The World's Best Exposed Pyrite Concretions Layers
Welcome!
Point Gratiot in Dunkirk, New York has one of the best exposed pyrite concretion layers known anywhere in the world. This EarthCache takes you to the beach at Point Gratiot to explore the origins of the pyrite concretions from a prominent pyrite-bearing layer of black shale in the Dunkirk Shale Member of the Canadaway Formation (Upper Devonian) along the Lake Erie shore.
Required Logging Tasks
To log this EarthCache, complete the following tasks:
- How do you think pyrite concretions formed at this site?
- Describe what you see in the rock face at the EarthCache coordinates. Focus on color, texture, and layering.
- Take a photo of yourself (or a personal item) in front of a pyrite layer at the site to confirm your visit.
Earth Science Lesson
What is a Pyrite?
Pyrite is a shiny, gold-colored mineral that looks a lot like real gold, which is why it's sometimes called "fool's gold." Even though it looks like gold, it's not valuable like real gold.
How does a Pyrite layer form in Shale?
A pyrite layer forms in shale over a long time, usually at the bottom of an old ocean, lake, or swamp. Here's how it happens:
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Dead plants and animals sink to the bottom and get covered by mud.
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As they break down, they use up the oxygen in the mud, creating a space with little or no oxygen.
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Tiny bacteria help break down the dead plants and animals and release a chemical called sulfur.
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Iron in the water or mud mixes with the sulfur and forms a new mineral called pyrite, also known as “fool’s gold.”
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Over time, more mud piles on top, pressing everything down. The pyrite starts to grow into tiny crystals.
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These crystals often form shiny layers or spots inside the shale.
What does a Pyrite layer look like in Shale?
In the picture below, near the top of the rock, you can see a yellow-gold streak running across the layers.
What is a Concretion?
A concretion is a hard, compact mass of mineral matter that forms within sedimentary rock, typically around a central core like a fossil, shell, or piece of organic material.
How do Concretions form?
They usually form when mineral-laden water moves through the sediment and deposits minerals (like calcite, silica, or iron oxide) around that core, cementing the particles together more tightly than the surrounding rock.
What do Concretions look like?
In the picture below, pyrite concretions can be seen as well as small black spores. Limonite hales appear due to weathering. Concretions often are spherical or oval in shape, but they can also be irregular. They come in a wide range of sizes. The slabs studied at the site were approximately 2 to 18 mm in diameter and 2 to 3 mm thick. Some pyrite concretions are compound, and therefore larger, up to 50 mm in maximum dimension.
Final Thoughts
Pyrite concretions in the Dunkirk Shale at Point Gratiot, New York, likely formed as large Protosalvinia spores decayed. These spores came in different sizes, but only the large ones developed pyrite around them. SEM and EDX analyses confirm that framboid-shaped pyrite is found with large spores, suggesting bacterial activity played a role. Similar pyrite beds in the Huron Shale of central Ohio appear to have formed in the same way and at a similar time. This points to a widespread Late Devonian event, possibly a tropical storm or hurricane, that scattered spores across the Appalachian Basin, creating these unique formations.
References
Geological Society of America (GSA) - EarthCache Guidelines
Origin of Pyrite Concretions From the Dunkirk Shale Member of The Canadaway Formation (Devonian), Western New York