Sunday, May 2, 2010

Mystery Devonian Rock

Mystery rock picked up in an area with Jeffersonville Limestone Devonian Period fossils. When I picked up the rock I assumed it was the inside of brachiopod. The inner part has a blue appearance. It reminds me of some of the crinoid stems found at Lake Cumberland, Kentucky. At one time, I thought it might be agate but there are no layer lines. Could it be some sort of blue opaque quartz or blue chalcedony? Then there is the crystal area, that I guess is calcite.

The area surrounding the nodule shape has a lot of dendrite patterns. I did not test the rest of the stone but I assume it is limestone.  Found in Clark County, Indiana.  I only found one half, not sure where the other part is.

What the nodule looks like from the back.


Dark Slander said...

Personally it looks like a chert nodule to men. I've found several last time I hunted the areas outside Louisville.

Fossil Detective said...

Thanks for the comment.

My curiosity leads me to wonder how did the nodule form with all the interesting features I pointed out in the post.

I hope you are enjoying your stay in the Louisville area. The heavy rains are making for nice fossil hunting.

Anonymous said...


This is an excellent example of what we geologists would call a geopetal structure--something that shows "which way is up". It records a sequence of events in its history:

1) Something (a brachiopod, or algal ball, perhaps?) was buried in the brown matrix. Apparently it was less resistant to dissolution than the matrix, and therefore it dissolved--carried away by percolating groundwater, leaving a hole (mold).

2) Later, a silica-rich groundwater percolated through, leaving the bluish chalcedony, lining the cavity and half-filling it. The fact that the cavity was only half-filled indicates that the rock was *above* the water-table at the time, so that the silica-rich solution settled by gravity on the bottom of the cavity, leaving an air gap in the upper half, like a tiny cave. Think of the thinner layer of chalcedony on the roof of the "cave" as a "stalactite" layer. This is where the "geopetal" comes in. The unfilled gap at the top shows which way was "up" (the correct orientation is shown in your first photo). Also note the lens of brown chalcedony on the bottom of the blue chalcedony. This could represent some particles of the brown matrix that settled on the bottom (another geopetal indicator!) and became embedded in the chalcedony.

3) A combination of a rising water table and a change in the chemistry of the groundwater resulted in the deposition of calcite, completely filling the old air gap.

4) Then you came along!


Anonymous said...

Looking at it again, there's more to the story!

3a) The ring of lighter-coloured rock surrounding the chalcedony shows that an "aura" of (probably) silica permeates the matrix around the nodule. Another change in groundwater chemistry resulted in the deposition of the black, dendritic mineral (pyrolusite?) in the surrounding matrix, but the "aura" of silica or calcite around the nodule sealed this area and prevented the deposition of the dendritic mineral.

4) THEN you came along!


Fossil Detective said...

Thanks Howard for that great analysis. I will print your story and place it in a bag with that specimen.

More than likely it was a brachiopod since those fossil fragments are spread around the other parts of the rock. Mostly small Devonochonetes and maybe Rhipidomella brachiopods appear to be the fossils.

I put some white vinegar on part of the matrix and it started to lightly fizz.