Sedimentary rocks. Sedimentary Rocks are produced from weathered debris of older rocks. Sedimentary rocks are produced by the following steps:

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1 Sedimentary rocks Most figures contained here are fro m Understand ing Earth Fourth Edition by F rank P ress, Raymond Sieve r, John Grotzinge r, and Thomas H. Jor dan Sedimentary Rocks are produced from weathered debris of older rocks Sedimentary rocks are produced by the following steps: Weathering: the breakdown of older rock (or any other mate rial) Transportation: moving the produces of weathering by a fluid (wind, water, ice, mud ) or under gravity (slides, falls ) Deposition: weathered products come to rest and are buried Lithification: process by which sediments are converted to rock. This involves burial, compaction and cementation. Weathering Chemical Weathering: rock is broken by solution, oxidation or some other chemical reaction or alteration. This either involves the rock being dissolved or converted to a different materi al (mineral ) by chemical alteration. Mechanical w eathering: rock is broken down to smaller pieces by physical processes but the resulting product are still the same minerals 1

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3 Major factors affecting the rate of weathering: Any process to change a rock from it present state requires the application of energy (heat, moving water ) or chemical reagents (wat er, acid ) to the rock. So anything that increases the available energy and water (acidic) or the ability to deliver the energy and water to the minerals of the rock (access to the rock) will increase the rate of w eathering or erosion. In short: wet climates will have faster weathering rates warm climates will have faster weathering rates previously cracked of fractured rock will weather faster exposed rock will weather faster the presence of organic matter inc reases ac id content (carbonic acid) and increases the rate of erosion. 3

4 Faster we athering Slow weathering Fastest weathering Chemical Weathering: rock is broken by solution, oxidation or some other chemical reaction or alteration. This either involves the rock being dissolved or converted to a different material (mineral) by chemical alteration. 4

5 clay Each feldspar may weathers to a different clay Weathering of feldspars Feldspar + CO 2 + H silica in solution + ion in solution This is an important source for sedimentary silica to form sedimentary quartz minerals and for cement Na, Ca, K depending on the type of feldspar This is a source for +ion for halides (salts) 5

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7 Weathering of limestone Calcite + CO 2 + H 2 0 Ca in solution CO 2 7

8 - Oxidation Oxidation Iron + Oxygen Iron Oxide or different iron oxides since the iron put into the system was probable already oxidized at a lower level Solution halides dissolve in water + O H H - Na Cl + O H H - Na + O H H + Cl Na Na Cl Na Cl H H O + Cl - H H O - + 8

9 In general all minerals are stable in conditions most like where they form Oxides are very stable since almost all surfac e location have free oxygen and are oxidizing environments Quartz and clay are very stable (w eather ve ry slowly) since they both form as sedimentary minerals Ferro magnesian silicates, micas and feldspars a re less stable (weather quickly) since they only form igneous or metamorphic minerals Halides and Calcite are stable where they form (in salty calcium ri ch water) but are not very stable (weather quickly) when exposed to the surface or fresh wate r. Quartz is most resistant: it only weathers to silica in solution Weathers to iron oxide, silica in solution and clay All feldspars weather to clay, +ions and silica in solution Most resistant to weathering K feldspar is the exception to this it is not very resistant to chemical weathering Least resistant to weathering 9

10 Mechanical weathering: rock is broken down to smaller pieces by physical processes but the resulting product are still the same minerals Rock particles can grind rock when propelled by a fluid (wind, water ). Jointing (fractures), most igneous rock have cooling joints but jointing in all rock it can also be due to tectonic stress or pressure release. 10

11 Organic activity: plant roots can further fracture rock, animal activity (i.e. hoofs ) can also break up rock Ice wedging, ice expands when it freezes and helps to fracture rock Exfoliation (onion skin) can be due to pressure release 11

12 Exfoliation (onion skin) can also be due to sacrificial heating and cooling (daily temperature variation only penetrate centimeters but seasonal affects may penetrate meters ) Weathering products of granite due to both mechanical and chemi cal weathering 12

13 Sedimentary en vironmen t is determined by the type, exposure and distribution of fluid (transportation agent) and energy (heat and fluid velocity) involved in the transportation and deposition of sediment. Energy include: temperature, gravity (steep vs shallow slope), wave energy. Fluid includes: wind, water, ice, mud Exposure or distribution includes: seasonal variations, fluid source (rain, tides, floods, channels ) MARINE ENVIRONMENTS Organic Geograp hic location and plate tecton ic s etting Transpor t agent and medium processes and organisms that mod ify sediments Climate 4 Glac ier Sedimenta ry envi ronmen t 1 Lake 2 3 Rivers De se rt Lake 5 De lta De se rt 6 7 Beach Tidal Flat 9 Sediments deposi ted Continental shelf 11 Continental margin/ slope 10 Orga ni c reef Turbidity currents 8 Deep sea 13

14 Chemical sedimentary rocks Biochemical sedimentary rocks: are the product of organic activity (shells, wood..). are the most abundant type of chemical sedimentary rock (limestone and dolomite). Chemical sedimentary rocks (inorganic): are deposited directly by water or are altered in place. Clay The previous slide assumes clay is clastic (detrital) sediment. Clay is the product of chemical weathering but it is rarely deposited where it was formed. Once clay has been produced mechanical weathering will break it apart and redeposit it in which case we call it a detrital sediment. 14

15 Biochemical sedimentary rocks Wood and soft tissue: If wood or other soft tissue are buried rapidly in an anoxic environment (swamp, deep basins with poor circulation, lagoonal or deltaic where clay rapidly buries small organisms) it can produce coal, oil or gas in proper temperatu re and pressure. Biochemical sedimentary rocks Shells: Microscopic organisms: Some have calcite (or aragonite) shells others have siliceous shells. These fill the plankton layer throughout the oceans and are primary photosynthesizes. These make up most deep sea sediment as calcareous oozes (resulting in fine grained sandstone) and siliceous oozes (chert). Large shell such as clams, oysters, etc. and skeletal fragments (coral etc.) can be deposited and cemented. If the shells have been reworked these may be classified as bioclastic limestone. Coralline algae: many algae types secrete calcium carbonate. Up to 50% of the calcium carbonate in a coral reef can be from coralline algea. 15

16 Chemical sediment: Dolomite is formed in place by the replacement of calciu m in a limestone with magnesium. Oxides can also form in place through oxidation and concretion. Evaporites (Halite, gypsum.) form by the evaporation of sea water or other salty water bodies (land locked lakes that dry out) 16

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19 MARINE ENVIRONMENTS Organic Geograp hic location and plate tecton ic s etting Transpor t agent and medium processes and organisms that mod ify sediments Climate 4 Glac ier Sedimenta ry envi ronmen t 1 Lake 2 3 Rivers De se rt Lake 5 De lta De se rt 6 7 Beach Tidal Flat 9 Sediments deposi ted Continental shelf 11 Continental margin/ slope 10 Orga ni c reef Turbidity currents 8 Deep sea 19

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23 Clastic sedimentary rocks Clastic or detrital sedimenta ry rocks are composed of the products of mechanical weathering. The naming of sedimentary rocks reflective of the texture of the rock and to a lesser extent the composition. The texture of a sedimentary rock is dependent on the amount of transportation, the energy of the transporting fluid and the way in which the transporting fluid looses its energy. 23

24 Fluids transport rock fragments in two ways Bed load (saltation): grain are pushed by the fluid and bounce along the fluid Earth interface. Suspended load rock fragments are ca rried up in the fluid column Grain size Grain size is dependent on the amount of energy in the transporting fluid. High energy fluids (ice, mud, flash flood of water) will move all sized particles. Low energy fluids can only transport clay (i.e. slow moving water, wind will move sand in its bed load; silt and clay in the suspended load) 24

25 The energy of the fluid in relation to its ability to move sediment is related to its density, viscosity and velocity High viscosity and density means high energy Low viscosity means low energy. High velocity will imply higher energy (but a rapidly moving low viscosity fluid may still have medium to low energy). Low velocity implies lower energy (but a very dense high viscosity fluid will still have high energy) Fluid type energy level why Ice high high viscosity and density moves all grain sizes Mud high high viscosity and density moves all grain sizes Water low to high depends on its velocity The size particle that moves depends on water velocity generally, water moves sand through clay in its suspended load and boulder through sand in its bed load. Wind low density is very low so energy is dependent soly on velocity Carries silt and clay in the suspended load (sand in storms) and sand in the bed load. sorting 25

26 Sorting is indicative of how the fluid looses its energy Well sorted sediment indicates the fluid has a narrow range of energy levels or a very bracketed energy system. Ice and mud can only produce poorly sorted sediment because they go from high to low energy as soon as the stop moving with no bracketing in energy levels. Water can produced both well and poorly sorted sediment. In flash flood areas the sediment will be very poorly sorted because a flood looses energy quickly. In a long stream channel the water has very bracketed the energy level (e.g. on steep gradients the water is fast and high energy and in flat areas it is slow and low energy) Wind produces well sorted sediment because it is low energy only Rounding, grain shape Rounding indicates long exposure to fluid and energy which usually means much transportation 26

27 Lithification includes compaction and cementation. Lithification converts sediment to sedimentary rock 27

28 Clastic rock types Conglomerate clay through gravel sized grains, well rounded typical of perennial mountain streams Breccia clay through gravel sized grains, angular, typical in arroyos and ephemeral streams where there are flashfloods (alluvial fans) sandstone sand sized grains, well rounded typical of perennial mountain streams arkose sand sized grains, angular, typical of arroyos and ephemeral streams where there are flashfloods (alluvial fans) Silt stone- silt sized grains, usually in perennial streams or wind blown sediments Shale clay stone, usually in perennial streams or wind blown sediments 28

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30 Other sedimentary features 30

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