We've seen in our previous class the facies model for rimmed platform. We've also discussed the
more complex nature of the distribution of facies at the small scale on those rimmed platforms.
What we still need to do is to look at ramps. And, we're going to do that in this
class.
Yes, that's right my field location for this class is not a field location at all; it's a city, it's
the city of Doha in the state of Qatar. Qatar is a state that thrived on pearl fishing in the past,
but now sits on one of the largest gas reservoirs in the world, which is a Permian carbonate. And,
Qatar also sits on the Arabian Gulf or the Persian Gulf, which is one of the best modern examples
of carbonate ramps; hence my choice of Qatar. So, let's look at the facies model for
carbonate ramps and on this diagram, you can see that on carbonate ramps, we of course do not have
a well developed reef. This does not mean that we don't have reefs, you will see that in the Persian
gulf we have reefs, but just the reefs do not form a barrier. So, what's the barrier to energy
of waves? Well, it's the shoal. So, waves come and crash on the shoal. That shoal is typically sandy
it can be skeletal sand or it can be original sand, it doesn't really matter, it's one or the
other. But, typically, you have shoals that form the barrier to energy. And,
the shoal is what allows us to distinguish between different parts of a carbonate ramp.
So, we have the outer ramp, which is below the storm wave base.
And, because it's below the storm wave base, it's dominated by shales, pelagic limestones, and mudstones,
in terms of Dunham textures. Then, we have the mid-ramp, which is still before the shoal. Now, the
mid-ramp is below fair weather-wave base, but above storm wave base. So, what you end up having are
thinly bedded limestone, which represents storm deposits, and you can also have mud mounds present.
The typical distribution of Dunham texture is more complex there, we expect grainstones, rudstones,
perhaps floatstones, or even wackestones. And then, we have the inner ramp. The inner ramp
comprises the shoal itself and everything that is more landwards from the shoal.
So, the shoal is wave dominated, of course, and it forms a beach barrier, or a strand plane, if you
want. So, essentially it's a massive body of sand that can contains patch reefs, but do not form
continuous reefs, because if it formed continuous reefs you'd have a rimmed
platform. And, the main texture you can expect are grainstones. And then, beyond into the inner ramp
you're in a protected environment where you can have suberial deposits, it's a very, very
low angle dipping system. And, you typically have lagoons, tidal flats, supratidal sabkha, for instance,
evaporites, palaeosoils, palaeokarsts, etc. And, the range of fabric Dunham
textures you expect are from wackestones to mudstones. And again, really the shoal is
where the wave breaks, and so you always want to know where the shoal is to know whether you are
in the inner ramp, the mid ramp or the outer ramp. So, let's look at a map of the Middle
East, here you have the state of Qatar that I point with an arrow. And, if you look at the detail of
the bathymetry of the Persian or Arabian Gulf, you see that it's actually very shallow maximum depth
in the central basin, it's maybe 80 metres, but more typical depth is about 20 metres, 20 to 40 metres, so
very shallow water conditions on this broad shelf, that is you know kind of the best example we
have of ancient ramps. Now, the Arabian Gulf is a relatively modern feature, it did not exist for
very long, in fact it became a shallow sea thanks to melting of ice after the Last Glacial Maximum.
And, this animation shows you how sea level rose from 12,000 BP to roughly 4,000 BP,
and giving rise now to what we know today as the Persian Gulf/Arabian Gulf.
So, relatively recent feature and it's dominated largely by carbonates, which makes it a very
interesting place to to study. Of course, it's no secret that the Middle East is very hot,
so these are hot shallow water, so ideal for tropical carbonates. So, let's look at some of
the distribution and types of facies we can find. There's been multiple studies on the Persian Gulf.
And, we'll start with looking at the coastal system. So, Stephen Lokier has done a lot of work on this,
and if you look at the coastline of Abu Dhabi, you can immediately recognise here that there are some
tidal channels, so tidal channels are very obvious and easy to see. But, if you look at the beach
the details of the beach things that you start to see are things like storm spit so that's sediments
that are moved by storm and here's an example of a storm spit that prograded land ward in one year by
over 20 meters so so very fast moving sand bodies on that very shallow geometry here of of the ramp
and storms are very important on these settings to move sediments you also have in the bit more
distal location some mud flats and as their name indicate those are really flat large extent of mud
and they are actually very broad extent and this is a beautiful fish's belt so to speak
but what's striking is in a lot of these areas you can see that you have a little bit of sediment it
can be those storm spit or some mud but that sediment sits on top of hard grounds and these
hard grounds are indicative of non-deposition at this location now the details of the high
ground is that you can see grains of carbonate and some nice aragonite cements that that basically
cement this hard ground together and if you look at the distribution of those hard ground in the
gulf there actually are located mostly in the shallow areas so where you have shallow water
and not a lot of sediments being deposited essentially you have enough time to cement
the seafloor and you obtain a lot of hard ground so that's one feature of the gulf
another thing that you also see in the shallow areas are mangrove now i showed you some beautiful
mangrove in the last class unfortunately in the united arab emirates the mangroves are declining
and that important ecosystem is lost but historically it used to be a very
broad belt of the position now it tends to be built over with the constructions
and of course because it's so arid you also have sabha we'll talk about sabka in
a different class but in general it's an area of net evaporation and you can see that this creates
ridges expansion ridges where minerals grow like evaporite minerals grow and basically
displace the sediment so a very arid environment here in in the gulf and subra are important
because it's one of the few place where we know of dolomite formation modern dolomite formation
you