Millboro Formation shale in outcrop and in hand sample

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Another site from the GMU sedimentology field trip in April: An outcrop on Route 33 in Brandywine, West Virginia, showing the Millboro Formation. It’s mostly shale, with some intriguing sandstones, too. There are fossils and diagenetic carbonate nodules (concretions).

Here’s the outcrop, the largest GigaPan I’ve taken so far (7.9 billion pixels):
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The shale itself looks… like shale. It’s fine-grained, and dark (high carbon content, suggesting low oxygen levels when deposited). It contains articulate brachiopods that are also suggestive of less-than-ideal-conditions-for-animals conditions. Here’s a look down on a bedding plane, for instance:

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The little bumps are brachiopods. The trained eye will also pick out some subvertical crenulation cleavage ‘wrapping’ around these little guys:

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This next slab has the brachiopods, too but also seems to contain a few snails (spiral shapes):

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Another fossil is a long, spear-point-shaped thing. Does anyone have any idea what it is?

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Maybe acanthodian spines? (early sharks)

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Here’s a GIGAmacro image of this fossil suite (tiny articulate brachiopods and some long spear-like things)
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The shale breaks apart with a characteristic pattern called “pencil cleavage”:

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Some close ups of the sandstone layers, which show cross-bedding, climbing ripples and convoluted bedding, suggesting rapid deposition:

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This one is a boffo example of convoluted bedding:

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Annotated, with laminations internal to the sandstone bed traced out in blue:

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Note that the shale above and below is NOT deformed – this is soft sediment deformation rather than tectonic deformation of rock. Look closer:

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Here is a sample that had weathered out of the outcrop. I took it, of course.

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Explore it in this  GIGAmacro image:
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Another sandstone package, lower in the outcrop (older):

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This next shot is a close-up of the upper left corner of the previous image:

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Further along strike, apparent climbing ripples could be seen:

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Here’s a loose climbing ripple sample, with out-of-focus West Virginia scenery in the background:

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Close-up showing the changing in angle of the bedding in this same sample:

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Annotated:

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Here is a GIGAmacro image of the climbing ripples, side (cross-sectional) view:
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Sandstone chunk showing cross-bedding and plane laminations:

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Carbonate nodules appear as positively-weathering features in certain horizons:

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Here is a little tiny one:

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In outcrop:

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Here is a GIGAmacro image of a carbonate nodule, top view:
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What fun! There’s a lot to see at this site.

0 thoughts on “Millboro Formation shale in outcrop and in hand sample”

    • Mike Huggins wrote to me via email about this (unable to post directly for some reason):

      Your Millboro Shale photos are great. With regard to those cone-shaped fossils you found – I had nearly identical fossils in some of my MS thesis rocks down in Washington County along the South Fork of the Holston River (just off Route 80 between Lindell and Hayter’s Gap; “Turkey Hollow” on Google). The rocks in question were younger than the Millboro Formation. In this case, they were black to dark gray shales of the mid-Mississippian Little Valley Formation (lowest unit of the Greenbrier Limestone or Group in the Greendale Syncline; just above the Maccrady Formation and below the Hillsdale Limestone).

      My specimens were of a similar size to yours. They were smooth and also characterized by noticeable longitudinal, linear cracks that appear to have been formed when the coned shell was collapsed from burial and overburden pressure. I had the impression based on this feature that they were very pretty thin-shelled. I could not find any concentric latitudinal growth rings on mine either. Because of this (lack of growth rings or septa) I was initially reluctant to call them orthocones. I looked at other cone-shaped fossils, like styliolineds or hyolithids, but in the end I ended up calling them orthocone nautiloids, on my measured section, like Ron Schott has for your specimens. I wasn’t doing invertebrate paleontology of the unit (conodont biostrat and carbonate facies) so I didn’t do a lot of research, but they were frustrating to figure out. I archived a bunch of the specimens in the Va Tech Geo Dept. Paleo collections; however, I don’t know if these were ever included in the specimens that were transferred to the Virginia Museum of Natural History when they got the collections. (Alton Dooley emailed me that a lot of the VPI stuff did make it into their collections, but for these, who knows?)

      Anyhoo, it would be pretty neat to hear from a cephalopod expert about your specimens. I am pretty sure they are the same as the ones I had.

      Reply

  3. Test. That was my long-winded email to Callan. In short, it’s amazing how close these “orthocones” look to my 35-55 my younger cones. I didn’t recall seeing anything quite like them before in Appalachian rocks.

    Reply

  4. Oh, the accompanying fauna with my Mississippian versions were pelecypods Aviculopecten, Phestia, & Sedgwickia? (the dominant fauna); brachiopods Tetracamera, Lingula, and a productid; and an ostracode.

    Reply

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