Fig' 6CO, p' 138

1
Weathering and Soil Sediment and Sedimentary
Rocks
Fig. 6-CO, p. 138
2
Weathering and Soil

• Weathering Processes
• Mechanical
• Chemical
• Weathering Products
• Soil
• Resources
• Sediment

3
Weathering

• Weathering
• Disintegration (physical breakdown) and
  decomposition (chemical alteration) of rock
  (parent material) at or near the surface
  (exposure to air and water).
• Breakdown of parent rock into sediment and
  dissolved ions.

4
Weathering is an important process in the rock
cycle. A transition process between rocks
(igneous, metamorphic, pre-existing sedimentary
rocks) and sediment. Sediment is the source for
new sedimentary rocks.
5
Weathering vs. Erosion

• Weathering is related to erosion, but they are
  NOT the same.
• Weathering occurs in place. No movement of
  material.
• Erosion implies the transport (movement) of the
  material made during weathering.

6
Fig. 6-1a, p. 140
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Fig. 6-1b, p. 140
8
Fig. 6-2, p. 141
9
Weathering

• Weathering produces the source material for
• Sedimentary rocks
• Soil
• Some natural resources
• Weathering is critical for the evolution of the
  landscape all around us.

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Mechanical Weathering

• Physical breakdown of rock (disintegration).
• Processes involve breaking rocks and minerals
  into smaller pieces.
• Composition of the pieces is identical to the
  source rock.
• Just change in size and shape

11
Mechanical Weathering

• Frost action repeated freezing and thawing of
  water in cracks and crevices.
• Frost wedging Water expands by 9 in volume when
  it freezes ? substantial tensional forces
  produced within the rock, sufficient to detach
  pieces.
• Frost heaving lifting lowering of layers due
  to ice formation.
• Most effective where have (a) water and (b)
  sufficient seasonal/diurnal temperature
  variations.

12
Frost Action
Talus accumulations of rock fragments at the
base of a cliff or slope that is actively
weathering (by any process).
Fig. 6-3a, p. 142
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Fig. 6-3b, p. 142
14
Mechanical Weathering

• Crystal growth growth of salt (and other
  crystals) in cracks and voids. New minerals grow
  (volume change) and induce stresses in the rock
  leading to fracture.
• Just like frost action
• Action of salt crystals is most common in arid
  and coastal areas.

15
Mechanical Weathering

• Pressure release Rocks formed at depth are
  stable at elevated ambient pressures.
• When rock is exhumed (uplifted exposed at the
  surface somehow), the pressure is decreased.
• Stresses in the rock increase the rock will
  fracture.
• Fractures (sheet joints) form parallel to the
  surface of the rock. Material peels off ?
  exfoliation.
• Problematic in mines

16
Fig. 6-4a, p. 143
17
Fig. 6-4b, p. 143
18
Fig. 6-4c, p. 143
19
Mechanical Weathering

• Thermal effects
• Materials change volume as they heat up and cool
  down. Rocks tend to expand when heated.
  Contract when cool.
• Volume change in a solid tends to cause stresses
  sufficient to fracture, especially if
  heating/cooling is uneven.
• Day-night seasonal cycles not sufficient to
  make this a large effect

20
Mechanical Weathering

• Abrasion wearing away of rock surfaces by the
  action of water-, wind-, and ice-bourne
  particles. Even the action of water by itself
• Critical factors
• Frictional forces electromagnetic interaction
  of surfaces in contact.
• Shear stresses the amount of force applied to a
  surface due to changes in velocity.

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Mechanical Weathering Processes

• Biological factors
• Plant roots forcing open cracks
• Animals burrowing
• These processes physically disrupt geologic
  materials. Also enable and cause other,
  non-mechanical weathering processes

23
Fig. 6-5a, p. 144
24
Fig. 6-5b, p. 144
25
Chemical Weathering

• Minerals are decomposed by chemical alteration.
  New minerals replace pre-existing ones according
  to stability.
• Variety of chemical reactions
• Dissolution (dissolving)
• Oxidation (rusting)
• Hydrolysis (just add water)

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Chemical Weathering

• Dissolution dissolving of minerals into a
  solution (usually aqueous e.g. water).
• Few minerals are very soluble in pure water.
  Acidic water (carbonic, hydrochloric, sulfuric,
  etc.) helps things along
• Carbonates dissolve readily ? chemically
  made/weathered rocks (chemical sediment)

27
Chemical Weathering

• Polar nature of water makes it a good solvent.
• One half of water molecule is more positive
  (hydrogen side) than the other (oxygen side).

28
Fig. 6-6, p. 145
29
Chemical Weathering

• Acids are good solvent due to amount of H
  cations. More H, more acidic, better solvent.
• Carbonic acid forms naturally due to equilibrium
  between atmosphere and surface water. Biology
  helps (decay). Climate (wet, hot) affects it too

30
CaCO3 H20 CO2 -gt CaCO3 H HCO3- -gt Ca2
2HCO3-
31
Chemical Weathering

• Oxidation minerals react with oxygen. Metal
  cations form oxides/hydroxides that are more
  stable at surface free-oxygen concentrations.
  Rusting.
• Important way of altering ferromagnesian
  minerals.
• Generally slow. Water helps the process.

32
4Fe 3O2 -gt 2Fe2O3
33
Chemical Weathering

• Hydrolysis reaction of minerals and water ions
  (H and OH-).
• H can replace cations in minerals. OH- can
  substitute for anions.
• Mineral becomes hydrated with water in its
  structure.
• Ions are released
• Mineral structure/properties may change
• Mineral may become soluble
• Example hydrolysis of feldspar produces clay
  minerals

34
2KAlSi3O8 2H 2HCO3- H2O -gt
Al2Si2O5(OH)4 2K 2HCO3- 4SiO2
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Weathering Rates

• Rates of weathering vary over time and from place
  to place and depending on the material being
  weathered.
• Mineral stability
• Climate
• Temperature
• Moisture
• pH
• Surface-area vs. volume

38
Mineral Stability

• Rocks formed in the crust are made of minerals
  chemically stable at elevated temperatures,
  elevated pressures, and certain amounts of oxygen
  and water.
• Bring those rocks to the surface where those
  conditions are different, and the minerals in the
  rocks will weather in order to attain chemical
  equilibrium with the new surroundings

39

• The minerals most susceptible to weathering at
  Earths surface are those that crystallize early
  in Bowens reaction series.
• Example Quartz is the most stable. Olivine is
  the least stable.

40
Environmental Factors

• Higher temperatures tend to accelerate chemical
  reactions.
• Wetter climates promote chemical weathering.
• Biology contributes to both mechanical and
  chemical weathering.
• ? Weathering is faster in the tropics than in
  deserts

41
Surface Area Volume

• Weathering is a process that acts on surfaces.
• The more surface area available, the more damage
  weathering (especially chemical weathering) can
  do.

42
Surface Area Volume

• A cube 1 m on a side
• Volume 1 m3
• SA 6 m2
• Break it into 8 pieces, each 0.5 m on a side
• Total volume 1 m3
• Total SA 12 m2
• etc...

43
Fig. 6-8, p. 146
44
Surface Area Volume

• Small objects have a higher surface-area-to-volume
  ratio than larger objects.
• Mechanical weathering acts on surfaces, but tends
  to reduce the size (volume) of particles.
• It therefore gives chemical weathering more
  surface area for it to act on

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Corners are unstable mechanically and chemically
Fig. 6-9, p. 147
47
Fig. 6-7, p. 146
48
Soils

• Regolith unconsolidated rock and mineral
  fragments
• Soil regolith with weathered material, water,
  air, organic matter that can support plant
  growth
• Humus organic material processed by bacterial
  decay ? enriched in N, less C than original
  organic matter
• Sand, silt, clay make up 45 of fertile soil.
• Sand silt mostly quartz create pore spaces
• Clay mostly weathered feldspar important
  nutrient supply

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Soils

• Residual soil formed by in-place weathering of
  parent material.
• Transported soil weathered material is
  transported to another place where it is
  converted to soil.

51
Soil Profile

• A soil profile is a horizontal stack of layers
  (horizons).
• A mature soil has well-developed horizons. An
  immature soil does not have well-developed
  horizons.

52
Fig. 6-10, p. 148
53
Soil Profile

• O horizon organic zone
• contains humus
• only a few cm thick
• A horizon leaching zone
• water percolating down - removes soluble material
• high biological activity
• B horizon accumulation zone
• clay and oxides precipitated from A horizon
  leachate
• fragipan either claypan (dense clay) or hardpan
  (carbonate)
• C horizon transition zone
• actively weathering rock
• saprolite (rotten rock)

54
Soil Formation

• Climate is the most important factor
• moisture temperature controls on weathering
• climate determines extent of soil transport
• Parent material helps determine soil type and
  depth and fertility
• Organisms are active soil formers
• Topography (relief and slope and elevation)
  affects (micro) climate and has mechanical
  controls (slope stability, etc.)
• Time weathering processes/soil forming
  processes act at different rates and compete with
  soil degradational processes/erosion

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Fig. 6-11, p. 149
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Soil Types

• Soil classifications in use are very complicated,
  but can be simplified to three types, based on
  climate
• pedalfer Al and Fe soil humid temperate
  climates dark A clay- oxide-rich B acidic
  fertile
• pedocal Ca soil arid/semi-arid climates
  light, less leached A hardpan B (caliche)
  alkaline difficult
• laterite tropical soils extreme leaching
  removes all but clay and Fe oxides ? not fertile
  valuable Al and Fe resources in some places

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Soil Degradation

• Decrease in soil fertility/volume by
• Removal of soil by erosion
• Physical or chemical deterioration

63
Soil Degradation

• Erosion transport of sediment/soil by the
  action of water, wind, ice
• Big problem in mismanaged farmlands
• Rain forest clearing removes vegetation that
  stabilizes topsoil.
• 25 of US cropland is eroding faster than being
  renewed

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Soil Degradation

• Water erosion by
• Sheet erosion overland flow of water as evenly
  distributed sheet. Removes thin layers of soil
• Rill erosion flow of water in channels (rills
  or gullies). If severe, may make cropland
  unusable.

66
Fig. 6-13a, p. 152
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Fig. 6-13b, p. 152
68
Fig. 6-14, p. 153
69
Soil Degradation

• Chemical deterioration nutrients depleted and
  not renewed ? productivity decreases
• Overuse
• Misuse of fertilizers (too little or too much)
• Salinization (big issue in arid/semi-arid places)
• Pollution by industrial wastes, insecticides,
  pesticides, etc.

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Soil Degradation

• Physical deterioration soil structure disrupted
  ? soil productivity decreases
• Compaction
• elimination of pore spaces
• increase in hardness (hard to plow, erosion by
  water)
• Sun-baking (problem in tropics) ? clear-cut areas
  of laterite (clay-rich soil) baked into brick in
  short time!

71
Soil Resources

• Laterites derived from Al-rich parent
• Al hydroxides accumulate in B horizon (residual
  concentration)
• These deposits are called bauxite (very important
  Al ore)
• US has little bauxite. Most imported from
  Brazil, Australia, etc.
• Fe, Mn, clays, Ni, P, Sn, diamond, gold also
  found in residual deposits

72
Fig. 6-12, p. 149
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Sediment Sedimentary Rocks

• Sediment
• Types
• Transport
• Deposition
• Lithificaton
• Sedimentary Rocks
• Types
• Facies and Environment
• Resources

76
Fig. 1-12, p. 20
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Sediment

• Raw materials for sedimentary rocks.
• Derived from weathering of parent rocks
• Detrital rock fragments mineral grains
  liberated by mechanical weathering and
  transported/deposited by mechanical processes.
  Terrigenous or clastic (clasts)
• Chemical formed of material dissolved during
  chemical weathering and deposited by biological
  or non-biological chemical reactions.
  Precipitates (solutions)

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Sediment

• What sorts of material?
• Clay minerals
• Quartz
• Calcite and dolomite ? chemical
• Feldspars
• Fe oxides and sulfides ? chemical
• Salts and gypsum ? chemical
• Volcanic debris (tephra)
• Organic matter

81
Sediment Sizes

• Detrital sediment is classified according to
  size.
• Detrital sedimentary rocks can contain a wide
  range of sizes, from mud to boulders

• boulder gt 256 mm
• cobble 64 to 56 mm
• pebble 2 to 64 mm
• sand 1/16 to 2 mm
• silt 1/256 to 1/16 mm
• clay lt 1/256 mm

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Table 6-1, p. 147
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Sedimentary Processes

• Weathering is the first step (form sediment).
• Unconsolidated detrital sediment is often eroded
  and transported and deposited.
• Products of chemical weathering are transported
  in solution.
• Sediment is then transformed into sedimentary
  rock by lithification. Chemical sedimentary
  rocks form directly from solution
  (precipitatation).

84
Sedimentary Processes

• Detrital sediment eroded and carried by water,
  ice, wind.
• Key factor is energy. Detrital sediment of a
  given size will be carried by an agent with
  sufficient energy.
• Example I can blow dust and sand by blowing on
  it. I cant move gravel or a boulder by blowing
  on it

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Sedimentary Processes

• Abrasion during transport reduces particle size
  and changes particle shape.
• Particles become rounder the longer they are in
  the transport phase
• Sorting of particles (distribution of sizes) is
  also affected by transport. Largest and heaviest
  particles settle out first (not transported as
  far).

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Fig. 6-15a, p. 154
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Fig. 6-15b, p. 154
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Fig. 6-15c, p. 154
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Sedimentary Processes

• Sediment eventually gets put somewhere. It
  ceases to be transported and is deposited.
• Deposition of detrital sediment occurs when
  energy necessary for transport is no longer
  available. Chemical deposition (precipitation)
  occurs when environmental (chemical) conditions
  change.
• Depositional environment geographic area where
  sediment is deposited.
• Continental
• Transitional
• Marine

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Fig. 6-16, p. 155
93
Sedimentary Processes

• Lithification is the process by which sediment
  becomes sedimentary rock.
• Relatively simple for chemical sediments as they
  simply precipitate out of solution and directly
  form a rock
• Detrital sediments are compacted (dewatered),
  cemented, recrystallized (diagenesis)

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Sedimentary Processes

• Deposited sediment has pore spaces between
  particles.
• Compaction due to pressure of overlying sediment,
  etc. reduces the volume of the deposit
• Reduces pore space (50 volume reduction)
• Forces fluid out (dewatering)

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Sedimentary Processes

• Cementation is the process by which clasts are
  stuck together.
• Compaction does some of the job (e.g. for mud),
  but for larger grainsizes, a cement or matrix is
  needed.
• The cement is like glue. Usually it is a
  chemical sediment (calcium carbonate or silica or
  Fe oxides) that precipitates in pore spaces

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Sedimentary Rocks

• Single most important characteristic of sediments
  and sedimentary rocks they are layered ?
  history.
• Layering of sediment records history of
  sedimentary process, climate, tectonics, life,
  etc

99
Original Horizontality

• Sedimentary rocks are formed in layers (strata)
  which were originally horizontal.
• Flat strata are probably undisturbed
• Tilted strata have been affected by tectonics.

100
Superposition

• Oldest sedimentary rocks at the bottom C
  deposited first
• Younger sedimentary rocks on top B then A.

101
Superposition

• Crosscutting igneous rocks are younger than what
  they intrude.
• Faults are younger than what they cut.

102
Lateral Continuity

• When they form, horizontal strata continue in all
  directions until they
• Thin to nothing,
• Change into something else,
• Abut against a barrier.

103
Principle of Inclusion

• Erosion surfaces exist in the rock record...
• Fragments within strata above are derived from
  the older strata below ? they are older than the
  strata containing them.
• The strata containing the fragments are younger
  than the strata the fragments came from.

104
Conformable Contact

• Layers of rock that have been deposited without
  any interruption.
• No gaps in time.
• No missing record due to erosion, non-deposition,
  etc.

105
Unconformity

• 3 types of break in the rock record.
• Such surfaces represent
• A hiatus in deposition and/or
• A period of erosion.
• ?Missing time
• ?Significant events.

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Angular Unconformity

• A sharp discontinuity in the rock record
  separating strata that are not parallel.
• Indicates that during the break, a period of
  deformation occurred.

107
Disconformity

• A break in the rock record across which there is
  little change in stratal orientation.
• Often just a pause in deposition (subtle).
• May also be obvious erosion surface.

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Nonconformity

• Horizontal sedimentary rocks on top of eroded
  crystalline rocks (metamorphic or igneous).
• Requires erosion to bring crystalline rocks to
  the surface.
• Buried topography.
• Intrusive contact is different!

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Sedimentary Rock Types

• Sedimentary rocks are classified by the particles
  that make them up
• Detrital (terrigenous or clastic) sedimentary
  rocks
• Chemical sedimentary rocks

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Clastic Sedimentary Rocks

• Generally consist of clasts (large pieces) within
  a matrix (finer grained pieces, all held together
  by a cement.
• Classified according to the size of the clasts.
• Classified by the angularity of the clasts
  (breccia vs. conglomerate).
• Classified by other characteristics (fissility,
  composition).

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Fig. 6-17, p. 156
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Massive claystone (not fissile)
Shale (fissile claystone)
Siltstone
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Fig. 6-18d, p. 157
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Fig. 6-18c, p. 157
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Quartz sandstone
Greywacke (rock fragment) sandstone
Arkose (gt 25 feldspar) sandstone
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Breccia
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Fig. 6-18b, p. 157
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Conglomerate
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Fig. 6-18a, p. 157
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Chemical Sedimentary Rocks

• Result from the precipitation of minerals from a
  solution. Material in solution originated in the
  weathering environment.
• Precipitation processes either biological or
  non-biological. Biochemical sedimentary rocks
  result from chemical processes of organisms.
• Chemical sedimentary rocks classified based on
  composition/chemical process

124
Chemical Sedimentary Rocks

• Carbonates (limestone)
• biochemical
• fossiliferous limestone
• coquina
• chalk
• oolitic limestone
• non-biochemical
• Evaporites (evaporation)
• rock salt
• gypsum

• Other
• chert
• amber
• coal

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Table 6-2, p. 156
126
Carbonate Sedimentary Rocks

• Carbonates are composed of calcite (CaCO3) and/or
  dolomite (CaMg(CO3)2). Dolomite is probably
  altered calcite
• Most are biochemical (but not all).
• Chemical process is the dissolution and
  precipitation of carbonate from water/carbonic
  acid solutions.

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Biochemical Carbonate Sedimentary Rocks

• Fossiliferous limestone deposition of carbonate
  matrix with organically-produced shells and other
  fossils included
• Coquina broken shells held together by
  carbonate cement (dense fossiliferous limestone).
• Chalk microscopic shells held together by
  carbonate cement (microscopic fossiliferous
  limestone).
• Oolitic limestone sandstone formed of
  cemented together spheres of carbonate (ooids).

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Fig. 6-19a, p. 158
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Fig. 6-19b, p. 158
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Fig. 6-19c, p. 158
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Fig. 6-19d, p. 158
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Carbonate Sedimentary Rocks

• A couple of examples of non-biological
  carbonates
• Caliche B soil horizion deposit of carbonate
  formed when alkaliine groundwater rises to the
  surface and evaporates into a crumbly powder.
• Travertine calcareous deposits from groundwater
  (freshwater)
• cave deposits (speleothems)
• surface mound deposits around hot springs, desert
  lakes (tufa)

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Evaporites

• Form by inorganic chemical precipitation of
  minerals (usually salts) from solution.
• Include halite (rock salt) and gypsum
• Why precipitate? Solution becomes saturated
  (solvent cant hold any more dissolved ions).

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Fig. 6-20a, p. 159
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Fig. 6-20b, p. 159
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Siliceous Chemical Sediments

• Dominated by silica (SiO2). Formed by
• Accumulation of silica-secreting organisms such
  as diatoms, radiolarians, or some types of
  sponges ? diatomaceous earth, layered chert
  (ooze).
• Chemical reactions of dissolved silica replacing
  carbonate in limestones ? chert interbeds

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Fig. 6-20c, p. 159
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Organic Chemical Sediments

• Coal compressed, altered remains of ancient
  organisms.
• Forms in oxygen-deficient (reducing)
  environments. Bacterial decay is impeded.
• Dead plant junk ? peat ? lignite ? bitumen ?
  anthracite.
• Each step in this diagenetic process involves
• Compression and dewatering
• Removal of more volatile elements (O, N, H)
• Concentration of C

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Fig. 6-20d, p. 159
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Other

• Phospates. Nodules on the searfloor (ooze).
  Fossilized guano (bird crap).
• Iron formations. Sulfides. Bog iron.
  Laterites. Placer deposits. Oolites! Banded
  iron formation.
• Mn nodules on ocean floor.

149
Sedimentary Facies

• Sedimentary rocks reflect the environment of
  deposition.
• The distribution and depositional patterns of
  sedimentary rocks (facies changes) tell us about
  environment.
• Sedimentary structures also tell us about
  environment and process.

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Fig. 6-16, p. 155
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Sedimentary Facies

• A given horizon of sediment deposited at a single
  time (a geologic timeline) will extend in all
  directions.
• But, as you trace a sediment stratum laterally
  (over large enough distances), it will change in
  composition and/or texture. Why?
• Moving towards/away from different sources.
• Moving across different environments with
  different processes (agent energy).
• At any one time, different environments are
  operating in different places, and each will have
  characteristic deposits ? facies.

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Fig. 6-27b, p. 165
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Fig. 6-21a, p. 160
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Fig. 6-21b, p. 160
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Fig. 6-21c, p. 160
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Fig. 6-21d, p. 160
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Fig. 6-21e, p. 160
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Fig. 6-21f, p. 160
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Fig. 6-21g, p. 160
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Fig. 6-21h, p. 160
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Transgressions Regressions

• The seas came inthe seas went out again
• Sea level fluctuations can cause changes in
  facies over large areas as time passes ? sediment
  changes vertically in a column.
• Transgression coarse to fine. Ocean comes in.
• Regression fine to coarse. Ocean goes out
  again.

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Fig. 6-21, p. 160
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Fig. 6-27b, p. 165
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Sedimentary Structures

• Keys to the past. Allow us to determine facies,
  environment, and process.
• Shapes and morphology (tool marks, etc.)
• Textures (coarse or fine well sorted or not).
• Bedforms (ripples, dunes, etc.)
• Three-dimensional geometry of deposits (sheets or
  channels)
• Fossils
• Interpreted based on comparison with present-day
  processes.

166
Sedimentary Structures

• Strata (bedding) horizontal layers (mm to m
  scale). Represent timelines. Separated by
  bedding planes transitions in sediment size,
  composition, etc.
• Graded bedding upward decrease in sediment size
  within a single bed. Indicative of order of
  settling of particles from suspension as in a
  turbidity current. Lateral grading in bedding,
  too (facies changes)
• Cross bedding bedding at angles to the
  horizontal at time of deposition. Indicative of
  dunes, river deltas, submarine environments where
  flow direction changes ? clues to paleocurrents.

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Fig. 6-22b, p. 161
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Fig. 6-22a, p. 161
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Varves climatically driven laminations
(seasonal) light (coarse grained) is summer,
dark (fine grained) is winter
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Fig. 6-23a, p. 162
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Fig. 6-23b, p. 162
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Sedimentary Structures

• Bedforms regularly repeating features on a
  bedding plane of sediment (sand) that is (was)
  being moved.
• Ripples
• Dunes
• Indicate environment (water or wind river or
  shore)
• Indicate flow direction and energy
• Indicate paleo-water temperature??

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Fig. 6-24a, p. 163
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Fig. 6-24b, p. 163
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Fig. 6-27a, p. 165
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Fig. 6-24c, p. 163
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Fig. 6-24d, p. 163
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Sedimentary Structures

• Mud cracks polygonal cracks formed as mud
  dried indicates shallow water used to show
  which way up.
• Raindrops impressions of falling rain in
  mudstone. Used to tell environment which way
  is up.
• Flute marks depressions scoured out by the flow.
• Tool marks tracks left by objects dragged along
  bedding surface by flow.
• Sole marks casts (fillings) of primary
  structures (like flute and tool marks). Used to
  tell which way is up.

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Fig. 6-25a, p. 163
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Fig. 6-25b, p. 163
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Sedimentary Structures

• Soft sediment deformation
• Flame structures
• Slump
• Disrupted/folded/convoluted bedding
• Indicate something happened to sediment after
  deposited but before it was lithified
  (dewatering, storms, earthquakes, dropped
  boulders)

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Fossils Trace Fossils

• Body fossils
• Footprints
• Trails
• Burrows
• Stromatolites

190
Fig. 6-26a, p. 165
191
Fig. 6-26b, p. 165
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195
Sedimentary Resources

• Gravel and sand ? construction, placer deposits
• Clay ? ceramics
• Limestone ? cement
• Evaporites ? chemicals
• Phosphates ? fertilizers
• Iron formations ? metal ores

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Sedimentary Resources

• Coal, petroleum natural gas for fuel
  chemicals.
• Deep sedimentary source rocks (contain dead plant
  and animal matter) ? heat pressure ?
  hydrocarbons.
• Hydrocarbons accumulate in reservoir rocks with
  high porosity permeability.
• Cap rocks with low permeability trap
  hydrocarbons.
• Facies changes and structural features (faults)
  important for geometries of reservoirs, caps, and
  traps.

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198
Fig. 6-28a, p. 167
199
Fig. 6-28b, p. 167