#craton

Precambrian Geologic History of Brazil field trip

Last week, we went to Minas Gerais to study the Brazilian Precambrian Geology. We visited parts of the São Francisco Craton and the Araçuaí Mobile Belt, between the Serra do Cipó and the town of Bom Despacho.

Gondwana Cratons A Craton is an ancient piece of earth’s continental crust, one that has been stable for billions of years. The Earth naturally contains elements that give off heat, such as radioactive potassium, thorium, and uranium. When the mantle melts, those elements move into the melt, extracting them from the mantle and moving them upward. Once those elements are concentrated in the crust, the heat can rapidly escape out into space. Once that heat is removed, the crust can become cold and stable, hard to deform.

View from the edge of Barron Canyon situated in the Algonquin Provincial Park, Ontario.This breathtaking landscape is a result of many geological processes among which erosion is the most recent and one which shaped the canyon into its present form.

Erosion started at the end of the last Ice Age, about 11,00 years ago when the region “been exposed by the northward retreat of the melting Laurentide Ice Sheet which had covered almost all of Canada and parts of the northern U.S.A.”*

Water from the prehistoric Algonquin Lake, one of the lakes created by the melting glacier, was carried through the fault (break) in the earth’s crust at thundering speeds causing the widening and deepening of the fault.The fault itself thought is a result of geological events that occurred over millions of years before the retread of the glacier.

Exposed rocks of the canyon are part of the Canadian Shield which is a revealed portion of the North American craton, a stable part of the earth’s crust that holds continents together while floating on magma or the liquid rock deep beneath the surface of the earth. 

The canyon is slowly but constantly expanding as the walls of the canyon are being constantly eroded. Subzero temperatures contribute to the cracking of the rocks which shattered debris accumulate on so-called talus slopes which are visible on both of the images.

Barron Canyon, Algonquin Provincial Park

Ontario, Canada

27.09.2020

© Blazej Marczak / www.bmarczak.com

Tectonic Timeline Out of the solid, rocky worlds in our solar system, only 1 has a crust broken into pieces that change position relative to each other: Earth. We call that motion plate tectonics, and over time it has redistributed and reshaped the continents and oceans again and again. But, the Earth formed extremely hot, as a giant, molten ball of rock out in space, and so there must have been a time when plate tectonics didn’t exist and a point where it started. Since no humans were around 4 billion years ago and none of the other worlds in the solar system show anything like plate tectonics on Earth, how and when tectonics began on Earth remains a geological mystery.

A deeper look

Lurking beneath all the more recent layers of rocks that drape our continents are their roots, the oldest of which are the remnants of cratons, the first nuclei of the light buoyant surface rocks that float atop the underlying mantle. When they pop into visibility at the surface, the most amazing patterns reveal themselves. These rocks have been repeatedly heated, melted and compressed as the continents moved back and forth, colliding and separating into supercontinents in their Wilson cycles.

Each mountain building event leaves its impression in the folding and melting of the rocks, forming a palimpsest of what geologists dryly term thermal and deformation events. Not a very poetic way, I know, to describe such tremendous forces that shaped the surface of long gone landscapes, but there you have it.

The photo shows boudinage, from the French for blood sausage. The semi molten rocks reacted to stress by straining, sliding around in a plastic manner known as ductile. Large chunks of darker low silica volcanic rock have been broken apart and intermingled with others, revealing something of the pattern of the forces. These rocks were melting while this was happening, and the white veins are the resulting new born granite. As the dark chunks separated, lower pressures resulted on one side (abive the dark mass in the photo), and the grey gneiss flowed into the area, producing the folded structure. The white veins are even more intricately folded, revealing further moments of pressure while the rocks were soft.

The photo was snapped in the Prince Charles mountains of Antarctica, 1 400 km long range whose main surface expression are nunataks (mountain tops poking out of an ice sheet). They are one of the few areas with extensive rock exposure in the whole continent, where a block of rock measuring 600x300km has sunk downwards between two faults forming a graben, a bit like Africa's great rift valley.

The event that shaped these rocks is thought to have occurred around a billion years back, with a second event around 500 million years ago with at least three recognised deformation events.

Loz

Image credit: Adrian F Corvino/Outcropedia

The Legacy of Sir John Rae

The Rae craton forms a large portion of the Northwest Canadian Shield and is comprised mostly of Archean-aged meta-plutonic rocks commonly overlain by Paleoproterozoic meta-volcanic and meta-sedimentary rocks. The Shield forms the heart of North America, spanning from Baffin Island into the continental US. It is built of several Archean (4 to 2.5 billion years old) cratons (i.e. Slave, Rae, Hearne, Superior) which were amalgamated during Paleoproterozoic (2.5 to 1.6 billion years old) collisional events. These rocks are some of the most extensively reworked continental crust on Earth and have an extremely complex metamorphic history, with some parts of the shield having seen more than eight separate tectonic events.

The Rae Craton is named for Sir John Rae (1813 – 1893), one of the first Europeans to explore the area nearly 200 years ago. Rae was born in Orkney (Scotland) and later attended medical school in Edinburgh. In 1833, he travelled to Canada aboard a Hudson’s Bay Company (HBC) ship to Moose Factory, a small outpost on the shores of Hudson’s Bay (Ontario, Canada) where he resided for 10 years. His first chance to lead an arctic expedition came in 1844 when he was selected by Sir George Simpson to survey unexplored areas of the Canadian Arctic coastline for HBC. The harsh landscape and weather made survival in the arctic challenging, and Rae stood out for his eagerness to learn traditional methods of survival from the local Inuit populations, going against the accepted code of conduct for European explorers of the time.

Between 1846 and 1854 Rae made four more Arctic expeditions to what is now Nunavut, travelling more than 10,000 miles on foot or in a small boat, and surveying a staggering 1,800 miles of previously uncharted arctic coastline. Rae and his crew surveyed many previously uncharted areas of the Arctic coast including Committee Bay, Simpson Peninsula, Melville Peninsula, and Pelly Bay. Rae’s most famous discovery came in 1854, when he realized King William Land was not a peninsula but an Island, discovering the Rae Strait and the last link in the famed Northwest Passage.

Controversy surrounding Rae began in 1848, when it became clear that the infamous Northwest Passage expedition lead by Sir John Franklin would not return. In their search for the missing crew, Rae and Sir John Richardson made several overland expeditions. They found little evidence of Franklins crew; only a few pieces of lumber and several artifacts in the possession of the local Inuit. It was the Inuit who told Rae that Franklin’s desperate crew had resorted to cannibalism. The inclusion of this in his 1854 reports lead to his defamation and discreditation, lead by Lady Jane Franklin and Charles Dickens to preserve Franklins name. Later surveys, including that in 2014 which located Franklin’s ship the Erebus, confirmed Rae’s findings.

Rae is regarded as one of the greatest arctic explorers of all time; a testament to both the great lengths of coastline he mapped and his respect for Inuit knowledge and traditions.

CD

References https://bit.ly/2x5QbI1 https://bit.ly/31wtzhz https://bit.ly/2KRwM5C https://bit.ly/2MPvLgO https://arcticreturn.com/john-rae/

Bass Formation

In the Grand Canyon, the oldest rocks are highly metamorphosed schists and gneisses – considered parts of the continental basement. These rocks are only reached in the inner gorge of the Colorado River, where the river has eroded through the entire sedimentary sequence of the western United States. The top of those metamorphic units is a called a nonconformity – it is a rough, erosional surface, created when the ancient metamorphic rocks were exposed at the surface for millions of years. The metamorphic rocks of the Grand Canyon formed about 2-1.7 billion years ago, then 10% of the age of the planet passed before anything else was recorded at this site. Atop those igneous and metamorphic rocks sits a small set of tilted sedimentary rocks known as the Bass Formation or the Bass Limestone. This is the first fairly pristine sedimentary rock we find while walking up the Grand Canyon. The features in the first photo are mostly stromatolites; layers of limestone and other sediments believed to be created as mats of bacteria grew on the floors of shallow, warm oceans. The second photo was taken at the Phantom Ranch Boat Launch deep in the Canyon’s inner gorge. Look around the people – all the rocks are pretty massive, there’s no obvious layering anywhere near the people. The only place where you see layered rocks is atop the ridge in the distance – those rocks are the Bass formation. All of the lower rocks are the igneous and metamorphic rocks of the inner gorge. The Bass Formation is the lowermost member of what is known as the Grand Canyon Supergroup. The Supergroup is a series of sedimentary rocks formed in the late Precambrian, exposed at the bottom of the canyon as a sequence of rocks that have been tilted and dip off to the northeast.

The Bass is a sequence of sedimentary rocks. It contains many layers of dolomite that probably originally formed in the ocean as limestone and then were altered to dolostone after they were buried, with thin layers of sandstone, siltstone, and the occasional coarse grained conglomerate. The stromatolites are fossil bacteria colonies that formed in the ocean This sedimentary sequence indicates that water levels were changing – from rivers that deposited the conglomerates to shallow ocean waters that formed the dolomite. This unit marks the first step in a transgressive sequence – water levels were rising to cover the exposed Vishnu Schist basement rocks, and those rising waters produced this rock unit.

Geologists look for layers of volcanic ash in sedimentary rocks like this one because we are easily able to produce age dates by measuring isotope ratios in those layers. An isotope measurement on this rock gave an age of 1254 million years old. After the big tectonic events that formed the crust of the western US, that’s when the next stage – the sedimentary stage – began.

-JBB

Image credit: NPS https://www.flickr.com/photos/grand_canyon_nps/6705376193/in/photostream/ https://www.flickr.com/photos/grand_canyon_nps/8229636485/

Reference: https://bit.ly/2REaVCr By the way, we’re going to be in the Grand Canyon Supergroup for a while.

The core of Northern Africa

I found this image while searching yesterday and thought it was a particularly insightful view into the assembly of one of the continents today. Africa as we know it today is mostly surrounded by spreading centers. It isn’t colliding with anything, it isn’t running into other continents, and there is no active subduction beneath Africa. However, it wasn’t always that way, and this simple image tells a portion of that story, at least for the northern ½ of the continent.

Africa was part of the supercontinent Gondwanaland along with India, Australia, Antarctica, and South America. Each of those continents we recognize today was itself an almagamation, created from much more ancient building blocks.

These building blocks are what geoscientists call “Cratons”. They are slices of ancient continental crust, many of them established more than 2 billion years ago. In-between them are the active zones or “mobile belts”, the areas that are deformed when the cratons collide.

2 of the main cratons shown in this image can be traced all the way back to the Archaean, before the rise of oxygen in Earth’s atmosphere. The oldest rocks exposed in the West African craton are 3.5 billion years old and the oldest rocks in the Congo craton are 2.9 billion years old. These cratons started off as something like modern day island arcs, and grew outwards as more things collided with them. Each of them shows more than a billion years of volcanism and metamorphism, but both of them had been established as blocks of somewhat stable, continent-like crust by about 2 billion years ago.

The Saharan block is called a Metacraton because it is thought to have been active at its core much more recently than the other cratons. Much of that area is covered by the Sahara Desert and the thick Nubian Sandstone and by igneous rocks that are about 750-550 million years old. This area could be a recently assembled craton or it could have been a piece of older crust where the assembly of Gondwanaland caused volcanism that overprinted many of the older rocks.

In-between these 3 main blocks are the mobile belts. Sometimes a thin sliver that broke off of a different continent gets trapped in these areas by running into the edge of a growing craton – the Tuareg block and the Oubanguides Belt are examples of these as both contain Precambrian and Archaean rocks but they are stuck in the middle of provinces that were deformed much more recently.

The areas in-between these blocks were deformed and overprinted during what is called the Pan-African Orogeny, from about 750-550 million years ago. These cratons collided at that time to make up the supercontinent Gondwanaland, which then lasted for 300 million years.

There are other cratons in Africa and in ancient Gondwanaland that aren’t shown on this image – just happens to be the one I found. The Kaapvaal craton in South Africa, for example, contains rocks that are at least 3.7 billion years old, some of the oldest continental rocks on Earth.

-JBB

Image credit: http://bit.ly/2wFF4TA