Lecture5 - What is a sedimentary rock? - ex. of sediments -...

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Unformatted text preview: What is a sedimentary rock? - ex. of sediments - basically any unconsolidated materials at the earth's surface 1 What is a sedimentary rock? Sediments - rock fragments that settle & accumulate in layers after being transported or precipitated Latin sedimentum – to settle • originates from mechanical / chemical breakdown • 5% (by volume) of Earthʼs outer 15 km & 75% of exposed rocks • contain evidence of past environments • tectonics, climate & life (fossils) preserved in sedimentary rock - 2 Sedimentary process Types of sedimentary rocks Rock types are based on the source of the material • detrital/clastic rocks: transported sediment as solid particles • chemical rocks: sediment that precipitates from solʼn or extracted from water by organisms (converted to shells/skeletons) Physical features in rock tell how seds were deposited & environmental conditions of deposition 1. weathering (breakdown) 2. erosion (mass wasting) rock sediment 3. transport 4. deposition 5. lithification (compaction & cementation) - detrital - ex. sand dune, delta into ocean, river sediment -- all would be turned into clastic or detrital rocks relocation rock - to form a clastic sedimentary rock, need a rock to begin with -- broke down... became another clastic sedimentary rock 3 Weathering Physical & chemical changes (weakening & break down) that occur in rocks when they are exposed to the atmosphere & biosphere Physical breakdown Factors which control weathering: 1. parent rock (e.g. granite vs salt) 2. climate (temperature and rain; e.g. desert vs rainforest) 3. soil – why? 4. time - granite makes up most of the mountain ranges in Cali... etc. - hot or warm temps will accelerate chemical weathering - precipitation will increase weathering processes - biological organisms (ex. roots of trees...) in soil can increase weathering - thick covering of soil will slow down weathering - time - more time, more weathering 4 Clastic sedimentary rocks Chemical breakdown Chief constituents of clastic rocks: • Clay minerals • Quartz • Feldspars • Micas ..those that are stable on the Earth's surface - limestone headstone - acid rain... - red colour - rocks oxidized - iron oxidized into rust - clastic rocks main constitiuents are: potassium feldspar, muscovite mica, quartz -- all stable at the earth's surface - 5 Classification of clastic rocks Texture (reflects process): • grain size – related to properties of parent rock – proportional to energy level of transport medium (hi-E = smaller grains) Classification of clastic rocks Texture (reflects process): • grain shape – angular to rounded – proportional to energy level or distance traveled (more collisions = rounded) • grain sorting – process by which a transport medium ʻselectsʼ particles of different sizes, shapes, densities – well-sorted to poor-sorted typically lighter, smaller = deposited farthest & last = more rounded - composition and their structure - texture reflects the process of how the sedimentary rock was formed ---grain size - igneous rock with coarse grains - if hasn't transported far, then it will still have coarse grains - hi-E -- ie. constant weathering - waves pounding on rock on beach ---grain sorting - a well-sorted sedimentary rock would be one where the grains are all the same size -poor sorted will be diff sizes altogether 6 Classification of clastic rocks Transportation & deposition leads to characteristic rock textures • Mudstone (shale & siltstones) – Clay to silt-sized particles in thin layers – Deposited in calm waters (lakes, lagoon, deep ocean) – >50% of sed rocks – Variable color: red (Fe & O rich), green (Fe & O poor), black (not enough O to decompose organic matter - velocity vs. grain size graph - faster moving speeds can move larger material - mudstone made of mud - black mudstone - organic matter that hasn't broken down yet 7 • Sandstone (graywacke, arenite, arkose) – Composed of sand-sized particles; mostly qtz – Forms in a variety of environments (rivers, beaches, glaciers, mountains, deserts..) - sandstone - medium-sized clastic sedimentary rocks • Conglomerate and breccia – particles > 2 mm in diameter in a finer grained matrix – conglomerate - rounded gravels – breccia - angular particles - very coarse - large pieces in finer grained material 8 Chemical sedimentary rocks Consist of precipitated material that was once in solution Precipitation of material occurs in two ways • Inorganic processes (precipitate from water .. sea or fresh) • Organic processes (biogenic origin) • Conglomerate and breccia – particles > 2 mm in diameter in a finer grained matrix – conglomerate - rounded gravels – breccia - angular particles end of clastic sedimentary rocks - only one slide on composition, usually focus on texture - mineral does not necessarily NEED to be inorganic -- some ex.'s: bones, teeth... 9 Chemical sedimentary rocks fossiliferous limestone Common chemical sedimentary rocks • Limestone – 10-15% of sed rocks – composed chiefly of the calcite- CaCO3 – marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) – inorganic limestones include travertine & speleothems - mostly found in Ottawa limestone coquina - limestone - pretty smooth - coquina - kind of looks like conglomerate - most of the statues are composed of limestone - easy to sculpt --- BUT, very susceptible to acid rain because of the calcite that is present 10 Chemical sedimentary rocks Factors controlling CaCO3 precipitation: 1) controlled by solubility of CaCO3 (proportional to CO2 in water) 2) cold water has more CO2 as water warms CaCO3 precipitates sediment size = ~ 2.8 cm 3) also effected by agitation of water (tides), photosynthesis (uses CO2), and water depth sediment size = ~ 2.0 cm - 2) this is why you won't get warm beaches in the Maritimes - What does this difference in shell size tell us about energy level fluctuations? NOTHING - the critter that secreted the shell was bigger - composed of the mineral calcite 11 Chemical sedimentary rocks Common chemical sedimentary rocks • Chert marine; ex. sponges – microcrystalline quartz – silica? ..skeletons of small animals – varieties include flint and jasper (banded form is called agate) Chemical sedimentary rocks Common chemical sedimentary rocks • Evaporites – evaporation triggers deposition of inorganic chemical precipitates – requires restricted basin + warm temps (Great Lakes salt, Mediterranean Sea) – examples include rock salt & rock gypsum - don't have lot of inflow and outflow in basin - 12 Chemical sedimentary rocks Fossil Fuels Common chemical sedimentary rocks • Coal derived from the sun’s energy via decayed organic matter Principle energy source in the world! organic carbon oxidized to form heat energy higher C content = higher energy = cleaner burning - 13 Fossil Fuels The Process of Forming Coal Coal From the decay and compression of land plants rich in resins, waxes, and lignins Organic matter accumulated in swamps (anoxic environment) -warm, moist, calm, little O2, rapid sedimentation Burial results in peat-lignite-bituminous and anthracite coal Widespread distribution, e.g. Carboniferous-age (c. 300 Ma) units in N.S. & B.C. Plentiful supply but pollution?? Peat - Lignite Bituminous Coal Anthracite - first stage of making coal: Peat - anthracite - really good stuff - low sulfur content, lots of carbon content 14 U.S. Coal Fields Fossil Fuels Oil & Gas Unlike coal - derived from the remains of marine plants and animals rich in proteins lipids & carbohydrates Formation complex & incompletely known Deposition of sediments rich in plant & animal remains under anoxic conditions Burial over millions of years produces hydrocarbons dt/dp must be correct petroleum window - MAIN DIFF - derived from marine plants and animals --- COAL IS derived from terrestrial plants and animals - 15 Oil & Gas Petroleum Window: dt/dp - middle triangle forms the the petroleum window Fossil Fuels Oil & Gas Cont’d. Most petroleum formed in shales from where it migrates upward Reservoirs must be porous & permeable Oil-pools!! Trap (anticline, fault or stratigraphic) that is an impermeable seal is necessary to hold fluid -shale - simple mudstone - 16 Types of Oil Traps Fossil Fuels Oil Shale Contains kerogen, a waxy substance from which fuels can be derived Found in abundance in western US Deposited in vast shallow lakes c. 50 Ma Exploitation energy intensive Gas Shale ..emerging industry.. - not going to talk about until three lectures later 17 Fossil Fuels Tar Sands Estimated worldwide oil reserves by region at the end of 2000 Viscous and “immature” oil that cannot be pumped Also found in shales and limestones Alberta has major resources: ~10% can be mined; remainder heated and pumped - immature hydrocarbons that have not broken down enough - 18 What is a sedimentary rock? Economic considerations: • coal • petroleum & natural gas • sources of iron, aluminum, & manganese • evaporites – salt, gypsum • phosphates – fertilizers Science considerations: • fossils (ancient life) • geologic histories / reconstructions find in textbook!!! _______________________________________________ 19 Metamorphism Metamorphism Greek meta (to change) & morphe (form) lo-T equilibrium = mineral stability hi-T Rocks do not suffer metamorphism, they enjoy it! - very little economic consideration - in Gatineau - mostly metamorphic rock - very uncommon on surface of Earth Process by which T, P, & chemical reactions alter mineral content & structure of pre-existing rock (w/o melting .. solid-state only) Occurs when T & P break some atomic bonds Metamorphic rocks produced from: • igneous rocks • sedimentary rocks • other metamorphic rocks - Need a preexisting rock to form a metamorphic rock just like clastic - NOT MELTING - bonds still there 20 Metamorphism All metamorphic rocks start as some other rock, so 3 questions: 1) What is rock now (mineralogy, texture)? 2) What was it before (protolith)? 3) What caused the change (tectonics)? - 1) what is it composed of, and texture? - 2) prior to change; protolith - "before rock" - 3) fluid involvement... Controls of metamorphism 1) Temperature • Not absolute T, but change in T • Rocks under new environmental conditions come into equilibrium with surrounding conditions • Recrystallization results in new, stable minerals (at those new conditions) • Geothermal gradient varies from 10°C/km in stable parts of continents to >100°C/km in tectonically active areas - happy at high temps, but brought onto surface will weather... etc, breaking down rock - not at equilibrium - new minerals constantly forming because of temp changes - 21 Controls of metamorphism Development of foliation due to confining pressure 2) Pressure (stress) lithostatic/confining • pushes on rock equally from all directions • rock becomes more dense; no shape change • increases at a rate of 0.3 kbar/km depth directed • pressure is greatest in one direction • change in shape • new mineral growth aligns perpendicular to P = foliation - temp prob most dominant because it allows us to change composition random arrangement - 1) - lithostatic/confining/hydrostatic 2) second rock has foliation - directed minerals ----equal pressure from all sides perpendicular to pressure direction - directed --- horizontal pressure in depicted image 22 Pressure in metamorphism Controls of metamorphism 3) Fluid activity two most abundant fluids crust • H2O & CO2 in Earth's • enhances migration of ions • increases potential for reaction All of these control the process (degree) of metamorphism, but the composition is controlled by the protolith - 23 Example of metamorphism sandy limestone metamorphosed to marble Example of metamorphism At 200°C, sandy limestone is made of: quartz & calcite At 600°C, minerals no longer stable together - each can exist alone, but together they form a new mineral: wollastonite SiO2 + CaCO3 = CaSiO3 + CO2 (volatile) - less than 200 degrees, limestone is happy - at equilibrium - two ingredients - one new mineral --- rearranged the mineral system 24 Metamorphic environments Example of metamorphism SiO2 + CaCO3 = CaSiO3 + CO2 -relative amounts of elements do not change, only restructuring and regrowth -CO2 is released & can leave rock (usually involves decarbonation; or H2O released in dehydration) -if CO2 leaves, reaction cannot go back when rock cools (cannot revert to qtz + cc unless CO2 is added) @ plate boundaries! - plate boundaries are where most of these processes occur 25 Metamorphic environments Impact metamorphism Orogenic metamorphism Contact metamorphism Metamorphic environments 1) Regional metamorphism Contact metamorphism Orogenic metamorphism a) Burial - T & lithostatic P within sedimentary basins (Mississippi Delta, Bay of Bengal) • progressive increase in P exerted by the growing pile of overlying sediments & sedimentary rocks and the increase in T with increased depth of burial Seafloor metamorphism Burial metamorphism - - large regional extent 26 Metamorphic environments lithification Diagenesis grades into burial metamorphism at depth Burial metamorphism: depths of 2-3 km T of 100 to 200°C Metamorphic environments 1) Regional metamorphism b) Dynamothermal (orogenic) - occurs in response to P&T changes induced by large-scale tectonics; directed P (foliated rocks) (convergence: Himalaya, Appalachians) Burial metamorphism defines the economic basement of oil & gas resources (organic matter is converted to methane & CO2 at >130°C - not crude oil or natural gas) - where sediments are actually turned into rock ---- very shallow depth and low temps - orogenic - happens in mountain belts 27 Metamorphic environments Regional Metamorphism Metamorphic environments 2) Contact metamorphism • response to T change produced by intrusion of magma into cooler rock • local in scale • an aureole, zone of alteration, forms in the rock surrounding the magma contact aureole metamorphic grade 28 Metamorphic environments Metamorphic environments 3) Hydrothermal metamorphism • a.k.a. metasomatism • Chemical alteration caused when hot, ion-rich fluids circulate through fissures & cracks that develop in rock • Most widespread along the axis of the mid-ocean ridge system, thus… most of ocean crust is metamorphosed Influx of cold seawater 1 2°C 7 1 1 2 3 Alteration of oceanic crust at <150°C 4 pH drops to ~3 (acidic) 5 Heating of seawater to ~450°C 150°C 6 Leaching of Cu, Zn, Fe, Au and S from surrounding rocks 400-450°C 7 Formation of hydrothermal precipitates in contact with cold seawater 2 60°C 3 6 4 5 1200°C Magma - 4) this is where we get the concentration of the elemental metal compounds, ex. gold, copper 29 Classification Classification of metamorphic rocks based on texture Foliated rocks: • Degree of metamorphism reflected in texture -metamorphism increases grain size (in the absence of deformation - directed P; deformation decreases grain size) • Composition of rock reflected in mineralogy - composition of rock is controlled by the protolith • Size of crystals • Nature of foliation • Degree to which light & dark minerals are segregated Granoblastic rocks: • Because granoblastic rocks are non-foliated, classification is based exclusively on mineral composition Porphyroblasts may occur in both, foliated & granoblastic rocks (name of the porphyroblastic mineral is added to the rock name) - foliated rock - texture of rock is aligned because of directed pressure - Granoblastic - don't have texture of rock that is aligned - Porphyroblast - bigger crystal in fine matrix 30 Classification New metamorphic minerals may grow into large crystals surrounded by fine matrix – these crystals are called porphyroblasts (garnet & staurolite) Generally, metamorphic rocks are classified based on the occurrence of foliated & granoblastic textures Some metamorphic rocks do not show foliation, due to the lack of platy minerals Granoblastic crystals that grow in interlocking equant (equidimensional) shapes; typical of monomineralic rocks: - calcite = marble - quartz = quartzite - simple igneous rocks (metabasalt) EXAM UP TO HERE 31 Classification Granoblastic metamorphic rocks Hornfels - high-T contact metamorphic rock of uniform grain size that has undergone little deformation • Typified by a granular texture, but commonly contains some pyroxene & mica Classification Granoblastic metamorphic rocks Quartzite - very hard, nonfoliated white rock derived from quartz-rich sandstone • Massive, may contain preserved bedding • Commonly contains thin bands of slate or schist (relicts of clay or shale layers) NOT RESPONSIBLE FOR THIS PAGE AND ONWARD!! 32 Classification Granoblastic metamorphic rocks Marble - metamorphic products of T & P acting on limestone or dolostone • White & pure marbles (e.g. Carrara marbles, Italy) are prized by sculptors • White & smooth, even textures, banded, or mottled Classification Metamorphism imprints new textures on the rock that it alters (sizes, shapes, & arrangement of minerals) Most common textural feature is foliation Platy minerals: mica -set of flat or wavy parallel planes produced by directed pressure / deformation -minerals are rotated or recrystallized Needle-like minerals: hornblende Orientation of platy minerals is perpendicular to the main direction of force 33 Classification Classification Foliated metamorphic rocks Foliated metamorphic rocks Slate - foliated rock that forms at low T & P Phyllite - forms at slightly higher T & P than slate • Very fine-grained – • Characterized by a individual minerals cannot be seen w/o a microscope 200-300°C glossy sheen resulting from mica crystals that are larger than those in slates • Typically forms from shales or volcanic ash • Used for roofing tiles • In contrast to shales, 300-400°C phyllite cannot be split into sheets 34 Classification Classification Foliated metamorphic rocks Schist - more intensely metamorphosed rock with platy crystals that are large enough to be visible to the naked eye slate • Minerals are typically phyllite segregated in lighter & darker bands 400-600°C • Characterized by a pervasive coarse, wavy foliation referred to as schistosity 35 Classification Classification Foliated metamorphic rocks Gneiss - coarse grained rock that forms under high T & P • Consists of light & dark >650°C layers resulting from segregation of minerals • Granular : platy minerals is higher than in schists • Poor schistosity & little Biotite schist Garnet-mica schist tendency to split 36 Classification Foliated metamorphic rocks Migmatite - forms at very high T where parts of the precursor rock begin to melt feldspar porphyroblasts • Typically highly deformed & contorted – contain many veins, small pods & lenses that represent solidified partial melts >750°C augen gneiss 37 Classification Mineralogy & Texture migmatite formation Granoblastic oatmeal cookie Foliated peanut butter cookie Chocolate chip (porphyroblastic) cookie 38 ...
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