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Part 1 Sedimentary Rocks

Lecture 6-7 Sedimentary and Metamorphic Rocks. Part 1 Sedimentary Rocks. Kyanite, Sillimanite, and Andalucite. What is a sedimentary rock?. Sedimentary rocks result from mechanical and chemical weathering Comprise ~ 5% of Earth’s upper crust About 75% of rocks at surface

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Part 1 Sedimentary Rocks

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  1. Lecture 6-7Sedimentary and Metamorphic Rocks Part 1 Sedimentary Rocks Kyanite, Sillimanite, and Andalucite

  2. What is a sedimentary rock? • Sedimentary rocks result from mechanical and chemical weathering • Comprise ~ 5% of Earth’s upper crust • About 75% of rocks at surface • Contain evidence of past environments • Record how sediment is transported • Often contain fossils

  3. What is the economic importance of sedimentary rocks? • They are important for economic reasons because they contain • Coal • Petroleum and natural gas • Iron, aluminum, uranium and manganese • Geologists use them to read Earth’s history

  4. Cementation • Precipitation of chemicals dissolved in water binds grains of a sediment together. • After the cements solidify, compaction drives out the excess water. • Important part of Lithification • Remember where cements come from?

  5. Types of sedimentary rocks • Chemical rocks – sediment from ions that were once in solution • Detrital rocks –sediment transported as solid particles

  6. Detrital sedimentary rocks • Constituents of detrital rocks can include • Clay minerals • Quartz • Feldspars • Micas • Particle size is used to distinguish among the various types of detrital rocks

  7. Detrital sedimentary rocks • Mudrocks: less than .063 mm • 1. Mud: small particles easily kept in suspension • Settles in quiet water • Includes Shale: mud-sized particles <.004 mm deposited in thin bedding layers called laminae Most common sedimentary rock 2. Larger mudrock grains called silts silt-sized particles .004-.063 mm Gritty grains can be felt

  8. Detrital sedimentary rocks • Sandstone • Made of sand-sized particles larger than .063 mm and less than 2mm • Forms in a variety of environments • Sorting, angularity and composition of grains can be used to interpret the rock’s history • Quartz is the predominant mineral (due to its durable nature)

  9. Detrital sedimentary rocks • Conglomerate and breccia • Both composed of particles > 2mm in diameter • Conglomerateconsists largely of rounded clasts. Rounded pebbles in high velocity areas • Breccia is composed of large angular particles Breccia is made of shattered rock that accumulates at the base of a cliff

  10. Energy • Coarse sediments are deposited in high energy (fast water) environments such as under breaking waves at the beach, or in the beds of fast streams. • Fine sediments are deposited in low energy environments, e.g. the slow water of deep lagoons, the abyssal plain, etc.

  11. Chemical sedimentary rocks • Precipitated material once in solution • Precipitation of material occurs two ways: • Inorganic processes: the minerals precipitate out of water • Organic processes: animals and plants precipitate the minerals to use as shells or skeletons http://www.ucmp.berkeley.edu/collections/micro.html

  12. Common chemical sedimentary rocks • Limestone • Most abundant chemical rock • Made of the mineral calcite CaCO3 • Marine biochemical limestones form as coral reefs,coquina (broken shells), and chalk (microscopic organisms) • Inorganic limestones include travertine(caves) and oolitic limestone (Bahamas)

  13. http://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpghttp://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpg • Common chemical sedimentary rocks • Evaporites • Evaporation triggers deposition of chemical precipitates • Examples include rock salt and rock gypsum

  14. Chemical Sediments: Coal

  15. Sedimentary environment determines roundness sorting, mineral diversity 6_5 Character of detrital sediments depends on time, distance, and energy. For example, in streams: Particles are large and irregular, and consist of a variety of lithologies, including the least resistant. Particles are mid-sized and of intermediate sphericity, and include resistant and nonresistant lithologies. Particles are small and nearly spherical, and consist mainly of the most resistant lithologies, such as quartz. HIGHLANDS LOWLANDS NEAR-COASTAL

  16. Graded bedding Floods change the local conditions 6_6 Fine-grained sediment On floodplain Older sediment 1 Pre-flood Bounders on bottom, sands and muds suspended Flood water Erosion of uppermost fine-grained sediment 2 Flood stage Waning flow Fine-grained above Coarse-grained below Bedding plane 3 Post-flood

  17. Sedimentary Facies • Different sediments accumulate next to each other at same time • Each unit (called a facies) possesses a distinctive characteristics reflecting the conditions in a particular environment • The merging of adjacent facies tends to be a gradual transition

  18. Some Facies in an oversimplified drawing Abyssal Ooze Stillwater muds Nearshore sands

  19. Strata- Bedding Planes

  20. Slabs of eroding sandstone with ripple marks

  21. Cross Beds areripples in cross section • Irregularities lead to ripples, dunes, sand bars. • In cross section these look like lines at an angle to the horizontal – “cross beds” • Ripples can indicate direction of air or water flow if asymmetrical, a tidal environment if symmetrical. Size and shape indicate fluid velocity.

  22. Cross bedding in Sand Dune deposits Navaho Sandstone Sandstone deposited in ancient sand dunes Frosted Grains, well sorted

  23. Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood. Useful for right-side up

  24. Terms for Marine (i.e. Ocean) Environments and some characteristic sediment facies 6_27 Continental slope Muds Continental shelf Sands Shallow marine Abyssal Plain Turbidite Graded Beds Ooze Deep marine Define Graded Beds Submarine volcanoes

  25. Fossils are traces of prehistoric life generally preserved in sedimentary rock

  26. Dinosaur footprint in mudstone

  27. End of Sedimentary Rocks

  28. Part 2 Metamorphism and Metamorphic Rocks

  29. Metamorphism • … is the transformation of rock by high temperatures (heat) and pressure • Metamorphic rocks are produced by transformation of: • Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks • These are the source of many important minerals – Talc (lubricant, insulators, refractories), Corundum, Garnet (abrasives), Kyanite (ceramics), Micas (insulators), Chrysotile (“asbestos” for fireproofing), etc., etc.

  30. Minerals do not melt during metamorphism Sedimentary rock 0km Metamorphic rock Igneous Sediment rock 10 km ~200ºC Sedimentary rock Metamorphismoccurs between about 10 and 50 km of depth Metamorphism Increasing depth and temperature 50km Melting ~800ºC

  31. What causes metamorphism? 1. Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Increasing Heat with Depth

  32. What causes metamorphism? • 2. Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially • All Directions = “Confining Pressure” • Differential = “Directed Pressure”

  33. Origin of pressure in metamorphism (Burial) (Convergent Margin)

  34. Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Source:Kenneth Murray/Photo Researchers Inc.

  35. Main factors affecting metamorphism 3. Parent rock • Metamorphic rocks usually have the same chemical composition as the rock they were formed from. • Different minerals, but made of the same atoms. • Exception: water carries in new atoms and removes others. Important at MOR and in subduction zones.

  36. Metamorphic Settings • Three types of metamorphic settings: • Contact metamorphism – due heat from adjacent rocks • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes: • Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated • Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes - foliated

  37. Contact metamorphism Produced mostly by local heat source

  38. 2.Hydrothermal Metamorphism • Due circulation of water near Magma • Important at mid-ocean ridge

  39. Hydrothermal Metamorphism

  40. Metamorphism in a Subduction Zone Shallow Lithosphere Near trench Deep Lithosphere

  41. Metamorphic Grade and Index Minerals • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

  42. Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form Notice Quartz and Feldspars are useless Note Quartz and Feldspar are not index minerals: Why? Some index minerals give us temperature info

  43. Some Useful as Thermometers and Pressure Gauges Sillimanite Kyanite Polymorphs of Al2SiO5 Andalusite

  44. 7_21 CANADA New England Dynamothermal Metamorphism Mapped by index minerals  MAINE Augusta CANADA  U.S.A. Montpelier NEW HAMPSHIRE VERMONT  Concord ATLANTIC OCEAN  Boston  Albany MASSACHUSETTS  NEW YORK  R.I. Hartford  Providence Binghamton CONNECTICUT Unmetamorphosed y e  Chlorite/muscovite zone Low grade l a l PENNSYLVANIA v  Biotite zone Scranton Long Island Garnet zone Medium grade t f Staurolite zone i r NEW Newark High grade Sillimanite zone JERSEY Increasing pressure and temperature LOW GRADE DIAGENESIS HIGH GRADE MELTING INTERMEDIATE GRADE Chlorite and muscovite Biotite Garnet Staurolite Sillimanite

  45. Common metamorphic rocks • 1. Nonfoliated rocks • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Forms in intermediate T, P conditions

  46. Common metamorphic rocks • Nonfoliated rocks (cont.) • Marble • Coarse, crystalline • Parent rock usually limestone • Composed of calcite crystals • Fabric can be random or oriented

  47. Change in metamorphic grade with depth Metamorphism of a mudstone Increasing Directed Pressure and increasing Temps

  48. A mica garnet schist Definition: Schist Garnets are abrasives, long lasting bearings, and jewels

  49. Gneiss displays bands of light and dark minerals

  50. Development of foliation due to directed pressure Granodiorite Gneiss

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