Mid-Silurian Mystery

[Miocene_Mason]

Hello Everyone!

Found this plate at a Maryland fossil site of Mid-Silurian age in the McKenzie Formation/Rochester shale. First thought was plant but plant fossils in north America first show up in the late Silurian. microscope pictures don't show any cellular structure, though early plants would not have xylem. When rubbed a black dust comes of it, but not super crumbly like low grade coal. Could be hematized worm burrow? Not the typical type of preservation here. The environment is shallow water, with the main fossils being ostracods, brachiopods, tentaculitids, and some worm burrows that look quite different from this. Next to it is a silvery fossil (typical preservation, don't know what mineral it is) with many little holes in it. What say you?

 

 

First up, micro shots

[Miocene_Mason]

Now macro:

[Miocene_Mason]

[KimTexan]

Is that macro of shale? It looks a bit more like sandstone from the pic, but pics can be deceiving.

I have seen inclusions like this in iron sandstone that usually we’re not identifiable other than a mineral/geologic precipitation in the rock. Other times I have found them to be inclusions of plant material within the iron sandstone. I’ve found petrified wood in iron sandstone, but it was Eocene I think. Other plant materials I have seen were Carboniferous in Arkansas.

[Miocene_Mason]

4 minutes ago, KimTexan said:

Is that macro of shale? It looks a bit more like sandstone from the pic, but pics can be deceiving.

I have seen inclusions like this in iron sandstone that usually we’re not identifiable other than a mineral/geologic precipitation in the rock. Other times I have found them to be inclusions of plant material within the iron sandstone. I’ve found petrified wood in iron sandstone, but it was Eocene I think. Other plant materials I have seen were Carboniferous in Arkansas.

Its more of a thickly bedded, hard shale. It's not like the soft, pliable, thin stuff I find slightly to the east. The outcrop has a mix of both shale and sandstone and the lines kinda blur. I don't think its just a mineral inclusion, but it could be. I was on the way to a carboniferous site when I stopped there, so I was expecting to find plants that trip, just not in the Silurian! I'm skeptical of it being plant though.

[ynot]

Looks carbonized, hematite would not have a "black dust" come off of it.

Maybe an algae (seaweed).

[Miocene_Mason]

12 minutes ago, ynot said:

Looks carbonized, hematite would not have a "black dust" come off of it.

Maybe an algae (seaweed).

Yeah, thought it was strange. That's a good idea! didn't think of that possibility.

[Miocene_Mason]

Forgot to tag @EMP 

[EMP]

Hmm...very interesting for sure. 

 

I'm also skeptical this is just a hematite mineralization. Like you said that mode of preservation isn't common in the McKenzie, at least in the layer you found this in. The fact that it rubs off on you also seems to rule this out. 

 

My guess would be some kind of carbonized fossil. Maybe something got covered in an algae crust so to speak and then that got carbonized?

 

PS: Actually, now that I think of it, what are the odds that it was preserved in iron but became heavily oxidized and what's rubbing off on you is rust flakes? Is what rubs on your finger like a powder or something else? There seems to be some kind of veining/bioturbation going on around the specimen, so maybe that's a clue as to what it is. Infilled burrow preserved in iron? Maybe partially carbonized burrow? 

[Miocene_Mason]

2 minutes ago, EMP said:

PS: Actually, now that I think of it, what are the odds that it was preserved in iron but became heavily oxidized and what's rubbing off on you is rust flakes? Is what rubs on your finger like a powder or something else? There seems to be some kind of veining/bioturbation going on around the specimen, so maybe that's a clue as to what it is. Infilled burrow preserved in iron? Maybe partially carbonized burr

That would be orange, though perhaps limonite is sometimes blackish. It’s just tiny black particles. I’ve never seen or heard of a carbonized burrow but I don’t see why not.

[piranha]

Swartz, C.K., & Prouty, W.F. 1923

Maryland Geological Survey (Silurian).

Johns Hopkins Press, 794 pp.

 

[EMP]

10 minutes ago, WhodamanHD said:

That would be orange, though perhaps limonite is sometimes blackish. It’s just tiny black particles. I’ve never seen or heard of a carbonized burrow but I don’t see why not.

 

It looks small, so the "dust" like appearance could just be that it's too small to produce larger flakes. 

 

There looks to be iron oxidation going on around it (the yellows and oranges).  There are also other similarly shaped objects near it too. 

 

I think it's pretty likely that it could be a burrow/trace of some sort. The question is really down to whether it's carbonized or oxidized iron. 

 

Another interesting possibility is that it could be some kind of hydrocarbon on the fossil, like this find from the forum:

 

 

This shows a couple of brachiopods covered in petroleum from Pennsylvanian marine deposits around Kansas City (link:

 

Maybe something similar is going on here; you have the actual fossil (a burrow or brachiopod fragment) that's been coated in some kind of hydrocarbon. In your case not petroleum, but maybe some kind of low grade lignite or something else, possibly from algae covering or from the animal itself? Just a thought I had. I think there's some good evidence that could point to either iron preservation or carbonization. 

 

There are some bad storms so sorry for the rushed post. 

 

Of course Piranha posts right as I'm writing . I'm not convinced it's that, but it's an interesting article. 

[ynot]

9 minutes ago, WhodamanHD said:

That would be orange, though perhaps limonite is sometimes blackish. It’s just tiny black particles. I’ve never seen or heard of a carbonized burrow but I don’t see why not.

Yes, limonite would be a reddish brown color (never black).

A burrow would have to be full of organic material for it to be carbonized. (not likely.)

It could be other minerals, like manganese.

[piranha]

[Miocene_Mason]

@EMP @ynot @piranha

Now I'm not sure what to think as a few inches away on the same plate I discovered a  line but it was filled with a green mineral, could this prove these are infilled burrows or is it a completely different phenomenon? picture below and yes I see the mite above it, I took this picture outside so the little critter decided to take a look. Not far from that another black segment. Piranha, that paper is quite interesting. It seems to stos short of saying it is plant stems. 

[piranha]

Here is the entire Swartz & Prouty monograph: LINK

[EMP]

28 minutes ago, piranha said:

 

Is this for the McKenzie or Rose Hill? 

33 minutes ago, piranha said:

 

Piranha, I'm sorry but that's the wrong formation. That description is for a section of the Rose Hill Formation south of Cumberland, Whodaman's find is from elsewhere and from the McKenzie Formation. 

 

Here is the section of the McKenzie:

 

McKenzie Formation 1 Thin-bedded argillaceous sandstone; greenish above, dark below with numerous worm borings parallel to bedding   ;  6 0 294 7

 

Dark shale, fossiliferous above, worm borings below... 4 0 288 7

 

Fissile drab shale with thin bands of limestone above, concealed below. The upper 50 feet of this unit con- tains Dizygopleura acuminata. D. swartzi, D. cari- nata, Euklcedenella punctillosa. The middle part of the unit bears JHchmina depressa, Klcedenella nitida, K. scapha-hrevicula, K. immersa, Dizygopleura per- rugosa, Eukl'Menella sinuata proclivis, Bythocypris obesa     175 0 284 7

 

The section is continued in the bluff overlooking the Western Maryland Railway. Shale, faint reddish tone, exposed 313 feet east of Keefer sandstone. Thickness about   5 0 109 7

 

Shale with numerous thin beds of limestone. The upper part of this unit abounds in Hormatoma mary- landica, H. hopkinsi, Orthoceras sp. About 25 feet above the base of this unit occur Dizygopleura subdivisa, D. intermedia. Thickness about  100 0 104 7

 

Dark limestone with a bed of shale 11 inches thick in its middle  •.  2 9 4 7

 

Drab shale. At its base is a bed of dark-gray limestone 9 inches thick with many calcite veins. Surface weathering light gray, in part a limestone conglom- erate     1 10 1 10

 

Total thickness of McKenzie formation  294 7

 

Note the occurrence of the Hormotoma sp. gastropods, which Whodaman found at the site. 

[Miocene_Mason]

11 minutes ago, EMP said:

 

Note the occurrence of the Hormotoma sp. gastropods, which Whodaman found at the site. 

Well I guess that confirms that ID, lucky guess. You (and everyone else if they so prefer) can call me Mason btw.

So I'm getting the feeling they are worm burrows filled with exotic substances? 

[EMP]

I'm still open to carbonization as the explanation for this, but personally it seems that it's becoming more likely it is indeed some strange iron/manganese mineralization going on. 

 

First I'll just lay out some points.

 

Carbonized:

 

- It rubs off on your fingers, a trait associated with hydrocarbons such as lignite 

- Reports of organic material in the formation (algae) that could in theory become carbonized

- General lack of extensive iron deposits in this particular bed

 

Iron mineralization:

 

- Presence of iron oxides around the specimen, appearing as orangish colored splotches

- Some of your Tentaculites sp. from the same exposure are partially coated in iron minerals

- Iron oxides are known and abundant in overlying and underlying layers. It's possible this has been weathered from some other, more iron rich layer. 

- This "dust", as I understand, is somewhat flaky, which could be do to brittle, oxidized pieces of iron minerals being removed by your fingers. 

 

At any rate there are similarly shaped objects all around the specimen, which to me indicates it's likely part of either some kind of branching organism (like what Piranha posted) or part of a burrow/trace structure. 

 

As for the green mineral....that's strange. I'm trying to find an explanation, but the only iron mineral I've been able to come up with that's green is fougerite, but it's only known from France it looks like. However, the mineral chamosite (part of the chlorite group) may offer an explanation:

 

"Chamosite is the Fe²⁺end member of the chlorite group. A hydrous aluminium silicate of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore. Like other chlorites, it is a product of the hydrothermal alteration of pyroxenes, amphiboles and biotite in igneous rock. The composition of chlorite is often related to that of the original igneous mineral so that more Fe-rich chlorites are commonly found as replacements of the Fe-rich ferromagnesian minerals (Deer et al., 1992)." From Wikipedia's page on the mineral. 

 

The McKenzie and Rochester in particular are known for their oolite deposits, as noted in the report posted above. Now, the only problem is that chlorites aren't often associated with sedimentary rocks, but the article goes on to say that:

 

"The Mamu-Nkporo formation in the locality of Okigwe, Nigeria was studied by Akande and Mucke (1993), and they concluded that the carbonate discovered with associated chamosite was formed in a shallow marine subtidal to intertidal environment developed during periods of rise and fall in sea level. Formation of chamosite bearing oolites record periods of increasing wave energy corresponding to storm conditions between quiet shallow marine sedimentation,(Akande and Mucke, 1993)." 

 

The McKenzie/Rochester meets the conditions from this site. It is a shallow marine unit, with carbonates present, and in an area with fluctuations in sea level. 

 

Wikipedia also states that "Chlorite is also a common metamorphic mineral, usually indicative of low-grade metamorphism" and this seems to be the case with these rocks. As John Means states in his book Roadside Geology of Maryland, Delaware, and Washington DC, "the rock layers of these provinces [Blue Ridge and Valley and Ridge] have been tightly folded into wavelike structures called anticlines and synclines by the great tectonic pressure exerted when the African Plate shoved into the North American Plat during the Alleghanian mountain building event between 320 and 250 million years ago." Perhaps this folding could have resulted in regional metamorphism? Geology.com states that chlorite group minerals "often form in clay-rich sedimentary rocks that are buried in deep sedimentary basins or subjected to regional metamorphism at a convergent plate boundary," which would seem to be the case for the McKenzie. 

 

I can't say definitively if it is a chlorite mineral, but it's a good guess I'd say, and at least provides a starting point. On the flip side, green is usually indicative of copper oxidation, so perhaps it could be a copper oxide (though I find this less likely, after all most source copper minerals like bornite or chalcopyrite usually occur in hydrothermal deposits, not sedimentary). 

 

 

Edit: Chamsonite is a greenish mineral, I forgot to add/

 

 

 

[abyssunder]

It might be something in the line of Phycodes / Treptichnus igen, in the baroque style of "fucoids", like the one in the comparative picture labeled as Phycodes palmatus.

 

 

comparative picture from here

 

 

 

 

 

 

[EMP]

I am very late to this, but I have some new info on this @WhodamanHD. 

 

It is not the McKenzie Formation. It is actually from the upper zone of the Rose Hill Formation, in the Liocalymene limestone beds near the contact with the Keefer Sandstone (possibly exposed at the very top of the exposure). The plant material described in the report is from the basal shale beds near the Rose Hill-Tuscarora contact around Pinto Station IIRC, and that is not exposed in the Evitts Mountain anticline. 

 

The weird squiggly fossils are actually overturned brachiopods, with their front edges sticking out of the shale towards the viewer. It's possible some of them are also trilobite molt fragments (likely L. clintoni). 

 

As for the mysterious fossil, I have found something similar from the argillaceous limestone above the upper shale beds overlying the Cresaptown Ironstone. Shiny black, stick like objects that are "striated" (for lack of a better term) top to bottom and not across. After comparing them with images from elsewhere online, it's possible they are actually fish remains, possibly from acanthodian fin spines, I'm not sure. After about an hour searching I found several shiny black, possibly enamel "fish" remains in argillaceous limestone float scattered amongst the upper shale beds. I also found a possible coprolite associated with such a "fish" remain in a coquina section further below the limestone bed. My fossils are not carbonaceous, however they are phosphatic, black, and have clear "groves" down their length, roughly similar to the macro photo, but ending in a clear point instead of rounded. 

 

Given the fauna of the piece this likely derives from the upper shale layer below the argillaceous limestone and above the Cresaptown. So a little lower stratigraphically then where I found my fossils from, but I would still keep it open as a possibility. Other possibilities include fragments off of trilobite shells (possibly the genal spine of a Dalmanites limulurus(?) which is reported from the pre-Rochester Clinton Group), or a hinge from a brachiopod (possibly strophomenid given it's apparent size). It might also be part of a thoracic segment of a trilobite, which given the abundance of trilobite remains in the shale and limestone beds below the Keefer Sandstone contact is a strong possibility. Such iron stained modes of preservation are relatively common in the Rose Hill, and it could also be that it was later stained by manganese or some other impurity. 

 

The many orange blobs are not mineralization - they are molds of ostracode fossils, which are made partially of limonite(?). There is no iron mineralization in the bed besides oxide staining, and given the lithology of the rocks there is unlikely to be any hematite crystals that large remaining in them. Some minor calcite veining, likely deriving from the shell coquinas, is present in the argillaceous limestone, but it is a white color. 

 

The green "mineral" could be a modern mold, lichen, or some other plant in the gap between two overturned shells. The depositional environment of the Rose Hill was likely a shallower water, high energy region given the disarticulated nature of the trilobites and ostracodes, and given that many of them are overturned and clumped together, possibly representing shallow depressions in the sea floor in which they congregated because of wave/current action. Cross bedding reported in the sandstone elsewhere also points to this. 

[Miocene_Mason]

27 minutes ago, EMP said:

 

I am very late to this, but I have some new info on this @WhodamanHD. 

 

Hey @EMP! Hope you’ve been well. 
 

Great info, going to have to change some labels! Reminds me I need to get back out there. I’ve been volunteering with the CMM and hitting the Paleocene/Eocene/Miocene for the past year. I’ve got some new paleo friends in WV and Dinosaur Park, time to hit start hitting those formations again. Still haven’t found that nice MD trilobite!

[EMP]

2 minutes ago, WhodamanHD said:

Hey @EMP! Hope you’ve been well. 
 

Great info, going to have to change some labels! Reminds me I need to get back out there. I’ve been volunteering with the CMM and hitting the Paleocene/Eocene/Miocene for the past year. I’ve got some new paleo friends in WV and Dinosaur Park, time to hit start hitting those formations again. Still haven’t found that nice MD trilobite!

 

I've been pretty busy this past while, haven't gone out collecting as much as I'd wanted. 

 

Sounds like you've been busy then! Eocene's pretty cool, I haven't seen too much of that stuff from MD. Usually lots of Paleocene, Miocene, and some Cretaceous stuff. 

 

As for MD trilobites, I have a post coming up about that