October 14. The North Atlantic and North Sea

As we discussed on October 7, the North Atlantic Ocean was very definitely forming during the Jurassic. In late Triassic time, you could probably have walked on dry land from what is now New York City to Casablanca, Morocco. You’d have had to deal with lakes, rivers, swamps, and volcanic eruptions, but probably no ocean. By Late Jurassic time, there was an ocean 1000 kilometers wide between the coasts of New York and Morocco.

But today I want to go a little further north, where Europe and North America were breaking apart but were still pretty close together. The initial rifting between Greenland and Scandinavia may date back to the Permian, barely after Pangaea was finally assembled. But it didn’t take off and separate the continents in a big way for millions of years. Even by late Jurassic time, there was probably nothing but a relatively narrow oceanic strait between Greenland and Scandinavia. Further south, Europe was a complex of islands. Map from Ziegler 

Some of those islands were high-standing, continental blocks, like Iberia and Ireland, but most were segmented and separated from each other by some degree of tectonic activity, analogous to the Triassic grabens of eastern North America. By late Jurassic time, active faulting and rifting were producing oceanic troughs between what is now Iberia and Brittany in France – the modern Bay of Biscay – as well as between smaller blocks such as the Grand Banks, off Newfoundland, and the Flemish Cap, Orphan Knoll, and Rockhall-Hatton Bank, all under water today. A narrow island extended approximately from where London is today across the English Channel into central Germany and the Czech Republic today. Together with another block in northern England and Scotland, those areas were separated from Scandinavia by a developing rift system beneath what is now the North Sea.

Some of these rifts would fail more or less quickly – the one between the Grand Banks and Newfoundland, for example, more or less ceased to be active by sometime in the Cretaceous, and the Grand Banks essentially became an extension of Newfoundland, itself part of North America. The Avalon Peninsula of Newfoundland is another example, like Florida, of rocks that were originally part of another continent – Avalonia – that was accreted to North America during the construction of Pangaea but was left back on the North American continent when the rifting began in Jurassic time. The main rift that formed the Atlantic Ocean might have gone through the zone between the Grand Banks and Newfoundland – but it didn’t. It was just a little bit further east, between the Grand Banks-Flemish Cap blocks and Iberia. That’s where the Atlantic Ocean opened eventually.

North Sea Oil and Gas Fields (USGS)

Let’s go back up to the North Sea area. The rift that formed there was pretty aggressive, producing a deep trough between Britain and Norway. That rift also ultimately failed – if it had not, Scotland today might still be attached to Greenland, or it might have become a microcontinent, independent of either Europe or North America, something like Madagascar today. But it did stay attached to Europe.

The heritage of the North Sea rift probably dates back to the Permian, an extension of that early break between Greenland and Scandinavia. The Permian salt beneath the modern North Sea and across northern Germany – the Zechstein, which we talked about on August 22 - was probably related to tectonic sagging that became a real rift system by Jurassic time.

A low-lying sag accumulates a lot of sediment, and in a restricted basin, it can be starved of oxygen in the deep marine waters, so organic matter can build up without being dispersed. That happened in the Jurassic rifts under the modern North Sea. As faulting continued, high and low ridges and basins formed. If you bury all of that under thick piles of later sediment, you have an excellent oil province. Much of the oil in the North Sea today was sourced in Jurassic rocks, and about half of the oil is in Jurassic reservoir rocks.

Because the rift was active, but not extending to produce a wide open ocean as was happening between Africa and North America, abundant sources of sediment were available throughout most of the Jurassic. This provided material for source rocks, reservoirs, and seals – the elements necessary for hydrocarbon accumulations – and later burial provided the thermal maturation that completed the picture. The North Sea oil province was explored beginning in 1964, although the Groningen gas field was discovered just offshore the Netherlands in 1959. Numerous discoveries in the late 1960s and 1970s made the North Sea into a world-class oil province. The North Sea has produced at least 30 billion barrels of oil over time. It’s a mature oil province, meaning that it is in decline in terms of oil production. At its peak of production in the late 1990s the province produced about 6,000,000 barrels per day. Natural gas production appears to have not yet reached its peak.

By the end of the Jurassic, it was apparent that the far northern Atlantic Ocean was forming, but it wouldn’t be for 40 million years or more, well into the Cretaceous, that separation between Europe and North America was significant.

—Richard I. Gibson

References:
J. R. Underhill, Jurassic – Chapter 8 in Petroleum Geology of the North Sea (K.W. Glennie, ed.). Blackwell Science, 1998.

Evolution of the Arctic-North Atlantic and the Western Tethys, by Peter Ziegler (1988, AAPG Memoir 43) 

Map by D.L. Gautier (USGS, public domain)