Archive for March, 2011

Here is a rock that was presented to me in lab this week that I found interesting. It is a sample of the New Creek Limestone Formation found in western Virginia; also known as the Coeymans in New Jersey. It is a Devonian aged light gray, fossiliferous limestone, with very coarse grained calcite crystals. It is the fossils in the sample that I found most interesting. For some reason or another I have always been partial to crinoids. It might be because that was the first fossil I found on my own. With that said, the crinoid stem fossil found in this New Creek sample has retained its shape remarkably well, and it has a lot to do with diagenetic process of replacement. At some point during the process of this sediment becoming a rock the structure of the crinoid fossil (possibly aragonite) was replaced by calcite which is more stable. This process is sometimes called calcitization. If you look on the right half of the first picture below, you can make out the radial grooves in the fossil.

Mechanical pencil tip for scale.

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Outside the American Museum of Natural History in New York City, just inside Central Park, are a few outcrops of exposed Manhattan schist, a similar rock to what is seen in Washington, D.C. Both were formed during the Taconian Orogeny 460 million years ago. Below is a picture of one part of the outcrop that caught my eye.

Do you see what I am seeing? Here is annotated version to help:

That is a kink fold. Typically formed in strongly foliated rocks, kink folds are short, angular folds distinguishable by two folds creating a short limb sandwiched by two longer limbs. This particular S-shaped fold is considered sinistral. The “short-limbed” aspect of the fold is referred to as the kink band and has a kink plane on each side. Much like an axial plane, the kink plane propagates down through the hinge line of each fold.

Kink folds form when cohesively bonded layers are subjected to strain. Here is my interpretation of which direction the strain would have come from for this example.

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