28 July 2015

Millboro Formation shale in outcrop and in hand sample

Posted by Callan

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.