Episode 382 The Riversleigh Lagerstätte

Today’s Episode takes us to Australia, the home of the Riversleigh Lagerstätte. Lagerstätten, you may recall, are fossil assemblages that typically have extraordinary diversity as well as extraordinary preservation. The Riversleigh Lagerstätte fits that definition well, containing fossils of 15- to 25-million-year-old mammals, including an extinct giant platypus, as well as birds, reptiles, frogs, and lungfish. But the story begins about 500 million years ago, not 25 million years ago.

In Early to Middle Cambrian time, more than half a billion years ago, the supercontinent of Gondwana was pretty much assembled, with South America, Africa, India, Antarctica, and Australia comprising a wide, long continent that stretched from the South Pole to north of the Equator. The prong of the continent including Australia was the part that was in the tropics, and much of what is now northern Australia was covered by a warm, shallow sea.

In what is now northwestern Queensland, where the Riversleigh Lagerstätte is found, the shallow sea transgressed and regressed – meaning it came and went – and among other things, sediments included grainy shallow-water limestones. The resulting Thorntonia Limestone contains fossils that I’m sure are interesting and informative, including trilobites, brachiopods, and stromatolites, pretty typical of Cambrian rocks. But that’s not the lagerstätte.

Nimbadon, a koala-like tree-dweller. From Wikipedia.

Fast forward about 475 million years, to the Oligocene epoch of the Paleogene Period of the Cenozoic Era. The rocks of northern Queensland were dissolving in a warm, wet, tropical environment, and the limestones of the Thorntonia formation were developing karst topography – caves and surficial pools rich in dissolved calcium carbonate. Starting about 25 million years ago, near the end of the Oligocene, animals began to die in those pools and become trapped in the caves, where ongoing calcium carbonate deposition preserved them remarkably well. So even though the origin of the calcium carbonate was a really ancient rock, the Cambrian Thorntonia formation, it was the much more recent dissolution of those rocks that provided the material that preserved the fossils at Riversleigh. Ages of the fossils extend from late Oligocene time into Miocene, around 15 million years ago, but there are younger fossils as well since the caves are still there and deposition is continuing.  

The Riversleigh site is not just one location, but many, spread out over a hundred-square-kilometer area that was named a World Heritage Site in 1994.

Riversleigh contains the richest assemblage of bat fossils in the world – at least 35 different species. Besides bats, mammal fossils include extinct koalas, marsupial lions, wombats, herbivores the size of sheep, at least 14 species of opossum, and 15 different kangaroo species. While many fossils are remarkably well preserved, some, such as Yalkaparidon, are only known from a few teeth and scattered bones, making their relationships to other families uncertain. In fact, some are grouped colloquially as Thingodonta, meaning “toothed thing”. Some researchers have suggested Yalkaparidon was a mammalian woodpecker.

Bird fossils at Riversleigh range from extinct flightless rails to storks and lyrebirds. Reptiles are represented by tree-dwelling crocodiles, horned turtles, and dragon lizards, which were probably related to iguanas. There are even snakes and frogs and two species of lungfish, all of which are really rare in the fossil record, especially as long ago as the Riversleigh assemblage, 15 to 25 million years.

Some lagerstätten record essentially an instant in geologic time, or a relatively short period. An example of such an assemblage is the fish of the Green River Formation in Wyoming, many of which may have been killed in a single event – a heavy fall of ash into the lake where they lived, from an erupting volcano. So Riversleigh, in addition to its diversity and excellent preservation is also special because it spans such a long period of time – more than 10 million years – offering a remarkable insight into the evolution of the unique fauna of Australia.

—Richard I. Gibson

Links:

Cambrian fossils in the Thorntonia

A good overview

Lists of species

Episode 381 Zealandia

Today for episode 381, we’re going to Zealandia. No, it’s not a quirky TV show modeled after Portlandia. It’s the 7^th largest continent on earth.

We’re not talking continents in the geographic sense, really large land masses like Africa and South America, but we are talking about continental material in the geologic sense, even though most of it is submerged beneath the ocean.

Public Domain via Wikipedia

Zealandia is centered on New Zealand, and in some ways, it’s been known for decades, ever since good charts of subsurface bathymetry were created. Those data show a complicated mix of rises and troughs on the sea floor around New Zealand, including the Lord Howe Rise in the Tasman Sea between New Zealand and Australia, the Campbell Plateau east of New Zealand, and other highs and lows.

Although the ocean floor around the globe is diverse, with seamounts and trenches, fault zones and piles of volcanic rocks here and there, on the whole oceanic crust is deep and uniform, or at least varying broadly and predictably. These relatively small-scale high plateaus on the ocean floor around New Zealand are unusual.

Let’s define a continent. Geographically, it usually means a large land mass. “Large” is subjective, but Australia, at about 9 million square kilometers, is considered to be the smallest continent. And the count depends on whether you separate Europe from Asia, or just say Eurasia as one, and other conventions. Many would say that a degree of isolation and separation also defines a continent, so Eurasia should be one, together with Africa, North America, South America, Antarctica, and Australia, for a total of six. Forget all that. We’re looking at places made of continental crust, the generally lighter, silica-rich material in contrast to oceanic crust, denser and more iron-rich.

By that geologic definition, all the conventional continents really are mostly continental crust too. All well and good. There are numerous smaller, separated areas of continental crust that are usually called microcontinents. Madagascar is probably the best example of a microcontinent, but there are lots of them, and you can get into plenty of arguments about whether a piece like, say, Greenland, is or is not fully a part of the North American continent.

So, let’s go to Zealandia. In 2017, Nick Mortimer and colleagues, writing in GSA Today, pretty much laid any argument to rest. There’s abundant evidence to say that there is a large, diverse terrain centered on New Zealand but 94% submerged, that is mostly continental crust. It adds up to about 5 million square kilometers, comparable in size to Arabia and India, sub-continents that are connected in one way or another with full-fledged continents. Zealandia is close to, but clearly separated from Australia, and Mortimer and others make a convincing case that it should be considered the seventh continent, six times larger than Madagascar, the biggest microcontinent.

So if continental rocks are lighter, which accounts for them being for the most part above sea level, why is most of Zealandia under water? The crust there is thinner, so it doesn’t rise as high as that on most continents, and it has been stretched and broken, partly rifted apart during a long complex history.

Zealandia was probably originally part of the supercontinent of Gondwana, attached to eastern Australia and West Antarctica, when those two continents were fully attached to each other about 105 million years ago, in Early Cretaceous time. That margin of Gondwana, thousands of kilometers long, was a subduction zone somewhat like today’s Andes, with an old Pacific oceanic plate diving beneath the Gondwana continental crust. There would have been a volcanic mountain range there, recorded in Zealandia as a surviving string of granitic rocks, batholiths similar to those that developed in western North and South America during Mesozoic and Cenozoic time and continuing to this day.

Gondwana began to be rifted apart by about 85 million years ago, Late Cretaceous time. It appears that one of the first parts to go from this part of the supercontinent was what is now Zealandia, a long narrow ribbon along the coast of the continent. Australia and Antarctica probably began to separate at about the same time, but that was initially a rather slow break-up that wasn’t complete for close to fifty million years.

The idea of a long narrow ribbon of continental crust rifting off a continent has plenty of precedents. During the Ordovician, the Avalonian ribbon rifted away from what is now northwest Africa and ultimately collided with North America, where it forms parts of Newfoundland, Nova Scotia, and New England today. More recently, a string of continental fragments rifted off what is now eastern Arabia and Africa, to collide with Eurasia to form the cores of Turkey, Iran and Afghanistan today. It’s also possible that one or more “ribbon” continents broke off what is now northern India, to begin to amalgamate with Eurasia in today’s Himalayas, even before India itself collided.

The rifting that split Zealandia off Gondwana is recorded in sedimentary rocks. The initial rifting may have begun the crustal thinning and breaking of Zealandia that we see today, but some of that may have developed later. Because Zealandia was (and is) in a zone of complex interactions among small and large continental and oceanic plates, it’s not really surprising that it would have undergone quite a bit of tectonic and crustal modification. Today, the main part of Zealandia that’s above sea level, New Zealand, contains the Alpine Fault, a huge strike-slip fault similar to the San Andreas. The Alpine Fault is ripping Zealandia apart, but even that helps us recognize Zealandia as a small continent. Mortimer and his colleagues identified rock types in the sea floor on both sides of the Alpine Fault – rock types that can be correlated with each other, and that reflect the predicted offsets you’d expect from the Alpine Fault.

I think Mortimer and his colleagues have made the case quite convincingly that we should indeed think of Zealandia as a continent, geologically speaking, even though most of it is under water. 

Mortimer et al. paper 

Book, Zealandia:Our Continent Revealed, published in 2014. 

—Richard I. Gibson

Episode 380 Makoshika State Park, Montana

The badlands at Makoshika State Park are in Montana, and so am I, but Montana’s big. The park is just outside of Glendive, almost at the eastern border of the state. It took me and my friend about seven hours to get there from Butte, but it was worth it.

Eastern Montana is quite different from western Montana, geologically. The west is broken, thrusted, pulled apart, and intruded, while the east is – with exceptions – largely more or less flat-lying, relatively undisturbed sedimentary rocks piled upon each other, with the oldest at considerable depth and the younger rocks at or near the surface.

At Makoshika, the rocks are from the last part of the Cretaceous Period and the early part of the Cenozoic Era, what we used to call the Tertiary. So the famous K-T boundary, the extinction point for the dinosaurs and a lot more, is in these rocks. Nowadays we call the earliest period of the Cenozoic the Paleogene, and if you’re really interested in the story of the names of the Cenozoic periods, check the podcast episodes from early December 2014.

Erosional features in the Fort Union Formation at Makoshika.
Photo by Dick Gibson.

There’s actually a coal bed near the K-T boundary at Makoshika, but the coolest thing about the rocks in my opinion is their weird and fanciful erosional shapes. The younger Tertiary rocks, the Fort Union Formation, are alternating sandstones and siltstones and shales, with a wide range of cementation, so the rocks respond to erosion very differently. Some erode easily while others stand out in rugged relief. Erosion creates pedestals and balanced rocks, natural bridges and sharp ridges, as well as softer slopes incised by rivulets large and small when it rains.

The sediments that came to make up the Fort Union Formation were deposited in rivers, lakes, and swamps around 65 to 60 million years ago, Paleocene time. There’s a bit more about the Fort Union Formation in the episode for December 4, 2014, which you can find on the blog, history of the earth calendar.blogspot.com, together with transcripts for this and older episodes. The Fort Union Formation’s rocks included many highly vegetated swamps which over time dehydrated and were compressed into thick coal beds, among the most important coal producers in the United States today. U.S. demand for coal is decreasing, as cheaper, more abundant fuels for generating electricity, most notably natural gas, are used more, but in 2017 burning coal was still the source for about 30% of US electricity. Just ten years ago, that proportion was close to 50%. Coal mines in the Fort Union Formation in Wyoming, south of Makoshika, produce about 40% of all US coal.

The Cretaceous rocks underlying the Fort Union Formation are similar kinds of rocks, but they contain dinosaur bones, especially further west, and they’re called the Hell Creek Formation. Triceratops and Tyrannosaur fossils have been found in the Cretaceous rocks at Makoshika, along with a nearly complete Thescelosaurus, a small, 10-foot-long probable herbivore discovered in 1997.

The name Makoshika is from the Lakota words maco sica, meaning 'bad land' or 'land of bad spirits,' but despite that I find the land remarkably beautiful. One of the reasons the landscape displays such spectacular erosional features is the fact that this area is gently uplifted. The Cedar Creek Anticline, a long, linear fold above a deep-seated fault trending northwest through this part of southeastern Montana, reaches its northern end near Makoshika.

You’d never think of this as a mountain uplift, but older, more erodible rocks of the Hell Creek Formation have been warped to the surface here, where wind and water have sculpted them for many thousands of years. Features like natural bridges and balanced rocks and delicately carved monuments are ephemeral in geologic terms, usually surviving for hundreds or a few thousand years at most. But they are continually replaced by other features, until the rocks with their variable resistance are all gone, washed down the rivers. That happens eventually, even in semi-arid country like eastern Montana.

—Richard I. Gibson