June 8, 2017

Dispatches from Field Camp: How about some rocks?

Posted by llipuma

By Brian Balta

Well, I’ve casually jumped around other field camp topics, let’s get into the heart of it. Here’s an intro to some of the rocks we’re actually mapping.

Credit: Brian Balta.

This photo was from our first day in the field – we walked the section of rocks exposed in this area from oldest to the youngest rocks we had time to get to. In this shot, without realizing it, the students are literally standing on 2.5 billion years or so of Earth’s history – more than half the time this planet has existed is sitting right there as an unconformity. The rocks to the left are Archaean in age, over 3 billion years old – possibly even older, they’re so heavily metamorphosed it’s hard to get a perfect age. The rocks to the right are the Cambrian aged flathead sandstone – the first record of the incursion of the sea in this area in the Phanerozoic. If there were any rocks formed or deposited between those two times, at this site there is no record of them – an unconformity more than half the age of the planet in one site.

The rocks students have been mapping with me are a mix of the oldest rocks – the Archaean and Precambrian rocks, and a handful of additional igneous intrusions. Let’s take a look at a few neat ones.

Credit: Brian Balta.

This is an example of the Archaean rocks in this area. They’re part of the Wyoming Province – a small slice of cratonic continental crust that stuck onto the growing core of the continent found today in Canada. The oldest rocks in the Wyoming Province are at its northwest corner and we’re in that area, so these are potentially the oldest rocks exposed in the United States. Canada of course has these beat by potentially another billion years.

The rocks have been heavily metamorphosed. Each layer you see is a foliation – a completely flattened, recrystallized layer. They are mostly feldspar and silica gneisses today, with different minerals segregated into layers during heating and deformation. It’s a little difficult to even know whether the protolith of these rocks was igneous or sedimentary or a little of both. They have been so highly strained that each of those tiny thin layers could be the remnant of a rock that was once meters in size. This particular one, photographed with my hammer, which I’ve had since my own field camp, was a neat site since I found a couple augen. These larger lens-shaped features are coarser crystals that were more resistant to the deformation and almost rolled around as the rock was being metamorphosed.

Credit: Brian Balta.

Here’s another rock. This one comes from what we call the “Big Sky Orogeny,” a mountain building event 1.8 billion years ago. These rocks are metamorphosed sediments. They were likely formed in a foreland basin as the mountain range was growing, thrust into the core of the mountain range, foliated, and even partially melted. This particular one may contain the remnants of an igneous dike. The darker stuff tends to be stronger and more resistant during strain, so it was potentially fractured while the lighter stuff, containing a lot of quartz, was more easily able to flow around it.

I’ll chime in with some more neat Big Sky aged rocks tomorrow. From Field Camp, thanks for checking in again!

— Brian Balta is a a visiting professor of petrology at Texas A&M University. Follow his twitter feed at https://twitter.com/theearthstory for more content.