Out in the (Gold)field

This semester I’m taking an undergraduate field geology class called Field Geology I. It’s intended to be a geology student’s first course in geologic field studies.

Since I didn’t do my undergraduate degree in geology (I used to be an engineer), I never took such a course. Following my qualifying exam for my PhD, my committee made a recommendation that I take the course. I took my committee’s recommendation seriously and decided to enroll in the course and I’m happy that I did.

Geologic field studies are an important part of being a geologist. Some even say “the best geologist is the one who’s seen the most rocks”. The field is where the products of geologic processes can be observed at various scales: from an entire outcropping of rock to a sample of rock that can be held in your hand, and even to what can be seen through a magnifying hand lens. There are of course other scales at which geologic observations can be made (from the global scale to the microscopic), but the three mentioned above are most relevant for field studies.

There are a lot of geologic skills that are at play when working in the field. One thing that we’ve been doing in the course thus far though is field mapping. Mapping basically consists of identifying rock types and geologic structures, figuring out the relationships between the different rock units and structures, and finally constructing a geologic history based on your observations.

This may seem simple: how hard can it be for a geologist to identify a rock? One might ask. But I’ve found that nothing is quite as simple as it may first seem when it comes to field studies.

For example, on our first field trip of the semester we went out to a place called Goldfield in central Arizona. Perhaps you’ve heard of Goldfield from it being a “ghost town” or perhaps even visited the tourist attractions there. But Goldfield is also a locality rich in interesting geology.

The rocks there are mostly igneous rocks (formed from molten rock) but there are also some sedimentary rocks (formed from sediments) present. There are also so-called geologic structures (specifically, faults) that make the field relationships a little more interesting.

For the rest of this post I’ll write about one stop that we found particularly interesting. I’ll start with a picture of my mapping partner Logan, approaching the feature we identified from afar. He’s trying to get a better look. He also provides a scale, which is important when thinking about the relative sizes of features. Logan is about 2 m (6.5 feet) tall.

Notice anything? Maybe the picture isn’t that great. I did leave my camera on the bus and I was trying to use Logan’s camera for the first time. But what we noticed first was that there were two different rock types here: the reddish brown type (what we called the red unit) on the left side of the photo and the grey to brown rock (what we called the brown unit) on the right side of the photo. The surface of the red unit (seen just above Logan’s head) is also distinctive in that it is unusually flat and close to vertical and the rocks surrounding the “contact” between the two units seem to be broken up a little more than rocks farther away from the contact.

I annotate the photo to make some of these observations a little clearer, the red color is the red unit, the brown color is the brown unit, and the black lines show the fractured zones.

Now, I said the red unit is distinct from the brown unit but how is it distinct, exactly? Well they clearly look different, so what does that mean?

The red unit appears to be composed of pieces of other rocks (clasts), which means it’s sedimentary. In the following photo, I crop some of the red unit out and highlight this feature by outlining some of the cobble-sized clasts of which the red unit is comprised. The arrow is pointing to a cavity where it looks like one of these clasts weathered out. You can also see that these clasts are very rounded or smooth. You also see there are not just larger clasts but also smaller ones as well. Finally, there’s a finer grained matrix that holds everything together.

Have you guessed the name of this rock yet? It’s called a conglomerate. The sizes of the clasts that make up the rock a variable (poor sorting) and the clasts are fairly well rounded.

When we look closer at the types of clasts that make up the conglomerate we see that a lot of them are made of crystals that are visible to the naked eye, these crystals are light in color and lack any type of preferred orientation. These characteristics lead us to believe that most of the clasts are made of granite. When geologists try to surmise the potential sources of sediment it’s called provenance study. So these clasts came from an area that had a lot of granite around.

Another thing that can be inferred from this rock is the environment of deposition. If we think about the sizes of the clasts and the roundness, we can conclude that the environment provided enough energy to transport large pieces for long enough (and therefore far enough away) to round out the clasts. One example of a setting with these characteristics is a river.

So what about the brown unit? Well, I don’t have a better picture of that unit, unfortunately. But, the rock is made of medium to dark grey crystals. Some of these crystals can be seen with the naked eye, but most cannot. We can also identify some of the minerals that are more visible and find that the rock is composed of weathered olivine, pyroxenes, hornblende, and some feldspar minerals. We call this rock basalt, which is an extrusive igneous rock. That means it formed from molten rock that was above the surface of the Earth, like from a volcano.

Now we ask ourselves, how did these two rock types get placed next to each other in the orientation that they currently have? One of the basic principles of geology is that rocks get deposited or emplaced in a horizontal position; however, these rock units are nearly vertical! Also, we know from other field relationships that the red unit is younger than the brown unit and there isn’t a history shown in the rocks of the transition from a volcanic environment to a river (fluvial) environment. Therefore, questions remain about the temporal relationships of these units. Well, luckily there’s another feature that helps explain some of this.

If you remember, I mentioned that the rocks are fractured a lot more closer to the contact between the two units. We also see evidence of slight polishing of the red unit on the left hand side of the contact. This and other evidence in other places nearby allow us to conclude that these two units were placed next to each other by a fault. The fault is oriented nearly vertically in most places but has a slightly shallower dip in this location.

A principle known as cross-cutting relationships allows us to date the fault relative to the rock units. We know that the rocks had to exist prior to being faulted. Therefore, the faulting of the rocks happened after the deposition of the two units.

These rocks (and perhaps the fault as well) were tilted at some point also. I mentioned before that rocks get deposited horizontally (the principal of original horizontality). But these rocks were rotated to become nearly vertical. This may have happened at the same time as faulting of the units or the timing may be different. I can’t really say for sure at this point but with more field study and research about the area I could probably form a more complete hypothesis.

That’s one fun thing about geologic field studies, though. Sometimes the relationships and the associated geologic history aren’t clear after only a day or two of field observations. Luckily we have other tools to help address uncertainties, some being field-based and others being laboratory or modeling based.

The professor of my field course has us end our field trip with a summary of what we did, what we observed, how we interpret it, and finally, what questions remain. I like ending with the questions part because it reminds me that with science, there’s always more to explore.