K-Bentonite For Dummies Like Me

[Bev]

So, I think I finally recognized a K-bentonite layer in a rock formation for sure - it's an orange colored clay in this instance. And I have been trying to figure out its significance for me as a lay person fossil hunter. I've been on the internet attempting this research, but my poor little pea brain is not understanding what they are saying...

K-bentonite is evidence of a mass volcanic eruption?

K-bentonite looks different in different areas because of the interaction with the local rock deposits and their chemical compositions?

K-bentonite is important to geology because it can be used to date rock formations?

K-bentonite is an indicator of a mass extinction event?

And yes I did do a search in Q&A for K-bentonite, but it didn't answer my questions.

Thanks for putting up with a dummy like me!

[painshill]

You’re no dummy Bev but maybe you’ve got the wires a little crossed here. The simple answers to your questions (or, at least my answers) are:

K-bentonite is evidence of a mass volcanic eruption?

Not exactly. Bentonite is an impure clay. It’s composed mainly of the mineral montmorillonite – a highly absorbent material that was widely used in cat-litter (because of its absorbent properties) until artificial materials began to take over. It’s a silicate and may be sodium-rich (Na), Calcium-rich (Ca) or potassium rich (K). The potassium-rich version is derived from illite and most usually the result of aqueous alteration of volcanic ash deposits. Those deposits may have accumulated either as an ash blanket from direct atmospheric fall or indirectly as wind-blown or water-transported sediments at some later time(s). So, yes, K-bentonite has its roots in volcanic eruptions but not necessarily a “mass” eruption nor necessarily a single geological event at a specific time.

K-bentonite looks different in different areas because of the interaction with the local rock deposits and their chemical compositions?

Yes. And also because exposures are prone to weathering which changes their composition and appearance. The common weathering colours for K-bentonite are orange through to yellow.

K-bentonite is important to geology because it can be used to date rock formations?

Yes and no. It’s not really any more useful than any other rocks which have an igneous history. Every time a rock (or rather the minerals within it) undergo a significant thermal event (such as melting) then – in a sense – the geological clock is re-set in a manner that’s really helpful for determination of the date for that event. The most useful and accurate information comes from rocks that are essentially unaltered subsequent to that event. So, for example, future geologists will be able to precisely date the 1980 Mount St Helens eruption from virgin areas of deeply compacted ash-fall, but less precisely so from altered and weathered deposits.

K-bentonite is an indicator of a mass extinction event?

No, it isn’t. K-bentonite layers of volcanic origin occur everywhere in the world with different depositional depths and of different ages. All they tell us is that volcanic ash went up into the air somewhere and came down again somewhere. The depth of the layers may or may not be an indication of how severe and prolonged a volcanic event might have been, but secondary deposition can easily destroy that relationship. What you may be thinking of is a specific K-bentonite layer of Ordovician age. That layer is found across several continents, is rather thick in some formations and has the same geological age in those places where it is found – roughly 448 to 455 million years old give or take a couple of million years.

That broadly corresponds to the date for the Ordovician-Silurian extinction event at roughly 443.4 (440 to 450) million years ago when about half of the faunal genera living in the ocean disappeared. There were no land animals of course and terrestrial life was limited to primitive moss-like plants just beginning to establish themselves out of semi-aquatic environments. But the volcanism had actually ceased a little while before the extinction and we don’t actually know what caused it. Normally volcanism raises the level of carbon dioxide in the atmosphere and the temperature rises from the greenhouse effect (that’s what had been happening). But carbon dioxide levels actually fell in the final stage of the Ordovician and there was a mini ice age - probably because carbon dioxide was pulled from the atmosphere in large quantities by weathering of the uplift that became the Appalachian Mountains. Essentially those rocks absorbed carbon dioxide as they weathered. Long-lived atmospheric ash clouds would also cause cooling if they were extensive enough but we don’t have the answer to that question.

At around the same time there may also have been a gamma-ray burst from a hypernova in the Milky Way that could have stripped Earth of much its protective ozone layer. There had also been a meteoritic bombardment of Earth (the “Ordovician meteor event”) arising from the break-up of an L-Chondrite asteroid around 470 million years ago. Although there were near-simultaneous impacts of moderate size across what is now North America, Canada and Northern Europe they were probably too early and too small to have influenced the extinction.

[Bev]

The cut on 44 near Spring Grove where I found the K-bentonite.

Continued...

[Bev]

Closeup of the segment of that cut.

Continued...

[Bev]

Closeup of what I believe is the K-bentonite layer.

[painshill]

Just a reminder than “on 44 near Spring Grove” doesn’t mean as much to everyone else as it may to you.

Nevertheless, I believe I know the area you are referring to and – although Ordovician – it has nothing at all to do with the Ordovician-Silurian (or any other) extinction event. I assume the location is the exposure of the Decorah Formation at the Spring Grove-Roverud Quarry in Minnesota (or someplace thereabouts).

The K-bentonite layer there is from prolonged secondary deposition. Around 540 million years ago, a string of volcanic islands had formed offshore on the eastern side of what is now the North American continent. Large amounts of volcanic ash were carried by prevailing winds and deposited in the shallow inland (epeiric) North American sea to the west of those islands. The sediments ultimately consolidated into a hydrated clay layer of K-bentonite. So, although the deposits are late Ordovician, the ash that represents their origin was generated by volcanism from the beginning of the Cambrian onwards.

The geology of this period is known as the “Taconian orogeny” and the ash production stopped around 440 million years ago at the end of the Ordovician when the volcanic islands collided with the continental land mass to create a huge chain of mountains. All that’s left of those (as a result of millions of years of erosion) are the hills of the Eastern Piedmont.

Edited June 29, 2014 by painshill

[Bev]

WOW, you are just sooo smart! You always AMAZE ME!!!

[squali]

Bentonite ( Montmorillinite) is an amazing geologic product that is also used as a component of geosynthetic clay liners to prevent the migration of liquids including water and hydrocarbons.

[Herb]

also known as "drillers mud" when added to water to seal soil drilling holes from leaking water.

[RichW9090]

Montmorillinite is also a key component in the mysterious blue pigment used by the Mayans in painting some of their pottery and wall murals. When combined with the sap of a certain tree (Guiacan) it turns blue.

Rich

[painshill]

The “Maya blue” pigment did indeed contain the clay montmorillonite, but as a minor component together with other minor clay constituents probably present as impurities. The formulation and process are now better understood as a result of modern analyses and the intentional variations in components seem to have been determined by whether it was used for body paint, wall paint, pottery decoration, fabric dying or whatever together with local accessibility/availability of materials.

The principal clay component for Maya blue is actually palygorskite (known to the Maya as “sak lu’um”) which also has remarkable binding properties. It acts primarily as a “fulling agent” to improve penetration of the dye into fabrics and also contributes to the remarkable stability of the colour over time when used on murals or for pottery. The Maya also used it as a temper for pottery and for medicinal purposes (and still do today).

The blue colouration derives from indigo dyes extracted from the leaves of the añil plant (Indigofera suffruticosa). This extract from: “The first direct evidence of pre-columbian sources of palygorskite for Maya Blue” (Arnold, Bohor, Neff, Feinman, Williams, Dussubieux & Bishop; 1981)”:

“Up until the 1960s, the composition of Maya Blue was a mystery, and details of its production unknown. Then, palygorskite (then called attapulgite) was found in the pigment using Xray Diffraction (Gettens, 1962). That same year Shepard (1962) and Shepard and Gottlieb (1962) suggested that the pigment was a clay organic complex, and indeed, five years later, the remaining component was identified as the organic colorant, indigo (Kleber et al., 1967).”

As an aside, in Georgia, palygorskite is known as attapulgite since it is extensively mined in the vicinity of the city of Attapulgus. There is great controversy over whether the Maya sourced the material from Georgia (the analytical and other evidence doesn’t support that) and even greater controversy over the claimed existence (by Richard Thornton et al.) of Mayan ruins at the Track Rock terrace complex in the mountains of Georgia. Criticising Thornton’s claims (which he inferred were supported by archaeological work), Jannie Loubser (the archaeologist who surveyed the site in 2000) responded: “Please read the article that I co-authored with Douglas Frink on our excavations at the stone feature complex near Track Rock Gap and see that nowhere did we mention the Maya”. Mark Williams added: “I am the archaeologist [Mark Williams] mentioned in this article [by Thornton]. This is total and complete bunk. There is no evidence of Maya in Georgia. Move along now”.

[RichW9090]

Take a look at this, Painshill:

http://www.jstor.org/discover/10.2307/30247023?uid=3739552&uid=2&uid=4&uid=3739256&sid=21104523759043

and this:

http://www.jstor.org/discover/10.2307/30247024?uid=3739552&uid=2&uid=4&uid=3739256&sid=21104523759043

Rich

[painshill]

Thanks Rich. Indeed the “Seri blue” pigment used principally for face-painting is as you described but relates only to the Seri people indigenous to Sonora, Mexico. It’s not the same as the remarkably stable pigment used for more diverse purposes by the Maya, known as “Maya blue”.

Both of the Seri-related papers were published in 1964, so their respective authors would have been aware that Maya blue was palygorskite (attapulgite)-based rather than montmorillonite-based (from Gettens Xray diffraction work in 1962). Peirce acknowledges that fact in the second paper.

The speculation about the other components of Maya blue (principally by Peirce) would however not have revolved around indigo as a component since Kleber et al. didn’t conclusively identify it until 3 years after Peirce published.

[Bev]

The big extinction event at the end of the Ordovician, if it is not related to K-bentonite, what would I be looking for in the rocks that would indicate that event?

Does anyone know what that event was fairly conclusively?

Or, is there no record of the event in the rock, just in the mass extinction left in its wake? I've read speculation that it was a sudden freeze which the rock would not show. Thoughts?

[painshill]

The trigger for the extinction was probably the movement of Gondwana into the south polar region leading to global cooling, a mini-ice-age and consequent fall in sea levels as water turned to ice. The extensive volcanism preceding the event would be expected to create higher levels of carbon dioxide in the atmosphere and lead to greenhouse warming, but it didn’t. It seems to have been offset (and overtaken) by huge amounts of carbon dioxide being absorbed through the weathering of the Appalachian Mountains. Lower carbon dioxide levels in the atmosphere then contributed to the cooling. If there was enough ash in the atmosphere for long enough (we don’t know the answer to that) then this would have further contributed to cooling, as well as interrupting plant life dependent on photosynthesis. The effect of a postulated gamma-ray burst from the Milky Way that I mentioned earlier can’t be ruled out, but the evidence is not strong.

So, in simple terms:

- There is a time-related association between some specific thick deposits of K-bentonite volcanic ash (in several places across the Northern hemisphere) and the Ordovician-Silurian extinction.

- Such deposits – although evidence of large-scale volcanism – did not actually cause the extinction event. At most, the ash clouds may have contributed to it.

- The particular layer of K-bentonite you are exploring overlaps into that time period but represents a mixed accumulation of ash as secondary deposits which includes material from much older periods.

As with most extinction events, the evidence in the rocks (unless you are performing sophisticated petrological testing) generally comes from comparisons of fossil abundance and diversity in strata of known ages… ie before and after the extinction event. In this case, what the fossil evidence shows is that around 100 marine organism families representing almost half of the genera that existed at the time pretty much disappeared in a very short space of time (geologically speaking).

That is, these fauna are abundant in strata than can be dated as preceding the 450 to 440 mya time-frame and decimated or absent from strata after that period. Bryozoa and brachiopods were particularly heavily hit and many graptolites, conodonts (eel-like chordates) and trilobites too. The evidence also suggests that the extinction happened in two cumulative bursts, separated by about a million years.

Edited July 25, 2014 by painshill