shale/mudstone/claystone: what differences?
Steve Bastable
Hello Sebastian Luening, in "shale/mudstone/claystone: what differences?" you
worte...
>Can anybody tell me the (subtle) differences between shale,
>(non-carbonate-) mudstone and claystone? How can I differentiate between
>carbonate mudstones (sensu DUNHAM) and non-carbonate mudstones in
>literature descriptions?
>Sebastian Luening
>Univ. Bremen
>E-mail: lue...@zfn.uni-bremen.de
The following definitions are from "Sedimentary Petrology: An
Introduction" (Tucker, 1981 Blackwell Scientific Publications):
Mudstone - The indurated equivalent of mud. A blocky, non-fissile rock.
Shale - As above BUT usually laminated and fissile.
Argillite - A more indurated mudrock.
Claystone - A sedimentary rock of clay-grade material only.
Siltstone - Contains more silt grade particles than clay.
Marl - Calcareous mudrock.
A table on page 120 (fig 4.30) classifies limestones. A definition of
a carbonate mudstone from this table would be a rock with the following
characteristics:
Original components not organically bound during composition.
Less than 10% of the allochems > 2mm diameter.
Containing carbonate mud (particles less than 0.003 mm diameter).
Mud-supported
Less than 10% grains.
The mudstone definition grades through to wackestone, packstone.... Eg
a wackestone is as above BUT with more than 10% grains.
Appologies for taking this direct(ish) from the text but I can never
remember it!!!!!
Cheers,
Steve.
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"It Is Better To Have Planned And Cancelled Than Never To Have
Planned At All"
Louis Hissink Jnr.
>
> In our book "Origin of Sedimentary Rocks" we tried to popularize the idea
> that Mudrock was the general term, shale if it was fissile and siltstone,
> mudstone, claystones for nonfissile mudrocks with different proportional
> of silt and clay. Seemed like a good idea at the time.
> --
> Gerry Middleton
> Department of Geology, McMaster University
> Tel: (905) 525-9140 ext 24187 FAX 522-3141
That suggest that shale is almost a metamorphic rock, and I stress almost,
but it depends on where diagenesis stops and metamorphism starts.
Either way, surely by now we have an authoritative standard nomenclature
for these rocks ?
Therefore if shale is fissile, then claystone/mudstone could be called generic terms,
and shale a term for a more lithified state.
Louis Hissink(hiss...@tmx.com.au)
Gerard Middleton
Fissility does not imply metamorphism: even some recent sediments that
have never been very deeply buried are fissile. On the origin of
fissilty (generally diagenetic) I refer you once again to our (now
outdated) textbook ("Origin of Sedimentary Rocks").
As for "authoritative definitions" what do you want? Rules laid down by
an international committee? Some people regard the AGI Glossary as
"authoritative" -- and indeed it does give a rather good description of
the way the term has been used, and stresses that it is
"...a thinly laminated or fissile claystone, siltstone or mudstone."
Andrew Phillips
Dear Gerry,
This is not a serious comment, just a tongue-in-cheek observation :-).
Call them
all Aleurolites, it seems to work quite well in Russia, for all 2,137
varieties of them.
Best wishes,
Andrew Phillips