Content deleted Content added
Reverting edit(s) by 49.144.205.59 (talk) to rev. 991102881 by ClueBot NG: non-constructive (RW 16) |
m Replaced obsolete center tags and reduced Lint errors |
||
| (16 intermediate revisions by 8 users not shown) | |||
Line 1:
{{Short description|Sedimentary rock strata at differing angles}}
{{stack|
[[Image:Cross-bedding Of Sandstone Near Mt Carmel Road Zion Canyon Utah.jpg|thumb|300px|right|Cross-bedding of [[sandstone]] near Mt. Carmel road, [[Zion National Park|Zion Canyon]], indicating wind action and sand [[dune]] formation had occurred prior to formation of the rock.]]
[[Image:DryForkDome.jpg|thumb|right|300px|
[[Image:Outcrop of Navajo Sandstone on the Island in the Sky District of Canyonlands, June 2005.jpg|right|thumb|300px|Sand dune cross-beds can be large, such as in the [[Jurassic]]-age [[Erg (landform)|erg]] deposits of [[Navajo Sandstone]] in [[Canyonlands National Park]]. [[Aztec Butte]] shown here]]
[[File:Formation of cross-stratification.png|thumb|Formation of cross-stratification]]
[[File:Formation of cross-bedding.jpg|thumb|Schematic of eolian cross-bedding]]
[[Image:Logan Formation Cross Bedding Scour.jpg|thumb|300px|right|Close up of cross-bedding and [[sole marking|scour]], [[Logan Formation]], Ohio]]
[[Image:Tabular xbed mcr2.JPG|thumb|300px|right|Tabular cross-bedding in the Navajo Sandstone in [[Zion National Park]]]]
Line 21 ⟶ 23:
==Formation==
Cross-bedding is formed by the downstream migration of bedforms such as ripples or dunes<ref name="Boggs">Boggs, S., 2006, Principles of Sedimentology and Stratigraphy (4th ed): Pearson Prentice Hall, Upper Saddle River, NJ, XXX p.</ref> in a flowing fluid.
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.<ref name="Reesink">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." ''[[Sedimentary Geology (journal)|Sedimentary Geology]]'', 202, 1-2, p. 281-296 {{doi|10.1016/j.sedgeo.2007.02.00508/2002}}.</ref>
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.<ref name="Bourke">Bourke, Lawrence, and McGarva, Roddy. "Go With The Flow: Part I Palaeotransport Analysis ." Task Geoscience. N.p., 08/2002. Web. 2 Nov 2010. <{{cite web |url=http://www.taskgeoscience.com/page.asp?id=1 |title=
===Internal sorting patterns===
Line 38 ⟶ 40:
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.<ref name="Boggs"/>
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.<ref name="Stow">Stow, A.V., 2009, Sedimentary rocks in the field. A color guide (3rd ed.) print.</ref> 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 typically in granular sediments, especially [[sandstone]], and indicate that sediments were deposited as ripples or dunes, which advanced due to a water or air current.<ref name= "Hurlbut">Hurlbut, C. 1976. The Planet We Live On, An Illustrated Encyclopedia of the Earth Sciences. NY: Harry N. Abrams, Inc., Print.</ref>
====Trough cross-beds====
Line 51 ⟶ 53:
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). 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.<ref name= "Ashley"/>
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 expressed as mean velocity or [[shear stress]].<ref name= "Ashley"/>
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.
Line 57 ⟶ 59:
===Tide-dominated===
Tide dominated environments include:
*Coastal water bodies that are partially enclosed by [[topography]], yet have a free connection to the sea.
*Coast lines that have a tidal range of greater than one meter.
*Areas in which the water run-off volume is low relative to the tidal volume or impact.
Line 66 ⟶ 68:
===Shallow marine===
Large scale bed forms occur on shallow, terrigenous or carbonate clastic continental shelves and epicontinental platforms which are affected by strong
=== Aeolian ===
In an aeolian environment, cross-beds often exhibit [[inverse grading]] due to their deposition by [[grain
==References==
| |||