Six Layers -- No Icing

I count 1,2,3,4,5,6 layers of the same rock type?

I always wondered what defined layer boundaries in rock beds of the same composition.

I remember vividly from 40+ years ago seeing a waterfall cutting away a shale bed with exactly this kind of layering.

wait a minute...

Here it is:

Say rock guys, what creates the layers in a mono mineral deposit like shale?

Layers in a homogenous rock type are caused by sumple variation in texture or mineralogy that may not be visible to the naked eye. These results from environmental effects during deposition, like variations in sediment supply or source, changes in grainsize from variation in energy etc. This is why petrography (studying thin sections of rocks under a microscope) is such an important tool. It allows us to identify what they layering actually is. If you were to look at a finely parted shale you would fine various in clay as opposed to detrital quartz content, or changes between proporiytions of different clays(smecitie/illite/kaolinite, for example.

With the Martian examples we know that the layers conatin silicate, suphate, and oxide phases. Variations between the proporitions is one way of getting the layering. Another is depositional events causing changes in grainsize. we have seen cross lamination and parallel bedding indicating such processes have occurred. There is no reason why both depositional and compositional layering cannot occur in the same rock. In fact, it ususally does.

Cheers

Jon

Cheers

Jon

That was me

Jon

Uh, thanks for the explanation Jon.

Can the layering be tied to a time scale?

Can the variations in temperature, etc. responsible for the layering be related to, say, climate or orbital parameter time-scales? Or are the driving parameters typically "local"?

IE, do shales of the same age have a common worldwide pattern of thickness?

Wondering about that slate, where is that spot? I'd like to visit there.

Kakabeka Falls

Now, slate and shale are different. Shale is entirely sedimentary, whereas slate is a metamorphosed sedimentary rock. With shale, the cleavage follows the original bedding planes, whereas with slate, it usually does not.

Er, yes, the rock in picture 2, reply 0 is shale. There is also an interesting outcrop of fossil bearing Gunflint chert in the area.

Now about those Martian layers...

Hi horton

The answer is both. Layering can be due to purely local events or global, or a mixture of the two. To determine which you first need an understanding of the time scale represented by the section, second, the nature of the processes causing the layers, and third what the global cycles are. Even on earth where the time scale of global cycles from 1st to 6th order (and shorter)are well understood it is not always easy to differentiate them in an individual outcrop. On Mars we are going to have to learn from scratch once more.

Cheers

Jon

Do a search for the term varve. The mechanism of formation of varves in glacial lakes is quite illuminating. We get this baklava-like layering effect, with incredibly thing layers.

I think that what we're seeing at the Columbia Hills is volcanic. I could be wrong.

Hi Aldabaran

Varves are an excellent example of a local feature produced by annual events and capable of recording global controls of the order of several thousands to 10's of thousands of years - at least on earth.

The problem at Columbia Hills is that all we can see is parallel layering. Until we have information on grainsize, boundary relationships, chemistry and mineralogy, we just can't go further. Fine laying like this can include: wind-borne dust or sand, water laid sand (transitional flow regime), silt or mud (low energy), impact fall out, airborne ash, ash surges. But we can rule out some processes - very high energy water laid deposits, proximal impact or pyroclastic facies, and coherent volcanic flows, for example. But such data should soon be forthcoming with close ups and spectral/Mossbauer info.

Cheers

Jon

More data will be helpful.

As Sherlock Holmes said, Eliminate the impossible and whatever remains, however improbable, must be the truth.

I think everyone must know that we are in a very early stage of data acquisition, so any and all not impossible explanations are equally likely.

hort
I enjoyed the discussion on slate and shale.
Does the girl know you posted her picture on the internet?

Moby, the girl no longer exists.

That beautiful creature is my former wife, the mother of my four children. I should have paid more attention to her than I did to rocks and planets and butterflies and computers.

(Sigh of regret.)

Layers can also indicate repeated similar events over time. For example, multiple volcanic events produce multiple ash layers, mudflows produce debris laers, etc. The events are often periodic, but the time scale can be anything from days to eons, unfortunately.

I'd like to know what the MARTIAN layers are made of, then we can guess at what deposited them. MY gut feel is they are lakebottom deposits. IT looks like they were made out of muddy stuff or silt, fine particle sizes.

Mars Global Surveyor images at [link] show many examples of large-scale layering.

The image at [link] for an example of cyclicity in a lake bed in the Triassic of New Jersey.

More incredible is a likely delta shown at http://www.msss.com/mars_images/moc/2003/11/13/ . This may give clues about sedimentary processes at Meridiani. Under the delta, there are light-colored layers that very well could be lake beds.

I would think this delta site would be a top priority for the next rover mission, assuming the rough terrain wouldn't be a hinderance to traversing. It may have nearly every type of sedimentary deposit all at one convenient site.

Just no more basalt plains please!

Time out! The text between two links apparently gets eaten when single spaces are used next to html. Now lets try this again......

Mars Global Surveyor images at

[link]

show many examples of large-scale layering.

The image at

http://www.msss.com/mars_images/moc/2003/04/26/

is quite striking. These rhythmic beds may have resulted from orbital effects on climate. It is possible that the beds at Meridiani have similar cyclicity. See the article at

[link]

for an example of cyclicity in a lake bed in the Triassic of New Jersey.

More incredible is a likely delta shown at
http://www.msss.com/mars_images/moc/2003/11/13/

This may give clues about sedimentary processes at Meridiani. Under the delta, there are light-colored layers that very well could be lake beds.

I would think this delta site would be a top priority for the next rover mission, assuming the rough terrain wouldn't be a hinderance to traversing. It may have nearly every type of sedimentary deposit all at one convenient site.

Just no more basalt plains please!

JonClarke - I should have been more specific. I think that varve-like features were seen at Meridiani (the wafer-like features that we have seen in Endurance, and in Eagle crater before that) and I think they reflect an ancient freeze-thaw cycle, characteristic of very short-duration thaw cycles.

The layers on the Columbia Hills are, I believe, volcanic in origin, although clearly, this has been altered by water.

Hi Aldebaran

That is ceratinly possible. But as yet there is no possible evidence I have seen that the waters were freezing at the time of deposition. We also have no evidence of the time scale either, so we don't know the time scale of each lamination. So I reserve judgement.

Cheers

Jon

Gentlemen,

good thread....At meridiani (and to no surprise) I believe the original texture has been overprinted with secondary and possibly tertiary diagenetic event(s) then by a decent size impact, which I believe is often overlooked. In the sense that the rock was dehydrated by the impact which wiped out further any original fabric. I'm still working on that one.

At Gusev I think we're a distance from the delta of Maadim Vallis and the thin beds seen at columbia hills look somewhat like overbank deposits...yet, with the original fabric overprinted again by diagenetic events such as mineralization and shock. Varve comparisons could be quite relative. In order to figure this one out I think it's time to review again Mike Malins photos to define a relationship of the hills with the Maadim Vallis delta.

Certainly at eagle crater there was extensive preservation of primary depositional fabrics like parallel lamination, scour and fill structures, and ripple cross lamination. This was despite the growth of sulphate crustals and haematite concretions during two phases of diagenesis. So we should not overemphasise the importance of diagenesis.

the same goes to the impact. there has been extenseive fracturing and tilting, but within the blocks the pre-impact textures are preserved. Impacts of the size of the one that formed endurance are just too small to cause melting or plastic flow. Certainly my visit to the largest Henbury crater (180 m) showed lost of intact primary features, although admittedly the substrate was much more lithfied.

Cheers

Jon