All my faults are normal, but I can reverse them…is an old joke among us structural geology nerds. But there is no reason to reverse this prettier-than-normal fault, which actually is in an outcrop on my way to work, in the middle of Oslo. It is also an example of a few pictures telling more geo-history than a thousand papers.
The dark rock in the outcrop is a late Cambrian shale, nicknamed the “Alun shale”. It is nearly black from a high content of organic matter. In the Oslo area, it is burnt out – more on that in a second – but towards the Baltic and Poland, it is an important source rock for petroleum production.
In striking contrast, the lighter rocks are intrusives, sills, composed of syneite (geochemists, please forgive my sloppiness with Streckeisen diagrams), that has followed the layers of the shale. It intruded when the Oslo rift made this area a lookalike to the Great Rift Valley in east Africa, at times a volcanic inferno during the late Carboniferous to early Permian. The Oslo graben is the reason a fjord exist, and therefore why Oslo is located where it is. No Permian volcanism, no fjord for Vikings to settle in, no capital. Another amazing perspective on geological time is that at around 300 million years, the intrusive is less that two-thirds as old as the shale’s ca. 490 m.y. Nevertheless, the whole shebang was faulted in the end, probably during a late phase of the Oslo rift.
Taking a closer look, the fault also displays a thin, but distinct damage zone, where the shale is bent along the fault, and flakes of the sill are dragged in and parallel to the fault plane.
Some of the people that walked by asked of what on Earth I took pictures. I explained that I took pictures of something in Earth. But, since all geology images should have a proper scale, I had fun making some instant portraits of people walking by the Earth in the early spring rain.
And then it rained even more, and I went inside a café to have a cinnamon bun and hot chocolate.