The Cretaceous rocks we saw yesterday here in Victoria, Australia gave us a reason to ponder temperature and climate, but in a much grander way than just saying the words, “global climate change.” I cogitated on this matter while enduring the harsh, ravaging effects of the Victorian winter here.
Thinking cold thoughts, post-field work version. Sorry for the omission of a beer, if at least for scale.
Yet despite what we observe here today, this part of southern Australia was connected to northern Antarctica 120 million years ago – during the Early Cretaceous Period – and the rocks here hold clues that reflect environments close to 75° S. (Just in case one of my students is reading this and furiously converting those degrees into Fahrenheit on his/her iPhone, those degrees are not for temperature, but latitude, and the South Pole is 90° S of the equator.)
Paleogeographic map from about 130 million years ago, during the Early Cretaceous Period. So if you think Australia is “Down Under” now, go back to the Cretaceous and see what you think, mate! Map from Ron Blakley’s fantastic paleogeographic map site, hosted by Northern Arizona University.
The Cretaceous outcrops we have walked across for the past eight days are now at 38° S. This means Australia has moved steadily north in the past 120 million years, and at this rate will reach the equator in another 100 million years or so. This is Reason #1,243 why plate tectonics, as a process, rules.
Granted, mean annual air temperature (MAAT) was much higher during the Cretaceous Period than today, at about 20°C, versus 14.6°C today. (OK, now you can do those conversions to Fahrenheit: that’s 58°F versus 68°F.) Still, it got very cold here during the winters, accompanied by months of darkness each winter, cold enough to freeze the ground, and even form permafrost. (Of course, it’s not frozen now, so it’s not really fulfilling the “permanent” part of permafrost. Unless you count rocks as “frozen,” which I don’t.)
What evidence do they have of frozen conditions here? Well, we could use stable oxygen isotope ratios, which are routinely applied to figure out paleotemperatures. Alas, we are mere underfunded field geologists, not the immortals on Olympus who use geochemistry and other means of witchcraft to pry thermally related secrets from the rocks.
We could also use paleobotany, as fossil plants have growth rings, leaf margins, and the assemblage of plants themselves that reveal much about the environments. Indeed, that has been done, and they match plant assemblages that are typical of MAAT of 8-10° C. But we also are not paleobotanists, however much we wish we were (because paleobotanists so cool, they inspire songs).
So how about good old geological evidence, observable in the field using our unaided eyes and not requiring any fancy equipment, or paleobotanical know-how? Hey, that’s a great idea! Sure enough, three lines of geological evidence support that these environments – inhabited by the likes of dinosaurs, mammals, and the “monster newt” Koolasuchus – were indeed frozen at times. These are:
• “Soil drops.”
• Hummocky ground.
• Ice wedges.
All of these collectively could be termed as cryoturbation structures, which may sound a little naughty, and for that reason alone they are probably illegal in much of the southeastern U.S., regardless of their identity. Nonetheless, the name literally means, “cold mixing,” which implies that cold and frozen conditions, along with thawing of ice, can mix sediments, like sand and mud.
Here are the “soil drops” viewed in cross section in an outcrop, in which darker sediments (mud) settled into the lighter colored sand. This was caused by ice melting above, which made the mud heavier, then it plopped into the less-dense sand. These structures are at Flat Rocks, the site of the Dinosaur Dreaming dig.
“Soil drops” at Flat Rocks, near Inverloch, Victoria. Photo scale is a wee one, only 10 cm (4 in) long.
Note that they all have flat bottoms, and they flatten out on some unseen surface about 20 centimeters (8 inches) below the top of the “drops.” This imaginary plane is where the permafrost below them halted their progress. Think of the permafrost acting like the plastic liner of a kiddie pool, thus preventing chocolate pudding from going below it while two consenting adults wrestle above. (OK, I have to work on that metaphor.) Anyway, I have looked carefully at these structures at Flat Rocks during several previous visits there, and lacking any other reasonable explanation, I am convinced that they are what other geologists have interpreted.
Hummocky ground forms the same way as the “soil drops,” only they are more symmetrical, with the troughs of the “hummocks” bottoming out above a permafrost layer. Yesterday, we saw a beautifully exposed example of such a horizon in an outcrop near Kilcunda, Victoria, outlined by a thin, black layer of coal.
A hummocky-ground horizon in vertical section, with its bottom marked by the black line; scale is about 20 cm (8 in) long.
Very close to these at the same outcrop, less than 100 meters (330 feet) away, were the ice wedges. These, of course, are not to be confused with “ice wedgies,” which I suffered at the hands of bullies all through elementary school. (Yeah, but who’s laughing now?). Ice wedges are formed by water freezing in sediments that, as the ice expands, wedges it apart and forms fractures. Here are some of the structures, filled with lighter-colored sandstone, which makes them stand out in the finer-grained and darker shale.
Ice wedge, filled with sandstone in shale, vertical section. (It’s that snaky looking thing in the middle.) Scale is the same as before.
Ice wedge, filled with sandstone in shale, but this time seen from above (what we geologists suggestively call the “bedding plane.") Yup, same scale.
So at the end of this little summary of what may be the most obscure topic you’ve ever had the gumption to read, you might justifiably ask, “So what?” The short answer is, these are the only examples of cryoturbation structures known from the Mesozoic Era in the world. Good enough for you? Also, the fact that they are in the same rocks that contain the remains of dinosaurs, mammals, amphibians, and other critters backs up the interpretations that these animals were not only living in polar environments, but environments that were at least occasionally frozen.
Dinosaurs out foraging on a snowy landscape during the Cretaceous winter in southern Australia, while a small mammal watches, remembering all of those “ice wedgies” the dinosaurs gave him. More gorgeous paleo-art by Peter Trusler; originally in Smithsonian Magazine.
And that’s significant, too: no other polar dinosaur site in the world shows such geological evidence, so clearly indicated and exposed such that geologists can just walk up to an outcrop and interpret them. Not only that, we can later that same day go back to the motel and take in some solar therapy, all while imagining those colder times in the Cretaceous of Australia, helped considerably by holding a cold Tasmanian beer.
(Much of the original research done on these cryoturbation structures is described in this peer-reviewed paper: Constantine, A., Chinsamy, A., Vickers-Rich, P., Rich, T., 1998, Periglacial environments and polar dinosaurs: South African Journal of Science, 94 (3).)