles can become rounded. Breccias typically form where the particles are not transported a significant distance in water, such as alluvial fans and talus slopes. Some examples of clastic sedimentary rocks are shown on Figure 6.1.8.\r\n\r\n<\/div>\r\n<\/div>\r\n
\r\n
[caption id=\"attachment_255\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> Figure 6.1.8 Examples of various clastic sedimentary rocks. [Image Description]<\/a>[\/caption]<\/div>\r\n
\r\n
\r\n

Exercise 6.2 Classifying sandstones<\/p>\r\n\r\n<\/header>\r\n

\r\n
\r\n\r\nTable 6.3 below shows magnified thin sections of three sandstones, along with descriptions of their compositions. Using Table 6.1 and Figure 6.1.6, find an appropriate name for each of these rocks.\r\n\r\n<\/div>\r\n
\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 6.3 Classifying sandstones<\/caption>\r\n
Magnified Thin Section<\/th>\r\nDescription<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
\"Magnified<\/td>\r\nAngular sand-sized grains are approximately 85% quartz and 15% feldspar. Silt and clay make up less than 5% of the rock.<\/td>\r\n<\/tr>\r\n
\"Magnified<\/td>\r\nRounded sand-sized grains are approximately 99% quartz and 1% feldspar. Silt and clay make up less than 2% of the rock.<\/td>\r\n<\/tr>\r\n
\"Magnified<\/td>\r\nAngular sand-sized grains are approximately 70% quartz, 20% lithic, and 10% feldspar. Silt and clay make up about 20% of the rock.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nSee Appendix 3 for Exercise 6.2 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n \r\n

Image Descriptions<\/h3>\r\nFigure 6.1.8 image description:<\/strong> (A) Mudrock with bivalve impressions, Cretaceous Nanaimo group, Browns River, Vancouver Island. A very fine-grained rock with shell impressions. (B) Coarse sandstone with cross-bedding, Cambrian Tapeats Formation Chino Valley, Arizona. (C) Conglomerate with imbricate (aligned, tilted down to the left) cobbles, Cretaceous Geoffrey Formation, Hornby Island, BC. (D) Sedimentary breccia, the Pre-Cambrian Toby Formation, east of Castlegar, BC. [Return to Figure 6.1.8]<\/a>\r\n

Media Attributions<\/h3>\r\n