Clues to original position of rock layers
1. "Sometimes tectonics forces overturn rock layers. In cross-bedded layers of sedimentary rocks, sediments are deposited in curved sheets at an angle to the bedding plane. The tops of cross-bedded layers are often eroded before new layers ar deposited. Thus, the cross-beds are still curved at the bottom but the tops are cut off. By studying the shape of the bedding planes, original position of cross-bedded layers can be determined."
I don't quite understand the bolded parts...Does cross-bedding simply means tilted (non-horizontal) rock layers created by sediments being deposited at an inclined surface? So tilted layers (not horizontal) are called cross-bedding?
"In cross-bedded layers of sedimentary rocks, sediments are deposited in curved sheets at an angle to the bedding plane" <---What does it mean curved sheets at an angle to bedding palne? Why is it curved?
2. "Ripple marks can also be helpful in determining the order of rock layers. In undisturbed sedimentary rock layers, the peaks of the ripple marks point upwards. So scientists can establish the original arrangement of the layers."
The peaks point upwards because of ocean waves moving back and forth? How about if these ripple marks are created by current flow, would the cusps (peaks) still point upwards?
I hope someone can explain. I would really appreciate!
Last edited by kingwinner; 12-14-05 at 12:43 PM.
To answer this one, we need to look at a picture of some cross bedding:
Originally Posted by kingwinner
This is a picture of some cross bedding caused by movement of sand dunes in a river. The curved surface that the text mentions is the obvious "lines" running from the top right to the lower left. In this photo youn see that in some occasions the top is truncated (beds to the right), but sometimes it isn't (left). Cross beds form as sand falls down the face and the dune essentially moves forward (progradation). In the above picture the dune moved from right to left.
Peaks always point upward on ripples, because that is what a peak is. The only difference between ripples caused by a current and those by waves is that wave ripples are symmetrical in cross section, current riplpes are assymetrical with the steeper side being the lee side and the shallow side being a the stoss side. The main thing about ripple marks is that they form on the top of a bed and are therefore way-up indicators as your quoted text eludes to.
Originally Posted by kingwinner
Here's a pic of some wave ripples on the top of a bed:
Hope this helps!
1) What is the meaning of cross-bedding? I am really confused! I tried to google definitions but I can't understand what they are saying, the wording is very fancy!
Cross bedding: A sequence of beds inclined at an angle to the main bedding planes in granular sediments.
What does it mean by a "main" bedding plane?
For example, the diagram above, the layers are tilted, not horizontal. It is likely that the layers are deposited at an inclined surface, not a flat, horizontal surface. Would it be considered cross-bedding, then?
"The curved surface that the text mentions is the obvious "lines" running from the top right to the lower left"
Do you mean the "thin" lines in the middle, between the 2 "thick lines? What does the 2 "thick" lines represent?
"In this photo youn see that in some occasions the top is truncated (beds to the right)"
How do you know that the top is truncated (eroded) for beds to the right, is it because the lines are shorter?
In your diagram, how many beds (layers) are there...is the region between the 2 thick lines considered to be one single bed only?
Thanks for explaning to me!
Last edited by kingwinner; 12-15-05 at 01:38 AM.
The "peak" is the sharp point or cusp of the ripple mark, right? So for the left figure, the peaks/cusps are clearly pointing upward. If a rock layer with the peaks/cusps pointing downward, we know that it has been overturned. By knowing this, we can determine the original arrangement of the rock layers (just flip it 180 degrees) and the relative ages of the rocks can then be determined. That's what I think the text/quote is trying to say...
But is it only referring to back and forth motion of waves, because like the right figure (current flowing in one direction), the ripple marks have no peaks/cusps at all...??
Originally Posted by kingwinner
The two "thick" lines in the photo are the bedding planes. The "thin" lines are the cross-bedding causes by the dune migrating. So on the photo there are 1 "bed" (the one that runs near-horizontal) and this beds shows cross-bedding (the diagonal lines). Cross-bedding is a sedimentary structure. To simplify things:
o Sediments are laid down horizontal
o The processes that causes sediment to be deposited can cause sedimentary structures - cross-bedding is one example. Ripples are another. Others are things like rip-up clasts, graded bedding, etc, etc, etc
In your diagram the beds are not cross bedding as the layers themselves have been titled. They would have been deposited horizontally and then later tilted by tectonic forces. Cross-bedding occurs within beds, within a formation. The scale can be centimetres to tens of metres (Google Zion national park for some large scale ones). There are some photos at the bottom of this page with more structures:
Any of the formations in your diagram (the different colours) could contain beds which have cross-bedding in them, but the formations themselves cannot be part of cross-bedding.
Hope this clears things up
Quite right, if ripples have been overturned you can tell that they have (sometimes tricky, but still possible). The current ripples do have cusps and peaks, but they are not as sharp. They are still there though and it is still possible to use them as way-up indicators. Convinced?
Originally Posted by kingwinner
Cross bedding and Folding: Cross-bedding is defined as an inclined angular bedding to the original layer of bedding.
"Description: Fluvial cross beds of Devonian age at Pease Bay, Scotland. Note how some of the complete cross beds have been preserved, with topsets, foresets and downlap all seen on the sets to the left of the picture. This gives the original height of the aqueous dune. The beds to the right have been truncated by the upper erosion surface." http://www.geologyrocks.co.uk/picdetail.php?id=40
This is an exmple where you can see cross-bedding in the upper right, and extreme folding in the lower left at the the Huttenberg Sand Pit in the rim of the Huttenberg crater (200-250,000 years ago) at the Eifel region in western Germany.
"bedding within the sedimentary strata (layers) - Laminations(thin layers) - indicate environments where the water is very still (and anoxic): lagoons, the interiors of lakes, and the like. In contrast, cross-bedded layers and ripple marks indicate that there was some form of higher energy environment: the channels of streams and rivers, desert sand dunes, the shores of lakes and rivers. In the case of the shores of lakes and rivers the ripple marks will be symmetrical around their crest, indicating that they are bi-directional (produced by water moving to-and-fro, as the water laps onto shore and drains back again). In contrast, in any given layer of a stream channel or sand dune deposit, the ripples will be unidirectional, indicating a single dirrection of the current. Because winds keep on shifting in deserts, though, these unidirectional cross-beds willoften show shifts in the direction of the current: these are called trough cross-beds. Bedding and ripple marks are just two types of sedimentary structure. There are others that also give clues to the environment of deposition....A layer of rocks rich in coal is covered by a layer of rocks with trough cross-bedded sandstones. This indicatessome kind of major climate change took place."
Source: "GEOL 104 Dinosaurs: A Natural History Geology Assignment"
Thanks a lot for explaning! So cross-bedding actually refers to things within a layer.
I found a law of deposition in a web site:
"Laws of Stratigraphy
Original Horizontality - sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface. Thus rocks that we now see inclined or folded have been disturbed since their original deposition. "
Why is it so? Aren't cross-bedding depositions originally already inclined? They are not deposited horizontally...this kind of contradicts the law?
Ok ok ok...
Firstly, The Principle of Original Horizontality states that, the deposition of sediments occurs as essentially horizontal beds.
This is always your starting point!!! Everything gets deposited in horizontal layers...this is of course due to gravity and the settling properties of sediments. ALWAYS...well ok waves and wind action change things alittle bit but to make things simple we'll leave it at that.
Secondly, cross bedding refers to something actually being tilted from its original horizontal position. Thats it....its that easy.
Kingwinner- I can understand your confusion.
Cross-beds seem to contradict original horizontality. Looking at the very small scale- the tilt of the cross beds are not horizontal, nor are they parallel with the surface of the earth. If you step back and look at the larger scale, you find that cross beds are small internal sedimentary structures compared to the overall strata (formation) in which they occur. The overall strata will conform to the principles of stratigraphy.
As an example, Zion National Park in Utah is famous for the myraid shapes and angles of cross beds. However, these cross beds are all contained within a stratigraphic unit called the Navajo Sandstone. While the cross beds, on the small scale, seem to defy original horizontality, the Navajo sandstone is quite conformable with the strata both above and below.
An example of inclined cross-bedding terminating at the bottom bedding planes.
Cross-beddings are rocks or sedimentary structures formed by currents of wind or water. They are characterized by relatively thin layers of sediment that are inclined at an angle to the dominant bedding.
"As the dune continues to form and advance sand from the top is moved by down the slope, eroding the old top of the dune slope. So over time a sedimentary deposit is created with inclined surfaces bounded on the top by an erosion surface. We can recognise this in the rock record as cross-bedding. The erosion surfaces represent the principal bedding planes and these are continuous. In contrast the planes of cross bedding terminate at the bedding planes."
"Once sand begins to pile up, ripples and dunes can form. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the properties of the material it's made of, such as the grain size and roundness. Ripples grow into dunes with increase of wind and sand input."
"The repeating cycle of sand inching up the windward side to the dune crest, then slipping down the dune's slip face allows the dune to inch forward, migrating in the direction the wind blows. As you might guess, all of this climbing then slipping leaves its mark on the internal structure of the dune. The image on the right shows fossil sand dune structure preserved in the Merced Formation at Fort Funston, Golden Gate National Recreation Area. The sloping lines or laminations you see are the preserved slip faces of a migrating sand dune. This structure is called cross-bedding, and can be the result of either wind or water currents. The larger the cross-bedded structure, however, the more likely it is to be formed by wind, rather than water."