Sedimentary RocksSedimentary Rocks

TerminologyTerminology

► Sedimentary RockSedimentary Rock - - a rock formed a rock formed through the deposition of sediments from through the deposition of sediments from weathering or biologic activity.weathering or biologic activity.

►DiagenesisDiagenesis-- Refers to all the chemical, Refers to all the chemical, physical, and biological changes that occur physical, and biological changes that occur after sediment is deposited and before and after sediment is deposited and before and after after lithificationlithification

► LithificationLithification - - The process by which The process by which unconsolidated sediments are transformed unconsolidated sediments are transformed into sedimentary rocks.into sedimentary rocks.

Lithification:Lithification:

► Two Processes control Lithification: Two Processes control Lithification: Compaction and CementationCompaction and Cementation

CompactionCompaction- - As sediment accumulates the As sediment accumulates the weight of the overlying material compresses the weight of the overlying material compresses the lower levels.lower levels.►This leads to a reduction in pore space (Imagine a This leads to a reduction in pore space (Imagine a

garbage compressor)garbage compressor) The size and shape of the grains that make up the The size and shape of the grains that make up the

sediment will directly determine the amount of sediment will directly determine the amount of compaction that occurs.compaction that occurs.

Sands and other course-grained sediment are less Sands and other course-grained sediment are less compressible than fine grained sediments such as clay.compressible than fine grained sediments such as clay.

CementationCementation

► CementationCementation- - Refers toRefers to the glue the glue (cement) that holds the sediments (cement) that holds the sediments together) together) For the most part, cementing materials are the For the most part, cementing materials are the

by product of chemical weathering (They are by product of chemical weathering (They are the dissolved components of other rocks).the dissolved components of other rocks).

Important Cementing agents:Important Cementing agents:► Calcite (CaCO3)Calcite (CaCO3)► Silica (SiO4)Silica (SiO4)► Iron Oxide (Fe2O3, FeO)Iron Oxide (Fe2O3, FeO)

These materials precipitate out of solution and These materials precipitate out of solution and become deposited within the pore spaces become deposited within the pore spaces between the sediment grainsbetween the sediment grains

Types of Sedimentary Types of Sedimentary RocksRocks

►DetritalDetrital- - Accumulated material that is Accumulated material that is transported as solid particlestransported as solid particles that that originated from chemical or originated from chemical or mechanical weathering.mechanical weathering.

►ChemicalChemical- - FormedFormed as previously as previously dissolved minerals precipitate out of dissolved minerals precipitate out of solution.solution.

Identification of Identification of Sedimentary RocksSedimentary Rocks

► Detrital Detrital sedimentary rocks sedimentary rocks are identified by are identified by both their both their composition and composition and their grain sizetheir grain size

► Chemical Chemical Sedimentary rocks Sedimentary rocks are classified by are classified by their chemical their chemical compositioncomposition

Sedimentary Sedimentary EnvironmentsEnvironments

► Sedimentary rocks form in many different Sedimentary rocks form in many different depositional environments ie. beaches, depositional environments ie. beaches, oceans, mountainsoceans, mountains

► Each area depositing a similar material is Each area depositing a similar material is considered a considered a sedimentary faciessedimentary facies. . As we move from the Continental Environment As we move from the Continental Environment

to the Marine Environment there will be a to the Marine Environment there will be a gradual transition between the facies due to the gradual transition between the facies due to the matter of transport and other factors affecting matter of transport and other factors affecting deposition.deposition.

► Detrital grain size gives some indication of the Detrital grain size gives some indication of the energy conditions during transport and energy conditions during transport and depositiondeposition

High-energyHigh-energy processes processes such as swift-flowing such as swift-flowing streams and waves are needed to transport streams and waves are needed to transport large grains such as gravellarge grains such as gravel

►Conglomerate (composed mostly of gravel) must Conglomerate (composed mostly of gravel) must have been deposited in areas of high energyhave been deposited in areas of high energy

►Sand transport also requires vigorous currentsSand transport also requires vigorous currents

Silt and clay are transported by weak currents and Silt and clay are transported by weak currents and and therefore only accumulate under and therefore only accumulate under low-energylow-energy conditions such as in lakes and lagoonsconditions such as in lakes and lagoons

Grain Size vs. Sedimentary Grain Size vs. Sedimentary EnvironmentEnvironment

Examples of Sed. FaciesExamples of Sed. Facies

Continental Continental EnvironmentsEnvironments

► Dominated by erosion and deposition associated Dominated by erosion and deposition associated with streamswith streams Channel DepositsChannel Deposits Alluvial FansAlluvial Fans Flood PlainsFlood Plains

► In colder areas glacial movement takes the place of In colder areas glacial movement takes the place of rivers and streams rivers and streams MorrainsMorrains

► In more arid areas wind is the more dominant factorIn more arid areas wind is the more dominant factor DunesDunes Playa LakesPlaya Lakes

Transitional EnvironmentsTransitional Environments (the shore line)(the shore line)

► BeachesBeaches► Tidal flatsTidal flats► LagoonsLagoons► Spits, bars and Spits, bars and

barrier islandsbarrier islands

MarineMarine

►Divided according to depthDivided according to depth Shallow Marine-Shallow Marine-

► reaches to depths of about 200 meters (~700ft)reaches to depths of about 200 meters (~700ft)►Extends from the shore to the outer edge of the Extends from the shore to the outer edge of the

continental shelfcontinental shelf► In some areas can reach up to 1500km, however, In some areas can reach up to 1500km, however,

average is about 80 kmaverage is about 80 km►Types of sediment deposited depend on several Types of sediment deposited depend on several

factorsfactors Distance from shoreDistance from shore Elevation of adjacent landElevation of adjacent land Water depthWater depth Water temperatureWater temperature ClimateClimate

Deep Marine –Deep Marine –

►Seaward of the continental shelf and reaches Seaward of the continental shelf and reaches depth greater than 200 mdepth greater than 200 m

►Deposition in these environments are Deposition in these environments are composed of tiny particles that drift in the composed of tiny particles that drift in the ocean current as well as skeletal remains of ocean current as well as skeletal remains of microscopic animals such as diatoms.microscopic animals such as diatoms.

► Sorting and rounding are two textural Sorting and rounding are two textural features of detrital sedimentary rocks that features of detrital sedimentary rocks that aid in determining depositional processesaid in determining depositional processes

► SortingSorting refers to the variation in size of refers to the variation in size of particles making up sediment or particles making up sediment or sedimentary rockssedimentary rocks It results from processes that selectively It results from processes that selectively

transport and deposit sediments of particular transport and deposit sediments of particular sizessizes

Sorting and RoundingSorting and Rounding

► If the size range is not very great, the If the size range is not very great, the sediment or rock is sediment or rock is well sortedwell sorted

► If they have a wide range of sizes, they are If they have a wide range of sizes, they are poorly sortedpoorly sorted

► For exampleFor example Wind has a limited ability to transport sediment so Wind has a limited ability to transport sediment so

dune sand tends to be well sorteddune sand tends to be well sorted Glaciers can carry any sized particles, because of Glaciers can carry any sized particles, because of

their transport power, so glacier deposits are poorly their transport power, so glacier deposits are poorly sortedsorted

SortingSorting

► RoundingRounding is the degree to which detrital is the degree to which detrital particles have their sharp corners and edges particles have their sharp corners and edges warn away by abrasionwarn away by abrasion Gravel in transport is rounded very quickly as the Gravel in transport is rounded very quickly as the

particles collide with one anotherparticles collide with one another Sand becomes rounded with considerably more Sand becomes rounded with considerably more

transporttransport

RoundingRounding

► A deposit of well A deposit of well rounded and well rounded and well sorted gravelsorted gravel

Rounding and SortingRounding and Sorting

► Angular, poorly Angular, poorly sorted gravelsorted gravel

► Sedimentary structuresSedimentary structures are visible features are visible features that formed at the time of deposition or shortly that formed at the time of deposition or shortly thereafterthereafter

Represent manifestations of the physical and Represent manifestations of the physical and biological processes that operated in depositional biological processes that operated in depositional environmentsenvironments

Structures seen in present-day environments or Structures seen in present-day environments or produced in experiments help provide information produced in experiments help provide information about depositional environments of rocks with similar about depositional environments of rocks with similar structuresstructures

Sedimentary StructuresSedimentary Structures

► Sedimentary rocks generally have Sedimentary rocks generally have beddingbedding or or stratificationstratification

BeddingBedding

Individual layers Individual layers less than 1 cm less than 1 cm thick are thick are laminationslaminations ►common in common in

mudrocksmudrocks BedsBeds are thicker are thicker

than 1 cmthan 1 cm►common in common in

rocks with rocks with coarser grainscoarser grains

► Some beds show an upward gradual decrease in Some beds show an upward gradual decrease in grain size, known as grain size, known as graded beddinggraded bedding

Graded BeddingGraded Bedding

► Graded bedding is Graded bedding is common in turbidity common in turbidity current depositscurrent deposits

form when sediment-form when sediment-water mixtures flow water mixtures flow along the seaflooralong the seafloor

As they slow, the As they slow, the largest particles largest particles settle out then settle out then smaller onessmaller ones

► Cross-beddingCross-bedding forms when layers come to rest forms when layers come to rest at an angle to the surface upon which they at an angle to the surface upon which they accumulate as on the downwind side of a sand accumulate as on the downwind side of a sand dunedune

► Cross-beds result from transport by either water Cross-beds result from transport by either water or windor wind The beds are inclined or dip downward in the direction of the The beds are inclined or dip downward in the direction of the

prevailing currentprevailing current They indicate ancient current directions, or They indicate ancient current directions, or

paleocurrentspaleocurrents► useful for relative dating of deformed useful for relative dating of deformed

sedimentary rockssedimentary rocks

Cross-BeddingCross-Bedding

► Small-scale alternating ridges and troughs known as Small-scale alternating ridges and troughs known as ripple marksripple marks are common on bedding planes, are common on bedding planes, especially in sandstoneespecially in sandstone

► Current ripple marksCurrent ripple marks form in response to water or form in response to water or wind currents flowing in one directionwind currents flowing in one direction have asymmetric profiles allowing geologists to determine have asymmetric profiles allowing geologists to determine

paleocurrent directionspaleocurrent directions

► Wave-formed ripple marksWave-formed ripple marks result from the to-and- result from the to-and-fro motion of wavesfro motion of waves tend to be symmetricaltend to be symmetrical

► Useful for relative dating of deformed sedimentary Useful for relative dating of deformed sedimentary rocksrocks

Ripple MarksRipple Marks

► Ripples with an Ripples with an asymmetrical shapeasymmetrical shape

► In the close-up of one In the close-up of one ripple, the internal ripple, the internal structure shows small-structure shows small-scale cross-beddingscale cross-bedding

► The photo shows The photo shows current ripples that current ripples that formed in a small formed in a small stream channel with stream channel with flow from right to leftflow from right to left

Current Ripple MarksCurrent Ripple Marks

► As the waves As the waves wash back and wash back and forth, forth, symmetrical symmetrical ripples formripples form

► The photo The photo shows wave-shows wave-formed ripple formed ripple marks in marks in shallow shallow seawaterseawater

Wave-Formed RipplesWave-Formed Ripples

► Tabular cross-bedding forms Tabular cross-bedding forms by deposition on sand wavesby deposition on sand waves

► Typically produce where Typically produce where streams enter large bodies streams enter large bodies of water (AKA Deltas)of water (AKA Deltas)

Cross-Cross-BeddingBedding

► Tabular cross-bedding in Tabular cross-bedding in the Upper Cretaceousthe Upper Cretaceous

►Animation of crossbeddingAnimation of crossbedding

Cross-Cross-BeddingBedding

► Trough cross-bedding Trough cross-bedding formed by migrating dunesformed by migrating dunes

► Trough cross-beds in the Trough cross-beds in the Pliocene Six Mile Creek Pliocene Six Mile Creek Formation, MontanaFormation, Montana

► When clay-rich sediments dry, they shrink and When clay-rich sediments dry, they shrink and crack into polygonal patterns bounded by crack into polygonal patterns bounded by fractures called fractures called mud cracksmud cracks

► Mud cracks require wetting and drying to form,Mud cracks require wetting and drying to form,

Mud CracksMud Cracks

lakeshore lakeshore river flood plain river flood plain or where mud is or where mud is

exposed at low exposed at low tide along a tide along a seashoreseashore

► Mud cracks in Mud cracks in ancient rocks in ancient rocks in Glacier National Glacier National Park, MontanaPark, Montana

► Mud cracks Mud cracks typically fill in typically fill in with sediment with sediment when they are when they are preserved as preserved as seen hereseen here

Ancient Mud CracksAncient Mud Cracks

► Biogenic sedimentary structuresBiogenic sedimentary structures include include trackstracks burrowsburrows trailstrails

► Also called Also called trace fossilstrace fossils► Extensive burrowing by organisms is called Extensive burrowing by organisms is called

bioturbationbioturbation may alter sediments so thoroughly that other may alter sediments so thoroughly that other

structures are disrupted or destroyedstructures are disrupted or destroyed

Biogenic Sedimentary Biogenic Sedimentary StructuresStructures

Sedimentary StructuresSedimentary Structures

► Sedimentary rocks form as layer upon layer of Sedimentary rocks form as layer upon layer of sediment accumulates in various environments.sediment accumulates in various environments.

► These layers are known as These layers are known as StrataStrata or or BedsBeds..

• Each Stratum or bed of a sedimentary rock is unique

•Texture and composition of the bed reflects the different conditions under which each layer was deposited.

Between the LinesBetween the Lines► Separating each strata are Separating each strata are bedding planesbedding planes..

Bedding Planes are flat surfaces along which the Bedding Planes are flat surfaces along which the rock tends to break. rock tends to break.

►Caused by changes in grain size.Caused by changes in grain size.►May also occur after pauses in deposition.May also occur after pauses in deposition.

In general most deposition occurs due to In general most deposition occurs due to sediments settling out of water therefore most sediments settling out of water therefore most bedding is horizontal.bedding is horizontal.

In wind driven deposition, layers are not always horizontal. In wind driven deposition, layers are not always horizontal. Sand dunes are a prime example. In these cases the Sand dunes are a prime example. In these cases the bedding is described as bedding is described as Cross BeddingCross Bedding

Bedding PlanesBedding Planes