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Tuesday, October 11, 2011

Stromatoporid(?) from the Keyser ls.

Recently I acid etched the specimen below out of some limestone from the Silurian aged Keyser formation. At first I thought it was a Stromatoporid which are calcareous sponges that lived in the oceans during the Paleozoic era. They were important reef builders along with Rugose and Tabulate corals and are known to occur in the Keyser and at the same location where I found this piece.


It is kind of nondescript and lumpy but when turned you can see some of the layering.


It was this view that made me think it was a Stromatoporid or possibly a Stromatolite (an algal reef formation).
Bottom of the specimen

As I looked at the specimen more closely I saw many small cylindrical shapes.


Now I am less sure that it was a Stromatoporid as I don't think they have this kind of feature. Maybe it's a Bryozoan colony instead? Could each of those cylinders represent where one animal lived?

 Another interesting feature of this fossil is that it's hollow inside. I don't think that has anything to do with the fossil itself but rather the fossilization process. You see, the fossil was originally preserved as a Calcite skeleton but over the millenia some of the calcium carbonate was replaced by silicon dioxide, or Quartz. When I soaked the limestone in muriatic acid, the calcium carbonate was dissolved away. This left a "shell" behind that was Quartz while the interior was dissolved away since it had not been entirely replaced.


I've blogged about some of the true Stromatoporids I've found within the Keyser formation before in this post.
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4 comments:

  1. AnonymousOctober 11, 2011 at 4:58 PM

    Interesting specimen! I'd say it's definitely a stromatoporoid. They are an important Devonian reef-building fossil where I live, in Alberta (many of our oil deposits are hosted in stromatoporoid reefs). The characteristic microstructure of stromatoporoids is the "pillar and floor" structure, of horizontal layers ("floors") separated by masses of closely-spaced vertical "pillars". The little cylinders in your photos are the pillars that have been etched out by the acid, and is an excellent illustration of the structure, which usually isn't seen this clearly.

    --Howard

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  2. DaveOctober 12, 2011 at 4:07 AM

    Howard,

    Thank you for the confirmation that what I found is a Stromatoporid. I hadn't ever seen the structure before in such detail so this is an important specimen to me. Most times I see just the horizontal layers or exterior surface.

    -Dave

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  3. AnonymousOctober 12, 2011 at 10:15 AM

    Thanks for showing me this pic...Just curious to know that how does replacement of Silica dioxide takes place? 1. Below Calcite compensation depth [CCD] after certain depth? or quartz flow in it? Hydrothermal fluid or etc?

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  4. AnonymousOctober 12, 2011 at 10:32 PM

    Anonymous--I can't give you details on the mechanism of silica replacement, which probably involves chemistry that's more complex than I'm comfortable with. But it's closer to your second guess, the hydrothermal fluid mechanism. Basically, you're dealing with a groundwater that is oversaturated with respect to silica, but undersaturated with respect to calcite. Why the silica selectively replaces some microstructures (pillars) rather than others (the presumed calcite matrix between pillars) is a good question; presumably it's related to subtle differences in the chemistry of calcite (perhaps solubility differences between high-Mg calcite versus low-Mg calcite, for example, or even calcite vs. aragonite, calcite vs. dolomite, or something along those lines). I wouldn't be surprised if even geochemists (of which I'm not one) would have questions about the process.

    --Howard

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