Sedimentologi Kamal Roslan Mohamed

Sedimentologi Kamal Roslan Mohamed CONTINENTS: SOURCES OF SEDIMENT

INTRODUCTION The ultimate source of the clastic and chemical deposits on land and in the oceans is the continental realm, where weathering and erosion generate the sediment that is carried as bedload, in suspension or as dissolved salts to environments of deposition. Thermal and tectonic processes in the Earth’s mantle and crust generate regions of uplift and subsidence, which respectively act as sources and sinks for sediment. Weathering and erosion processes acting on bedrock exposed in uplifted regions are strongly controlled by climate and topography.

FROM SOURCE OF SEDIMENT TO FORMATION OF STRATA In the creation of sediments and sedimentary rocks the ultimate source of most sediment is bedrock exposed on the continents. The starting point is the uplift of pre-existing bedrock of igneous, metamorphic or sedimentary origin. Once elevated this bedrock undergoes weathering at the land surface to create clastic detritus and release ions into solution in surface and nearsurface waters. The pathway of processes involved in the formation of a succession of clastic sedimentary rocks, part of the rock cycle.

FROM SOURCE OF SEDIMENT TO FORMATION OF STRATA Erosion follows, the process of removal of the weathered material from the bedrock surface, allowing the transport of material as dissolved or particulate matter by a variety of mechanisms. Eventually the sediment will be deposited by physical, chemical and biogenic processes in a sedimentary environment on land or in the sea. The final stage is the lithification of the sediment to form sedimentary rocks, which may then be exposed at the surface by tectonic processes. The pathway of processes involved in the formation of a succession of clastic sedimentary rocks, part of the rock cycle. These processes are part of the sequence of events referred to as the rock cycle.

MOUNTAIN-BUILDING PROCESSES Plate tectonic theory provides a framework of understanding the processes that lead to the formation of mountains, aswell as providing an explanation for how all the main morphological features of the crust have formed throughout most of Earth history. The boundaries of the present-day principal tectonic plates. Plate movements and associated igneous activity create the topographic contours of the surface of the Earth that are then modified by erosion and deposition. Areas of high ground on the surface of the globe today can be related to plate boundaries.

GLOBAL CLIMATE The climate belts around the world are principally controlled by latitude. The amount of energy from the Sun per unit area is less in polar regions than in the equatorial zones so there is a temperature gradient from each pole to the Equator. The present-day world climate belts. These temperature variations determine the atmospheric pressure belts: high pressure regions occur at the poles where cold air sinks and low pressure at the Equator where the air is heated up, expands and rises. These differences in pressure give rise to winds, which move air masses between areas of high pressure in the subtropical and polar zones to regions of low pressure in between them.

WEATHERING PROCESSES Rock that is close to the land surface is subject to physical and chemical modification by a number of different weathering processes. These processes generally start with water percolating down into joints formed by stress release as the rock comes close to the surface, and are most intense at the surface and in the soil profile. Weathering is the breakdown and alteration of bedrock by mechanical and chemical processes that create a regolith (layer of loose material), which is then available for transport away from the site.

Physical weathering Chemical weathering • These are processes that break the solid rock into pieces and may separate the different minerals without involving any chemical reactions. • - Freeze–thaw action • Salt growth • Temperature changes • These processes involve changes to the minerals that make up a rock. • Solution • Hydrolysis • Oxidation

Physical weathering -Freeze–thaw action Water entering cracks in rock expands upon freezing, forcing the cracks to widen; this process is also known as frost shattering and it is extremely effective in areas that regularly fluctuate around 08C, such as high mountains in temperate climates and in polar regions

Physical weathering - Salt growth Seawater or other water containing dissolved salts may also penetrate into cracks, especially in coastal areas. Upon evaporation of the water, salt crystals form and their growth generates localised, but significant, forces that can further open cracks in the rock.

Physical weathering - Temperature changes Changes in temperature probably play a role in the physical breakdown of rock. Rapid changes in temperature occur in some desert areas where the temperature can fluctuate by several tens of degrees Celsius between day and night; if different minerals expand and contract at different rates, the internal forces created could cause the rock to split. This process is referred to as exfoliation, as thin layers break off the surface of the rock.

Chemical weathering - Solution Most rock-forming silicate minerals have very low solubility in pure water at the temperatures at the Earth’s surface and so most rock types are not susceptible to rapid solution. It is only under conditions of strongly alkaline waters that silica becomes moderately soluble. Carbonate minerals are moderately soluble, especially if the groundwater (water passing through bedrock close to the surface) is acidic. Most soluble are evaporite minerals such as halite (sodium chloride) and gypsum, which locally can form an important component of sedimentary bedrock.

Chemical weathering - Hydrolysis Hydrolysis reactions depend upon the dissociation of H2O into Hþ and OH ions that occurs when there is an acidifying agent present. Natural acids that are important in promoting hydrolysis include carbonic acid (formed by the solution of carbon dioxide in water) and humic acids, a range of acids formed by the bacterial breakdown of organic matter in soils. Many silicates undergo hydrolysis reactions, for example the formation of kaolinite (a clay mineral) from orthoclase (a feldspar) by reaction with water.

Chemical weathering - Oxidation The most widespread evidence of oxidation is the formation of iron oxides and hydroxides from minerals containing iron. The distinctive red-orange rust colour of ferric iron oxides may be seen in many rocks exposed at the surface, even though the amount of iron present may be very small.

The products of weathering Material produced by weathering and erosion of material exposed on continental land masses is referred to as terrigenous (meaning derived from land). Terrigenous clastic detritus comprises minerals weathered out of bedrock, lithic fragments and new minerals formed by weathering processes. Stable minerals such as quartz are relatively unaffected by chemical weathering processes and physical weathering simply separates the quartz crystals from each other and from other minerals in the rock. Micas and orthoclase feldspars are relatively resistant to these processes, whereas plagioclase feldspars, amphiboles, pyroxenes and olivines all react very readily under surface conditions and are only rarely carried away from the site of weathering in an unaltered state.

The products of weathering The most important products of the chemical weathering of silicates are clay minerals. A wide range of clay minerals form as a result of the breakdown of different bedrock minerals under different chemical conditions; the most common are kaolinite, illite, chlorite and montmorillonite. Oxides of aluminium (bauxite) and iron (mainly haematite) also form under conditions of extreme chemical weathering. The relative stability of common silicate minerals under chemical weathering.

Soil development Soil formation is an important stage in the transformation of bedrock and regolith into detritus available for transport and deposition. In situ (in place) physical and chemical weathering of bedrock creates a soil that may be further modified by biogenic processes. An in situ soil profile with a division into different horizons according to presence of organic matter and degree of breakdown of the regolith.

EROSION AND TRANSPORT Weathering is the in situ breakdown of bedrock and erosion is the removal of regolith material. Loose material on the land surface may be transported downslope under gravity, it may be washed by water, blown away by wind, scoured by ice or moved by a combination of these processes. Falls, slides and slumps are responsible for moving vast quantities of material downslope in mountain areas but they do not move detritus very far, only down to the floor of the valleys. The transport of detritus over greater distances normally involves water, although ice and wind also play an important role in some environments

DENUDATION AND LANDSCAPE EVOLUTION The lowering of the land surface by the combination of weathering and erosion is termed denudation. Weathering and erosion processes are to some extent interdependent: it is the combination of these processes that are of most relevance to sedimentary geology, namely the rates and magnitudes at which denudation occurs and the implications that this has on the supply of material to sedimentary environments. Rates of denudation are determined by a combination of topographic and climatic factors, which in turn influence soil development and vegetation, both of which also affect weathering and erosion. In addition, different bedrock lithologies respond in different ways to these combinations of physical, chemical and biological processes.

Topography and relief A distinction needs to be made between the altitude of a terrain and its relief, which is the change in the height of the ground over the area. A plateau region may be thousands of metres above sea level but if it is flat there may be little difference in the rates of denudation across the plateau and a lowland region with a comparable climate. With increasing relief the mechanical denudation rate increases as erosion processes are more efficient. Rock falls and landslides are clearly more frequent on steep slopes than in areas of subdued topography: stream flow and overland water flow are faster across steeper slopes and hence have more erosive power.

Climate controls on denudation processes Chemical weathering processes are affected by factors that control the rate and the pathway of the reactions. Water is essential to all chemical weathering processes and hence these reactions are suppressed where water is scarce (e.g. in deserts). Temperature is also important, because most chemical reactions are more vigorous at higher temperatures; hot climates therefore favour chemical weathering. Water chemistry affects the reactions: the presence of acids enhances hydrolysis and dissolved oxidising agents facilitate oxidation reactions. The rates and efficiency of the reactions vary with different bedrock types.

Bedrock lithology and denudation The type of bedrock is a fundamental control on the rates and patterns of denudation. The greatest variability is seen in humid climates where chemical weathering processes are dominant because different lithologies are broken down, and hence eroded, at widely different rates. Quartz-rich rocks are least susceptible to breakdown, whereas mafic rocks such as basalts are rapidly weathered and eroded. Limestone bedrock is primarily weathered by dissolution, and the pattern of denudation is therefore dominated by development of karst scenery.

Soils and denudation Soil development has an important role in weathering processes. Water is retained in soils and hence the thickness of the soil profile influences how much water is available. Biochemical reactions in soils create acids, collectively known as humic acids, which increase rates of solution of carbonate bedrock. Soils are host to plants and animals, which also play a role in breaking down bedrock, especially roots that can penetrate deep into the rock and widen fractures.

Vegetation and denudation The types of vegetation and the coverage they have over the land surface are determined by the climate regime, which is in turn influenced by the latitude and altitude. A dense vegetation cover is very effective at protecting the bedrock and its overlying regolith from erosion by rain impact and overland flow of water. Even steep mountain slopes can be effectively stabilised by plants.

TECTONICS AND DENUDATION The creation of the topography of the continental land surface is fundamentally controlled by plate tectonic processes and mantle behaviour but surface processes, particularly erosion, play an important role in modifying the landscape. Denudation results in the removal of material from the uplifted bedrock and this reduces the mass of material in these areas. This removal of mass results in isostatic uplift. Uplift due to thickening of the crust followed by erosion results in isostatic compensation as the load of the rock mass eroded is removed. If the erosion is uneven then locally the removal of mass from valleys can result in uplift of the mountain peaks between.

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Sedimentologi Kamal Roslan Mohamed