Physical Geography - Version 1 Unit 10 Sedimentary Rocks

' Va . Figure Red Rock State Park , in California . Image by Jeremy is used under a license . UNIT 10 SEDIMENTARY ROCKS Goals Objectives of this unit Describe the differences between cobbles , pebbles , sand , silt , and clay and explain the relationship between size and the extent to which can be transported by moving water or by the wind . Explain the differences in the characteristics and depositional environments of various types of chemical sedimentary rocks . Differentiate between various sedimentary depositional environments in both terrestrial and marine environments and explain how the formation of sedimentary basins can be related to plate tectonic processes . Apply your understanding of the features of sedimentary rocks , including grain characteristics , sedimentary structures , and fossils , to the interpretation of past depositional environments and climates . GEOGRAPHY

WHAT ARE SEDIMENTARY ROCKS ?

Transportation is the movement of sediments or dissolved ions from the site of erosion to a site of deposition this can be by wind , flowing water , glacial ice , or mass movement down a slope . The deposition takes place where the conditions change enough so that sediments being transported can no longer be transported , like when a current slows . Burial occurs when more sediments are piled onto existing sediments , and layers formed earlier are covered and compacted . Lithification is what happens at depths of hundreds to thousands of meters when those compacted sediments become cemented together to form solid sedimentary rock . In this unit , we divide sedimentary rocks into two main types clastic and chemical . Clastic sedimentary rocks are mainly composed of a material that has been transported as solid fragments ( Chemical sedimentary rocks are mainly composed of a material that has been transported as ions in solution . It important not to assume that mechanical weathering leads only to clastic sedimentary rocks , while chemical weathering leads only to chemical sedimentary rocks . In most cases , millions of years separate the weathering and depositional processes , and both types of sedimentary rocks tend to include at least some material derived from both types of weathering . Clastic Sedimentary A is a fragment of rock or mineral , ranging in size from less than a micron ( too small to see ) to as big as an apartment block . The smaller tend to be composed of a single mineral crystal , and the larger ones are typically composed of pieces of rock . Most are made of quartz because quartz is more resistant to weathering than any other common mineral . Most that are smaller than sand size ( are made of clay minerals . Most larger than sand size ( are actual fragments of rock , and commonly these might be rock like basalt or andesite , or if they are bigger , rock like granite or gneiss . CLASSIFICATION There are six main categories five are broken down into , clay being the exception . The diameter limits for each successive subcategory are twice as large as the one beneath it . In general , a boulder is bigger than a toaster and difficult to lift . There is no upper limit to the size of a boulder . A small cobble will fit in one hand , a large one in two hands . A pebble is something that you could throw quite easily . The smaller ones , known as granules , are gravel size , but still , you could throw one . But you ca throw a single grain of sand . Sand ranges from down to , and its key characteristic is that it feels sandy or gritty between your fingers even the finest sand grains feel that way . Silt is essentially too small for individual grains to be visible , and while sand feels sandy to your fingers , silt feels GEOGRAPHY

smooth to your fingers but gritty in your mouth . Clay is so fine that it feels smooth even in your mouth . By utilizing models , students could identify features on models , which include glacial , fluvial , and costal . Students could also assess how the features are formed and interpret materials and processes that shape the lithosphere , hydrosphere , atmosphere , and solar system . Size Range in Description to large no limit Boulder medium 512 1024 small 256 512 large 128 236 Cobble small 128 very coarse 32 64 coarse 16 32 ) medium 16 fine Size in microns very fine from to ' coarse coarse 500 Sand medium 250 500 fine 125 250 very fine 63 125 Silt very coarse 32 63 coarse 16 32 medium 16 fine fine Clay clay Figure The Scale for Classifying Sediments the Grains that make up Sedimentary Racks . Image is used under a Creative Commons Attribution International License . GEOGRAPHY

TA TION One ofthe key principles of sedimentary geology is that the ability of a moving medium ( air or water ) to move sedimentary particles and keep them moving , is dependent on the velocity of flow . The faster the medium flows , the larger the particles it can move . 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 . If 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 . Dissolved Load Suspended Load Bed Load Figure Examples ofhow Material Moves in A River Environment , 50 moves by Saltation , Solution Load Equally Transport the Remaining 50 . Image is in the public domain . During peak discharge at this location , the water is high enough to flow over the embankment on the right , and it flows fast enough to move the boulders that can not be moved during low flows . Large are pushed ( by traction ) or bounced along the bottom ( saltation ) while smaller are suspended in the water and kept there by the turbulence of the flow . As the GEOGRAPHY

flow velocity changes , may be either incorporated into the flow or deposited on the bottom . At various places along a river , there are always some 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 . I I A . Figure An Example of Notice That the are too Large to Move . Image by Jeremy is used under a license . Other 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 . Clastic sediments are deposited in a wide range of environments , including glaciers , slope failures , rivers , both fast and slow , lakes , deltas , and ocean environments , both shallow and deep . Depending on the grain size in particular , they may eventually form into rocks ranging from fine mudstone to coarse breccia and conglomerate . TION Lithification is the term used to describe several different processes that take place within a deposit of sediment to turn it into solid rock . 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 are all touching one another . Cementation is the process of crystallization of minerals within the pores between the small , and also at the points of contact between the larger ( sand size and larger ) Depending on the pressure , temperature , and chemical conditions , these crystals might include calcite , hematite , quartz , clay minerals , or a range of other minerals . GEOGRAPHY

The characteristics and distinguishing features of clastic sedimentary rocks are summarized in the image below . Table The Main Types of Clastic Sedimentary Rocks Their Characteristics GROUP EXAMPLES CHARACTERISTICS Mudstone 75 silt and clay , not bedded Shale 75 COAL Dominated by fragments of partially decayed plant matter , often enclosed between beds of sandstone or SANDSTONE Quartz Sandstone Dominated by sand , 90 quartz Arkose Dominated by sand , 10 feldspar Lithic Wacke Dominated by sand , 10 rock fragments , 15 silt and clay CONGLOMERATE Dominated by rounded , pebble size and larger BRECCIA Dominated by angular , pebble size and larger is composed of at least 75 and fragments . If it is dominated by clay , it is called . If it shows evidence of bedding or fine , it is shale otherwise , it is mudstone . form in very low energy environments , such as lakes , river backwaters , and the deep ocean . Most coal forms in fluvial or delta environments where vegetation growth is vigorous and where decaying plant matter accumulates in 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 . It is important to note that in some textbooks coal is described as an organic sedimentary In this book , coal is classified with the clastic rocks for two reasons first , 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 or sandstone . In other words , coal accumulates in environments where other clastic rocks accumulate . GEOGRAPHY

It worth taking a closer look at the different types of sandstone because sandstone is a common and important sedimentary rock . The term applies to a clean sandstone , meaning one with less than 15 silt and clay . Considering the grains only , with 90 or more quartz are called quartz . If they have more than 10 feldspar and more feldspar than rock fragments , they are called feldspathic or arkosic ( orjust arkose ) If they have more than 10 rock fragments , and more rock fragments than feldspar , they are lithic . A sandstone with more than 15 silt or clay is called a wacke , pronounced ) The terms quartz wacke , lithic wacke , and feldspathic wacke are used . Another name for a lithic wacke is . Clastic sedimentary rocks in which a significant proportion of the are larger than are known as conglomerate ifthe are well rounded , and breccia if they are angular . Conglomerates form in environments where the particles can become rounded , such as rivers . Breccia typically formed where the particles are not transported a significant distance in the water , such as alluvial fans and talus slopes . Figure Breccia from Mosaic Canyon , Death Valley California . Size between . Image by Jeremy is used under a license . GEOGRAPHY

CHEMICAL SEDIMENTARY ROCKS Whereas clastic sedimentary rocks are dominated by components that have been transported as solid ( clay , silt , sand , etc . chemical sedimentary rocks are dominated by components that have been transported as ions in solution ( Na , Ca , There is some overlap between the two because almost all clastic sedimentary rocks contain cement formed from dissolved ions , and many chemical sedimentary rocks include some . Since ions can stay in solution for tens of years ( some much longer ) and can travel for tens of thousands of kilometers , it is virtually impossible to relate chemical sediments to their source rocks . The most common chemical sedimentary rock , by far , is limestone . Others include chert , banded iron formations , and a variety of rocks that form when bodies of water evaporate . Biological processes are important in the formation of some chemical sedimentary rocks , especially limestone and chert . For example , limestone is made up almost entirely of fragments of marine organisms that manufacture calcite for their shells and other hard parts , and most chert includes at least some of the silica tests ( shells ) of tiny marine organisms such as diatoms . Limestone Almost all limestone forms in the oceans and most of that form on the shallow continental shelves , especially in tropical regions with coral reefs . Reefs are highly productive ecosystems populated by a wide range of organisms , many of which use calcium and bicarbonate ions in seawater to make carbonate minerals ( especially calcite ) for their shells and other structures . These include corals , of course , but also green and red algae , urchins , sponges , mollusks , and crustaceans . Especially after they die , but even while they are still alive , these organisms are eroded by waves and currents to produce carbonate fragments that accumulate in the surrounding region Reefs tend to form near the edges of steep because the reef organisms thrive on upwelling currents . As the reef builds up , it is eroded by waves and currents to produce carbonate sediments that are transported into the steep offshore area and the shallower inshore area . These sediments are dominated by carbonate fragments of all sizes , including mud . Calcite can also form on land in several environments . Tufa forms at springs and travertine ( which is less porous ) forms at hot springs . Similar material within limestone caves to form stalactites , stalagmites , and a wide range of other . GEOGRAPHY

Figure Tufa Towers in Mono Lake , California . Image by Jeremy is used under a license . Dolomite Dolomite is another carbonate mineral , but dolomite is also the name for a rock composed of the mineral dolomite ( although some geologists use the term to avoid confusion ) Dolomite rock is quite common ( there a whole Italian mountain range named after it ) which is surprising since marine organisms do make dolomite . All ofthe dolomite found in ancient rocks has been formed through magnesium replacing some of the calcium in the calcite in carbonate and sands . This process is known as dolomitization , and it is thought to take place where water through the sediments in carbonate tidal flat environments . used under a license . GEOGRAPHY

Chert Banded Iron Formations As we seen , not all marine organisms make their hard parts out of calcite some , like diatoms , use silica , and when they die their tiny shells ( or tests ) settle slowly to the bottom where they accumulate as chert . In some cases , chert is deposited along with limestone in the moderately deep ocean , but the two tend to remain separate , so chert beds within limestone are quite common , as are nodules , link the flint nodules ofthe Cretaceous chalk of southeastern England . In other situations , and especially in very deep water , chert accumulates on its own , commonly in thin beds . Some ancient chert beds , most dating to between 1800 and 2400 Ma , are also combined with a rock known as banded iron formation ( a deep deposit of iron oxide that is a common ore of iron . forms when iron dissolved in seawater is oxidized , becomes insoluble , and sinks to the bottom in the same way that silica tests do to form chert . The prevalence of in rocks dating from 2400 to 1800 Ma is due to the changes in the atmosphere and oceans that took place over that time period . Photosynthetic bacteria ( cyanobacteria ) consume carbon dioxide from the atmosphere and use solar energy to convert it to oxygen . These bacteria first evolved around 3500 Ma , and for the next billion years , almost all of that free oxygen was used up by chemical and biological processes , but by 2400 Ma free oxygen levels started to increase in the atmosphere and the oceans . Over a period of 600 million years , that oxygen gradually converted soluble ferrous iron to insoluble ferric iron , which combined with oxygen to form the mineral hematite , leading to the accumulation of . After 1800 Ma , little dissolved iron was left in the oceans and the formation of essentially stopped . In arid regions , lakes and inland seas typically have no stream outlet and the water that flows into them is removed only by evaporation . Under these conditions , the water becomes increasingly concentrated with dissolved salts , and eventually , some of these salts reach saturation levels and start to crystallize . Although all evaporate deposits are unique because of differences in the chemistry of the water , in most cases minor amounts of start to precipitate when the solution is reduced to about 50 of its original volume . Gypsum at about 20 of the original volume and halite at 10 . Other important evaporate minerals include borax . GEOGRAPHY

Figure Student Licking Halite from Basin , Death Valley California . image by Jeremy is used under a license . SEDIMENTARY STRUCTURES Through careful observation over the past few centuries , geologists have discovered that the accumulation of sediments and sedimentary rocks takes place according to some important geological principles , as follows The principle of original horizontality states that sediments accumulate in essentially horizontal layers . The implication is that tilted sedimentary layers observed today must have been subjected to tectonic forces . The principle of superposition states that sedimentary layers are deposited in sequence and that unless the entire sequence has been turned over by tectonic processes , the layers at the bottom are older than those at the top . The principle of inclusions states that any rock fragments in a sedimentary layer must be older than the layer . For example , the cobbles in a conglomerate must have been formed before the conglomerate . The principle of faunal succession states that there is a order in which organisms have evolved through geological time , and therefore the identification of specific fossils in a rock can be used to determine its age . By understanding the origins of these features , we can make some very useful inferences about the processes that led to the deposition of the rocks that we are studying . GEOGRAPHY

Bedding Bedding , for example , is the separation of sediments into layers that either differs from one another in textures , composition , color , or weathering characteristics or are separated by partings , narrow gaps between adjacent beds . Bedding is an indication of changes in depositional processes that may be related to seasonal differences , changes in climate , changes in locations of rivers or deltas , or tectonic changes . Partings may represent periods of that could range from a few decades to a few centuries . Bedding can form in almost any depositional environment . is bedding that contains angled layers and forms when sediments are deposited by flowing water or wind . in streams tends to be on the scale of centimeters to tens of centimeters , while those in ( wind deposited ) sediments can be on the scale of meters to several meters . Jurassic Navajo Formation sandstone at Zion National Park , Utah . In most of the layers , the dip down toward the right , implying wind direction from right to left during deposition . One bed dips in the opposite direction , implying an abnormal wind . form as sediments is deposited on the leading edge of an advancing ripple or dune . Each layer is related to a different ripple that advances in the flow direction and is partially eroded by the following ripple . is a very important sedimentary structure to GEOGRAPHY

recognize because it can provide information on the direction of current flows and , when analyzed in detail , on other features like the rate of flow and the amount of sediment available . A ) Figure Example , Zion National Park Utah . Image by Steven is under a By license . Pin It ! View this to learn more abo and are developed . Ripples Ripples , which are associated with the formation of , may be preserved on the surfaces of sedimentary beds . Ripples can also help to determine flow direction as they tend to have their steepest surface facing downflow . is used under a license . GEOGRAPHY

In a stream environment , boulders , cobbles , and pebbles can become imbricated , meaning that they are generally tilted in the same direction . in streams tend to tilt with their upper ends pointing downstream because this is the most stable position with respect to the . Mud Cracks Mud cracks form when a shallow body of water ( a tidal flat or pond ) into which muddy sediments have been deposited , dries up and cracks . This happens because the clay in the upper mud layer tends to shrink on drying , and so it cracks because it occupies less space when it is dry . The various structures described above are critical to understanding and interpreting the formation of sedimentary rocks . In addition to these , geologists also look very closely at sedimentary grains to determine their mineralogy or lithology ( in order to make inferences about the type of source rock and the weathering processes ) their degree of rounding , their sizes , and the extent to which they have been sorted by transportation and depositional processes . Figure Mud Cracks in Valley . Image by Jeremy is used under a license . The word fossil comes from the Latin term , meaning dug up . Fossils are formed when an organism is buried by water containing debris and minerals and through the effects of wind or gravity . Most fossils are found in sedimentary rocks . Fossils can also be found in metamorphic rock or rock that has been altered by heat or pressure . Rarely are fossils found in GEOGRAPHY

igneous rock , which is formed when magma flows and hardens . The five most often cited types of fossils are mold , cast , imprint , and trace fossils . Mold or Impression A mold or impression fossil is formed when the plant or animal decays completely but leaves behind an impression of itself , like a hollow mold . No organic material is present and the organism itself is not copied . Mold or impression fossils can form in several ways , but generally enough air must be present to allow the organic material to completely decompose , which prevents the fossilization of the organism . These fossils are usually formed in sand or clay . Cast Fossils Cast fossils are the type people are most familiar with , as they make up the spectacular dinosaur skeletons on view in museums . Cast fossils occur when minerals deposit into the mold left by the rotting organic material , resulting in a replica of the hard structures ofthe plant or animal . Imprint Fossils Imprint fossils are found in silt or clay , like the mold or impression fossils , but they leave behind just a imprint . These fossils are sometimes found on exposed rock surfaces or when the layers in the rock are broken , revealing the fossil inside . Figure Imprint Fossil of an Ammonite , 65 Million Years Old . Image on . GEOGRAPHY

In , or petrified , fossils , each part of the organism is replaced by minerals , leaving a stone copy of the organism . Bones , teeth and even woody plant materials such as trees are sometimes preserved in this manner . One famous example of petrification is the hundreds of petrified trees in the Petrified Forest in , Arizona . ver 300 Million Years Old . Image by Jeremy is used under a license . Figure A Fossilized Trilobite , Trace Fossils Trace fossils usually show tracks that animals made while moving across soft sediment . This sediment later hardens to become sedimentary rock . Trace fossils are valuable to paleontologists because by studying these footprints , scientists can discover how the animals moved , which in turn gives important information about the structure and even the life of the species . GEOGRAPHY

UNIT 10 SUMMARY Sedimentary are classified based on their size , and variations in size have important implications for transportation and deposition . Clastic sedimentary rocks range from conglomerate to mudstone . size , sorting , composition , and shape are important features that allow us to differentiate clastic rocks and understand the processes that took place during their deposition . Chemical sedimentary rocks form from ions that were transported in solution and then converted into minerals by biological chemical processes . The most common chemical rock , limestone , typically forms in shallow tropical environments , where biological activity is a very important factor . Chert and banded iron formation are sedimentary rocks . form where the water of lakes and inland seas becomes due to evaporation . The deposition of sedimentary rocks takes place according to a series of important principles , including original horizontality , superposition , and faunal succession . Sedimentary rocks can also have distinctive structures that are important in determining their depositional environments . Fossils are useful for determining the age of a rock , the depositional environment , and the climate at the time of deposition . GEOGRAPHY