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Sedimentary Structures
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particle sizes are deposited on top of each other. These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick. Sedimentary structures such as cross-bedding, graded bedding, and ripple marks are utilized in stratigraphic studies. They help indicate the original position of strata in geologically complex terrains. They also provide insight into the depositional environment of the sediment. Flow structures There are two kinds of flow structures: bidirectional (multiple directions, back-and-forth) and unidirectional. Flow regimes in single-direction (typically fluvial) flow, which at varying speeds and velocities produce different structures, are called bedforms. In the ''lower flow regime'', ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Megaripple
In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''unidirectional ripples'', or ''asymmetrical ripple marks'' are asymmetrical in profile, with a gentle up-current slope and a steeper down-current slope. The down-current slope is the angle of repose, which depends on the shape of the sediment. These commonly form in fluvial and Aeolian processes, aeolian depositional environments, and are a signifier of the lower part of the Lower Flow Regime. * Ripple cross-laminae forms when deposition takes place during migration of current or wave ripples. A series of cross-laminae are produced by superimposing migrating ripples. The ripples form lateral to one another, such that the crests of vertically succeeding laminae are out of phase and appear to be advancing ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lower Plane Bed
A bedform is a geological feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the rock record as a result of being present in a depositional setting. Bedforms are often characteristic to the flow parameters, and may be used to infer flow depth and velocity, and therefore the Froude number. Bedforms Initiation Bedforms are omnipresent in many environments (e.g., fluvial, eolian, glaciofluvial, deltaic and deep sea), although there is still some debate on how they develop. There are two separate, though not mutually exclusive, models of bedform initiation: defect initiation and instantaneous initiation. Defect Initiation The defect theory proposes that the turbulent sweeps that are generated in turbulent flows entrain sediment that upon deposition generates defects in a non-cohesive material. These deposits then pro ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Palaeocurrent
A paleocurrent or paleocurrent indicator is a geological feature (typically a sedimentary structure) that helps one determine the direction of flowing water in the geologic past. This is an invaluable tool in the reconstruction of ancient depositional environments. There are two main types of paleocurrent indicators: *''Unidirectional'', which give a clear, single direction of flow *''Bidirectional'', which give a good linear direction, but it is unclear which direction along the linear trend the water flowed. Paleocurrents are usually measured with an azimuth, or as a rake on a bedding plane, and displayed with a Rose Diagram to show the dominant direction(s) of flow. This is needed because in some depositional environments, like meandering rivers, the paleocurrent resulting from natural sinuosity has a natural variation of 180 degrees or more. Below are a partial list of common paleocurrent indicators: Unidirectional *Cross-bedding – the axis of a trough cross bed or the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Interference Ripples
Interference ripples are a type of sedimentary structure Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different parti ... made up of two sets of ripples formed at right-angles to each other as a result of there being two dominant paleocurrents. These ripples may be formed in the beds of intermittent streams. References Further reading * Prothero, D. R. and Schwab, F., 1996, ''Sedimentary Geology'', pg. 43-64, Sedimentology Patterned grounds Sedimentary structures {{Sedimentology-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Flaser Bedding
Flaser beds are a sedimentary, bi-directional, bedding Bedding, also called bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environment ... pattern created when a sediment is exposed to intermittent flows, leading to alternating sand and mud layers. While flaser beds typically form in tidal environments, they can (rarely) form in fluvial conditions - on point bars or in ephemeral streams, or also in deep water environments when turbiditic sediments are reworked by seasonal bottom-currents. Individual sand ripples are created, which are later infilled by mud during quieter flow periods. These mud drapes are typically a minor constituent of the deposit; they can consolidate within three hours, protecting the underlying layer from erosion. Flaser bedding typically forms in high-energy environments. (with photograph) I ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Herringbone Cross-stratification
Herringbone cross-stratification is a type of sedimentary structure formed in tidal areas, such as tidal flats, where the current periodically flows in the opposite direction. Formation During the conventional formation process of cross-stratification, sand grains saltate up the upstream side of the dune, collecting 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 eventually form the sedimentary structure known as cross-stratification, with the structure dipping in the direction of the paleocurrent. In tidal areas, which have bidirectional flow, structures are formed with alternating layers of cross-beds dipping in opposite directions that reflect the alternating paleocurrent. These sedimentary structures are not common because they require the current to be equal in both directions, wh ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ripple Mark
In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''unidirectional ripples'', or ''asymmetrical ripple marks'' are asymmetrical in profile, with a gentle up-current slope and a steeper down-current slope. The down-current slope is the angle of repose, which depends on the shape of the sediment. These commonly form in fluvial and Aeolian processes, aeolian depositional environments, and are a signifier of the lower part of the Lower Flow Regime. * Ripple cross-laminae forms when deposition takes place during migration of current or wave ripples. A series of cross-laminae are produced by superimposing migrating ripples. The ripples form lateral to one another, such that the crests of vertically succeeding laminae are out of phase and appear to be advancing ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Antidunes
An antidune is a bedform found in fluvial and other channeled environments. Antidunes occur in supercritical flow, meaning that the Froude number is greater than 1.0 or the flow velocity exceeds the wave velocity; this is also known as upper flow regime. In antidunes, sediment is deposited on the upstream (stoss) side and eroded from the downstream (lee) side, opposite lower flow regime bedforms. As a result, antidunes migrate in an upstream direction, counter to the current flow. Antidunes are called in-phase bedforms, meaning that the water surface elevation mimics the bed elevation; this is due to the supercritical flow regime. Antidune bedforms evolve rapidly, growing in amplitude as they migrate upstream. The resultant wave at the water's surface also increases in amplitude. When that wave becomes unstable, breaks and washes downstream, much of the antidune bedform may be destroyed. Formation Antidunes are typically found in fluvial environments in shallow areas with a high ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Parting Lineation
Parting lineation (also known as current lineation or primary current lineation) is a subtle sedimentary structure in which sand grains are aligned in parallel lines or grooves on the surface of a body of sand (or lithified as a sandstone). The orientation of the lineation is used as a paleocurrent indicator, although the precise flow direction (i.e. upstream vs. downstream) is often indeterminable. They are also the primary indicator of the lower part of the upper flow regime bedform. Description Parting lineation is a sedimentary structure commonly found on the surface of parallel-laminated sandstones. It is aligned with the current direction, the alignment holding out in places over several square meters. The lineation is formed by flat, parallel ridges which are separated by depressions or grooves. The height of the ridges rarely exceeds several grain diameters. In profile the depressions are flat-bottomed and the ridges are rounded. Ridges and depressions are arranged en e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Upper Plane Bed
A bedform is a geological feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the rock record as a result of being present in a depositional setting. Bedforms are often characteristic to the flow parameters, and may be used to infer flow depth and velocity, and therefore the Froude number. Bedforms Initiation Bedforms are omnipresent in many environments (e.g., fluvial, eolian, glaciofluvial, deltaic and deep sea), although there is still some debate on how they develop. There are two separate, though not mutually exclusive, models of bedform initiation: defect initiation and instantaneous initiation. Defect Initiation The defect theory proposes that the turbulent sweeps that are generated in turbulent flows entrain sediment that upon deposition generates defects in a non-cohesive material. These deposits then pro ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dune
A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat regions covered with wind-swept sand or dunes, with little or no vegetation, are called ''Erg (landform), ergs'' or ''sand seas''. Dunes occur in different shapes and sizes, but most kinds of dunes are longer on the wiktionary:stoss, stoss (upflow) side, where the sand is pushed up the dune, and have a shorter ''slip face'' in the lee side. The valley or trough between dunes is called a ''dune slack''. Dunes are most common in desert environments, where the lack of moisture hinders the growth of vegetation that would otherwise interfere with the development of dunes. However, sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of Outwash plain, glacial outwash ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
