Episode 388 Folds in Algeria

You may have seen some of the spectacular images of the earth in southern Algeria, curves and colors like some Picasso in the opposite of his cubist period. If you haven’t, check out the one from NASA, below. 

The ovals and swirls, with their concentric bands, are immediately obvious to a geologist as patterns of folds, but not just linear folds like many anticlines and synclines form. These closed ovals represent domes and basins – imagine a large scale warping, both up and down, in a thick succession of diverse sedimentary rocks, like sets of nested bowls, some of them right-side up and some inverted, then all sliced off halfway through.

But “obvious to a geologist” has plenty of limitations in a space image. Without knowing more information, it’s difficult to be sure if an oval is a basin or a dome. And you can speculate, but without some ground truth, it’s challenging to be sure what the rock types are.

Ahnet-Mouydir, Hoggar Mountains, Algeria. NASA image - source

This area, called the Ahnet-Mouydir, on the flank of the Hoggar Mountains close to the middle of the Sahara Desert, is remote, inhospitable, and arid, and called the “land of terror” for a reason. The rocks represent a thick sequence of marine sandstones, shales, and limestones, spanning a huge range of ages, from at least the Ordovician to the early Carboniferous – 150 million years or more, a great chunk of the Paleozoic era.

The core of the Hoggar Mountains is an old Precambrian block, not as big as the cratons and shields that form the hearts of most of the continents, but otherwise similar. It might have been something like a microcontinent that became amalgamated into the growing supercontinent of Gondwana about 600 million years ago. After that amalgamation, seas came and went much like they did in western North America throughout much of the Paleozoic era, laying down the sediments that became the rocks we see today in the northern Hoggar Mountains.

That’s all well and good – but here’s the next question, how did the rocks get deformed into these oval domes and basins? If you imagine the kinds of collisions that are typical on earth, you think of linear or curvilinear things – island arcs, edges of continents and such – that when they collide, are likely to make linear belts of deformation. This is why so many mountain ranges are long, l