Jurrasic shale cylinders with a square hole down the center (??)

[Richard Beale]

I live in the Western Sierra Nevada region of California near (on) the Mariposa Formation.  I find ammonites, belemnites and buchia (common Jurassic bivalve).  I also find cigar shaped somethings (fossils?) in the same strata.  They come in many sizes with the larges about the diameter of a US quarter (as are the ones in the picture), down to only a few mm diameter, and their length is one to six cm.  The always have a very square hole running down the length of the center.  The first time I found one I thought there had been a pyrite crystal that had been chemically dissolved from the center of a concretion, but the wholes are only square in cross section, they pretty much always go the length of the cigar shape.

These come from Oxfordian Jurassic shale with the other common fossils listed above. 

I'll be going back to the main site to collect more Tuesday and hop to post more.  Any identification help would be appreciated!

 

Richard

[Ludwigia]

Remarkable! Do the holes run all the way through, or do they end at both ends in the center? Is there at least some kind of leftover residue, like a powder, in them when you excavate them, or are they completely barren?

[Spongy Joe]

Are there any markings on the surfaces of the hole? If this was Palaeozoic, I'd be suggesting conulariids... but it's not.

[supertramp]

they seem to be cherty;

if so, they could be concretions as shown here (see "Concretions")

 

http://trailventuresbc.com/southern-chilcotin-geology/

 

but the question had no answer :-)

"Some concretions from the map area are elongated (cylindrical) with a square hole down the center – what could have been the nucleus?

 

ciao

[Miocene_Mason]

Reminds me very much of rhizoliths. 

[GeschWhat]

Now that is something you don't see every day!

 

EDIT: If you have some that are unbroken, it would be interesting to get a cast of the interior.

[supertramp]

just a suggestion: maybe ikaite crystals (a "melting mineral") https://en.wikipedia.org/wiki/Ikaite 

not replaced by some pseudomorphs https://www.mindat.org/min-27112.html

see also the piece (top left) at 4:03 here 

 

 

 

anyway I don't think those structures were biological in origin

 

ciao

[doushantuo]

Ikaite is formed/preserved under VERY special circumstances,because principally it's a metastable mineral(carbonate hexahydrate)

[abyssunder]

The holes are not quite quadrate (some of them might be). Considering that they may come from an area with volcanic-plutonic activity in the past, I'm wondering if they can't be thundereggs (lithophysae) with the missing cores (biconoids / multiconoids).

 

" The mid Jurassic apparently was a time of tectonic transition, involving major changes in plate motions and the onset of a substantial component of convergence. If the resultant calcalkaline arc and apron of derived volcanogenic sediments initially formed as a continuous curvilinear belt, this time interval evidently attended a progressive 200 km outboard migration of the Klamath terrane amalgam relative to the Sierra Nevada [Ernst et al., 2008]. Initiation of volcanism-plutonism and the clastic detritus shed oceanward from the growing arc include Middle and Late Jurassic granitoids [Stern et al., 1981; Bateman, 1992; Dunne et al., 1998; Dickinson, 2008] and their outboard erosional products such as the western Klamath Galice Formation [Harper et al., 1994; Miller and Saleeby, 1995; Gray, 2006] and the western Sierran Foothills Mariposa Formation [Sharp, 1988; Edelman and Sharp, 1989], both of Oxfordian- Kimmeridgian age. Dating the onset of construction of this Upper Jurassic sequence in terms of Mariposa sedimentation, based on macrofossils and microfossils reported by Imlay [1961], Clark [1964], and Graymer and Jones [1994], may well define the change from earlier, chiefly strike-slip plate motion to later transpression involving a major component of lithospheric convergence along the Californian sector of the North American margin. " -  W. G. Ernst et al, 2009

 

 

The mystery of biconoids

 

Edited May 27, 2018 by abyssunder

[doushantuo]

I will be back in this thread,but Abyssunder is right,I think.

These strucutures might be the result of a geophysical(thermal)process(or even several processes),principically involving high heat flow and magmatic degassing.

Lithophysae are frequently found in (often fluorine-rich) silicic(alkaline) magmas,and are defined by their concentric zoning,but NOT by their square(-/ish) shape.

The fluorine changes the rheology of the magma,and the silica,when fractionally crystallizing out of the magma,might shield the internal cavity from e.g.weathering

The squarish shape MIGHT also be due to subsequent faulting,but I am not sure how the factors PRECISELY interrelate.

There might have been a large topaz crystal in the centre of those cylinders.....**

Minerals that MIGHT be close would be quartz,pseudobrookite,specularite,bixbyite..

**Say "fluorine-rich silicic magma" around any gemmologist,and he/she's likely to grab your lapels,and shout in your face "Where?"

 

 

 

[Richard Beale]

Thank you all for the very interesting ideas and replies.  The picture posted by abyssunder (with the coin in it) is certainly more examples of what I am finding. 

This is only found sedimentary Jurassic rock (in my location). I couldn't tell which part of abyssunder's text referred to the picture of the fossils(?) that matched mine. 

Could you tell us where that picture comes from?

Concerning Lithophysae, can this occur in sedimentary layers (as these are found)?  The central hole is almost always sharp lined.  The first I saw came from Copperopolis, CA.  At first I thought it was hardened clay that had a square nail removed (common in our gold rush area) - it looked man made, but then a friend and I started pulling them from sedimentary layers.

Richard

[supertramp]

Hi Richard,

have a look here

https://www.mindat.org/photo-680937.html

https://www.mindat.org/photo-681361.html

from here

https://www.mindat.org/gl/14996

ciao

[abyssunder]

Richard, these are not fossils, but geological wonders of what Mother Nature can create, in my opinion.
I don't know exactly which kind of processes make them to look like concretions / nodules with those angular cavities. I supposed they might be lithophysae with missing cores, but I'm not 100% convinced of this. The pictures of the links posted before by our friend supertramp looks exactly like yours, but there's no convincing explanation or evidence to prove what they really are and how they have that shape.
The reference, which I omitted to mention, is reachable here .

 

Maybe more of them (from your finds) will be more convincing of what they might be. Can you post high-res pictures of them, eventually unopened specimens in longitudinal and transverse section?

[Richard Beale]

Here are some more fossils from the same formation (but I am convinced by the members on this page who have argued that the "square holes" are not fossils).

[Richard Beale]

I found a few more examples of the unusual square void which I'll post this weekend

[doushantuo]

I provided AT LEAST part of the explanation,  but then again you have to know at least something* about the relation between high pressure phenomena in

igneous petrology /volcanology and structural geology.

*and I am NOT even saying I "know something".

I think Supertramp knows his structural geology as well, BTW (not entirely sure)

 

A lot of studies on lithophysae were done a long while ago by e.g.Hauer,Harker,Iddings.Jenzsch

 

 

 

 

 

 

 

 

 

[supertramp]

Hi doushantuo,

I think those are concretions formed around some calcite pseudomorphs after ikaite ("glendonite") in a sedimentary environment; 
the encasements of the dissolved calcite crystalls have been preserved from dissolution probably thanks to their siliceous composition 
(as for most of chert nodules or concretions with respect to their limestone encasement); 
so I think they could probably be defined as chert concretion l.s. (with a peculiar precipitation-inducing nucleus – i.e. a crystal).

ciao

[Plax]

A termination in the cavity would be good to see in this case. Or possibly use the broken halves of the concretion to make a cast of the square void. While these were collected in a sedimentary environment they could be part of a coarse accumulation of transported clasts like a conglomerate. Not saying that this is the case. Supertramp has the best explanation in my opinion.

[abyssunder]

On 6/5/2018 at 4:50 AM, Richard Beale said:

I found a few more examples of the unusual square void which I'll post this weekend