Durango Bill’s
Ancestral Rivers of the World
Ancestral Rivers in
Australia
by
Bill Butler
Antecedence and superimposition are geologic processes
that explain how and why rivers can cut through mountain systems
instead of going around them. The examples here (including pictures)
are from Australia, but other examples can be found throughout the
world.
Featured areas
Arroyos - the Great Outback (3 pictures)
Coongan River
Fitzroy River ( 2 pictures)
Ord River
Victoria River
Arroyos - the Great
Outback (3 pictures), Northern Territory
Australia does have rivers with real
water, but Australia would not be the same if it did not have the
“Great Outback”. You could spend days trying to drive
across the “Outback”, and it might seem as though there was
nothing there except “miles and miles” of nothing but
“miles and miles”.
Aerial views reveal a different and fascinating world. The
pictures below are an excerpt from a geologic story that started long
ago when the “Outback” was different.
The picture above looks west-northwest over the western
end of the MacDonnell Ranges some 75 miles west of Alice Springs. There
is a small motel at Glen Helen just below the center of the picture
where the dry stream bed enters the main ridge.
The dry steam bed actually has a name, the Finke River,
and on rare occasions it does flow (from right to left). Close
examination of the Google Earth photographs show there was no flowing
water at the time the picture was taken, and there were just a few
stagnant mud puddles.
Many hundreds of millions of years ago, there were
horizontal layers of sand and silt laid down in what is the current
field of view. Gradually they hardened into layers of sandstone and
siltstone. The MacDonnell Ranges were subsequently uplifted some 300 to
400 million years ago. (The highest portions were off the right side of
the picture.) What had been horizontal sedimentary layers were bent
upward over the core of the uplifted mountain range. Mt. Sonder (right
edge of the picture) is a minor remnant of this former mountain range.
River drainage that developed off the south side of these mountains
eroded valleys, and the remnants of these rivers persist to today. One
of these ancient drainage valleys is the ancestor of today’s
“Finke River”.
As the upper layers of the range erode away, the upturned
layers along the flanks are gradually exposed. The harder sandstone
layers resist erosion and are exposed as long ridges.
Even though the Finke “River” only flows
occasionally, it still has enough erosion power to abrade down through
these harder ridges. Thus today, it occupies a series of gorges that
cut through the upturned sandstone layers. The gorge just below the
center of the picture is over 500 feet deep.
The picture above is some 40 miles south-southeast of the
first picture, and looks westward where the Finke River cuts across
more upturned ridges. (In this view it again flows from right to left.)
The landscape was crumpled many millions of years ago, and the
structural deformations probably date back to the uplift of the
MacDonnell Ranges. A couple of these beveled crumples can be classified.
The structure in the lower right hand corner is the
beveled remnant of an anticline. In an anticline, the center is lifted
upward, and the overlying sedimentary layers are tilted so that they
tilt upwards toward the center of the anticline.
The next “crumple”, that runs vertically
through the center of the picture is a syncline. Here the layers are
bent down toward the center of the structure. In-between the anticline
and the syncline, the layers dip down sharply as you go from right to
left.
Finally there is another beveled anticline in the lower
left corner. (The portion of the picture in this area has a different
tint and lower resolution.) Once again, the layers were bent over the
top of this anticline similar to the anticline in the lower right
corner.
The structural folds can be traced for many miles into the
distance. The Finke River cuts across these folds since it was in place
first, and has had enough erosion power to keep its ancestral path.
The view above looks east-northeast in another area some
125 miles west-northwest of Ayers Rock (Uluru). Another dry arroyo is
cutting through a mountain range. The mountain range is identified by
Google Earth as the Dean Range (part of the Petermann Ranges), and some
searching through atlases found the name for the small town in the
lower left quadrant as “Docker River”. (A dirt airstrip is
somewhat more visible on the far side of the “river” almost
directly above the town.)
The Dean Range is an upturned sandstone ridge that was
bent upward over the top of a larger dome structure centered some 10 to
20 miles further to the south-southeast (off the right edge). There is
very little left of the dome except slightly higher ground that serves
as a source drainage area for the “Docker River”, and
exposed rock layers that look much older than the sandstone in the Dean
Ranges.
As for the “Docker River”, a close examination
of the Google Earth (satellite) photos couldn’t even find a mud
puddle. However, given the very occasional times the river does flow
combined with millions of years of geologic time, the Docker River has
cut a gap through the Dean Range that is over 1,000 feet deep.
Coongan River, Western
Australia
The majority of Australia’s “Ancestral
Rivers” are found in Western Australia. The picture above looks
north, and shows the Coongan River (left) and a tributary cutting
through what appears to be another upturned sandstone ridge north of
the Chichester Range.
The thin dark line that snakes upward in the right side of
the picture appears to be a volcanic dike. At some point after the
sedimentary layers were laid down, and before erosion exposed the
current surface, there was some kind of force that pulled the upper
layers of the crust slightly apart. Molten lava seeped upward into the
resulting crack, and formed a thin wall of basaltic rock when it
cooled. When it is exposed, it is more resistant to erosion, and thus
produces a small ridge. The dark rock in the lower right corner looks
as though it is part of the same intrusion.
When a mountain range is uplifted, river systems develop
radially outward. They thus ignore layers of sedimentary rock that were
bent up over the rising range. With time, erosion removes the upper
layers, and the lower layers are revealed. Frequently, river systems
are able to maintain their paths, and cut gaps through the more
resistant elements within these sedimentary layers.
If you stood where the rivers are cutting through the
ridge, you would see a cross section of these resistant layers. In this
particular view, they would be tilted up toward the viewpoint for the
picture, and tilted downward toward the distant horizon.
Fitzroy River (2
pictures), Western Australia
The picture above is a vertical view of a limestone dome
(the Gieke Range) where the Fitzroy River has eroded down through the
dome to form Geike Gorge (Geikie Gorge). The Fitzroy enters the field
of view from the right edge and flows left through the dome. The small
town of Fitzroy Crossing is downstream off the left edge.
The thin dark line for the river is bigger than you might
think. The area is a local tourist attraction where tour boats take
tourists through the gorge.
The lighter rocks in the picture are Devonian age
limestones - part of an ancient 350 million to 400 million year old
reef system. The uplift history of the reefs (which are on the southern
edge of the King Leopold Ranges) is not known, nor is the history of
the Fitzroy River known. However, the river has cut a gorge over 100
feet deep in the old limestone reefs.
About 35 linear miles upstream, the Fitzroy has cut down
through other limestone layers. The limestone layers in Geike Gorge
(first picture) are essentially horizontal as they are found on the top
of a dome. The layers in this picture have been tilted as they are on
the southwest side of a syncline. (The axis of the syncline runs from
the top to the bottom of the picture to the right of the center.)
The tilting has exposed a sequence of layers, and where
these more erosion resistant layers are exposed, they produce long
ridges. The Fitzroy enters from the right edge and has eroded down
through these ridges before it exits the field of view on the left
edge. There are other gorge systems and caves in these limestone reefs
that may be of interest if you ever take a trip to the area.
Ord River, Lake
Argyle, Western Australia
The picture above shows the extreme north end of
Lake Argyle and its outlet, the Ord River. Lake Argyle is a large
man-made reservoir that backs up from the Ord Dam.
The Ord River cuts through the Carr Boyd Ranges (left and
above the lake) at about 150 feet above sea level. The tops of the Carr
Boyd Mountains are 900 to 1,200 feet above seal level. There is an
alternate path that the Ord River could have originally taken to the
right of the fault (suture) zone (extends through the center of the
picture) where the river would never have to travel across terrain that
exceeded 340 feet above sea level. The overflow spillway for the lake
was built through this lower alternate route. If you look closely at
the extreme north end of the lake, you can trace the route of this
overflow spillway.
Why did the river take a path through the higher mountains?
There are 2 possible reasons for the illogical course of
the Ord River. In both cases the mountain route would have been lower
at the time the river established its course.
Case 1) The area to the right of the fault looks like the
soil/rock is less resistant to erosion than the rock to the left of the
fault. Thus it erodes more rapidly. If we ran the clock backward and
replaced everything that was eroded, it’s quite possible that not
too far in the distant past, that this area was higher than the current
Carr Boyd Mountains. This seems possible, but there are multiple other
potential escape routes for the Ord River that stay under the 900-foot
level, and these would also have to be blocked in the past.
Case 2) A more logical explanation is that the old fault
was reactivated at some time in the last 10 to 20 million years. In
this scenario the current height of the Carr Boyd Mountains is due to
relatively recent uplift. Thus the Ord River was in place first at a
low elevation to the left of the fault. As the mountains were uplifted,
the Ord River eroded away material that kept getting in the way. Thus
it could keep its original path.
There is a piece of evidence for case two. The Ord Canyon
below the dam has steep sides. This is characteristic of something that
has been cut in the last 10 million years.
Victoria River,
Northern Territory, Australia
The picture above looks to the north where the Victoria
River follows a relatively narrow passageway into and through a
low plateau in the northern part of the Northern Territory. The
passageway is nowhere nears as spectacular as other canyons that we
have looked at, and the view has used a vertical exaggeration factor of
3 to emphasize the relief. The river elevation as it enters the
passageway is slightly under 200 feet above sea level while the top of
the very flat plateau is constant between 750 and 800 feet.
What is of interest is why did the river take this path instead of
going somewhere else?
The terrain to the south (in back of the field of view) is
exceedingly flat. You can travel 100 miles to the south from the
observation point and never exceed 600 feet above sea level. (Remember
the plateau is 750 to 800 feet.) It seems highly unlikely that this
huge area to the south could have eroded down to under 600 feet while
the plateau was essentially untouched by erosion except in the
immediate vicinity of the river.
There is an alternate escape route for the river. You
could go 30 to 40 miles to the east of the observation point and then
follow virtually level ground north into another drainage system and
stay under 600 feet. The river didn’t take that route either.
Instead it went north over a plateau that is currently more than 750
feet above sea level until you erode down to the river’s current
elevation.
It appears that the river established its path when the
plateau was somewhat lower than it is today. The plateau has been
subsequently uplifted a few hundred feet, but the river was able to
erode down as fast as the plateau rose.
The river never was at the 750-foot elevation of the
current top of the plateau. It was always at its current elevation of
just a couple of hundred feet above sea level. As the plateau rose, the
river simply cut a groove into the rising block of material.
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