This is geology B with all of us trumple.
Rivers can seem very ephemeral often changing course or drying
up entirely yet. Some Rivers have persisted for
tens, or even hundreds of millions of years, even
testifying to the breakup of Pangaea, the most recent,
supercontinent about 200 million years ago, on the one hand,
their courses may be determined by tectonic processes, such as
the formation of mountain belts, and on the other, they
themselves can affect tectonic processes bike.
Rating continents scale features such as giant submarine fans.
Martin gambling has spent a lifetime studying rivers and
river sediments around the world.
He is Emeritus professor in the department of Earth and
environmental Sciences at Dalhousie University in Halifax,
Nova Scotia in Canada. This episode is the first of two
of our conversation about rivers.
In this episode, we talk about fluvial deposits in the
geological record and the impact of the breakup of Pangaea on
river systems. In the second episode, we talked
specifically about the history of rivers of Europe and the
Americas as well as the impact of recent ice ages.
We end by considering how humans have changed rivers and their
deposits throughout human history.
Martin giggling, welcome to geology B.
Thanks very much Oliver. It's a great pleasure to talk
with you. How far back into the geological
record, do fluvial deposits. Go.
They go back a surprisingly long way.
The oldest fluvial deposits about which we are really
certain are in the mood is group of southern Africa.
And those date back to about 3.2 billion years and that's a long
way back towards the origin of the earth.
That 4.6 billion the May well be older fluvial Rocks.
But up to now they've been to altered and deformed for anyone
to be certain about. However, it's worth saying that
in rocks of Western Australia, researchers have found Zircon
grains that date back to about four point four billion years
that takes us shortly after the formation of the earth and very
surprisingly. They yield isotopic evidence for
surface temperatures that were very much like those of today
that implies liquid water on the surface of the Earth.
It in turn implies the presence of flowing water and rivers.
So we have to assume that Rivers have been with us since the very
earliest days of planet Earth. Okay.
But with those icons that's quite a complicated chain of
inferences. How do we actually distinguish
an intact fluvial deposits in the geological record?
From other waterborne deposits? Such as those lay down on Lake
beds or on the seashore and those deposited by wind?
Yes you have to get into the details.
The Rock record and the strata. And there were looking at the
organizational architecture as it's called of the strata, as
well as particular sedimentary features and fossils that are
present. And of course, if you're far
back in time, you won't have the fossils to guide you.
And if you've got very deformed or altered rocks, then you're
struggling to make sense of the sedimentary features, but what
you can often see in younger strata.
Its places where River channels, have eroded down into the strata
below and you get surfaces that look very much like Channel
forms filled in with sand and gravel.
What's filling them in is often cross beds laid down on the
Avalanche phases of Ripples and dunes and those allow you to
measure the flow directions at the time, the water was flowing.
And in the case of river channels, they are
unidirectional, they're all going in One Direction down the
channel, which is strong evidence that these were Fluval
and then you can find things like rain, prints and mud cracks
in the strata. Jinda, Kate that these were
exposures on the terrestrial surface and then in younger
beds, you get terrestrial fossils.
Roots, Woody material, freshwater, fish shells, and
things like, trackways made by giant arthropods, and also by
dinosaurs, of course, so all these lines of evidence can make
you pretty certain that you're dealing with fluvial deposits.
Wouldn't most of those also possibly appear in wind-borne
deposit, or is it that it's unidirectional with rivers.
So, so that rules out win because it couldn't blow this
way and that it can. But you can, of course, get
systematic. Directions of wind flow that
would generate uni-directional crust strata.
But in those cases you wouldn't be seeing the channel basis.
Sometimes with gravel lining the surfaces as a kind of a lag
where the channels eroded in and that's where the architecture of
the strata and their organization come into play.
You would also get mud layers in River Channel.
That you wouldn't be expecting to get in Dune systems, for
example, in that previous episode of geology, B Neil
Davis, Describe the impact on the landscape of the first land
plants during the middle division to the mid devonian.
Can you remind us as to how fluvial deposits changed when
plants? First colonized the land?
Yes, Neil is made a wonderful job of setting the stage for
this. What happened?
When the Earth was greened, the great Green Landscape?
Ellucian. As I think of it, the earliest
land plants in the OR division are represented by spores
microscopic fragments of plants and they came from very small.
Plants with simple unbranched stems and minimal routing
structures. As we move on through the
silurian, they start to become a bit larger complex enough that
you get a bit of a macro fossil record.
And then when you hit the devonian things really take off
large routing systems evolved By the mid devonian and enough
biomass for it to get preserved in the fluvial, record along
with things like peat which indicate that vegetation is
really beginning to build up. Solidly in the case of rivers,
this led to a profound and dramatic increase in the amount
of mud that was preserved in fluvial, deposits and Neil.
And his colleagues are very nicely documented.
This really very dramatic Trend. What the mod implies is the
weathering was Densifying, almost certainly due to the
organic acids and the Carbonic acids associated with plants and
with the drawdown of CO2 from the atmosphere and groundwater
systems. And when you had mud being
produced in Greater abundance, you also had more of it
transported in river systems and then trapped and baffled by the
standing vegetation. And so it tends to build up in
the river Plains and become preserved rather than just
Passing through to our sir is Notions, the mud would have had
the effect of making river, sand.
More cohesive strengthening the the bed of the rivers
strengthening particular the Riverbanks and they would have
become more difficult to erode by flowing water.
So you start to get more obvious Channel, Banks and channel forms
in The Preserve record through the latest silurian and
devonian, are you suggesting that by the mid devonian, the
modern River had formed and that all the different plants that
Would have had an impact on Rivers.
How that was all sort of, pretty much the way it is today, or
have there been further diversification and have certain
regions of the planet being colonized?
That went really fully colonized by the mid devonian.
The bit devonian was when things really began to take off with a
sense of very large forest trees that were a meter or even a
couple of meters in diameter with very large root systems.
And there have been recent discoveries in Upstate New York.
Boa and elsewhere that have showed very beautifully.
Some of these major Forest systems but that was only really
the start and by the Carboniferous, you're starting
to get early gymnosperm plants, the ancestors of conifers of the
present-day and those were able to penetrate quite deeply down
even into dry land areas. And so you start to stabilize,
not just the Wetland environments, but also
relatively dry open Plains. It's and what's happening is
that Rivers diversify through to about the mid part of the
Carboniferous say, the start of the Pennsylvanian when they are
more or less covering the range of things that we see at the
present day. So, initially we had large sand
bed, River's water typically braided, rivers, with shifting
sandbars, and rather weak Banks, and those progressively move
into being more Meandering, Rivers.
With Muddy Banks, where the and has migrated much more slowly.
You start to get abandoned channels, you start to get
multi-channel anastomosis, river systems Wetlands with black,
organic stained water and the whole range of what we now know
is river systems, today is coming into existence through
that period of time especially with the riparian or Bank
vegetation that makes the whole system so much more intricate
Katyn complex and where life organisms simply began to evolve
at a much more rapid rate in these little niches along the
lines of the river? Corridors was there any
significant impact on the Fluval record?
When the planet started to be covered by large tracts of
tundra open. Grassland?
This was a major change and people are still trying to work
out how much that affected river systems you have the angiosperm
plants flowering plants coming in in the Cretaceous.
They typically greedy of water in many cases and then you have
the grasses coming in, in the, the cenozoic with, they're very
dense, root systems, and the way they bound the flood.
Plain surfaces must have have quite a lot of effect on river
systems. It's just a bit difficult to
figure out how exactly they affected things.
But, along with them, you have the rise of gigantic organisms
like the dinosaurs and the big herds of ungulates.
That you might see in the Serengeti at the present day and
the Buffalo before they were pretty much exterminated in
North America. And those would have trampled
through Riverbanks and in places like hippos do today, they would
have made Trails down to Rivers, which would have allowed rivers
to break out a long low points at times.
And people have even made a case for Dino avulsion so-called
where dinosaurs might have trampled their way through to
the rivers and promoted River breakout, well, Incredible image
of China or a hippo affecting the course of an entire River.
Yes, it's quite remarkable. And you mentioned the tundra
areas and the rise of permafrost, greatly affected
Northern river systems eventually in the ice ages.
And those of course are starting to melt away somewhat at the
present day and you're starting to see modifications of rivers,
as the thermal water begins to erode down into the permafrost.
So that were complex effects related to the glaciations even
in your book. Entitled River Planet, you have
a wonderful chart showing the longevity of modern Rivers.
What exactly do you mean when you talk about the age of a
river, that's a very good question.
Rainwater is always flowing downhill.
So, in reality, the flow Paths of water are always evolving
following tiny gullies, little reels moving together into
larger channels and trying to figure out the age of a It is
really a bit of a chimera it's like trying to say how old a
rainbow is for example, but it's helpful to think of the age of a
river as the length of time that the river Channel or course, has
been following pretty much the line that it has at present,
wouldn't be exactly the same course because channels, migrate
they break out, but the channel belt as a whole would be
occupying within a matter of a few kilometers.
Probably about the same. Mission, that can be very
difficult to, determine you might think it would be obvious,
but sometimes Rivers cease to exist for a while.
Maybe the river plane gets drowned out by the sea or lake
glaciers can overrun it. Lava flows, sand dunes can
invade the area. The river May cease to exist for
a while, but it then may re-establish itself in the same
course. And a good example of this is
the Columbia River in the western us, probably.
Originated about 17 million years ago in the Maya scene as
far as anybody can work it out. But periodically the river was
overrun by lavas and by pyroclastic, flows.
Coming off the Columbia Bassel plateau in the East and the
Cascade Range of volcanoes in the west.
And it seems that the river simply cease to exist in its
organized form for periods of time.
And then it reforged a new course, more or less where it's
been before. So, it gets to be a little
debatable, whether those kind of gaps in the record matter or
not, whether you reset the clock if for a little while, the river
ceases to exist. But in Broad terms, I think we
could say that with minor gaps there were rivers that have been
present for hundreds of millions of years in their courses and
that makes sense. So how old are some of the
oldest Rivers? Broadly, speaking that still run
in their ancestral course has the oldest rivers.
That anybody is really documented running an end That's
true courses. Go back to the Carboniferous
about 300 million years ago by no means back to the beginning
of the earth. The earth is an extremely
dynamic system and rivers have been destroyed in the way that
almost anything else has on Earth over geological time.
But some of the world's oldest rivers, are those of Eastern
Europe, the Volga? The dawn, the Dnieper, they may
be as old as the Carboniferous and they run down into the black
and Caspian Sea. These which are the last
remnants of the tethys ocean death is got squeezed out.
When Africa came up against Europe, Arabia, likewise, and
then India against the southern part of Asia and death is was
eliminated except for these minor Oceanic remnants.
And in that part of Eastern Europe, the deformation has been
south of the black and Caspian seas in turkey and the Caucasus.
And so, the rivers further north have not yet been Disrupted by
the rise of mountains to the South that affect them.
And so in course of time they marry very well reverse their
directions to flow north, as other of us have done, but they
haven't done that yet. So they're still in ancestral
courses quite remarkably or broadly within them.
And if you want an example in North America, there were Rivers
Running along the line of the modern Mississippi, west of the
Appalachians back into the Carboniferous about 300 million
years. Ago.
And this is a case where the Mississippi for a while in the
Triassic and Jurassic ceased to be in its present position, due
to faulty activity Marine flooding, but it's been running
along its present line down to the Gulf of Mexico since the
Jurassic a lot of these courses take us back to the time when we
had the supercontinent Pangaea. Wouldn't the subsequent breakup
of Pangaea have completely altered the drainage patterns
and therefore, the courses of the rivers, Yes, certainly.
So the breakup of Pangaea into smaller continents that drifted
apart had profound effects on, virtually all of the world's
Rivers, when Pangaea was in existence, there were rivers of
immense length. Some of them probably longer
than any in existence, today, they would have flowed out of
Mountain chains like the Appalachians in the eastern US
and the risk and Mountain chains Europe and those were equivalent
in scale to the modern and he's So we have to envisage, Rivers,
probably on the scale of the Amazon at time's running out
cross the adjoining Plains. And in the case of the southern
continents gondwanaland, there's evidence from age dates from
again. Zircon grains that there were
very large rivers that flowed out of Antarctica which was not
at that time under ice and they flowed out across Australia into
the Sydney Basin. For example, on the New South
Wales coast and then out into the sea, some part of India and
these continents, of course, and a thousands of kilometers apart.
But at that time, they were part of single very large river
systems that ran through. But once Pangaea began to break
up, all of those systems were dismembered to begin to change.
That's fascinating. So can we track a particular
river that used to be contiguous that now has been separated by
thousands of miles of ocean? I mean, do we look at fossils or
is it all the detrital zircons? How can we do that matching or
is that even possible? It's very difficult and there
aren't many cases where anybody has made a very precise match.
These are not for the most part rivers that exists of the
present day so they're not rivers with names in other words
that we apply to actual flowing water in the case of the, the
Sydney Basin in Australia. It's the grains that give you
the age signature of an eroding Upland in Antarctica and the,
the Sydney Basin has the Triassic.
Rocks of the Hawkesbury sandstone.
A very very big brahmaputra scale or nearly that scale of
river system. It's flowing in across a full
belt out into the what was the ocean at that time and it has
been matched to the highlands in Antarctica, but it's very
difficult to actually track it all the way back there.
There are gaps the erosion and so on and so forth.
So it's these kinds of lines of evidence of B circumstantial
that We build those stories. But I think some of those are
the most fascinating stories that you could pick up a river
in one continent, and potentially find its
continuation in another continent 5,000 kilometers away.
I think there's going to be a lot more coming through on that
kind of thing in the future. Yeah it's amazing.
And can you go even further? You said earlier that fluvial
deposits were characterized by being unidirectional.
So can you infer the direction of current flow and say okay?
Well Upstream there. There must have been a mountain
belt at that time. Yes.
A lot of This comes from where the fluvial deposits are located
and their flow directions. While people refer to, as paleo
current flow, the flow directions of the ancient
currents. And if you go on the field, you
can measure tens, or hundreds of cross bed, directions and figure
out which way the flows will going.
And you can begin to build up patterns regionally afloat.
Our actions did any major rivers actually survive the breakup of
Pangaea. Oh, do we have a whole fresh lot
of rivers? After that happened, the Quite a
lot of rivers that are relics There Are Places on some of the
plateau lands in Africa, and in parts of Asia are up on the
Tibetan Plateau. Where there are Remnant River
patterns, people have mapped them, they appear on satellite
images and they've been inferred to represent remnants of the
original pangaean drainage, but they're not now connected to
active systems. They've been broken through and
so they seem to be virtually all that we've got in the way of the
original drainage, except for those Rivers.
We talked about particularly like those of Eastern Europe,
which haven't yet had the effects of mountain building
from collisions where some Rivers born, as a direct result
of the breakup of Pangaea. Yes, they were.
When you have a supercontinent like Pangaea heat begins to
accumulate underneath it and you start to get doming mantle
plumes, volcanic activity that begins.
To generate domes Rifts of various sorts and these began to
affect virtually. All the continents, in different
ways, you have things like the Kuru plume and the Parana plume
across southern Africa and South America.
These domed up the crust and there's evidence from paleo flow
directions, that the rivers were radiating in places from those
very large domes and then in particular, once the Continents
began to separate new Rifts formed, which divided them
widening into oceans. And because the roofs were
relatively low elevation, they drew in the river systems and
generated and tie a new drainage networks.
And this is the case with the Niger.
Also, with the Rhine very recently and in the case of the
big rivers of Peninsular India, many of which occupy Cretaceous
Rifts, and you would think that. Then once a continent is Away,
nothing much would happen. But in point of fact, continents
are drifting over thousands of kilometers of mantle, which is
not uniform, it varies in it, density in its temperature to
other properties. And as the continents move
slowly across at a few centimeters a year, they rise,
they fall they tilt up and over and normal is in the mantle
below. And this is what's called
Dynamic, topography, link to the mantle properties.
And Of course, rivers are very, very sensitive to Elevation and
to gradient. And so you have good examples,
for example, in Africa, which is currently very high in the
southern areas. Its, anomalously high, a super
swell as people have called it. And this is affected the history
of the Zambezi Limpopo and orange Rivers, which have
altered their courses and directions to correspond to the
elevation of southern Africa as fascinating.
We actually did Analia podcast with Karina left guard
Barcelona. You talked about Dynamic
topography and another one with Becky flowers, which included
some discussion specifically about South Africa, and what we
can tell about the effect of dynamic topography there.
So the ongoing Collision of India with Asia that started
about 50 million years ago, must have had a dramatic effect on
the river systems there. Yes, this is one of the big
stories of river modification through the breakup of Pangaea
and it's absolutely fascinating. India is Is drifting northwards
are quite a rapid rate. Separated intern from Australia,
Antarctica Africa, elsewhere. And by, about 60 million years
ago. In the paleocene, India is
approaching the Southern Shores of Asia.
The tethys ocean is narrowing down to a narrow sea way and
then you have the start of collision and everything
changes. In fact, there are not many
rivers in this part of Asia. It go back more than about 20
million years and there are constant changes of Direction
and River capture tributary systems that affect them.
All, and if you think of Himalaya and the Tibetan
Plateau, there were Rivers radiating out in all directions,
it's called the water tower of Asia, and the thrust sheets are
continuing to move southwards and indeed, India has pushed
probably a couple of thousand kilometers into the body of
Asia. Why wouldn't it?
It have an enormous effects and the kind of effects go a bit
like this. Probably the oldest river in the
system is still the Indus which actually runs along the line of
the collision westwards and then cuts through the Himalaya.
That's the Western sin taxes where Asia curls around the edge
of India, as it indented into the continent.
And so, it curls around and runs down into the Arabian.
Sea and initially, The Ganges occupied the for Land Based on
so-called, on the south side of the Himalayas running westwards
into the Indus. So it was initially in in
distributary, and then, as India, move progressively,
northwards an old body of rock in the ravely, Hills, South of
Delhi began to impinge on the river basins, and separated them
off into two catchments. And the Ganges was diverted
eastwards about 15 million years ago in the Maya scene along, its
present, line down to the Bay of Bengal.
Also up in the Himalaya flowing, eastwards on the line of
collision was the yarlung tsangpo river and that initially
flowed on into the Irrawaddy and out through my admire.
And that is known because of fish populations that are
relatively similar in the two rivers but don't match
elsewhere. But the brahmaputra Steep
eroding head words into the Himalaya cut through and
captured the line of the song power of 18 million years ago
and brought it down into the Bay of Bengal.
Those are massive changes in river and drainage systems and
give us time and there will be more to come.
Because there are several Rivers like the Aaron and the Gann doc
that actually cut through the Himalayas and are now within
Striking Distance of capturing parts of the yelling.
Tsangpo system. They may eventually diverged
flow down into the Ganges and out to the Bay of Bengal by
different route. It's really fascinating.
Wow, it's incredible. Meanwhile, down in Peninsula
India. The rivers have not yet been
affected and you get a big Rivers still lying within
Jurassic and Cretaceous Rifts as they have been since The
Break-Up when India separated from other continents.
But as time passes and we're talking millions of years,
they're going to come closer. Closer to the mountain front and
will thrust front. And so as time passes, they are
going to be modified drawn under with subduction, they're going
to become tributaries to other systems or whatever.
So there's a lot more still to come.
And when we go further east into a share the big rivers of the
Yangtze, the Mekong, the salween All Began life, as little
Coastal, rivers, on the edge of the rising to bat and they then
eroded head word. Started to capture the Red
River, which may have been the biggest drainage system.
It's thought that the middle part of the Yangtze was,
originally a tributary of the Red River but reversed, its
course, cut the 3, Gorges in the miocene probably and then was
locked into place. So there's been a jockeying of
rivers, all very close together as the tectonics of the Himalaya
and Tibetan plateau of squeeze them, modified them,
structurally, change them. And these changes are still
Ongoing one thing that really struck me, as you were giving,
all these examples is how you can have a river.
Course that can be relatively unchanged for geological time,
then it can get more or less in a geological blink of an eye
captured by another River and then flow to somewhere
completely different on the planet essentially.
Yes. Yes.
And you can get rivers that are captured and then diverted
literally from one ocean into another for example, something.
Thousands of kilometers apart in the case of the Yangtze Mekong
and salween, they actually all run in a series of Gorges within
about 100 kilometers of each other.
And they all end up, diverging in two completely different
oceans. They've been likened to a bundle
of javelins in the hands of Zeus Arrangement pictures where you
have the javelins held by Zeus but their tips are radiating out
in different directions. Good example of this, by the
way, is the Niger in Western Africa, which just began in a
rift and was quite small. And then it took over an inland
drainage very suddenly to the west.
And then it went further west when a river that was running
out into the Sahara. Got blocked by sand dunes and
diverted into the upper part of the Niger.
So very, suddenly you had a river that rises very close to
the coast runs. North swings round in the Big
Bend at Timbuktu. And then Runs for some 4,000
kilometers and runs into the Atlantic on down the coast in
the original position. Absolutely fascinating.
The continuation of my conversation with Martin Giblin
is in the next episode of geology B will be talking about
the rivers of Europe and the Americas as well as the impact
of the ice ages and humans on River courses.
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