1_Sed Geol

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METAMORPHICETAMORPHIC SEDIMENTARYEDIMENTARY

IGNEOUSGNEOUS

melti

ng

heat

,pre

ssur

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ions

weathering,tra

nsp

orta

tion

lithific

atio

n

melting

weathering, transportation,lithification

heat, pressure, ions

The Rock Cycle

Rocks are naturally occurringcombinations or coherent

aggregates of minerals, fossils

or other hard materials. They

are classified by the way in

which they form. The three

rock types are: igneous,

sedimentary and

metamorphic.

All rocks on Earth are locked into

a system of cycling and re-

cycling known as theROCK

CYCLE.

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Study of sedimentary geology aims at:

The description and classification of clastic/nonclastic sedimentary rocks. Processes and their products in the sedimentary record that aid in

interpreting ancient sedimentary environments.

Age of rocks:

Based on relative age (relative to associated rocks)

orabsolute age (radiometric dating)

Earth History:The history of changing environments on Earth

Environmental interpretation of rocks + Age of rocks

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The Sediments and the Sedimentary Rocks

A very basic classification of

all sedimentary rocks isbased on the type of material

that is deposited and the

modes of deposition.

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The study of sediment and sedimentary rocks that are made up of particles that are thesolid products of weathering at or near the Earths surface.

66% of the surface of the Earth is covered by sediment or sedimentary rocks

Exploit resources from it.

Clastic SedimentologyGravel Sand Mud (silt and clay)

Conglomerate Sandstone Siltstone and Shale

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Environmental interpretation: The present is the key to the past

By examining the characteristics of various environments on Earth today,environments in which ancient sediments were deposited can be interpreted

The description and classification of clastic sediment along with the process(es)involved and their products in the sedimentary record aid in interpreting ancient

sedimentary environments

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Surface Processes: chemical and physical weatheringand sedimentary rocks

Questions

How do rocks get destroyed and recycled at the surfaceof the Earth?

At the other end of the transport system, how doweathered and eroded materials end up making the

various kinds of sedimentary rocks? What can observations of the sedimentary record reveal

about the tectonics, petrology, and climate of bothdepositional environments and upstream sourceenvironments?

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Weathering and Sedimentation in the Rock Cycle Geology so far has focused on internally-driven processes: plate

tectonics, magmatism, metamorphism, orogeny.

The rest of geology isdriven by surfaceprocesses: thehydrologic cycle(rainfall, streams, ice),gravity, aqueous

chemistry. Weathering and

erosion are theprocesses that formand transportsediment.

Sedimentation, burialand lithification arethe processes thattransform weatheringproducts intosedimentary rocks.

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Weathering: decomposition of rocks

There is a distinction betweenweathering and erosion:

Weathering converts exposed

rock to soil in place

Erosion transports dissolvedor fragmented material fromthe source area whereweathering is occurring to adepositional environment .

Most of the earths surface iscovered by exposure ofsediment or sedimentary rock,by area.

But the sediment layer is thinin most places, with respect to

overall crustal thickness, sosedimentary rock is a minorvolume fraction of the crust(in part by definition: onceburied to the mid-crust,sediments get cooked tometasediments).

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Weathering: chemical and physical

The destruction of rocks at the Earthssurface by weathering has twofundamental modes of operation:

Chemical weathering is dissolution oralteration of the original minerals,usually by reactions with aqueoussolutions

Chemical weathering puts ions fromthe source minerals into solution for

subsequent erosion by transport inflowing water as dissolved load.

Physical weathering is fragmentationinto progressively smaller particles,from intact outcrop to boulders and ondown to mineral fragments and sandgrains.

Physical weathering makes loosepieces of rock available for downslopemovement by mass wasting ortransport in flowing water as suspendedor bed load.

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Chemical Weathering

Chemical weathering is driven bythermodynamic energy minimization, justlike chemical reactions at hightemperature.

The system seeks the most stableassemblage of phases.

The differences are that (1) kinetics areslow and metastability is common; (2) thestable minerals under wet, ambient

conditions are different from those at highTandP; (3) solubility in water and itsdependence on water chemistry (notablypH) are major determinants in the stabilityof minerals in weathering.

A fresh rock made of olivine and

pyroxenes will end up as clays and ironoxides, with other elements in solution

A fresh rock made of feldspars and quartzwill end up as clays, hydroxides, andquartz in most waters.

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Chemical Weathering

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Chemical Weathering The most common alteration product of feldspars is kaolinite, Al2Si2O5(OH)4,

which serves as a model for the formation of clays by weathering generally.

The reactions of feldspars to kaolinite illustrate some of the basic trends:

K, Na, Ca are highly soluble and readily leachedby chemical weathering. Excess Si can be removed as silicic acid although quartz is relatively insoluble.

Al is extremely insoluble, and is essentially conserved as source rock is converted to clays.

Weathering is a hydration process, leaving H2O bound in the altered minerals.

2 KAlSi3O8 + 9 H2O + 2 H+ -> Al2Si2O5(OH)4 + 2 K

+ + 4 H4SiO4 Note the H+ on the left-hand sideonly acidic water can drive this reaction

Natural waters are acidic due to equilibrium of carbonic acid with CO2 in the atmosphere

CO2 (g) + H2O = H2CO3

2 KAlSi3O8 + 9 H2O + 2 H2CO3 - > Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4 + 2HCO3

Alteration of rock transforms acidic rainwater into neutral surface or ground water,with bicarbonate the dominant species (relative to CO2 and CO3

2).

Mg and Fe2+

are also readily leached, but Fe3+

is very insolublethe ultimate residueof alteration of mafic rocks is laterite.

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Chemical Weathering Some minerals are congruently soluble in acidic water, leaving no residue

The most abundant is calcite: CaCO3 + H2CO3 = Ca2+ + 2HCO3

(the Tums reaction)

Effects of dissolution (and precipitation) of calcite can be dramatic, to say the least.

Sinkhole Speleothems

Karst terrain

R t f Ch i l

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Rates of Chemical

Weathering

Many factors affect therate at which a rock willweather, as summarizedhere.

Some of these variables arelocal (e.g., source rock),some are global. Theseinclude temperature andpCO2, leading to the CO2-

weathering feedback cycle.

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Physical Weathering

Anything that promotes disaggregration of a rock so that pieces can form soil or beeroded away by wind, water, or gravity transport is physical weathering.

The distinction between physical weathering and erosion is subtle, but think of physicalweathering as fragmenting the rock and erosion as carrying the fragments away; attimes these may be the same event, of course.

Rocks that are jointed or faulted or have pre-existing weak zones are most easilyweathered.

Few of the stresses associated with physical weathering are significant compared to the

tensile strength of intact rocks; something, has to start the process, either initial cracksand weaknesses or chemical attack on mineral cohesion.

Organisms, especially plants (think tree roots), are fond of breaking up rocks.

Freeze-thaw, frost wedging, frost heavethe volume change between ice andwater is effective in widening cracks in rock in suitable climates.

Physical abrasion by flowing air or water, or more often by rock particles alreadymobilized by water or wind (think Fossil Falls).

Tectonicsrocks caught in a fault zone are definitely undergoing physicalweathering.

Etc.

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Weathering feedbacks: chemical and physical Physical weathering and

chemical weatheringgenerally proceed in

parallel in mostenvironments.

Physical and chemicalweathering promote oneanother: Formation of cracks by

physical weatheringincreases reactive surfacearea, promoting chemicalweathering.

Chemical weatheringreplaces intact

interlocking mineralswith weak clays or voidspace, making the rockeasier to physicallydisaggregate, promotingphysical weathering

W h i f db k

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Weathering feedbacks:

more generally

Weathering of both kinds playskey roles in several feedbacks.

Tectonics affects weatheringthrough slopes and elevations,climate affects weatheringthrough temperatures (viachemical kinetics and freeze-thaw), rainfall, pCO2, etc.

Conversely, weathering anderosion affect tectonics andclimate:

Denudation by erosion must beisostatically compensated andso affect vertical motions of the

crust

Weathering controls waterchemistry, courses of streamsand groundwater, removes CO2from the atmosphere, etc.

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Soil formation

A weathered surface develops astratified structure, with intact rockat the bottom (or inside) and

maximum weathering at the top . Leachable ions are transported

downwards by groundwater flow,possibly redeposited as waterchemistry adjusts towards

equilibrium with the developingsoil profile.

Chemically and physically weathered rock that is noteroded or transported but remains in place becomessoil.

Soil formation

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Soil formation The mineralogy and thickness of soil layers depends on

source rock, climate (temperature and rainfall), and age.

Which of these soil types would you rather farm?

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Erosion and Transport Between weathering and sedimentation, matter must be

transported from source to destination. This is erosion. We dealt with the landforms generated by erosion in the

geomorphology lecture; here our concern is with the effects oftransport on sedimentary rocks.

Modes of transport:

Gravity (short distances and steep slopes)

Wind (small particles only)

Glaciers Water

Surface runoff carries dissolved, suspended, and bed loads

Groundwater flow only carries dissolved load

All these mechanisms carry products of physicalweathering and insoluble residues of chemicalweathering.

Only water transport carries away leached solubleproducts of chemical weathering.

i d

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Erosion and Transport

Certain modes of transport physically modify andphysically and chemically sort particles en route.

Size sorting by surface water runoff flow:

Current of a givenvelocity can generallycarry all noncohesiveparticles smaller than a

critical size; sincecurrent velocity dropswith decreasing slopesfrom mountains tolowlands, it follows thatsediments evolve from

poorly sorted andcoarse-grained nearsource to well-sortedand finer grained withincreasing transportdistance.


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Chemical sorting withincreasing transport distanceis like a continuation ofchemical weathering during

intermittent times whenparticles are temporarilydeposited before furthertransport; most stableminerals are transported thefurthest.

Textures of particles aremodified by abrasion duringwind or water transport.Close to source particles are

angular; far from sourceparticles are rounded.

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Sedimentation

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Sedimentation Eventually transported particles and dissolved ions reach a place where they can

be permanently deposited and accumulated. This issedimentation.

The sedimentary rocks that result from this accumulation are controlled by andrecord thesedimentary environmentwhere they were deposited.

We interpret ancient sedimentary rocks by comparison to modernenvironments where we can observe ongoing sedimentary processes andrelate them to the composition, texture, and structure of the resulting rocks.

S di t ti

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Sedimentation Sediments and the

environments in which theyform are fundamentally dividedinto clasticand chemical:

Clastic sediments are made ofphysically transported anddeposited particles (they maylater gain chemically growncement during diagenesis)

Chemical sediments are grown

from solution, organically orinorganically; biochemicalsediment more specificallyrefers to minerals grown fromsolution by organisms

In some cases the relationship

between the environment andthe character of the sediment isabsolute and obvious (carbonatein reefs, boulder-strewn till inperiglacial deposit, etc.); othercases are more subtle.

Diagenesis

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Diagenesis

The process of modification ofnewly deposited sediments intosedimentary rocks is diagenesis or

lithification. Processes include:

physical compaction by the pressure ofoverburden, accompanied by expulsion ofpore waters

Growth of new diagenetic minerals andcontinued growth of chemical sedimentsfrom pore waters.

Dissolution of soluble elements of clasticrocks.

Recrystallization and remineralization aswater chemistry, pressure, andtemperature evolve.

At the high-TandPend, diagenesismerges smoothly into the low-TandPend of metamorphism. The distinction isarbitrary.

Sedimentary Rocks

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Sedimentary Rocks

The preserved end-result of weathering, erosion, transport,sedimentation, and diagenesis is sedimentary rocks.

Like sediments and sedimentary environments, the resulting rocks are

divided into clastic (or siliciclastic or volcaniclastic, etc.) and chemical (orbiochemical).

Clastic rocks are classified by particle size (and sorting) andcomposition.

S di t R k

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Sedimentary Rocks Chemical sediments are primarily classified, of course, by

mineralogical composition.

di k d i l i f i

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Sedimentary rocks and environmental information How do sedimentary rocks preserve information about their

depositional environments? By composition, mineralogy and grain size, obviously, but also

through sedimentarystructure Elements of sedimentary structure:

Bedding

Bed thickness, from finely laminated to massive

Vasquez formation: massive Burgess Shale:

fine

30 cm

30 m

S di

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Sedimentary structure

Cross-bedding indicates high and

unidirectional current velocity, oftenwinds in terrestrial settings, formingsand dune lee-slopes.

Character of bedding, from simple horizontal laminae to cross-bedding,ripples,soft-sediment deformation, orbioturbated.

Ripple marks record back-and-forth action by waves in shallow water.

Sedimentary Structure

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Mud cracks demonstratedrying-out of a thin layer ofsediment fine enough tohave significant cohesion.

Definite proof of terrestrialsetting or very shallow watermarginal marine.

MODERN ANCIENT

What about this structure?(Hint: it is not the surface of

the Moon)

S di S

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Bioturbation is the verticalmixing of sedimentary layers

by burrowing organisms.Evidence of such activitycan be preserved on beddingsurfaces as trace fossils.Indicative of water depth,availability of nutrients and

oxygen, etc.

Soft-sediment deformation indicates slumping or compression of layersbefore complete lithification.

S di S

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Alluvial settings, with wanderingchannels that fill up and becomeoverbank deposits

Continental slopes with turbidity

currents

Graded Bedding: sorting of particle sizes within bedsindicates time dependence and hence process of deposition

An environment in which a episodes of high-energy transport giveway to periods of low-energy transport gives normal graded beds:

C b t R k

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Carbonate Rocks

Most carbonate rocks are entirelybiochemical sediment, made up of the bodyparts of calcite or aragonite-precipitating

organisms Deep-sea carbonate ooze is made of foram shells

Reef carbonates are made of coral reefs (usually)

Stromatolites are formed by carbonateprecipitation by microorganisms

Tour of sedimentary environments

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Tour of sedimentary environments

Let us go through each of the major categories of sedimentaryenvironment, keeping in mind the relationship betweenobservable processes in modern settings and the preservedfeatures in ancient examples, and the ways in which

observation of a sedimentary rock formation can be used toinfer the type of setting and detailed information about it.

Sedimentary environments: Terrestrial

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Sedimentary environments: TerrestrialI. Fluvial (rivers and streams of all kinds and sizes)

a. Alluvial Fans

We saw alluvial fans on the field trip. They form where drainages exit

mountain fronts onto surrounding lowlands.Individual fans may merge to form a piedmont slope (like Pasadena).

In arid regions like California,sediment transport onalluvial fans is dominatedby debris flows likemudslides and landslides,and by periodic streamflows that divide the faninto channel deposits,overbank deposits.

Sorting is poor, but increasesdownstream; grain sizedecreases downstream;sediments are oftenoxidized and poor in fossilsor organic matter.


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I. Fluvialb. River systems

Rivers are classified into meandering or

braided, most often.Braiding is favored by high sediment

load, steep gradients, variable streamflow, and unstable poorly vegetatedbanks.

Meandering is favored by the opposite.


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I. Fluvialb. River systems

Meandering rivers develop in a fairly

regular pattern by channel migration,leaving a predictable sequence ofcyclic, fining-upward sedimentarydeposits. Braided river deposits aremore chaotic leave somewhat randomdeposits, since channels wander

randomly across the floodplain.


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Sustaineddeposition of

wind-blown dustmakes thickdeposits ofloess.

II. Desert environment

Deserts basins are basically alluvialfans, playas, and sand dunes. Theymay be dominated by wind transport

or by fluvial transport restricted torare, seasonal storms and floods

Alluvial fans are debris flow andstream flow deposits (as above).

Playas are dry or seasonal lake beds dominated byevaporites or fine-grained and finely laminatedmudstones and siltstones.

Sand dunes leave fascinating cross-bedded to massivesandstone deposits.


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III. Lacustrine (i.e., lakes)

Lakes are special, compared to rivers and oceans, in several ways:

Small size (no large waves), absence of tides, and low currents makes lakes verylow-energy sedimentary environments. Coarse sediments are limited to theirmargins.

Lakes generally keep all sediment that arrives from a large drainage area, sosedimentation rates are high, often ten times higher than marine settings.

Open lakes (with inlet and outlet streams) are usually fresh-water and generate

only clastic sediments. Closed basin lakes become saline and lead to chemical-dominated sedimentation. Many lake deposits show cyclic alternations betweenclosed and open conditions.

Varves

Annual variations in sediment supply

(especially if the lake freezes overeach winter) are often preserved inlow-energy lacustrine depositionalenvironments as countable annuallayers orvarves.


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IV. Glacial and peri-glacial

We saw some of the typical valley glacier deposits on the field trip.But there is more to the glacial environment than moraines and tills.

Glaciers generate characteristic river deposits (frequently braided) and lakedeposits (frequently varved) when they terminate on land, and characteristicmarine deposits when they terminate in the ocean (dropstones). They move largeboulders, but they also generate huge amounts of very fine rock flour that endsup as mud or loess.

Periglacial deposits, likemost sedimentarysequences, have severalfacies: a basal tilldeposited in front of theglacier is overlain by

moraines, lake sediments,glacio-fluvial deposits,and finally loess.

Sedimentary environments: Marginal Marine

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I. Deltaic environment: Deltas form wherever rivers empty intooceans or lakes. Much of the clastic load carried to the mouth of theriver is deposited in a restricted area at or near the coast, forming a

delta. Because deltasprograde outwards, they build deposits with reverse grading,

coarsening upwards as the delta moves past a given location.

The forces affecting sedimentation in a delta arefluvial, tidal, and waves, anddifferent deltas display effects of dominance by different forces.

TheMississippi delta isfluvial-dominated:

Both tides and waves are

weak in the Gulf of

Mexico, so distribution of

sediment is dominated by

the river itself, which formslong, relatively stable

channels (life span ~1000

years) with levees; each

channel narrows upwards

until it pinches off.


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I. Deltaic environment

Flow at the mouth of a fluvial-dominated delta is controlled by the relative densityof river outflow and ambient sea-water. Depending on river sediment load andtemperature (and on ocean salinity and temperature), the flow may be hyperpycnal

(river outflow denser), or hypopycnal (river outflow less dense).

Hyperpycnal flow leads to turbidite deposits from sediment-rich flows along thebottom. Hypopycnal flow leads to uniform, well-sorted sediments since in thiscase settling is controlled by flocculation of fine particles.

Sedimentary environments:

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Marginal Marine

The Sao Francisco river in Brazilis wave-dominated

Wave-energy here is 100 times thatat the Mississippi. Sedimentsreaching the mouth of the river arerapidly reworked and redistributedby longshore currents to buildbeaches, barriers, and lagoons,

similar to stretches of coast where noriver is present.

The Ganges-Brahmaputradelta is tide-dominated

Although the river outflow is

higher and more sediment-ladenthan the Mississippi, the tidalrange is large (about 4 meters).This type of delta breaks upinto sand bars and channelsoriented parallel to the tidal

inflow-outflow direction. Thereis a large, intermittentlyexposed, tidal flat.


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II. Beach-barrierenvironment

Any continental margin where

there is not a river mouth islikely to form a beach with asingle shoreface or a beach-barrier island-lagoon system

A beach produces adistinctively ordered set ofrecognizable facies, fromdune sands through the

surf zone, breaker zoneand into deeper water.

A barrier complex has alagoon and often a swampdeposit behind the barrier.


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II. Beach-barrierenvironment

If a simple beach is prograding,i.e. building out to sea anddepositing near-shore facieson top of distal facies, itmight produce a stratigraphiccolumn like this, coarseningupwards and hence clearlydistinct from any river

floodplain or continentalslope deposit.

Keep in mind the relationshipbetween the lateralsuccession of environmentsat any constant time across abeach and the verticalsuccession of sedimentsshown in a column like thisone.


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IV. Tidal flats

A wide, flat area of land between low-tide level and high-tide level is

a tidal flat.These are common environments for deposition of carbonates andevaporites. They may be associated with deltas, beaches, or estuaries

III. Estuarine environment

An estuary is a partly enclosed body of water at the mouth of a river.It may be part of a delta; it may be the lagoon behind a barrier-island. Generally,estuaries must have a connection to the open ocean at least at high tide. They areenvironments of mixing between seawater and freshwater. Example: San FranciscoBay

Sedimentary environments: Marine

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yI. Neritic environments

This term refers to depths below wave-base and low tide, and abovethe shelf-slope break.

At times of sea level highstand, when shallow seas cover the continentalplatforms, the neritic environment may encompass a significant fraction of theearths area.

The neritic environment is where carbonate reefs are built.

Sedimentary environments: MarineII. Oceanic environments

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Continental slope depositsare characterized by turbidites, cyclic fining-upward sedimentary sequences that form by turbidity flows of suspended sedimentdown the moderately steep slopes of the continental slope.

Deep sea (abyssal) deposits There is a clearregional pattern

with areas

dominated by

chemical sediment

(carbonate ooze or

siliceous ooze) or

by a very slow

accumulation of

fine clastic

particles (pelagic

clay).

We will develop

the ocean

chemistry and

geology to

understand thispattern...

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