Lecture 12_Sedimentary Rocks Part II_Students

Lecture 12_Sedimentary Rocks Part II_Students - Sedimentary...

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Unformatted text preview: Sedimentary Rocks. II. Figure 7.1 Sedimentary rocks form in layers: Oldest layers at the bo>om, youngest at the top = Principle of SuperposiCon Sedimentary Structures •  Layered like a cake: “straCfied.” –  Arranged in planar, close-­‐to-­‐horizontal “beds.” •  A series of beds are referred to as strata. •  FormaCon: Strata recognized on a regional scale. Sedimentary Structures Sedimentary Structures •  Bedding reflects changing condiCons during deposiCon. •  These can be changes in… –  Energy condiCons, and hence, grain size. DeposiConal Environments 1.  Terrestrial environments 2.  TransiConal environments (land meets sea) 3.  Marine environments Terrestrial: Glacial deposits •  Due to movement of ice. •  Ice carries and dumps every grain size. •  Creates glacial Cll; poorly sorted gravel, sand, silt, and clay. Terrestrial: Mountain streams •  Water carries large clasts during floods. •  During low flow, these cobbles and boulders are immobile. •  What type of rock? Course conglomerates, breccia Terrestrial: River & stream deposits –  Channel deposits •  Coarse, rounded gravel and sand –  Levees •  Fine sand or silt –  Floodplains •  Silt and clay Modern stream Ancient stream deposit (conglomerate) Terrestrial: rivers & streams Modern river and floodplain Ancient floodplain deposit (siltstone) Terrestrial: Desert environments Modern sand dune Ancient sand dune (sandstone) Cross bedding •  Created by ripple and dune migraCon. –  Sediment moves up the gentle side of a ripple or dune. –  Sediment piles up, then slips down the steep face. Terrestrial: Lake environments •  Very diverse! Depends where you are in the Modern lake… lake –  Gravel, sand –  Fine mud and silt Ancient lake sediments (siltstone –mudstone) TransiConal environments: Where land meets sea TransiConal: Deltas –  Velocity Drops, so does the sediment! –  Deltas grow over Cme, building out into the basin Beaches –  Deposited in surf zone. •  High energy, waves. •  A common result? Well-­‐sorted, well-­‐rounded medium sand. Modern sandy beach Ancient sandy beach (sandstone) Beach sandstones may preserve ripples. Marine environments •  Closer to shore, more course deposits •  Further from shore, finer grain sediments Modern marine sand Ancient marine sand (sandstone) Shallow marine carbonates •  Found in warm, tropical locaCons (30 °N to 30 °S) •  Warm, clear, shallow, normal salinity, low CO2 •  Biochemical calcite or aragonite that makes up the “hard parts” of marine plankton, corals, & other marine invertebrates (e.g. clams, oysters) Archer, 2000 Modern reef Ancient reef (limestone) ConCnental slope – transiCon to deep sea •  Turbidite deposits – submarine landslides! Deep sea sediments •  Abyssal plain – deep ocean floor •  Very fine grained sediments DistribuCon of deep-­‐sea sediments Where might these rocks have formed? A.  Alluvial fan B.  Desert C.  Coral reef D.  Deep marine E.  Fluvial (river) environment Fossiliferous limestone A.  B.  C.  D.  E.  Alluvial fan Desert Shallow marine Deep marine Fluvial (river) environment Conglomerate A.  B.  C.  D.  E.  Alluvial fan Desert Shallow marine Deep marine Fluvial (river) environment Sandstone A.  B.  C.  D.  E.  Alluvial fan Desert Shallow marine Deep marine Fluvial (river) environment Sandstone, siltstone, & shale A.  B.  C.  D.  E.  Alluvial fan Desert Shallow marine Deep marine Fluvial (river) environment Four steps required to form sedimentary rocks 1.  Weathering –  Produces raw materials 2.  Erosion & Transport by wind, water, or ice –  Affects shape, composi5on, and sor5ng (size distribu5on) of par5cles 3.  DeposiCon –  Caused by a change in energy of transport system 4.  LithificaCon –  Transforma5on of loose sediment in to rock LithificaCon •  Two steps: –  1. CompacCon. Overlying pressure squeezes out water and air between clasts. –  2. CementaCon. If the rock gets buried deep enough… •  METAMORPHOSIS!! Why studying metamorphic rocks is cool: •  Can tell us about the tectonic history of a region •  Impacts global processes (ever wonder where the CO2 in volcanoes comes from?) ...
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  • Winter '15
  • Geology, Sandy, coral  reef, A.  Alluvial

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