Hi, I’m on the Joides Resolution again and this time we're just offshore from the Great Barrier
Reef in Australia. This is the coral sea ODP leg 194. The goal of this leg is to reconstruct
the magnitude of Miocene sea level. So, it's one of many ODP IODP expedition that relies
on the principle of sequence stratigraphy to try to understand our natural world and the past
earth's condition better. So, let's talk about sequence stratigraphy.
So, in our first lecture we talked about unconformity and in the second lecture we
looked at chronostratigraphy and we've also talked about the fact that lithostratigraphic units
are not chronostratigraphic units. In this important lecture I will introduce the concept
of sequence stratigraphy, which is a modern way to look at sediments in space and time.
Now in a nutshell sequence stratigraphy is the science of looking at genetically related units
that are bounded by unconformities and their correlative conformities.
Now, this is a bit of a mouthful, but I will in the class introduce the notions that you need to understand this.
So, I think that the best way to understand sequence stratigraphy is to take a historical trip
down stratigraphy and how different notion came into being and how in the late seventies
these were assembled into the science of sequence stratigraphy that we know today
and perhaps one of the first pioneer in that way of thinking was Walter a German stratigrapher
who came up with Walter's law. Now Walter's law is essentially to recognize that the
stratigraphic record is subdivided by conformable and unconformable
surfaces and Walter's law states that to be conformable a succession needs to have a
juxtaposition of facies vertically in a log. That is similar to the juxtaposition of facies
you would find laterally in the depositional environment. So, effectively Walter's law
reflects on the sediments in space and in time. So, in this diagram what you see is a vertical log
with three facies, the upper shore face, middle shore face and the lower shore face and then we
also have a sense of the lateral distribution of those facies and Walter's law stipulates that to
be conformable the vertical succession of facies must also correspond to the lateral succession of
facies. So, for instance if we start with the lower shore face at the base of this section,
on top of this we expect the middle shore phase and on top of that we expect the upper shore face
and if that is the case then we most likely are looking at a conformable succession of facies. So,
in concrete terms what does that mean for environmental deposition and what we can expect to
find in the stratigraphic record. So, here's an example where we have a beach facies which is
sand, followed by silt, which is a finer facies, followed by clay, much deeper in the depositional
environment and finally deep in the basin we find carbonate mud. Now a conformable succession would
show the same succession of facies that means that a timeline would basically follow this dashed red
line. So, you can see then that it's very obvious if this red line is a dash, if this dash red
line is a timeline then lithostratigraphy the units that represents one lithology
are not chronostratigraphic units. So, lithofacies are time transgressive that's one very important
conclusion you can get from Walter’s law. It's also obvious that what we see here is
a shift in the position of facies, at first in blue we have a transgression
that's when base level, I will use the term base level which is essentially equates to sea level
and for simplification when base level rises or sea level rises the facies then migrates towards
the continent and that's a transgression and this is followed by a regression and that's a very big
concept in sequence stratigraphy that we will explore how and why we have transgression
regression cycles. So, if Walter's law is the first step in trying to understand sequence
stratigraphy, the second step a very important step is the notion that a surface, an erosional
surface can be a time surface because that's one of the tenet of sequence stratigraphy and
the first researcher to actually use stratigraphic surfaces as timeline is Elliot Blackwelder in 1909
where he published the use of unconformities as time markers. So, if you look at this figure what
he noticed is that the sediments that are folded at the base, we don't really know when they were
deposited it's very hard to find a surface in those sediments that represents a timeline.
Same thing for the sediments that are unconformably overlying this sequence,
however the unconformity itself marked here by the red arrow. That unconformity represents a
timeline. This is the timeline when essentially, we had erosion of the folded sediments.