September 27. West Siberian Rift

Today I’m calling on your memory again, to think back on the vast flood basalts that erupted in Siberia just about at the end of the Permian. They’ve been implicated in the extinction event then, the most devastating extinction the earth has seen. 

Today, the basalt flows are exposed over extensive areas of Siberia, but west of those exposures, in the relative lowlands occupied by two huge river systems, the Yenisey and the Ob, the basalt flows are found in the subsurface of what’s called the West Siberian Basin. 

W. Siberian Rifts (green) from USGS (Ulmishek)

A few million years after the start of the Triassic and probably continuing for millions of years into the Triassic, a rift began to form in this West Siberian area. It was perhaps something like the rifts, the grabens, that were forming in eastern United States, but it was unlike them in that the whole system ultimately failed – no ocean was created by the rifting here, whereas the Atlantic formed from the rifting between North America and Africa. The West Siberian rifts were pretty big, though – one extends continuously for at least 1,800 kilometers.

Now visualize something like Nevada’s basin and range – long linear rift valleys adjacent to long linear mountain ranges. I’ve mentioned repeatedly that when you have such a situation, the erosion begins to fill those valleys – but this time, let’s focus on the mountains that are being eroded. The Permian and early Triassic flood basalts covered this region in vast, continuous sheets. When the rifting began, the sheets were broken – some segments were dropped down into the grabens, while other segments of the sheets were left on top of the mountain ranges, which are technically called horsts, a German word for heap or pile, something standing higher than the surrounding land.

The basalt on the tops of the mountain ranges got eroded away, leaving the basalt in the down-dropped basins. An alternative interpretation is that the basalts were erupting as the basins and ranges were forming, so the lavas flowed into the low-lying basins and were never deposited on the mountain tops, but I think the preferred interpretation is that the basalt flows were eroded off the mountains. In any case, what’s left, deep in the subsurface, is linear basins full of basalt and linear uplifts barren of basalt. The whole thing subsided even more, millions of years later, so all that complex structure, basins and ranges, is completely buried by later sediments, and the surface of the West Siberian Basin today is fundamentally a flat marshy plain.

Cross-section by Richard Gibson - source

So how do we know the basalt flows are down there? One way of course is to drill into the subsurface and get samples. But that’s expensive. Another way is to recognize that basalt is a dark, iron-rich rock, and a good bit of that iron is in the form of magnetite – an iron oxide mineral that is highly magnetic. We can measure the earth’s magnetic field at a distance from the magnetic rocks, from an airborne magnetometer, so that we can infer the distribution of magnetite-rich rocks in the subsurface. When Soviet geophysicists did that in the 1950s and 1960s, they revealed long linear magnetic highs – representing zones where there was a lot of magnetite – alternating with long linear magnetic lows, where magnetite was absent or less abundant. You’ve probably guessed that what they were defining were the grabens filled with basalt and the buried uplifts where the basalt had been eroded away.

So what? Well, those uplifts are buried beneath Jurassic and Cretaceous rocks that include some rich hydrocarbon source rocks and excellent reservoirs. The sedimentary rocks draped over the deep Triassic uplifts contain some of the largest natural gas fields in the world. And the gas fields coincide almost perfectly with the low values in the magnetic data, the uplifts where basalt is absent. Many trillions of cubic feet of natural gas from the West Siberian Basin have heated homes in Russia and much of Europe for decades. Back in 1990 I did an extensive analysis of the magnetic map of the Soviet Union for oil exploration, and in the process discovered this correlation between magnetic lows and gas fields. The Soviets had known it for decades, of course, but it was pretty satisfying to unravel the details of the relationships myself.

I have a link below to a nice paper from the USGS if you are interested in more about the oil and gas in the West Siberian Basin. Or email me.

—Richard I. Gibson

Links:
Siberian basalt flows 
West Siberian Basin Oil  (PDF - source of Map)
Cross-section by Richard I. Gibson
Urengoy gas field