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Welcome to our first sedimentary geology stop!
I encourage you to zoom around on the map to the right. There are three locations along the Florida coast that we're going to visit, each of which is unique.
In terms of geologic time, Florida is very young. The oldest units we'll be showing here formed in the Pleistocene, though there are older units in different parts of Florida — some date all the way back to the Eocene!
Looking at this map on the left, you should notice that all the shades of yellow refer to a Quaternary unit, meaning these are the layers of sand/dirt that are sitting on top of other units. You may also notice that you see older units more as you look to the interior of the peninsula. That is because the recent sea-level changes in the last several thousand to several million years have allowed for erosion and subsequent deposition along the coastline. An indicator of sea-level change in Florida is the preserved shorelines found both above and below modern sea-level, called paleo-shorelines.
Florida stores a beautiful geologic history recorded by many fossils that range from terrestrial animals during times of low sea-level, all the way to microscopic fossils that make up some of the vital geologic units. Although this field trip covers coastal Florida, remember that this state is also full of Karst features (like sinkholes and caves).
Welcome to Washington Oaks Gardens State Park! Located south of Saint Augustine, Florida, Washington Oaks is known for its wonderful outcrops of coquina, right on the coast.
Coquina is a sedimentary rock that is made from the shell fragments of marine organisms and cemented with (in this case) calcite. As a brief history lesson, this special rock (which is actually a limestone!) was named after the small coquina clam, Donax variabilis. This clam, combined with plenty of microfossils and shells, formed in a slightly off-shore environment with thousands of years' worth of deposition. Following the end of the most recent Ice Age, sea-level fell and allowed the compacted shells to be subject to the elements. As acidic rain percolated through the shells, calcium carbonate formed and cemented the materials together!
While the map above shows that this area is mostly Quaternary alluvium (which the vast majority is!), what we're looking at here is a part of the Anastasia formation, which formed sometime in the Pleistocene.
I encourage you to watch the videos below, look through the pictures, and make general observations about some of the sedimentary features you see. In some of the videos, we'll also ask you questions!
If you have any paper or a notebook handy, please try to take field notes and sketch an outcrop from any of the pictures!
Tori :).MP4Roots in Coquina.MP4Strat relationships in Coquina.MP4Berm on Coquina beach.MP4Coquina textures/grainsized.MP4Clay.MP4
FIELD SKETCH
What do your field notes look like? Don't forget — sketching outcrops and taking notes is an essential step in the field! Sometimes it can be hard when you are facing harsh weather conditions or are tired, but as a good geologist, you should still make the effort. Notes do not need to be pretty. It's the content that matters! Above is an outcrop that displays clear layers, with a pencil for scale.
Pretty cool root system under these rocks to the right.
Above is an example of field notes for the outcrop to the left. Note that the sketch shows layers that are distinct and colored differently to show separation, as well as numbered to correspond with the notes below the sketch.
Welcome to the famous Saint Augustine fort - Castillo de San Marcos!
Castillo de San Marcos is the oldest stone fort in the US (it's over 315 years old)! Originally built by the Spanish, it has had many roles over the years and now serves as a historical monument that can be toured.
The fort is built out of the same coquina observed at Washington Oaks Gardens State Park. This rock was the perfect building tool for defending the fort from British attacks. The holes in the coquina shown below are the result of cannonballs. As you can see, the coquina easily absorbed the impact and protected the fort! Take a look at the photos and videos below — also look to see if you can find any bedding in the blocks. They aren't in situ (meaning, in their original location of deposition) but if they were, think about what that bedding would tell you.
Fort canon balls.MP4Here we are at our last location for this stop — Little Talbot Island State Park. As we move up north on the east coast of Florida, the depositional environment, and thus the geology, changes! Little Talbot Island is an Atlantic barrier island off the coast of Florida, north of Jacksonville. We wanted to include this last stop to show what a "typical" Southeast Atlantic beach looks like before we move on to lithified units.
As you flip through the pictures below, notice the different colored sand grains/bands. In nearly all the beaches of the world, sand is commonly made of three components: quartz, feldspars, and lithic (rock) fragments. The differing amounts of these three components help to tell us the sediment's provenance (where it's source rock, the rock that is eroding into sand grains, is located). Their amounts also tell us about the distance, climate, and processes at work to move and break down the grain/rock into tiny minerals.
When a rock is eroded, less resistant minerals (not quartz and feldspar) may be chemically weathered to form clays. However, it is also possible that a few of these less-resistant minerals may make it to the beachfront without being altered. Many of these minerals are referred to as heavy minerals, which tend to be darker in color.
A heavy mineral is a mineral with a density greater than 2.9 g/cm³. In order to show actually how heavy these minerals actually are, let's think about if you were to hold a pint of water. We know the density of water is 1 g/cm, so you would be holding a pound of water. However, if you were to take same pint, and instead managed to fill it with quartz, it would weigh a lot more (the density of quartz is 2.65 g/cm³). Finally, if you managed to fill that whole pint with magnetite, a magnetic mineral that is commonly used in magnets and has a density of 5.17 g/cm³, it would weigh a little over 5 lbs.!
The sands in this part of Florida (and much of the Atlantic coast) are sourced from the Appalachian Mountains. A few of the heavy minerals that are present on this beach, and many others, include ilmenite, rutile, zircon, and staurolite. All of these minerals, aside from zircon, tend to be darker in color. When you look at the photos below, the dark stripes of minerals that you see as we dig down are bands of these heavy minerals.
Let's talk about why heavy minerals form bands. If you think about trying to move an object, the heavier it is, the more energy it takes for you to move it. The same principles apply to these heavy minerals when they're in water. The heavier the mineral is, the more energy the water must use to move it. The ocean typically has the most energy during storm-surge events. The heavy mineral bands you see are often interpreted as storm-related events, or at the very least high-energy events.
Beach profile.MP4
When you're ready, let's move on! This stop was to introduce you to modern-day sedimentary environments and to get you thinking. In our next stop, we're going to go a little more in-depth into how we determine the depositional environment, like the one we just observed, from a much older sedimentary rock.
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