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Petrologi 8-sedimen
 

Petrologi 8-sedimen

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    Petrologi 8-sedimen Petrologi 8-sedimen Presentation Transcript

    • Semester 2, 2007-2008 petrologi 2 SKS teori 1 SKS praktikum by: hill. gendoet hartono Senin, jam 09.50 – 10.40 jam 10.45 – 11.35
    • Sedimentary Rocks Batuan Sedimen
    • Sediments and Sedimentary Rocks Rock Cycle
    • What is a sedimentary rock? • Sedimentary rocks result from mechanical and chemical weathering • Comprise ~ 5% of Earth’s upper crust • About 75% of exposed rocks • Contain evidence of past environments • Record how sediment is transported • Often contain fossils
    • Fossil Fish - 50 Million Year Old Lakes in southern Wyoming These fish tell us the Wyoming climate 50 million years ago. Sedimentary Rock made of fine-grained mudstone.
    • Overview ► sediment production ► types of sediment and sedimentary rocks ► sediment transport and deposition ► depositional systems ► stratigraphic architecture and basins ► chrono-, bio-, chemo-, and sequence stratigraphy ► Earth history
    • Sedimentary rocks are the creation product of the creation, transport, deposition, and diagenesis of detritus and solutes solutes derived from preexisting rocks.
    • Steps Involved in Formation of Clastic Sedimentary Rock Weathering Erosion Transport Deposition Lithification --Compaction --Cementation
    • Weathering
    • Steep terrain accelerates erosion
    • Turbidity Currents
    • Ions weather out of rock, are transported by groundwater to sediment layers below 6_11 Ions transported to lake or Water enters pore ocean spaces between sediment grains Ion-rich groundwater Dissolved ions precipitate to form cement between sediment grains
    •  Diagenesis includes:  Recrystallization – growth of stable minerals from less stable ones  Lithification – loose sediment is transformed into solid rock by compaction and cementation Natural cements: calcite, silica, and iron oxide. Formed from ions in solution in water.
    • ► Diagenesis – chemical and physical changes that take place after sediments are deposited ► Diagenesis varies with composition
    • Lithification ► Compaction: As more sediments are piled on top, compaction drives out the excess water. ► Cementation: Precipitation of chemicals dissolved in water binds grains of a sediment together. ► Remember where the dissolved chemicals come from?
    • Compaction and Cementation of Clastic Sediment into Rock Overburden Sediment Compaction Cementation
    • Lithification and compaction of shale NB volume loss during compaction
    • ► Source area - the locality from which the sediment was derived: factors used to evaluate source area include rock type, environment of deposition, direction (paleocurrents) and distance from source area
    • ► Depositional environment – where sediment is deposited. It can be determined by looking at sedimentary structures (including fossils), the bed shape and vertical sequences within the sedimentary layers, and grain composition
    • Sedimentary environments & plate tectonic settings Convergent boundaries – coarse-grained clastic sediments with abundant volcaniclastic and felsic material
    • Sedimentary environments & plate tectonic settings Divergent boundaries - thick wedges of gravel and coarse sand along margins,with lake bed deposits and associated evaporite rocks possible in bottoms of rift valleys
    • What is the economic importance of sedimentary rocks? ► They are important for economic reasons Remember this because they contain when we talk about ►Coal ►Petroleum and natural gas correlation. Note how beds pinch out or are offset by faults ►Iron, aluminum, uranium and manganese ►Geologists use them to read Earth’s history
    • SEDIMENTARY ROCKS ► WEATHERING PROCESSES BREAK ROCK INTO PIECES, SEDIMENT, READY FOR TRANSPORTATION DEPOSITION BURIAL LITHIFICATION INTO NEW ROCKS.
    • CLASSIFYING SEDIMENTARY ROCKS ► ► ► ► THREE SOURCES Detrital (or clastic) sediment is composed of transported solid fragments (or detritus) of pre-existing igneous, sedimentary or metamorphic rocks Chemical sediment forms from previously dissolved minerals that either precipitated from solution in water , or were extracted from water by living organisms Organic sedimentary rock consisting mainly of plant remains
    • CLASTIC SEDIMENTARY ROCKS CLASSIFIED ON GRAIN OR PARTICLE SIZE ► Shales: finest-grained ► Sandstones: medium-grained ► Conglomerates – Breccias: coarse-grained
    • SHALES ► SHALES: finest-grained – composed of very small particles (from <0.004-0.063 mm)  50% of all sedimentary rocks are Shales  Consist largely of Clay minerals  Subcategories: Claystones; Siltstones; Mudstones  Economic value: building material; china and ceramics; spark plug housings
    • SANDSTONES ► SANDSTONES: medium-grained; particle-size (0.063-2 mm) ► 25% of all sedimentary rocks fall into this category ► Three major kinds of Sandstone, based on mineral composition and appearance:    Quartz Arenite: >90% quartz grains Arkoses : more Feldspar minerals Graywackes :quartz and feldspar grains, and volcanics ► Economic value: glass; natural reservoirs for oil, gas, and groundwater
    • CONGLOMERATES BRECCIAS ► CONGLOMERATES AND BRECCIAS : ► The coarsest of all the detrital sedimentary rocks ► Composed of particles >2 mm in diameter  Conglomerate - the particles are rounded  Breccia - the particles are angular
    • CHEMICAL SEDIMENTARY ROCKS ► TWO CATEGORIES:  INORGANIC CHEMICAL SEDIMENTARY  ORGANIC CHEMICAL SEDIMENTARY
    • INORGANIC CHEMICAL SEDIMENTARY ROCKS ► Formed when dissolved products of chemical weathering precipitate from solution ► Most common types:  Inorganic limestones and cherts: precipitates directly from seawater and fresh water  Evaporites: precipitates when ion-rich water evaporates  Dolostones: Origin is still in debate
    • INORGANIC - LIMESTONES ► Limestones - account for 10% - 15% of all sedimentary rocks formed from Calcite or Calcium Carbonate (CaCO3). ► Formed as pure carbonate muds accumulate on the sea floor ► Also formed on land:  Tufa - a soft spongy inorganic limestone that forms where underground water surfaces  Travertine - forms in caves when droplets of carbonate-rich water on the ceiling, walls and floors precipitate a carbonate rock
    • ORGANIC LIMESTONES ► Formed with calcite from marine environment: CaCO 3 shells and internal/external skeletons of marine animals ► Coquina - “crushed” shell fragments cemented with CaCO3 Chalk - made from billions of microscopic carbonate-secreting organisms Coral Reefs - Formed from the skeletons of millions of tiny invertebrate animals who secrete a calcite-rich material. Live “condo” style while algae acts as the cement to create the large structures called “reefs”. Organic Chert - formed when silica-secreting microscopic marine organisms die (radiolaria {single-celled animals} and diatoms {skeletons of singled-celled plants}) Flint - an example of an Organic Chert ► ► ► ► ►
    • ORGANIC SEDIMENTARY ROCKS ► ► Coal - Organic sedimentary rock consisting mainly of plant remains Formation:  Burial of decaying vegetation;  Increasing pressure from the overlying layers expels water, CO2 and other gases;  Carbon accumulates. ► ► ► ► Peat - formed early in the process, when the original plant structure can still be distinguished. Lignite - a more hardened form of Peat Bituminous - more pressure and more heat produce this moderately hard coal. Anthracite - the hardest coal - formed from metamorphic processes under extreme heat and pressure - Hard - Shiny - the most desired as an energy resource.
    • SEDIMENTARY ENVIRONMENTS ► Lakes ► Lagoons ► Rivers ► Ocean bottoms ► Estuaries ► Salt Flats ► Playas ► Glacial environments
    • SEDIMENTARY PROCESSES ► LITHIFICATION : ► As sediment is buried several kilometers beneath the surface, heated from below, pressure from overlying layers and chemically-active water converts the loose sediment into solid sedimentary rock ► Compaction - volume of a sediment is reduced by application of pressure Cementation - sediment grains are bound to each other by materials originally dissolved during chemical weathering of preexisting rocks ►  typical chemicals include silica and calcium carbonate.