So, parasequences are interesting of their own accord, but they're even more powerful
if you look at their stacking, in other words how these different rhythmic sequences stack on top
of each other because this 1D view of the world can tell you a lot about the 3D nature of the
sedimentary system that you're investigating. So, here's an example of a parasequence. So,
in the middle here we have the location of a well, or potentially the location of an outcrop,
if you're looking at outcrop and what we can see is that we have the stacking of parasequences. So,
remember that the parasequences are topped with flooding surfaces. So, we have parasequence one,
parasequence two, three and four here and what's interesting is we see that with each parasequences
we have less and less of the shelf mudstone and more and more of the sand, this yellow colour sand
and at the last part of sequence we even have a little bit of that green facies, which represents
the coastal plain and the implication of this 1D view is profound because it is an indication
that the system is prograding because each parasequence represents the infilling of
accommodation, but the stacking of these different parasequences indicates a much greater design, or
much greater pattern, which is one of progradation and of course if you are in a progradational
parasequence set it implies that you're looking at something that is probably either an HST,
or an LST depending on where you are and depending on the facies that you find. So, being able to
look at the 1D stacking pattern of parasequences and deduct something about the 2D, or 3D
progradation-retrogradation and the type of system tract you're in is one of the very powerful things
that this thinking in terms of parasequences brings and at outcrop it looks something like
this, this is a picture of the Blackhawk, the Blackhawk Formation in the USA and you can see
that we have multiple parasequences and each one of these parasequences, becomes progressively
more enriched in the shallow water sand. So, this is a progradational parasequence set.
Of course, we have other examples of parasequences, so in this case here we have
multiple parasequence and notice that instead of becoming progressively more rich in sand,
these become progressively less rich in sand, which by extension indicates retrogradation.
So, again looking at the parasequence stacking pattern we're able to deduct
what the system is doing in the much larger scale, either prograding or in this case
retrograding and there's of course also the case where each parasequence is relatively equal. We
don't really change that much the proportion of the shelf mudstone to shallow marine sandstone
and that is indication of aggregation, or an aggradation or parasequence
set. So, what is the use of using parasequences and maybe more broadly sequence stratigraphy
in trying to determine the geometry of subsurface bodies. Remember, I said in my first lecture
that as stratigraphers we're interested in understanding the geometry and distribution
of subsurface formations. So, let's have a look here at one example that we will interpret using
both the sequence stratigraphic approach, using parasequence set and using just lithostratigraphy.
So, in this example here we have four wells. So, imagine that we have well A, B, C and D, now all
we have as an information is this log here, the four logs of the four wells. We don't really have
the connection between these different bodies, so the connection is really our interpretation.
Now, if you use the parasequence set you basically first determine what the age of your sediments are
and so you recognize flooding surfaces and because you have independent dating you can correlate
those flooding surfaces because you know that they are timelines. So, you obtain something like this,
where in blue you have those flooding surfaces, so the top of your parasequences and so by
extension you connect the sands together, like for instance, sand number four with sand number four
sand number three with sand number three in wells D, C and B etc and that gives you essentially
this geometry of a relatively connected body of sand, the yellow colour, but of course with some
complication because some tongue of sands go and enter are intertongued within the basinal
muds. Now, if you don't use the sequence stratigraphic approach, essentially what
you do is you line up your wells based on their depth at which depth you recover them
and then you look at the lithologies and so you find this green coastal plain lithology that you
correlate in one body and then it gets tricky when you look at the sand bodies because you find them
in every well, but you don't find the same number of sands you know in every well, you have less
sand in wells A and B than in C and D and so you end up with connecting the sand that seem to be at
the same stratigraphic depth and of course this is the wrong approach and it leads you to interpret
sand bodies that are not connected in your interpretation they end up being isolated sand
bodies and that has profound implications if you try to extract resources like water or hydrocarbon
from these sands because you'll assume that they're not connected. So, they are not available,
and we have another example here of a not connected sand body and a third example here of a
not connected sand body and I’ll show you now the two interpretations side by side. You can see that
they are extremely different, and the sequence stratigraphic interpretation is the correct one
whereas the lithostratigraphic interpretation is not. So, you can see that applying the concept of
parasequence set if you have wells or outcrop, but more broadly the sequence stratigraphic approach
that we've been covering in the last two lecture leads to a much more reasonable interpretation
of the interconnection of sand bodies in this case, but any facies really in the subsurface.