Sedimentary basins

Sedimentary basins

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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 1 Sedimentary Basins What are sedimentary basins? Sedimentary basins can be defined as a low area in the Earth’s crust of tectonic origin, in which sediments accumulate into successions of hundreds to thousands of meters in thickness over areas of thousands to millions of square kilometers and finally lithified to form compact sedimentary rocks irrespective of its source material. For sediment to accumulate in a basin there must be a vertical interval between the water surface and the basin bottom, termed as accommodation space. Basin forming mechanism: Sedimentary basins are a depression of some kind, capable of trapping sediment. Subsidence of the upper surface of the crust must take place to form such a depression. Mechanisms that can generate sufficient subsidence to create basins include crustal thinning, mantle-lithosphere thickening, sedimentary and volcanic loading, tectonic loading, subcrustal loading, asthenospheric flow and crustal densification. Thick sedimentary sequences may form where the weight of the sediment itself causes isostatic depression of the crust. Factors controlling sediment supply and basin fill: The rate at which a basin is infilled is a function of several variables in the basin and in the drainage catchment area:  Variables within the basin include the environment and its energy level, that is, the ability of wind, wave, and tidal currents to transport, segregate and deposit sediment.  Other critical intra-basinal variable include changes in sea level which affect the top of the accommodation space and tectonism, principally rate of subsidence which affects the floor of the accommodation space.  The rate of sediment supply into a basin is dependent on the type and intensity of weathering, erosion and transportation within the drainage catchment area. Most of these variables are closely related to climate.  Vegetation is another important controlling parameter, closely related to climate.  The chemical composition and physical parameters of the rock in the source area will also play an important part.
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 2 Sedimentary basin types and their tectonic settings (after Boggs, 2006): Basin types & Tectonic settings Example Divergent Settings Terrestrial rift valleys Rio Grande Rift (New Mexico) Proto-oceanic rift troughs Red Sea Intraplate Settings Continental rises and terraces East coast of USA Continental embankments Mississippi Gulf Coast Intracratonic basins Chad Basin (Africa) Continental platforms Barents Sea (Asia) Active ocean basins Pacific Ocean Oceanic islands, aseismic ridges and plateaus Emperor-Hawaii seamounts Dormant ocean basins Gulf of Mexico Convergent Settings Trenches Chile Trench Trench-slope basins Central America Trench Fore-arc basins Sumatra Intra-arc basins Lago de Nicaragua Back-arc basins Marianas Retro-arc foreland basins Andes foothills Remnant ocean basins Bay of Bengal Peripheral foreland basins Persian Gulf Piggyback basins Peshawar Basin (Pakistan) Foreland intermontane basins Sierras Pampeanas basins (Argentina) Transform Settings Transtensional basins Salton Sea (California) Transpressional basins Santa Barbara Basin (California) (foreland) Transrotational basins Western Aleutian fore-arc (?) Intracontinental wrench basins Quaidam Basin (China) Fig. 1: Diagram showing the interplay between the variables that control sediment supply and accommodation space (Selley, 2000).
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 3 Hybrid Settings Aulacogens Mississippi Embayment Impactogens Baikal Rift (Siberia) (distal) Successor basins Southern Basin and Range (Arizona) Basin classification: All the above mentioned different tectonic settings are the areas where sediment can accumulate and at a simple level, three main settings of basin can be recognized (Nichols, 2009): 1. Basins associated with regional extension within and between plates; 2. Basins related to convergent plate boundaries; 3. Basins associated with strike-slip plate boundaries. In the following discussion the main basin types and the transitions between them are considered in terms of the plate tectonic setting. 1. Basins related to lithospheric extension: Divergent tectonic settings are regions of Earth where tectonic plates are separating. In the early stages of tectonic extension, rifts form and are typically sites of continental sedimentation. If the stretching continues, the continental lithosphere may rupture completely and the injection of basaltic magmas results in the formation of new oceanic crust within the zone of extension. The principal types related to extensional settings are: a. Rift basins: In regions of extension, continental crust fractures to produce rifts which are structural valleys bound by normal faults. The axis of the rift lays more-or-less perpendicular to the direction of the stress. The down-faulted blocks are referred to as graben and the up-faulted areas as horsts. The bounding faults may be planar or listric and if the displacement is greater on one side they form asymmetric valleys referred to as half-graben. Uplift on the flanks of rifts due to regional high heat flow and the effect of relative movements on the rift-bounding faults creates local sediment sources for rift valleys (Fig. 2a). b. Intracratonic basins: Areas of broad subsidence within a continental block (craton) away from plate margins or regions of orogeny are known as intracratonic basins. The cratonic crust is typically ancient and with low relief. The area may be very large but the amount of subsidence is low and the rate is very slow. Subsidence is so slow that there seems to have been no depression of the upper surface of the lithosphere so depositional environments are mostly the same as those in surrounding areas; the succession is just thicker (Fig. 2b).
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 4 Fig. 2: Types of basins related to crustal extension, (2a) Rift basin, (2b) Intracratonic basin, (2c) Proto- oceanic basin, (2d) Passive continental and Oceanic basin (Nichols, 2009). c. Proto-oceanic troughs: the transition from rift to ocean: Continued extension within continental crust leads to thinning and eventual complete rupture. Basaltic magmas rise to the surface in the axis of the rift and start to form new oceanic crust. Where there is a thin strip of basaltic crust in between two halves of a rift system the basin is called a proto-oceanic trough. The basin will be wholly or partly flooded by seawater by the time this amount of extension has occurred and the trough has the form of a narrow seaway between continental blocks. Sediment supply to this seaway comes from the flanks of the trough which will still be relatively uplifted (Fig. 2c). d. Passive margins: The regions of continental crust and the transition to oceanic crust along the edges of spreading ocean basins are known as passive margins. The term ‘passive’ is used in this sense as the opposite to the ‘active’ margins between oceans and continents where subduction is occurring. Morphologically, the passive margin is the continental shelf and slope and the clastic sediment supply is largely from the adjacent continental land area. The climate, topography and drainage pattern on the continent therefore determines the nature and volume of material supplied to the shelf. Passive margins are important areas of accumulation of both carbonate and clastic sediment (Fig. 2d). e. Oceanic basins: Basaltic crust formed at mid-oceanic ridges is hot and relatively buoyant. As the basin grows in size new magmas created along the spreading ridges, older crust moves away from the hot mid-ocean ridge. Cooling of the crust increases its density and decreases relative buoyancy, so as crust moves away from the ridges, it sinks (Fig. 2d). 2a 2b 2c 2d
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 5 2. Basins related to subduction: Subduction related settings (Fig. 3) are features of seismically active continental margins such as the modern Pacific Ocean margin. These settings are characterized by a deep sea trench, an active volcanic arc and an arc-trench gap separating the two. The most important depositional sites in subduction related settings are deep sea trenches, forearc basins that lie within the arc-trench gap and backarc or marginal basins that lie behind the volcanic arc in some arc-trench systems. Fig. 3: Types of basins related to subduction (Nichols, 2009). a. Trenches: Ocean trenches are elongated gently curving troughs that form where an oceanic plate bends as it enters a subduction zone. The inner margin of the trench is formed by the leading edge of the overriding plate of the arc–trench system. Trenches formed along margins flanked by continental crust tend to be filled with sediment derived from the adjacent land areas. Transport of material into trenches is by mass flows, especially turbidity currents (Fig. 3). b. Accretionary prisms: The strata accumulated on the ocean crust and in a trench are not necessarily subducted along with the crust at a destructive plate boundary. The sediments may be wholly or partly scraped off the downgoing plate and accrete on the leading edge of the overriding plate to form an accretionary complex or accretionary prism. These prisms or wedges of oceanic and trench sediments are best developed where there are thick successions of sediment in the trench (Fig. 3). c. Forearc basins: The inner margin of a forearc basin is the edge of the volcanic arc and the outer limit is the accretionary complex formed on the leading edge of the upper plate. The width of a forearc basin will therefore be determined by the dimensions of the arc–trench gap which is in turn determined by the angle of subduction. The main source of sediment to the basin is the volcanic arc and, if the arc lies in continental crust, the hinterland of continental rocks (Fig. 3). d. Backarc basins: Extensional backarc basins form where the angle of subduction of the downgoing slab is steep and the rate of subduction is greater than the rate of plate convergence. The
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 6 principal source of sediment in a backarc basin formed in an oceanic plate will be the active volcanic arc. Backarc basins are typically underfilled, containing mainly deep water sediment of volcaniclastic and pelagic origin (Fig. 3). 3. Basins related to strike-slip tectonism: Zones of localised subsidence and uplift due to a network of transform faults create topographic depressions for sediment to accumulate and the source areas to supply them. Most basins in strike-slip belts are generally termed transtensional basins and are formed by three main mechanisms: First, the overlap of two separate faults can create regions of extension between them known as pull-apart basins. Such basins are typically rectangular or rhombic and are unusually deep (Fig. 4a). Second, where there is a branching of faults, a zone of extension exists between the two branches forming a basin (Fig. 4b). Third, the curvature of a single fault strand results in bends that are either restraining bends (locally compressive) or releasing bends (locally extensional): releasing bends form elliptical zones of subsidence (Fig. 4c). Fig. 4: Types of basins related to strike-slip tectonism, (4a) Pull-apart basin, (4b) Basin formed at fault branch, (4c) Basin formed at releasing bend (Nichols, 2009). Basin fill types: Basin type Basin fill Rift basins Coarse to fine siliciclastics, usually nonmarine; often lacustrine sediments; interbedded basalts. Intracratonic basins Shallow water cratonal sediments (carbonates, shales, sandstones). Passive margin basins Shallow marine siliciclastics and carbonates of the continental shelf, thickening seaward. These sediments pass gradually or abruptly into deeper marine fine sediments of the continental slope and rise often grading or interfingering seaward into deep-marine coarse and fine siliciclastics or resedimented carbonates in the form of turbidites building submarine fans at the base of the slope and filling the deepest parts of the ocean basin to form abyssal plains. Trenches Varies from thin pelagic sediments (fine abyssal muds, volcanic ash) to thick arc-derived coarse siliciclastics and volcaniclastics. 4b 4a 4c
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 7 Forearc basins Nonmarine siliciclastic fluvial to deltaic deposits at the arcward margin pass seaward into deep marine siliciclastics, all interbedded with arc-derived volcanic flows and pyroclastics. Backarc basins Sedimentation is strongly asymmetric with most of the sediment supplied from the active magmatic arc. From cores collected during the Deep Sea Drilling Project (DSDP) nine sediment types were found (Klein, 1985) in the back-arc basins of the western Pacific: Debris flows, Submarine fan systems, Pelagic clays, Biogenic pelagic silica sediments, Biogenic pelagic carbonates, Resedimented carbonates, Pyroclastics. Pull-apart basins Filled by thick nonmarine to marine, coarse to fine clastics, often as alluvial fans passing into lake deposits or into deposits of restricted marine environments. In some cases thick marine turbidites fill the distal parts of the basin. Remnant basins Very thick and highly varied with strong lateral facies changes; usually fluvial at the margins, commonly passing into deep marine sediment-gravity-flow deposits. Sedimentary basin analysis: Sedimentary basin analysis is the aspect of geology that considers all the controls on the accumulation of a succession of sedimentary rocks to develop a model for the evolution of the sedimentary basin as a whole. The purpose of basin analysis is to interpret basin fills to better understand sediment provenance, paleogeography, depositional environments and geologic history and to evaluate the economic potential of basin sediments. Techniques of sedimentary basin analysis: Analyzing the characteristics of sediments and sedimentary rocks that fill basins and interpreting these characteristics in terms of sediment and basin history, demands a variety of sedimentological and stratigraphic techniques. These techniques require the acquisition of data through outcrop studies and subsurface methods that can include deep drilling, magnetic polarity studies and geophysical exploration. In this section, we look briefly at the more common techniques of basin analysis. a. Measuring stratigraphic sections: To interpret Earth history through study of sedimentary rocks requires that we have detailed, accurate information about the thicknesses and lithology of the stratigraphic successions. To obtain this information, appropriate stratigraphic successions must be measured (Fig. 5a) and described in outcrop and/ or from subsurface drill cores and cuttings. The process also involves describing the lithology, bedding characteristics and other pertinent features of the rocks. Thus, measuring and describing stratigraphic sections is commonly the starting point for many geologic studies. b. Preparation of stratigraphic maps and cross sections: This can be done preparing the below mentioned maps/ daigrams: 1. Stratigraphic cross sections (Fig. 5b)/ Fence diagrams (Fig. 5c)
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 8 2. Structure-contour maps (Fig. 5d) 3. Isopach maps (Fig. 5e) 4. Paleogeologic maps 5. Lithofacies maps (Fig. 5f) 6. Computer-generated maps Fig. 5: Techniques of basin analysis, (5a) measuring stratigraphic successions, (5b) cross section, (5c) fence diagram, (5d) structure-contour maps, (5e) isopach maps, (5f) lithofacies maps. c. Paleocurrent analysis: Paleocurrent analysis is a technique used to determine the flow direction of ancient currents that transported sediment into and within a depositional basin which reflects the local or regional paleoslope and also the direction in which the sediment source area or areas lay. Further, it aids in understanding the geometry and trend of lithologic units. Paleocurrent analysis is 5a 5b 5c 5d 5e 5f
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 9 accomplished by measuring the orientation of directional features such as sedimentary structures (e.g., flute casts, ripple marks, cross-beds) or the long-axis orientation of pebbles. d. Provenance studies: Analysis of the particle composition of siliciclastic mineral assemblages (and rock fragments) provides a method of working backward to understand the nature of the source area. We commonly refer to such study as provenance study where provenance is considered to include the following: (1) the lithology of the source rocks, (2) the tectonic setting of the source area and (3) the climate, relief, and slope of the source area. Provenance studies provide important information about the paleoclimatology and paleogeography of the basin setting. d. Geophysical studies: Geophysical investigations, including both seismic and paleomagnetic studies of various kinds play an important role in basin analysis. Seismic techniques are used to document the regional structural trends and overall basin geometry as well as to identify local structural features such as anticlines and faults that may provide traps for hydrocarbons. The most widespread application of paleomagnetism in basin analysis is the study of magnetic polarity reversals as a tool for correlation. Applications of sedimentary basin analysis: a. Interpreting geologic history: One major goal of basin analysis is simply to develop a better understanding of Earth history as recorded in particular depositional basins. Through analysis of sedimentary textures, structures, particle and chemical composition, fossils and the stratigraphic characteristics of sedimentary rocks (as revealed by physical, biological, paleomagnetic and seismic reflection characteristics), geologists are able to interpret the important tectonic and sedimentologic events that transpired to generate and fill a particular sedimentary basin. Thus, these various kinds of basin studies, allow geologists to interpret past tectonic, climatic, and sedimentologic events and conditions (including source area characterization and interpretation of depositional environments) to reconstruct the paleogeography and paleogeology of Earth during specific times in the past. a. Economic Applications: The second goal of basin analysis is to use the principles and techniques described above to evaluate the economic importance of sedimentary rocks and identify economically exploitable deposits of minerals or fossil fuels. Basin analysis finds its greatest economic application in the fields of petroleum geology and to a lesser extent, hydrogeology. In spite of the fact that petroleum geologists have been trying for many years to locate petroleum (hydrocarbon) accumulations, no successful method has yet been developed for direct detection of hydrocarbon deposits. To find an oil or gas deposit, geologists must (1) explore basins that have the right conditions for the formation and migration of hydrocarbons and (2) locate a suitable trap such as a structural anticline, in which the hydrocarbons may have accumulated. Basin analysis to the petroleum geologist thus means locating within depositional basins suitable source rocks, reservoir rocks and traps. To do this successfully calls into play most of the principles of sedimentology and stratigraphy,
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  GS-522 Assignment Md.Shahadat Hossain, Roll-1651 Sedimentary Basins Page 10 References: Boggs S., 2006, Principles of Sedimentology and Stratigraphy, 4th Edition, Pearson Prentice Hall, USA. Klein G. D., 1985, The Control of Depositional Depth, Tectonic Uplift and Volcanism on Sedimentation Processes in The Back-Arc Basins of the Western Pacific Ocean, Journel of Geology, vol. 93, p. 1-25. Nichols G., 2009, Sedimentology and Stratigraphy, 2nd Edition, Blackwell Publishing, UK. Selley R. C., 2000, Applied Sedimentology, 2nd Edition, Academia Press, USA. Tucker, 2003, Sedimentary Petrology, 3rd Edition, Blackwell Publishing, UK.