Sedimentary Environments
- 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
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
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
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
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
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)
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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)
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
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
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
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
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
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
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
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
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
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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