So, what are the basic principles of stratigraphy? Let's start with the principle of uniformitarism,
this was defined by James Hutton in the 19th century and it's often referred to as the present
is the key to the past. In other words, the principles that apply today in modern sedimentary
system are still applicable in the deep past. So, we can understand processes in the deep past by
looking at modern principles and that is one of the key principles in stratigraphy, but it is not
the only principle. A key figure for stratigraphy perhaps an unlikely figure is actually an
ecclesiastic from Italy who lived in the 17th century and his name is Nicola Steno came up with
three principles that are still applicable today in stratigraphy. Steno's first principle
is known as the principle of superposition and I've already mentioned this earlier in this
class. It's to basically recognise that because sediments were deposited in the ocean
on a flat surface, the layers that are below are older than the layers that are above. Now this
sounds pretty simple for 21st century geologists, but in the 17th century this was a revolution and
Nicolas Steno introduced that principle and we can look at this principle here in the dolomite,
which is the region where Nicolas Steno lived, and the dolomites are Triassic in age. Their limestone
deposited in the Triassic and just by looking at the layers we can already know where the
older strata are and where the younger strata are and this is really the only thing we can
do with lithostratigraphy to determine time or an age. Steno's second principle is the principle
of original horizontality. He recognized that these were marine strata, marine sediments
that were deposited at a very low angle, maybe one two degree, in his mind probably flat and in the
field, it looks like this this is a picture I took in Oman. You can see it's a beautiful
anticline, actually it's a salt core anticline here in Jebel Salak and the principle of Steno's
law is that even though these layers are now tilted the top can still be recognised and
we can still recognise the fact that these were deposited flat and we know today that tectonic is
the main mechanism by which these layers are being upturned and then finally Steno's third principle,
is the principle of original lateral continuity. By this he means that when the layers were
deposited, they were continuous laterally, you could follow them to the edge of the basin,
but today if you look at modern sediments they are of course cut by valley, cut by rivers and they no
longer can be traced. But Nicolas Steno recognised that this was a feature of the modern landscape
and so by extension he introduced the notion that we could do correlation. That we could actually
trace rocks in space, if not in time. Now that principle of lateral continuity of course has its
limitations there is a point where the layers will stop to continue laterally. We know that layers
are not infinite in space they will for instance stop at the edge of the basin at the very least,
or they may pinch out, or just you may not have enough sediments to fill the entire accommodation.
So, we can recognise multiple types of different terminations. We can have either pinch out,
where the layers are essentially terminating by a pinch out geometry. We can have intertonguing,
where different sediments are mixed together as small tongues of each of the different types of
sediments, or we can also have lateral gradation, where the sediment essentially grade from one
type of sediment to another type of sediment. For instance, sand becoming progressively more rich
in clay and eventually we deposit a clay stone. Here's an example of intertonguing in Texas, this
is the Permian Basin and what you see here is different members of a limestone slash clastic
type of deposit. So, we have the Lamar member which is the slope of a carbonate platform here
and below we have the Cherry Canyon tong which is deeper clastic sands, and you can see very clearly
here that these two lithologies, these two members are intertonguing at a relatively small scale
at the outcrop. You're looking at maybe 60 meters of deposits. In the subsurface,
this is also visible and you have here an example of how those sands come from the basin and are
mixed with the slope carbonates and immediately you can see the value here of trying to understand
the distribution of sand and carbonates. If you're interested, for instance in finding water,
oil or other resources. You can see that the subsurface is more complicated than simply a nice
layer cake type of stratigraphy. So, that are, so these are the three principle from Nicolas Steno,
but these are not the only principle or laws of stratigraphy. Another, very important law is the
principle of cross-cutting relationships, which you must already be familiar with. This principle
simply states that something that cross cuts a fabric must be younger than that fabric. So,
in this case I'm showing you a dyke on a left and a fold on the right and both the dyke and the fold
must be younger than the host rock that contains them and that is common sense.
Another very important principle is the principle of inclusion which states that any rock or
feature that is included in a lithology must be younger than the bed in which it's deposited.