Why is dolomite so common in the Silurian?

[Hapchazzard]

I've noticed that a huge majority of the North American Silurian system is composed of dolomite. During the Ordovician, it seems to me that dolomite formation was sparse, but during the Silurian there is a massive surge of it, and in the succeeding Devonian it seems to have returned to pre-Silurian levels again. What was the reason for this? Is this just a North American phenomenon, or was it worldwide?

[Ridgehiker]

Interesting question. You have an inquiring mind!

 

Your question depends on what do you mean by North America during Lower Paleozoic times?  Continental plates shift and 'Laurentia' and the position of oceans, plates, etc were not as they are today.  

 

Today? You need define what formations you are referring to. The geology of North America is quite diverse. Also, different parts of North America have quite diverse geologic histories.   They had different influences from mountain building, ocean formation, etc.  

 

Anyways, 'North America' isnt a geologic unit.  For example Nova Scotia was attached to Morocco. parts of Western Canada to Asia. Later during Pangea the faunal  province was north south across today's Arctic Ocean.  East and West North 

America were complete separate faunal provinces.  There isnt anything to unite a common trend that distinguishes greater North American geology

 

 

[ynot]

The difference between dolomite and limestone is the presence of manganese in the rock, to wit....

(From wikipedia.)

Dolomite (pronunciation: /ˈdɒləmaɪt/) is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite.

 

Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and mollusks. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).

[Ridgehiker]

11 hours ago, ynot said:

The difference between dolomite and limestone is the presence of manganese in the rock, to wit....

(From wikipedia.)

Dolomite (pronunciation: /ˈdɒləmaɪt/) is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite.

 

Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and mollusks. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).

 

In geology the word 'dolomite' can be confusing and research papers will  sometimes write of 'dolomitic limestone'  as just a general description and no real tests are made. We never got too particular about the composition of matrix in studying corals, brachs, etc.  Sometimes sedimentary layers and matrix is  analyzed but in most cases it was educated guesstimates based on appearance. 

 

The irony is that some of the best outward appearing fossils have the least scientific value. If there is a lot of silicification, the speciman can be complete but crystals often replace the minutia of internal structural details.

[doushantuo]

Let me just post some JKW here:

 

dol

 

a fairly recent piece with some musings on the "dolomite problem":

 

 

 

dolomital_2015.pdf

[doushantuo]

a lesser known dolomite model(lesser known than the Machel and "DORAG" ones):

 

wrightdol2000(1).pdf

[doushantuo]

Less obvious origins:

 

Roberts gsamicromethDolomite.pdf

[doushantuo]

carbonates in general(GOOD comprehensive review):

 

car

[Hapchazzard]

First off - thanks for all of your replies!

 

7 hours ago, Canadawest said:

Interesting question. You have an inquiring mind!

 

Your question depends on what do you mean by North America during Lower Paleozoic times?  Continental plates shift and 'Laurentia' and the position of oceans, plates, etc were not as they are today.  

 

Today? You need define what formations you are referring to. The geology of North America is quite diverse. Also, different parts of North America have quite diverse geologic histories.   They had different influences from mountain building, ocean formation, etc.  

 

Anyways, 'North America' isnt a geologic unit.  For example Nova Scotia was attached to Morocco. parts of Western Canada to Asia. Later during Pangea the faunal  province was north south across today's Arctic Ocean.  East and West North 

America were complete separate faunal provinces.  There isnt anything to unite a common trend that distinguishes greater North American geology

 

 

 

I mean rocks from the (old) coast of the Laurentian craton - e.g. the Roberts Mountain formation in the Rockies and several formations from the Cincinnati arch. There are a lot of formations from this time, but in general I'm referring to those with the aforementioned characteristics.

 

6 hours ago, ynot said:

The difference between dolomite and limestone is the presence of manganese in the rock, to wit....

(From wikipedia.)

Dolomite (pronunciation: /ˈdɒləmaɪt/) is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite.

 

Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such as coral, forams and mollusks. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3).

 

Yes, and I'm really curious as to why Silurian coasts were apparently so Mg-rich compared to other periods.

 

3 hours ago, doushantuo said:

Let me just post some JKW here:

 

dol

 

a fairly recent piece with some musings on the "dolomite problem":

 

 

 

dolomital_2015.pdf ((and several other pieces))

 

Wow, thanks for all the literature! I was looking high and low across the internet for it, and couldn't really find something like this myself. I'll make sure to read through all of it. 

[doushantuo]

err ,that's just a small fraction of dolomite literature.

edit

Correction:very small fraction

Dolomite was pretty much widespread in the Neoproterozoic as well.

 

 

[ynot]

4 hours ago, Hapchazzard said:

Yes, and I'm really curious as to why Silurian coasts were apparently so Mg-rich compared to other periods.

There could be a multitude of reasons. There may have been more free manganese in the environment or it could have been something in the lithification of the rock. Or as canadawest said some of the rock labeled as dolomite may be limestone, so misidentification could also be a reason that dolomite seems to be more prevalent in some areas.

I think that doushantuo's papers and diagram cover it better than I can.

[Ludwigia]

This is a very interesting subject, and I don't profess to knowing all that much about it, but I do know that there were an awful lot of different factors involved. There certainly were times in the geological record where there seems to have been more magnesium available in the oceans to form dolomitic sediments than at other times and I believe that had to do with the amount of vulcanic release on the ocean floor at the time.

[ashcraft]

If you want to see Ordovician dolomite, come to eastern Missouri.  Much of the dolomite forms from replacement.  The original rock is calcium carbonate, then as magnesium bearing water percolates through it, the magnesium is exchanged for the calcium, as it is more reactive.  this destroys many fossils in the process.

 

Not fond of the grey dolostones of southeast Missouri

Brent Ashcraft

[Ludwigia]

That dolostone sure can be nasty to fossils.

[EMP]

It varies from region to region, since here in the Appalachians most of the Silurian rocks are thick shales and sandstones.

 

I think it's a combination of factors that included a more tropical location, low mountain building (so less disturbance of the sea bed), increased erosion from the Taconic Mountains (which may have released more magnesium?), and abundance of shelled life to help accumulate calcium carbonate deposits to be later turned into dolostone.

 

Aside from that I have no real clue.