Field Work Travelogue: Day 14 – Searching for the Red Beds #NTCave15

Today I worked on the surface collecting rock samples. I’m seeking an answer to a fairly simple question – one that was brought up by one of out caver colleagues.

Most of the so-called micro-vertebrate fossil (these are small animals like lizards, snakes, songbirds, mice, and shrews) come from a traceable layer in Natural Trap Cave that is brilliantly red.

A terrible flash photo, but you can clearly see the reddish layer. The hole in it is where I removed sediment for analysis.
A terrible flash photo, but you can clearly see the reddish layer. The hole in it is where I removed sediment for analysis.

So, why is the red layer red?

Having an answer to this question might help us understand why micro-vertebrates are so common within it.

We have two operating hypotheses:

1) The layer is red due to oxidation of the sediments, which could be a climate signal. Maybe it was warmer or wetter during a short period of time and the little animals flourished.

2) The layer is red because it is the result of the erosion and re-depostion of red rocks that were once around the cave mouth. Perhaps those red sediments were a better growth medium and habitat for plants and animals than the limestone beds that currently surround the cave.

The best way to approach this is to collect sediments from inside the cave, and then go out and collect rock samples from near the cave. We will compare the sediments and the rocks to see if they are similar or different.

There is a red rock layer nearby that sits over the limestone that the cave formed in. A limestone layer sits over the top of that. My job today was to collect the limestone of the cave, the red rocks, and the limestone that sits on top of all of it.

I had to walk quite a ways before finding a decent outcrop. It happens that the red rocks are fine-grained and loaded with clay, so you don’t often see actual rock on the surface.

Red layers are between limestone (the blocky) layers... but no actual red rock to be seen.
Red layers are between limestone (the blocky) layers… but no actual red rock to be seen.

I finally found the outcrops I was looking for. They weren’t great, mind you, but they were there at least.

The red rocks weather so easily they make only smooth hill slopes.
The red rocks weather so easily they make only smooth hill slopes.

I grabbed my pick-axe (because everyone carries a pick-axe right?) and dug into the slope to find some rock. The red rock itself wasn’t far down.

Red rocks with rock hammer for scale. The greenish blebs are where conditions were not oxidizing, so iron stays grey.
Red rocks with rock hammer for scale. The greenish blebs are where conditions were not oxidizing, so iron stays grey.

Above the red layer is a purplish layer. I decided to grab a sample of it, too.

I'm not sure why this rock is so purple. No doubt there's a mineralogical reason. The white layer above is unoxidized like in the red rocks.
I’m not sure why this rock is so purple. No doubt there’s a mineralogical reason. The white layer above is unoxidized like in the red rocks.

Above all that is another limestone that’s also purplish in color. I grabbed a sample but not a photo.

Then it was back to camp for lunch and then screenwashing. I’m washing sediments from the red layer, so there’s lots to find.

Little bones everywhere.
Little bones everywhere.

I had to chuckle when I found Pleistocene mammal parts right next to Paleozoic brachiopods. The Paleozoic fossils come from the limestone that forms the cave.

A brachiopod (a clam-like animal) next to what is likely the ear part of a mammal skull.
A brachiopod (a clam-like animal) just above what is likely the ear part of a mammal skull.

Then, as always…

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