This is a mystery which has existed for over a century and was first discovered by the famed
geologist John Wesley Powell in 1869. Geologists believe that layers of rock are deposited one on
top of the other, little by little over time. This means that when we examine the layers
in exposed rock faces you should generally be able to see that each successive layer is older
than the other with no gaps between them. The thing about the Great Unconformity is
that about a billion years of rock appears to be missing between the 3 billion-year-old sediment
and the relatively young, 550 million year layers sitting directly on top of this. To add to the
mystery the 550 million year strata are just a few million years before the Cambrian Explosion
which marked the widespread appearance of complex life on Earth. Let’s dive in and find out more…
It is believed that Unconforimities tend to reflect long term changes in the pattern
of the accumulation of sedimentary or igneous strata in low lying areas,
these are then thought to be uplifted and eroded, then subsequently subsiding,
and eventually being buried under younger sediment.
The first person to discover one of these was James Hutton in 1787 at Siccar Point in Scotland.
It is an angular unconformity that consists of gently dipping, reddish, Upper Devonian
and Lower Carboniferous breccias, sandstone, and conglomerates of the Old Red Sandstone
overlying deeply eroded, nearly-vertical, greyish, Silurian shales. The Silurian shales are thought
to be deposited by turbidity currents in a deep-sea environment 425 million years ago.
The overlying Devonian strata were deposited by rivers and streams about 345 million years ago.
There was therefore a gap of 80 million years.
Hutton was the first to demonstrate that there were significant breaks in the geological record.
This together with other unconformities this provided evidence for Hutton’s ideas
about the recycling of geological materials
and for unconformities representing very large time scales. He argued that these concepts
pointed to the great antiquity of the Earth and the vastness of the geological time scale.
Nearly a hundred years later John Powell would discover an unconformity of such a magnitude
that to this day it still cannot be fully explained. While studying the rocks in the
grand canyon he came across a break in the geological record between what is called the
Tonto Group which sits on the underlying rocks of the Grand Canyon Supergroup and the Vishnu
Basement Rocks. This unconformity represents a period as much as 1.2 to 1.6 billion years.
Although the Great Unconformity can be easily spotted in the Grand Canyon, similar breaks
are apparent in lots of other places. Charles Walcott stated the following about it in 1910:
I do not know of a case of proven conformity between Cambrian and pre-Cambrian Algonkian rocks
on the North American continent. In all localities where the contact is sufficiently extensive,
or where fossils have been found in the basal Cambrian beds or above the basal
conglomerate and coarser sandstone, an unconformity has been found to exist.
Stated in another way, the pre-Cambrian land surface was formed of sedimentary,
eruptive, and crystalline rocks that did not in any known instance immediately precede
in deposition or origin the Cambrian sediments. Everywhere there is a stratigraphic and time break
between the known pre-Cambrian rocks and Cambrian sediments of the North American continent.
Some geologists have hypothesised that whatever caused them
was some kind of global event. One proposal is that glaciation might have
caused the large parts of the surface to be eroded at roughly the same time.
The only way this could happen is if the Earth was entirely covered in ice like a snowball Earth.
A new study suggests that there may not have been one unconformity worldwide but rather a
series of them roughly coincident around the world and that they believe that it may have
been caused by an ancient supercontinent called Rodinia that formed about a billion years ago.
The new research is based on a dating technique called thermochronology. Thermochronology is
the science and practice of inferring thermal histories of minerals and rocks from chemical,
isotopic, or physical properties of minerals that are sensitive to both temperature and time.
It examines how temperature would affect the decay rate of certain elements and
uses this attempt to understand what might have happened to the rock over its lifetime.
So what did they discover? They believe that the lower layer had been thrust upward to the surface
about 700 million years ago, they think that it was then subjected to erosion
that scoured away its upper layers of rock. What could erode so many layers of rock?
They believe that the supercontinent Rodinia is responsible. This predates Pangaea
and is thought to have formed in a process called extrovert assembly,
here pieces of a prior supercontinent that had broken apart meet again after having travelled all
the way around the planet! They think that during this journey, the edges of the pieces experience
significant erosion before smashing back together. Where the continents collide, they think that
mountain belts form and that these would be responsible for the large amounts of erosion.
Both the birth and death of Rodinia may have wreaked havoc
all over the world as its pieces first came together and then eventually broke apart.
There is currently no widely accepted explanation for the Great Unconformity among geoscientists.
So what can we make of this? The first thing we need to consider is that of the dating of the
rocks and strata. The current assumption is that this is a slow process that takes a long time.
It is based on the following principles:
The principle of superposition. This means that each layer above a layer must be younger
and any layer lower it must be older The principle of continuity. This states
that strata owe their existence to sediments in a fluid. At the time when a stratum formed,
it was either circumscribed by another landmass or ran around the entire globe.
The principle of horizontality - at the time when a stratum formed,
it would be parallel to the lower surface onto which it was being deposited
and its upper surface would be parallel to the horizon.
And lastly, the idea that two strata with the same fossil content are the same age
I have previously looked at the work of Guy Berthault
who was a french sedimentologist who wanted to understand the mechanics of how this might work.
He conducted many experiments that showed that the view that sedimentation occurs slowly
and horizontally was wrong. His experiments showed that deposits are formed parallel to
the subsurface layer, not the horizon and that this process can occur very rapidly.
This casts doubt on the accepted formation process and dating of the layers of rock
but geologists also have radiometric dating of the rocks in these layers
that surely confirms the slower process. Here we also need to be careful as there
are problems with this type of measurement as it assumes a fixed decay rate for an isotope
that remains constant and it also makes the assumption on the abundance of these elements in
the initial rock. I have previously discussed some of the more recent findings that have shown that
the decay rate of certain unstable elements can be altered through low energy nuclear reactions
through either exposure to hydrogen isotope plasma and also through the work of Vladimir Vysotskii
who showed that certain bacteria were capable of converting Cs137
which has a half-life of 30 years into Barium138 which has a half-life of only 310 days.
In the first case we need only consider the idea
that increased electrical discharges through the rocks could therefore change this decay
rate making it appear like the rocks were older than they are. The second example also
makes me consider Eugene Bagashov’s idea of an Ultra Deep Biosphere and what role these
bacteria might play in altering the chemical composition of the rocks surrounding them.
In the latter case, this would be unlikely to alter the composition consistently across an
entire strata and would more likely create pockets. In the electrical discharge case,
I have previously discussed how this might explain why metals are found in veins
which terminate with heavier elements like Uranium. This too would lead to veins with
altered chemistry unless the discharge events were on a planet wide scale?
Could a layer laid down in these circumstances lead to what appears as a much older layer with
a large gap between this and the next layer laid down after this event when it had calmed down?
One interesting point to note is that the layer that sits above this
is the start of the formation of complex life on the planet known as the Cambrian Explosion.
One aspect that is also worth further investigation is what evidence we have
for how the landmasses looked in the past. This idea of the repeated formation of supercontinents
which collide and then break apart and then collide again.
What evidence is used to determine that this happened, their shape and their direction?
I have recently read Hapgood pole shift book and I think there are certain aspects of this that are
worth exploring in more detail. There are a number of different models to explain the formation of
the continents of which Hapgoods is just one. Previously I have looked at an expanding earth
model to explain the contents and their movement as well as potentially that of a shifting pole.
Hapgood’s model is worth exploring in more detail as it reveals elements that must be
considered both in terms of ice ages but also processes like mountain building.
I would like to therefore compare the different models to understand where the shortfalls for
each lie and what common ground can be found both in terms of ice ages but also formation process.