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Coral fossils in metamorphic rock?


Sky

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Greetings,
My lying eyes are struggling with the unlikely chance that the forms in this rock are biological in origin.

 

This looks like coral to me: I see rough impressions of corallites, polyp anatomy, septa, hexagonal and circular chambers, and signs of stacked chambers where masses are sheared perpendicular to the exterior surfaces. The average diameter of the corallite-like openings is ~.5 cm. Many are smaller. Some are up to 2 cm. Note the manner in which various tilted tubes erode to reveal their lengths and interiors; and the presence of angle-cornered walls that group fields of corallite-like forms (LooksLikeCoral1.jpg).

 

The bed from which these blocks and boulders emerged is sandwiched between crystalline limestone, garnet gneiss, and quartzite. These in turn are closely adjacent to a granitic intrusion. The rock shown here is metamorphic. It feels more dense than quartz, and seems at least as hard as quartz, and reacts very weakly with acid. At a broken edge, its separately fractured crystals reveals the nearly transparent vitreous luster, and the color is proportioned ~95% colorless and ~5% olive green. Examples of the most coherent masses of this rock are seen in LooksLikeCoral1.jpg and LooksLikeCoral2.jpg. A less coherent form of the rock includes a comparatively more easily eroded fibrous mineral that separates the tougher corallite-like layers (see LooksLikeCoral3.jpg). This fibrous mineral is composed of crystals which I believe have the same proportion of colorless and olive green. At the site of the rocks, there is a transition from coherent coral textured masses to less coherent: The fibrous fill increases in volume between ever more deeply curved layers of "remaining" hexagonal nuggets which gradually lose their walls. At one margin of the whole site, the fibrous filler is replaced with marble. And at another, where it approaches the granitic intrusion, there is a transition through stages of melt until the nugget layers are obliterated and mixed into a swirling fine grained mass, finally becoming indistinguishable from quartzite. The site is on privately owned land in the San Ysidro Mountain block of Eastern San Diego County, California. Quite some time ago, this block was determined to go back to the Ordovician, but no physical evidence for this claim has been found.

 

I will test specific gravity, and next week, sections from a sample will be cut and polished for a look inside. I will share the results.
In the meantime, it would be interesting to discuss:

 

If you didn't know this was metamorphic rock, would you think these textures and patterns have biological origin?
In the unlikely event this is fossil material, do the forms and patterns suggest an ID, and period?
If you are forced to speculate that this could indeed be fossil material, what tough mineral/s might have first replaced the biological material so that it could survive the heat, pressure and deformation of mountain building?
Just as interesting, can you suggest any non-biological processes that would result in forms and patterns like these?

Thanks in advance for replies.

 

 

LooksLikeCoral1.jpg

LooksLikeCoral2.jpg

LooksLikeCoral3.jpg

Edited by Sky
additional information, one typo repaired
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I have to start out by saying I'm on a low bandwidth connection and can only see 1/2 of the first photo, all of the second photo, and none of the third photo.

The hexagon and cup-like shapes sure do look like a colonial coral at first glance, but I'm not seeing any anatomical features to say it is coral. It might be my limited view, with the cropped photos I see. But, I think it is possibly a mineral deposit of aragonite. Aragonite can form these rhombic shapes, and the cup shapes are a bit odd, but they might be explained by the errosion of the aragonite.

To show it is coral, would need to look for detials more closely, otherwise I'm with this being a geological mineral deposit.

https://www.google.com/search?q=aragonite&btnG=Search&hl=en&gbv=1&tbm=isch

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Thanks.

I wonder if others cannot see all of the images?

Aragonite is much softer than this rock, which is very hard, possibly harder than quartz.

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Other people can see the images, I'm in the odd situation of using early 1990's technology in the back woods.

When you cut and polish this, I'd be intersted to see the results.

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" The Southern Otay Mountain and Western Otay Mountain Wilderness Study Areas are underlain by a sequence of weakly metamorphosed dacitic, andesitic, and rhyolitic volcanic rocks assigned to the Santiago Peak Volcanics. The rocks are resistant, thinly layered to locally massive flows and tuff with minor breccia, agglomerate, and volcanic conglomerate. Sandstone, siltstone, and mudstone form a minor part of the section. Fresh outcrops are typically dark gray to black or greenish gray, weathering to tan and reddish- and yellowish-brown colors due to the oxidation of iron-bearing sulfide minerals.
Finely disseminated pyrite is widespread throughout the section, and arsenopyrite is a less common replacement mineral in mineralized rocks (Weber, 1963, p. 124). Parts of the volcanic section have been completely altered to quartz-sericite schist and hornfels. Metamorphic recrystallization and widespread silicification impede the recognition of primary structures; porphyritic and fragmental volcanic textures are fairly common, and flow banding is seen locally. Some felsitic layers grade from silicified metatuff to tuffaceous quartzite and metasiltstone.
Volcanic rocks of the San Ysidro Mountains were intruded on the east by the Cretaceous Peninsular Ranges batholith, which in this region includes undifferentiated granitic rocks and may include gabbro (fig. 2). The gabbro pluton shown in figure 2 is chiefly fine grained and is partly surrounded by the Santiago Peak Volcanics.
Because the pluton is undated, it is not known whether it was a feeder for the Late Jurassic volcanic rocks or is part of the Cretaceous Peninsular Ranges batholith. It is possible that high-level plutons such as this one were transitional in age and geologic setting between superjacent volcanic rocks and deeper level plutons of the batholith.
The volcanic rocks in the southern part of the San Ysidro Mountains are cut by fine-grained to aphanitic felsic dikes, possibly related to the emplacement of the batholith.

Although the orientations of the dikes vary, the most common trends are northerly and easterly. The dikes appear to have utilized north- and east-trending fractures and faults. Locally, the slight offset of one dike by a second dike of the same set suggests that at least some faulting took place during batholithic emplacement. Quartz veins and stringers occur locally in mineralized zones. "

 

VICTORIA R. TODD et al. 1988. Mineral Resources of the Southern Otay Mountain and Western Otay Mountain Wilderness Study Areas, San Diego County, California. U.S. GEOLOGICAL SURVEY BULLETIN 1711 - E.

 

I have a geological vibe on this. Interestingly, the spherulitic felsite pattern shown in the below picture looks close to the specimen(s) in question.

 

1.jpg

 

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25 minutes ago, abyssunder said:

" ...The volcanic rocks in the southern part of the San Ysidro Mountains are cut by fine-grained to aphanitic felsic dikes, possibly related to the emplacement of the batholith.

Although the orientations of the dikes vary, the most common trends are northerly and easterly. The dikes appear to have utilized north- and east-trending fractures and faults. Locally, the slight offset of one dike by a second dike of the same set suggests that at least some faulting took place during batholithic emplacement. Quartz veins and stringers occur locally in mineralized zones. "

 

VICTORIA R. TODD et al. 1988. Mineral Resources of the Southern Otay Mountain and Western Otay Mountain Wilderness Study Areas, San Diego County, California. U.S. GEOLOGICAL SURVEY BULLETIN 1711 - E.

 

I have a geological vibe on this. Interestingly, the spherulitic felsite pattern shown in the below picture looks close to the specimen(s) in question.

 

The area where I found this is further north. It is close to Anza Borrego Desert State Park at a similar latitude to San Ysidro peak.

The spherulitic felsite is interesting. I need to get more detailed photos of the specimens for readers to compare.

 

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Here are closeups of one rock. BoxShaped1 reminds me of this Goniophyllum Fossil. In BoxShaped1, look at the orifice in the rotund structure that is centered in the box shaped surround. The lighting on the rounded structure reveals a flattened top, as if it was once a soft mass pressed in by a ceiling, like an operculum. BoxShaped2 is a broader view of the rock, which reveals similar structures. I realize what I am proposing is inconceivable.

BoxShaped1.jpgBoxShaped2.jpg

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hi sky,

a really unusual texture…

have you checked wether the pattern on blocks and boulders is actually only on their weathering surface?

 

ciao

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The exposed and buried sides of all blocks and boulders are consistent in stain. Fractured edges have different texture than the variety of textures of the non-fractured sides.

There are no dissolved surfaces as seen in the local marbles. Well stained old fractures on blocks are not decayed or textured, compared to fresher less stained broken edges.

 

Here is another impression found in one of the rocks. The diameter of the impression is about 1.5 inches.

 

As you can see, there is a good deal of variety in the impressions in these rocks. Each reminds me of something else. This one reminds me most of a sea scorpion.

InterestingRock5.jpg

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