Cross-bedding
In geology, cross-bedding known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are horizontal units composed of inclined layers; the original depositional layering is tilted, such tilting not being the result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers. Cross-bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunes. Examples of these bedforms are ripples, anti-dunes, sand waves, hummocks and delta slopes. Environments in which water movement is fast enough and deep enough to develop large-scale bed forms fall into three natural groupings: rivers, tide-dominated coastal and marine settings. Cross-beds can tell geologists much about; the direction the beds are dipping indicates paleocurrent, the rough direction of sediment transport. The type and condition of sediments can tell geologists the type of environment. Studying modern analogs allows geologists to draw conclusions about ancient environments.
Paleocurrent can be determined by seeing a cross-section of a set of cross-beds. However, to get a true reading, the axis of the beds must be visible, it is difficult to distinguish between the cross-beds of a dune and the cross-beds of an antidune. The direction of motion of the cross-beds can show ancient wind directions; the foresets are deposited at the angle of repose, so geologists are able to measure dip direction of the cross-bedded sediments and calculate the paleoflow direction. However, most cross-beds are not tabular, they are troughs. Since troughs can give a 180 degree variation of the dip of foresets, false paleocurrents can be taken by blindly measuring foresets. In this case, true paleocurrent direction is determined by the axis of the trough. Paleocurrent direction is important in reconstructing past climate and drainage patterns: sand dunes preserve the prevalent wind directions, current ripples show the direction rivers were moving. Cross-bedding is formed by the downstream migration of bedforms such as ripples or dunes in a flowing fluid.
The fluid flow causes sand grains to saltate up the upstream side of the bedform and collect at the peak until the angle of repose is reached. At this point, the crest of granular material has grown too large and will be overcome by the force of the depositing fluid, falling down the downstream side of the dune. Repeated avalanches will form the sedimentary structure known as cross-bedding, with the structure dipping in the direction of the paleocurrent; the sediment that goes on to form cross-stratification is sorted before and during deposition on the "lee" side of the dune, allowing cross-strata to be recognized in rocks and sediment deposits. The angle and direction of cross-beds are fairly consistent. Individual cross-beds can range in thickness from just a few tens of centimeters, up to hundreds of feet or more depending upon the depositional environment and the size of the bedform. Cross-bedding can form in any environment, it is most common in stream deposits, tidal areas, in aeolian dunes.
Cross-bedded sediments are recognized in the field by the many layers of "foresets", which are the series of layers that form on the lee side of the bedform. These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities. Cross-bedding can be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a flood, new bedforms are deposited in the scoured area. Cross-bedding can be subdivided according to the geometry of the sets and cross-strata into subcategories; the most described types are tabular cross-bedding and trough cross-bedding. Tabular cross-bedding, or planar bedding consists of cross-bedded units that are extensive horizontally relative to the set thickness and that have planar bounding surfaces. Trough cross-bedding, on the other hand, consists of cross-bedded units in which the bounding surfaces are curved, hence limited in horizontal extent. Tabular cross-beds consist of cross-bedded units that are large in horizontal extent relative to set thickness and that have planar bounding surfaces.
The foreset laminae of tabular cross-beds are curved so as to become tangential to the basal surface. Tabular cross-bedding is formed by migration of large-scale, straight-crested ripples and dunes, it forms during lower-flow regimes. Individual beds range in thickness from a few tens of centimeters to a meter or more, but bed thickness down to 10 centimeters has been observed. Where the set height is less than 6 centimeters and the cross-stratification layers are only a few millimeters thick, the term cross-lamination is used, rather than cross-bedding. Cross-bed sets occur in granular sediments sandstone, indicate that sediments were deposited as ripples or dunes, which advanced due to a water or air current. Cross-beds are layers of sediment that are inclined relative to the base and top of the bed they are associated with. Cross-beds can tell modern geologists many things about ancient environments such as- depositional environment, the direction of sediment transport and environmental conditions at the time of depos
Principle of original horizontality
The Principle of Original Horizontality states that layers of sediment are deposited horizontally under the action of gravity. It is a relative dating technique; the principle is important to the analysis of folded and tilted strata. It was first proposed by the Danish geological pioneer Nicholas Steno. From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics; as one of Steno's Laws, the Principle of Original Horizontality served well in the nascent days of geological science. However, it is now known. For instance, coarser grained sediments such as sand may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort; this is known as the angle of repose, a prime example is the surface of sand dunes. Sediments may drape over a pre-existing inclined surface: these sediments are deposited conformably to the pre-existing surface.
Sedimentary beds may pinch out along strike, implying that slight angles existed during their deposition. Thus the Principle of Original Horizontality is but not universally, applicable in the study of sedimentology and structural geology. Law of superposition Principle of cross-cutting relationships Principle of faunal succession Principle of lateral continuity
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. In geology, sedimentation is used as the opposite of erosion, i.e. the terminal end of sediment transport. In that sense, it includes the termination of transport by true bedload transport. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the termination of the settling process. In estuarine environments, settling can be influenced by the absence of vegetation. Trees such as mangroves are crucial to the attenuation of waves or currents, promoting the settlement of suspended particles. Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water to suspensions of dust and pollen particles to cellular suspensions to solutions of single molecules such as proteins and peptides.
Small molecules supply a sufficiently strong force to produce significant sedimentation. The term is used in geology to describe the deposition of sediment which results in the formation of sedimentary rock, but it is used in various chemical and environmental fields to describe the motion of often-smaller particles and molecules; this process is used in the biotech industry to separate cells from the culture media. In a sedimentation experiment, the applied force accelerates the particles to a terminal velocity v t e r m at which the applied force is canceled by an opposing drag force. For small enough particles, the drag force varies linearly with the terminal velocity, i.e. F d r a g = f v t e r m where f depends only on the properties of the particle and the surrounding fluid; the applied force varies linearly with some coupling constant that depends only on the properties of the particle, F a p p = q E a p p. Hence, it is possible to define a sedimentation coefficient s = d e f q / f that depends only on the properties of the particle and the surrounding fluid.
Thus, measuring s can reveal underlying properties of the particle. In many cases, the motion of the particles is blocked by a hard boundary; the concentration of particles at the boundary is opposed by the diffusion of the particles. The sedimentation of a single particle under gravity is described by the Mason–Weaver equation, which has a simple exact solution; the sedimentation coefficient s The sedimentation of a single particle under centrifugal force is described by the Lamm equation, which has an exact solution. The sedimentation coefficient s equals m b / f, where m b is the buoyant mass. However, the Lamm equation differs from the Mason–Weaver equation because the centrifugal force depends on radius from the origin of rotation, whereas in the Mason–Weaver equation gravity is constant; the Lamm equation has extra terms, since it pertains to sector-shaped cells, whereas the Mason–Weaver equation is one-dimensional. Classification of sedimentation: Type 1 sedimentation is characterized by particles that settle discretely at a constant settling velocity,or by the deposition of Iron-Rich minerals to streamlines down to the point source.
They do not flocculate or stick to other during settling. Example: sand and grit material Type 2 sedimentation is characterized by particles that flocculate during sedimentation and because of this their size is changing and therefore their settling velocity is changing. Example: alum or iron coagulation Type 3 sedimentation is known as zone sedimentation. In this process the particles are at a high concentration such that the particles tend to settle as a mass and a distinct clear zone and sludge zone are present. Zone settling occurs in lime-softening, active sludge sedimentation and sludge thickeners. In geology, sedimentation is the deposition of particles carried by a fluid flow. For suspended load, this can be expressed mathematically by the Exner equation, results in the formation of depositional landforms and the rocks that constitute sedimentary record. An undesired increased transport and sedimentation of suspended material is called siltation, it is a major source of pollution in waterways in some parts of the world.
High sedimentation rates can be a result of poor land management and a high frequency of flooding events. If not managed properly, it can be detrimental to fragile ecosystems on the receiving end, such as coral reefs. Climate change affects siltation rates. In chemistry, sedimentation has
Limestone
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
Mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, physical properties of minerals and mineralized artifacts. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization. Early writing on mineralogy on gemstones, comes from ancient Babylonia, the ancient Greco-Roman world and medieval China, Sanskrit texts from ancient India and the ancient Islamic World. Books on the subject included the Naturalis Historia of Pliny the Elder, which not only described many different minerals but explained many of their properties, Kitab al Jawahir by Persian scientist Al-Biruni; the German Renaissance specialist Georgius Agricola wrote works such as De re metallica and De Natura Fossilium which began the scientific approach to the subject. Systematic scientific studies of minerals and rocks developed in post-Renaissance Europe; the modern study of mineralogy was founded on the principles of crystallography and to the microscopic study of rock sections with the invention of the microscope in the 17th century.
Nicholas Steno first observed the law of constancy of interfacial angles in quartz crystals in 1669. This was generalized and established experimentally by Jean-Baptiste L. Romé de l'Islee in 1783. René Just Haüy, the "father of modern crystallography", showed that crystals are periodic and established that the orientations of crystal faces can be expressed in terms of rational numbers, as encoded in the Miller indices. In 1814, Jöns Jacob Berzelius introduced a classification of minerals based on their chemistry rather than their crystal structure. William Nicol developed the Nicol prism, which polarizes light, in 1827–1828 while studying fossilized wood. James D. Dana published his first edition of A System of Mineralogy in 1837, in a edition introduced a chemical classification, still the standard. X-ray diffraction was demonstrated by Max von Laue in 1912, developed into a tool for analyzing the crystal structure of minerals by the father/son team of William Henry Bragg and William Lawrence Bragg.
More driven by advances in experimental technique and available computational power, the latter of which has enabled accurate atomic-scale simulations of the behaviour of crystals, the science has branched out to consider more general problems in the fields of inorganic chemistry and solid-state physics. It, retains a focus on the crystal structures encountered in rock-forming minerals. In particular, the field has made great advances in the understanding of the relationship between the atomic-scale structure of minerals and their function. To this end, in their focus on the connection between atomic-scale phenomena and macroscopic properties, the mineral sciences display more of an overlap with materials science than any other discipline. An initial step in identifying a mineral is to examine its physical properties, many of which can be measured on a hand sample; these can be classified into density. Hardness is determined by comparison with other minerals. In the Mohs scale, a standard set of minerals are numbered in order of increasing hardness from 1 to 10.
A harder mineral will scratch a softer, so an unknown mineral can be placed in this scale by which minerals it scratches and which scratch it. A few minerals such as calcite and kyanite have a hardness that depends on direction. Hardness can be measured on an absolute scale using a sclerometer. Tenacity refers to the way a mineral behaves when it is broken, bent or torn. A mineral can be brittle, sectile, flexible or elastic. An important influence on tenacity is the type of chemical bond. Of the other measures of mechanical cohesion, cleavage is the tendency to break along certain crystallographic planes, it is described by the orientation of the plane in crystallographic nomenclature. Parting is the tendency to break along planes of weakness due to twinning or exsolution. Where these two kinds of break do not occur, fracture is a less orderly form that may be conchoidal, splintery, hackly, or uneven. If the mineral is well crystallized, it will have a distinctive crystal habit that reflects the crystal structure or internal arrangement of atoms.
It is affected by crystal defects and twinning. Many crystals are polymorphic, having more than
Depositional environment
In geology, depositional environment or sedimentary environment describes the combination of physical and biological processes associated with the deposition of a particular type of sediment and, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more that direct modern analogues are not available. Continental Alluvial Aeolian – Processes due to wind activity Fluvial LacustrineTransitional Deltaic – Silt deposition landform at the mouth of a river Tidal Lagoonal – A shallow body of water separated from a larger body of water by barrier islands or reefs Beach – Area of loose particles at the edge of the sea or other body of water Lake – A body of still water, in a basin surrounded by landMarine Shallow water marine environment Upper shoreface – The portion of the seafloor, shallow enough to be agitated by everyday wave action Lower shoreface – The portion of the seafloor, the sedimentary depositional environment, that lies below the everyday wave base Deep water marine environment – Flat area on the deep ocean floor Reef – A bar of rock, coral or similar material, lying beneath the surface of waterOthers Evaporite – A water-soluble mineral sediment formed by evaporation from an aqueous solution Glacial Volcanic Tsunami – Sedimentary unit deposited by a tsunami Depositional environments in ancient sediments are recognised using a combination of sedimentary facies, facies associations, sedimentary structures and fossils trace fossil assemblages, as they indicate the environment in which they lived.
Harold G. Reading. 1996. Sedimentary Environments: Processes and Stratigraphy. Blackwell Publishing Limited. Sedimentary Environments Classification Charts Depositional environments on e-notes
