Hi Ken, Yes, smectite clay minerals are characterisitic of bentonites, whhich result from volcanic ash deposited in an alkaline environment.  Certainly pumice and other relics of volcanic ash are also characterisitic. Thank you, Ken.

Hi Ken, I do not know about the presence of zeolites in the older volcanic deposits. Perhaps they have been converted to clays like the rest of the volcanic ash. I will be on the lookout for information on them . Best wishes, Paul.

Dear colleagues,

 I found smectite in ash tuffs (air fall tuffs) down to the Upper Carboniferous, but it was not very common in these beds and only preserved where its alteration was hampered by pore fluids such as oil (see figure). Zeolites in ash tuffs I know only from deposits as old as Cretaceous. As many of the Paleozoic rocks have already reached the low-grade stage of regional metamorphism I do not expect zeolites. The most successful approach can be taken using structures and textures. Even if most of the reactive SiO2 has been converted into the most stable minerals phase beta-quartz, the various textures related to devitrification, or bedding types are still visible and the only diagnostic features. Glass shards and some lapilli can be preserved, although transformed into quartz (the oldest one I know has been derived from an ash-lapilli tuff of early Ordovician age and compositional close to a quartz-keratophyric tuff). In Paleozoic rocks undergoing very-low grade to low-grade regional metamorphism even kaolinite disappears and shows up as pyrophyllite and the preservation potential of smectite goes down to almost nil. It depends upon the chemical composition but the stability field of reactive SiO2 and smectite is very narrow. Pumice and scoria may preserve their porous nature but not their original mineral phases. Perlites are converted into quartz but the onion-shell texture is well preserved down to the Upper Carboniferous and of diagnostic value.

Best regards

Harald G. Dill

Dear Harald, Thank you for your refined answer to my question.  I especially appreciate your comments on the preservation of zeolites.  In rhyolitic ash falls (tonsteins) of Pennsylvanian age there are a host of minerals that are preserved including euhedral beta quartz, as you mentioned, shard-like grains of beta quartz, sanidine, hexagons of  brookite (after ilmenite), rarely monazite in the brookite,  and euhedral zircon,  Rare earth element plots of the  tonsteins show a pronounced Eu anomaly.  The rock itself is normally a frint claystone. See Lyons et al., 1992, An Appalachian isochron: A kaolinized Carboniferouus air-fall volcanic-ash deposit (tonstein). Geol. Soc. America Bull., 104: 1515-1527..

Dear Paul,

your answer fully agrees with mine. All labile constituents such as glass were converted into the most stable modification beta-quartz. Kaolinite or mostly dickite is not anything out of the ordinary in tonsteins. They are well-known from the Coal Measures of the Ruhr District, but these Paleozoic rocks of Upper Carboniferous age did not reach the level of low-grade stage regional metamorphism which is common to the rocks undergoing the main structural deformation of  the Variscan Orogeny. In this case all kaolinite group modifications including nacrite and the more erratic tubular halloysite should have gone (> 400 to 450°C). The heavy minerals monazite and zircon can survive these conditions very easily and obviously are the sole host of LREE. Brookite is a little bit erratic and seems to me belonging to a later hydrothermal phase under slightly increased Eh, as the Fe was present in its trivalent stage.

Best regards

Harald

Dear Harald, Thank you for your comments.  Appalachian geologists have spent a lot of time looking for tonsteins beccause of their stratigraphic importance. We have found that in many cases there is more than one tonstein at the approximately the same stratigrpahic level..  We found this out by getting electron-probe data on glass inclusions mainly in volcanic quartz.that showed the pre-eruptive composition of the magma.  This was very surprising to me. The stratigraphers have considered all of them just one tonstein.

Dear Paul,

I feel that the tonstein stratigraphy offers much more and will unravel some more data as the EMP and other methods are applied. I am very much interested in this topic. It is a bit of my favourite projects called chemo- and minerostratigraphy.

Best regards

Harald

Dear Harald, I too am very iintnerested in tonstein stratigraphy. Yes, new chemical and mineralogical tools will help to decipher  tonstein stratigraphy that is much more complex than the early stratigraphers realized. Thank you for your input. Best wishes, Paul.

Dear Paul

You mention EMP analysis in the identification of indicative minerals but have you considered also the use of whole rock XRD analysis? We have used this for a variety of purposes. If you have the correct set-up this can be undertaken on a cut, or cut and polished, blocks (could be the off-cut from your thin section).

Best wishes

Jana 

Dear Jana, We have used whole-rock XRD analysis.  Also we have used the electron microprobe for determining the elemental chemistry of glass inclusions in volcanic quartz. This gives you the pre-eruptive chemistry of the magma and can be used to sort one eruption for another. Thanks very much for your input. Best wishes, Paul Lyons.

Dear Kenneth,

there are K-bentonites in Paleozoic rocks. This is correct. That is why I have always added the metamorphic conditions to the environment which I was talking about in my answer above. The best example is a simple comparison of the Central European Variscides and their marginal facies which went through their final metamorphic / kinematic disturbance during the Early/Late Carboniferous under mostly (very low) to low grade stage conditions. Contemporaneous Paleozoic sediments laid down in the marginal parts of the basin, close to the Baltic Shield were left almost unaffected by these thermal and kinematic processes and despite of their age look like epicontinental Mesozoic sediments of the Germanic Facies. Given a certain P/T regime of low-grade stage/ greenschist facies during orogenic processes does not allow the original smectites which have derived from devitrification to survive. Na-enriched smectite is replaced by phyllosilicates like paragonite and chlorite around 350°C/ 2 kbar. A relapse into smectite from muscovite/illite seems possible but this is a true epigenetic process after flushing with certain elements.

As far as the zeolites are concerned, the same holds true as to the temperature changes. I know a lot of occurrences of stilbite, laumontite .. in Paleozoic and even Precambrian rocks, but all of them resulted from younger processes mostly from low temperature hydrothermal processes < 300°C.

So your reference is correct, but smectite-group minerals and zeolites are rather sensitive to regional metamorphism and will not survive such T increases as exerted on the Paleozoic sediments. Heulandite is replaced by zeolites like laumontite at ca. 200 to 250°C and subsequently transformed into lawsonite.

The key question to all that is: Will we have to deal with Paleozoic platform-type sediments or rocks involved in orogenic disturbances ?

Best regards

Harald

Hi Ken, I asked the question to open discussion on how to recognize air-fall ash deposits in Paleozoic strata.  Air-fall ash beds have been called tonsteins, cinerites, and K-bentonites.  They are usually thin beds with a planar top and can be distributed over a large geographic area. Thus, they  are very important stratigraphdic markers. The original ash has been converted to clay, which is well-crystallized kaolinite under acidic conditions (tonsteins or cinerites) or mixed layer smectite-montmorillonite under alkaline condiitions (K-bentonites).  They contain relic minerals such as volcanic quuatz (sometimes euhedral beta quartz), sanidine, euhedral zircon, monazite, and secondary minerals such brookite (after ilmenite).  Electron microprobe analysis of the glass inclusons in volcanic quartz indicates that tonsteins are high-silica rhyolites.  Rare-earth element analysis indicates that all the tonsteins that we studied in the central Appalachian have a prounced negative Eu anomaly.  The well-crystallized kaolinite in tonsteins and cinerites give the rock the apperance of a flint claystone.

. 

I think ash has very high resistivity. In-situ geoelectric sounding test would detect layers at some conditions: layer thickness has to be some meters and depth is not so deep (some decades). Unfortunately conditions are not determined and advice, respectively, cannot be relevant.

Michael Ezersky

Dear colleagues,

if the term "tonstein" sensu Burger et al (1962), Guthorl et al. (1956) and Scheere (1958) had been mentioned in the question much of what I was talking about I would not have been referred to. It is a late Paleozoic volcaniclastic rock described from Coal Measures in the Variscan Foreland Depression in Germany, Belgium and the Netherlands. As it formed after the Sudetic deformation and was only subjected to the much lesser intensive Asturian disturbances all discussion about early Paleozoic rocks with low-grade stage metamorphism was not necessary. But who knows this ? It is a Paleozoic rock which paved the way from the orogeny into the platform setting. As such all what I was talking about in terms of the Upper Carboniferous is correct.The rest you can skip as it is unrelated to the "tonsteins" and only for those who are interested in a general approach.

I wished I had been confronted with a more focused question which could have enabled us to come to the point much faster.

Best regards

Harald

Thank you, Harald.  I only know of one report of where a tonstein has been traced into a bentonite.  You may know of others. Best wishes, Paul.

Hi Mikhail, Sounds like an interesting technique.  Unfortunately, most tonsteins and cinerites are less than 1 m thick.  Thanks you for  your input. Best wishes, Paul Lyons.

Ken Towe has indicated the presence of pumice is a way of recognizing air-fall volacanic ash deposits  Although very rare, this phenomenon has been reported in the cinerites of France (see Bouroz et al;. 1983, Review of the formation and evolution of pegtrographic markers in coal basins, Societe Geologique du Norld, Memoires, Tome XVI ( Pl. III,  Fig. 4). Thank you Ken. Best wishes, Paul.

Dear Paul,

There are a variety of approaches, but I found it interesting, as it tried to make the Estonian colleagues. Here is the link: http://www.kirj.ee/public/Estonian_Journal_of_Earth_Sciences/2014/issue_4/earth-2014-4-264-270.pdf

Best wishes, Victor podkovyrov

Dear Victor,, Thank you so much for this interesting reference (Tarmo et al., 2014, Estonian J. Earth Sciences, 63,4: 264-270. It shows that a thick Upper Ordovician bentonite can be correlated using trace amounts of Ti, Nb, Zr, and Th.  Correlation of bentonies is no easy matter and this approacch is very worhtwhile. Best wishes, Paul.Lyons.

Dear Paul

Correlation of Silurian bentonites in Wales has used a multimethod approach e.g.

 see:  https://www.researchgate.net/publication/268073484_Correlation_Silurian_K-_bentontites_from_Wales_and_the_Welsh_borderlands_using_trace_elements_and_isotope_analsyis?ev=prf_pub

Conference Paper Correlation Silurian K- bentontites from Wales and the Welsh...

Paul,

It depends how the ash bed is preserved (degree of alteration) - if perfectly preserved then all the usual field properties will hold (see attached paper Lowe 2011 Quat Geochron). But presumably highly unlikely to have that situation.

(1) One method might be to look for melt inclusions (glass) in host quartz crystals (Delano et al. ).

(2) I have also seen micrographs of pseudomorphs of pumice clasts or glass shards preserved in clayey material.

(3) Several studies of bentonites have analysed mineral chemistry, e.g. apatite, biotites

- Samson et al. 1995  used trace elements and Sr isotopes

- Huff 2008 used biotite gechemistry

(refs in Lowe 2011)

Regards

David Lowe

David, thank you for all the references.  My personal preference is electron microprobe microanalyis of glass inclusions because it gives you the pre-eruptive chemistry of the magma. Best wisihes, Paul.