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Chapter 6   sedimentary rocks
 

Chapter 6 sedimentary rocks

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    Chapter 6   sedimentary rocks Chapter 6 sedimentary rocks Presentation Transcript

    • Essentials of Geology 3 rd Edition Chapter 6 Norton Media Library
    • Pages of Earth’s Past: Sedimentary Rocks Prepared by: Ronald Parker , Senior Geologist Fronterra Geosciences Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén www.fronterrageo.com
    • Sedimentary Cover
      • Earth is covered by a thin ‘veneer’ of sediment.
      • The veneer caps ig neous and meta morphic “basement.”
      • Sediment cover varies in thickness from 0 to 20 km.
        • Thinner (or missing) where ig and meta rocks outcrop.
        • Thicker in sedimentary basins.
    • Sedimentary Rocks
      • Sediments are the building blocks of sedimentary rocks.
      • Sediments are diverse, as are the rocks made from them.
      • Four classes:
        • Clastic – Made from weathered rock fragments (clasts).
        • Biochemical – Cemented shells of organisms.
        • Organic – The carbon-rich remains of plants.
        • Chemical – Minerals that crystallize directly from water.
      Chemical Clastic Organic Biochemical
    • Clastic Sedimentary Rocks
      • Clastic sedimentary rocks reflect several processes.
        • Weathering – Generation of detritus via rock disintegration.
        • Erosion – Removal of sediment grains from rock.
        • Transportation – Dispersal by wind, water, and ice.
        • Deposition – Settling out of the transporting fluid.
        • Lithification – Transformation into solid rock.
      • Lithification – Transforms loose sediment into solid rock.
        • Burial – More sediment is added onto previous layers.
        • Compaction – Overburden weight reduces pore space.
          • Sand – 10 to 20%
          • Clay – 50 to 80%
        • Cementation – Minerals grow in pores, “gluing” sediments.
      Clastic Sedimentary Rocks
    • Clastic Sedimentary Rocks
      • Classified on the basis of texture and composition.
        • Clast (grain) size.
        • Clast composition.
        • Angularity and sphericity.
        • Sorting.
        • Character of cement.
      • These variables produce a diversity of clastic rocks.
    • Clastic Sedimentary Rocks
      • Clast (grain) size – The average diameter of clasts.
        • Range from very coarse to very fine.
        • Boulder, cobble, pebble, sand, silt, and clay.
        • With increasing transport, average grain size decreases.
    • Clastic Sedimentary Rocks
      • Clast composition – The mineral makeup of sediments.
        • May be individual minerals or rock fragments.
        • Mineral identities provide clues about…
          • The source of the sediment.
          • The environment of deposition.
    • Clastic Sedimentary Rocks
      • Angularity and sphericity – Indicate degree of transport.
      • Fresh detritus is usually angular and non-spherical.
      • Grain roundness and sphericity increases with transport.
        • Well-rounded – Long transport distances.
        • Angular – Negligible transport.
    • Clastic Sedimentary Rocks
      • Sorting – The uniformity of grain size.
        • Well-sorted – Uniform grain sizes.
        • Poorly sorted – Wide variety of grain sizes.
      • Sorting becomes better with distance from source.
    • Clastic Sedimentary Rocks
      • Cement – Minerals that fill sediment pores.
        • Fluids with dissolved solids flush through pore system.
        • Dissolved ions slowly crystallize and fill pores.
      • Cementation varies from weak to strong.
      • Common cements:
        • Quartz
        • Calcite
        • Hematite
        • Clay minerals
    • Clastic Sedimentary Rocks
      • Coarse clastics – Composed of gravel-sized clasts.
        • Breccia – Comprised of angular fragments.
          • Angularity indicates a lack of transport processing.
          • Deposited relatively close to source.
    • Clastic Sedimentary Rocks
      • Coarse clastics – Composed of gravel-sized clasts.
        • Conglomerate – Comprised of rounded gravel.
          • Indicates water transport.
            • Clasts bang together forcefully in flowing water.
            • Collisons round angular corners and edges of clasts.
          • Conglomerates are deposited at a distance from the source.
    • Clastic Sedimentary Rocks
      • Sandstone – Clastic rock made of sand-sized particles.
        • Forms in many depositional settings.
        • Quartz is, by far, the dominant mineral in sandstones.
      • Sandstone varieties.
        • Arkose – Contains abundant feldspar.
        • Quartz sandstone – Almost pure quartz.
    • Clastic Sedimentary Rocks
      • Fine clastics - Composed of silt and clay.
        • Silt-sized sediments are lithified to form siltstone.
        • Clay-sized particles form shale.
      • Fine clastics are deposited in quieter waters.
        • Floodplains, lagoons, mudflats, deltas, deep-water basins.
        • Organic-rich shales are the source of petroleum.
      • These are sediments derived from living organisms.
        • Biochemical – Hard mineral skeletons.
        • Organic – Cells of plants, algae, bacteria and plankton.
      Biochemical and Organic Rocks
    • Biochemical Rocks
      • Biochemical limestone – CaCO 3 skeletal (shell) remains.
        • Warm, tropical, shallow, clear, O 2 -rich, marine water.
        • Diverse organisms (plankton, corals, clams, snails, etc.).
        • Many textural varieties.
          • Reefs.
          • Shell debris.
          • Lime mud (micrite).
      • Chert – Rock made of cryptocrystalline quartz.
        • Formed from opalline silica (SiO 2 ) skeletons.
          • Diatoms.
          • Radiolarians.
        • Opalline silica added to bottom sediments dissolves.
        • Silica pore fluids solidify to form chert nodules or beds.
      Biochemical Rocks
      • Made from organic carbon.
        • Coal – Altered remains of fossil vegetation.
          • Accumulates in lush tropical wetland settings.
          • Requires deposition in the absence of oxygen.
        • Oil shale – Shale with heat altered organic matter.
      Organic Rocks
    • Chemical Sedimentary Rocks
      • Comprised of minerals precipitated from water solution.
      • Evaporites – Created from evaporated seawater.
        • Evaporation triggers deposition of chemical precipitates.
        • Examples include halite (rock salt) and gypsum.
    • Chemical Sedimentary Rocks
      • Travertine – Calcium carbonate (CaCO 3 ) precipitated from groundwater where it reaches the surface.
        • Dissolved calcium (Ca 2+ ) reacts with bicarbonate (HCO 3 - ).
        • CO 2 expelled into the air causes CaCO 3 to precipitate.
          • Thermal (hot) springs.
          • Caves.
    • Chemical Sedimentary Rocks
      • Dolostone – Limestone altered by Mg-rich fluids.
        • CaCO 3 altered to dolomite CaMg(CO 3 ) 2 by Mg 2+ -rich water.
        • Dolostone looks like limestone, except…
          • It has a sugary texture and a pervasive porosity.
          • It weathers to a buff, tan color.
    • Chemical Sedimentary Rocks
      • Replacement chert – Nonbiogenic in origin.
      • Many varieties.
        • Flint – Black or gray from organic matter.
        • Jasper – Red or yellow from Fe-oxides.
        • Petrified wood – Wood grain preserved by silica.
        • Agate – Concentrically layered rings.
    • Sedimentary Structures
      • Features imparted to sediments at or near deposition.
        • Layering.
        • Surface features on layers.
        • Arrangement of grains.
      • Help decipher conditions at or near time of deposition.
    • Sedimentary Structures
      • Sedimentary rocks are usually layered or “stratified.”
        • Arranged in planar, close-to-horizontal “beds.”
        • Bedding is often laterally continuous for long distances.
        • Beds are often similar in composition, color and texture.
    • Sedimentary Structures
      • Bedding reflects changing conditions during deposition.
      • These can be changes in…
        • Energy conditions, and hence, grain size.
        • Disturbance by organisms.
      • Bedding may also reflect non-deposition or erosion.
    • Sedimentary Structures
      • A series of beds are referred to as strata.
      • Formation: Strata recognized on a regional scale.
      • Geologic maps display the distribution of formations.
        • i.e. Coconino Formation
      Note the prominent white band of Coconino Sandstone clearly visible across Grand Canyon.
    • Sedimentary Structures
      • Water flowing over loose sediment creates bedforms.
      • Bedforms are linked to flow velocity and sediment size.
        • Ripples, cm-scale ridges, and troughs, indicate flow.
          • Asymmetric ripples – Unidirectional flow.
          • Symmetric ripples – Wave oscillation.
          • Ripples are commonly preserved in sedimentary rocks.
    • Bedforms
      • Cross beds – Created by ripple and dune migration.
        • Sediment moves up the gentle side of a ripple or dune.
        • Sediment piles up, then slips down the steep face.
          • The slip face continually moves downstream.
          • Added sediment forms sloping “cross-bedded” layers.
    • Bedforms
      • Dunes – Similar to ripples except much larger.
        • Form from wind-blown sand in desert or beach regions.
        • Often preserve large internal cross-laminations.
    • Bedforms
      • Turbidity currents.
        • Sediment moves on a slope as a pulse of turbid water.
        • As pulse wanes, water loses velocity and grains settle.
        • Coarsest material settles first, medium next, then fines.
      • This process forms graded beds in turbidite deposits.
    • Bed-Surface Markings
      • Occur after deposition while sediment is still soft.
        • Mudcracks – Polygonal desiccation features in wet mud.
          • Indicate alternating wet and dry conditions.
          • Necessitate deposition in a terrestrial setting.
        • Scour marks – Troughs eroded in soft mud by current flow.
        • Fossils – Evidence of past life.
          • Footprints.
          • Shell impressions.
      • Locations where sediment accumulates. They differ in…
        • Energy regime.
        • Sediment delivery, transport, and depositional conditions.
        • Chemical, physical and biological characteristics.
      • Environments range from terrestrial to marine.
      Depositional Environments
    • Depositional Environments
      • Terrestrial environments – Deposited above sea level.
        • Glacial – Due to movement of ice.
          • Ice carries and dumps every grain size.
          • Creates glacial till; poorly sorted gravel, sand, silt, and clay.
    • Depositional Environments
      • Terrestrial environments – Deposited above sea level.
        • Mountain streams.
          • Water carries large clasts during floods.
          • During low flow, these cobbles and boulders are immobile.
          • Course conglomerate is characteristic of this setting.
    • Depositional Environments
      • Terrestrial environments – Deposited above sea level.
        • Alluvial fan - Sediments that pile up at a mountain front.
          • Rapid drop in stream velocity creates a cone-shaped wedge.
          • Sediments are immature conglomerates and arkoses.
    • Depositional Environments
      • Terrestrial Environments– Deposited above sea level.
        • Sand dunes – Wind-blown piles of well-sorted sand.
          • Dunes move according to the prevailing winds.
          • Result in uniform sandstones with gigantic cross beds.
    • Depositional Environments
      • Terrestrial environments– Deposited above sea level.
        • Rivers – Channelized flow transports sediment.
          • Sand and gravel fill concave-upward channels.
          • Fine sand, silt, and clay are deposited on nearby floodplains.
    • Depositional Environments
      • Terrestrial environments– Deposited above sea level.
        • Lake – Large ponded bodies of water.
          • Gravels and sands trapped near shore.
          • Well-sorted muds deposited in deeper water.
          • Often capped with wetland muds.
    • Depositional Environments
      • Marine environments – Deposited at or below sea level.
        • Deltas – Sediments dropped where a river enters the sea.
          • Sediment carried by the river is dumped when velocity drops.
          • Deltas grow over time, building out into the basin.
          • Often develop a topset – foreset – bottomset geometry.
    • Depositional Environments
      • Marine environments – Deposited at or below sea level.
        • Coastal beaches – Surf zone.
          • Sediments are constantly being processed by wave attack.
          • A common result? Well-sorted, well-rounded medium sand.
          • Beach sandstones may preserve oscillation ripples.
    • Depositional Environments
      • Marine environments – Deposited at or below sea level.
        • Shallow marine – Finer version of beach sediment.
          • Fine silts and muds turn into siltstones and mudstones.
          • Usually support an active biotic community.
    • Depositional Environments
      • Marine environments – Deposited at or below sea level.
        • Shallow water carbonates – Tropical.
          • Skeletons of marine invertebrates.
          • Born in the carbonate factory.
          • Warm, clear, shallow, normal salinity, marine water.
    • Depositional Environments
      • Marine environments – Deposited at or below sea level.
        • Deep marine – Fines predominate far from land sources.
          • Skeletons of planktonic organisms make chalk or chert.
          • Fine silts and clays turn to shale.
    • Sedimentary Basins
      • Sediments vary in thickness across Earth’s surface.
        • Thin to zero edge where non-sedimentary rocks outcrop.
        • Thicken to 10 to 20+ km in sedimentary basins.
      • Subsidence – Sinking of the land during sedimentation.
        • Due to crustal flexure and faulting.
        • Compounded by the weight of added sediments.
      • Basins are important locations for natural resources.
        • Coal.
        • Petroleum.
        • Natural gas.
        • Uranium.
    • Sedimentary Basins
      • Basins form where tectonic activity creates space.
        • Rift basins – Divergent (pull-apart) plate boundaries.
          • Crust thins by stretching and rotational normal faulting.
          • Thinned crust subsides.
          • Sediment fills the down-dropped basin.
    • Sedimentary Basins
      • Basins form where tectonic activity creates space.
        • Passive margin basins – Non-plate-boundary continental edge.
          • Underlain by crust thinned by previous rifting.
          • Thinned crust
          • subsides as it cools.
    • Sedimentary Basins
      • Basins form where tectonic activity creates space.
        • Intracontinental basins – Interiors far from margins.
          • Result from differential thermal subsidence.
          • May be linked to failed crustal rifts.
    • Sedimentary Basins
      • Basins form where tectonic activity creates space.
        • Foreland basins – Craton side of collisional mountain belt.
          • Flexure of the crust from loading creates a downwarp.
          • Fills with debris eroded off of the mountains.
    • Sedimentary Basins
      • Sea-level changes.
        • Sedimentary deposition is strongly linked to sea level.
        • Changes in sea level are commonplace geologically.
          • Depositional belts shift landward or seaward in response.
          • Layers of strata record deepening or shallowing upward.
        • Transgression – Flooding due to sea-level rise.
          • Sediment belts shift landward; strata “deepen” upward.
    • Sedimentary Basins
      • Regression – Exposure due to sea level fall.
        • Depositional belts shift seaward; strata “shallow upward.”
        • Regression tied to erosion; less likely to be preserved.
      • Sea-level rise and fall creates a predictable pattern.
      Transgression – Regression Animation
    • Diagenesis
      • Physical, chemical, and biological changes to sediment.
        • Bioturbation.
        • Lithification.
        • Dissolution.
        • Mineral precipitation.
      • Temps between burial and metamorphism (~300 o C).
      • Integrates changes across the entire sediment history.
    • W. W. Norton & Company Independent and Employee-Owned
      • This concludes the Norton Media Library PowerPoint Slide Set for Chapter 6
      • Essentials of Geology
      • 3 rd Edition (2009)
      • by Stephen Marshak