Category: Sigmoidal cross bedding

GSA Bulletin ;; 97 4 : — The Curtis Formation in central Utah contains a rich suite of sedimentary structures that provides persuasive evidence for a tidal mode of origin. The most diagnostic of these structures are sigmoidal tidal bundles: sandstone packages deposited during one dominant tidal episode that comprise a set of large-scale cross-lamination enclosed by two relatively gently dipping, sigmoid-shaped pause planes. Bundles are especially recognizable as tidally generated features because they exhibit an internal progression, from gentle, drape-like sigmoids to avalanche-type cross-bed foresets back to sigmoidal drapes.

Cross-bedding

This progression reflects the sequential change in tidal flow velocities from slack water to maximum flow back to slack water. In addition, cyclic variability in the thickness and in internal structures among tidal bundles is clearly related to periodicity in the lunar month. These features of the Curtis Formation are remarkably similar to sedimentary structures observed in Holocene tidal sediments along the Dutch North Sea coast.

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Next Article. Article Navigation. Research Article April 01, Sigmoidal tidal bundles and other tide-generated sedimentary structures of the Curtis Formation, Utah R. Google Scholar. GSA Bulletin 97 4 : — Article history first online:. Abstract The Curtis Formation in central Utah contains a rich suite of sedimentary structures that provides persuasive evidence for a tidal mode of origin.

This content is PDF only. Please click on the PDF icon to access.Stop Curtis Formation Stop.

sigmoidal cross bedding

Figure 1A. Figure 1B. West flank San Rafael Swell on I, north side of interstate. Upper Jurassic. Rock Units:. Curtis Formationupper San Rafael Group. Features Present:. Depositional Environment:. Tidal, nearshore. Sigmoidal bundles and the regular alternating rhythmites are attributed to tidal processes. Channelized tidal flow has distinct slack-water periods that result in mud drapes like these in the Curtis Formation.

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The most complete and well-defined bundles occur during the spring phase while neap-tide bundles are less well developed. Kreisa and Moiola reported on 28 bundles with cyclic variation in thickness and sedimentary structures.

They noted that foresets developed during maximum flow of spring tides dip 25 o to 28 obut neap-tide foresets are more gently inclined 12 o to 25 o. Accelerating tidal currents caused the foreset lamination of bundles to steepen.

Figure 3: Tidal rhythmites, alternating layers of mud vs. For a complete list of references please go to the References page. A pdf version of this website is available on the Main Page.

sigmoidal cross bedding

Disclaimer: The information is property of the University of Utah. Unless cited, images and files found on this site have been taken or created by the Geology and Geophysics Department at the University of Utah. Any use of these images should be cited appropriately. The stratigraphic column is from: Mathis, A. Anderson and D. Sprinkel eds.In geologythe sedimentary structures known as cross-bedding refer to near- horizontal units that are internally composed of inclined layers.

Cross Stratification/Lamination Geometry

This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation. Cross-beds or "sets" are the groups of inclined layers, and the inclined layers are known as cross strata. Cross bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunesand indicates that the depositional environment contained a flowing medium typically water or wind.

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Examples of these bedforms are ripples, dunes, anti-dunes, sand waves, hummocks, bars, 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 what an area was like in ancient times. The direction the beds are dipping indicates paleocurrentthe rough direction of sediment transport.

The type and condition of sediments can tell geologists the type of environment rounding, sorting, composition…. 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.

Sigmoid function

It is also difficult to distinguish between the cross beds of a dune and the cross beds of an antidune. This could lead to misinterpretation since dunes dip downstream while antidunes dip upstream. The direction of motion of the cross-beds can show ancient flow or wind directions called paleocurrents. However, most cross-beds are not tabular, they are troughs Template:Citation needed. Since troughs can give a degree variation of the dip of foresets, false paleocurrents can be taken by blindly measureing 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, and current ripples show the direction rivers were moving.

Cross-bedding is formed by the downstream migration of bedforms such as ripples or dunes [3] in a flowing fluid. The fluid flow causes sand grains to saltate up the upstream "stoss" 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 "lee" side of the dune.

Repeated avalanches will eventually 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 generally 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 generally 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.

It is most common in stream deposits consisting of sand and graveltidal areas, and 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 ripple or dune. These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities. Cross-bedding can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area.To browse Academia.

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Skip to main content. Log In Sign Up. Download Free PDF. Fernando Vesely. Download PDF. A short summary of this paper. Sediments deposited during ice retreat or deglaciation phases constitute the bulk of the stratigraphic record in most of the basins, particularly in the case of epicontinental and passive marginal marine basins.

In these types of basins, characterized by low topographic gradient, normally glacial advance causes glacial abrasion and deposition essentially of thin lodgement till layers. Such deposits have a great potential of preservation in subaqueous environments because they are rapidly covered by transgressive deposits, especially in marine environments due to post-glacial rise in relative sea level.

Preservation in subaerial proglacial settings is less significant due to glacioisostatic rebound and reworking by fluvial and eolian processes. Quaternary deglaciation sequences or facies successions are widespread in North America and Eurasia.

The vertical profile was interpreted as the retrogradation of proglacial depositional systems due to the retreat of a grounded ice-margin. Particular attention has been focused on the nature of deglaciation stratigraphic record in the Paleozoic basins of Gondwana Santos et al. Investigating the Permo-Carboniferous Dwyka Group of the Karoo and Kalahari basins of southern South Africa, Visser described various unconformity-bounded diamictite units up to m thick and introduced the term deglaciation sequence to designate "an upward-thinning sediment body or package deposited seaward of the ice-grounding line during a major recessional phase of a marine ice margin".

The outcrop belt was analyzed and surface profiles were described and correlated with well logs. The deglaciation sequences were identified and their sedimentary facies are presented in this paper, as well as a discussion about the depositional tract system proposed to explain the vertical facies succession.

The megasequence is a transgressive-regressive second order cycle in the sense of Vail et al. These three formations have no distinct lithologic attributes but similar finingupward facies successions composed by thick basal sand-rich unit followed by a fine-grained section made up of diamictites, sandstones, rhythmites and shale. Similar occurrences were found in cores from boreholes drilled near the eastern basin margin by Quadros According to biostratigraphic data Daemon and Quadros, ;Souza et al.

In addition, a surface type-section, essential to correlation with well logs, is not yet available. To make possible the surface-subsurface stratigraphic correlation, the outcrop belt was investigated in order to find good places to measure stratigraphic sections. Based on this section, five depositional sequences bounded by disconformities were identified Vesely and Assine, Tillites and subglacial features like boulder beds are locally present at the sequence bases. The five sequences were recognized along kilometers across the basin in an approximately E-W section, from the outcrop belt to the far west well 2-ANPR Fig.

For chronostratigraphic correlation, the datum was placed in the fine-grained interval of sequence 4 that is the most easily traceable horizon in the stratigraphic section and correlative to the Lontras shale marker of Castro The sequences have the same basic stratigraphic signature in subsurface, exhibiting the typical fining-upward profile in their lower portions, a pattern characteristic of deglaciation facies succession.

The erosive sequence boundaries are clearly observed as abrupt changes in gamma-ray logs. Sequence 1 corresponds to the Lagoa Azul Formation, as well as sequence 5 corresponds to the Taciba Formation.

Sequence thickness is roughly the same in the stratigraphic section presented in Fig. The uppermost sequence is widespread due to the southwards expansion of the basin as a consequence of the Late Carboniferous to Early Permian transgression, an important event of marine flooding discussed earlier by Santos et al. The vertical facies succession is shown in more detail in Fig.A sigmoid function is a mathematical function having a characteristic "S"-shaped curve or sigmoid curve. A common example of a sigmoid function is the logistic function shown in the first figure and defined by the formula: [1].

Other standard sigmoid functions are given in the Examples section. Special cases of the sigmoid function include the Gompertz curve used in modeling systems that saturate at large values of x and the ogee curve used in the spillway of some dams. Sigmoid functions have domain of all real numberswith return response value commonly monotonically increasing but could be decreasing. Sigmoid functions most often show a return value y axis in the range 0 to 1. A wide variety of sigmoid functions including the logistic and hyperbolic tangent functions have been used as the activation function of artificial neurons.

Sigmoid curves are also common in statistics as cumulative distribution functions which go from 0 to 1such as the integrals of the logistic densitythe normal densityand Student's t probability density functions. The logistic sigmoid function is invertible, and its inverse is the logit function. A sigmoid function is a boundeddifferentiablereal function that is defined for all real input values and has a non-negative derivative at each point [1] and exactly one inflection point.

A sigmoid "function" and a sigmoid "curve" refer to the same object. In general, a sigmoid function is monotonicand has a first derivative which is bell shaped. Conversely, the integral of any continuous, non-negative, bell-shaped function with one local maximum and no local minimum, unless degenerate will be sigmoidal. Thus the cumulative distribution functions for many common probability distributions are sigmoidal. One such example is the error functionwhich is related to the cumulative distribution function of a normal distribution.

A sigmoid function is convex for values less than 0, and it is concave for values greater than 0. Many natural processes, such as those of complex system learning curvesexhibit a progression from small beginnings that accelerates and approaches a climax over time.

When a specific mathematical model is lacking, a sigmoid function is often used. The van Genuchten—Gupta model is based on an inverted S-curve and applied to the response of crop yield to soil salinity.

Examples of the application of the logistic S-curve to the response of crop yield wheat to both the soil salinity and depth to water table in the soil are shown in logistic function In agriculture: modeling crop response. In artificial neural networkssometimes non-smooth functions are used instead for efficiency; these are known as hard sigmoids.

In audio signal processingsigmoid functions are used as waveshaper transfer functions to emulate the sound of analog circuitry clipping. In biochemistry and pharmacologythe Hill equation and Hill—Langmuir equation are sigmoid functions.Tidal environments occur in a wide variety of settings e. Intertidal environments include sandy to muddy tidal flats where tidal rhytmites may form, commonly bordered by salt marshes or mangroves where muddy facies or peats accumulate.

Estuaries are transgressed, drowned, river valleys where fluvial, tide, and wave processes interact; they are characterized by a net landward movement of sediment in their seaward part. Tide-dominated estuaries contain tidal sand bars at the seaward end, separated from the fluvial zone by relatively fine-grained tidal flats e. Wave-dominated estuaries have a coastal barrier with a tidal inlet and flood- tidal delta, separated from a bayhead delta by a central basin where fine- grained sediments muds accumulate.

Tide-influenced sedimentary structures can take different shapes:. Herringbone cross stratification indicates bipolar flow directions, but it is rare. Tide-influenced sedimentary structures can take different shapes: Herringbone cross stratification indicates bipolar flow directions, but it is rare.

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Sigmoidal Cross-bedding; Mud-draped cross strata. Thinner, mud draped bundles represent neap-tide, thicker sets represent spring-tide higher tidal energy.In geologycross-beddingalso known as cross-stratificationis layering within a stratum and at an angle to the main bedding plane.

The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original depositional layering is tilted, such tilting not being the result of post-depositional deformation.

sigmoidal cross bedding

Cross-beds or "sets" are the groups of inclined layers, which are known as cross-strata. Cross-bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunes ; it indicates that the depositional environment contained a flowing medium typically water or wind. Examples of these bedforms are ripples, dunes, anti-dunes, sand waveshummocksbarsand delta slopes. Cross-beds can tell geologists much about what an area was like in ancient times.

The direction the beds are dipping indicates paleocurrentthe rough direction of sediment transport. The type and condition of sediments can tell geologists the type of environment rounding, sorting, composition 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 also difficult to distinguish between the cross-beds of a dune and the cross-beds of an antidune.

Dunes dip downstream while antidunes dip upstream. The direction of motion of the cross-beds can show ancient flow or wind directions called paleocurrents.

However, most cross-beds are not tabular, they are troughs [ citation needed ]. Since troughs can give a 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, and current ripples show the direction rivers were moving.

Cross-bedding is formed by the downstream migration of bedforms such as ripples or dunes [3] in a flowing fluid. The fluid flow causes sand grains to saltate up the stoss 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 moving water, falling down the lee downstream side of the dune. Repeated avalanches will eventually form the sedimentary structure known as cross-bedding, with the structure dipping in the direction of the paleocurrent.

Deglaciation sequences in the Permo-Carboniferous Itararé Group, Paraná Basin, southern Brazil

The sediment that goes on to form cross-stratification is generally 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 generally 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. It is most common in stream deposits consisting of sand and graveltidal areas, and 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 downstream or lee side of the bedform ripple or dune.

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These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities. Cross-bedding can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area.