Sedimentary environments

deep marine basin environments : ocean margins

ocean margins:

sediments transported by rivers accumulates on the continental shelf - eathquakes cause sediment to slip down continetal slope - turbidity current. causes deposition on the abyssal plain. The density of flow = high velocity down 4 degrees slope

clastic material transported includes coarse, medium particles and mud.

material deposited in submarine fans(continental rise) /ocean trenches/abyssal plain

heavier coarser material deposited first

turbidite - upward fining deposit of greywacke deposited froma turbidity current:                    coarse bed of pebbles & conglomerate in sandy matrix -> coarse then medium greywacke -> greywacke -> sandstone -> siltstone -> shale formed by fine mud 

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deep marine : sedimentary structures

graded bedding in greywackes - coarse particles settle faster    (way up indicator)                              flute clasts - eorded by turbidiity currents. infilled by sediment (found at base of greywacke beds) (Paleocurrent indicator) 

deposits get thinner with distance across abyssal plain                                                                        grain size decreases from source of sediment                                                                                            ratio of sand to mud decreases 

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deep marine basin environments : ocean basins

abyssal plain 3-5km below sea level. covered in sediment = flat, topography covered. mud carried by turibity currents, clay and silt transported by wind = pelagic clay

carbonate oozes contain skeletal remains of coccolithosphores, preserved at dpeths shallower than 4km - carbonate compensation depth. below this depth carbon dissolves due to higher c02 content and lower emp of water

siliceous oozes form from; skeletons of diatoms in deposits near poles & radiolaria in deposits near equator. silica dissolves at a slow rate in sea water.

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marine evaporites: intro

sea water salinity = 35 parts per thousand. As sea water evaporates it becomes saturated and then supersatured. Evaporite minerals crystallise now in reverse order of solubility

order of solubility: small quantities of calcite and dolomite, gypsum and anhydrite, large quantity of halite, potassium salts. 

barred basin - bay partly isolated from the open ocean by a bar

sabkhas - low lying coastal sand flats in hot arid regions with gentle slopes.

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marine evaporites formed in barred basins

as water evaporates from the basin the surface lowers therefore drawing more water in through the narrow channel at entrance.

evaporation of surface water increases salinity, especially at end furthest from the entrance of water. 

carbonates precipitated near entrance due to low solubility                                                          gypsum crystals precipitate at surface and dissolve as they fall through- desnity of water increased at bottom of basin                                                                                                                dense brine sinks                                                                                                                                           basin water saturated in gypsum = crystals form                                                                                (process repeats)

incomplete cycles form when normal sea water floods into the basin before complete evaporation takes place. 

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marine evaporites: evaporites formation in sabkhas

processes operating:                                                                                                                                  very strong evaporation - salinity increases as water approaches the coast                                        calcium carbonate in sea water removed by shelled organisms                                                            groundwater evaporates and draws sea water into sediments onshore                                          evaporites develop above high water line as groundwater evaporated                                               algae grow on shore between high and low tide - preserved as stromatolites                                  gypsum is the first mineral to crystallise within sediments followed by anhydrite (forming modules in the sediment) 

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clear shallow seas: chalk and micrite

micrite:

 fine, hard crystalline limestone                                                                                                                formed from calcite mud (undergone diagenesis)                                                                                 calcite mud produced by chemical precipitation from sea water or carbonate secreted algae       no fossils

chalk: 

white, hard, massive, well jointed.                                                                                                                biologically formed limestone composed of coccoliths.                                                                      may contain microfossils of foraminifera. macrofossils of bivavles                                            formed in low energy deep water shelf environments where little land sediment was transported to.                                                                                                                                                                     flint nodules common 

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clear shallow seas: oolitic limestone

formed from ooliths, form in shallow seas where tiny grains of sand/shell/pellets are rolled in carbonate mud by tidal currents, around this nucelus concentric layers of calcium carbonate (in the form of aragonite) 

worm tropical climates, water shallower than 2m deep where ave agitation occurs

typically white but can be stained yellow or orange from iron

current and wave action means they are often cross bedded

many fossils broken by high energy conditions

organisms which attach to sea floor: brachiopods and bivavles

organisms that burrow: echinoids 

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clear shallow sea: reef/bioclastic limestone

reef limestone:

carbonate secreting algae encrust the reef. reef limetsones dont have beds as they grow upwards. crinoids, brachiopods, echinoid, bivalve found in reef limestone

water temps = 25-29 degrees C, shallow waters (less than 25m)  well oxygenated tropical waters. 

volanic island colonised by coral, forms barrier reef around island, insland sinks = atoll forms

bioclastic limestone:

invertebrae skeletons + calcite mud = detrital mud                                                                              detrital - describes fragments derived from the mechanical weathering of roks                                  grey in colour, hard, well jointed, massive beds, often crumbly and fossils can be picked out whole

 crinoidal limestones contain broken remains of crinoids

bioclastic limestones common from the middle jurassic 

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clastic shallow seas: intro

spring tide - greatest tidal range every two weeks

tidal range - vertical distance between low tide and high tide

littoral zone - area between extreme low and extreme high water of the spring tide

longshore drift - combination of littoral dirft and beach drift

rip current - narrow fast current flowing seaward away from the coastline

currents are unidriectional 

tides are bi-directional 

transgression - occurs where the sea spreads over the land

regression - occurs where the sea retreats from the land

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clastic shallow seas: beach environments

sand, gravel, mud

sand well rounded due to attrition

common rock= orthoquartzite (entirely composed of quartz)

bivavles can survive the tides changing the environment​

bi-directional movement of tides - symmetrical ripple marks with crests parallel to crests of waves

further up the beach, cobbles, pebbles and beach gravel are less well sorted 

conglomerates common

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clastic shallow seas: shallow seas

rivers carry a suspended load of clay minerals, salts, sand & wind carries atmospheric dust deposited on the sea

continental shelf is a dynamic area:

sediment supply - low erosion/weathering or large rivers transported alot of sediment            changes in sea level - rise + flood land and zones of sediment move inland and mud deposited on sand (marine regression) / drop is sea level and sea retreats (marine regressions)                 fossil forests - submerged trees preserved by silica or calcite- evidence of the rise in sea level since the last glacial periods                                                                                                                 sediment transport increased by changes in current direction or rate of flow

daylight penetration decreased near shore due to sediments, abudance of life decreases. water depth of less than 100m sustains an abundance of life

glauconite - green potassium iron silicate found in sandstones in shallow seas. unidirectional currents form assymetrical ripply marks on these sandstones.

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glacial environments: boulder clay

rocks obstruct glacier movement, ice melts due to pressure, water flows downstream of rock, enters cracks, plucking occurs when water freezes

characteristics of boulder till:                                                                                                                       poorly sorted, angular fragments, large clasts, randomly orientated and scattered throughout clay, clasts may be striated, contain erratics

fragments of rock at base of glacier scratch striations on underlying rock surfaces

abrasion during transport produces fine grained crushed rock fragments

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glacial: fluvioglacial deposits and varves

fluvioglacial deposits:

transported by glacial meltwater,                                                                                                                  deposited on an outwash plain,                                                                                                    composed of gravels and sands with some coarser pebbles,                                                          sorted by fast flowing braided streams,                                                                                                      finer clay and silts carried further deposited as varves

varves:

glacier melts- lake forms - spring thaw transport of fine sand, silt, clay,                                          sand and silt settle first - silty layer of varves,                                                                                      clay settles when lake freezes again - clay layer of varves,                                                                pair of varves = one years deposition

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fluvial environments: alluvial fan

alluvial fan - fan shaped deposit of sediment built up by streams.                                                Mountain streams flow onto flat valley floor or plain - decrease in gradient and stream veolcity = lower energy. sediment deposited

breccias form from angular and coarse scree fragments 

conglomerates composed of rounded pebbles form in streams

arkose formed from sand sized material, 25% k feldspar (source= granite rock upstream) , deposited rapidly in air conditions 

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fluvial environments: channel meanders

meander - a winding curve/bend in a river as a result of deposition on the inside of a meander on the point bar and erosion on the outside of the bend. 

point bar - deposit of sand or coarser grained sediment on the inside of a meander bend

braided river - a river that consists of a network of small channels seperated by small islands

channel lag - coarse grained sediment deposit left in a channel after finer grained particles have been tranported away

upward fining cycles of sediment ;                                                                                                         conglomerates form from the channel lag, may show imbricate structure if clasts are disc/blade

sandstones form as the point bar deposits, often showing cross bedding, moderately well sorted with subrounded grains of quartz and muscovite mica, rare masses of lignite (fossil tree trunks)

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fluvial environments: flood plain clays and silts

high flow causes the river to flood surrounding flat area around the channel forming a flood plain

clays rich in organic material ared deposited from the suspended load of the river continaing plant fossils

siltstone shows small scale cross bedding 

mudstone may show evidence of sub-aerial exposure with deiscation cracks and soil development.

any fossils will be of terrestrial origin

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desert: wadi conglomerates and desert sandstone

wadi - desert stream channels with a source in the mountains with steep gradient. due to intensity of rainfall + steep gradient = high energy streams that can transport coarse grained fragments.

energy quickly lost when rain stops as rovk of channel is porous

deposition is rapid - poorly sorted conglomerates                                                                            grains may be subrounded if transported many times                                                                               red coating of oxidised iron minerals 

desert sandstones: 

sand grains 'millet-seed sand' transported by wind - attrition- characteristics:                                       very well sorted, very well rounded, high sphericity, frosted due to collisions with other grains     

made entirely of quartz  (unreactive and hard)                                                                                             other minerals removed by mechanical and chemical weathering                                                    grains coated by oxididsed iron                                                                                                              silica cement                                                                                                                                              show large scale cross bedding due to large(200m) crescent shaped dunes moving in direction of prevailing wind.    

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hot desert environments: playa lake evaporites

hot climate + infrequent rainfall + high rates of evaporation + high rates of evaporation = desert streams dont reach the sea but temporary playa lakes.

calcium, sodium, potassium in solution= products of weathered rocks upstream 

water evaporates due to heat, dissolved ions become more concentrated                                     least soluble are first to precipitate, most soluble at centre of playa lake

less soluble - calcite - precipitated first                                                                                                                                 gypsum                                                                                                                                                               halite                                                                                                                                         more soluble - k minerals - precipitated last 

fine grained sediments depoisted from suspension form mudstones, 

desication cracks, asymmetrical ripple marks, salt pseudomorphs and lenses - body of ore resembling a convex lens in cross section- of gypsum 

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deltaic environment: delta top

distributary channels - stream that diverges water from the main channel                                       swamps, bays, flood plains inbetween distibuataries                                                                             delta switching - channels change course frequently, leading to lateral changes in pattern of sedimentation

topsets - sediments deposited on the delta top                                                                                 coarse grained sands/ gravels form channel sandstones, showing cross bedding

clays deposited in areas between channels                      

in swamps anaerobic conditions allow peat formation, diagenesis forms coal

seat earth - sandy, clay fossil soil found beneath coal, representing the soil in which coal forming plants grew in 

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delta slope and offshore deposition

river meets sea - coarsest grains deposited first                                                                                deposition occurs on delta slope and into the sea                                                   

foresets - inclined layers formed on the delta slope, comonly consisting of cross bedded sandstones. coarser sand deposited at the top.

marine fossils may be found

low energy deeper water at bottom of delta slope                                                                          bottomsets- lowest horizontal layers of a delta, consisting of clays and silts which lithify into shales containing marine fossils 

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deltaic environment: deltaic sequences

COARSENING UPWARD SEQUENCE repeated many times - each is a cyclothem

marine shales, silts, fine to coarse sandstones, seat earth, coals and clay                                   limestone are sometimes found at the base of succession representing normal marine conditions

bioturbation - disturbance of sediment by the activities of organisms, marine fossils in bottomsets, terrestrial fossils in foresets and topsets. 

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