Chapter6 sedimentaryrocks-110708161605-phpapp01

Essentials of GeologyEssentials of Geology
33rdrd
EditionEdition
Chapter 6
Norton Media LibraryNorton Media Library

Pages of Earth’s Past:
Sedimentary Rocks
Prepared by:Prepared by:
Ronald ParkerRonald Parker,, Senior GeologistSenior Geologist
Fronterra GeosciencesFronterra Geosciences
Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, NeuquénHouston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén
www.fronterrageo.comwww.fronterrageo.com

Essentials of Geology, 3rd
edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks
Sedimentary CoverSedimentary Cover
 Earth is covered by a thin ‘veneer’ of sediment.Earth is covered by a thin ‘veneer’ of sediment.
 The veneer capsThe veneer caps igigneous andneous and metametamorphic “basement.”morphic “basement.”
 Sediment cover varies in thickness from 0 to 20 km.Sediment cover varies in thickness from 0 to 20 km.
 Thinner (or missing) where ig and meta rocks outcrop.Thinner (or missing) where ig and meta rocks outcrop.
 Thicker in sedimentary basins.Thicker in sedimentary basins.

Sedimentary RocksSedimentary Rocks
 Sediments are the building blocks of sedimentary rocks.Sediments are the building blocks of sedimentary rocks.
 Sediments are diverse, as are the rocks made from them.Sediments are diverse, as are the rocks made from them.
 Four classes:Four classes:
 Clastic – Made from weathered rock fragments (clasts).Clastic – Made from weathered rock fragments (clasts).
 Biochemical – Cemented shells of organisms.Biochemical – Cemented shells of organisms.
 Organic – The carbon-rich remains of plants.Organic – The carbon-rich remains of plants.
 Chemical – Minerals that crystallize directly from water.Chemical – Minerals that crystallize directly from water.
ChemicalChemicalClasticClastic OrganicOrganicBiochemicalBiochemical

Clastic Sedimentary RocksClastic Sedimentary Rocks
 Clastic sedimentary rocks reflect several processes.Clastic sedimentary rocks reflect several processes.
 Weathering – Generation of detritus via rock disintegration.Weathering – Generation of detritus via rock disintegration.
 Erosion – Removal of sediment grains from rock.Erosion – Removal of sediment grains from rock.
 Transportation – Dispersal by wind, water, and ice.Transportation – Dispersal by wind, water, and ice.
 Deposition – Settling out of the transporting fluid.Deposition – Settling out of the transporting fluid.
 Lithification – Transformation into solid rock.Lithification – Transformation into solid rock.

 Lithification – Transforms loose sediment into solid rock.Lithification – Transforms loose sediment into solid rock.
 Burial – More sediment is added onto previous layers.Burial – More sediment is added onto previous layers.
 Compaction – Overburden weight reduces pore space.Compaction – Overburden weight reduces pore space.
Sand – 10 to 20%Sand – 10 to 20%
Clay – 50 to 80%Clay – 50 to 80%
 Cementation – Minerals grow in pores, “gluing” sediments.Cementation – Minerals grow in pores, “gluing” sediments.

 Classified on the basis of texture and composition.Classified on the basis of texture and composition.
 Clast (grain) size.Clast (grain) size.
 Clast composition.Clast composition.
 Angularity and sphericity.Angularity and sphericity.
 Sorting.Sorting.
 Character of cement.Character of cement.
 These variables produce a diversity of clastic rocks.These variables produce a diversity of clastic rocks.

 Clast (grain) size – The average diameter of clasts.Clast (grain) size – The average diameter of clasts.
 Range from very coarse to very fine.Range from very coarse to very fine.
 Boulder, cobble, pebble, sand, silt, and clay.Boulder, cobble, pebble, sand, silt, and clay.
 With increasing transport, average grain size decreases.With increasing transport, average grain size decreases.

 Clast composition – The mineral makeup of sediments.Clast composition – The mineral makeup of sediments.
 May be individual minerals or rock fragments.May be individual minerals or rock fragments.
 Mineral identities provide clues about…Mineral identities provide clues about…
The source of the sediment.The source of the sediment.
The environment of deposition.The environment of deposition.

 Angularity and sphericity – Indicate degree of transport.Angularity and sphericity – Indicate degree of transport.
 Fresh detritus is usually angular and non-spherical.Fresh detritus is usually angular and non-spherical.
 Grain roundness and sphericity increases with transport.Grain roundness and sphericity increases with transport.
 Well-rounded – Long transport distances.Well-rounded – Long transport distances.
 Angular – Negligible transport.Angular – Negligible transport.

 Sorting – The uniformity of grain size.Sorting – The uniformity of grain size.
 Well-sorted – Uniform grain sizes.Well-sorted – Uniform grain sizes.
 Poorly sorted – Wide variety of grain sizes.Poorly sorted – Wide variety of grain sizes.
 Sorting becomes better with distance from source.Sorting becomes better with distance from source.

 Cement – Minerals that fill sediment pores.Cement – Minerals that fill sediment pores.
 Fluids with dissolved solids flush through pore system.Fluids with dissolved solids flush through pore system.
 Dissolved ions slowly crystallize and fill pores.Dissolved ions slowly crystallize and fill pores.
 Cementation varies from weak to strong.Cementation varies from weak to strong.
 Common cements:Common cements:
 QuartzQuartz
 CalciteCalcite
 HematiteHematite
 Clay mineralsClay minerals

 Coarse clastics – Composed of gravel-sized clasts.Coarse clastics – Composed of gravel-sized clasts.
 Breccia – Comprised of angular fragments.Breccia – Comprised of angular fragments.
Angularity indicates a lack of transport processing.Angularity indicates a lack of transport processing.
Deposited relatively close to source.Deposited relatively close to source.

 Coarse clastics – Composed of gravel-sized clasts.Coarse clastics – Composed of gravel-sized clasts.
 Conglomerate – Comprised of rounded gravel.Conglomerate – Comprised of rounded gravel.
Indicates water transport.Indicates water transport.
Clasts bang together forcefully in flowing water.Clasts bang together forcefully in flowing water.
Collisons round angular corners and edges of clasts.Collisons round angular corners and edges of clasts.
Conglomerates are deposited at a distance from the source.Conglomerates are deposited at a distance from the source.

 Sandstone – Clastic rock made of sand-sized particles.Sandstone – Clastic rock made of sand-sized particles.
 Forms in many depositional settings.Forms in many depositional settings.
 Quartz is, by far, the dominant mineral in sandstones.Quartz is, by far, the dominant mineral in sandstones.
 Sandstone varieties.Sandstone varieties.
 Arkose – Contains abundant feldspar.Arkose – Contains abundant feldspar.
 Quartz sandstone – Almost pure quartz.Quartz sandstone – Almost pure quartz.

 Fine clastics - Composed of silt and clay.Fine clastics - Composed of silt and clay.
 Silt-sized sediments are lithified to form siltstone.Silt-sized sediments are lithified to form siltstone.
 Clay-sized particles form shale.Clay-sized particles form shale.
 Fine clastics are deposited in quieter waters.Fine clastics are deposited in quieter waters.
 Floodplains, lagoons, mudflats, deltas, deep-water basins.Floodplains, lagoons, mudflats, deltas, deep-water basins.
 Organic-rich shales are the source of petroleum.Organic-rich shales are the source of petroleum.

 These are sediments derived from living organisms.These are sediments derived from living organisms.
 Biochemical – Hard mineral skeletons.Biochemical – Hard mineral skeletons.
 Organic – Cells of plants, algae, bacteria and plankton.Organic – Cells of plants, algae, bacteria and plankton.
Biochemical and Organic RocksBiochemical and Organic Rocks

Biochemical RocksBiochemical Rocks
 Biochemical limestone – CaCOBiochemical limestone – CaCO33 skeletal (shell) remains.skeletal (shell) remains.
 Warm, tropical, shallow, clear, OWarm, tropical, shallow, clear, O22-rich, marine water.-rich, marine water.
 Diverse organisms (plankton, corals, clams, snails, etc.).Diverse organisms (plankton, corals, clams, snails, etc.).
 Many textural varieties.Many textural varieties.
Reefs.Reefs.
Shell debris.Shell debris.
Lime mud (micrite).Lime mud (micrite).

 Chert – Rock made of cryptocrystalline quartz.Chert – Rock made of cryptocrystalline quartz.
 Formed from opalline silica (SiOFormed from opalline silica (SiO22) skeletons.) skeletons.
Diatoms.Diatoms.
Radiolarians.Radiolarians.
 Opalline silica added to bottom sediments dissolves.Opalline silica added to bottom sediments dissolves.
 Silica pore fluids solidify to form chert nodules or beds.Silica pore fluids solidify to form chert nodules or beds.
Biochemical RocksBiochemical Rocks

 Made from organic carbon.Made from organic carbon.
 Coal – Altered remains of fossil vegetation.Coal – Altered remains of fossil vegetation.
Accumulates in lush tropical wetland settings.Accumulates in lush tropical wetland settings.
Requires deposition in the absence of oxygen.Requires deposition in the absence of oxygen.
 Oil shale – Shale with heat altered organic matter.Oil shale – Shale with heat altered organic matter.
Organic RocksOrganic Rocks

Chemical Sedimentary RocksChemical Sedimentary Rocks
 Comprised of minerals precipitated from water solution.Comprised of minerals precipitated from water solution.
 Evaporites – Created from evaporated seawater.Evaporites – Created from evaporated seawater.
 Evaporation triggers deposition of chemical precipitates.Evaporation triggers deposition of chemical precipitates.
 Examples include halite (rock salt) and gypsum.Examples include halite (rock salt) and gypsum.

 Travertine – Calcium carbonate (CaCOTravertine – Calcium carbonate (CaCO3) precipitated from
groundwater where it reaches the surface.
 Dissolved calcium (Ca2+
)reacts with bicarbonate (HCO3
-
).
 CO2 expelled into the air causes CaCO3 to precipitate.
Thermal (hot) springs.
Caves.

 Dolostone – Limestone altered by Mg-rich fluids.Dolostone – Limestone altered by Mg-rich fluids.
 CaCOCaCO33 altered to dolomite CaMg(COaltered to dolomite CaMg(CO33))22 by Mgby Mg2+2+
-rich water.-rich water.
 Dolostone looks like limestone, except…Dolostone looks like limestone, except…
It has a sugary texture and a pervasive porosity.It has a sugary texture and a pervasive porosity.
It weathers to a buff, tan color.It weathers to a buff, tan color.

 Replacement chert – Nonbiogenic in origin.Replacement chert – Nonbiogenic in origin.
 Many varieties.Many varieties.
 Flint – Black or gray from organic matter.Flint – Black or gray from organic matter.
 Jasper – Red or yellow from Fe-oxides.Jasper – Red or yellow from Fe-oxides.
 Petrified wood – Wood grain preserved by silica.Petrified wood – Wood grain preserved by silica.
 Agate – Concentrically layered rings.Agate – Concentrically layered rings.

Sedimentary StructuresSedimentary Structures
 Features imparted to sediments at or near deposition.Features imparted to sediments at or near deposition.
 Layering.Layering.
 Surface features on layers.Surface features on layers.
 Arrangement of grains.Arrangement of grains.
 Help decipher conditions at or near time of deposition.Help decipher conditions at or near time of deposition.

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

 Bedding reflects changing conditions during deposition.Bedding reflects changing conditions during deposition.
 These can be changes in…These can be changes in…
 Energy conditions, and hence, grain size.Energy conditions, and hence, grain size.
 Disturbance by organisms.Disturbance by organisms.
 Bedding may also reflect non-deposition or erosion.Bedding may also reflect non-deposition or erosion.

 A series of beds are referred to as strata.A series of beds are referred to as strata.
 Formation: Strata recognized on a regional scale.Formation: Strata recognized on a regional scale.
 Geologic maps display the distribution of formations.Geologic maps display the distribution of formations.
 i.e. Coconino Formationi.e. Coconino Formation
Note the prominent white band of Coconino
Sandstone clearly visible across Grand Canyon.
Note the prominent white band of Coconino
Sandstone clearly visible across Grand Canyon.

 Water flowing over loose sediment creates bedforms.Water flowing over loose sediment creates bedforms.
 Bedforms are linked to flow velocity and sediment size.Bedforms are linked to flow velocity and sediment size.
 Ripples, cm-scale ridges, and troughs, indicate flow.Ripples, cm-scale ridges, and troughs, indicate flow.
Asymmetric ripples – Unidirectional flow.Asymmetric ripples – Unidirectional flow.
Symmetric ripples – Wave oscillation.Symmetric ripples – Wave oscillation.
Ripples are commonly preserved in sedimentary rocks.Ripples are commonly preserved in sedimentary rocks.

BedformsBedforms
 Cross beds – Created by ripple and dune migration.Cross beds – Created by ripple and dune migration.
 Sediment moves up the gentle side of a ripple or dune.Sediment moves up the gentle side of a ripple or dune.
 Sediment piles up, then slips down the steep face.Sediment piles up, then slips down the steep face.
The slip face continually moves downstream.The slip face continually moves downstream.
Added sediment forms sloping “cross-bedded” layers.Added sediment forms sloping “cross-bedded” layers.

BedformsBedforms
 Dunes – Similar to ripples except much larger.Dunes – Similar to ripples except much larger.
 Form from wind-blown sand in desert or beach regions.Form from wind-blown sand in desert or beach regions.
 Often preserve large internal cross-laminations.Often preserve large internal cross-laminations.

BedformsBedforms
 Turbidity currents.Turbidity currents.
 Sediment moves on a slope as a pulse of turbid water.Sediment moves on a slope as a pulse of turbid water.
 As pulse wanes, water loses velocity and grains settle.As pulse wanes, water loses velocity and grains settle.
 Coarsest material settles first, medium next, then fines.Coarsest material settles first, medium next, then fines.
 This process forms graded beds in turbidite deposits.This process forms graded beds in turbidite deposits.

Bed-Surface MarkingsBed-Surface Markings
 Occur after deposition while sediment is still soft.Occur after deposition while sediment is still soft.
 Mudcracks – Polygonal desiccation features in wet mud.Mudcracks – Polygonal desiccation features in wet mud.
Indicate alternating wet and dry conditions.Indicate alternating wet and dry conditions.
Necessitate deposition in a terrestrial setting.Necessitate deposition in a terrestrial setting.
 Scour marks – Troughs eroded in soft mud by current flow.Scour marks – Troughs eroded in soft mud by current flow.
 Fossils – Evidence of past life.Fossils – Evidence of past life.
Footprints.Footprints.
Shell impressions.Shell impressions.

 Locations where sediment accumulates. They differ in…Locations where sediment accumulates. They differ in…
 Energy regime.Energy regime.
 Sediment delivery, transport, and depositional conditions.Sediment delivery, transport, and depositional conditions.
 Chemical, physical and biological characteristics.Chemical, physical and biological characteristics.
 Environments range from terrestrial to marine.Environments range from terrestrial to marine.
Depositional EnvironmentsDepositional Environments

 Terrestrial environments – Deposited above sea level.Terrestrial environments – Deposited above sea level.
 Glacial – Due to movement of ice.Glacial – Due to movement of ice.
Ice carries and dumps every grain size.Ice carries and dumps every grain size.
Creates glacial till; poorly sorted gravel, sand, silt, and clay.Creates glacial till; poorly sorted gravel, sand, silt, and clay.

 Mountain streams.Mountain streams.
Water carries large clasts during floods.Water carries large clasts during floods.
During low flow, these cobbles and boulders are immobile.During low flow, these cobbles and boulders are immobile.
Course conglomerate is characteristic of this setting.Course conglomerate is characteristic of this setting.

 Alluvial fan - Sediments that pile up at a mountain front.Alluvial fan - Sediments that pile up at a mountain front.
Rapid drop in stream velocity creates a cone-shaped wedge.Rapid drop in stream velocity creates a cone-shaped wedge.
Sediments are immature conglomerates and arkoses.Sediments are immature conglomerates and arkoses.

 Terrestrial Environments– Deposited above sea level.Terrestrial Environments– Deposited above sea level.
 Sand dunes – Wind-blown piles of well-sorted sand.Sand dunes – Wind-blown piles of well-sorted sand.
Dunes move according to the prevailing winds.Dunes move according to the prevailing winds.
Result in uniform sandstones with gigantic cross beds.Result in uniform sandstones with gigantic cross beds.

 Terrestrial environments– Deposited above sea level.Terrestrial environments– Deposited above sea level.
 Rivers – Channelized flow transports sediment.Rivers – Channelized flow transports sediment.
Sand and gravel fill concave-upward channels.Sand and gravel fill concave-upward channels.
Fine sand, silt, and clay are deposited on nearby floodplains.Fine sand, silt, and clay are deposited on nearby floodplains.

 Terrestrial environments– Deposited above sea level.Terrestrial environments– Deposited above sea level.
 Lake – Large ponded bodies of water.Lake – Large ponded bodies of water.
Gravels and sands trapped near shore.Gravels and sands trapped near shore.
Well-sorted muds deposited in deeper water.Well-sorted muds deposited in deeper water.
Often capped with wetland muds.Often capped with wetland muds.

 Marine environments – Deposited at or below sea level.Marine environments – Deposited at or below sea level.
 Deltas – Sediments dropped where a river enters the sea.Deltas – Sediments dropped where a river enters the sea.
Sediment carried by the river is dumped when velocity drops.Sediment carried by the river is dumped when velocity drops.
Deltas grow over time, building out into the basin.Deltas grow over time, building out into the basin.
Often develop a topset – foreset – bottomset geometry.Often develop a topset – foreset – bottomset geometry.

 Coastal beaches – Surf zone.Coastal beaches – Surf zone.
Sediments are constantly being processed by wave attack.Sediments are constantly being processed by wave attack.
A common result? Well-sorted, well-rounded medium sand.A common result? Well-sorted, well-rounded medium sand.
Beach sandstones may preserve oscillation ripples.Beach sandstones may preserve oscillation ripples.

 Shallow marine – Finer version of beach sediment.Shallow marine – Finer version of beach sediment.
Fine silts and muds turn into siltstones and mudstones.Fine silts and muds turn into siltstones and mudstones.
Usually support an active biotic community.Usually support an active biotic community.

 Shallow water carbonates – Tropical.Shallow water carbonates – Tropical.
Skeletons of marine invertebrates.Skeletons of marine invertebrates.
Born in the carbonate factory.Born in the carbonate factory.
Warm, clear, shallow, normal salinity, marine water.Warm, clear, shallow, normal salinity, marine water.

 Deep marine – Fines predominate far from land sources.Deep marine – Fines predominate far from land sources.
Skeletons of planktonic organisms make chalk or chert.Skeletons of planktonic organisms make chalk or chert.
Fine silts and clays turn to shale.Fine silts and clays turn to shale.

Sedimentary BasinsSedimentary Basins
 Sediments vary in thickness across Earth’s surface.Sediments vary in thickness across Earth’s surface.
 Thin to zero edge where non-sedimentary rocks outcrop.Thin to zero edge where non-sedimentary rocks outcrop.
 Thicken to 10 to 20+ km in sedimentary basins.Thicken to 10 to 20+ km in sedimentary basins.
 Subsidence – Sinking of the land during sedimentation.Subsidence – Sinking of the land during sedimentation.
 Due to crustal flexure and faulting.Due to crustal flexure and faulting.
 Compounded by the weight of added sediments.Compounded by the weight of added sediments.
 Basins are important locations for natural resources.Basins are important locations for natural resources.
 Coal.Coal.
 Petroleum.Petroleum.
 Natural gas.Natural gas.
 Uranium.Uranium.

 Basins form where tectonic activity creates space.Basins form where tectonic activity creates space.
 Rift basins – Divergent (pull-apart) plate boundaries.Rift basins – Divergent (pull-apart) plate boundaries.
Crust thins by stretching and rotational normal faulting.Crust thins by stretching and rotational normal faulting.
Thinned crust subsides.Thinned crust subsides.
Sediment fills the down-dropped basin.Sediment fills the down-dropped basin.

 Passive margin basins – Non-plate-boundary continentalPassive margin basins – Non-plate-boundary continental
edge.edge.
Underlain by crust thinned by previous rifting.Underlain by crust thinned by previous rifting.
Thinned crustThinned crust
subsides as it cools.subsides as it cools.

 Intracontinental basins – Interiors far from margins.Intracontinental basins – Interiors far from margins.
Result from differential thermal subsidence.Result from differential thermal subsidence.
May be linked to failed crustal rifts.May be linked to failed crustal rifts.

 Foreland basins – Craton side of collisional mountain belt.Foreland basins – Craton side of collisional mountain belt.
Flexure of the crust from loading creates a downwarp.Flexure of the crust from loading creates a downwarp.
Fills with debris eroded off of the mountains.Fills with debris eroded off of the mountains.

 Sea-level changes.Sea-level changes.
 Sedimentary deposition is strongly linked to sea level.Sedimentary deposition is strongly linked to sea level.
 Changes in sea level are commonplace geologically.Changes in sea level are commonplace geologically.
Depositional belts shift landward or seaward in response.Depositional belts shift landward or seaward in response.
Layers of strata record deepening or shallowing upward.Layers of strata record deepening or shallowing upward.
 Transgression – Flooding due to sea-level rise.Transgression – Flooding due to sea-level rise.
Sediment belts shift landward; strata “deepen” upward.Sediment belts shift landward; strata “deepen” upward.

 Regression – Exposure due to sea level fall.Regression – Exposure due to sea level fall.
 Depositional belts shift seaward; strata “shallow upward.”Depositional belts shift seaward; strata “shallow upward.”
 Regression tied to erosion; less likely to be preserved.Regression tied to erosion; less likely to be preserved.
 Sea-level rise and fall creates a predictable pattern.Sea-level rise and fall creates a predictable pattern.
Transgression – Regression AnimationTransgression – Regression Animation

DiagenesisDiagenesis
 Physical, chemical, and biological changes to sediment.Physical, chemical, and biological changes to sediment.
 Bioturbation.Bioturbation.
 Lithification.Lithification.
 Dissolution.Dissolution.
 Mineral precipitation.Mineral precipitation.
 Temps between burial and metamorphism (~300Temps between burial and metamorphism (~300oo
C).C).
 Integrates changes across the entire sediment history.Integrates changes across the entire sediment history.

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