November 4. Paleogeography and climate

Because it’s so long – 80 million years – it’s definitely not cool to imply anything constant about the distribution of lands and seas nor about the climate of the Cretaceous, but as usual I’m happy to make some generalizations. As we said at the end of the Jurassic, the globe was beginning to become recognizable in terms of the modern continents. We’ll talk about some specifics, like the opening of the South Atlantic Ocean, over the course of the month, but at least by the middle of the Cretaceous, about 100 million years ago, the basic shapes of most of the continents were close to their modern forms.

Cretaceous globe (105 million years ago) by Ron Blakey, used under CC-BY-SA & GDFL. 

India and Australia were still way south of the Equator, having just separated from Antarctica, and the Tethys Ocean was still between them and the main mass of Asia. Antarctica itself was near or at the South Pole as it is today, and the northern margins of North America and Eurasia were in the north polar region as they are today.


But the climate was much less extreme than it is today, with only a few notable exceptions, throughout most of the 80 million years the Cretaceous lasted. There were no widespread continental glaciers during the Cretaceous, although there were probably mountain glaciers in high latitudes, but even that was probably limited to the very early Cretaceous in what is now Australia. Most of the rest of the period was warm, and the temperature change from poles to equator was not extreme. One probable consequence of the weak temperature gradient would be less extreme winds and perhaps fewer intense storms like modern hurricanes. That, in turn could lead to oceans with poorer circulation – though the general distribution of land masses, at least until the late Cretaceous, might have a greater control on that. This is a possible explanation for the extensive black shales, related to anoxic events in the oceans, which are observed in the Cretaceous rock record.

The equable climate at the poles is reflected in the presence of dinosaurs there, as well as temperate plants. Southernmost Australia today was within the Antarctic Circle in early Cretaceous time, and mud flats that formed there in the rift between Australia and Antarctica have yielded diverse dinosaur bones, and Cretaceous dinosaurs are also known from Antarctica itself.

Temperate forests were present within a few degrees of the poles. Remember that whatever the temperature, the poles still had long seasons of darkness just as they do today. Cretaceous greenhouse conditions allowed plants to grow in polar regions, but it might have been the light and dark cycles that provided evolutionary pressure for plants to evolve. The early Cretaceous forests were much like those of the Jurassic, with cycads, ferns, and conifers. But within 15 or 20 million years, dramatic evolution of the angiosperms, flowering plants, had begun. We’ll talk more about that in a few days.

By late Cretaceous time, average temperatures had risen even more, to perhaps 4 to 6 degrees Centigrade warmer than today. This time is called the Cretaceous Thermal Maximum, and it occurred during the Turonian stage. Exactly what caused it is debated, but two favored factors are both related to rifting. As more and more rifts formed, and oceanic crust was generated, the volcanism associated with rifting could have put more and more CO2 into the atmosphere, enhancing an already strong greenhouse effect. It’s also possible that changing patterns of land and sea allowed for more efficient movement of warm surface water around the globe. There is considerable variation in the patterns of temperature even during the Turonian, so this event is definitely not fully understood.

In North America, the Cretaceous was dominated by the Western Interior Seaway, a long shallow sea that extended from Alaska to Alberta to Colorado to West Texas and into Mexico. We’ll talk more about that when we talk about the mountain-building events that were defining the western margin of North America, later this month.

—Richard I. Gibson

LINKS:
Rifting and Cretaceous Thermal Maximum 

Cretaceous Thermal Maximum

Dinosaur Cove, Australia 

Cretaceous globe (105 million years ago) by Ron Blakey, used under CC-BY-SA & GDFL. 

September 8. Pangaea begins to break up

The other day, I said that in the Triassic, Pangaea was pretty much still intact as a single huge continent, with the big embayment on the east side, the Tethys Ocean. But I’ve also said there were hints of the great breakup that was about to begin.  

As the Cimmerian blocks began to rift away from the northeastern coast of Pangaea, the southern portion of the supercontinent, the old Gondwana, was probably rotating a bit, so that where Africa and Europe were attached, they began to pull apart at least to some extent, but it’s not really completely clear exactly what was going on there during the Triassic. Further north and west, through the complex mountain ranges that formed during the Caledonian, Alleghenian, and Appalachian Orogenies, over many millions of years, the compression due to collision was giving way to extension.

Triassic Globe by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)

Even as early as the late Permian, a rift, a pull-apart, had begun to form between what is now northeastern Greenland and northwestern Norway. That narrow strait might have allowed sea water to invade the basins of northern Germany and the Netherlands, where the Permian Zechstein salt formed. By the middle of the Triassic, the rifting seems to have extended a long way into the combined North America-European continent.

Tethys reconstruction globe from Stampfli & Borel 2002

How do we know this? There are extensive deposits of terrestrial sediments scattered through the region – mostly offshore today – from southern Greenland to west of Ireland and France and Iberia, and on the North America side, from east of Newfoundland around the margin to south of Nova Scotia – which was still attached to Africa, about where Morocco is today. This was not a complete seaway, but the rifting was making basins, similar perhaps to the basins that received the Old Red Sandstone back in the Devonian, after the first big mountains formed through this zone, the Caledonian Mountains. It was a complex array of zig-zagging rift basins, and I really think it’s fair to think of it like the East African Rift today – long, linear interconnected rift zones, in places with lakes, in places just lowlands receiving eroded debris off the adjacent highlands.

Let’s take a break for a minute and talk about rifts. When I say “rift,” I mean a major break in a continent, where two parts of the continent pull apart from each other. Ultimately, such a rift might become an ocean basin, with the two continental fragments bordering it on each side. This process is often driven by the generation of new oceanic crust at a mid-ocean ridge. Heat rising in convection currents from the deep mantle brings molten material to the surface – or at least near the surface – in a linear zone. As more and more such material rises, the previous material has to move out of the way – and the crust of the ocean spreads apart, away from the mid-ocean ridge. If that ridge started beneath a continent, the inexorable force of rising heat and magma will eventually break even thick continental crust. That’s what’s happening today in East Africa, and it’s what was beginning to happen during the Triassic where North America and Europe were attached. This is the birth of the modern Atlantic Ocean.

Rifting. (I have tried and failed to
determine the owner of this image;
if you are the copyright owner, please let me know.)

But during the early and middle Triassic, we didn’t have much in the way of open oceans yet. Probably just that narrow strait next to Greenland in the north, and possibly some ocean between North Africa and southern Europe. The rest was a diverse lowland, a sag, with linear mountains surrounding lake basins and continental river systems. Think of it like a big mass of cold caramel – soft enough to stretch some, but brittle enough to break eventually. As you pull the caramel apart, the middle will sag, and finally will break with a pretty sharp edge, assuming the consistency is just right. Since most of the rocks are under the Atlantic Ocean today, they are known only from remote sensing studies, including seismic data, and from wells drilled for oil and gas exploration. Not quite the same as having them exposed for geologists to take rock hammers to.

I think two big questions might be occurring to you at this point. First, what makes a rift start? And second, in this case, why did the rift run more or less along the zone where the original collision had created a huge mountain uplift that went from northern Greenland all the way to West Texas and probably beyond?

Oceanic rifts start where the linear edges of mantle convection currents rise toward the surface. The ultimate controls on the geometry, size, and position of convection currents are poorly understood – it’s the distribution of heat down in the mantle, and the complex response of the solid earth to that. And the solid earth is not uniform, so variety will be the name of the game. It’s also possible that some rifts begin because isolated mantle plumes, or hot spots like those at Yellowstone and Iceland, rise and weaken the crust, essentially encouraging the rift to radiate from that location. To break continental crust, much thicker and stronger than oceanic crust, probably depends on some special circumstances, such as a pre-existing state of stress, but it seems possible that a mantle plume might initiate such a continental break-up. This is still a controversial topic. Here's a 2014 paper on this idea, and see also this paper from 2014 for an opposing view.

As for the second question, why did the Atlantic Rift begin to form pretty much right along the zone where the continents had come together, one simple rationale is that such a zone, full of faults and inhomogeneities, would be the weak point in the system. The central cores of the continents – the cratons, which we outlined in January, and the word craton means “strong” – would have been much more resistant to breaking apart than the collision belt. You might argue that the collision zone made the crust even thicker, and with lots of igneous rocks and metamorphism, the suture zone, where the continents were welded together, ought to be the strongest part. Maybe it was. But it’s an observational fact that the break-up of Pangaea – at least between Europe and North America – followed the old collision, more or less. There are some interesting exceptions that we’ll talk about as the break-up proceeds over the next month or so.

* * *

Today’s birthday is Raphael Pumpelly, born September 8, 1837, in Oswego, New York. His geological work was wide-ranging, from Chinese coal fields to the copper country of Michigan, but he focused on economic geology of mineral deposits. He was the first to explore the Gobi Desert scientifically and he was also in charge of the Northern Transcontinental Survey of Dakota, Montana, and Washington Territories in the early 1880s. Pumpellyite, a low-grade metamorphic calcium-iron silicate mineral, was named for him.

—Richard I. Gibson

Tethys reconstruction globe from Stampfli & Borel 2002:    http://www-sst.unil.ch/research/plate_tecto/alp_tet_main.htm#Introduction 

Globe by Ron Blakey, NAU Geology, under Creative Commons license (notes by Gibson)


References: P.A. Ziegler, Evolution of the Arctic-North Atlantic and the Western Tethys, AAPG Memoir 43, 1988.

Mantle plumes cause rifts? 

July 15. Pennsylvanian paleogeography

By late Pennsylvanian time, the ongoing collision between the southern supercontinent of Gondwana and the northern continent of Laurasia, North America plus Baltica or Europe, was well underway. There was probably still a bit of ocean between the far northwestern part of Gondwana, which is South America today, and what is now southwestern North America. Siberia and other terranes probably hadn’t quite attached themselves to the eastern margin of Baltica, and there were still some continental blocks remaining unattached, including microcontinents that would become parts of China today. But the assembly of Pangea, whose name means “all earth” was clearly approaching its climax. 

Late Pennsylvanian Map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

We’ve talked about some of the tectonic events in North America, resulting from the collision of Gondwana with what is now eastern and southern United States. The Appalachian-Alleghenian Orogeny had created another high mountain range approximately where the modern Appalachians stand, and the southwestern extension of that collision produced the Ouachita Mountains in what is now Arkansas and Oklahoma, extending as far west as the Marathon Region of West Texas and beyond. Deformation within the western part of the continent broke the crust to create the Ancestral Rocky Mountains (see July 6) Especially across the eastern interior of what is now the United States, Pennsylvania south to Alabama and west to Illinois, the uplifts confined tropical lowlands that became the vast rainforests and swamps whose plants ultimately produced the coal for which the region is famous.

The Pennsylvanian equator ran approximately from modern San Francisco to Nova Scotia and into Britain and France, which were attached to the eastern side of Canada. The huge southern continent, Gondwana, extended from its tropical to sub-tropical attachment to North America and Europe all the way to the south pole, and by late Pennsylvanian glaciers were forming there. The episodic advance and retreat of glaciers probably contributed to sea-level changes that produced the alternations between swamps, during low stands of sea level, and incursions of the sea when the water was deeper as the ice melted. This alternation is what produced the rhythmically alternating layers of coal and sediment in Pennsylvanian or Late Carboniferous strata around the world.

—Richard I. Gibson

Map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

Another Map

June 22. Late Mississippian Lands and Seas

By late Mississippian time, around 325 million years ago, the extensive warm shallow seas that covered much of North America were becoming restricted. The sedimentary rocks were less often the limestones we heard so much about for most of this month, and more the shales we discussed on June 17. More clastic sediment reflects the onset of uplift of lands around the marine areas.

In North America, in both the east and south-central part of the continent, collision with the northwestern margin of Gondwana was beginning. In the east, this was largely along the same zone where previous collisions, from the Taconic Orogeny at the end of the Ordovician to the Caledonian and Acadian Orogenies in the Silurian and Devonian had produced episodic ongoing uplifts. This is the beginning of the Appalachian or Alleghenian Orogeny, the main continent-continent collision between North America and Gondwana.

Another aspect of that collision was beginning in what is now Oklahoma, Texas, and Arkansas, where it is called the Ouachita Orogeny. The uplifts there led to the deposition of the Barnett Shale in the Fort Worth Basin, which today is a significant source of natural gas production.

The Tethys Ocean between Eurasia and Gondwana was also closing, and aspects of that collision during the latter part of the early Carboniferous, the equivalent of the Mississippian Period, are the Variscan Orogeny in Europe. This was definitely not a simple collision, as there were many isolated microcontinental pieces, oceanic crust, and island arcs involved, so parts of the Tethys remained open while other parts were closing. Parts of the Tethys Ocean still exist today, beneath the Black Sea, the southern Caspian Sea, and in the deeper basins of the Mediterranean.

While the timing of all these events varies by a few tens of millions of years, I think it’s safe to say that the collision between Laurasia – North America plus Eurasia – and Gondwana was definitely underway by late Mississippian time. We’ll hear more about it over the next month or so.

—Richard I. Gibson

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

June 15. Mississippian Plate Tectonics

I’ve said for months that Gondwana is coming toward southeastern North America. It’s still coming, but the remaining ocean between the two continents was really pretty narrow by middle Mississippian time.

The Supercontinent of Gondwana included South America, Africa, Arabia, India, Antarctica, and Australia, plus a few additional bits. One of the most interesting bits, from North America’s point of view, was a triangular zone between what is now west Africa and northern South America, along the Venezuelan coast. That triangular zone included what is now Florida and southern Georgia and parts of Alabama and the Bahamas. It was absolutely part of Gondwana at this time, but fear not, we’ll get it fairly soon.  

North America was tilted relative to its present geography, so that the Mississippian equator ran more or less through the middle of the continent, with what is now the east coast forming a southern coastal edge to the continent. The northeastern part of North America – the Maritime Provinces of Canada, Newfoundland, Labrador, and Greenland, were still pretty firmly attached to Europe by the remnants of the Caledonian Mountains, even though they were fairly old by Mississippian time.

The central part of Europe, France, central Germany, and points east, were assembling as a result of small continental blocks that had been shed off the northern flank of Gondwana colliding with the margins of Baltica. What’s now the Iberian Peninsula, Spain and Portugal, was involved in a complex mountain belt called the Variscan Orogeny, which won’t really culminate until the late Carboniferous and Permian Periods, 20 to 50 million years after the end of the Mississippian or early Carboniferous.

* * *

On June 15, 1991, Mt. Pinatubo on Luzon Island in the Philippines reached the climactic phase of a multi-day eruption. Pinatubo had been inactive in recent memory until the 1991 events, and the mountain was deeply eroded and supported extensive forests and human populations.

The eruption ejected huge quantities of ash, and the reported death toll of about 900 was largely the result of buildings collapsing under the weight of the ash.

Evacuations saved thousands, but the lives of pretty much everyone in the region of the eruption were severely disrupted. The volume of ejected material was the second greatest in the 20th century, estimated to be a bit less than the eruption at Katmai, Alaska, in 1912, which we discussed a week ago. The ash and especially the aerosols Pinatubo put into the atmosphere did have global climatic impacts, reducing the average global temperature by almost a degree Fahrenheit or a half degree Centigrade.

—Richard I. Gibson

Early Mississippian map

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

U.S. Geological Survey Photograph of Pinatubo eruption in 1991 taken by Richard P. Hoblitt.

June 7. Lands and Seas

The Mississippian Period was much like the earlier part of the Paleozoic Era in North America – much of the continent was covered by warm shallow seas in which abundant life contributed to the thick sheets of limestone that were laid down. Eastern North America was still a highland, the result of the ongoing collisions with Europe and the microcontinents that were rifting away from Gondwana, and the ocean between North America and Gondwana was getting narrower and narrower.  

In western North America, the Antler uplift that we discussed at the end of May was still there, and it was shedding sediment into the western seaway. The combined weight of those sediments and the piles of thrusted rocks pushed over the western edge of North America by the Antler Orogeny pushed the earth’s crust downward, creating what’s called a foreland basin. The Antler foreland basin extended through what is now eastern Nevada, western Utah, and into parts of Idaho and California. It was a relatively deep trough, and the mud that found its way into it contained a lot of organic material, washing in from both the west and the east. The resulting rock is called the Chainman Shale – a rock we mentioned in the Devonian, in May, as the source rock for the oil fields of Nevada. The Chainman is an excellent source rock, as much as 8% total organic carbon in some places.

There was some tectonic activity in what is now Alaska’s North Slope and across the Arctic islands of northern Canada – it’s called the Innuitan or Ellsmerian Orogeny and it was mostly taking place toward the end of the Devonian and into the Mississippian Period.

In Europe, Gondwana was pretty much encroaching on the southern margin of Baltica, but it was a complex interaction with lots of smaller blocks colliding. The seaway between Gondwana and Europe, called the Tethys Sea, is actually still there. We call it the Mediterranean Sea today. 

—Richard I. Gibson

Links to Paleogeographic maps:
North America
Western North America
Europe

Article:

Innuitan Orogeny

May 7. Devonian paleogeography

If you’ve been following along, you know that Laurentia, the core of North America (NA in map below), and Baltica, the Scandinavian heart of Europe (E), fused together during the long tectonic episode called the Caledonian Orogeny. The assembled continent is sometimes called Laurasia, or Larussia. But don’t think of it as a perfect, or complete union – there were still stringers and islands and other bits of continents impacting, and Gondwana was still coming. 

Late Devonian map by Ron Blakey via Wikipedia, used under Creative Commons Attribution license

Another block that was on its way was Siberia (S on the map). Siberia, together with various complications like island arcs and microcontinents, was north of the combined North America-Europe continent of Laurasia. Laurasia was pretty much along the Devonian equator for much of the period, which meant that any shallow seas would likely be warm and would support plenty of diverse life, and that’s what the fossil record shows in those areas.

There was plenty of variation over the 50 million years and more of the Devonian. At times, much of what is now the interior of North America was above sea level, but by the middle to late part of the Devonian, extensive shallow seas covered most of what is now the United States. It wasn’t a perfectly smooth, uniform shallow sea – troughs like the Appalachian Basin and deeper zones like the Michigan Basin made for plenty of variety in the marine environments. Maybe that variety, in North America and Europe, with warm tropical seas, contributed to the relatively rapid evolution and diversification of the fishes. But remember, life was definitely present on land during the Devonian, and there were many many niches there for life to adapt to.

—Richard I. Gibson

Globe image by Ron Blakey via Wikipedia, used under Creative Commons Attribution license.

April 4. Silurian lands and seas

The low sea levels of the Ordovician glacial period rebounded in the Silurian as the ice melted. Although much of Gondwana was still over the south pole, it appears that conditions were not right for glaciers, and the ice caps, if any, were much smaller during most of the Silurian than they were during the Ordovician glacial time.

Map by Ron Blakey, via Wikipedia, used under CC-A-SA license.

Still, sea level wasn’t quite as high as during much of the Ordovician, at least in parts of North America. We’ll discuss some of the variations later in the month.

The Silurian map above shows Baltica, the Precambrian core of Europe, getting pretty close to North America, and the long narrow microcontinent, Avalonia, is getting caught between them. Siberia is to the northeast, and Gondwana is on the margins of the globe in the map above. Plenty of fodder for mountain building. We’ll talk about that, too, later in April as we get further into the Silurian Period.

—Richard I. Gibson

Map by Ron Blakey, via Wikipedia, used under CC-A-SA license.

March 16. Ordovician paleogeography

Plate tectonics, the process driven by heat convection down in the mantle, has operated pretty much continuously at least since the late Precambrian, and probably even longer than that. During the Ordovician, it seems that there may have been a little more “action” than at some other times – probably the normal variation in the intensity of tectonic activity.

Most of the supercontinent of Gondwana, which we first discussed on February 9, was finally assembled during the Ordovician, and it would remain together for the next 200 million years or more. At least by the late part of the Ordovician, Gondwana was situated near the south pole.

But apart from Gondwana, several important continental blocks, including Laurentia, which is the core of North America, Baltica, the heart of northern Europe, and Siberia, were still moving around pretty much independently. And moving continents means subduction zones, volcanic island arcs, and small continental blocks fragmenting and colliding. Those three major continental blocks, Laurentia, Baltica, and Siberia, were relatively close to each other, but there were also some complex island chains not too far away. One of the most important of those is called Avalonia, named for the Avalon Peninsula of Newfoundland.

The long, linear Avalon terrane today is found in Massachusetts, Maritime Canada, Newfoundland, southern Ireland and Britain, and maybe some bits of northwestern Europe. But during the Ordovician, it was separate from the major continental blocks. I think of Avalonia as something like the western Pacific today – from the Kamchatka Peninsula to Japan to the Philippines. A discontinuous string of continental fragments, volcanic islands, oceanic crust, and more – a real mess.  This string began to collide with North America during the Ordovician Period, probably causing the buckling of the crust that formed the Cincinnati Arch and Michigan Basin, which we talked about earlier this month. We’ll talk about the culmination of this collision, called the Taconic Orogeny, at the end of March.

You can imagine that such a complex strip of diverse geological settings wasn’t necessarily just sitting there idly, even before it collided. We’ll talk about some of the consequences of the plate tectonic events related to Avalonia in a few days.

—Richard I. Gibson

Ordovician 450 million years ago, map by Ron Blakey, via Wikipedia, public domain.

See also http://jan.ucc.nau.edu/~rcb7/global_history.html