Suggestively Shaped Nodule - Deposition Information?

[Kane]

As I was collecting at Arkona in the Widder Fm yesterday, my eye was drawn to this piece. It had a lot going for it - apparent texture, concentric ribbing, suggestive shape. Not knowing for sure at that moment, and not wanting to spend valuable collecting time deliberating on it when I had a lot of shale to move, I simply put it in the bucket so that I could take a closer look when I got home.

 

My question for those who know more about taphonomy and such would be this: can anyone tell me a bit more on the processes that produce this kind of nodular form? The concentric patterns are kind of throwing me, almost as though there was some reworking due to tidal turbulence, like mixing dough in a bowl. I'd certainly appreciate learning a bit more about this phenomenon, as it provides me a bit more by way of clues as I move from layer to layer.

[ynot]

First off - it looks more like a limestone than shale to Me.

There are several ways that a rock can end up with concentric layers in a rounded shape....

Folding of the original layers, weathering from the outside in (defoliation.), difference in the deposition of concretion growth, laminations of soils by algal growth.

[Kane]

Thanks, @ynot! I do know that the Widder has a lot of evidence of some turbidity that is reflected in how some of the fossils get clustered. The lighting on the picture was not ideal, but it is shale (the limestone occurs at the base of the unit). Sadly, whenever I encounter a nodule in these layers, they tend to be blank.  The nodule appears to coil upwards, like a flatter gastropod spire. I don't know if that helps better pinpoint which of those processes are occurring. The outside "texture" appears to be some kind of mineral staining.

[ynot]

It could be caused by a combination of any or all of the mentioned processes.

[doushantuo]

When i was fairly new here,I was almost laughed off this forum by Tony for mentioning pot and gutter casts .

here they are again

They are more common in lowstand wedges,if you know your sequence stratigraphy

[doushantuo]

 

Paul Myrow has some Paleozoic examples,but the PDF is too Rorschach-y

 

 

 

[Al Dente]

Could it be a nautiloid? Looks like a siphuncle in the middle.

[Kane]

Just now, Al Dente said:

Could it be a nautiloid? Looks like a siphuncle in the middle.

That was my initial hope when I found it, although I am not familiar with any coiled nautiloids in this layer (at best, little ammonoids like Tornoceras). I'll still hold out a small bit of hope that it is something more than just a nodule! 

[ynot]

43 minutes ago, doushantuo said:

When i was fairly new here,I was almost laughed off this forum by Tony for mentioning pot and gutter casts .

here they are again

I do not remember doing anything like that (must have been Ynot). 

Can You link to the thread where this atrocity occurred?

[TqB]

1 hour ago, Al Dente said:

Could it be a nautiloid? Looks like a siphuncle in the middle.

 

My first thought too.

[FossilDAWG]

It's a nautiloid.  Nice find!

 

Don

[Kane]

51 minutes ago, FossilDAWG said:

It's a nautiloid.  Nice find!

 

Don

Really? That certainly makes my day! Never find a coiled one this large at this site before. My next task will be to ID this little monster. 

 

Moral of the story: always trust your first instincts and just put the uncertain stuff in the bucket!

[FossilDAWG]

I have one from Arkona, but it is not as nicely preserved as yours.  I recall large coiled nautiloids are also known from the Dundee or Lucas Formations.  

 

BTW here's what I see:

 

Don

[Ludwigia]

Don's got it. That was my first thought as well.

[ynot]

6 hours ago, Kane said:

My question for those who know more about taphonomy and such would be this: can anyone tell me a bit more on the processes that produce this kind of nodular form? The concentric patterns

And then there is always the layers caused by an organic skell being fossilized.