Physical Geology – Arbuckle Mountains – Field Trip, Part 1

>>Dr. Doug Elmore: We are on our field trip
to the Arbuckle Mountains in southern Oklahoma. We are along Interstate 35, so you’re going
to hear some road noise as cars and trucks go by including an occasional honk or two.
I also have some students with me: Shannon, Dustin, and Ally. This is our first stop on
the field trip and what we’re going to look at here is what’s called an angular unconformity,
which is right up here. And let me explain what we have here. First
you’ll notice the lower part of this outcrop you can see some beds. And these beds are
dipping like this. [gestures with hands] They’re dipping to the south, which is that direction.
They’re dipping fairly steeply. They’re Ordovician limestones and some shales. But if you go
up the outcrop you’ll notice, there’s a line up there and at that line you don’t see dipping
rocks anymore you see a different rock type. That rock type up there is a conglomerate,
pieces of other rocks. You’ll notice that some of them are rounded. That contact between
the conglomerate and the rocks underneath is called an angular unconformity. So we’re going to talk now about, and I’m
going to have the students help me go through this, about the events that occurred to produce
this angular unconformity. But before I do that, a little bit about an angular unconformity.
Why is it important? Because it tells us about the history of the rock and the succession,
and it also represents a time gap. Something’s missing. And we’ll come back to that point.
So let’s now talk about the succession of events that produced this particular outcrop
in southern Oklahoma in the Arbuckle Mountains. So I’ll ask the students: what was the first
event that formed here, that started off this sequence? Deposition of what?>>Shannon: These underlined beds.>>Dr. Doug Elmore: Okay, which are lime stones,
alright? Where do limestones form today?>>Shannon: In the ocean.>>Dr. Doug Elmore: Where in the ocean?>>Shannon and Ally: Shallow waters.>>Dr. Doug Elmore: Shallow, warm waters. So
if you want to go study where these limestones are forming today where would you go?>>Shannon and Ally: The Bahamas.>>Dr. Doug Elmore: Okay, tough places to go
for field trips. That’ll be our next field trip. Alright, so the lime stones were formed
and they were laid down horizontally. Then what happened?>>Dustin: Deformation.>>Dr. Doug Elmore: Okay, what’s deformation?>>Dustin: The tilting of the layers?>>Dr. Doug Elmore: The tilting of the layers.
What caused those tilting? Well, you had a deformational or an orogenic event, and that
event was probably the Ouachita orogeny which occurred about 300 million years ago. And
that’s when South America and Africa hit North America, or what is now North America, and
caused deformation and compression and tilted these rocks. So the rocks were tilted up and
then what happened?>>Shannon: It planed off of at the top.>Dr. Doug Elmore: By what process?>>Ally: By erosion.>>Dr. Doug Elmore: By erosion. Okay, so you
had uplifts, the internal processes we’ve talked about in the class before, then you
had erosion, those external processes which are caused by water and so on. You had erosion
and you produced conglomerate, which deposited right on top of these tilting beds. And the
conglomerate’s called the “Collings Ranch conglomerate.” We’re going to see it in the
next stop as well.