Friday, January 25, 2008

The Whaleback

Outside of Shamokin, Pennsylvania, is a coal strip mine that has had the coal stripped away. Under the coal was a Pennsylvanian (in the time sense of the word) carbonaceous shale (the Llewellyn Formation), which is now preserved in lovely undulating Appalachian folds. Thanks to the removal of the coal, these fold surfaces appear in three dimensions -- a rarity for structural geologists like myself. The area is known as "The Whaleback" because of one anticline (center) with a shape that evokes a surfacing cetacean:

I went to the Whaleback last fall on a fossil-hunting trip with the The Calvert Marine Museum Fossil Club. In today's post, I'll take a look at the structure, and in a later post, I'll show you some photos of the fossils themselves. Here's some of the guys on the trip:

At the north end of the excavation, a cross-sectional view of the absent upper levels is preserved, showing this syncline. It once continued towards the camera's perspective in the air, a downflung fold between the Whaleback anticline and the neighboring anticline which made up the background "wall" in the first photo.

This is a closer look at the limb of the biggest anticline, dipping down into the Whaleback's open pit. Note that it appears to have a bad case of acne. Other observers have likened it to appearing as if it were "shot full of cannon balls." Note the person (lower left) walking along the Whaleback's fold axis, for scale.





















This last shot shows a close-up of one of these "cannon balls." These are nodules of hematite -- concretions that wrap around some initial point of nucleation and serve as a chemical point of precipitation, encouraging more hematite to glom on and lay down a new layer. Because they're hematite, they rust when exposed at the surface. This phenomenon is a diagenetic one -- that is, these nodules formed as this layer of organics & mud was being compressed into the Llewellyn Shale. (These nodules were not rolling around the Pennsylvanian swamp bottom.) Their random but regular dispersal throughout the layer really impressed me: it was almost the same pattern that might result if an artist were stippling a drawing to shade it.
Okay, that's it for today. Tune in soon for the fossiliferous sequel.

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4 Comments:

Blogger Ron Schott said...

Site of my very first (admittedly crude) Gigapan image! Look familiar?

January 25, 2008 10:22 AM  
Blogger Mel said...

Do you think those concretions were originally pyrite and subsequently had O2 rich fluids pass through and converted to hematite? Do you see any evidence of the volumetric change that occurs from pyrite --> hematite?

January 25, 2008 3:47 PM  
Blogger Callan Bentley said...

Mel,

Quite possibly they were originally pyrite. Which way does the volume change in that reaction? To my eye, the nodules looked like they were "burstingly full" -- swollen. Bedding distorts around them, etc. So if the volume increases from pyrite to hematite, that sounds like a good guess to me.

January 25, 2008 5:11 PM  
Blogger Mel said...

I believe so. I'm recalling a field trip to the Taconic Mnts as an undergrad. We stopped at a shale quarry (the quarry was abandoned when they ran into a fold) and in the shale were pyrite cubes. But because of the regional stresses involved the grains had been rotated from their original position but the "gaps" were filled with hematite. It was pretty cool stuff. I found an example of quartz and pyrite doing this, but haven't found my original example yet.
http://www.minsocam.org/MSA/collectors_corner/arc/pressure_shadows.htm

January 26, 2008 12:20 AM  

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