I went fishing for the first time today, and I’d love to write about it because it was REALLY fun – but I only managed to get one photo, and I technically was in between casts, so I wasn’t really fishing. Just sitting on a boat looking cool with a dog. Oops. So stay tuned for a post about fishing later, after I’ve had a few goes at it (and taken some more photos!).
Instead, I’m going to attempt to explain sequence stratigraphy and my graduate thesis project to non-sedimentary geologists!
First, some Geology 101:
Sedimentary rocks are rocks that are made up of sediment (gravel, sand, clay, mud, etc. derived from other rocks that were weathered and eroded). Water (and other things, like wind, but mostly water) moved this sediment around via rivers and waves and tides. Eventually, this sediment settles down for the long haul and slowly gets buried by more and more sediment. This burial causes the sediment to squish together and compact. At a certain point, the sediment “lithifies” and becomes a sedimentary rock. Welcome to my favorite part of the rock cycle.
Sedimentary rocks are conglomerates (gravel-sized sediment held together by smaller sediment), sandstones (basically sand that has become a rock via the process described above), shales (really fine grained stuff, generally too small to see without some kind of magnifier), and mudstones (the finest grained sediment).
The type of sedimentary rock you’re looking at, the size of the sediment grains, and any sedimentary structures that were preserved (like ripples, crossbeds, planar beds, etc,) can tell you what kind of environment the sediment was deposited in. Composition of the sediment grains (minerals and rock fragments) can also help, but sometimes the grains are too small to determine that without a microscope. You might imagine that a sedimentary rock created by a lake (mostly very fine-grained sediments like mud and clay) would be very different from one created in a beach environment (mostly sand), which would in turn be very different from a rock from a river environment (gravel and pebbles). Of course, all of these environments can be variable, but I’ll get to that in a minute.
Stratigraphy is the study of these sedimentary deposits/rocks, and how they are layered.
One more thing before we start putting things together into sequence stratigraphy. Over time, sea level around the world rises and falls for a variety of reasons. Sometimes it’s climate – either global (eustatic) or local. Sometimes it’s caused by plate tectonics – the movement and interaction between crustal plates.
Now, let’s put these concepts together:
I’ve talked about depositional environments, and I’ve talked about sea level change. At the most basic level, putting these two concepts together is sequence stratigraphy. Burial is what ties them together.
I’m going to put this into context with a coastline, because that’s what I work with, and that’s what makes the most sense to me. On a coastline you might have the river meeting the ocean, a beach, a tidal flat, and the deeper, offshore environments.
Imagine you’re on a beach, and sea level begins to rise. Pretend you can breathe under water/sediment, and you’re immortal, so you can totally watch things change on a geological time scale.
First, your beach sand would get buried by finer grained sand from the tidal flat, and as the water continued to get deeper, the beach sediment deposit and the tidal flat deposit would get buried by the deep offshore deposits (really fine muds with maybe a little really fine sand).
That, my friend, is a sequence. If you cut a slice into the sediment right where you were standing when sea level began to rise, you would see this stratigraphic sequence, and the sediment would be getting finer closer to the top. We call this a “transgressive” cycle, because the shoreline is “transgressing” across the land – it is moving landward. Also, the furthest the shoreline extends at the end of transgression creates a surface called the “maximum flooding surface” – hopefully this seems pretty obvious: as sea level rises, you are flooding the environments that were there before sea level began to rise.
Now, pretend you are still standing in the same place on that beach (now buried under quite a lot of sediment), and sea level begins to drop. You might see the return of tidal flat deposits, and eventually you’d see the beach again, and if sea level drops far enough, you might even see the river environments at the very top. This is what we call a “regressive” cycle – the shoreline is regressing away from the land and moving seaward.
If you were to step back and take a slice out of this whole sequence I have described – from the first beach deposit to the fluvial deposit, and then studied how these depositional environments changed laterally and vertically, you would be studying sequence stratigraphy.
Of course, it is a TON more complicated than this, especially since these processes are often erosive, so you don’t always get a perfect sequence that records an entire cycle of sea level rise and fall, but hopefully you get the idea. There are also these things called “significant surfaces” (the maximum flooding surface is one of them), which help us define sequences. They are usually created by some form of erosion – either transgressive or regressive, but involve some kind of shift in either the direction or speed of sea level rise or fall.
One important aspect of sequence stratigraphy is the source of sediment, and this is where the focus of my thesis project lies. You can hopefully imagine that river sediments come from somewhere upstream, while beach or tidal flat deposits might be sourced from somewhere else on the coastline, or they might get sediment from the ocean. Sediment comes from all over the place.
In my field area, the previous graduate student identified three different sources of sediment. I’m going to be looking at the composition of all the sandstones (sorry, sand can be found in many different environments, try not to thing about it too hard) in my rock formation and comparing them to see if there are significant differences between these different sources – and if the sands from the same source have similar compositions. Again, it’s a bit more complicated than that, but that’s the general idea. Also there are GREEN minerals in my sandstones (not a common sedimentary mineral color). I get to identify them – I’m pretty stoked.
Sequence stratigraphy is like studying history, but it’s history of the earth rather than of people, and that’s what I love about it. Sequences of depositional environments is very intuitive to me. Plus, looking at things in a powerful microscope (a few different kinds actually), is really fun.
That thing up in the right corner that looks like plaid? That’s called “tartan twinning.” It’s a potassium feldspar grain. It GREW like that. Plaid is found in nature, guys. Chew on that.
If you’re curious, or you need me to explain something differently, please feel free to leave questions in the comments, and I’ll do my best to help you understand! This stuff comes as second nature to me (and I already find it fascinating), so it’s difficult (as any specialty can be) to break it down and keep it interesting. I hope you at least learned something about sedimentology by reading this post.
Fun fact: “Sedimentology” is not recognized by computer dictionaries. My entire area of study does not exist to technological devices.