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GordonC

Stromatolite encrustation

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GordonC

Side view of stromatolite piece (~10")DSCN1170.thumb.JPG.6a6fac3d83a3d28d432b9aef28cee077.JPG

DSCN1169.thumb.JPG.249759a4c75e3777af1c503765b1a949.JPG and view of partial encrustation for ID please. Thanks, Gordon

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Auspex

This is an interesting piece.

I'm struggling to understand how an encrusting organism would wrap around the apparently broken end of a stromatolite.

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ynot

I do not see a stromatolite.

I see a piece of limestone with 2 grades of lithification.

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DPS Ammonite

I'm thinking that the "encrustation" is a piece of chert. Limestone is often partially replaced by chert during diagenisis. The acid resistant chert stands in positive relief against the limestone that is dissolving away. Note how the chert appears to go into the rock in the lower left side of both photos.

 

Do do some simple tests. See if the limestone and the "chert" area bubble with when vinegar or pool acid is placed on them. Also scratch both areas with a knife blade. What areas are scratched? 

 

Although the chert is not a fossil per se, it may have some fossil molds in it - I see some cavities. Show us a close up photo of the chert.

 

 

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abyssunder

" The Manlius Formation of the Lower Devonian Helderberg Group in central New York State contains decimeter-scale thrombolites. Internally, thrombolites comprise centimeter-scale, digitate to irregularly-shaped clots surrounded by skeletal wackestone. The clots are composed of variable amounts of: 1) millimeter-scale hemispheroids with radial and concentric structure that resemble Keega; 2)Renalcis-like forms; 3) stromatolitic laminae of mudstone; 4) peloidal mudstone laminae with filament molds; and 5) rare fossils. The thrombolites themselves are interbedded with grainstones, mudstones, graded beds, and intraclastic conglomerates. A low diversity fauna is found in many of these associated rocks.

Manlius Formation thrombolites are interpreted to have been rigid structures built by calcified cyanobacterial communities and encrusting and framework-building skeletal algae. Thrombolites are interpreted to have formed in a subtidal, restricted lagoonal setting. The low diversity of associated skeletal remains suggests that the environment was unfavorable for most organisms which would compete for space or consume the mound-building organisms. "

 

5ab6d387815c6_Fig.1.thumb.jpg.f2e3ed396884ac45021c266fa68b27ec.jpg

 

excerpts from K. M. Browne & R. V. Demicco. 1988. Thrombolites of the lower devonian Manlius Formation of central New York. Carbonates Evaporites 2: 149-155

 

 

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GordonC
On 3/23/2018 at 8:44 PM, DPS Ammonite said:

I'm thinking that the "encrustation" is a piece of chert. Limestone is often partially replaced by chert during diagenisis. The acid resistant chert stands in positive relief against the limestone that is dissolving away. Note how the chert appears to go into the rock in the lower left side of both photos.

 

Do do some simple tests. See if the limestone and the "chert" area bubble with when vinegar or pool acid is placed on them. Also scratch both areas with a knife blade. What areas are scratched? 

 

Although the chert is not a fossil per se, it may have some fossil molds in it - I see some cavities. Show us a close up photo of the chert.

 

Here a section of another stromatolite (13" max dim.),2. encrusted exterior and 3.detail. The encrustation does not react to muriatic acid.

The stromatolite does vigorously react to muriatic acid.

Thanks,Gordon

DSCN1179.JPG.f8c2b01c631d347740d93cbc6202a3b2.JPGDSCN1181.JPG.6e463a1f7cb734d6f7aa2599987b8c6a.JPGDSCN1182.thumb.JPG.768847c002243b65ca0b28e4b3ea2302.JPG

 

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