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Author Topic:   Depositional Models of Sea Transgressions/Regressions - Walther's Law
Minnemooseus
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Posts: 3880
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 4.0


(1)
Message 1 of 533 (724524)
04-17-2014 9:49 PM


This attempt at a new topic is a result of message 347 at the "Why is evolution so controversial?" topic. That topic has a pretty good discussion going. Unfortunately that discussion has wandered very far from the original topic theme.

That message 347 writes:

Tanypteryx writes:

Minnemooseus writes:

Pretty damn bogus reply.

I can't see AT ALL how the deep ocean basin deposition model really has anything to do with any with the sediment deposition of the Grand Canyon area rocks. At best, MAYBE the bottom-most pre-Cambrian rocks that are now high grade metamorphics MIGHT have some sort of deep ocean origins. MAYBE.

Maybe you are right.

I assumed that the limestones and shales were from an ocean deposition and I thought the sandstones were from deposition closer to continental shelf. Obviously, the crossbedded sanddune deposits were deposited when it was dry land.

Can you tell me what the depositional environment was?

Faith writes:

The accumulations of sediment you can point to here and there are paltry little collections by comparison.

It seems to me that deposition happening in the present in the Pacific cannot be described as here and there or paltry on a timescale of millions of years.

thanks.

Now, some of the sediments of the Grand Canyon and are are of non-marine (above sea level) origin, but here I'm going to try to focus on depositions of on to/off of the continents sea transgressions/regressions.

A (the?) central focus of said are sedimentary facies and Walther's Law. I've tried in the past to find good Walther's Law references and graphics, and have never been real happy with what I've found. Go ahead and Google "Walther's Law" and see what you can find.

One (sort of) good starting point I did find is facies. Please also look at this.

Basically, Walther's Law is that, in sea transgressions/regressions, the coarsest clastic sediments (sand to become sandstone) are deposited nearest to the shoreline. As you go further from shore (and deeper) the clastics get progressively finer to silts (siltstone) and then clays (shale). As you get far enough from shore the clastic sediments become less and less until what sedimentation you have left happening is carbonate (limestone).

For a sea transgression, this results in a vertical sequence of upward fining - Sand at the bottom, carbonate at the top. For the sea regression, the order is opposite - Carbonate at the bottom, sand at the top.

The transgressive sequence:

ADDED BY EDIT - The above diagram is deceiving in that the silt, clay, and carbonate symbology implies that the bedding planes are parallel to the contacts. The bedding planes are horizontal or near horizontal. See the diagram in Percy's message 9 for a clearer (and better annotated) illustration. Also (for the non-geologist types), note that the diagram has a substantial vertical exaggeration (The vertical distances are actually small relative to the horizontal distances). - END OF ADDED BY EDIT

Source is "Please also look at this" link, above. There are also other diagrams and explanations at that site.

Well, not a prize winning PNT, but perhaps it will work to get geology discussion back into a geology topic.

Moose

Added by edit: Some maybe relevant messages at Dr. Adequate's "Introduction To Geology" topic:
Nearshore Sediments
Marine sediments
Turbidites

Edited by Minnemooseus, : "vertical sequence of upward fining" not "vertical sequence of upward coursing(sic)"

Edited by Minnemooseus, : Added by edit.

Edited by Minnemooseus, : ADDED BY EDIT.


Replies to this message:
 Message 3 by Tanypteryx, posted 04-18-2014 10:59 AM Minnemooseus has taken no action
 Message 4 by Faith, posted 04-19-2014 3:18 PM Minnemooseus has taken no action
 Message 12 by Dr Adequate, posted 04-21-2014 1:43 AM Minnemooseus has seen this message

  
Minnemooseus
Member
Posts: 3880
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 4.0


(1)
Message 21 of 533 (725495)
04-27-2014 7:36 PM
Reply to: Message 18 by Percy
04-26-2014 9:41 AM


Some limestone thoughts
Distances will be highly variable and dependent upon local conditions, but of course sand is deposited close to the coast (perhaps within a mile), mud is deposited anywhere from a hundred yards from shore to a few miles out, carbonate sediments (limestone) can be deposited in any warm shallow sea regardless of distance from a coast, and coccolith foram ooze means pelagic sediments, i.e., deep ocean sediments, that are high in calcium carbonates.

The old geologic cliche that I heard was "You get limestone because nothing was happening".

Basically, you get limestone deposition because no clastic sediments are making it there. As we have been saying, that's because it's a long way from shore. Another aspect, however, is whether there has been any area tectonic activity raising land areas. If so, then you have more hills and mountains, and more weathering and erosion to supply clastic sediments. No hills and mountains, then you get a reduced supply of clastics.

Per "deep ocean sediments" - Yes, what you say is true, to a point. But I remind all of the carbonate compensation depth concept:

quote:
Calcite compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite (calcium carbonate) lags behind the rate of solvation, such that no calcite is preserved. Aragonite compensation depth (hence ACD) describes the same behaviour in reference to aragonitic carbonates. Aragonite is more soluble than calcite, so the aragonite compensation depth is generally shallower than the calcite compensation depth.

For the non-mineralogists, both Aragonite and Calcite are CaCO3, only with different crystal structures.

The CCD is because of the odd(?) characteristic of CaCO3, in that it is more soluble in colder water (there are also other factors; See the above link for more), as opposed to most substances being less soluble in colder water. The bottom line is, in the deepest ocean depths, you don't get carbonate deposition. But that is irrelevant to shallow continental sea deposition.

quote:
Calcium carbonate is essentially insoluble in sea surface waters today.

I point out that the bulk of limestone deposition is thought to be from biogenic actions - The limestone is seashells etc., much of which is degraded into lime sand, silt, and mud. So, in a sense, even limestones are clastic rocks.

Moose


This message is a reply to:
 Message 18 by Percy, posted 04-26-2014 9:41 AM Percy has seen this message

  
Minnemooseus
Member
Posts: 3880
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 4.0


Message 311 of 533 (727153)
05-15-2014 10:38 PM
Reply to: Message 278 by Percy
05-15-2014 9:06 AM


Re: the great unconformity

I find it curious (for lack of a better term) that the Tapeats appears to be (and is) angularly unconformable in the left 2/3rds of the photo, but appears to be conformable in the right 1/3rd of the photo. It must be that the lower units must be striking* parallel to the unconformity exposure in the right 1/3rd.

Which goes to show you, that a wrong perspective can give you a very wrong image of what is there.

Moose

* Strike - The bearing (compass direction) of a horizontal line on a bedding plane, a fault plane, or some other planar structural feature.


This message is a reply to:
 Message 278 by Percy, posted 05-15-2014 9:06 AM Percy has replied

Replies to this message:
 Message 312 by edge, posted 05-15-2014 10:52 PM Minnemooseus has seen this message
 Message 314 by Percy, posted 05-16-2014 8:33 AM Minnemooseus has seen this message

  
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