Geology of Southwestern Sweden with Mark Johnson

Posted on May 16th, 2013 by

On April 30th, the group headed two hours west from our dorm in Jönköping to a hostel in Gothenburg. On our afternoon of arrival, we met Mark Johnson, a former professor of Geology at Gustavus. He began teaching at Gustavus in 1985, and moved to Gothenburg in the early 2000s to teach at Gothenburg University. He is currently the head of the Geology Department there and is also the president of Sweden’s Geological Survey. Mark took us all around southwest Sweden, telling a narrative of the geology of the area from the Precambrian period (~4500-541 million years ago) to the last Ice Age (~10-12,000 years ago). We were with Mark until May 3rd. The field experience was a component of our natural science course. Being the geology major on the trip, I’m going to do a quick run-through of some of the story we learned from Mark. If I were to tell the entire story, it would take far too many pages to do it here. But by discussing layers and a few landforms, I hope I can give a sense of how geologists construct geologic history. Unfortunately, pictures of the stuff I’m talking about were weirdly scarce (and I didn’t know I’d be doing this sort of thing later), so I’m just going to throw a few random ones in at the end.

Geologists typically begin a narrative by discussing the oldest layer and work forward through time from there. This reflects the principle of superposition in Geology, which states something seemingly obvious, but is nonetheless extremely useful: rock layers are deposited in a time sequence, with the oldest layer on bottom and the youngest on top. Conditions do occur in which this principle does not apply, but we did not encounter such conditions during our trip. The day before we headed out, Mark taught us a pneumonic device for remembering the order of the six layers we’d be seeing—“USA klockan tre.”

• Urberg Gnejs (gneiss)
• Sandsten (sandstone)
• Alunskiffer (Alun shale)
• Kalksten (limestone)
• Lerskiffer (Ler shale)
• Trappa (diabase, or solidified magma)

I’m going to start by briefly describing the Urberg gneiss, since it’s the oldest layer we saw, formed 1500Ma (abbreviation for millions of years ago). Gneiss is a foliated (essentially layered) metamorphic rock characterized by alternating dark and light bands. The important thing about this is that we know metamorphic rocks form under conditions of high pressure and temperature—in this case, the gneiss formed 20km under an ancient mountain chain. The mountains must have subsequently eroded to a nearly-flat surface, or peneplain, over the past 1.5 billion years. Tremendous pressure on the gneiss while the mountains were there created folds, faults, and fractures which were weathered and widened, still visible today in this fracture valley landscape, which was not affected much by recent glaciation.

Interestingly, this entire layer extends all the way from the coast into the mainland, but while it is fractured on the coast, it is nearly completely flat beginning 40km from the coast. We got to stand on this peneplain, and a picture will follow, but there’s a pretty distinct reason as to why this is. That requires a brief synopsis of the next four layers in our series—sandstone, shale, limestone, and another shale. There are two things we can quickly pull from this. The first is that these are all sedimentary rocks which are formed in low-energy shallow marine environments. A sequence of rocks like these indicates changes in sea level. This is where the law of superposition comes in to help interpret the layers we see. Sandstone is made of fine-grained beach sand, meaning that this area of Sweden once had a similar climate to the Bahamas! In fact, at the time this sandstone formed (during the Cambrian period about 500Ma), Sweden would have been pretty close to the equator—very similar to Minnesota’s story. Next, we have the Alun shale, which is made of very tiny clay particles (clay particles being less than 0.004mm). Shale forms in deeper marine environments than sandstone; therefore, we can conclude that at the time this shale was formed (also in the Cambrian), sea level had risen since the sandstone had formed. Next, we have a limestone layer, which was formed during the Ordovician period (~460Ma) in an environment 15-20m under water, between that of shale- and sandstone-forming depth. The water must have had good shell-forming conditions, because the limestone here was largely composed of shell fragments from ancient organisms, which contain calcite (or aragonite, depending on overall ocean chemistry).

So we know that sea level has been changing for millions of years, causing the formation of various types of sedimentary rock on top of the preexisting flattened Urberg gneiss. Sea level change happens for a couple reasons. Ice ages happen in intervals due to Milankovitch cycles, which are large-scale fluctuations (such as changes in Earth’s tilt and orbital eccentricity (shape)) that impact climate. Water gets locked up in the massive ice sheets, causing a drop in sea level. Ice is also really, really heavy. Enough to cause Earth’s crust to sink down on the plastic-like mantle. When the ice melts, the water eventually makes it back into the oceans, but the crust also rebounds in such a way that may outpace sea level rise. This makes sea level fluctuation in some areas hard to visualize.

Now I’ll head toward wrapping this post up. I said earlier that we got to stand on this peneplain. So that means all of these sedimentary layers are gone, and the peneplain is the original peneplain—not the fracture valley landscape I talked about previously. There are three questions left: 1) Why is the gneiss flat here? 2) How do we know these sedimentary layers existed on top of the gneiss? 3) Where did these layers go?

The gneiss is flat in this region because the peneplain was protected from the Mesozoic weathering that caused fractures and folds to widen near the coast. As far as why it’s so smooth, there are, according to Mark, various “ocean theories” to explain the flatness of the gneiss, but it isn’t known for sure. But knowing the age of the sedimentary layers by using radioactive dating techniques, geologists know that the sedimentary layers were on top of the gneiss in this region during the Mesozoic, protecting it. But where did these layers go? There are two hills in this region, called Billingen and Kinekulle. These two hills contain all of the “USA klockan tre” layers. They still stand today because they were capped by cooled and solified magma (diabase), which is very resistant to erosion and weathering. The area between the two eroded away, because sedimentary rock is essentially just glued together, thus vulnerable to wind, water, and other forces. The reason that we know these layers extend between Kinekulle and Billingen, therefore protecting the peneplain, is due to the principle of lateral continuity, which states that layers originally extend uninterrupted horizontally (getting thinner and thinner away from their source material). As a cap on this portion of the geologic story, we found that the peneplain had striations on it, which indicates the sedimentary material was all gone before the most recent glaciation. Striations are caused when rocks plucked by a glacier scrape along a bedrock surface and create long “scratches” in the bedrock which are parallel to the direction of glacial flow. Where we were standing, the glacier would have been about 1-2km thick.

Here we were standing on the peneplain. Photo by Cami Andersen.

Here we were standing on the peneplain. Those big “cuts” in the rock are not striations. Photo by Cami Andersen.

Conveniently, that brings us fairly close to the most recent stuff that Mark showed us and I’ll call it off there. Thanks for reading, and I hope you learned a bit! There’s so much more that we saw and talked about, I wish I could reasonably get through it all here. Others have/will address other aspects of our excursion with Mark.

-Will

Here we were in a gravel pit, and Mark was explaining some of the layering and patterns found in the sandstone and clay.

Here we were in a gravel pit, and Mark was explaining some of the layering and patterns found in the sandstone and clay.

This picture was taken on the granite island called Hållö, which was our first stop. These are p-forms, which are caused by high-pressure water movement under the thick glacier.

This picture was taken on the granite island called Hållö, which was our first stop. These are p-forms, which are caused by high-pressure water movement under the thick glacier.

 

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