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Author Topic:   Salt of the Earth (on salt domes and beds)
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 1 of 81 (433000)
11-09-2007 1:54 PM


I would like to get some of the geologists that come here to EvC to educate me on Salt Domes and Salt Glaciers.
Where are they found?
How big are they?
I'm sure that I will have other questions as we go along, but we can start there.
Just for my benefit, can we start with Spindletop?
Edited by jar, : add to sub-title

Aslan is not a Tame Lion

  
AdminPD
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Message 2 of 81 (433014)
11-09-2007 3:06 PM


Thread moved here from the Proposed New Topics forum.

  
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 3 of 81 (433223)
11-10-2007 5:10 PM


bump looking for geologists
Any of our geologists out there?

Aslan is not a Tame Lion

Replies to this message:
 Message 4 by jar, posted 11-12-2007 7:10 PM jar has not replied

  
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 4 of 81 (433686)
11-12-2007 7:10 PM
Reply to: Message 3 by jar
11-10-2007 5:10 PM


Re: bump looking for geologists
No geologists home?

Aslan is not a Tame Lion

This message is a reply to:
 Message 3 by jar, posted 11-10-2007 5:10 PM jar has not replied

  
IrishRockhound
Member (Idle past 4426 days)
Posts: 569
From: Ireland
Joined: 05-19-2003


Message 5 of 81 (433708)
11-12-2007 7:32 PM


Ok, I'll start. Salt domes and salt glaciers. They're essentially the same thing.
From wikipedia:
"A salt dome is formed when a thick bed of evaporite minerals (mainly salt, or halite) found at depth intrudes vertically into surrounding rock strata. The salt that forms these domes was deposited within restricted marine basins. Due to restricted flow of water into a basin, evaporation occurs resulting in the precipitation of salts from solution, depositing evaporites. It is recognised that a single evaporation event is rarely enough to produce the vast quantities of salt found in evaporite deposits indicating that a sustained period of episodic flooding and evaportation of the basin must occur, as can be seen from the example of the Mediterranean Messinian salinity crisis."
Ok so, pretty much what this means is that a salt dome starts as a salt flat, which is produced by repeated floods followed by evaporation, which causes salts in the water to precipitate and form sediments. So many little floods leave pools and expanses of standing water which evaporate instead of draining away, and they leave a crust of salt. Many crusts build up into a layer of evaporite deposits, like any other sedimentary rock.
"Over time, the salt is covered with sediment and becomes buried. Since the density of salt is generally less than that of surrounding material, it has a tendency to move upward toward the surface, forming large bulbous domes, diapirs , sheets, pillars and other structures as it rises. If the rising salt diapir breaches the surface, it can become a flowing salt glacier. In cross section, these large domes may be anywhere from 1 to 10 kilometers across and extend as far down as 6.5 kilometers.
One example of an island formed by a salt dome is Avery Island in Louisiana, except for the fact that at present ocean levels, it's no longer surrounded by the sea but it is surrounded by bayous on all sides, so it technically remains an island."
So - if the low density evaporite deposits are covered with other, denser sediment, they will tend to rise to the surface like a cork floating in water. This produces the salt domes and other structures as the salt bulges into the overlying rock. The salt can deform and break the strata above it, and can form oil and gas reservoirs as they are trapped under the impermeable salt layer.
For reference: this is a picture of a geological cross-section of the Northern Basin in Germany, showing the salt domes in blue bulging into the upper strata.
File:Salt dome hg.png - Wikipedia
In the US the Spindletop oil field is an example of an oil reservoir that collected under a salt dome. Unfortunately I can't find a geological map of it online.
Edited by IrishRockhound, : fixed image link

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jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 6 of 81 (433913)
11-13-2007 3:28 PM
Reply to: Message 5 by IrishRockhound
11-12-2007 7:32 PM


on evaporation
Ok so, pretty much what this means is that a salt dome starts as a salt flat, which is produced by repeated floods followed by evaporation, which causes salts in the water to precipitate and form sediments.
It would seem to me as a non-geologist, that there must be slightly more there, so please excuse me if I seem dull or uninformed.
First, to get the conditions you describe, it would appear that the water would have to be pretty much enclosed, not flowing, to allow the evaporation to lay down the layer of salt.
Is that correct?
Second, since we are precipitating salt out of the water, the salt layer would seem to be less than whatever the depth of the water was originally.
Is that correct?
So based on those two factors, it would seem that several things are needed:
  • we need an enclosed body of water, a lake or inland sea.
  • we need any outflow from the lake to be less than or equal to any inflow.
  • the trapped water must stay relatively undisturbed long enough for the water to evaporate.
Is that correct?

Aslan is not a Tame Lion

This message is a reply to:
 Message 5 by IrishRockhound, posted 11-12-2007 7:32 PM IrishRockhound has not replied

Replies to this message:
 Message 7 by The Matt, posted 11-13-2007 6:28 PM jar has replied

  
The Matt
Member (Idle past 5532 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 7 of 81 (433949)
11-13-2007 6:28 PM
Reply to: Message 6 by jar
11-13-2007 3:28 PM


Re: on evaporation
First, to get the conditions you describe, it would appear that the water would have to be pretty much enclosed, not flowing, to allow the evaporation to lay down the layer of salt.
Is that correct?
Yup. Not always enclosed entirely, but in open ocean you don't get evapourites.
Second, since we are precipitating salt out of the water, the salt layer would seem to be less than whatever the depth of the water was originally.
Is that correct?
Yes for an individual layer, however you can get repeated inundation and evaporation occurring at the same place, so you might end up with a sequence much thicker than the maximum depth of water at any one time.
So based on those two factors, it would seem that several things are needed:
* we need an enclosed body of water, a lake or inland sea.
You can also get salt deposits in:
-Coastal areas that are periodically flooded but are usually separated from the sea.
-areas where groundwater upwells and evaporates.
-semi-enclosed marine basins in arid environments, as in those that are separated from open water by a shallow strait or rock ledge. Water becomes denser as salinity increases so when evapouration occurs, the more saline remaining water sinks and you get a very saline water body trapped behind the barrier that encloses the basin.
So I'd say you need an enclosed or semi-enclosed body of water.
* we need any outflow from the lake to be less than or equal to any inflow.
* the trapped water must stay relatively undisturbed long enough for the water to evaporate.
I'm not 100% sure what you mean here. I'd just say you need losses by evaporation to be equal to or greater than water inputs (as even fresh water will contain some dissolved salts).

This message is a reply to:
 Message 6 by jar, posted 11-13-2007 3:28 PM jar has replied

Replies to this message:
 Message 8 by jar, posted 11-13-2007 6:43 PM The Matt has replied

  
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 8 of 81 (433951)
11-13-2007 6:43 PM
Reply to: Message 7 by The Matt
11-13-2007 6:28 PM


Re: on evaporation
Yes for an individual layer, however you can get repeated inundation and evaporation occurring at the same place, so you might end up with a sequence much thicker than the maximum depth of water at any one time.
But will those show as layers?
I'm not 100% sure what you mean here. I'd just say you need losses by evaporation to be equal to or greater than water inputs (as even fresh water will contain some dissolved salts).
Okay, that is certainly clearer, evaporation greater than inputs. That was kinda what I was trying (not to effectively) to ask.
So I'd say you need an enclosed or semi-enclosed body of water.
Okay. But large open bodies such as an ocean or flowing water like a river won't lay down salt beds.
Is that correct?
So the next question is if it is laid down in layers, what is the likelihood of there being deposits on top of a salt layer?

Aslan is not a Tame Lion

This message is a reply to:
 Message 7 by The Matt, posted 11-13-2007 6:28 PM The Matt has replied

Replies to this message:
 Message 9 by The Matt, posted 11-14-2007 2:28 PM jar has replied

  
The Matt
Member (Idle past 5532 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 9 of 81 (434103)
11-14-2007 2:28 PM
Reply to: Message 8 by jar
11-13-2007 6:43 PM


Re: on evaporation
quote:
But will those show as layers?
In an ideal situation you get very clear evidence of layering. Salts precipitate out in order from least soluble to most soluble. Calcite first, followed by gypsum and anhydrite, halite, and last potassium/magnesium salt. In a pool that dries completely, in its deepest parts you will see all of these salts in that order (with calcite deepest) and be able to count how many inundations/dryings there have been. Things probably won't be that clear in most deposits though. Only the deepest parts (the last remaining puddles) will accumulate the potassium and magnesium salts, shallower areas left high and dry long before they begin to precipitate. I am not sure if it is possible to differentiate between layers in an area that only ever accumulates calcite- you'd have to ask someone more in the know. I'm also not sure about semi-enclosed marine basins.
quote:
Okay. But large open bodies such as an ocean or flowing water like a river won't lay down salt beds
You're right, they won't.
quote:
So the next question is if it is laid down in layers, what is the likelihood of there being deposits on top of a salt layer?
It could happen quite easily, but likewise I guess they could become dissolved again and leave no trace of their existence. I don't know too much about this so can't say much more with any authority, but my guess is that they'd stand the best chance of survival if covered by a relatively impermeable layer like a clay or soil that would restrict water movement.

This message is a reply to:
 Message 8 by jar, posted 11-13-2007 6:43 PM jar has replied

Replies to this message:
 Message 10 by jar, posted 11-14-2007 3:13 PM The Matt has replied

  
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 10 of 81 (434109)
11-14-2007 3:13 PM
Reply to: Message 9 by The Matt
11-14-2007 2:28 PM


Re: on evaporation
Okay, so far. Thanks for keeping it simple enough for this old mind.
Now I once visited Detroit. Under the city is an old salt mine. The salt bed is hundreds and hundreds of feet thick. The salt bed actually covers something like 170,000 square miles, underlying parts of Michigan, Ontario, Ohio, Pennsylvania, New York and West Virginia.
So according to you, the way the salt bed was created was that it was an enclosed body of water where the water evaporated leaving salt.
I found a few pictures from there showing the some of the salt:
So correct me if I am wrong.
Sticking just to the one salt bed, the one lying under Michigan, Ontario, Ohio, Pennsylvania, New York and West Virginia, it appears that what we see is a fairly long series of some enclosed area, inland sea, being flooded, then evaporating, drying up and depositing the salt, only to be inundated again and repeating the process over and over.
It does appear to be individual layers as well and that there seems to be repetitions of materials.
Is that a reasonable summary?

Aslan is not a Tame Lion

This message is a reply to:
 Message 9 by The Matt, posted 11-14-2007 2:28 PM The Matt has replied

Replies to this message:
 Message 11 by The Matt, posted 11-14-2007 5:14 PM jar has replied

  
The Matt
Member (Idle past 5532 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 11 of 81 (434150)
11-14-2007 5:14 PM
Reply to: Message 10 by jar
11-14-2007 3:13 PM


Re: on evaporation
I'm not familiar with the deposit myself, but a quick google has found this:
quote:
The Michigan Formation, exposed in abandoned gypsum mines around Grand Rapids, Kent County, consists of cyclically layered, thick units of gypsum, and thin beds of shale, dolomitic sandstones/siltstones, and detritus-rich dolomite. Based on the presence of mud cracks, ripple marks, angular to subrounded sand grains, and detrital, terrestrial plant debris, the depositional environment is interpreted to have been a shallow, marginal sea with periodic input of floodwaters from streams
http://gsa.confex.com/...4AM/finalprogram/abstract_75403.htm
So your interpretation sounds about right.
Edited by The Matt, : No reason given.

This message is a reply to:
 Message 10 by jar, posted 11-14-2007 3:13 PM jar has replied

Replies to this message:
 Message 12 by jar, posted 11-14-2007 5:59 PM The Matt has replied

  
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 12 of 81 (434164)
11-14-2007 5:59 PM
Reply to: Message 11 by The Matt
11-14-2007 5:14 PM


on area
Great. That makes me feel better.
Now again it seems this is a pretty large area, 170,000 square miles, so it would take a while to evaporate all the water of that large an area. Is that a reasonable assumption?
Edited by jar, : fix subtitle

Aslan is not a Tame Lion

This message is a reply to:
 Message 11 by The Matt, posted 11-14-2007 5:14 PM The Matt has replied

Replies to this message:
 Message 13 by The Matt, posted 11-14-2007 6:08 PM jar has replied

  
The Matt
Member (Idle past 5532 days)
Posts: 99
From: U.K.
Joined: 06-07-2007


Message 13 of 81 (434165)
11-14-2007 6:08 PM
Reply to: Message 12 by jar
11-14-2007 5:59 PM


Re: on area
Why does the area make a difference? It's the surface area to volume ratio of the water body and the climate that's important.

This message is a reply to:
 Message 12 by jar, posted 11-14-2007 5:59 PM jar has replied

Replies to this message:
 Message 14 by jar, posted 11-14-2007 6:15 PM The Matt has not replied

  
jar
Member (Idle past 384 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 14 of 81 (434166)
11-14-2007 6:15 PM
Reply to: Message 13 by The Matt
11-14-2007 6:08 PM


Re: on area
Because it is the surface area. The area of deposition would I imagine correspond fairly closely to the surface area. As to volume, we have only an indirect way to estimate that, the depth would I imagine be something greater than the thickness of a given layer of salt.
I wonder if it would be possible to work backwards from that to some initial "MINIMUM" depth? There must be some limit to how super saturated water at one atmosphere pressure could be? Could that then give us an estimate of the minimum amount of water that had to be present?

Aslan is not a Tame Lion

This message is a reply to:
 Message 13 by The Matt, posted 11-14-2007 6:08 PM The Matt has not replied

Replies to this message:
 Message 16 by EighteenDelta, posted 11-14-2007 6:53 PM jar has replied

  
Coragyps
Member (Idle past 725 days)
Posts: 5553
From: Snyder, Texas, USA
Joined: 11-12-2002


Message 15 of 81 (434172)
11-14-2007 6:52 PM


FWIW, you can see one of these evaporite pans forming not far from here. Cedar Lake (sic) is a big white expanse of salt/gypsum near Seminole, Texas that was named probably by a land developer in a real wet year. It's only a couple of dozen square miles in extent, though. I have no clue how thick the salts are.

Replies to this message:
 Message 23 by jar, posted 11-14-2007 11:07 PM Coragyps has replied

  
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