Continental depositional environment

Brief information about Continental depositional environment

1. MUHAMMAD HAFEEZ Roll num :------ ASSIGNMENT: Continental Depositional Environments Department of Earth Science

2. Depositional environments Landscapes form and constantly change due to weathering and sedimentation. the area where the sediments accumulates and is later buried by the other sediments is known as depositional environments Depositional environments are also separated into three general types or setting Continental environment /Terrestrial (on land) or Transitional environment /( marginal marine)/ (coastal environments) Marine environment /(open ocean)

3. Depositional Environments Continental environments Transitional environments Marine environments

4. Fluvial Environments: In fluvial system sediments deposited by streams and rivers, or it is associated with river and stream and the deposit and landform created by them. Rivers systems can also be depositional, accumulating sediments within the channel and on floodplain. Fluvial and alluvial deposits in the stratigraphic record provide evidence of tectonic activity and indications of the paleoclimate at the time of deposition. Comparisons between modern ancient river systems should be carried out with care because continental environments have change dramatically through geological time as land plant and animal communities have envolved .

5. Zones of fluvial systems: Three Geomorphology zones can be recognized with in the fluvial and alluvial systems Erosional zones: In the erosional zone the stream are actively down cutting removing bedrock from the valley floor and from the valley sides via downslope movement material into the stream bed. Transfer zones: In the transfer zones the gradient is lower, stream and river are not actively eroding, but nor is this a site of deposition. Depositional zone: The lower part of the system is called depositional zone, where sediment is deposited in the river channels and on floodplains of fluvial system or on the surface of alluvial fan These three components are not present in all systems: some may be wholly erosional as far as the sea or lake, and others may not display a transfer zone. . The area of the ground that supplies water to a river system is the catchment area ( sometime also referred to as a drainage basin) .

6. Factors controlling the catchment area Two factors are important in controlling the supply of water to a river system. First the size of the catchment area: a small area has a more limited capacity for storing water in the soil and as groundwater than a large catchment area. The second factor is the climate: catchment areas in temperate or tropical regions where is regular rainfall remains wets throughout the year and keep the river supplied with water. A large river system with a catchment area that experiences year-round rainfall is constantly supplied with water and the discharge (the volume of water flowing in a river in a time period) show only moderate variation through the year. These are called Perennial fluvial system. In contrast, rivers that have much smaller drainage areas and/or seasonal rainfall may have highly variable discharge. If the rivers are dry for long period of time and only experience flow after there has been suffient rain in the catchment area they are considered to be Ephemeral Rivers. The deposits of these ephemeral flows are characteristically poorly sorted, consisting of angular or sub angular gravel clasts in a matrix of sand and mud.

7. FLOODPLAIN DEPOSITION When the discharge exceeds the capacity of the channel, water flows over the banks and out onto the floodplain where overbank or floodplain deposition occurs. Most of the sediment carried out onto the floodplain is suspended load that will be mainly clay- and silt sized debris but may include fine sand if the flow is rapid enough to carry sand in suspension. As water leaves the confines of the channel it spreads out and loses velocity very quickly. The drop in velocity prompts the deposition of the sandy and silty suspended load, leaving only clay in suspension. The sand and silt is deposited as a thin sheet over the floodplain, which may show current ripple or horizontal lamination: rapid deposition may result in the formation of climbing ripple cross lamination. The remaining suspended load will be deposited as the floodwaters dry out and soak away after the flow has subsided.

8. Sheets of sand and silt deposited during floods are thickest near to the channel bank because coarser suspended load is dumped quickly by the floodwaters as soon as they start flowing away from the channel. Repeated deposition of sand close to the channel edge leads to the formation of a levee , a bank of sediment at the channel edge which is higher than the level of the floodplain. Through time the level of the bottom of the channel can become raised by sedimentation in the channel and the level of water at bank full flow becomes higher than the floodplain level. When the levee breaks, water laden with sediment is carried out onto the floodplain to form a crevasse splay

9. Floodplain structures The primary depositional structures commonly observed in floodplain sediments are: A very thin and thin beds normally graded from sand to mud; The evidence of initial rapid flow (plane parallel lamination) quickly waning and accompanied by rapid deposition (climbing ripple lamination); A thin sheets of sediment, often only a few centimeters thick but extending for tens to hundreds of meters; The erosion at the base of the overbank sheet sandstone beds is normally localized to areas near the channel where the flow is most vigorous (Strong). The evidence of soil formation

10. FLUVIAL TYPES Fluvial environment Further divides into; River and alluvial fans River: In river include it includes the following types: a) Braided river system b) Meander river system c) Anastomosing Braided Rivers: A braided river contain mid-channel bars that are covered at bank-full flow. River with a high proportional of sediments carried by rolling and salting along the channel floor are referred as bed Load Rivers. Where the bed load deposited as bars of sand or gravel in the channel the flow is divided to give the river a braided form.

11. The bars in the river braided river channel are exposed at low flow stages, but are covered when the flow is at bank-full level. The bar within the channel may vary in shape and size and there are different types of bars. Longitudinal Bars: are elongate along the axis of the channel. Transverse Bars: Those bars that are wider than they are long, spreading across the channel are called transverse bars. Linguoid Bars: crescentic bars within their apex pointing downstream are Linguoid bars. compound bars: Bars may consist of sand, gravel or a mixture of both ranges of clast size (compound bars).

12. Braided Stream Deposits Braided stream deposits consist of conglomerate cross-bedded sandstone but mudstone is rare or absent

13. Anastomosing It is also known as anabranching river, which consists of multiple, interconnected channels that are separated by areas of floodplain. Both braided and anastomosing river channels can be sinuous, and sinuous rivers that have depositional bars only on the insides of bends are called meandering. Anastomosing or anabranching rivers are seen today mostly in places where the banks are stabilized by vegetation, which inhibits the lateral migration of channels ( Smith 1983), but anastomosing rivers are also known from more arid regions with sparse vegetation. The positions of channels tend to remain fairly fixed but new channels may develop as a consequence of flooding as the water makes a new course across the floodplain, leaving an old channel abandoned.

14. A distinction between river sinuosity and meandering: A distinction between river sinuosity and meandering form should be recognised : a river is considered to be sinuous if the distance measured along a stretch of channel divided by the direct distance between those points is greater than 1.5. A river is considered to be meandering if there is accumulation of sediment on the inside of bends .

15. Meander River Both braided and anastomosing river channels can be sinuous and sinuous rivers that have depositional bars only on the insides of bends are called Meandering. Meandering rivers transport and deposit a mixture of suspended and bedload (mix load).the bedload is carried by the flow in the channel, with the coarser material carried in the deepest part of the channel. Fine bedload is also carried in shallows part of the flow and is deposited along the inner bend of the meander loop where fraction reduce the flow velocity .

16. Meandering Stream Deposits In meandering stream deposits, mudstone deposited in a floodplain is common sandstones are point bar deposits channel conglomerate is minor

17. ALLUVIAL FANS An alluvial fan is a triangular shaped or covex upward cross-sectional profile, that deposit of gravel, sand, and even smaller pieces or sediment, such as silt. this sediment is called alluvium. Alluviam fans are usually created as flowing water interacts with mountains, hills, or the steep wall of the canyons. Usually alluvial fans are particularly common in high relief mostly at the base of the mountain where abundant supply of supply of sediments available. Sediments of alluvial fans are mostly poor sorted, includes abundant gravel size detritus. Differnent depositional process, involving for making the alluvial fans including debris fluid dominated fans and stream flow dominated fans, some formed in small scales with steep slope angle is (1.5-25°) that were deposit mainly in sediment gravity flow, particularly debris flow and upper flow regime fluid flow.

18. Scree cones formed primarily of rock fall and rock avalanche are commonly associated with alluvial fan deposits at the basin margin. Sediment bodies that consist of a mixture of talus deposits and debris- flow deposits are sometimes called colluvial fans: these features are common in subpolar regions where gravity processes are augmented by wet mass flows of debris .

19. Characteristics of fluvial and alluvial fan deposits Lithologies – conglomerate, sandstone and mudstone Mineralogy – variable, often compositionally immature Texture – very poor in debris flows to moderate in river sands Bed Geometry – sheets on fans, lens shaped river channel units Sedimentary Structures – cross-bedding and lamination in channel deposits Palaeocurrents – indicate direction of flow and depositional slope Fossils – fauna uncommon, plant fossils may be common in floodplain facies Colour – yellow, red and brown due to oxidizing conditions Facies Associations – alluvial fan deposits may be associated with ephemeral lake and aeolian dunes, rivers may be associated with lake, delta or estuarine facies .

20. GLACIAL ENVIRONMENTS The sediments or particles which are deposit in the glacial is called glacial environments. Glacial are very important agents of erosion of bedrock and mechanisms of transportation of detritus in mountain regions. TYPES OF GLACIER : there are three main types of glacial terrains: temperate (or mountain) glacial and polar ice caps. Temperate or mountain glaciers: It form in area of relatively high altitude where precipitation in the winter is mainly in the form of snow. Accumulating snow compacts and starts to form ice especially in the upper parts of valleys, and a glacier forms if the summer melt is insufficient to remove all of the mass added each winter.

21. Once formed, the weight of snow accumulating in the upper part of the glacier (the accumulation zone of the glacier) causes it to move downslope, where it reaches lower altitudes and higher temperatures. The lower part of the glacier is the ablation zone where the glacier melts during the summer. A cooling of the climate reduces the rate of melting and there will be Glacial Advance down the valley, whereas under a warmer climate the melting will exceed the rate of addition of snow and there will be Glacial Retreat .

22. Polar Glaciers It occur at the north and south poles, which are regions of low precipitation (Antarctica is the driest continent): the addition to the glaciers from snow is quite small each year, but the year-round low temperatures mean that little melting occurs. Permanent ice in the polar continental areas forms large ice sheets and domed ice caps covering tens to hundreds of thousands of square kilometers. These may completely or partially bury the topography, and the hills or mountains that protrude above the ice as areas of bare rock are called Nunataks .

23. DEPOSITION BY CONTINENTAL GLACIERS The general term for all deposits directly deposited by ice is Till if it is unconsolidated or Tillite if it is lithified. Tills can be divided into a number of different types depending on their origin. Meltout tills: are deposited by melting ice as accumulations of material at a glacier front. Lodgement tills: are formed by the plastering of debris at the base of a moving glacier, and the shearing process during the ice movement may result in a flow-parallel clast orientation fabric. Basal Tills: Collectively meltout and lodgement tills are sometimes called basal tills. Flow Tills are accumulations of glacial sediment reworked by gravity flows.

24. Origin of glacial drift Moraines and poorly sorted till

25. Moraines Accumulations of till formed directly at the margins of a glacier are known as moraine. Types of moraines Several different types of moraine can be recognized Terminal moraines: It is also called End moraines, mark the limit of glacial advance and are typically ridges that lie across the valley. Push moraines: Push moraines are formed where a glacier front acts as a bulldozer scraping sediment from the valley floor and piling it up at the glacier front. Dump moraines: Dump moraines form at the snout of the glacier where the melting of the ice keeps pace with glacial advance. If a glacier retreats the melting releases the detritus that has accumulated at the sides of the glacier where it is deposited as a lateral moraine. Ressional moraines: moraines located behind the outermost edge of a glacier, formed when the glacier linger in one spot for a long time. Glacial moraines: gently rolling hills and plain deposited by ice.

26. Ablation morains : a moraines formed from the material that fell upon the glacier Lateral moraine: Lateral moraines form ridges along the sides of glaciated valleys, parallel to the valley walls. Where two glaciers in tributary valleys converge detritus from the sides of each is trapped in the center of the amalgamated glacier Medial moraine: Medial moraine along the center of a glaciated valley. When a cold glacier retreats, the snout of the glacier is often left with a carapace(cover) of detritus left behind as the glacier front has been melting.

27. MARINE GLACIAL ENVIRONMENTS: Where a continental ice sheet reaches the shoreline the ice may extend out to sea as an Ice Shelf . Modern ice shelves around the Antarctic continent Modern ice shelves around the Antarctic continent extend hundreds of kilometres out to sea forming areas of floating ice which cover several hundred thousand square kilometres . Ice shelves break up at the edges to form icebergs and melt at the base in contact with seawater.

28. AEOLIAN ENVIRONMENT The term aeolian (or eolian in North American usage) is used to describe the processes of transport of fine sediment up to sand size by the wind, and aeolian environments are those in which the deposits are made up mainly of wind-blown material. The wind is a movement of air from one part of the Earth’s surface to another and is driven by differences in air pressure between two places. Air masses move from areas of high pressure towards areas of low pressure, and the speed at which the air moves will be determined by the pressure difference. The main contrast in temperature is between the Equator, which receives the most energy from the Sun, and the poles, which receive the least. Heat is transferred between these regions by air movement (as well as oceanic circulation). katabatic winds :Air masses blowing over mountain ranges are forced upwards and are cooled, and similarly the air is chilled when winds blow over ice caps: this results in katabatic winds.

29. Aeolian transport processes A flow of air over a loose grain of sand exerts a lift force on the particle and with increasing velocity the force may increase to the point where the grain rolls or saltates . Winds of 55m/ s or more are recorded during hurricanes, but strong winds over land areas are typically around 30m/s, and at these velocities the upper limit on the size of quartz grains moved by the wind is around a half a millimeter in diameter, that is, medium sand size storms DESERTS AND ERGS: A desert is a continental area that receives little precipitation: they are arid areas that receive less than 250mm/ yr precipitation. (Areas that receive average precipitation of between 250 and 500mm/y are defined as semi-arid and are not usually considered to be true deserts.) The lack of vegetation is an important influence on surface processes because without a plant cover detritus lies loose on the surface where it is subject to aeolian activity.

30. Ergs: An erg is an area where sand has accumulated as a result of aeolian processes (Brookfield 1992): these regions are also sometimes inappropriately(not sutible ) referred to as a ‘sand sea’. Ergs are prominent features of some deserts, but in fact most deserts are not sandy but are large barren areas known as rocky deserts. Erg is the area of the desert which cover 125 square kilometers. Desert processes: sand dunes

31. AEOLIAN BEDFORMS The processes of transport and deposition by wind produce bedforms that are in some ways similar to subaqueous bedforms, but with some important differences that can be used to help distinguish Aeolian from subaqueous sands. Difference between Aeolian bedform and subaqueous bedform: Three groups can be separated on the basis of their size: Aeolian ripples Dunes Draas .

32. Aeolian ripple bedforms : As wind blows across a bed of sand, grains will move by saltation forming a thin carpet of moving sand grains The grains are only in temporary suspension, and as each grain lands, it has sufficient energy to knock impacted grains up into the free stream of air, continuing the process of saltation. Irregularities in the surface of the sand and the turbulence of the air flow will create patches where the grains are slightly more piled up. The result is a series of piles of grains aligned perpendicular to the wind and spaced equal distances apart. These are the crests of aeolian ripples The troughs in between are shadow zones where grains will not be picked up by the air flow and where few saltating grains land. Aeolian ripples have extremely variable wavelengths (crest to crest distance) ranging from a few centimetres to several meters .

33. Coarser grains tend to be concentrated at the crests, where the finer grains are winnowed away by the wind, and as aeolian ripples migrate they may form a layer of inversely graded sand. Where a crest becomes well developed grains may avalanche down into the adjacent trough forming cross-lamination, but this is less common in aeolian ripples than in their subaqueous counterparts.

34. Aeolian dune bedforms Aeolian dunes are bedforms that range from 3m to 600m in wavelength and are between 10 cm and 100m high. They migrate by the saltation of sand up the stoss (upwind) side of the dune to the crest. This saltation may result in the formation of aeolian ripples which are commonly seen on the stoss sides of dunes. Sand accumulating at the crest of the dune is unstable and will cascade down the lee slope as an avalanche or grain flow to form an inclined layer of sand. Repeated avalanches build up a set of crossbeds that may be preserved if there is a net accumulation of sand.

35. Types of dunes Barchan Dunes , which are lunate structures with arcuate slip faces forming trough crossbedding. linear or seif dunes: Under circumstances where there are two prominent wind directions at approximately 90 to each other, linear or seif dunes form. Star dunes: In areas of multiple wind directions Star Dunes have slip faces in many orientations and hence the cross-bedding directions display a similar variability.

36. Draa bedforms : When an ergs is viewed from high altitudes in aerial photographs or satellite images, it is possible to see a in amplitude. These structures are known as draas and there is again evidence that they are a distinct, larger bedform separate from the dunes that may be superimposed on them.

37. LACUSTRINE/LAKE ENVIRONMENTS Lakes form where there is a supply of water to a topographic low on the land surface. A lake is an inland body of water. Although some modern lakes may be referred to as ‘inland seas’ Lakes form where there is a depression on the land surface which is bounded by a sill such that water accumulating in the depression is retained Sand and mud are the most common components of lake deposits, although almost any other type of sediment can accumulate in lacustrine (lake) environments, including limestone, evaporites and organic material. Plants and animals living in a lake may be preserved as fossils in lacustrine deposits, and concentrations of organic material can form beds of coal or oil and gas source rocks. The study of modern lakes is referred to as limnology .

38. LAKE FORMATION The most important processes for the creation of lake basins are those of continental extension to generate rifts. Lakes can also be created where thrust faults locally uplift part of the land surface and create a dam across the path of a river. Glacial processes can also create lakes by building up a natural dam of detritus across a valley floor through the formation of a terminal moraine. Volcanic activity can also create large lakes by caldera collapse and explosive eruptions that remove large quantities of material from the centers of a volcanic edifice, leaving a remnant rim within which a crater lake can form.

39. Crater lake Glacial lake Rift-graben lake Oxbow lakes

40. Lake Hydrology . The supply of water to a lake is through streams, groundwater and by direct rainfall on the lake surface. If there is no loss of water from the lake, the level will rise through time until it reaches the spill point, which is the top of the sill or barrier around the lake basin. A lake is considered to be hydrologically open if it is filled to the spill point and there is a balance of water supply into and out of the basin If the rate of evaporation exceeds or balances the rate of water supply there is no outflow from the lake and it is considered to be hydrologically closed . These types of lake basin are also sometimes referred to as endorheic and are basins of internal drainage.

41. Types of lakes From a sedimentological point of view, three types of lake can be considered, irrespective of their mode of formation or hydrology. Fresh waterlakes Saline lakes Ephemeral lakes Freshwater lakes: Freshwater lakes have low salinity waters and are either hydrologically open, or are hydrologically closed with a low supply of dissolved ions allowing the water to remain fresh. The majority of large modern lakes are freshwater.

42. 2. SALINE LAKES: Saline lakes are hydrologically closed and are perennial water bodies in which dissolved ions have become concentrated by evaporation. The chemistry of saline lake waters is determined by the nature of the salts dissolved from the bedrock of the catchment area of the river systems that supply the lake. Types of saline lakes: Three main saline lake types are recognised according to the composition of the brines (ion-rich waters) in them Soda lakes: Soda lakes have brines with high concentrations of bicarbonate ions and sodium carbonate minerals such as trona and natron : these minerals are not precipitated from marine waters and are therefore exclusive indicators of non-marine evaporite deposition Sulphate lakes: Sulphate lake brines have lower concentrations of bicarbonate but are relatively enriched in magnesium and calcium: they precipitate mainly sulphate minerals such as gypsum and mirabilite (a sodium sulphate ). Salt lakes: Salt lakes or chloride lakes such as the Dead Sea are similar in mineral composition to marine evaporites .

43. 3. EPHEMERAL LAKES Ephemeral lakes mainly occur in arid climatic settings and are temporary bodies of water that exist for a few months or years after large rainstorms in the catchment area, but are otherwise dry although the term playa lake is also commonly used. Terms such as ‘ saline pan’ are also sometimes used to describe these temporary lake environments. Evaporite minerals also form within the sediments surrounding ephemeral lakes.

44. Which condition lacustrine is converted into fluvial: The rate of sediment supply is significant in all lacustrine environments. If the rate of deposition of clastic, carbonate and evaporite deposits is greater than the rate of basin subsidence the lake basin will gradually fill. In overfilled lake settings this will result in a change from lacustrine to fluvial deposition as the river waters no longer pond in the lake but instead flow straight through the former lake area with channel and overbank deposits accumulating. Balanced fill and underfilled basins will also gradually fill with sediment, sometimes to the level of the sill such that they also become areas of fluvial deposition.

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