In finding marine fossils locally in the Glenshaw Formation, I have noticed three distinct preservation types within limestone. This does not indicate that there are only three, but these are the most common I have identified. All of these involve the preservation of marine creatures’ remains in limestone. Marine limestone is a sedimentary rock that forms in warm shallow seas. Non-marine limestone forms in lakes, tufa deposits, etc.
Marine animals use calcium carbonate to create their shells, for living in and general protection. This calcium carbonate also precipitates, like continuous rain, over the seafloor. This material combines with sediment on the seafloor into limestones I find locally that is a natural cement. The lime from limestone is used to make cement, a key ingredient in concrete. This rock is formed after hundreds of millions of years underground with more and more sedimentary rocks adding compression from the crushing weight above.
The First Type: Standard Limestone
This natural cement is very hard and can be difficult to work with for extracting fossils. However, I have found three different preservation types of this rock, depending on where it is located. In the Brush Creek limestone, there is standard limestone, which is very hard, heavy, and not prone to breaking. While there are some cleavage planes through these boulders, you need a very heavy hammer to get them to split. This limestone is no more than a foot thick in Parks Township, but it can be several feet thick in other places. This depends slightly on how long the sea persisted, and more on the rate of sedimentation.
Specimens within have some unique properties. Steinkerns (molded representation of a creature created by the filling hollow portions with sediments during the fossilization process) are solid, made of this type of limestone. Any marine creatures that are preserved as shells are difficult to extract as intact specimens. The outside, textured portion of shells adhere strongly to the matrix, while the smooth portions that face inward separate easily. So when you split a rock, and there is a fossil cephalopod, brachiopod, or gastropod on the plane of cleavage, the shell will remain to one side, and the exposed steinkern on the other.
Unique Brush Creek specimen preservation
One unique trait for these finds is the preservation of shell material on brachiopods that are recovered from this limestone. These shells appear as a bright white color against the dark gray color of the limestone. Their shells from from calcium carbonate, but it is one of the three naturally occurring crystal forms. It is a white color likely due to recrystallization of the original calcite shell. This unique preservation also occurs within the Pine Creek limestone, just not in the exact same way.
The Second Type: Weathered Limestone
While the standard limestone is very hard, locally is breaks naturally. Especially over large areas. These pieces can be several different sizes, depending on where they are located. Some pieces are as large as a school desk, some as long and wide as a bus. I believe these occur throughout the layer, however, it may be only towards the exposed edges. Locally there are synclines and anticlines, which are up and down folds (respectfully) of the stratum. I believe this manipulation of previously level layers causes stress fracturing.
Nevertheless, these fractures are margins in the rock that allow water to seep through. Over a very long period of time, this water creates a sort of rind on the outside of the rock. Float limestone lying in water goes through a similar transformation. The dark color turns to a brown, and most of the calcite within appears to dissolve away. There are many variations of these, with some still being a dark color, and others being very light and brown. The variation in color probably depends on a combination of how long the rock was exposed to chemical weather agents like mineral-laden water.
Fossils found in this layer have some unique properties. They are often fragile, like the matrix they come from. They can be only a surviving steinkern or a shell that has changed properties. A find that prompted me to write this article was of a Metacoceras that had separated camerae (chambers in the shell). In standard-type limestone, these specimens hold together as one piece. In the first type if limestone, there is a discernable calcite layer, called the septum between them. This septum is nearly absent in this type of stone. This type of preservation displays intricate details of each camera of the cephalopod’s shell. The formerly empty chambers were filled with sediment and now each is a steinkern.
The Third Type: Fissile Limestone
The third type is similar to the standard limestone. The key difference is that it is very fissile. This has been my experience in working with the Pine Creek limestone in the Kittanning area. This material splits into many different planes. It is similar to shale. Specimens in this matrix preserve the aragonite as a white chalky material, similar to the first type. There are advantages to fissile limestone, but there can be problems with it as well.
Using cephalopods as an example, I have been able to recover preserved portions of the shell. The fissile nature of the rock helps it not to adhere to the shell, and I have been able to discover three-dimensional pieces of shells of cephalopods. Gastropods are another type that is easy to recover. The compact nature of the shells allows them to stay intact and simply fall out of the matrix when searching through it.
- Geology.com – Limestone