so that means geologists can also use subsurface drill cores try to understand what is going
on, and that is the hardest one to do, because on the surface you can see all the relationships
clearly but in the subsurface all you are
relying on the are 3-4 inch of drill core samples,
a lot harder but it can be done. Most of the geology folks at the oil drilling sites, they
do this a lot. They have these huge tables
where they lay down the core samples and they
go every inch of which studied to
understand the rocks and all that. All right,
so we have some geologic events here, just
focusing on the ones labeled here, the geologic
events and I will draw circles next to them and we are gonna put numbers
inside them, each circle is gonna be a number
inside and we are going to be putting them
in sequence of events, order of events. And
also Dike B and sill together, those are two
events together because as you know the intrusion
of magma could be cutting across the layers
as well as could be sandwiched between the layers.
and that is basically what happens with sill.
Sill intruded between the layers, it is not
like a sedimentary layer, so we can't use
the law of superposition for an igneous sill.
sometimes it could be confusing but as long
as you identify the igneous rocks, obviously
it must have intruded. But only caution for
this one is lava flows, lava flows can be
considered for superposition because as you
can imagine lava flows and spills onto the
surface like a layer which could be considered
for superposition, same thing with the pyroclastics
like volcanic ash, volcanic breccia, those
could also be considered for law of superposition.
Keep in mind there are more sedimentary layers
but we are just focusing on the ones labeled,
there are obviously more layers. So how many
circles did I draw? Altogether, I have three,
five, seven, nine. So basically we have nine
geologic events. 9 geologic events. So we
are basically going to put these in an order,
starting with the oldest one, 1 is going to
be the oldest one, as you go up, it will be
younger. the youngest is going to be represented
by number 9, the oldest one will be represented
by number 1. All we are doing here is relative
dating here, we are not really concerned about
the age of the rocks, that is going to come
much later like numerical dating, that is
why you have to also collect samples, you
have to collect samples, you have to take
them into the lab, analyze them and write
a nice geology report, make a map, so you
also have to build a map to see where the
rocks are..so many many things to keep you
busy as a geologist for months, years sometimes,
depending on how complex the geology of the
area. All right, so using the law of superposition,
which of those layers are oldest? as you can
see the bottom layers are the oldest, top
layers are the youngest, we know among the
sedimentary layers, the shale is the youngest
one, and the limestone is the oldest one.
now also we look at the fault here, go ahead
and use a different color to indicate the
relationship. if you look at here, fault A
cuts across, fault A cuts across, cuts across
limestone and sandstone, limestone and sandstone.
therefore it is younger, fault A is younger,
so whatever is cutting across is younger,
whatever being cut is older according to the
crosscutting relationships, that means we
are going to put Fault A on the top and we
are going to put the sandstone underneath,
because sandstone is relatively younger than
limestone, so we are going to put sandstone
underneath, and then we are going to put the
limestone. now if you go back to where I drew
the circle there, red circle, notice the Fault
A doesn't go across the conglomerate, so it
doesn't break thru conglomerate, that tells
us that conglomerate must be younger, Fault
A is older. So Fault A is older than conglomerate.
Okay, that means we are going to have to put
the conglomerate, let me do that here again,
so we are going to put shale, obviously shale
is younger than conglomerate, so we are going
to put shale on top, put conglomerate underneath,
and then the Fault A underneath, so that is
basically the sequence there, obviously these
two will be added down here, let me bring
it down here, you can actually put all those
layers on top right here, okay. So that is
the story there. Now if you look at the Fault
B, moving over to this side, Fault B cuts
across all the layers, as you can see cuts
across limestone, cuts across sandstone, cuts
across conglomerate, shale and everything.
Fault B is younger than all sedimentary layers,
that means Fault B will be on top of shale,
Fault B is going to be right there, on top.
All right, those are basically the relationships
between the faults. Let's look at the intrusions,
intrusions as you can see, you can look at
the relationships here, notice that Dike B
and Sill cuts across the Fault B, so therefore
Dike B is younger than Fault B. Okay. As you
can see, we can probably go ahead and oldest
is going to be the limestone, limestone is
the oldest one because we started from the
bottom right here, limestone, and then the
second oldest one is going to be the sandstone,
and then Fault A will come, and the fourth
one is going to be conglomerate, and then
comes the shale. okay. Now since, on top is
going to be Fault B because Fault B is younger
than all these other layers, also the Fault
A because of the crosscutting relationships.
Now we know the Dike B is cutting across Fault
B, therefore Dike B and Sill will be on top
of the Fault B, but we also have to look at
the relationships between Fault B and the
Batholith. That also the batholith is younger,
cuts across Fault B, also notice the Dike
B cuts across the batholith, therefore Dike
B is younger than the batholith. What is going
to happen is the next one you are going to
have is batholith because the batholith is
older than Dike B, so you are going to put
the number 7 as the batholith. Dike B and
sill cuts across the batholith, and then notice
here Dike A cuts across sill, and the sill
is part of Dike B, therefore Dike A is younger
than
Dike B and sill. Okay. So therefore, we are
going to put Dike B and sill right after the
batholith, and then the youngest one here
will be Dike A. Okay. So if were going to
put the numbers in there, you are going to
go with the numbers, number 1 limestone, number
2 sandstone, number 3 Fault A, number 4 conglomerate,
number 5 is shale, number 6 is going to be
your Fault B, 7 is going to be the batholith,
and then 8 is going to be Dike B and sill,
and then 9 is going to be your Dike A. Okay.
So that is basically how we analyze the relationships
in different places and you can basically
decide which one is before, which one is after,
you have to look at many different locations
and analyze different relationships, then
put them all together. Of course crosscutting
relationships come very handy, especially
the intrusions, intrusions like this one,
as you can see here the dark-colored rock
cuts across a light-colored rock, clearly
they are two different types of igneous rocks,
we know that the dark-colored rock cuts across
is younger, the lighter-colored rock is older,
it was there, it gets cut across. Those are
dikes and sills. And also remember lava flows
and pyroclastics could be considered law of
superposition. Igneous intrusions, you can't
use that but for those you can use the law
of superposition. But mainly it applies to
sedimentary rocks, remember sedimentary rocks
those principles or laws are written for.
But again, lava flows could be considered
in that regard.
