How does structure effects on coal seam and its mining?

Introduction
Any significant lateral or vertical structural change in a coal seam has a direct bearing on its thickness, quality and mine ability. Such changes can be on a small or large scale, affect the internal character of the coal, or simply displace the coal spatially, replacing it with non-coal sediment, or, in certain circumstances, with igneous intrusives. Disruption to coal seam thickness and continuity can lead to the interruption or cessation of mining, which will have economic repercussions, particularly in underground mines where mining flexibility is reduced. Therefore an understanding of the structural character of a coal deposit is essential in order to perform stratigraphic correlation, to calculate coal resource/reserves, and to determine the distribution of coal quality prior to mine planning.

Syndepositional effects
The majority of coal-bearing sediments are deposited in or on the margins of tectonic basins. Such a structural environment has a profound influence on the accumulating sediments both in terms of the nature and the amount of supply of detrital material required to form such sequences, and on the distribution and character of the environments of sedimentation. In addition, diagenetic effects within the accumulating sediments produce structural deformation; this may be due to downward pressure from the overlying strata, and may be combined with water loss from the sediments when still in a non-indurated or plastic state.

Microstructural effects
The combination of thick sediment accumulation and rapid basin subsidence can produce instability particularly along the basin margins. The effects on coal-bearing sediments are frequently seen in the form of slumping and loading structures, and liquifaction effects, with the latter being characterized by the disruption of bedding laminae and the injection of sediment into the layer above and below. Under such loading effects, coal may be squeezed into over lying strata and the original seam structure may be completely disrupted. In addition, coals may be injected by surrounding sediment in the form of sedimentary dykes. Inter-bedded sequences of mudstone, sandstone and coal that have undergone loading deformation exhibit a variety of structures such as accentuated loading on the bases of erosives and stones, flame structures, distorted and dislocated ripples, and folded and contorted bedding. Instability within environments of deposition, whether induced by fault activity or simply by overloading of accumulated sediment, can produce movement of sediments in the form of gravity flows. If a coal is transported in this fashion, the result can be an admixture of coal material and other sediment with no obvious bedding characteristics. 

Macrostructural effects
Within sedimentary basins, existing faults in the underlying basement may continue to be active and influence the location, thickness and character of the sedimentary sequence. Many coal-bearing basinal sediments display evidence of growth faulting. In West Virginia and Pennsylvania, United States, broad-scale tectonic features have caused local thickening of the sequence in response to an increased rate of subsidence, as distinct from more stable platform areas (less rapidly subsiding), where sedimentation prograded rapidly over the shelf. In South Wales, United Kingdom, growth faults have again influenced sedimentation in addition to active basement elements, faults are developed that owe their origin to gravity sliding within the sedimentary pile. Over-pressured, non-compacted argillaceous sediments initiate faults on gentle gradients. Such faults tend to have a curved cross-sectional profile, steep at the top and flatten progressively into bedding plane faults, often along the roof of a coal. In many cases such faults are partially eroded before the succeeding sediments are laid down. Seam splitting can also, in certain circumstances, be attributed to growth faulting. Reactivation of faults with changes in the sense of movement can result in the down warping of sections of peat beds, this is then followed by non-peat deposition on the down warped section, and then peat deposition resumed at the original level of the first peat. Periodic changes in base level in deltaic areas through fault activation will result in changes in the development and character of coals. With emergence, coals may become more extensive and, where the influx of detritus is curtailed, have a lower ash content. If submergence occurs, coals may be restricted areally, or receive increased amounts of detritus, which may increase the ash content or even cease to develop at all. Furthermore, submerged coals may be contaminated with marine waters, which could result in a higher sulphur content in the uppermost parts of the seam. Growth folds also influence the deposition patterns in coal basins, local up warping can accelerate the rates of erosion and deposition in some parts of a basin, but can also have the effect of cutting off sediment supply by uplift or by producing a barrier to the influx of detritus. In very thick sedimentary sequences, the continued growth of such folds can result in the production of over steepened fold axes. Where this occurs, over-pressured mudstone at depth may be forced upwards and actually breach the anticlinal axial areas, this can be seen by the breaking up of the surface strata and the intrusion of material from below. Such diapiric intrusion breccia can be found in East Kalimantan, Indonesia, and these are often accompanied by the development of mud volcanoes along the axial region of the anticlines. Development of diapiric structures can disrupt as well as distort coal beds; in the Bełchat´ow opencast coal mine in Poland, a large diapir has intruded into the coal-bearing sequence, dividing the coal reserve into two distinct areas. In the Kutei Basin in East Kalimantan, Indonesia, the established structural pattern continually evolved throughout the Paleogene–Neogene Periods. In this area, the anticlines are tight with steep or overturned dips accompanied by steep reverse and normal faults in the complex axial regions. The synclines are broad and wide with very low dips, the transition between the two structures can be abrupt, now represented by steep reverse faults. These growth folds are thought to have been further accentuated by gravity sliding associated with very thick accumulation of sediment (up to 9000 m) in the Kutei Basin, and rifting in the Makassar Strait to the east. The structural grain and the paleo-strike were roughly parallel in this region, and the resultant sequence is characterized in its upper part by upper delta plain and alluvial plain sedimentation with numerous coals. Penecontemporaneous volcanism can also have a profound effect on the character of coals. Large amounts of airborne ash and dust together with water borne volcanic detritus may result in the deposition of characteristic dark lithic sandstones, possible increases in the ash content in the peat mires, and the formation of tonstein horizons.

Post-depositional effectsAll coal-bearing sequences have undergone some structural change since diagenesis. This can range from gentle warping and jointing up to complex thrusted and folded coalfields usually containing high rank coals. These post-depositional structural elements can be simply summarized as faults, joints (cleat), folds and igneous associations. Mineral precipitation may also produce some changes in the original form and bedding of coal-bearing sequences.

Jointing/cleat in coalCoal, and in particular all ranks of black coal, is noted for the development of its jointing, more commonly referred to as cleat. This regular pattern of cracking in the coal may have originated during coalification, the burial, compaction and continued diagenesis of the organic constituents results in the progressive reduction of porosity and permeability. At this stage micro-fracturing of the coal is thought to be generated. The surfaces and spaces thus created may be coated and filled with mineral precipitates, chiefly carbonates and sulphides. Cleats are fractures that occur in two sets that are, in most cases, mutually perpendicular, and also perpendicular to bedding. Abutting relations between cleats generally show one set pre-dates the other. Through-going cleats formed first and are referred to as face cleats, cleats that end at intersections with through going cleats formed later and are called butt cleats. These fracture sets and partings along bedding planes impart a blocky character to coal. Cleats are subvertical in flat-lying beds and are usually orientated at right angles to the bedding even when strata are folded. In a number of cases, cleats are confined to individual coal beds, or to layers composed of a particular maceral type. These are usually uniform in strike and arranged in sub-parallel sets that have regional trends. Tectonism may obliterate previously formed cleats in anthracites, if this is so, then regular variations in cleat spacing with rank in the range lignite to high volatile bituminous coal might not occur. 

Faulting
The development of strong joint and fault patterns in coal bearing sequences is the commonest post-depositional structural expression; principal fault types are described briefly in the following paragraphs. Normal faults, these are produced by dominantly vertical stress resulting in the reduction of horizontal compression, leaving gravity as the active compression, which results in the horizontal extension of the rock sequence. This form of faulting is common, movements can be in the order of a few metres to hundreds of metres. The dip of normal faults ranges widely, in coalfields most are thought to be in the region of 60–70◦. Some normal faults die out along their length by a decrease of throw towards either one or both ends of the fault. Again a fault may pass into a mono-clinal flexure, particularly in overlying softer strata. Such faulting also produces drag along the fault plane, the country rock being pulled along in the direction of movement. Where large faults have moved on more than one occasion, and this applies to all kinds of faulting, a zone of crushed coal and rock may extend along the fault plane and have a width of several metres. Large-scale normal faults are produced by tensional forces pulling apart or spreading the crustal layer; where these faults run parallel, with the down faulted areas in between, they are known as graben structures. Many coalfields are preserved in such structures: the brown coalfields of northern Germany and eastern Europe, and the Gondwana coalfields of India and Bangladesh are examples. Low-angle faults with normal fault displacements are known as lag faults. They originate from retardation of the hanging wall during regional movement. Lag faults are common in the coalfield of South Wales, United Kingdom. Reverse faults are produced by horizontal stress with little vertical compression, which results in the shortening of the rock section in the direction of maximum compression. Very high-angle reverse faults are usually large structures, associated with regional uplift and accompanying igneous activity. In coal geology, those reverse faults with low angles <45◦ are more significant. When the angle is very low, and the lateral displacement is very pronounced, such faults are termed thrust faults. The shape of such low-angle reverse faults is controlled by the nature of the faulted rocks, especially when a thrust plane may prefer to follow the bedding plane rather than to cut across them. In typical sequences of coal, seatearth and mudstone with subordinate sandstone, such low-angle faults often follow the roof and/or the floor of coal seams as these allow ease of movement, the seatearths often acting as a lubricant. One detrimental effect is the contamination of the coal seam with surrounding country rock, thereby reducing its quality and, in some cases, its mine ability. In highly tectonized coal deposits, a great number of coal seam contacts have undergone some movement and shearing, in some cases the whole seam will have been compressed and moved. Thrusting is also accentuated where coal and mudstone sequences are sandwiched between thick sequences of coarse clastic rocks, the upper and lower portions of the sequence reacting to compressive forces quite differently to the incompetent coals and mudstones. Strike-slip faults have maximum and minimum stress in the two horizontal planes normal to one another. This has the effect of producing a horizontal movement either in a clockwise (dextral) or anticlockwise (sinistral) sense. Strike-slip faulting is usually found on a regional scale, and although important, has a lesser influence on the analysis of small coal deposits and mine lease areas. Evidence of faulting on the rock surface can be seen in the form of slicken sides, which are striations on the fault plane parallel to the sense of movement. Some fault planes have a polished appearance, particularly where high rank coal has been compressed along the fault plane. Conical shear surfaces are characteristically developed in coal, which are known as cone-in-cone structures, and are the result of compression between the top and bottom of the coal. Coal responds in a highly brittle manner to increasing deformation by undergoing failure and subsequent displacement along ever increasing numbers of fracture surfaces. 

Folding
Coals in coal-bearing sequences may be folded into any number of fold styles. In coalfield evaluation, the axial planes of the folds need to be located and the dips on the limbs of the folds calculated. In poorly exposed country the problem of both true and apparent dips being seen has to be carefully examined. Also in dissected terrain, dips taken at exposures on valley sides may not give a true reflection of the structural attitude of the beds at this locality, many valley sides are unstable areas and mass movement of strata is common, resulting in the recording of over-steepened dips. This is characteristic of areas of thick vegetation cover where a view of the valley side is obscured and any evidence of movement may be concealed. In underground operations, if the dip of the coal seams steepens, it can make the working of the coal difficult, and in the case of long wall mining, prevent further extraction. Therefore it is important to be sure that all readings taken reflect the true nature of the structure in the area of investigation. Compression of coal seams during folding can produce tight anticlinal folds with thrusting along the nose of the fold, these have been termed queue anticlines. Coal seams can be pinched out along the fold limbs and appear to have flowed into the axial areas of the anticlines. Where this occurs from two directions approximately normal to one another, coals can be concentrated in ‘pepperpot’ type structures. Such features are usually found only in highly tectonized coal fields and examples of such intense deformation.

Igneous associationsIn many coal fields associated igneous activity has resulted in dykes and sills being intruded into the coal-bearing sequence. The intrusion of hot molten rock into the coals produces a cindering of the coal and a marked loss in volatile matter content which has been driven off by heat. This can have the effect of locally raising the rank of lower rank coals, and can therefore in certain circumstances make the coal attractive for exploitation. Such ‘amelioration’ of coal seams is a common feature in areas of igneous activity, and good examples are found in Indonesia and the Philippines where Paleogene–Neogene sub-bituminous coals have been ameliorated up to low volatile bituminous and some even to anthracite rank. The majority of dykes and sills are doleritic in composition, as in the case of South African and Indian coal fields, but occasionally other types are found. Igneous sills have a tendency to jump from one coal seam to another so that close spaced drilling is often required to identify precisely the nature and position of such intrusions. Igneous intrusions are found in coal sequences worldwide, but in particular are a common feature of South African coal workings. Where such igneous bodies exist, the coal geologist must identify the areas occupied by igneous material within the mine area, and also those seams affected by igneous activity. In addition, the possibility of methane gas driven off during intrusion may have collected in intervening or overlying porous sandstones. Mine operatives need to investigate this possibility when entering an intruded area of coal.

Mineral precipitatesA common feature of coal-bearing sequences is the formation of ironstone, either as bands or as nodules. They usually consist of siderite (FeCO3) and can be extremely hard. Where iron stone nucleation and development takes place either in, or in close proximity to a coal seam, this can deform the coal, cause mining difficulties, and, because of the difficulty in separating coal and ironstone when mining, will have an effect on the quality of the run-of-mine product. Iron sulphide (FeS) in the form of iron pyrite may be precipitated as disseminated particles, as thin bands, or as is more common, as coatings on cleat and bedding surfaces. Inorganic sulphur held in this form in coal can be removed by crushing and passing the coal through a heavy liquid medium. Organic sulphur held elsewhere in the coal cannot be readily removed, and remains an inherent constituent of the coal. Other mineral precipitates usually are in the form of carbonates, coating cleat surfaces, or occasionally as mineral veins. Where quartz veining occurs, this has the detrimental effects of being hard, liable to produce sparks in an underground environment where gas is a hazard and also when crushed is an industrial respiratory health hazard.

Geologic Principles for Defining Relative Age

Building from the work of Steno, Hutton, and others, the British geologist Charles Lyell (1797–1875) laid out a set of formal, usable geologic principles. These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages.

Uniformitarianism: The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past.

Original horizontality: The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief (such as floodplains or the sea floor) in a gravitational field. If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified. With this principle in mind, geologists conclude that examples of folds and tilted beds represent the consequences of deformation after deposition.

Law of Superposition: The principle of superposition states that in a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect. Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest. 

Lateral continuity: The principle of lateral continuity states that sediments generally accumulate in continuous sheets within a given region. If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon. 

Cross cutting relations: The principle of cross-cutting relations states that if one geologic feature cuts across another, the feature that has been cut is older. For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike. If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers. But if a layer of sediment buries a fault, the sediment must be younger than the fault. 

 

Principle of baked contacts: The principle of baked contacts states that an igneous intrusion “bakes” (metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusion.

Principle of inclusions: The principle of inclusions states that a rock containing an inclusion (fragment of another rock) must be younger than the inclusion. For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone. 

Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location. They then go further by interpreting the formation of each feature to be the consequence of a specific geologic event.

Examples of geologic events include: Deposition of sedimentary beds; erosion of the land surface; intrusion or extrusion of igneous rocks; deformation (folding and/or faulting); and episodes of metamorphism. The succession of events in order of relative age that have produced the rock, structure, and landscape of a region is called the geologic history of the region. We can use these principles to determine relative ages of the features. We develop a geologic history of the region, defining the relative ages of events that took place there.

Fossil Succession
As Britain entered the industrial revolution in the late 18th and early 19th centuries, new factories demanded coal to fire their steam engines and needed an inexpensive means to transport goods. Investors decided to construct a network of canals to transport coal and iron, and hired an engineer named William Smith (1769–1839) to survey some of the excavations. Canal digging provided fresh exposures of bedrock, which previously had been covered by vegetation. Smith learned to recognize distinctive layers of sedimentary rock and to identify the fossil assemblage (the group of fossil species) that they contained. He also realized that a particular assemblage can be found only in a limited interval of strata, and not above or below this interval. Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever. Smith’s observation has been repeated at millions of locations around the world, and has been codified as the principle of fossil succession. It provides the geologic underpinning for the theory of evolution.

Example: Bed 1 at the base contains fossil species A, Bed 2 contains fossil species A and B, Bed 3 contains B and C, Bed 4 contains C, and so on. From these data, we can define the range of specific fossils in the sequence, meaning the interval in the sequence in which the fossils occur. The sequence contains a definable succession of fossils (A, B, C, D, E, F), that the range in which a particular species occurs may overlap with the range of other species, and that once a species vanishes, it does not reappear higher in the sequence. Once the relative ages of a number of fossils have been determined, the fossils can be used to determine the relative age of the beds containing them. For example, if a bed contains Fossil F (from the succession specified above), geologists can say the bed is older than a bed containing Fossil A, even if the two beds do not crop out in the same area. As we will see, painstaking work over many years eventually allowed geologists to assign numerical age ranges to fossil species. Of note, some fossil species are widespread, but survived only for a relatively short interval of geologic time. Such species are called index fossils (or guide fossils), because they can be used by geologists to associate the strata with the specific time interval.

How does sedimentary environments effects on rock geomechanical behaviours?

Sedimentary environments are important for investigating rock physical, geological, and geomechanical behaviours. Different from other mineral resources, coal-bearing formations mainly formed in different ancient coal accumulation environments. Its characters are controlled by the ancient geologic environment and its transition when the peat was piled up. Because of different sedimentary environments and sedimentary features, the thicknesses of the coal seam roof and floor formed under different environments changes greatly in both vertical and lateral directions. These lead to heterogeneity and discontinuity of coal-bearing formations. During mining process of the coal seams, roof stability becomes worse in these weak formations. Roof caving, bottom heaving, and rock burst accidents often occur in the transition zone between the sandstone and mudstone of the roof. During mine development and production, in order to meet the requirements of transportation and ventilation, a safe, stable, and complete shaft and tunnel network system need to be established. However, because the lithologies of the shaft and tunnel change significantly in the lateral direction, soft or weak formations can be encountered. In this case, shaft and tunnel construction and maintenance become difficult. In the past, mine designers often considered that the rocks distribute constantly in both thickness and lithology in the lateral direction. This design was not always true and led to the failure of the original design. Therefore, it is necessary to study rock sedimentary environments, to understand rock lithology distribution in the studied area, and to make the appropriate design according to geological conditions. Formations making up the Earth's crust are described by the term, facies. Sedimentary facies is the most interested facies with regard to fluid flow. Broadly divided into sandy facies, shaly facies, and carbonate facies, sedimentary facies is related to the environment in which their sediments are deposited. In general, sedimentary environments include alluvial fan sediments, fluvial deposits, delta deposits, lake deposits, barrier island deposits, and lagoon deposits.

Alluvial fan sediments
Alluvial fan sediments are deposits of sediments in regions of high relief, generally where streams issue from mountains onto a level plain. The fan starts at the apex, the source of sediments from regions of higher relief. Sediment transport from the apex tends to follow the steepest slope downward, and the sediments, therefore, spread out in a fan. The largest boulders or pebbles are deposited near the apex. Downslope, the fan channel splits up into a number of smaller channels. This reduces the velocity of the water flow, and capacity for carrying sediment is lowered. Therefore, sediments become finer-grained down-slope, even if there is no reduction in steepness of the slope. Further downslope it becomes braided streams and lake deposit. In front of the apex the large faulting sedimentary basin usually are formed. This phenomenon is quite common in the Mesozoic coal-bearing strata in the eastern China.  
 Near the apex the sediments are primarily consisted of conglomerates, and their main components are coarse gravels and boulders. The gravels are filled with clay, silt, and sand. The conglomerate layer distributes in strips and is parallel to the direction of the water flow. Downslope to the middle fan, the sedimentary rocks are mainly comprised of sandstone with some gravels. In the fan tail the sediments are much finer. The sedimentary rocks are consisted of sandstones, siltstones, claystones, and coal seams.
The main geomechanical features of the alluvial fan sediments are as follows:

• The rocks are blocky with great thickness and high strength. 
• The sediment lithology is complex with coarse granularity, and the rocks are easily weathered. 
• On the end of an alluvial fan, the coarse rocks have high porosity and it may be a good aquifer.

River sediments

Fluvial deposits are sediments deposited as the result of rivers. Fluvial sediments include deposits of braided streams, meandering rivers, and anastomosing streams. Braided streams have branched channels because the river channel is not very stable. This usually occurs with steeper stream gradients and an abundant supply of sediments. Braided streams favour the deposition of coarse sediments containing coarse sand and gravel with little clay and silt. Meandering rivers move in loops, with the greatest velocity at the outer bank where erosion occurs and lower velocity at the inner bank where deposition occurs. The fining upward sequence from sand to silt to clay is typical of meandering river facies. This sequence is the result of the water velocity decreasing as the river, over a given spot, migrates from the outer bank to the inner bank. If the spot is no longer in the river but in the flood plain, then only clay and silt from nearly stagnant water are deposited. An anastomosing stream is defined as a branching, interlacing stream having a net like appearance. Along the U.S. Gulf Coast is a typical example of bayous and slough in regions of very low stream gradient and with subsidence. Meandering river sediments vary from coarse to fine grains upwards. During the basin deposit process, thick sediments may form due to the continuous development of the meandering rivers. In the meandering river sedimentary system, channel sands act as the skeleton in the rock mass. Generally, it forms laterally many strips of sandstones surrounded by the flood basin deposits. In the lateral direction some layers in the strata were thickened, thinned, or even disappeared. Vertically, the sandstones array or overlap each other in lens shape, and lithology varied cyclically. Therefore, in the meandering river deposit system, fine-grained sediments always surround sand sediments. These made rock mechanical properties anisotropic and heterogeneous.    
The rocks in fluvial deposits are sandstone, siltstones, shales, mudstones, and claystones and have the following geomechanical behaviours:

• The rocks are stratified layers and inter-bedded and alternated with soft and hard layers. 
• The sandstones are weak in weathering resistance, and the strength changes gradually from the bottom to the top. 
• The strength of the sandstone in which the sandstone becomes thinner and near the dead-end is the lowest. 

Delta sediments

Deltas form where rivers carrying a large supply of sediments empty into a sea coast where the sediments cannot be transported away as fast as it is deposited. Thus, deltas lie in the transit region between the fluvial and marine environments. The Mississippi River Delta is a good example.    
The rocks in delta deposits have the following geomechanical behaviours:

• The rocks are stratified with significant variations of lithology and thickness in the lateral direction. 
• The rocks are weak in weathering resistance, and the rock strength increases from the bottom to the top as the granularity increases. 

Lake sediments 

Lake sediments are deposited in a lake accumulated on the lake shore and on lake floor. They are deposited in a terrestrial environment and contain organic and inorganic particles, microfossils such as pollen and algae, and macro fossils such as leaves and seeds. Deposit speed in lake environment is faster than that in marine environment, because of a smaller wave in the lake. Lake shore deposits are generally well-sorted sands. The sediments load of a stream entering a lake will be dropped as the stream’s velocity and transporting ability suddenly decrease. The resulting deposit, which extends outward into the lake, is a delta. Inclined, generally well-sorted layers on the front of a delta pass downward and outward into thinner, finer, evenly laminated layers on the lake floor. Most lake sediments are layered, in which the layers/strata are defined by colour variations. In the deeper parts of the lake, the sedimentary layers are very thin, and deeper-water sediments are fine-grained while those in shallow water are coarse.
The strata in lake deposits have the following geomechanical behaviours:

• The rocks have alternately soft and hard layers deposited. Periodic changes of the lake level generate cyclical soft and hard strata.  
• The rock layers are continuous with little change in thickness and have low strength. 
• Most strata are impermeable layers. 

Barrier island sediments 

Barrier islands or spits are long, narrow, offshore deposits of sand or sediments that parallel the coast line. The islands are separated from the main land by a shallow sound, bay or lagoon. Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets. The rising waters carried sediments from those beach ridges and deposited them along shallow areas just off the new coast lines. Waves and currents continued to bring in sediments that built up, forming the barrier islands. In addition, rivers washed sediments from the mainland that settled behind the islands and helped build them up. The sediments of barrier islands are well-sorted, generally medium to fine grained sandstones with silicate cementation. These sandstones have very high strength. Some strata of barrier island sediments can extend for 160 km or more. They are very hard strata. If a coal seam roof is this kind of strata for long wall caving mining, it is very easy to form a large area of un-caving strata; therefore it is likely to have rock burst.

Lagoon and tidal lagoon sediments 

Lagoon, tidal lagoon, and barrier islands or spits are sedimentary elements that parallel to the coast line. Lagoon environment belongs to a shallow basin that are separated from the ocean by barrier islands or barrier spits and jointed with the ocean through tidal inlets. In the places where tide develops, a lagoon is a shallow depression full of water even in the period of low tide. If there is sufficient sediment supply, a coastal lagoon can gradually develop into a tidal lagoon or swamp. Therefore, lagoon deposit is closely associated with tidal lagoon and swamp deposits. They transit vertically and are contiguous horizontally.  Lagoon sediments generally are laminated fine-grained sediments, such as clay and silt. In humid and semi-humid regions where coal measures form, these fine sediments often are rich in organic substances. Tidal lagoon is a wide and flat region around the lagoon and depends on the difference of the low and high tides and the ground slope. Near low tide line in the intertidal zone, due to the strong hydraulic activity, flat sand deposits can be formed and developed to be large slaty or sphenoid cross-beddings. Near high tide line, the sediments are mostly mud and silt with horizontal lamination and current lamination.   Sediments from the lagoon are uniform in mineralogical and mechanical composition. The geomechanical features of the sediments in this sedimentary mode are as follows: 

• The rocks have alternately soft and hard layers deposited. Weak interfaces exist between hard and soft layers. 
• The rock layers are continuous with little change in thickness and have low strength.  
• The formations usually contain clay minerals, which are most likely to swell and weaken, particularly when they are exposed to water.     

Analyses of sedimentary environments are important and applicable to rock engineering. Through investigation of sedimentary environments of the rocks one can understand rock structure, lithologic characteristics, and strata sedimentary sequences. This investigation is beneficial to rock engineering design and construction.

How can Paleocurrents direction be interpretted?

A paleocurrent is just what the term implies: a current, of water or wind, that existed at some time in the past.

Much effort has gone into developing ways of figuring out paths of dispersal of sedimentary material in basins. One of the standard ways is to measure paleocurrent directions recorded locally in the rocks.  Knowledge of paleocurrents is helpful in solving both local and regional problems of sedimentary basins. Locally, paleocurrent directions can help you to figure out or predict, indirectly, the shape and orientation of sediment bodies, like channel sandstones. This has obvious advantages in petroleum exploration. Regionally, paleocurrent directions can help establish paleoslope and source of sediment supply to the basin.

Cross stratification: Measure the local orientation of laminae in the cross sets, on the theory that the local down dip direction, which presumably is the direction of progradation of the foreset slope, is likely to represent fairly closely the local current direction. That's true, however, only if the bed forms were reasonably two dimensional. If the bed forms were three dimensional, resulting in trough cross stratification, measurement of foreset dip directions at local places in the cross sets can be very misleading; it's much better to try to ascertain the orientation of the trough fills themselves, although it takes good outcrops to do that. Seeing rib and furrow is by far the most reliable way of obtaining a paleocurrent direction from cross-stratified deposits, but unfortunately it’s uncommon to see on outcrop. 

Bed forms: If you are lucky enough to see bedding planes covered with symmetrical ripples or dunes, you can get an excellent measurement of current direction. 

Clast orientation: Long axes of the larger clasts in a clastic deposit, whether gravel or sand, are commonly oriented by the current, although the orientation may be rather subtle. The problem is that the orientation relative to the current depends on the flow itself in ways not well understood. So beware of clast orientation in and of itself. Pebble imbrication is an exception, and should always be sought in gravels and conglomerates.  

Sole marks: Flutes and grooves at the bases of turbidites and other strong current event beds give excellent evidence of the direction of the initial, eroding current. But keep in mind that the later current that did the depositing did not necessarily flow in exactly the same direction.

Parting lineation: Parting lineation is thought to reflect a subtle anisotropy in rock strength caused by a statistical tendency toward alignment of sand grains in a sandstone parallel to the current direction. It gives excellent evidence of the orientation of the current, but unfortunately not the direction.

Classification of sedimentary basins

Sedimentary basins are classified by tectonic (and, specifically, plate-tectonic) setting. That's fairly easy to do for modern basins, but it's rather difficult to do for ancient basins. List of some of the important criteria that could be used, ranging from more descriptive at the top of the list to more genetic at the bottom of the list:

• nature of fill
• geometry paleogeography
• tectonic setting 

Intracratonic Basins 

Location and tectonic setting: in anorogenic areas on cratons.

Tectonic and sedimentary processes: These basins have no apparent connection with plate tectonics. They are thought to reflect very slow thermal subsidence (for times of the order of a hundred million years) after a heating event under the continental lithosphere. But the reasons for depression below the original crustal level are not understood. Erosion during the thermal uplift seems untenable, as does lithospheric stretching. 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. These successions are also more complete, however, there are fewer and smaller diastems (A diastem is a brief interruption in sedimentation, with little or no erosion before sedimentation resumes), so at times the basin must have remained under water while surrounding areas were emergent.

Size, shape: rounded, equidimensional, hundreds of kilometres across.

Sediment fill: shallow-water cratonal sediments (carbonates, shales, sandstones), thicker and more complete than in adjacent areas of the craton but still relatively thin, hundreds of meters.

Aulacogens 

Location and tectonic setting: extending from the margins toward the interiors of cratons.

Tectonic and sedimentary processes: Aulacogens are thought to represent the third, failed arm of a three-armed rift, two of whose arms continued to open to form an ocean basin. In modern settings, aulacogens end at the passive continental margin. An example is the Benue Trough, underlying the Congo River Basin in West Africa. The ocean eventually closes to form an orogenic belt, so in ancient settings aulacogens end at an orogenic belt; an example is the basin filled by the Pahrump Group (Proterozoic) in Nevada and California.

Size, shape: long, narrow, linear; tens of kilometres wide, many hundreds of kilometres long

Sediment fill: very thick (up to several thousand meters); coarse to fine siliciclastics, mostly coarse, minor carbonates; mostly non-marine, some marine; contemporaneous folding and faulting; the succession often passes upward, with or without major unconformity, into thinner and more widespread shallow-marine cratonal siliciclastics and carbonates.

Rift Basins

Location and tectonic setting: Within continental lithosphere on cratons.

Tectonic and sedimentary processes: Lithospheric extension on a craton, presumably by regional sub-lithospheric heating, causes major rifts. Some such rifts continue to open and eventually become ocean basins floored by oceanic rather than continental crust; the basin description here then applies to this earliest stage of the rifting. In other cases, the rifts fail to open fully into ocean basins (perhaps some adjacent and parallel rift becomes the master rift), so they remain floored by thinned continental crust rather than new oceanic crust. A modern example: the East African rift valleys. An ancient example: the Triassic–Jurassic Connecticut and Newark basins in eastern North America. Sediment supply from the adjacent highlands of the uplifted fault blocks is usually abundant, although in the East African rifts the land slope is away from the rim of the highlands, and surprisingly little sediment reaches the rift basins.

Size, shape: long, narrow, linear; tens of kilometres wide, up to a few thousand kilometres long

Sediment fill: Coarse to fine siliciclastics, usually non-marine; often lacustrine sediments; inter-bedded basalts.

Oceanic Rift Basins 

Location and tectonic setting: In a narrow and newly opening ocean.

Processes: This category of basins is transitional between intracontinental rift basins and passive-margin basins. Basins have opened wide enough to begin to be floored with oceanic crust but are still so narrow that the environment is either still non-marine or, if marine, has restricted circulation. Modern examples are the Red Sea and the Gulf of Aden. In the ancient, the sediment fill of such a basin is likely to underlie passive-margin sediments deposited later in the history of ocean opening.

Size, shape: long, narrow; straight or piecewise straight; tens to a few hundreds of kilometres wide, up to a few thousand kilometres long.

Sediment fill: mafic, volcanics and coarse to fine non-marine siliciclastics, as in intracratonic rift basins described above, passing upward and laterally into evaporites, lacustrine deposits, and fine marine sediments, often metal-rich from hydrothermal activity at the spreading ridge.

Passive Margin Basins 

Location and tectonic setting: Along passive continental margins, approximately over the transition from continental to oceanic crust formed by rifting and opening of a full-scale ocean basin

Processes: As an ocean basin opens by spreading, the zone of heating and extensional thinning of continental crust on either side of the ocean basin subsides slowly by cooling. Sediments, either siliciclastics derived from land or carbonates generated in place, cover this subsiding transition from continental crust to oceanic crust with a wedge of sediment to build what we see today as the continental shelf and slope. In the context of the ancient, this represents the miogeocline. The subsidence is accentuated by loading of the deposited sediments, resulting in a prominent down bowing of the continental margin. Deposition itself therefore does not take place in a basinal geometry, but the base of the deposit is distinctively concave upward.

Size, shape: Straight to piecewise straight, often with considerable irregularity in detail; a few hundreds of kilometres wide, thousands of kilometres long.

Sediment fill: Overlying and overlapping the earlier deposits laid down earlier during rifting and initial opening are extensive 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 inter-fingering seaward into deep marine coarse and fine siliciclastics or re-sedimented 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 

Location and tectonic setting: In the abyssal ocean, at the line of initial down bending of the subducted oceanic-crust plate in a subduction zone.

Tectonic and sedimentary processes: (1) Open-ocean pelagic sediments (mainly abyssal brown clay and organic oozes) are conveyor-belted to the trench, and underlie the sediments, thin to thick, deposited in the trench itself. (2) What happens to the sediments delivered to or deposited in the trench? While still within the trench they are little deformed, but they don't stay that way long. They are either scraped off the descending plate to form an accretionary wedge, whose structure ranges from chaotically mixed material in a subduction mélange to a fairly regular imbricated succession of under-thrusted sheets dipping toward the arc (the thrust sheets themselves get younger downward, but within a given plate the sediments get younger upward), or they are dragged down the subduction zone. 

Size, shape: long and narrow (tens of kilometres wide, thousands of kilometres long), arcuate, with convex side toward the oncoming subducted plate.

Sediment fill: varies from thin (hundreds of meters) pelagic sediments (fine abyssal muds, volcanic ash) to thick (thousands of meters) arc-derived coarse siliciclastics and volcaniclastics, as local fans built perpendicular to the trench axis or oblong fan like bodies built parallel to trench axis.

 

Trench-Slope Basins 

Location and tectonic setting: On the inner (arcward) wall of subduction-zone trenches.

Tectonic and sedimentary processes: The basins are formed as low areas, with closed contours, between adjacent thrust sheets in the growing accretionary wedge. Near-surface folding of sediment in the accretionary wedge may also be a factor in the development of the basins. These basins intercept some of the sediment carried as turbidity currents from upraised older parts of the accretionary complex, or from the more distant arc itself.

Size, shape: Small (no larger than kilometres wide, tens of kilometres long, often smaller); linear, and elongated parallel to the trench.

Sediment fill: deep-marine silts and muds sedimented directly into the basins or slumped into the basins from higher on the slope; also coarser siliciclastics supplied from farther upslope by turbidity currents.

 

Fore-Arc Basins 

Location and tectonic setting: In subduction zones; between the upraised subduction complex just inboard of the trench and the volcanic arc (in the case of ocean-ocean subduction) or the overriding continent (in the case of ocean–continent subduction).

Processes: As subduction proceeds, a relatively low area, usually below sea level, is formed between the relatively high outer arc upraised by subduction and the inner volcanic arc built by subduction magmatism. Ancient examples of such fore-arc basins are likely to be tectonically isolated from the originally adjacent areas. After an arc-continent collision, another variant of fore-arc basin can be formed between the outer arc and the overriding continent. These basins are likely to be filled mainly from the high land of the tectonically active continent. Later continent-continent collision would make direct reconstruction of their tectonic setting difficult.

Size, shape: tens of kilometres to over one hundred kilometres wide, up to thousands of kilometres long; commonly arcuate.

Sediment fill: non-marine, siliciclastic, fluvial to deltaic deposits at the arcward margin pass seaward into deep marine siliciclastics, mainly sediment-gravity flow deposits, all inter-bedded with arc derived volcanics flows and pyroclastics. Section thickness can be many thousands of meters.

Foreland Basins 

Location, tectonic setting, processes: There are two kinds of foreland basins: retro-arc foreland basins, which are formed on stable continental crust by loading by thrust sheets moving toward the continental interior as a result of compression and crustal shortening in an ocean-continent subduction zone, and peripheral foreland basins, formed after continent-continent collisions by loading of the continental crust of the subducted plate by development of thrust sheets in the continental crust of the subducted plate directed back away from the subduction zone. Both kinds tend to be asymmetrical, with their deepest parts nearest the emplaced thrust sheets. They tend to migrate away from the arc or suture zone with time. They are filled by sediments derived from the mountainous terrain associated with the compression and thrusting.

Size, shape: tens to a few hundreds of kilometres wide, hundreds to thousands of kilometres long; often with varying development along their length; commonly arcuate or piecewise arcuate, reflecting the geometry of subduction.

Sediment fill: Coarse, fluvial, siliciclastics, mainly as alluvial fans, thinning and fining away from the arc or suture, often passing into shallow-marine sandstone-shale successions if sea level is high enough to flood the basin. Thickness are up to many thousands of meters. The classic molasse facies, thick non-marine conglomerates, is deposited in foreland basins.

 

Remnant Basins 

Location and tectonic setting: within suture zones formed by continent-continent collision.

Processes: Continental margins and subduction zones are (for different reasons, connected with geometry of rifting and geometry of subduction) commonly irregular in plan rather than straight, so when continent-continent collision eventually comes to pass, certain salient of continental crust encounter the subduction zone before re-entrants. With further subduction and suturing, this creates isolated basins still floored by residual oceanic crust, which receive abundant sediment from adjacent strongly uplifted crust.

Size, shape: many tens to hundreds of kilometres across; irregular in shape.

Sediment fill: very thick and highly varied, with strong lateral facies changes; usually fluvial at the margins, commonly passing into deep-marine sediment gravity-flow deposits; sometimes the basin becomes sealed off from the ocean, so that facies include lacustrine sediments.

 

Pull-Apart Basins 

Location and tectonic setting: Locally along major strike-slip or transform faults, either in continental crust or in oceanic crust.

Processes: If a strike-slip fault is stepped or curved rather than straight, movement along it tends to produce tension, where the sense of the curvature and movement are such that the walls of the fault are pulled apart from one another (this kind of regime is described as transtensile), or compression, where the sense of the curvature and movement are such that the walls are pushed against one another (this kind of regime is described as transpressive). In the tensional segments, gaps or basins are produced which are filled with sediment from adjacent high crust.

Size, shape: There is a strong tendency for pull-apart basins to be rhomboidal. They range from approximately equidimensional early in their history to elongated later. Widths are kilometres to a few tens of kilometres, and lengths are kilometres to many tens of kilometres. Some basins are even smaller than this.

Sediment fill: The continental crust basins, which are the most significant sedimentologically, are filled by thick non-marine 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. There is usually sharp variation in facies laterally, and the thickness of the lithologic units may be not much greater than the lateral extent, or even less. Deposition is concurrent with elongation of the basin, so be wary of total section thickness computed by bed-by-bed measurements of the section.

How are sedimentary basins formed?

Sedimentary basin: A low area in the Earth’s crust, of tectonic origin, in which sediments accumulate. Sedimentary basins range in size from as small as hundreds of meters to large parts of ocean basins. The essential element of the concept is tectonic creation of relief, to provide both a source of sediment and a relatively low place for the deposition of that sediment. Tectonics is the most important control on sedimentation; climate is a rather distant second. The important effects of tectonics on sedimentation, direct or indirect, include the following:

•  nature of sediment
• rate of sediment supply 
• rate of deposition 
• depositional environment 
• nature of source rocks 
• nature of vertical succession

The only basins that are preserved in their entirety are those that lie entirely in the subsurface! Basins exposed at the surface are undergoing destruction and loss of record by erosion. So there’s an ironic trade-off between having more complete preservation in the subsurface but less satisfactory observations. How do you gather data on sedimentary basins?.

Master cross sections: With the present land surface as the most natural datum, construct several detailed physical cross sections through the basin to show its geometry and sediment fill.

Stratigraphic sections: Construct a graph, with time along the vertical axis, showing the time correlations of all the major rock units along some generalized traverse across the basin. Such a section includes hiatuses, during which there was non deposition or erosion.

Isopach maps: With some distinctive stratigraphic horizon near the top of the section as datum, draw a contour map showing isopachs (isopachs are loci of equal total sediment thickness) in the basin.

Lithofacies maps: For one or a series of times, draw a map showing distribution of sediment types being deposited at that time.

Ratio maps: Compute things like sand/shale ratio, integrated over the entire section or restricted to some time interval, and plot a contour map of the values.

Paleocurrent maps: For one or a series of times, draw a map showing the direction of paleo-currents in the basin at that time.

Grain-size maps: For the entire basin fill, averaged vertically, or for some stratigraphic interval or time interval, draw a map that shows the areal distribution of sediment grain size. This is especially useful for conglomeratic basins.

In one sense, the origin of sedimentary basins boils down to the question of how relief on the Earth is created. Basically, there are only a few ways, described in the following sections.

Local

On a small scale, hundreds to thousands of meters laterally, fault movements can create relief of hundreds to thousands of meters, resulting in small but often deep basins (some of these are called intermontane basins). Along strike-slip faults. can produce small pull-apart basins; more on them later. Relief of this kind is on such a small scale that it tends not to be isostatically compensated. It’s like setting a block of granite out on your driveway; the flexural rigidity of your driveway is great enough compared with the imposed load that the granite block is prevented from finding its buoyant equilibrium position.

Regional

Basin relief can be created mechanically on a regional scale in two very important ways: thermally or flexurally, or by a combination of those two effects). Each of these is discussed briefly below. Keep in mind that basins can also be made just by making mountain ranges, on land or in the ocean, by volcanism. 

Thermal

If the lithosphere is heated from below, it expands slightly and thus becomes less dense. This less dense lithosphere adjusts isostatically to float higher in the asthenosphere, producing what we see at the Earth’s surface as crustal uplift. If the lithosphere cools back to its original temperature, there’s isostatic subsidence back to the original level.

 But suppose that some erosion took place while the crust was elevated. The crust is thinned where the erosion took place (and thickened somewhere else, where there was deposition; that might be far away, at the mouth of some long river system), so when the crust cools again it subsides to a position lower than where it started, thus creating a basin available for filling by sediments.

 But the magnitude of crustal lowering by this mechanism is less than is often observed in basins thought to be created thermally. It has therefore been proposed, and widely accepted, that in many cases extensional thinning of the lithosphere accompanies the heating. Then, upon re-cooling, the elevation of the top of the lithosphere is less than before the heating and extension. This kind of subsidence has been invoked to explain many sedimentary basins.        

                   

Flexural

Another important way to make basins is to park a large load on some area of the lithosphere. The new load causes that lithosphere to subside by isostatic adjustment. But because the lithosphere has considerable flexural rigidity, adjacent lithosphere is bowed down also. The region between the high-standing load and the lithosphere in the far field (in the parlance of geophysics, that just means far away!) is thus depressed to form a basin. This model has been very successful in accounting for the features of foreland basins, which are formed ahead of large thrust sheets that move out from orogenic areas onto previously undeformed cratonal lithosphere.

How to construct a cross section using the Kink method?

Kink method: The basic method is to allow each dip measurement to define a zone where the dip is constant. The boundaries of the dip zones are the lines that bisect angles between adjacent dips. The example below begins with three different ways to find the bisector.

We first find the line L12 that bisects the angle between dips 1 and 2. 

One way is to construct lines perpendicular to dips 1 and 2 (in red). They intersect at C12. Bisect the angle formed by the two perpendiculars (shown in green).

Extend the dips out to L12. When you cross the line, continue with the dip on the opposite side. That is, use dip angle 1 on the left side of line L12 and dip angle 2 on the right.

Here's another way: halfway between datum points 2 and 3, draw lines parallel to dips 2 and 3 (shown in red). Bisect the angle to find line L23.

Extend dips 2 and 3 to L23, then continue on the opposite side of the line with the appropriate dip. For example, extend dip 2, then where it intersects L23, continue with dip 3.

We can also extend the line that originates at datum point 1. Between L12 and L23 it has dip 2, then on the right of L23 it has dip 3.

Yet another approach is to average two adjacent dips. The desired line passes through the midway point between the two datum points. Draw a vertical line through that point and measure the average angle from the vertical line. Pay careful attention to dip direction so that the line has the proper slope.

Lines left of L34 have dip 3, to the right they have dip 4. Continue the lines that originated at datum points 1 and 2 by extending them through L34 with the appropriate dips.

L45 constructed. Lines are extended to L34 and the dip beyond is indicated. Dips 4 and 5 are very close to already existing lines and are unlikely to give any new information, so we don't need to construct lines through dips 4 and 5 for now.

L56 constructed. The rocks at datum point 6 are stratigraphically lower than any we have so far seen, so we extend dip 6 to line L56, then continue to the left parallel to the previously drawn lines.

L67 constructed. The rocks at datum point 7 are even lower than those at datum point 6, so we also extend dip 7 to line L67, then continue to the left parallel to the previously drawn lines.

L78 constructed. The rocks at datum point 8 are still lower , so we also extend dip 8 to line L78, then continue to the left parallel to the previously drawn lines.

L89 constructed. Datum point 9 lies between two already constructed lines and is not likely to tell us anything we don't already know. For now we omit drawing a line through it.

L910 completed. Since point 10 falls close to an already drawn line, there is no real need to construct another line for it, either.

Here we have indicated the stratigraphy. It is virtually certain  when you draw a cross section using strictly geometric methods that the contacts will not match exactly with their predicted positions.

What we need to do now is redraw the folds so the cross-section matches both the dips and the stratigraphy.

Here the cross section lines are subdued. 

Most of the time you can modify the fold shapes by hand to match the stratigraphy without too much trouble. Modified contacts are in black.