Ch.6 Sedimentary Rocks PDF

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Ch.6 Sedimentary Rocks Turning Sediment into Sedimentary Rock: Diagenesis and Lithification - Change can occur to sediment from time it is deposited until it become a sedimentary rock - Diagenesis - Collective term for all the chemical, physical and biologic changes that take place after sediments are deposited but before metamorphism - Burial promotes diagenesis because as sediments are buried, they are subjected to increasingly higher temperatures and pressures - Re- crystallization is an example of diagenetic change - The development of more stable minerals from less stable one's - Includes lithification - Lithification - Process by which unconsolidated sediments are transformed into solid sedimentary rocks - Basic lithification processes include compaction and cementation - Compaction - As sediment accumulates, the weight of overlying material compresses the deeper sediments - The deeper the sediment is buried, the more compacted and firm its becomes - As grains are pressed closer together, there is considerable reduction in pore space - Cementation - Most important process by which sediments are converted to sedimentary rock - Chemical diagenetic change - Involves precipitation of minerals among the individual sediment grains - Cementing materials are carried in solution by water percolating through the pore spaces between particles - Most sedimentary rocks are lithified by means of compaction and cementation Sedimentary Environments - A sedimentary environment is a geographic setting where sediment is accumulating - Each site is characterized by particular combination of geologic processes and environmental conditions - Geographic setting and environmental conditions of a sedimentary environment determine the nature of the sediments that accumulate - Geologists study sediments in present-day depositional environments because the features they find can also be observed in ancient sedimentary rocks - Types of Sedimentary Environments - Sedimentary rocks are commonly placed in 3 categories - 1. Continental

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- 2. Transitional (shoreline) - 3. Marine Continental Environments - Dominated by erosion and deposition associated with streams - In cold regions, moving masses of glacial ice replace running water as dominant process - In arid regions, wind takes a greater importance - Nature of sediments deposited in continental environments is strongly influenced by climate - Streams are dominated agent of landscape alteration - Agitation of sediment by running water tends to separate sedimentary particles according to density and grain size - Aeolian - Work of wind and its resulting deposits - Aeolian sediments are well sorted - Where winds are strong and surface is not anchored by vegetation, sand transported close to the ground accumulates in dunes - Desert basins are areas where dunes develop and sites at which shallow playa lakes occasionally form allowing heavy rain periods of snow melt in adjacent mountains Transitional Environments - In transition zones fine grained sediment may be alternately deposited under shallow sheets of water and exposed to air producing mud dominated tidal flats - Where shoreline is characterized by energetic wave and current action, sediments delivered to the sea by stream can be repeatedly sorted and deposited to produce a sand or gravel beach - Reworking and redistribution of sand by waves and currents along a shoreline can produce linear sand bodies - Deltas are the most significant deposits associated with transitional environments Marine Environments - Marine depositional environments are divided according to depth - Shallow marine environment borders all of the worlds continents - Because of ongoing erosion of adjacent continent, the shallow marine environment receives huge quantities of land derived sediment - Coral reefs are also associated with warm, shallow marine environments - In hot regions where the sea occupies a basin with restricted circulation, evaporation triggers the precipitation of soluble materials - Deep marine environments include all the floors of the deep ocean Sedimentary Facies

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By studying series of sedimentary layers, we see changes in environmental conditions - Changes in past environments can be seen when a single unit of sedimentary rock is traced laterally - In a zone where biologic activity is high and land derived sediments are scarce, the deposits consist largely of calcareous remains of small organisms - Sedimentary facies - The characteristics of a rock unit that reflect its environment of deposition and differentiate the rock unit from time equivalent units adjacent to it - When a sedimentary unit is examined in cross section from one end to the other, each facies can grade laterally into another that formed at the same time but exhibit different characteristics Types of Sedimentary Rocks - Sediment has 3 principal sources - 1. Sediment can originate as solid particles derived from both mechanical and chemical weathering of existing rocks - Deposits of this kind are called detrital and the sedimentary rocks they form are called detrital sedimentary rocks - Rocks that form from accumulation of materials that originate and are transported as solid particles derived from both mechanical and chemical weathering - 2. Soluble material produced largely by chemical weathering - The rocks formed are called chemical sedimentary rocks - Sedimentary rock consisting of material that was precipitated from water by either inorganic or organic means - 3. Organic matter derived from carbon rich tissues of once living things - Ex. plant remains that accumulate in a swamp can be buried and altered over time to produce coal which is an organic sedimentary rock - A sedimentary rock mainly composed of altered organic matter ( ex coal, consisting of lithified plant remains) Detrital Sedimentary Rocks - Silicate minerals clay and quartz are chief constituents of most sedimentary rocks in this category - These rocks can be referred to as siliciclastic sedimentary rocks - Clay minerals are most abundant product of chemical weathering of silicate minerals - Other common minerals in detrital rocks are feldspars and micas - Because chemical weathering rapidly transforms these minerals into new substances, their presence in sedimentary rocks indicates that erosion and deposition were fast enough to preserve primary minerals - Particle size is primary basis for distinguishing various detrital sedimentary rocks - Sizes and components of grains provides information about environments of deposition

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- Stronger the current, the larger the particle size carried Very little energy is needed to transport clay so it settles slowly Common detrital sedimentary rocks in order of increasing particle size are…. Shale and Other Mudrocks - Mudrocks are the group of sedimentary rocks that include shale, mudstone and siltstone - Consists of clay to silt sized particles and account for more than half of all sedimentary rocks - Tiny grains in mudrocks indicagte that deposition is the result of gradual settling from non turbulent currents - Much of the clay is deposited only after the individual particles coalesce to form larger aggregates - Sometimes chemical composition of the rock provides additional information - As silt and clay accumulate, they tend to form thin layers called laminae - Particles of laminae initially are oriented randomly and abundant pore space is filled with water or other fluids - During compaction clay and silt particles can align almost in parallel and become compact - Rearrangement of grains reduces the size of the pore spaces and force water out - Inability of water to migrate between microscopic pore spaces explains why shale forms barriers to subsurface movement of water and petroleum - Shale is a type of mudrock that can be split into thin layers along closely spaced planes - This is called fissility - If the rock breaks into chunks or blocks, the name mudstone is applied - Siltstone is a fine grained rock that is often grouped with shale but lacks fissility - Mudstones are most common sedimentary rock but are usually not as noticeable because they do not form prominent outcrops as do sandstone and limestone Sandstone - Sandstone is the name given to rocks in which sand size grains dominate - After shale, sandstone is the most abundant sedimentary rock - Form in variety of environments - Contain significant clues about their origin, including sorting, particle shape and composition - Sorting - Degree of similarity in particle size in a sedimentary rock - If all the grains in a sample of sandstone are about the same size, the sand is considered well sorted - If the rock is a mixture of large and small particles, the sand is considered poorly sorted - Deposits of windblown sand are usually better sorted

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Sediments that exhibit poor sorting usually result when particles are transported for only a relatively short time - Roundness of sand grains, reflecting the degree to which corners and edges of grains have been smothered down, also help decipher the history of sandstone - Degree of roundness reflects the distance, time or repetition of disturbance involved in the transportation of sediment - Very angular grains imply that the materials were transported only a short distance before they were deposited and that some other medium may have transported them - Substantial weathering, long transport and reworking lead to the gradual destruction of weaker and less stable minerals - Origin and history of sandstone can often be deduced by examining the sorting, roundness and mineral composition of its constituent grains - Owing to its durability, quartz is the predominant mineral in most sandstones - This is called quartz sandstone - When a sandstone contains appreciable quantities of feldspar, the rock is called arkose - Wacke is a third variety of sandstone and is a dark coloured rock containing abundant rock fragments and matrix - Matrix refers to the silt and clay sized particles found in spaces between larger sand grains - Poor sorting and angular grains characteristic of wacke suggest that the particles were transported only a relatively short distance from their source area Conglomerate and Breccia - Conglomerate consists largely of rounded pebbles and cobbles - These particles can range in size from large boulders to particles as small as green peas - Particles are commonly large enough to be identified as distinctive rock types - Many conglomerates are poorly sorted because the spaces between the large gravely particles contain trapped sand or mud - Gravels accumulate in a variety of environments and usually indicate sediment flows down steep slopes - Coarse particles in conglomerate may reflect the action of energetic mountain streams or strong wave activity - If the large particles are angular rather than rounded the rock is called breccia - Conglomerates and breccia contain clues to their history - Their particle sizes reveal the strength of the currents that transported them - The degree of rounding indicates how far or how long the particles travelled

Chemical Sedimentary Rocks - Chemical sediments derive from material that is carried in solution to lakes and seas where some of it precipitates to form chemical sediments - Precipitation occurs in 2 ways

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Inorganic processes such as evaporation and chemical activity Organic processes where aquatic organisms form chemical sediments of biochemical origin Limestone - Most abundant chemical sedimentary rock - Composed of Calcite and forms either inorganic or biochemical processes - Mineral composition of all limestone is similar yet many different types exist because limestones are produced under a variety of conditions - Those forms of biochemical origin are the most common - Organic limestones - Corals produce large quantities of marine limestone - Although most limestone is the product of biologic processes, shells and skeletons can re-crystallize before becoming lithified into rock - Coquina - One easily identified biochemical limestone - Composed of poorly cemented shells and shell fragments - Fossiliferous limestone - Limestone consisting of visible shells, shell fragments and other skeletal components - Chalk - Another form of limestone - Soft, poruous rock made up almost entirely of the hand parts of microscopic marine organisms - Inorganic limestones - These limestones form when chemical changes, high water temperatures and/or microbial activity increases the concentration of calcium carbonate - Travertine - An example found in caves where groundwater is the source of calcium carbonate - Oolitic limestone - Composed of small spherical grains called ooids - Ooids form in shallow marine waters as tiny “seed” particles and are moved back and forth by currents Dolostone - Composed of calcium magnesium carbonate mineral dolomite - Can rarely form by direct precipitation from seawater, most dolostone probably originates when magnesium, dissolved in seawater or ground water, replaces half the calcium in limestone - Dolomite is less prone to being dissolved by acidic rain water than is calcite Chert - Same name used for a number of hard rocks made of microcrystalline silica including - Flint

- Whose dark colour results from the organic matter within it Jasper - A red variety coloured by iron oxide - Agate - Chert deposits are commonly found in one of two situations - 1. As irregularly shaped nodules in limestone/ dolostone - 2. As tabular layers of rock - Silica composing many chert nodules may have been deposited inorganically by direct precipitation from water - Most layers of chert originated primarily as biochemical sediment - Some bedded cherts occur together with lava flows and layers of volcanic ash and not from biochemical sources - Chert is also found interbedded with iron-rich deposits in banded iron formation, indicating there was more dissolved silica in sea during precambrian times - Evaporites - Evaporation is the mechanism triggering deposition of chemical precipitates in places - Minerals commonly precipitated in this fashion include halite, and gypsum - Halite is familiar aka salt - Gypsum is basic ingredient of plaster of paris and is used extensively in the construction industry for drywall and interior plaster - Evaporites - A sedimentary rock formed of material deposition from solution by evaporation of the water - When a body of seawater evaporates, the minerals that precipitate do so in a sequence that is determined by their solubility - Less soluble minerals precipitate first and more soluble minerals precipitate later Coal: An Organic Sedimentary Rock - Coal is made from organic matter - Often reveals fossil remnants of leaves, bark and wood - Coal is the end product of large amounts of plant material buried for millions of years - Coal formation requires large quantities of plant remains to accumulate in a swamp - Partial decomposition of plant remains in an oxygen poor swamp creates a layer of peat - A soft brown material in which plant structures are still easily recognized - With shallow burial, peat slowly changes to lignite, which is a soft brown coal - Higher temperatures bring about chemical reactions within the plant materials and yield water and organic gases - The greater the carbon content, the greater is the coal’s energy ranking as a fuel - Lignite and bituminous coals are sedimentary rocks - When sedimentary layers are subjected to the folding and deformation associated with mountain building, the heat and pressure cause a further loss of volatiles - Coal is a major energy resource Classification of Sedimentary Rocks -

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Classification scheme divided sedimentary rocks into two major groups - 1. Detrital - 2. Chemical - The main criterion for subdividing the detrital rocks is particle size, whereas the primary basis for distinguishing among rocks in chemical group is their mineral composition - Rigid scheme but in reality many sedimentary rocks contain at least small quantities of detrital sediment - Texture is a part of sedimentary rock classification - Two major textures used in the classification of sedimentary rocks - 1. Clastic - Rocks that display a clastic texture consist of discrete fragments and particles that are cemented and compacted together - All detrital rocks have a clastic texture - Some chemical sedimentary rocks exhibit this texture too - 2. Non clastic - Some chemical sedimentary rocks have a non clastic texture - The minerals form a pattern of interlocking crystals - The crystal may be small or large - Materials that make up many non clastic rocks may have originated as detrital deposits - Consists of intergrown crystals, and some may resemble igneous rocks which are also crystalline - Minerals contained in non clastic sedimentary rocks are unlike those found in igneous rocks Sedimentary Structures - Sediments exhibit a variety of structures - Sedimentary rocks form as layer upon layer of sediment accumulates in various depositional environments - These layers are called strata o  r beds - Probably the single most common and characteristic feature of sedimentary rocks - Each stratum is unique - Variations in texture, composition and thickness reflect the different conditions under which each layer was deposited - Thickness of beds ranges from microscopically thin tens of metres thick - Bedding planes - Separate the strate - Flat surfaces along which rocks tend to separate or break - Changes in grain size or composition of sediment being deposited can create bedding planes - Because most sediments accumulate as particles that settle from a fluid, most strate are originally deposited as horizontal layers - Cross bedding

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Occurs when sometimes a bed of sedimentary rock is examined and we see layers within in that are inclined to the horizontal - Graded beds - Represent another special type of bedding - The particles within a single sedimentary layer gradually change from coarse at the bottom to fine at the top - Graded beds are most characteristic of rapid deposition from water containing sediment of varying sizes - Deposition of a graded bed is most commonly associated with a turbidity current - Graded beds can also be produced when sediment particles are thrown into suspension during a storm then settle out after the storm passes - Other features in sedimentary rocks give clues to the past - Wet mud is observed to dry and shrink on exposure to air, forming mud cracks - Ripple marks are small waves of sand that develop on the surface of a sediment layer by the action of moving water or air - Significance of storms in deposition of sediment in the sea has become apparent in the past couple of decades - Storms are capable of eroding, transporting and depositing large volumes of sediment in a short time - Hummocky cross-stratification - A common type of sedimentary structure found in sand storm deposits - Characterized by low angle laminations that define wavelike undulations - Erosion and sorting of sediment on the sea floor by storms can also form concentrations of pebbles , shells and other coarse grained debris Fossils: Evidence of Past Life - Fossils - The remains or traces of prehistoric life - Fossils are important inclusions in sediment and sedimentary rocks - Can provide information on sedimentary environments - Fossils are important time indicators and play a key role in correlating rocks of similar ages among different areas of the world - Fossils can be classified into 2 categories - 1. Body fossils - Which preserve evidence of the tissues of an organism - 2. Trace fossils - Which preserve evidence of an organism's activities - Normally, the remains of an animal or plant are totally destroyed - The preservation of any organic remains requires their isolation from scavengers and microbes that consume and disturb dead tissue and from the physical and chemical agents that assist in the breakdown of mineralized tissue - Fossil preservation is more likely to occur when the remains are rapidly entombed or better yet, when rapid entombment is accompanied by the suppression of scaven...