Sediment: an accumulation of loose mineral grains or

Transcription

1 07_00.jpg Sediment: : an accumulation of loose mineral grains or very small fragments of rocks, such as boulders, pebbles, sand, silt, or mud, that are not cemented together. Sedimentary rocks: : rock that forms either by the cementing together of fragments broken off preexisting rock or by the precipitation of mineral crystals out of water solutions at or near the Earth s surface.

2 PC version Cover: : sedimentary rocks that occur on top of, or overlain older igneous or metamorphic rocks. Basement: : The older igneous and metamorphic rocks that sedimentary rocks lay over.

3 Sedimentary rocks are made out of grains also called detritus (the chunks and smaller grains of rock broken off outcrops by physical weathering). These detritus can be classified by their diameters. boulders cobbles pebbles sand silt mud (clay) more than 256 mm between 64 mm and 256 mm between 2 mm and 64 mm between 1/16 mm and 2 mm between 1/256 mm and 1/16 mm less than 1/256 mm

4 07_03.jpg Weathering: : The processes that break up and corrode solid rock, eventually transforming it into sediment. There is two types of weathering: 1) Physical and 2) chemical.

5 Physical weathering: : The process in which intact rock breaks into smaller grains or chunks by mechanical means. There are several different types. frost wedging: The process in which water trapped in a joint freezes, forces the joint open, and may cause the joint to grow.

6 Root wedging: : Roots extend along joints. As the tree grew, the roots broke the rock apart. Salt wedging: The process in arid climates by which dissolved salt in groundwater crystallizes and grows in open pore spaces in rocks and pushes apart the surrounding grains.

7 Other types of physical weathering include: Thermal expansion: : heating of a rock causes it to expand and break apart. biological means: : Biological organisms such as insects can cause rocks to break apart. Most weathering occurs along open fractures in rocks called joints.

8 07_04.jpg Exfoliation: : The process by which an outcrop of rock splits apart into onion-like sheets along joints that lie parallel to the ground surface. This is the result of stress being released from the basement rock due to the erosion of the cover rocks. Erosion: : The grinding away and removal of Earth s surface materials by moving water, air, or ice.

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10 Chemical weathering: : The process in which chemical reactions alter or destroy minerals when rock comes in contact with water solutions and/or air. Saprolite: : A layer of rotten rock created by chemical weathering in warm, wet climates. Saprolites tend to resemble the rock that they were altered from. Chemical weathering can occur in four different fashions: 1) Dissolution: : minerals and rocks are dissolved in water

11 Dissolved Cl - pitted crystal face Weathering by dissolution H + + H + O 2- water molecule showing polar nature From Earth: Portrait of a Planet by S. Marshak

12 Hydrolysis: : The process in which water chemically reacts with minerals and breaks them down. e.g., Feldspar + water fi clay mineral + ions (e.g., Ca2+, Na+, K+). Clay minerals: many types - clay size (sheet silicates); Ions: carried into groundwater fi streams fi ocean to make water salty. Oxidation: A reaction in which an element loses electrons; an example is the reaction of iron with air to form rust. Hydration: : The absorption of water into the crystal structure of minerals.

13 Fastest weathering Slowest weathering Halite Calcite Olivine Ca-plagioclase Pyroxene Amphibole Na-plagioclase Biotite Orthoclase Muscovite Clay Quartz Gibbsite Hematite Least stable minerals Most stable minerals

14 06_06.jpg Mineral stability is Bowen s reaction series in reverse.

15 07_08and09.jpg The greater the surface area, the more weathering can take place. As a rock gets weathered, its surface area increases, so it weathers faster.

16 07_08and09.jpg Chemical and physical weathering work hand in hand when rocks are being broken down into sediment. Below, first the rock is broken apart, then the biotite is altered through chemical processes into clay.

17 07_10c.jpg Some rocks are more resistant to weathering than others. Above, some layers of rock are weathering faster than others resulting in layers that stick- out.

18 07_12.jpg Soils are formed when weathered material is altered through leaching of water through it and by biological means.

19 Humus: partially decayed organic matter. O horizon: soil layer that consists entirely of organic material (humus). A horizon: layer of soil characterized by downward movement of water; also called the zone of leaching. The water leaches material out of the soil here, and carries it deeper into the soil. A1 horizon: more organic rich layer of the leaching zone. A2 horizon: less organic/more mineral layer of the leaching zone. B horizon: layer of soil, above the bedrock, where material leached from above accumulates. Also called the zone of accumulation. C horizon: soil layer composed of incompletely weathered parent material (bedrock).

20 Horizons Organic (humus) 07_13a.jpg A 1 horizon (contains some humus) A 2 horizon (lighter colored, little humus) B horizon (accumulation) C horizon (weathered bedrock)

21 07_14abc.jpg pedalfer pedocal laterite (highly leached) Temperate humid climate Dry climate Tropical climate

22 O horizon A 1 horizon A 2 horizon Well developed soil developed on granite (similar to a pedalfer but more oxidized as evident from red color). Road cut in Georgia. B horizon C horizon From Interpreting the Earth by R. Compton Deeper part of soil (lower part of C horizon) Note weathering of granite along joints.

23 There are four types of sedimentary rocks: 1) Clastic sedimentary rock: : Sedimentary rock consisting of cemented-together detritus derived from the weathering of preexisting rock.

24 07_21c.jpg Biochemical sedimentary rocks: : Sedimentary rock formed from material (such as shells) produced by living organisms.

25 Organic sedimentary rock: Sedimentary rock (such as coal) formed from carbon- rich relicts of organisms.

26 07_25.jpg Chemical sedimentary rock: : Sedimentary rocks made up of minerals that precipitate directly from water solution.

27 The process to go from a loose sediment to a sedimentary rock takes five steps: Weathering Erosion Transportation moving of materials by wind, water or ice Deposition sediment settles out of a transporting medium Lithification transformation of loose sediment into solid rock through compaction and cementation. Lithification usually involves compaction which is the phase of lithification in which the pressure of the overburden on the buried rock squeezes out water and air that was trapped between clasts, and the clasts press tightly together.

28 source area 07_16.jpg Area of deposition source area Area of deposition This diagram shows the first 4 of the 5 processes that form sedimentary rocks. The further away from a source area sediment is carried, the finer grained it gets.

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30 07_20.jpg The further a sediment is carried away from its source area, the more mature it becomes (more cohesive)

31 In describing and classifying sedimentary rocks five physical features are needed: Clasts size Composition of the clasts what the clasts are made out of. Angularity (degree to which grains have sharp or rounded edges or corners) and sphericity (measure of the degree to which a clast approaches the shape of a sphere) Sorting (1) The range of clast sizes in a collection of sediment; (2) the degree to which sediment has been separated by flowing currents into different-size fractions. Character of cement: cement is what holds the rock together, and it can be made out of different chemicals or smaller size clasts.

32 Angular clasts are not transported as far as rounded clasts. The transportation medium is what rounds the clasts. 07_18.jpg Poorly sorted clasts are not transported as far as well sorted clasts.

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34 07_19c.jpg Conglomerate: : Very coarse-grained sedimentary rock consisting of rounded clasts.

35 07_19a.jpg Breccia: : Coarse sedimentary rock consisting of angular fragments; or rock broken into angular fragments by faulting.

36 07_19h.jpg Sandstone: : Coarse-grained sedimentary rock consisting almost entirely of quartz.

37 07_19d.jpg Arkose: : A type of sandstone that is made up of quartz and feldspar grains.

38 07_19j.jpg Shale: : Very fine-grained sedimentary rock that breaks into thin sheets.

39 07_21b.jpg Limestone: : Sedimentary rock composed of calcite CaCo3. It also represents a former proliferation of life.

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41 07_23ac.jpg Evaporites: : are chemical sedimentary rocks that form in arid environments from the evaporation of water that is rich in salt, or in a restricted basin.

42 Evaporite deposit. 07_23b.jpg

43 Dolostone: : Similar to limestone but made of CaMg[CO3]2. Mostly occurs as a reaction between limestone and Mg-rich water.

44 Travertine: : composed of crystalline calcite (CaCO3) that seeps out of the ground. Usually found to form from hot springs.

45 07_26.jpg There are several different sedimentary rock structures: Bed: : A single layer of a sedimentary rock. Bedding: : Layering or stratification in sedimentary rocks. Bedding plane: : surface between beds.

46 07_27.jpg Strata: : Several beds together. Stratigraphic formation: : A recognizable layer of a specific sedimentary rock type or set of rock types, deposited during a certain time interval, that can be traced over a broad region.

47 07_28ab.jpg Ripple marks: : small elongate ridges that form on a bed surface at right angles to the direction of current flow.

48 07_28c.jpg Modern ripples that are occurring at a beach.

49 07_28d.jpg Ripple marks that are occurring in a rock along a bedding plane.

50 07_29a.jpg master bed Cross beds: : form as a sand dune or ripple migrates in the direction of flow.

51 07_29b.jpg Cross beds in sandstone located in Zion national park

52 07_31.jpg Mud cracks: : hexagonal plates that curve up at their edges. The result of water drying after a clay rich sediment is deposited. Occurs on the surface of beds.

53 Scour marks: : small troughs abraded out of a bed by sediment in water as it flows over that surface. The marks are in the same direction as the current flow. These also occur on bedding surfaces.

54 07_30.jpg Turbidity currents are formed when an earthquake, storm, or underwater landslide causes sediment of different sizes to mix together.

55 07_30a.jpg Once the turbidity currents begin to settle out of the water, the larger material will settle out first, and the finest grained material will settle out last. This is called graded bedding.

56 07_30b.jpg A real world example of a graded bed formed by a turbidite.

57 There are a series of sedimentary environments: : conditions under which a sediment was deposited. They can be broken up into nonmarine (formed on land) and marine (formed under water). Examples of nonmarine sedimentary environments are: 1) Glacial: : areas where it is very cold and snow and ice are present

58 07_32b.jpg 2) Mountain stream: : fast moving, turbulent water with the ability to carry large boulders.

59 07_32c.jpg 3) Alluvial fan: : where stream empty out onto plains.

60 07_33a.jpg 4) Lakes: : fine clays are deposited in still bodies of water.

61 5) Sand-dune: : in deserts with high winds.

62 07_33b.jpg 6) river environments: : sand silt and clay get deposited by the meandering rivers as they carve flood plains.

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64 Examples of marine sedimentary environments are: 1) Marine delta: : mouth of a river where sediment empties into a sea or ocean.

65 2) Coastal beach: : where the oceans/seas meet the land. Low energy, lots of sand.

66 3) Shallow-marine: : just off shore, the water is not very deep. Sand to silt size particles.

67 07_35a.jpg 4) Shallow-water carbonate: : shallow water settings just off shore, where coral reefs are forminf.

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69 07_36b.jpg 5) Deep water deposits: : very fine grain sediment.

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71 Glacial Mountain stream Lakes Alluvial fan Sand-dune river Coastal Shallow-water Marine delta Deep water

72 07_37a.jpg transgression: : The inland migration of shoreline resulting from a rise in sea level.

73 07_37b.jpg regression: : The seaward migration of a shoreline caused by a lowering of sea level.