Black Rock Beach

Welcome to the last stop of our sedimentary geology chapter! At this stop, we're going to try to use all the skills we've covered thus far to practice creating a stratigraphy column and putting together a geologic history of the area.

This is Black Rock Beach, north of Talbot Island, and it is composed of a black quartz arenite. If you remember from our Lineville Falls stop, a quartz arenite is a sandstone made of 90% (or more) quartz grains.

As a coastline evolves over time, the composition of the sedimentary deposits changes — meaning, if you look at the layering of the beds, you can see the change of the depositional environment over time. If you think about out west (say, Utah, or New Mexico, or the Grand Canyon), there are large outcrops full of different colored layers. When there are big changes in depositional environments, it gives us these changes in color (which is a change of lithology). Sometimes the change isn't a sudden one (where we go from a red sandstone right to a yellow mudstone) but instead is a gradual change.

We're going to be reconstructing the stratographic sequence of this area by looking at the compositions of the lithified and non-lithified units. By establishing a stratigraphy order, we'll be able to then have a discussion about the formation of this coast and how it's changed over time.

Let's introduce you to our first unit - a black sandstone:

Blackrock beack intro.MP4

Next, lets look at the unit that's below the black sandstone:

BRB strat 1.MP4

Now lets take a look at the units above the black sandstone:

Last, let's look at some erosional/secondary features:

Paleosol.MP4

mangrove graveyard.MP4

Let's now talk about how we divide units. Generally, we want to divide these units based on the ones that have a different depositional setting; however, a depositional setting isn't something we observe, but rather something we infer based on our observations.

So what are some observations that can help us divide units?

Color is a big one — but not one we want to rely completely on if at all possible. Although some units might be the same color, they may be compositionally different.

Grain size is another important one — we shouldn't have mud/silt particles being deposited on a beach environment (remember, the smaller, light minerals get carried further out to sea where there is lower energy). So if we see something that is just sand particles, and something that has a combination of sand and mud, or a unit with just mud, we know that something had to have changed.

Mineral composition — if we see a unit that is a quartz arenite, and another that is a lithic arenite, we know that something had to have changed (even though they're both sandstones).

Friability/lithification — If you were to be at the grand canyon, you'd notice that some layers form cliffs, while others form slopes. This is often fundamentally tied to grain size, but can also be tied to the type and amount of cement present. Cement is the mineral that is existing between the grains of sand to hold them together as a rock and can take on many compositions (quartz, calcite, hematite, etc.). In addition to this, especially when looking at young stratigraphy (such as this stop) different units have existed for different amounts of time, and thus may be at different stages of lithification (we would expect the older, deeper units to behave more like a rock, and the younger, shallower units to behave more like unconsolidated sediment.

With these things in mind, let's take a look at the stratigraphic column here:

BRB strat 2.MP4

Cross bedding BRB.MP4

Here is a close up of the hummocky cross-beds.

Congratulations! You finished Part 2! Now would be a great time for a break, and when you're ready for more, we're going to really change directions and talk about structural geology in the field.

Topic 3: An Introduction to Structural Geology

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