GL4 E1GL4 E1

Key Idea 2 b iiiKey Idea 2 b iii SEDIMENTARY ROCKS SEDIMENTARY ROCKS The mineralogy and texture of The mineralogy and texture of

sedimentary rocks are controlled by sedimentary rocks are controlled by processes of weathering, erosion and processes of weathering, erosion and depositiondeposition

GL4 E1 KI2 b (iii) DepositionGL4 E1 KI2 b (iii) Deposition

DEPOSITION, selectively concentrates DEPOSITION, selectively concentrates products in particular environments – products in particular environments – grain size related to energy of grain size related to energy of depositional environment; dominance depositional environment; dominance of quartz and muscovite in coarse of quartz and muscovite in coarse fraction and clay minerals in fine fraction and clay minerals in fine fraction; flocculation; precipitation; fraction; flocculation; precipitation; concentration of biogenic material in concentration of biogenic material in particular environmentsparticular environments

Horizontal layersHorizontal layers

Credit: British Geological Survey

Horizontal layers

What is deposition?What is deposition? Transporting agents can only carry Transporting agents can only carry

sediment when they have energysediment when they have energy When energy is reduced, some of load When energy is reduced, some of load

must be dropped (deposited)must be dropped (deposited)

Energy lost all of a sudden = load Energy lost all of a sudden = load dumped, no sortingdumped, no sorting

Energy lost gradually = load gradually Energy lost gradually = load gradually dumped, largest particles first then dumped, largest particles first then smaller (graded bedding)smaller (graded bedding)

Hjulstrom’s diagramHjulstrom’s diagram

Grain size/energy currentGrain size/energy current

Big link between these!Big link between these! Hence the grain sizes found in a rock Hence the grain sizes found in a rock

tell us the energy current which tell us the energy current which existed just before it was depositedexisted just before it was deposited

Depositional environmentsDepositional environments River flowing into a lakeRiver flowing into a lake Precipitation from Precipitation from

evaporation of sea waterevaporation of sea water Glacier meltingGlacier melting River flood plainRiver flood plain BeachBeach Estuary deltaEstuary delta SeabedSeabed Desert Desert

River depositionRiver depositionProfile along a river from source to mouthSource

Mouth

Particles deposited here are large and irregular and consist of a variety of lithologies, including the least resistant

Particles deposited here are mid-sized and of intermediate sphericity, and include resistant and non-resistant lithologies

Particles deposited here are small and nearly spherical, and consist mainly of the most resistant lithologies

The effects of siltation in rivers The effects of siltation in rivers and streamsand streams

Siltation is a leading pollution problem. Over the short term: silt can kill fish, destroy spawning beds, increase water turbidity (and reduce rates of photosynthesis).Over the long term: unchecked siltation can alter habitat and affect aquatic life.

Credit: U.S. Geological Survey

Average annual concentrations of suspended sediment, nitrogen, and

phosphorus input into the reservoir system and output to the Chesapeake Bay.

RiversRivers Flowing water (current flow depends on Flowing water (current flow depends on

gradient)gradient) Deposition in lower course of riverDeposition in lower course of river Meanders (outside = erosion; inside = Meanders (outside = erosion; inside =

deposition)deposition) Levees and flood plain – “flood dumping”Levees and flood plain – “flood dumping” Alluvial fans (change in slope Alluvial fans (change in slope energy energy

change)change) Deltas (build out into still body of water – Deltas (build out into still body of water –

prograde)prograde)

River deposition featuresRiver deposition features

AsymmetricAsymmetric ripple ripple marksmarks

Cross laminationCross lamination Point bar depositsPoint bar deposits (Alignment due to (Alignment due to

river flow)river flow) Gradual rounding Gradual rounding

of fragments of fragments downriverdownriver

Credit: British Geological Survey

DesertDesert Aeolian (wind) transportAeolian (wind) transport Small range of grain sizes Small range of grain sizes

transported (~ 0.3mm)transported (~ 0.3mm) Frosted grains (wind blasted)Frosted grains (wind blasted)

Credit: U.S. Geological Survey

Desert deposition featuresDesert deposition features Large scale cross beddingLarge scale cross bedding Red colour (oxidised iron matrix)Red colour (oxidised iron matrix) Flash floods bring in larger materialFlash floods bring in larger material Lakes dried up quickly – evaporation Lakes dried up quickly – evaporation

features (eg. mud cracks)features (eg. mud cracks)

Credit: British Geological Survey Credit: British Geological Survey

Land environmentLand environment

Mainly erosional, not depositionalMainly erosional, not depositional Swamp landsSwamp lands Glacial depositionGlacial deposition

Land depositional featuresLand depositional features Glaciers – erratics, drumlins, Glaciers – erratics, drumlins,

till/boulder clay, outwash sands and till/boulder clay, outwash sands and gravelsgravels

Swamps – coal depositsSwamps – coal deposits

Pleistocene glacial deposits,Thurstaston, Wirral

Sea/coastsSea/coasts

Storm beach – high energy sea (large Storm beach – high energy sea (large fragments)fragments)

Wave action/long shore drift – rolling Wave action/long shore drift – rolling of particles – roundingof particles – rounding

Lagoons – conditions more still, and Lagoons – conditions more still, and may also be evaporationmay also be evaporation

Limestone reefsLimestone reefs Turbidity currentsTurbidity currents

Sea/coast deposition Sea/coast deposition featuresfeatures

Graded beddingGraded bedding Finest laminations far offshore, Finest laminations far offshore,

clay/shale depositionclay/shale deposition

Credit: British Geological SurveyCredit: British Geological Survey

Other featuresOther features Sole marks/groove casts – Sole marks/groove casts –

pebbles/animals bouncing on a pebbles/animals bouncing on a sediment surface, marks left infilled sediment surface, marks left infilled by sediment deposited on topby sediment deposited on top

Scour marks/flute casts – erosion of Scour marks/flute casts – erosion of surface by current, infilledsurface by current, infilled

Flame structures/mud volcanoes – Flame structures/mud volcanoes – sand deposited on top of mud, sand sand deposited on top of mud, sand weighs down and mud erupts up weighs down and mud erupts up through sand layerthrough sand layer

What else?What else?

dominance of quartz and muscovite dominance of quartz and muscovite in coarse fraction and clay minerals in coarse fraction and clay minerals in fine fraction; in fine fraction;

Go back to Bowen’s reaction series Go back to Bowen’s reaction series for the reason why quartz is for the reason why quartz is resistant/survivesresistant/survives

Clay minerals from broken down Clay minerals from broken down feldsparfeldspar

Other processesOther processes

flocculation; precipitation; flocculation; precipitation;

FlocculationFlocculation Fine particles begin to stick Fine particles begin to stick

together (due to addition of a together (due to addition of a catalyst – e.g. when freshwater catalyst – e.g. when freshwater meets saltwater in an estuary)meets saltwater in an estuary)

Flocs are heavy so sinkFlocs are heavy so sink

PrecipitationPrecipitation

EvaporatesEvaporates CarbonatesCarbonates

EvaporatesEvaporates

Salt waterSalt water High temperatures, little rainfallHigh temperatures, little rainfall Calcium carbonate first to precipitate Calcium carbonate first to precipitate

(least soluble)(least soluble) In shallow lagoons the following can In shallow lagoons the following can

precipitate after limestone:precipitate after limestone: Gypsum CaSOGypsum CaSO44.2H.2H22O (must lose 80% of O (must lose 80% of

water for this to occur)water for this to occur) Then Halite (NaCl) (90% water gone)Then Halite (NaCl) (90% water gone) Finally potassium and magnesium saltsFinally potassium and magnesium salts

LimestoneLimestone

Calcium carbonateCalcium carbonate Reef structures (calcium carbonate Reef structures (calcium carbonate

built by shelled creatures)built by shelled creatures) Lagoons behind reef collect fine Lagoons behind reef collect fine

calcium “muds”calcium “muds”

Evaporates 2Evaporates 2

300cm depth of seawater yields just 300cm depth of seawater yields just 5cm of salts5cm of salts

Calculate how much water was Calculate how much water was evaporated to form the Stassfurt evaporated to form the Stassfurt sequence in Germany which is sequence in Germany which is 1000m thick1000m thick

Right, the maths bit!!Right, the maths bit!!

1000m x 100cm = 100,000cm1000m x 100cm = 100,000cm 100,000cm 100,000cm ÷÷5cm = 20,000 units5cm = 20,000 units 20,000 units each of 3m of water20,000 units each of 3m of water

= = 60,000m of sea water60,000m of sea water

= 60km of sea depth= 60km of sea depth

Is this feasible?????Is this feasible?????

How deep are our oceans?How deep are our oceans?

Seas aren’t deep enough!Seas aren’t deep enough!

Refill/top-up theory – sea basin (like Refill/top-up theory – sea basin (like the Mediterranean) evaporates and the Mediterranean) evaporates and then is refilledthen is refilled

Carbonate Compensation DepthCarbonate Compensation Depth

Carbonate dissolves when water is Carbonate dissolves when water is deeper than 4km deeper than 4km

Concentration of biogenic Concentration of biogenic materialmaterial

Only in selected environmentsOnly in selected environments Trees (coal) – swamps (and delta tops), Trees (coal) – swamps (and delta tops),

rapid burial essential (to preserve rapid burial essential (to preserve material by excluding oxygen to stop material by excluding oxygen to stop biogenic material rotting)biogenic material rotting)

Credit: British Geological Survey

Shells – where current collects Shells – where current collects material (eg. only one valve from material (eg. only one valve from bivalves due to eating habits or a bivalves due to eating habits or a winnowing current)winnowing current)

Credit: British Geological Survey