Sedimentary Environments

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  • Created by: Aisling
  • Created on: 12-05-13 21:44

Deep Marine Basin Environments

TURBIDITE SEQUENCE:

HIGH VELOCITY

1) Coarse bed of pebbles to conglomerate in a sandy matrix (Graded Bedding)

2) Coarse, Medium sandstone (greywacke) (Parallel Lamination, Graded Bedding)

3) Sandstone (greywacke) (cross bedding)

4) Fine Sandstone then Siltstone (Ripples, cross bedding)

5) Shale, may contain pelagic marine fossils e.g. Graptolites (Parallel Laminations)

INTERTURBIDITE

TURBIDITE DEPOSITS: show lateral changes. Get thinner with distance. Sand: mud decreases

STRUCTURES: Graded Bedding + Flute Clasts

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Ocean Basins Sediment

FLAT because sediment covers the underlyig topography

Mud is transported by Tubidity Currents + Clay/Silt is transported by Wind. The wind blown sediment is deposited to form Pelagic Clay

CARBONATE OOZES:

Contains Skeletal remains of Coccolithophores + foraminifera. Carbonate Compensation Depth is the depth they are only preserved at depths shallower than 4km. Below the CCD, any carbonate material dissolves due to high CO2 content and Lower temperature of water

SILICEOUS OOZES:

Form from the skeletons of Diatoms (near Poles) + Radiolara (Equator). Silica dissolves at a slow rate in sea water. Ooozes accumulate where the rate of deposition is greater than the rate of solution.

Sedimentation rate 1mm per 1000 years

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Marine Evaporites

Form in SHALLOW MARINE/ PLAYA LAKES environments

Normal sea water contains a variety of ions in solution giving it a salinity of 35 parts per thousand

As sea water evaporates, the brine produced first be SATURATED, then SUPERSATURATED, at which point evaporite mineral begin to crystallise in REVERSE order of solubility:

1) Calcite + Dolomite are precipitated (<1%)

2)Gypsum + Anhydrite (3%)

3) Halite (78%)

4) Potassium salts

Occurs in:

Barred Basins + Sabkhas

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Formation of evaporited in Barred Basins

BARRED BASIN- Is a bay partly isolated from the open ocean by a bar. In hot arid conditions, water  evaporates from the basin, causing lowering of the surface, drawing more water through the narrow channel at the entrance

Evaporation of surface water increases salt concentration:

1) Carbonates are precipitated near the entrance (low solubility)

2) Gypsum is precipitated at the surface due to evaporation further from the entrance + dissolve as they fall through the water below, increasing the density at the bottom of the basin

3) Dense Brine sinks from the surface to the bottom of the basin + the concentration of Gypsum increases as crystals fall frim the surface + dissolve

4) Eventually, the water in the basin is saturated in Gypsum. Gypsum forms + sinks without dissolving

5) The process is repeated for the other evaporite minerals

-Incomplete cycles are formed when normal sea water floods into the basin before evaporation

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Formation of Evaporites In Sabkhas

SABKHAS: Low lying coastal sand flats in hot arid regions. Very gentle slops means during extreme high tides the whole coastline is  under water.

PROCESSES:

1) Very strong evaporation, so as sea water approaches the coast salinity increases

2) Calcium Carbonate in sea water is removed by shell organisms

3) Evaporation of froundwater from the supratidal area draws sea water into the sediment along the shore

4) Evaporites develop above the high water line as groundwater is evaporated. Algae grow on the shore between high + low tides + are preserved as Stromatolites

5) Evaporite minerals are found above the Strimatolites

6) First mineral to crystallise is GYPSUM, then ANHYDRITE forming NODULES in the sediment

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Clear Shallow Seas

-Tropical Islands which have a lcak of TERRIGENOUS sediment (no sand/mud). Cut off from areas where there is supply of sediment by deep water. A wide range of orhanisms thrive.

-Rocks are Biologically, Chemically formed LIMESTONES

-All Limestones consist of Calicum Carbonate so react with HCL. They contain Macrofossils/ Microfossils

-Some are composed of Calcium Carbonate that has directly precipitated from sea water

BIOCLASTIC/ FOSSILIFEROUS LIMESTONE:

-75% of Bioclastic rock may be composed of the remains of invertebrate skeletons from organisms e.g. Crinoids, Brachiopods, mixture of fossils

-Rest of the rock is formed of Calcite mud + detrial mud

-Grey, Hard, Well Jointed, Thick, Massive Beds

-May contain remains of Stem Segments of Crinoids (Crinoidal Limestones)

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Reef Limestone

-CORALS make most of the reef but other organisma live on or around reefs

-Carbonate secreting algae encrust the reef, cementing it together

-They are typically Unbedded because reefs form growing upwards

-Live in warm, shallow, high energy, well oxygenated tropical waters

-Water temps need to be between 15 + 29C

-Do not grow well at depths greater than 25m

-Corals only live in shallow water near landmasses

-In the Pacific, volcanic Islands sink and a ring of coral called an ATOLL forms

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Chalk

WHITE, HARD, MASSIVE, WELL JOINTED

BIOLOGICALLY formed

Composed of COCCOLITHS (tiny calcareous disc/ oval shaped platelets that form part of the skeletons of single celled algae)

May contain microfossils of forminifera e.g. Globigerina

May be macrofossils also of Echinoids, Belemnites, Brachiopods + Bivalves

Formed in LOW ENERGY, DEEP WATER SHELF environments

FLINT NODULES are common

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Oolitic Limestone

Formed from OOLITHS

Form in SHALLOW SEAS where tiny grains of sand/ shell fragments/ pellets are rolled in Carbonate Mud by Tidal current/ Wave action

Around the nucleus CONCENTRIC LAYERS OF CALCIUM CARBONATE (ARAGONITE) are precipitated from the sea water

Formed by a CHEMICAL process in WARM TROPICAL climates

Most form in water less than 2m deep (wave agitation/ tidal movemet active)

Tidal Action- CROSS BEDDED

FOSSILS are common

Many fossils broken by HIGH ENERGY environments

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Micrite

FINE, HARD CRYSTALLINE LIMESTONE

Formed from CALCITE MUD which has undergone DIAGENESIS

Calcite mud is produced by CHEMICAL PRECIPITATION from SEA WATER/ CARBONATE SECRETING ALGAE

NO FOSSIL PRESENT

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Clastic Shallow Seas

Most Shallow Seas recieve large amounts of CLASTIC SEDIMENT transported from land by RIVERS

LITTORAL ZONE- Area between high water + low water marks and includes beaches. It is covered by sea for part of the time + this HIGH ENERGY TIDAL AREA accumulates sediment

SAND/ FINER grained sediment- deposited on BEACHES/SHALLOW SEAS has been transported to sea by RIVERS

COARSER grained sediment- supplied by EROSION of COASTAL CLIFFS

PEBBLES/COBBLES- found at the back of beaches, where they have been transported by HIGH ENERGY WAVES during STORMS

Transport- LONGSHORE DRIFT (littoral drift + beach drift) or RIP CURRENTS (fast current flowing away from the coastline)

Currents- UNI-DIRECTIONAL. Tides- BI-DIRECTIONAL

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Beach Environments

Mainly composed of SAND/GRAVEL, some may contain MUD

SAND- picked up + moved around by waves= WELL SORTED, ROUNDED (result of attrition + wave action)

LIGHT= ORTHOQUARTZITE

BURROWING organisms, BIVALVES can survive

Tracks, trails and burrows of beach organisms/ broken shells may be found scattered through the sand/shell banks

BI-DIRECTIONAL movement of WATER/TIDES leads to the formation of SYMMETRICAL RIPPLE MARKS

Crests are PARALLEL to the crest of waves

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Shallow Seas

Rivers- carry SUSPENDED load of CLAY MINERALS/  DISSOLVED SALTS / SAND

Wind- ATMOSPHERIC DUST, can be deposited anywhere in the sea

Below Littoral zone- sediments affected by CURRENTS

SEDIMENT SIZE DECREASES AS DEPTH + DISTANCE AWAY FROM COAST INCREASES

Sea water contains SUSPENDED sediment, reducing daylight penetration closer to the shore. Where water depth is less than 100m Abundant life exists

GLAUCONITE- distinctive green, potassium iron silicate mineral found in some SANDSTONES formed in shallow seas.

UNI-DIRECTIONAL CURRENTS- form ASSYMETRICAL RIPPLE MARKS in sandstone

MUDSTONES/CLAYS form in LOWER ENERGY OFFSHORE + contain FOSSILS

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Continental Shelves sediment deposition

Continental Shelves have a number of key varibles that increase the variety in sediment deposition:

1) SEDIMENT SUPPLY CONTROLS RATE OF SEDIMENTATION

-Where erosion is high + large rivers are transporting sediment, rate of deposition is HIGH

2) CHANGES IN SEA LEVEL

-Rise in sea level causes the sea to flood the land + the zones of sediment move inland so MUD is deposited on top of SAND, this is called MARINE TRANGRESSION

-Drop in sea level means that sea retreats, causing MARINE REGRESSION + renewed erosion on land takes plce bringing more sediment to the sea

3) CHANGES IN CURRENT DIRECTION/ RATE OF FLOW

-Sediment transport can be increases

-Many offshore sand bars can move several 100m in a year, causing a danger to shipping

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Deltaic Environments

DELTAS- occur where a river flows into a sea/lake + deposits its load of sediment

Must be LITTLE WAVES/ TIDAL ACTION

DECREASED ENERGY thats causes deposition- COARSEST settle out first

Build up of sediment causes the river channels to become blocked so that the river switches its course + splits into DISTRIBUTARY channels

Deltas are just above sea level + inbetween the distrbutary channels there are SWAMPS

In Equatorial + humid tropical climates vegetation grows very rapidly + is abundant

Plant remains require ANAEROBIC conditions in order to be PRESERVED

This is why ANCIENT PLANT REMAINS are preserved as COAL in equatorial deltaic sequences

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The Delta Top

Is dominated by DISTRIBUTARY CHANNELS with areas of SWAMPS, BAYS, FLOOD PLAINS in between

Channels changecourse frequently, process known as DELTA SWITCHING that leads to lateral changes in the pattern of sedimentation

Sediments deposited on the delta tip are called TOPSETS

1) COARSE SANDS/ GRAVELS- deposited in the channels and form CHANNEL SANDSTONES, may contain CROSS BEDDING

2) CLAYS, deposited between channels.

Swamps, anaerobic conditions allow peat formation.

DIAGNETIC processes lead to formation of COAL

Soils in which the trees grew may be preserved as SEAT EARTH

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The Delta Slope

As river meets the sea, COARSEST SAND grains are deposited first + the FINEST SILT last

Depostions occurs on the DELTA SLOPE

As the front of the delta advances into the sea, a VERTICAL SUCCESION FORMS

Sediments deposited on the delta slope are called the FORESETS

.

COARSE SAND- DEPOSITED ON TOP- CROSS BEDDED

FINER SANDS/ SILTS- LOWER DOWN

Marine Fossils may be found

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Offshore Deposition

Takes place in LOW ENERGY DEEP WATER at the BOTTOM of the delta slope

Sediments are called BOTTOMSETS

Consist mainly of CLAYS/ SILTS which are THINLY BEDDED + LACK SEDIMENTARY STRUCTURES

Lithify to form SHALES, which may contain marine fossils

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Deltaic Sequences

Delta Sequence= COARSENING UPWARDS sequence. It is known as a CYCLOTHEM.

ORDER OF SEQUENCE:

 1) MARINE SHALES + SILTS

2) FINE TO COARSE SANDSTONES

3) COALS, SANDSTONES, SEAT EARTH + CLAY

LIMESTONES are sometimes found at the base of the SUCCESSIONS, representing normal marine conditions before the deltaic conditions begin

BIOTURBATION + TRACE FOSSILS are common. Marine fossils in BOTTOMSET + non marine in higher parts

Often rapid deposition of MUD covering the plant material before it decays

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Cyclothem

Cyclothem Sequence may be repeated in succession due to SUBSIDENCE/ EMERGENCE, leading to marine TRANGRESSIONS/ REGRESSIONS.

These are caused by:

1) Changes in Sea Level

 

2) Isostatic Changes, due to mass of sediment causing the sea floor to sink

 

3) Changes in position of the delta lobes, due to migration of channels

 

4) Changes in the rate of sedimentation, allowing the delta to build out or be inundated by sea

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Hot Desert Environments- Wadi Conglomerates

WADIS- Desert stream channels which have a steep gradient

Together with INTENSE RAINFALL, the streams have VERY HIGH ENERGY + so they transport COARSE (gravel) grained fragments

Energy is quickly lost when the rain stops + because the water sinks into the POROUS ROCKS

Deposition is RAPID, leaving:

1) POORLY SORTED CONGLOMERATES

2) SUB ROUNDED- if been transported on several occasions

3) ANGULAR- only been breifly transported

4) RED COATING- Oxidised iron minerals

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Desert Sandstones

SAND GRAINS are transported by HIGH ENERGY WINDS + affected by ATTRITION. Making them:

1) VERY WELL SORTED

2) VERY WELL ROUNDED

3) HIGH SPHERICITY

4) FROSTED due to collisions between grains

Composed entirely of QUARTZ- other minerals been removed by mechanical/ chemical weathering

Show LARGE SCALE CROSS BEDDING= often deposited in LARGE DUNES

DUNES- crecent shape Barchan dunes/ straight seif dunes (some dunes are 200m high)

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Playa Lake Evaporites

PLAYA LAKE- Temporary lake formed by storm run-off in deserts having inland drainage

Stream water flowing into lakes contains IONS OF CALCIUM, SODIUM, POTASSIUM in SOLUTION

PRODUCTS of WEATHERING ROCKS UPSTREAM

Water evaporates- dissolved ions become more concentrated- least soluble 1st to precipitate to form EVAPORITE MINERALS (Sedimentary rocks resulting from the evaporation of saline water)

ORDER (LESS SOLUBLE--> MORE SOLUBLE)

1) CALCITE 2) GYPSUM 3) HALITE 4) K SALTS

FINE GRAINED sediment is deposited from SUSPENSION to form MUDSTONES

May contain DESICCATION CRACKS, RIPPLE MARKS, SALT PSEUDOMORPHS

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Fluvial Environments

Include all parts of rivers- STREAMS, RIVERS, ESTUARIES.

Rivers are TRANSPORT SYSTEM for sediment, moving MUD, SAND, ROCK

 

LONGER PARTICLES ARE TRANSPORTED FOR- THEY GET SMALLER + ROUNDER DUE TO ATTRITION + ABRASION

 

HIGH ENERGY

Flood Plain- LOW ENERGY, not enough of a STRONG CURRENT

Heavy Rainfall= Energy Level RISES

Dry Conditions= Little water will flow so energy levels will be LOW

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Fluvial Environments- Velocity + Deposition

Where Velocity is REDUCED, DEPOSITION TAKES PLACE:

 

1) Where there is a SUDDEN CHANGE IN SLOPE

 

2) On the INSIDE OF MEANDER BENDS

 

3) Where a TRIBUTARY JOINS A RIVER

 

4) Where rivers enter LAKES/ SEAS that DO NOT HAVE STRONG CURRENTS 

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Alluvial Fan Breccias, Conglomerates + Arkoses

Where mountain streams flow onto a FLAT VALLEY PLAIN, there is a marked DECREASE in GRADIENT + REDUCTION in stream VELOCITY + ENERGY. Lare volumes of sediment are deposited to form ALLUVIAL FANS

BRECCIAS:

-ANGULAR, COARSE SCREE FRAGMENTS

CONGLOMERATES:

- ROUNDED PEBBLES form in STREAMS

ARKOSE:

-SAND SIZE material deposited rapidlt in arid conditions, contains more than 25% feldspar, which shows LITTLE SIGNS OF WEATHERING,

SOURCE- GRANITE ROCK UPSTREAM

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Channel Conglomerates + Sandstones in Meandering R

Meandering Streams- flow in channels with looping curves

Outside= ERODED  Inside= DEPOSITIONS occurs on POINT BAR

Result= the channel migrates LATERALLY

Meandering rivers result in UPWARD FINING (Average grain size decreases upwards)

CHANNEL LEG:

- CONGLOMERATES, IMBRICATE STRUCTURE

POINT BAR:

-SANDSTONES, CROSS BEDDED, MODERATELY WELL SORTED, SUB ROUNDED, have rare masses of LIGNITE (fossil tree trunks washed into rivers)

FLOOD PLAIN:

-CLAYS/ MUDSTONES, LOW ENERGY

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Flood Plain Clays + Silts

High flow- rivers flood surrounding flat area adjacent to channel, forming a flood plain

 

CLAYS rich in ORGANIC material are deposited from the SUSPENDED load of the river + may contain FOSSIL PLANTS

 

SILTSTONE is common, shows SMALL SCALE CROSS BEDDING

 

MUDSTONES may show DESICCATION CRACKS + THE DEVELOPMENT OF SOIL

 

Any fossil will be TERESTRIAL e.g. VERTEBRATE FOOTPRINTS/ PLANT FOSSIL

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Glacial Environments- Processes + Products

Sediment was ERODED + transported by the actions of ICE, WATER + WIND during the QUATURNARY

 

Material beneath glaciers + Ice Sheets= TILL/ BOULDER CLAY

 

Melting of Ice Sheets + Glaciers= SAND/ GRAVEL were transported along river valley/ deposited along the edges of the ice sheets

 

Large Ice-dammed lakes- CLAYS + SANDS

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Boulder Clay

When ice melts it deposits POORLY SORTED material in the form of BOULDER CLAY (TILL) which may become lithifies to form TILLITE.

Boulder Clay/ Till is composed of the UNSORTED materials of glacial erosion:

1) POOR SORTING

2) ANGULAR

3) LARGE CLASTS RANDOMLY ORIENTATED + SCATTERED THROUGHOUT CLAY

4) CLASTS MAY BE STRIATED

5) MAY CONTAIN ERRATICS (clasts of different rock type from that found locally + therefore transported from another area

Fragments of rock at the base of a glacier SCRATCH striations on underlying rock surfaces. ABRASION during transport produces FINE, CRUSHED ROCK FRAGMENTS

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Fluvio-Glacial Deposits

Sediments that have been transported by glacial MELTWATER STREAMS before being deposited on an outwash plain some distance away from the melting ice.

 

Mainly composed of GRAVELS/ SANDS but can contain COARSER PEBBLES

 

SORTED by fast flowing braided streams

 

May show CROSS BEDDING + GRADED BEDDING

 

FINER MUD, SILT carried further away- to lakes- deposited as VARVES

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Varves

VARVES- forms when glacier ice melts forming lakes

During SPRING THAW, streams transport FINE SAND/ SILT/ CLAY

SAND/ SILT settle first

VERY FINE remain in SUSPENSION, settling when the lake FREEZES in winter

This produces FINE GRAINED LAMINATED SEDIMENTS called VARVES

SILTY LAYER= SUMMER DEPOSITION

CLAY LAYER= WINTER DEPOSITION

Have been counted + used to date events at the end of last Ice age

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