Now, Dunham is a great classification but, like I said, it's based on thin sections.
And if you work on reefs (here's a nice example of a reef) you know that some of those reef grains
can be extremely large. Here, we're looking at a metric grain. And, Embry and Klovan in the 70s
worked on reefs and they noticed that the Dunham classification was very powerful; they wanted to
use it, but it was also not so useful in reefs because everything they looked at was actually
much coarser than what is typically classified in the Dunham texture. So, they had to come up
with an alternative classification, that we will explore now, which is known as the Embry and Klovan
classification that came up in 1971. So, we'll build this classification from the ground up.
Know though, that it was made for reefs, for people working on reefs, that were looking at very coarse
sediments and, I will preempt things a little bit here and say that, we can actually combine this
classification with Dunham. So, we'll do that later. Let's build this classification from the ground
up. What is the first question that comes up? So, the first question in this classification is
whether you're looking at an originally bound together component or a loose sediment and
are locked into sediments. And if it is and are locked into sediment, then the next question is:
Do you have more than 10% of grains that are larger than 2 millimetres? Notice that this
limit of 2 millimetres is arbitrary, but it's important because if you don't have more than
more than 10% of grains larger than two millimetres, you cannot use this classification.
Now, if you do, the next question that comes is whether you are matrix supported or grain supported.
In other words, whether the grains smaller than two millimetres support your framework or whether
the grains larger than two millimetres support your framework. So, if you are matrix supported, so
in other words supported by the grains that are smaller than two millimetres, then we talk about
a floatstone. And, you can see that we have those large, larger than two millimetre grains
floating into the matrix; hence the term floatstone. And here's an example of a floatstone.
Notice the bar at one millimetre, here. And, I will highlight for you the grains. And, you can see
clearly that we have more than 10% grains in terms of surface of this thin section that are
larger than two millimetres. But, these grains do not form a framework, they float in the matrix that is
smaller than two millimetres, and that matrix, by the way, can be micrite, but it can also be just smaller grains.
Now, what if we are supported by the grains that are larger than two millimetres?
Well, then we're looking at what is known as a rudstone, where the large grains form the framework
And, here's an example of a rudstone. Again, I'll highlight the grains for you. And, you can see these
grains basically form the framework of the rock. And, in between these grains, we have the smaller
matrix that can either be grainy or muddy. Now, let's look at another case. If the components are
actually bound together at deposition. And, this is really one of the reasons this classification
was invented; it's to basically look at reefs. Then, we have three cases that are possible under this
classification; either the organism forms a baffle, so a vertical barrier, and then we talk about
bafflestones. Here's an example of an ancient bafflestone. This is basically a branching coral.
And, you can see that these corals form a baffle, so we would talk about a bafflestone in this case.
If the organisms are encrusting or binding, so they're much more like encrusting organisms, then
we have what is known as a bindstone. And, here's an example of a bindstone. This is effectively a
algal mat that binds the sediment together. And, finally, you can also have organisms that build a
rigid framework, so like a full reef, and then we'll talk about a framestone. And, here's an
example of a framestone. This is an ancient reef that effectively is interconnected and forms a
framestone. Now, what is, what usually people do, and the classification we will be using as a class,
is a combination, or an expansion, of the Dunham classification with the Embry and
Klovan classification. So, from now on, when I say we'll use the Dunham classification, I mean the
expanded Dunham classification, that I'm about to show you. So, we start by looking at the
component, and we look at whether or not the components are bound together at time of
deposition. If the answer is they're not, then the next question is: do we have more or less grains
that are larger than two millimetres? If the answer is "less than 10% of grains are larger
than two millimetres", then we use our traditional Dunham classification, that we've covered before.
If we have more than 10% of grains larger than two millimetres, then we use the
terms floatstone and rudstone, as introduced before. And, of course, if we have autochthonous sediments, so
they're not transported, they're bound together at time of deposition, then we use the bafflestone,
bindstone, framestone terminology. And, we can combine this and have one unified classification.
That is effectively what Embry and Klovan did; they combined their classification with Dunham to refer
to reefs. So, when you hear or read about the Dunham classification applied to carbonates, usually it
means this expanded Dunham classification. And, again, please use this one in this course.
I do not mind if you want to simplify your life, if you just call everything that's bound
together at time of deposition as a boundstone, because in most of the examples that we will see,
in ancient examples, it's very difficult to say whether it's a bafflestone, a bindstone, or a
framestone; because you need to be able to look at very large areas of the reef, which is not always
possible. So, I'm perfectly fine if you want to simplify your life and call all the textures
that are bound together at the time of deposition as a boundstone. Of course, if you can use those
more specific terms, it's better. But, otherwise it doesn't really matter, I think.
And, again, you can also remember that this classification is very useful because if we can apply it,
we have an indication of the hydrodynamism of the environment of deposition; how much energy
was present when that rock was formed. And, that is key to trying to understand the palaeoenvironment of
deposition. That, and the fossil content, are the two things we'll use to do palaeo-reconstruction of the
environment of deposition.
Okay, so that brings me to my summary. So ,what have we learned in this class?
Well, we've learned that we will apply the Dunham classification expanded with the Embry and Klovan classification.
We learned that these classifications are based on preserved textures at time of deposition.
So, be careful with this notion, because it does mean that sometimes you will need to make an interpretation on the texture that is present in the rock,
because you will have to strip away any trace of diagenesis.
These textures are indicative of the hydrodynamic energy of the environment of deposition, which is why it's so useful.
And, that gives us an indication of the depositional condition, and we can use
this, then, to build our interpretation in terms of sedimentology, or in terms of sequence stratigraphy,
of the particular system that we are studying.
So, that's it for now. In the next class, we'll look at another type of classification, which is the classification of porosity.