So far in this course we've looked at the distribution of facies on modern
ramps and modern platforms. And we've discussed controls on carbonates
but we have not looked at changes in base level and sequence stratigraphy.
So we're going to start now with an introduction of sequence stratigraphy for carbonates.
So we're back in Oman Mountain but in a very remote area of the Oman Mountain. This is a
road leading to the village of Yasib. And the reason I brought you here is because you have
this beautiful unconformity. And right on top of this unconformity you have Permian limestone.
And the very base of those limestone is characterized by these spectacular corals. You can
see they're absolutely ornate corals, absolutely recognizable corals and of course these corals
can only have been deposited in very shallow waters. And if you study or if you do a log of
the carbonates above the corals, you will quickly realize that in the Permian, water death went up .
And the corals were basically drowned and replaced by much more distal ramp type of deposits.
And that is a beautiful place then
to try to understand what happened to carbonates during sequence stratigraphy.
So let's start with some theoretical concept and work out how this impacts the carbonates.
Let's start with the basic reminder! If you take a simplified cross-section of the earth, one that is
basically focused on sequence stratigraphy. We recognize three main things: one is basement,
two is sediment and three in blue is water in the ocean. We can also put on this slide
the center of the earth as a reference point. So now let's define some of the terminology
we will be using. You should already be familiar with this but it doesn't hurt to repeat it.
The distance between basement and the centre of the earth is controlled by tectonic.
Subsidence will create space and you'll have substance of the basement towards the center
of the earth and uplift of course will be the reverse. You will push the basement
away from the centre of the earth. So the centre of the of the earth is our absolute reference frame.
We can also speak about eustasy or eustatic sea level or absolute sea level.
These things are equivalent. So eustasy or eustatic sea level is defined
as the distance between the center of the earth and the top of the water column.
And then we recognize the relative sea level which is the distance between the top of the basement
and the top of the water column. So these things are not similar. Eustatic sea level and relative
sea level are measured relative to a different datum. And that's a very important concept.
Of course sediment can have a certain thickness, so we have sediment thickness.
And then we also have a given water depth. So that leads us to this famous equation which is
known as the accommodation equation,
which states that the tectonic subsidence plus the eustatic component
must be equal to the thickness of the sediment plus the water depth. Okay?
And that is an equation that controls what happens to sediments whether a system prograde, aggrade,
retrogrades when we think about sequence stratigraphy. So this is absolutely fundamental and
something you you probably are already familiar with. So now let's look at processes and what their
implication is for the rock record. So here on the top you have two types of processes, you have
Holocene sea level change and long-term subsidence.
And these are rate of eustatic sea level change
and rates of long-term subsidence. Okay, in meters per million years. And if you look at the record
what we see is that eustatic changes, so short-term eustatic changes
are at the same rate than modern Holocene coral reef deposition and ooid deposition.
In fact, if you look carefully at the rate of coral reef growth, it far exceeds Holocene sea level
changes which means that coral reefs, healthy coral reefs,
should be able to accommodate any eustatic changes.
We'll see it's not always the case but in theory it is possible. But the main point is that
those short-term process control short term thickness of modern sediments. But if you look at
a longer term thickness of ancient platforms. So like the Apennine platform, Triassic Alps
or The Bahamas from the Jurassic to the Recent. These are the red curve at the bottom
you see that they are much lower in terms of rates. They're not as high as the green rate and that's
because on the long term what controls sediment accumulation ultimately really is tectonic.
It's subsidence. So it's a long-term substance rate that ultimately control at the geological time scale,
the thickness of the sediments that you can accumulate. So these are important concept to keep in mind.
Another important concept to keep in mind is that carbonates are by are
biologically mediated. And so that implies that
their biological response to a change in the environment
will dictate partially at least the volume of sediment produced. So let's look at the rate law
for ecology. And this is a general diagram that is applicable to coral but is also applicable to mice
population, viruses, whatever you you can imagine. So it's really an ecological concept. So imagine
if you look at this graph that we have growth of either a niche, so that means like an environment
in which animals can live or growth of the population, so that's a vertical axis versus time
on the horizontal axis. So in green the dashed line represents here a line of a creation of a new
niche. So in terms of shallow water carbonates, a niche would be warm waters in shallow waters.
Okay! that would be an ideal niche.
So imagine that we have a very rapid increase in a niche. That a niche is created all of a sudden.
Let's say you drown a flat top surface and you create hundreds of square kilometers
of that perfect environment for corals. So that's a very rapid growth of the niche that
then eventually plateaus at the top. Now the corals of course or the carbonates will want to colonize
this environment and they will do so. But at first because the population is slow, each generation
when they reproduce can only feel so much of the niche as they can. Okay? So this is a
exponential growth of population. So at first because the creation of the niche
is much faster than the rate at which the small population can reproduce we are in what is known
as a lag phase. The population lags behind the equation of the the niche. In terms of sequence
stratigraphy, we talk about the startup phase, when essentially the population starts up to
fill the niche. And then this curve goes into a log phase. When the population becomes sufficiently
large, their reproduction means that they will be able to feel some of the niche more rapidly
and in fact they might even have a rate of growth that is greater than the rate of increase of the niche.
And that's known as the log phase. And in terms of sequence stratigraphy, we talk about the
catch-up phase in this case. And then there comes a point where this the population is so large
that the space available. The niche itself becomes the limiting factor to their growth
because it stopped growing, the niche. So then we are in what is known as the keep up phase
of the sigmoidal growth law. Now this law is very important
if you want to understand how carbonate respond to eustatic or or base level change.