Op ch07 lecture_earth3, sedimentary rocks

Chapter 7
A Surface Veneer: Sediments, Soils, and
Sedimentary Rocks

LECTURE OUTLINE

earth

Portrait of a Planet

Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak

Chapter 7: A Surface Veneer: Sediments, Soils and Sedimentary Rocks

A Surface Veneer:
Sediments, Soils, and Sedimentary Rocks
Prepared by

Ron Parker
Earlham College Department of Geosciences
Richmond, Indiana

Sediments
 The Nile flows over a canyon filled by thick sediments.
 The size of Grand Canyon.
 The canyon was carved by the Nile.
 Indicates the Mediterranean has evaporated and re-filled.

 Sediment:
 Rock and mineral fragments.
 Shells.
 Mineral precipitates.

 Cemented into rocks.



Sedimentary Cover
 Earth is covered by a thin “veneer” of sediment.
 The veneer caps igneous and metamorphic “basement.”
 Sediment cover varies in thickness from 0 to 20 km.
 Thinner (or missing) where ig and meta rocks outcrop.
 Thicker in sedimentary basins.



Weathering
 Processes that break up rock to create sediment.
 Physical - Mechanical breakage and disintegration.
 Chemical - Decomposition by reaction with water.

 Weathering processes occur at Earth’s surface.
 Rocks react with hydrosphere, atmosphere, and biosphere.
 Low temperature and pressure.



Physical Weathering
 Mechanical breakup; doesn’t change mineral makeup.
 Creates broken fragments or “detritus.”
 Detrital fragments classified by size.
 Coarse-grained – Boulders, cobbles, and pebbles.
 Medium-grained – Sand-sized.
 Fine-grained – Silt and clay (mud).



Physical Weathering
 Types of physical weathering.
 Jointing.
 Frost wedging.
 Root wedging.
 Salt wedging.
 Thermal expansion.
 Animal activity.

 Physically weathered rock fragments move by gravity.
 Large blocks often accumulate as talus below a cliff.
 Smaller fragments are carried away by water and wind.


Jointing
 Deep crustal rocks are hot and under high pressure.
 Erosion removes material; deep rocks become exposed.
 At the surface, crustal rocks cool and expand.
 This causes through-going fractures called joints.
 Joints may exhibit a variety of geometries.



Jointing
 Igneous plutons crack in onionlike “exfoliation” layers.
 These layers break off as sheets that slide off of a

pluton.
 Over time, this process creates domed remnants.
 Examples: Half-Dome (Ca.) and Stone Mountain (Ga.).

W. W. Norton



Chemical Weathering
 Reaction with water disintegrates many minerals.
 Water is the universal solvent.
 Maximized under warm and wet conditions.

Tropical weathering is intensive.
Turns rock into heavily decomposed “saprolite.”
 Chemical weathering is virtually absent in deserts.



Chemical Weathering
 Forms stable minerals from unstable precursors.
 Common chemical weathering reactions include:
 Dissolution.
 Hydrolysis.
 Oxidation.
 Hydration.



Chemical Weathering
 Dissolution
 Some minerals (halite, gypsum, calcite) dissolve.

CaCO3 + H2CO3 +2H+ = 2H2O + 2CO2 +Ca2+
(calcite) + (carbonic acid) + (hydrogen ion) = (water) + (carbon dioxide) + (calcium ion)

 Acidity (i.e. acid rain) enhances this effect.



Hydrolysis
 Water breaks cation bonds in silicate minerals. Yields…
 Dissolved cations.
 Alteration residues.

Clay minerals.
Iron oxides (rust).



Chemical Weathering
 Oxidation
 A reaction whereby a metal loses electrons.
 Important process in mafic silicate decomposition.
 Rusting is a familiar example.

 Hydration
 Absorption of water into a mineral structure.
 Results in a volume increase (expansion).
 Important processes in some clay minerals.


Chemical Weathering
 Minerals differ in their tendency to weather chemically.
 Silicate weathering susceptibility predicted by stability.
 Temperature (T) and pressure (P) are a dominant control.

High T and P minerals tend to weather quickly at low T and P.
Low T and P minerals are more stable at Earth’s surface.



Biological Weathering
 Organisms often important chemical weathering agents.
 Plant roots.
 Fungi.
 Lichens.
 Bacteria.

 Organic acids

attack minerals.



Weathering Feedbacks
 Weathering processes often work as a positive feedback.
 Physical weathering speeds chemical weathering. How?
 An increase in surface area accelerates chemical attack.
 Chemical weakening increases surface area via breakage.



Chemical Weathering Rate
 Block geometry influences weathering.
 Corners weather fastest; 3 sides of attack.
 Edges weather at a moderate rate (2 sides).
 Flat faces weather slowest (1 side).

 Cube-shaped rocks develop “spheroidal weathering.”



Differential Weathering
 Rates of weathering attack vary due to changes in…
 Mineral stability.
 Degree of compaction or cementation.
 Subtle differences in texture, etc.

 Major control on surficial expression.



Soil
 “Soil consists of rock and sediment that has been

modified by physical and chemical interaction with
organic material and rainwater, over time, to produce a
substrate that can support the growth of plants.”
 Soil-forming processes require long periods of time.
 Soil may be easily destroyed by human activities.
 Soil is a crucial natural resource in need of protection.



The Soil Profile
 Soil forms a vertical sequence of layers called a profile.
 Profiles develop from the surface downward.
 Weathering generates detritus.
 Rainwater alters the detritus.

 Individual layers are horizons.
 The upper horizons form topsoil.
 The only fertile part of the profile.
 Easily removed by erosion.
 1000s of years in the making.



Soil-Forming Processes
 Coffee is a direct analogue.
 Zone of leaching – Upper soil

profile.
 Ions from chemical weathering
 Fine silts and clays infiltrate.

 Zone of accumulation –

Lower soil profile.
Ions form new minerals.
Silts and clays clog pore

spaces.


Soil Horizons
 Distinct horizons reflect soil-

forming processes.
 O Horizon – Dark organic

matter-rich surface layer.
 A Horizon – Organic and mineral
matter.
 E Horizon – Transitional layer
leached by organic acids.
 B Horizon – Organic-poor
mineral rich layer.
 C Horizon – Slightly altered
bedrock.



Soil-Forming Factors
 Soil genesis influenced by a number of factors.
 Climate – Amount of water and warmth.
 Substrate composition – Soil parent minerals.
 Slope steepness – Soils develop best on low slopes.
 Drainage – Wet soils are more organic-rich.
 Time – Older soils are more developed.
 Vegetation – Controls type of organic matter added.



Soil vs. Climate
 Climate exerts a dominant control on soil development.
 Latitude.
 Elevation.



Soil Use and Misuse
 Human activities can make soil unproductive.
 Crops can strip soil nutrients.
 Topsoil erodes away when vegetation is removed.

 Topsoil is sometimes mined and sold; usually a bad

idea.
 Soil quality is degraded by…
 Overuse of pesticides.
 Industrial contamination.
 Salt build-up.

 It is wise to protect soils.



Sedimentary Rocks
 Sediments are the building blocks of sedimentary rocks.
 Sediments are diverse, as are the rocks made from them.
 4 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.
Clastic

Biochemical


Organic

Chemical


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 – Accumulation after transport stops.
 Lithification – Transformation into solid rock.



 Lithification – Transforms loose sediment into solid rock.
 Burial – More sediment is added onto a previous layer.
 Compaction – Overburden weight reduces pore space.

Sand – 10-20%
Clay – 50-80%

 Cement – Minerals from groundwater that “glue” sediment.


 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.


 Clast (grain) size – The average diameter of clasts.
 Range from very coarse to very fine.
 Boulder, cobble, pebble, and pea gravel.
 Coarse, medium, and fine sand.
 Coarse, medium, and fine silt.
 Coarse and fine clay.

 With increasing transport, average grain size decreases.



 Clast composition – Mineral makeup of clasts.
 May be individual minerals or rock fragments.
 Mineral identity important.

Stable end-products of weathering (quartz, Fe-oxides, clays).
Unstable minerals signify special conditions (feldspars).
 Composition helps to decipher depositional history.



 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.



 Sorting – The uniformity of grain size.
 Well-sorted – Uniform grain size.
 Poorly sorted – Wide variety of grain sizes.

 Sorting indicates the constancy of environmental energy.
 Well-sorted – Uniform energy (i.e., a beach).
 Poorly sorted – Variable energy (i.e., an alluvial fan).



 Cement – Minerals that fill sediment pores.
 Fluids with dissolved solids flush through pore system.
 Dissolved ions slowly crystallize in (and fill) pores.

 Cementation varies in degree.
 Weakly cemented (friable) – Grains easily pulled from rock.
 Strongly cemented (indurated) – Grains hard to dislodge.
 Common cements:
 Quartz
 Calcite
 Hematite
 Clay minerals



 Maturity – A measure of the degree of “processing.”
 Textural maturity – Degree of roundness and sorting.
 Mineral maturity – Degree of unstable mineral removal.

 Time and transport causes sediment evolution.
 Texture - Average grain size  ; roundness and sorting  .
 Composition - Unstable minerals  ; Stable minerals  .

 Maturity is used to reconstruct depositional conditions.



 Coarse clastics - Composed of gravel-sized clasts.
 Breccia – Comprised of angular clasts.

Indicates lack of transport processing.
Deposited close to source.
 Conglomerate – Comprised of rounded clasts.

Indicates processing by transport.
Deposited away from source area.



Biochemical and Organic Rocks
 These are sediments derived from living organisms.
 Biochemical limestone – Made from CaCO3 shell remains.
 Carbonate grains accumulate in the “carbonate factory.”

Warm (tropical and subtropical).
Normal-salinity marine water.
Wave-agitated.
Oxygenated.
Shallow.
Clear.



Biochemical and Organic Rocks
 Organic 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.



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.



Sedimentary Structures
 Sedimentary rocks are usually layered or “stratified.”
 Arranged in planar, horizontal “beds.”
 Bedding is often laterally continuous for long distances.
 Beds are often similar in composition, color, and texture.



 Bedding caused by 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.



 A series of beds are referred to as strata.
 A sequence of strata that is sufficiently unique to be

recognized on a regional scale is termed a formation.
 The formation is the fundamental geologic mapping unit.

Example: Note how the prominent
white cliff-forming band of the
Coconino Formation can be visibly
traced across Grand Canyon.



 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
 Dunes – Similar to ripples except much larger.
 Cross-bedding – 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
 Graded Beds – Bedding layers that fine upward.
 Transition from coarse to medium to fine grain sizes.

Base of coarse sand and gravel.
Mid-level sands.
A fine silt and clay cap.
Abrupt contact with overlying coarse base.



Depositional Environments
 Locations where sediment accumulates. They differ in…
 Energy regime (mostly comes from the sun; gravitational
and chemical potential energy)
 Sediment delivery, transport and depositional conditions.
 Chemical, physical and biological characteristics.
 Environments range from continental to marine.



 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.



 Terrestrial environments.
 Mountain streams.

Water carries large clasts during floods.
During low flow, boulders are immobile.
Course conglomerate is characteristic.



 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.



 Rivers – Channelized flow transports sediment.

Sand and gravel fills concave-up channels.
Fine sand, silt, and clay is deposited on flood plains.



 Marine environments – Deposited at or below sea-level.
 Deltas – Sediments dropped where a river enters the sea.

River flow halts at the ocean.
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.



 Marine environments.
 Shallow marine – Finer version of beach sediment.

Fine silts and muds.
Biologically active.
Siltstone and mudstone.



 Shallow water carbonates – Tropical.

Skeletons of marine invertebrates.
Born in the carbonate factory.
Warm, clear, shallow, normal salinity marine water.



 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 a zero edge where non-sedimentary rocks outcrop.
 Thicken to 10 to 20+ km where the surface has subsided.

 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 margins – Non-plate boundary continental edge.

Underlain by crust thinned by previous rifting.
Thinned crust subsides as it cools.



Diagenesis
 Physical, chemical, and biological changes to sediment.
 Bioturbation (The stirring or mixing of sediment or soil by

organisms)
 Lithification.
 Dissolution.
 Mineral precipitation.
 Pressure solution.
 Temp range between burial and metamorphism (~300oC).
 Integrates changes across the entire sediment history.



This concludes the
Chapter 7
A Surface Veneer: Sediments, Soils, and
Sedimentary Rocks

LECTURE OUTLINE

earth

Portrait of a Planet

Third Edition
©2008 W. W. Norton & Company, Inc.


Op ch07 lecture_earth3, sedimentary rocks

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