If you’ve been with us since May 3, maybe you remember the Old Red Sandstone. Those were the sands laid down in largely terrestrial environments, in basins scattered through the Caledonian Mountain Range that resulted from Baltica – Europe, especially the British Isles and Scandinavia – colliding with North America, specifically Greenland, Labrador, and Newfoundland. The early European geologists recognized two red sandstones, and applying the law of superposition, also recognized that one was very definitely older than the other. Hence Old Red and New Red Sandstones.
The Old Red became assigned to the Devonian Period as the early and middle Paleozoic strata were sorted out. The New Red was above, and therefore younger than the distinctive coal measures of the Carboniferous. Ultimately the rocks of the New Red Sandstone were recognized as being of Permian and Triassic age. In Britain, much of the package is quartz sandstone cemented by hematite, iron oxide, which gives it the red color. As we’ve indicated several times, oxidized iron usually indicates that the rock was alternately under water and exposed to the atmosphere for the oxidation to happen. This gives us some good clues about the environment, and like the Old Red, the New Red was laid down in largely terrestrial environments, including rivers, deltas, and ephemeral seas and lakes.
East of Britain, beneath the waters of today’s North Sea, and extending into the Netherlands, Germany, and Poland, the later New Red equivalents include the Zechstein salt that we talked about a few days ago. Salt and other evaporites, including gypsum and sulfates, indicate a restricted sea or large lake in a relatively hot, arid environment. Sea level changes would alternately flood the land or recede leaving mud flats and sandy delta plains exposed to the atmosphere so the iron in the sediment could oxidize and the salts could precipitate out. Today this environment is called a sabkha, an Arabic word for salt flat.
The Zechstein formed in Late Permian time, and the older equivalents of Britain’s New Red are called the Rotliegend Group in Germany, the Netherlands, and in the subsurface of the North Sea. Rotliegend means “the underlying red,” for their presence beneath the distinctive Zechstein evaporites. The basin containing the New Red Sandstone and the Rotliegend Group was surrounded by the mountains uplifted when Gondwana collided with central Europe, so the setting was very similar to that of the Old Red Sandstone – just with a somewhat different geography of mountain ranges.
|Carboniferous-Rotliegend oil and gas fields (from USGS Bulletin 2211)|
There are quite a few oil and gas fields in the North Sea whose reservoirs are in the Permian Rotliegend rocks. The complex changing environments during the deposition of the Zechstein also make for abundant source and reservoir rocks, and the salt can flow and move in ways that help create significant traps for oil and gas. The Rotliegend strata probably spanned a considerable time during the early to middle Permian, perhaps as much as 30 million years. Zechstein deposition was more constrained, maybe only 5 to 7 million years, very near the end of the Permian Period. It’s possible that the incursion of salt water into the Rotliegend basin represents the very first pulse of the break-up of Pangaea. Some reconstructions show a narrow rift beginning between Greenland and Scandinavia by Late Permian Zechstein time, but alternatively, the sea could have come in through a trough extending southeast from Poland to the Tethys Ocean.
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Laurence McKinley Gould was born August 22, 1896, at Lacota, Michigan. He was a geological explorer of the Arctic and Antarctic, working as Richard Byrd’s chief scientist on Byrd’s first trip to the Antarctic. Gould was on the faculty at Carelton College in Minnesota for 30 years.
—Richard I. Gibson
- Permian and Triassic red beds p.11, 17
- Petroleum Geology of the North Sea, K.W. Glennie, ed. (1998, Blackwell Science)
- Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter Ziegler (1988, AAPG Memoir 43)
- Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary by Donald Gautier, USGS Bulletin 2211