If you could watch a time-lapse of the earth over the 50 or so million years of Jurassic time, I think it would be evident that the supercontinent of Pangaea was coming apart at the seams. In some cases, the break-up was literally along the seams, the sutures where Pangaea was assembled over the course of half of the Paleozoic era, but in others, the breaks were either in more or less brand-new zones or they followed much older sutures. And nowhere did they follow the old zones precisely.
At the start of the Jurassic about 200 million years ago, the most evident break would probably have been the one we’ve been tracking in eastern North America, where Africa was beginning to separate. I’ve mentioned several times the block-faulted basins, and low-lying depressions and igneous intrusions that marked that initial extension.
|Middle Jurassic map (170 million years ago) by Ron Blakey used under Creative Commons license|
The other big break that was beginning was between the two main parts of the southern portion of Pangaea, in the old continent of Gondwana. There, West Gondwana, which was pretty much Africa plus South America, was beginning to separate from East Gondwana, which included Antarctica, India, and Australia, along with a lot of smaller blocks such as New Zealand.
The initial crack between the two parts of Gondwana spread by about 180 million years ago so that the Karoo-Ferrar volcanics were erupting along a long incipient rift between South Africa and Patagonia in South America, and between South Africa and adjacent Antarctica. The volcanic belt extended through Antarctica, between what is now East Antarctica, the main mass of that continent, and the multiple terranes of West Antarctica. It continued to southeastern Australia and Tasmania, starting the break between that area and the small continental block that is New Zealand today. One of the largest exposed areas of igneous diabase (called dolerite in British English) covers a vast area of Tasmania, evidence of this Jurassic eruptive event about 180 to 183 million years ago.
By late Jurassic time, it would have been clear that Pangaea was done for. North America was essentially completely separated from Europe, Africa, and South America, and the cracks between Africa and South America were apparent, both in the north and the south.
And the break-up had changed the geography of southeastern North America. Where it was formerly a reasonably smooth, curving edge, swinging through the present-day Appalachians to curve through what is now central Georgia and west through Alabama and Mississippi, the southeastern corner of North America now included a fairly long, pointed triangle – Florida.
If you remember the African Suture, also called the Brunswick Magnetic Anomaly, which we talked about on July 26, that was the line that marked the collision, the amalgamation, of Gondwana and North America in one of the major events of the assembly of Pangaea. Well, when the rift happened that created the Atlantic Ocean between North America and Gondwana, it didn’t follow the old suture. It broke further east, in a nearly north-south line though rocks that were originally part of Gondwana. Then it zig-zagged to the west, dismembering the accreted terranes that came from Gondwana, so that some bits were left with North America and some became fragments with a life of their own.
Florida, southern Georgia and Alabama and Mississippi, and probably most of the Bahamas and possibly some more bits even further offshore to the southeast of Florida are really parts of Gondwana that were left behind when Africa took off in the Jurassic. And one of the big fragments to the west? That became Yucatan, a continental block that was originally part of Gondwana, but was attached to North America during the amalgamation of Pangaea. When the rifting happened in the Jurassic, Yucatan broke on both sides. As it moved away from North America, the Gulf of Mexico opened, and we’ll have more to say about that later this month. The other side, toward Gondwana, also broke, and that edge ultimately became the Caribbean Sea as South America pulled away.
This is all obviously easier to visualize with maps than from my words, so I have links below to some of the excellent paleogeographic maps prepared by Ron Blakey at Northern Arizona University. They will help you visualize the ongoing changes to Pangaea during the Jurassic.
Bottom line, by the end of the Jurassic, much of the North Atlantic Ocean was very much evident, even though it was much narrower than it is today. In the far north, Greenland and Scandinavia, and some small blocks in between, were still pretty close together, although probably separated by oceanic straits. The South Atlantic Ocean was beginning with those narrow cracks separating Africa and South America, but it won’t be until next month, the Cretaceous, that the South Atlantic really began to open.
—Richard I. Gibson
Middle Jurassic map above by Ron Blakey used under Creative Commons license
Tasmanian volcanic flows