Introduction to sedimentary environments MAHBOOB AHMED
Landscapes form and constantly change due to weathering and sedimentation. The area where sediment accumulates and is later buried by other sediment is known as its depositional environment. Depositional environments are often separated into three general types, or settings: terrestrial (on land), marginal marine (coastal), and marine (open ocean). Examples of each of these three regional depositional settings are as follows: terrestrial-alluvial fans, glacial valleys, lakes. marginal marine- beaches, deltas, estuaries, tidal mud and sand flats. marine-coral reefs, continental slope and deep marine deposition. During deposition of sediments, physical structures form that are indicative of the conditions that created them. These are known as sedimentary structures. They may provide information about water depth, current speed, environmental setting or a variety of other factors. Among the more common of these are: bedding planes, beds, channels, cross-beds, ripples, and mud cracks.
Continental/terrestrial depositional environmentsContinental environments Fluvial bed forms Continental environments are those in which sediments are deposited on land or in fresh water.Fluvial environmentsIn fluvial system sediments deposited byStreams and rivers.Fluvial deposits include cross-beddedand rippled river sandstones and parallelor cross-bedded floodplain contains mudstones (siltstones and clay shales).
Braided andmeandering streamsFluvial environmentsinclude braided andmeandering river andstream systems. Riverchannels, bars, levees, andfloodplains are parts of thefluvial environment.Channel deposits consist ofcoarse, rounded gravel,and sand. Bars are made ofsand or gravel. Levees aremade of fine sand or silt.Floodplains are covered bysilt and clay.
Braided riversRivers with a high proportionof sediments , sand or gravelin thechannel the flow is divided togive the river a braided form.The bars in a braided riverchannel are exposed at lowflow stages.The bars within the channelmay vary in shape lithologyand sizes.
Alluvial environmentsAlluvial fans Alluvial fans are fan shapeddeposits formed at the baseof mountains due to fastflowing stream, which areflattens, slows, and spreadstypically onto a flatter plain. These are characterized bypoorly sorted, boulder andgravel dominated, debris flowconglomerates.
DesertsenvironmentsAeolianDeposited by wind in desertsDeposits.Usually contain vast areaswhere sand is deposited indunes. Dune sands are cross-bedded, well sorted, and wellrounded, without associatedgravel or clay.Aeolian sandstones frequentlydisplay large scale (1 to 3meter) cross bed sets.
LacustrineenvironmentsSediments are deposited inlack .Lack deposits may be large orsmall, shallow or deep, andfilled with terrigenous,carbonate, or evaporiticsediments.Fine sediment and organicmatter settling in some lakesproduced laminated oil shalesand coal form.
Glacial environmentsSediments deposited by aglacier.Sediments which aredeposited by a Glacier arepoorly sorted ,angularsediments .
Deltaic environmentsDeltas are fan-shapeddeposits formed where a riverflows into a standing body ofwater, such as a lake or sea . Coarser sediment (sand)tends to be deposited nearthe mouth of the river; finersediment is carried seawardand deposited in deeperwater.
Marginal marineenvironmentsMarginal marineenvironments lies along theboundary betweencontinental and marinedepositionalEnvironments.A wide variety of sedimentsincluding Conglomerates ,sandstone s , shalescarbonates , and evaporitescan accumulate in thesevarious marginal marineenvironments.
Beach and barrierislandsThese are shoreline depositsexposed to wave energy anddominated by sand with amarine fauna. Barrier islands are separatedfrom the mainland by alagoon. They are commonlyassociated with tidal flatsdeposits.
Lagoonal environmentsLagoons are coastal bodies ofwater that have very limitedconnection to the openocean.Lagoons generally developalong coasts where there is awave-formed barrier and arelargely protected from thepower of open ocean waves.A lagoonal succession istypically mudstone, oftenorganic-rich, with thin, wave-rippled sand beds.
EstaurineenvironmentsAn estuary is the marine-influenced portion of adrowned valley .A drowned valley is the seawardportion of a river valley thatbecomes flooded with seawaterwhen there is arelative rise in sea level.They are regions of mixing offresh and seawater.Sediment supply to the estuaryis from both river and marinesources, and the processes thattransport and deposit thissediment are a combination ofriver and wave and/or tidalprocesses.
Tidal flatsTidal flat are formed whenmud is deposits by tides orrivers.Tidal flats are the border oflagoons and estuarineenvironments. Tidal flats are areas of lowrelief, cut by meandering tidalchannels. Laminated orrippled clay, silt, and fine sand(either terrigenous orcarbonate) may be depositedby a tidal flat.
Marine environmentsMarine environmentsare in the seas oroceans. Marineenvironments includereefs, the continentalshelf, slope, rise, andabyssal plain.Continental shelfThe continental shelfis the flooded edge ofthe continent. It isrelatively flat (with aslope of less than0.1o), shallow (lessthan 200 m or 600 ftdeep), and may be upto hundreds of mileswide. Continentalshelves are exposedto waves, tides, andcurrents, and arecovered by sand, silt,mud, and gravel.
Reef depositsReef are wave-resistant,mound-like structures madeof the calcareous skeletons oforganisms such as corals andcertain types of algae. Mostmodern reefs are in warm,clear, shallow, tropical seas,between the latitudes of30oN and 30oS of theequator.
Continental slopedepositThe continental slope arelocated seaward of thecontinental shelf.The continental slope is thesteep (5- 25o) "drop-off "atthe edge of the continent.The continental slope passesseaward into the continentalrise, which has a moregradual slope.
Continental riseContinental rise locatedbetween the continentalslope and the abyssal plain.The continental rise is the siteof deposition of thickaccumulations of sediment,much of which is in submarinefans, deposited by turbiditycurrents at the base ofcontinental rise. Turbiditycurrent deposits are calledturbidities are characterizedBy graded bedding.Continental slope andcontinental rise are show deepmarine deposition.
Abyssal plainAbyssal plain is the deepocean floor. It is basically flat,and is covered by very fine-grained sediment, consistingprimarily of clay and the shellsof microscopic organisms(such as foraminifera,radiolarians, and diatoms).Abyssal plain sediments mayinclude chalk, diatomite, andshale, deposited over thebasaltic ocean crust.