Howdy partners! We're back in the Guadalupe Mountains to talk about what happens
to carbonate and ramps attached to continents but deposited under arid conditions.
I'm standing here at the beginning of the Permian Reef Trail.
This is a very famous um trail in the in the Guadalupe Mountain,
I used to take students there for teaching because it shows some really clear evidence
of all the different facies that you can find in this Permian system.
And it also contains evidence that this was a very arid climate when the rocks were deposited.
So in the Permian, in the Late Permian
we had still the supercontinent Pangaea. And the ocean surrounding Pangaea
is known as the Panthalassic Ocean and our location was really on the western margin
of Pangaea, of that super continent.
I'll indicate it here with the star and it was a shallow water location.
And in this particular location, we had subsidence going on.
So we had tectonic subsidence going on.
And the formation of three mini basins: The Midland Basin which is currently
in the subsurface where you find all the oil and gas, The Delaware basin which is partially in the
subsurface but also partially exposed. In fact I'm standing on the edge here of the Delaware Basin
Where, you know, at the outcrop. And we also had the Orogrande Basin in Mexico
which is not shown on this picture.
The point I want to make is that there's a lot of oil in these Permian rocks.
In fact it's the Permian Basin of Texas, it's one of the oldest oil plays in the world
where a fortune was made.
And we're going to try to understand why this is the case because it is a carbonate system.
So let's look theoretically first on this block diagram to what happens to carbonates when they
are exposed during lowstand but in arid condition. And that's a stark difference between what we saw
previously and what happened in arid condition.
The main difference of course is that during lowstand you typically have aeolian dunes
migrating on top of the exposed carbonates.
The sand, the clastic sand is a good evidence for arid conditions.
but when you say arid condition, it doesn't necessarily mean that there is no water.
In fact we have evidence for some water, maybe seasonal water,
maybe during the wet season you had more rainfall.
Because if you look at the interdune area,
we see karstification, minor karstification, but karstification is present in the system as well.
And this is like a nice karst filled with silt and some debris of the surrounding limestone.
Now during the transgressive system tract, you have all the sands on the shelf
that are basically being reworked
So you have a a very classic rich shelf and you have evidence for grain shoal on the
edge of the system and potentially reefs and of course you can still have mass transport deposits.
And in the Permian Basin, you have really good evidence for those shelf sand. Here is a picture:
this is literally a few meters behind me
You can see that we have something interesting happening
We have the gray rock at the bottom, that's a limestone, that's the Permian limestone we have
a clear surface that's a sequence stratigraphic surface, in fact this is a sequence boundary.
This is the this evidence for dissolution.
If you looked closely at the surface you would see also a desert varnish.
So evidence that this was exposed to a desert and then above the surface, so above
the pen here what you see is this more reddish brown lithology.
And this is actually an interesting lithology because it's a sandstone.
It's a very fine well-sorted sandstone.
It's pitted, it's well sorted, it's small, it's clearly an aeolian sandstone.
But what's strange is that this aeolian sandstone contains carbonate grains,
contains marine organisms as well.
So it's a mix of aeolian sand and submarine carbonate
So what is it? Well effectively it's the transgressive
tract and the transgressive tract here in this case is made up of the lowstand tract sands,
aeolian sand that were mixed with carbonates during the transgression.
So you completely rework the lowstand which was the sand
and you make it into the new transgressive system tract.
So the surface we're looking at really is both the sequence boundary and the transgressive surface.
So how does that relate now to the Permian Basin
and what's the important concept here?
Well the important concept is that during lowstand, what we tend to have
is migration of clastic on top of the platform, and this clastic is dumped or shunted into the basin.
That's what we saw previously.
We saw those thick sands in the basin that form good reservoirs, those are lowstand sand.
And on top, we have the of the platform we have the dunes.
Now comes the transgression we rework those sands
And we've seen a a picture of those sands before
and then during the highstand, we deposit more of the carbonate.
So then during the the highstand we tend to be dominated by carbonates on the shelf
and because during highstand carbonate production is maximum,
those carbonates are also exported to the basin, whereas during lowstand we
tend to be dominated by clastic on both the shelf and the basin.
And this is really characteristic for carbonate system in arid environments
And it's known as reciprocal sedimentation.
Reciprocal because you have carbonates in highstand
both on the shelf and the basin but you have clastic during lowstand
both on the shelf and basin. That's the reciprocal part of this equation.
Now during highstand because we're in arid condition, typically what we have is saline
mudflat at the base of that system and salinas. So very saline lagoon if you want.
That's not really a system where you form a lot of tidal flats,
contrary to the wet system,
but you tend to have those salinas.
And if you look at those salinas close from shore: Here's an example again from the
Permian Basin, you can actually have deposition of evaporitic minerals, in this case we see here.
It's a combination of gypsum and halite but mostly dominated by gypsum.
And you also have in that gypsum: some clastic silt.
So it's a mix of clastic silt gypsum and a little bit of of halite in this system.
And that's very characteristic for the back of the depositional system of
the Permian Basin carbonates.
If you go a little bit closer, a little bit more into the open lagoon
you have evidence for high salinities because despite the fact that you don't have necessarily
deposition of evaporative minerals, you see that these stromatolite grow here:
this is stromatolitic limestone.
Now to grow stromatolite, you need to avoid grazing by other organisms, notably by gastropods.
And gastropods don't do well in high salinity. So that's why in modern environments
like Shark Bay, you tend to have stromatolites when you don't have grazers: when you have high salinity
So again in the Permian Basin we have evidence for high salinity in the the saline lake, the salinas.
They're not lake they're actually marine system but in the salinas because we see the deposition
of those stromatolytic limestones. And as we get closer to the open ocean,
the lagoon becomes less saline.
We are more in an open ocean and we start to have just muddy deposits with
evidence for again deposition of open marine fauna like in this case: this large gastropod.