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environmental conditions at the time of deposition. Typically, units in the rock record are referred to as beds, while the constituent layers that make up the bed are referred to as laminae, when they are less than 1 cm thick and strata when they are greater than 1 cm in thickness. Cross-beds are angled relative to either the base or the top of the surrounding beds. As opposed to angled beds, cross-beds are deposited at an angle rather than deposited horizontally and deformed later on. Trough cross-beds have lower surfaces which are curved or scoop shaped and truncate the underlying beds. The foreset beds are also curved and merge tangentially with the lower surface. They are associated with sand dune migration.
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For example: well-rounded, and well-sorted sand that is mostly composed of quartz grains is commonly found in beach environments, far from the source of the sediment. Poorly sorted and angular sediment that is composed of a diversity of minerals is more commonly found in rivers, near the source of the sediment. However, older sedimentary deposits are frequently eroded and re-mobilized. Thus, a river may well erode an older formation of well-rounded, well-sorted beach sands of nearly pure quartz.
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Bed forms are relatively dynamic sediment storage bodies with response times that are short relative to major changes in flow characteristics. Large scale bed forms are periodic and occur in the channel (scaled to depth). Their presence and morphologic variability have been related to flow strength
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Tabular cross-bedding is formed mainly 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
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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. Cross-bedding can form in any environment in
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Flows are characterized by climate (snows, rain, and ice melting) and gradient. Discharge variations measured on a variety of time scales can change water depth, and speed. Some rivers can be characterized by a predictable seasonably controlled hydrograph (reflecting snow melt or rainy season).
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The shape of the grains and the sorting and composition of sediment can provide additional information on the history of cross-beds. Roundness of the grains, limited variation in grain size, and high quartz contents are generally attributed to longer histories of weathering and sediment transport.
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Although the flow direction reverses regularly, the flow patterns of flood on ebb currents commonly do not coincide. Consequently, the water and transport sediment may follow a roundabout route in and out of the estuary. This leads to spatially varied systems where some parts of the estuary are
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Cross-bedding can be subdivided according to the geometry of the sets and cross-strata into subcategories. The most commonly 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
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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
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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
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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 (paleocurrent) and even
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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
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Tabular (planar) cross-beds consist of cross-bedded units that are large in horizontal extent relative to set thickness and that have essentially planar bounding surfaces. The foreset laminae of tabular cross-beds are curved so as to become tangential to the basal surface.
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In a fluvial environment, the water in a stream loses energy and its ability transport sediment. The sediment "falls" out of the water and is deposited along a point bar. Over time the river may dry up or avulse and the point bar may be preserved as cross-bedding.
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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.
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relative to the set thickness and that have essentially planar bounding surfaces. Trough cross-bedding, on the other hand, consists of cross-bedded units in which the bounding surfaces are curved, and hence limited in horizontal extent.
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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 typically in granular sediments, especially
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Others are dominated by durational variations characteristic of alpine glaciers run-off or random storm events, which produce flashy discharge. Few rivers have a long term record of steady flow in the rock record.
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flood dominated and other parts are ebb dominated. The temporal and spatial variability of flow and sediment transport, coupled with regular fluctuating water levels creates a variety of bed form morphology.
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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.
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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.
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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.
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In general, the greater the tidal range the greater the maximum flow strength. Cross-stratification in tidal-dominated areas can lead to the formation of
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Reesink, A.J.H. and Bridge, J.S., 2007 "Influence of superimposed bedforms and flow unsteadiness on formation of cross strata in dunes and unit bars."
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Large scale bed forms occur on shallow, terrigenous or carbonate clastic continental shelves and epicontinental platforms which are affected by strong
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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
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Bourke, Lawrence, and McGarva, Roddy. "Go With The Flow: Part I Palaeotransport
Analysis ." Task Geoscience. N.p., 08/2002. Web. 2 Nov 2010. <
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will eventually form the sedimentary structure known as cross-bedding, with the structure dipping in the direction of the paleocurrent.
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Collinson, J.D., Thompson, D.B., 1989, Sedimentary
Structures (2nd ed): Academic Division of Unwin Hyman Ltd, Winchester, MA, XXX p.
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Hurlbut, C. 1976. The Planet We Live On, An
Illustrated Encyclopedia of the Earth Sciences. NY: Harry N. Abrams, Inc., Print.
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Boggs, S., 2006, Principles of
Sedimentology and Stratigraphy (4th ed): Pearson Prentice Hall, Upper Saddle River, NJ, XXX p.
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contained a flowing medium (typically water or wind). Examples of these bedforms are ripples, dunes, anti-dunes,
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The direction of motion of the cross-beds can show ancient flow or wind directions (called paleocurrents). The
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Cross-beds can tell geologists much about what an area was like in ancient times. The direction the beds are
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are deposited at the angle of repose (~34 degrees from the horizontal), so geologists are able to measure
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Ashley, G. (1990) "Classification of Large-Scale
Subaqueous Bedforms: A New Look At An Old Problem."
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Middleton, G., 2003, Encyclopedia of
Sedimentary Rocks : <MPG Books, Cornwall, GB, XXX p.
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Areas in which the water run-off volume is low relative to the tidal volume or impact.
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Stow, A.V., 2009, Sedimentary rocks in the field. A color guide (3rd ed.) print.
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which result are roughly horizontal units composed of inclined layers. The
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Cross-bedded sediments are recognized in the field by the many layers of "
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which a fluid flows over a bed with mobile material. It is most common in
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Prothero, D. R. and Schwab, F., 1996, Sedimentary
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Cross-bedding forms during deposition on the inclined surfaces of
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is tilted, such tilting not being the result of post-depositional
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Trough cross-bedding in the
Waddens Cove Formation in Nova Scotia
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Trough cross-bedding in the Lower Cove
Formation in Nova Scotia
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Coast lines that have a tidal range of greater than one meter.
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deposits (consisting of sand and gravel), tidal areas, and in
558:"Task Geoscience - the Borehole Image and Dipmeter Experts"
651:, Society for Sedimentary Geology (SEPM), Atlas Series
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Animation showing deposition and erosion of cross-beds
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formation had occurred prior to formation of the rock.
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In an aeolian environment, cross-beds often exhibit
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Coastal water bodies that are partially enclosed by
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Tabular cross-bedding in the South Bar
Formation in
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The Changing Earth: Exploring Geology and Evolution
70:Sand dune cross-beds can be large, such as in the
130:Tabular cross-bedding in the Navajo Sandstone in
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27:Sedimentary rock strata at differing angles
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401:Tide dominated environments include:
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688:The Bibliography of Aeolian Research
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683:Cross-bedding animation from USGS
106:Schematic of eolian cross-bedding
98:Formation of cross-stratification
505:Journal of Sedimentary Petrology
421:Herringbone cross-stratification
385:expressed as mean velocity or
196:original depositional layering
114:Close up of cross-bedding and
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262:and the cross-beds of an
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219:depositional environment
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566:. Retrieved
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241:Significance
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78:deposits of
18:Crossbedding
457:grain flows
200:deformation
143:Nova Scotia
88:Aztec Butte
42:Zion Canyon
697:Categories
568:2010-12-02
467:References
407:topography
327:Geometries
249:indicates
223:sand waves
90:shown here
347:sandstone
282:Formation
38:sandstone
362:Sediment
317:foresets
271:foresets
264:antidune
227:hummocks
209:such as
207:bedforms
72:Jurassic
447:Aeolian
307:dunes.
305:aeolian
288:saltate
255:analogs
247:dipping
211:ripples
188:stratum
176:geology
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376:Rivers
301:stream
233:, and
122:, Ohio
235:delta
215:dunes
116:scour
74:-age
667:ISBN
657:ISBN
638:ISBN
620:ISBN
573:>
260:dune
231:bars
213:and
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540:doi
275:dip
174:In
82:in
76:erg
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