I was hiking along the shore of the North Saskatchewan River here in Edmonton a few weeks ago and came across some pretty stellar leaf fossils.
These are so finely detailed they look like they could have been made yesterday, but how old are they? That’s always my first question when I find a fossil, and it can be fun to figure it out. Not because it’s always scientifically important so much as because it’s cool to think about.
So How Old Are They?
These stones were broken off from the same layer of rock in the walls of the river valley, but I found them about 100 M apart from each other. The stones in the river valley are often a jumble of broken pieces from different layers, so it’s not always easy to figure out where they came from. In this case, it looks like these stones came from a layer not far above the water line.
The above picture is the area I found the fossils in. You can see a collection of similar reddish sandstone broken off and partially embedded in the hillside. The stones could have been transported from somewhere else, but they were both among this sandstone. There were no other visible sources of such rock in the hillside, so I’m fairly confident they are from this layer.
With that in mind, it’s possible to sort out how old they might be.
The geology of the river valley isn’t too complicated. It’s mostly just debris churned up by glaciers placed on top of the Horseshoe Canyon formation. The Edmonton Geological Society has a neat poster summarizing the geology. This particular layer is close to the water line, so it’s part of the Horseshoe Canyon formation and not the glacial deposits above it.
So how old is this formation? It’s not possible to use radiometric dating on some layers of rock, including the sandstone these fossils are in. Fortunately there are tools that can be used to sort out an age bracket.
An easy tool for this (for me to look up the results from) is magnetostratigraphy, where magnetic particles in rocks are examined to see which way they were facing when they settled. The Earth’s magnetic field makes some grains of sediment act like tiny compasses and align with magnetic North. As it turns out, Earth’s magnetic field has reversed many times in its history. Magnetic North became South, and back again many times. That story is visible if you look at the magnetic grains in sediments all over the world. Geologists have carefully studied this, and have been able to piece together where magnetic North was at different times in history.
You can see the most updated chart of the Earth’s magnetic history as published by the Geological Society of America (GSA). It’s a pretty cool piece of work.
So the trick to finding out the age of my leaf fossils is as simple as figuring out which magnetic period(s) they could have been buried in. At least one layer of the Horseshoe Canyon formation here in Edmonton has been dated to ~71.9 million years (Davies et al. 2014). Scientists looking at the formation have used similar dating as a marker along with looking at the magnetic signatures in the rocks from top to bottom to figure out the Horseshoe Canyon formation’s time of deposition. As it turns out, the rock formed during the time periods (Magnetozones) 30n through 33n (Davies et al. 2014).
Periods 30n-33n work out to be between 69-74 million years (Davies et al. 2014). So that’s the time constraint on my leaves based on the formation they were found in: they must be between 69-74 million years old if they came from the layer I think they did. It would probably be possible for me to go looking for more data and figure out more exactly how old they are based on specific layers in the formation, but I’m happy knowing within 5 million years.
I think that’s pretty amazing. These fossils lived with dinosaurs 69 million years ago, were buried in sand, and found by me today. I doubt that very many of us will leave such an impression as these leaves in 69 million years time. I find that both amazing and humbling.
Davies JHFL, Wotzlaw JF, Wolfe AP, Heaman LM. 2014. Assessing the age of the Late Cretaceous Danek Bonebed with U-Pb geochronology. Can J Earth Sci. 51: 982-986.