[Rockhounds] Re: Pennsylvania fern fossils --Llewellyn Fmn

[Mr EMan]

You asked how you get "shale-like" fossils next to a coal seam and that is fairly much how the are always found.  If you meant "slate-like" fossils that too is consistent with the Llewellyn but challenges a common belief about anthracite deposits. 

The St Clair fern deposits are within the Llewellyn Formation which also contains a full range of sedimentary rock types. Shale is the appropriate term for the fern bearing matrix and is wholly consistent with coal sequences: delta deposited highly graded milky quartz pebbles which become conglomerates, peat bogs which are coal-forming layers, sandy beaches becoming sandstones, mud-bottomed offshore ledges yielding fossil laden slates and deep water siltgoing to siltstones. Fossils are found in all the above but preservation is best in the very fine-grained slates.  

The fact that mica grains are rarely found with macro fossils suggests that the slilt particle sizes the ferns formed in were carried even farther out off shore by remaining in suspension longer.  Ergo the ferns must also have sloshed around and floated for sometime before sinking.  yet I've never found a bent stem and they should be common if it was violent storms which ripped them from the bogs and carried them out to sea. There are many small plants, seed pods, and even insect wings in the St Clair location.  Larger limbs and broad long leaves are also sorted but are rare in the St Clair shale deposit being more common closer to shore. Whilst ferns are more rare in courser, near-shore deposits.  This suggests the fern stems either float better, else are more easily destroyed by larger grain sizes and either way is fodder for a PHd thesis.

So in answer again; shale is very common in coal measures. I've never seen any coal without some associated shale.  There has long been a belief that anthracite formation conditions will destroy all textures along with any fossil traces but St Clair is clear proof that this isn't always true with regard to adjacent deposits of non-coal rocks. I've some amazing corporlites from the Llewellyn which could have been dropped there last week from their appearance.

In anthracite regions, slate seems to consistently harder and may be a combination of burial depth, reduced weathering, more recent exposure from strip mining and/or organic cementing. Many exposures have a hacky fracture-- across bedding planes and could be legitimately called near-slates. BTW, other local shales include the Martinsberg(sp) and Chattanooga.   The use of the term "slate" is used by locals and possibly stems from the deceiving similar appearance to the adjacent "Slate Belt" workings and workers migrating between the two industries 

Be it remembered that coal producing bogs were at the margins twixt un-vegetated inland highlands and coastal lagoons. In a time of no soil nor plant-supporting Eco-systems on the highlands, even routine rainstorms created very violent frequent flooding which ripped through the bogs and washed vegetation out to sea regularly. Stream beds of quartz pebbles are very common in coal seams. This cycle provided a constant load of silt off shore.

The fern bearing shales of the St Clair represent vegetation that was washed out into the relatively quiet muddy bottoms where silt accumulated in what was oxygen starved conditions(aka reducing)that retarded decay. The constant supply of silt provided for rapid burial and the finely detailed fossils we know today.

 The white mineral coating St Clair ferns(pyrophyllite) is believed to be a weathering replacement of the pyrite that formed in the plant tissue cavities during the fossilization sequence. The pyrite was derived from sulphur rich organic matter in a reducing environment.  In fact the pyriteoids I find there almost always contain a white dust I assume is pyrophillite.

 There might be some slate-like affinities within the Llewellyn but by far it is a classic high organic shale.  I don't have the graphs at hand but there are temperature and depth plots that confirms that for anthracite to form, it had to have a very deep burial (5-8 miles?)  This is sufficient to produce slate but the whole conditions weren't met for  slate to form, near the coal seams anyway. Theoretically, heat and depth conditions should have destroyed all traces of fossils in forming slate but, 300 million years later we have the beautiful white fern fossils of St Clair. I surmise that something in the coalification process prevents the conversion of shale to slate. Just 10-20 miles from St Clair are true slate beds which produced, what anyone over 40, probably remembers as the "chalk-board" they used in grade school.

Speaking of the Llewellyn formation, are there any coal experts watching?  I've found balls of sandy oolitic-looking objects within shales near Hazelton/Latimer which resemble sponges. I would like someone that might have a potential identification on what these are.

Elton

--- On Mon, 11/3/08, smkell45 at aol.com <smkell45 at aol.com> wrote:

> I've been reading about St. Clair material. People keep
> calling the matrix "slate". My specimens from
> there frequently are very detailed and not at all distorted
> as they might be if there was? more than a little? slate
> producing metamorphism. Also a thought that slate splits
> along planes perpendicular to the pressure, not along
> sedimentary layers containing the fern fossils. I also was
> told that this was a worked out anthracite location. How do
> you get shale type fossils and matrix next to an anthracite
> location.? Somebody set me straight. smkell
>