- Created by: Ajay Wynne
- Created on: 14-04-17 20:00
deep marine basin environments : 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
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
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.
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.
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.
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)
clear shallow seas: chalk and 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
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
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
clear shallow sea: reef/bioclastic 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
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
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
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
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.
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
glacial: fluvioglacial deposits and varves
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
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
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
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)
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
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
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.
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
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
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
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.