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Chapter 6 Sediments and !Sedimentary Rocks!Image source: NOAA-GEOS-12 (1 km)!Hurricane Ivan: 09-16-04, 15:15 UTC!rock cycle!sediment: (sedimentum: settling)!easy to see sedimentation in action!• fragments of solid material that settle and accumulate!• fragments settle in layers after being transported!fragments are products of weathering and erosion!sedimentary rock: “lithified” sediment; “lithos” -- rock!• 75% of all rock exposed on Earth’s land surface!• principal source of coal, oil, natural gas, cement, Al, Fe, groundwater!• composition is weathering products of rocks or biological matter!detrital sediment: fragments of pre-existing rocks!3 types of sediments and sedimentary rocks:!chemical sediment: precipitated from dissolved ions in water!organic matter: formed from accumulation of biological debris (swamps)!relationship to Earth’s systems!• atmosphere !-- most sediments produced by weathering in air!-- sand and dust transported by wind!• hydrosphere !-- water is primary agent in sediment production, ! transportation, deposition, cementation, and ! formation of sedimentary rocks!• biosphere !-- biological activity key to formation!-- petroleum and coal resources have biological origin!sediment: loose, solid particles!-- weathering and erosion of pre-existing rocks!-- chemical precipitation from solution, including !"secretion by organisms in water!originate from:!sediment is classified by particle size!• boulder > 256 mm!• cobble: 64 to 256 mm!• pebble: 2 to 64 mm!• sand: 1/16 to 2 mm!• silt: 1/256 to 1/16 mm!• clay: < 1/256 mm!turning sediment into sedimentary rock!transportation!movement of sediment away from its source,! typically by water, wind or ice!rounding occurs by abrasion during transport! “particles knocking against each other”!sorting occurs as sediment is separated by size! by transporting agents, such as water!--sediment size decreases with increased transport distance--!angular fragments!rounded beach boulders!sediments may be angular or rounded!particle size sorting by rivers !headwaters! mouth!grade = change in elevation/length!as flow slows down, river can no longer carry larger particles!current velocity decreases!Gravel!River!Plain!Mountains!Sand!River!Plain!Mountains!Gravel!Plain!Silt and clay!Mountains!Sand!River!Plain!Gravel!gravel/boulders at headwaters!normally!deposited!in or near!steep!mountains!at river!headwaters!sand/silt downstream, closer to mouth of river!Distance from Source!sediments may be poorly sorted or well sorted!wind: most selective transporting agent!"-- only picks up sand thus deposits are well sorted!windblown sand forms dunes!note well sorted and rounded grains!glaciers and flooding rivers are unselective transporting agents!poorly sorted river sediment!poorly sorted glacial sediment! flooding rivers -- fast currents!…transport all sizes…!deposition!• settling of transported material!• accumulation of chemical or organic sediments,!"particularly in water!turning sediment into sedimentary rock!sediment transported!from canyon and!deposited at !canyon mouth!at the!base of the slope!sediments and sedimentary rocks form in many settings!environment of deposition is setting of deposition!preservation!• sediments must be preserved to turn into rock!• burial by additional sediments deposited on top!turning sediment into sedimentary rock!lithification!• sediments must be hardened into rock!• compaction and cementation occur!turning sediment into sedimentary rock!water is expelled from pore space! other fluids cement grains!Basic Clastic Rock Types!• Breccias: angular particles!• Conglomerates: rounded particles!• Sandstones!– Quartz sandstone: dominated by quartz grains!– Arkose sandstone: composed of qtz & fsp grains!– Graywacke: dominated by lithic (rock) fragments!• Shales: mud and silt particles!Coarse Breccia!NB Angular fragments!Conglomerate!NB Rounded particles & dissimilar lithologies!Quartz Sandstone - > 90% qtz!Arkose Sandstone - Feldspar + Qtz!Graywacke - lithic fragments in dark fine grained matrix!Poorly sorted, “dirty” sandstone!Sorting!can be!based on!particle!size and/or!composition;!It yields !important !constraints!on provenance!of the clastic!particles.!Shale outcrop, Pittsburgh, PA!Shale hand specimens!NB very fine grained and thin laminations!Lithification and compaction of shale!NB volume loss during compaction!Chemical Sedimentary Rocks!• Carbonate Rocks (e.g. CaCO3)!• Chert (e.g. SiO2)!• Evaporites (e.g. NaCl; CaSO4•2H2O)!Carbonate atoll, Bora Bora!Patch Reefs!Global Reef Distribution!Carbonate Beach - Lagoon Environments!Bioclastic Limestones!Clearly visible fossils! Fine grained carbonate mud"Coralline algae - Lagoon Environment!Fossil Rich Limestone - Coquina!Marl - Chalk (pelagic ooze)!SEM images of forams and coccoliths (CaCO3)!Photo credits: V. Pariente, J. Lundquist, D. Greig !Carbonate Bank - Shallow Currents!Oolitic Limestone!Nodular Chert "(poorly crystalline SiO2)!Redwall limestone, Grand Canyon, AZ!Bedded Chert!SEM images of diatom fustules (SiO2) - unicellular plants!From: http://www.uni-mainz.de/FB/Geøpalaeontologie/griessdiato.html!100 m! 100 m!Evaporites!Bonneville salt flats, UT!Organic Sedimentary Rocks!• Coal!Sedimentary Structures!Environments of DepositionCoal Bed - Trinidad, CO!Composed of decayed plant remains "- compressed peat bogs!Sedimentary Structures!• Bedding - series of visible layers in sedimentary rocks!– Principle of Original Horizontality!– Superposition!– Cross-cutting relationships!• Grading!• Mud Cracks!• Ripple Marks!• Fossils!Horizontal Bedding!Cross Bedding!Cross Bedding Models!Normal Grading!Grading Model!Turbidity Currents!Mud Cracks!Preserved mud cracks!Ripple Marks Model!Symmetric"Asymmetric!Tidal Flat Ripples!Current Direction"Preserved Ripple Marks!Preserved Bivalve Fossils!Vertebrate (Fish) Fossil!Dinosaur Footprint!Interpretation of Sedimentary Rocks!• Source Area!• Environment of Deposition!• Plate Tectonics and Sedimentary Rocks## !Sedimentary Formation Definitions!• Body of rock of considerable thickness!• Characteristics must be distinguishable from adjacent units!• Usually composed of one or more beds!• Often based on rock type (lithology or lithofacies)!• Contacts are the boundary surfaces between two rock units of different


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