Cambrian Fossils in Maryland

[EMP]

This is a retcon of an earlier post I had. 

 

Cambrian fossils aren't something one thinks of when they think of Maryland fossil hunting, and perhaps for good reason. The Cambrian rocks of the state are poorly exposed, those few areas where they do outcrop usually being gobbled up in urban sprawl. Compared to sites elsewhere like in Utah or York, Pennsylvania, the Maryland Cambrian is also rather barren. You could probably count on both hands the number of macrospecies in the entire early and middle Cambrian section of the state. But this rarity only makes collecting in it that much more interesting! Luckily for me I'm pretty close by most of these formations, so I have a decent knowledge of the area and outcrops, but even then it took a decent amount of time researching and scouting to find a site. 

 

The most recent formation I visited was the Araby Formation. Up until the mid 20th century the Araby was considered part of the Antietam Sandstone further west in the Blue Ridge, but after some more studies done on the formation it was found that it's lithological character was distinct enough to warrant it being a separate unit. Whereas the Antietam is a white quartz sandstone (much like the Oriskany I posted about yesterday) deposited in a beach-like environment, the Araby was deposited in deeper water (compared to the Antietam) and is more a mixture of siltstones, shales, phyllites, and slates. Together with the Antietam the Araby has some of the oldest fossils in the state dating back to the early Cambrian period some 540 million years ago. This makes it the oldest formation in the Frederick Valley. 

 

For those that don't know the Frederick Valley is a predominantly limestone syncline in west central Maryland (I consider it western Maryland, but most people probably wouldn't). At it's core is the early Ordovician Grove Limestone (which has practically no fossils), and on it's flanks are the late Cambrian Frederick Limestone (fossiliferous in parts, but those parts are very rare) and finally the Araby Formation. The Araby takes up positions along the far flanks of the valley, and it's eastern boundary with the metamorphic rocks of the Westminster Terrane marks the Martic Fault (no Washingtonians you don't need to worry about a San Andrea, from what I've read the Martic has been inactive for a long, long time). Due to it's sediment type and that of the surrounding rocks, the Araby is also a minor ridge forming unit, holding up the series of hills that flank Frederick Valley's eastern edge. These hills are nicely visible from the grounds of Monocacy National Battlefield, which is also of interest for marking the site of the northernmost Confederate victory (July 9, 1864 for those who're curious) in the Civil War. This ridge forming aspect means that, although very thin and covering a very small area, the Araby Formation has multiple exposures throughout the Frederick Valley. Some of the better ones are visible along I-70 just east of it's crossing over Monocacy River (an MGS team found some trilobites there) and MD-355 as you drive through the woods before hitting Araby Church Road (I believe the namesake for the formation is actually the Araby Church). 

 

In terms of fossils the Araby is almost exclusively dominated by the trace fossil Skolithos linearis, an annelid worm burrow. Other fossils found in it, however, include echinoderms and Olenellus sp. trilobites. As another aside the Cash Smith Shale, once held as an independent formation, also has trilobites and I believe inarticulate brachiopods reported from it, however it is no longer considered an independent formation but rather a member of the Araby Formation. 

 

Almost all of my fossils were the worm burrows, still cool but for everyone's sake I won't constantly repeat what they are this time around. 

 

Image 1: The largest burrow I've found. I originally thought it was a genal spine from a trilobite due to it's size. 

 

Image 2: Cross section of a burrow, outlined by the iron oxide stain. 

 

Image 3: Another burrow, this one roughly outlined by the iron oxide. 

 

Image 4: The large tubular structure covered in iron oxide (you might be noticing a pattern here with the oxides and burrows. I can't say definitively if they're connected in some way, but oftentimes you'll find the one with the other). 

 

 

[EMP]

Image 1: I didn't actually find this at the Araby site, but rather at the Cretaceous one. It's still a burrow, but I believe from the Antietam Sandstone, not the Araby Formation. 

 

Image 2: Cross section outlined in iron oxide. 

 

Image 3: The tubular structure partially coated in iron. 

 

Image 4: Pretty deformed due to metamorphism, but you can sort of make out the trace of a burrow. 

[EMP]

Image 1: Cross sections in white. 

 

Image 2: Tubular structure near the bottom. 

 

Image 3: Small burrow to the left of finger. 

 

Image 4: Cross section in white, vaguely resembling one of those old feather pens. 

 

 

[EMP]

Image 1: Large burrow. 

 

Image 2: Large burrow. 

 

Image 3: Possible echinoderm. 

 

Image 4: Possible Olenellus sp. cephalon? 

[EMP]

Image 1: What looks to be a cheek from an Olenellus sp. trilobite. 

[Kane]

Unlikely to be an Olenellus sp. fragment. Looks more like some form of mineral staining. What diagnostic characteristics do you think makes this a trilobite fragment? Perhaps you are seeing something we are not?  @piranha

[FossilDAWG]

I see some burrows (trace fossils), but unfortunately I don't see any echinoderm or trilobite bits.  Sorry.

 

Don

[Kane]

Perhaps someone with more knowledge and experience in these formations can identify these. 

[Wrangellian]

Might need better shots to confirm some of them (better lighting and higher magnification)... I'm curious about that 'feather pen' item.

[Plax]

The native rock in the area is partially metamorphosed. Also this rock is a bit sandy as opposed to shale which does a better job of holding preservation. In this case I believe that suggestive shapes are indeed fossils. I think that most trilobite collectors are accustomed to only seeing well preserved trilobites or crappy distinctive ones at worst. The professional literature has at least a few papers which include only poor examples. The South Carolina Cambrian trilos come to mind (Southeastern Geology). The New Hope PA occurrence is also poorly preserved but includes some better quality specimens.

[EMP]

1 hour ago, Plax said:

The native rock in the area is partially metamorphosed. Also this rock is a bit sandy as opposed to shale which does a better job of holding preservation. In this case I believe that suggestive shapes are indeed fossils. I think that most trilobite collectors are accustomed to only seeing well preserved trilobites or crappy distinctive ones at worst. The professional literature has at least a few papers which include only poor examples. The South Carolina Cambrian trilos come to mind (Southeastern Geology). The New Hope PA occurrence is also poorly preserved but includes some better quality specimens.

 

Yes, I forgot to mention it, but I sort of alluded to it when I mentioned the Martic Fault. The Araby was pretty close to the Taconic Orogeny and was partially deformed by it, hence the phyllite/slate and metasiltstone layers (technically these are metasiltstones, not regular siltstone). Still, the most heavily deformed rocks were just across the fault in the uplands section of the Piedmont - those were fully metamorphosed whereas the Araby was just deformed. 

 

And for those wondering, the MGS found some Olenellus specimens from the Araby, but they were fragmentary and deformed. 

 

Edit: Pelmatozoans are also reported from the formation, and a quick Google search showed that this is what they were

 

http://www.fossilmuseum.net/fossils/starfish/Stromatocystites/Stromatocystites.htm

 

I've definitely found stuff like that, should I post it here or in the ID forum?

[Fossildude19]

2 hours ago, Plax said:

The native rock in the area is partially metamorphosed. Also this rock is a bit sandy as opposed to shale which does a better job of holding preservation. In this case I believe that suggestive shapes are indeed fossils. I think that most trilobite collectors are accustomed to only seeing well preserved trilobites or crappy distinctive ones at worst. The professional literature has at least a few papers which include only poor examples. The South Carolina Cambrian trilos come to mind (Southeastern Geology). The New Hope PA occurrence is also poorly preserved but includes some better quality specimens.

 

Any plates/pictures from that literature? 

I am having trouble making out any kind of trilobite shapes, even, ... on the two examples provided.    

 

Maybe someone could point out what they are calling trilobite pieces?   

 

I reversed Black and White to get differing contrast, but I just don't see any trilo-bits here.

 I         

 

 

 

 

               

 

 

[Plax]

Hey Tim,

  I have some of the papers as paper from Southeastern Geology but as we know papers that can't be googled for pdfs are in effect non existent to today's researchers. I believe that Southeastern Geology's list of publications is available but haven't seen the actual papers as pdf. You might try researchgate and authors names such as Patricia Weaver whom I know was a co-author on one of the SC papers. Most of these impressions look like fossils to me but the one just above this looks like a fortuitous chip.

Don

[piranha]

9 hours ago, Plax said:

 The South Carolina Cambrian trilos come to mind (Southeastern Geology). 

 

 

There are a total of 3 Southeastern Geology papers with trilobites:

 

McMenamin, M.A.S., & Weaver, P.G. 2002
Proterozoic-Cambrian paleobiogeography of the Carolina Terrane.

Southeastern Geology, 41(2):119-128   PDF LINK

 

McMenamin, M.A.S., & Weaver, P.G. 2004
Middle Cambrian polymeroid trilobites and correlation of the Carolina and Augusta terranes.
Southeastern Geology, 43(1):21-38   PDF LINK

 

Schwimmer, D.R., & Montante, W.M. 2012
An Aphelaspis zone (Upper Cambrian, Paibian) trilobite faunule in the central Conasauga River valley, north Georgia, USA.

Southeastern Geology, 49(1):31-42   PDF LINK

 

 

These papers also have info on the South Carolina (Asbill Pond) trilobites:

 

Maher, H.D., Palmer, A.R., Secor, D.T., & Snoke, A.W. 1981

New trilobite locality in the Piedmont of South Carolina, and its regional implications.

Geological Society of America Geology, 9(1):34-36

 

McMenamin, M.A.S. 2002
The Ptychoparioid Trilobite Skehanos gen.nov. from the Middle Cambrian of Avalonian Massachusetts and the Carolina Slate Belt, USA.
Northeastern Geology and Environmental Sciences, 24(4):276-281   PDF LINK

 

Samson, S.L. 1984
Middle Cambrian fauna of the Carolina slate belt, central South Carolina.
M.Sc. Thesis, University of North Carolina at Wilmington, 54 pp.

 

Samson, S.L., Palmer, A.R., Robison, R.A., & Secor, D.T. 1990

Biogeographical significance of Cambrian trilobites from the Carolina slate belt.

Geological Society of America Bulletin, 102:1459-1470

 

Secor, D.T., Samson, S.L., Snoke, A.W., & Palmer, A.R. 1983

Confirmation of the Carolina slate belt as an exotic terrane.

Science, 221:649-651

[Plax]

you're good Piranha! I talk too much without giving accurate references...