These daily podcasts build upon previous episodes, so the best way to work through them is by starting with the oldest, January 1. But you don't have to do that.
Most episodes are two to 10 minutes long. It's September, so we're moving through the Triassic Period.

Friday, September 19, 2014

September 19. Triassic Grabens: the Newark Group





A few days ago we started talking about the extension that was pulling Pangaea apart. The rift zone was following, more or less, the old collision zone between Europe and North America and Africa and North America – the Caledonian and Appalachian Mountains.


Map of Eastern North America Rift Basins, via Wikipedia
The extension that would ultimately become the Atlantic Ocean was not along a single, sharp, linear fault zone, however. The pulling apart affected a wide area before the final snap that separated what is now Africa from the eastern United States. Parts of the eastern U.S. must have looked somewhat like today’s Nevada, with long mountain ranges alternating with deep troughs or basins between them. The troughs are called grabens, a German word for trench. The grabens were fault-bounded down-dropped valleys that resulted from the pull-apart stress that was imposed in Triassic time on a vast region, from at least Nova Scotia south to Georgia.

As with any valley alongside a mountain range, these valleys began to accumulate sediments. The Triassic sediments that filled these fault-bounded valleys are called the Newark Supergroup, from exposures near Newark, New Jersey. The rocks are mostly terrestrial, derived from rivers, floodplains, alluvial fans, lakes, and swamps. Many of the sandstones and shales are red, indicating periodic exposure to air to oxidize the iron, and the rocks contain mud cracks, ripple marks, raindrop impressions, and dinosaur footprints – all indicative of wet flood plains and similar environments.

The Newark Supergroup rocks are very thick, up to 6 kilometers or nearly 20,000 feet – almost four miles of sediment. This is typical of rift valleys, where relatively rapid uplift and subsidence can make such huge piles of sediment. The modern rift at Lake Baikal, in Russia, is close to 10 kilometers or more than 30,000 feet deep.

The extension that produced the grabens of eastern North America was active about 220 million years ago, during the late Triassic. All of the basins are technically “failed rifts” – the successful rift is the Atlantic Ocean. It’s fair to think of the whole thing a pulling apart, breaking things here and there, until at last an irregular, jagged break formed where the extensional forces were greatest, or the breaking rocks were weakest, or some combination of both of those factors.

There are at least 20 distinct, separate grabens or fault-bounded valleys of Triassic age along the east coast, from the Bay of Fundy south to North Carolina. There is at least one more large one buried in the subsurface of South Carolina and Georgia, along the Savannah River.

Source: National Park Service - Newark Basin 
Rifting thins the continental crust. Ultimately, it will thin to nothing, and what’s left is oceanic crust. The whole process results from upwelling heat and magma, forcing older material at the rift axis apart – remember the discussion we had on September 8, about rifting in general. That means that molten material is often associated with rifting, even with failed rifting. The Newark Supergroup rocks are injected with igneous rocks, mostly in thin but often laterally extensive sheets. If those sheets cut across the pre-existing rocks, they are called dikes, and if they force their way between the beds of the older sedimentary rocks, they are called sills. Some flows on the surface would end up as sill-like bodies after they were buried by later sediment. Dikes and sills are common in the Triassic grabens of eastern North America, especially in the Connecticut Graben and the Newark Graben.  One prominent sill, the Palisades Sill, will be our topic early next month.

The basalt lava flows in the Triassic grabens of eastern North America are called trap rocks, from the Swedish word trappa, meaning step. We encountered that word at the end of the Permian, when we talked about the Siberian traps. The alternating, step-like topography of trap rocks can be found in New Jersey, too. In northeastern New Jersey, the basaltic trap rocks contain interesting and rare minerals, including zeolites, franklinite, an iron-manganese-zinc oxide, and pectolite, a calcium-sodium silicate. Many of the rocks from Franklin, New Jersey are fluorescent in spectacular ways.

Body fossils of animals are not common in the Newark Supergroup rocks, although footprints are abundant. One of the most famous fossils is Icarosaurus, a small gliding reptile about 7 inches long including the tail, with a 10-inch wingspan. It was discovered in 1960 by a teenage collector in a quarry in North Bergen, New Jersey. There’s only one specimen that is definitely this animal. Thanks to Steve Henderson for pointing me to the information about Icarosaurus, whose name means “Icarus lizard” for the mythological flying man, Icarus.

—Richard I. Gibson

Links:
Map of Eastern North America Rift Basins, via Wikipedia


Newark Basin 

Mesozoic Basins 

Thursday, September 18, 2014

September 18. The Petrified Forest: Chinle Formation



As we get later into the Triassic, environments were changing from the common hot arid desert settings to more complex systems. In what is now Arizona, the variegated Chinle Formation was laid down. It did include extensive wind-blown desert sands, but it also was a time for lakes, swamps, and river systems. The Chinle and its equivalent, the Dockum Group, extend from western Kansas west across the Colorado Plateau into Nevada, and south from Colorado and Utah into Arizona and New Mexico. Stratigraphically, the Chinle typically lies on top of the Moenkopi Formation that we talked about September 7, but there is usually an unconformity between the two packages of rock, indicating either a period of non-deposition, or deposition and erosion, or both. 

Fossil animals in the Chinle formation include various reptiles, including semi-aquatic crocodile-like phytosaurs, small dinosaurs including Coelophysis which we mentioned yesterday, amphibians, lungfish, and sharks. But probably the most famous life forms in the Chinle are trees.

Petrified wood, Arizona. Photo by Kumar Appaiah used under Creative Commons license


The Petrified Forest of Arizona is in the Chinle Formation. Fossil logs represent trees that grew up to 200 feet high and two feet in diameter. They were mostly conifers, like modern pines. The trees grew along river channels, and when they died and fell into the rivers, logjams sometimes developed. The region was also one where occasional volcanic ash falls occurred. Groundwater dissolved silica from the volcanic ash and carried it into buried tree trunks, where the wood was replaced by silica in the form of multicolored agate. The diverse colors reflect trace elements such as iron. In some cases, the replacement was so delicate, practically on a molecular scale, that bark and tree rings are preserved. Since silica is quartz, a resistant mineral, the petrified logs typically weather out of the soft shale that constitutes much of the Chinle formation.

The Petrified Forest contains at least 200 different plant species, making it the richest Triassic plant fossil locality in the world. The most common tree fossils are conifers.

The nearby Painted Desert is also in the Chinle Formation. The alternating shales, mudstones, sands, lakebeds, and volcanic ash are colored mostly by iron and manganese in various amounts.
—Richard I. Gibson


Photo by Kumar Appaiah used under Creative Commons license

Wednesday, September 17, 2014

September 17. Dinosaurs



A week ago we talked about the archosaurs, the reptilian ancestors of dinosaurs, and I mentioned the oldest known dinosaur, Eoraptor, dated to 231 million years ago, in the late middle Triassic. By late Triassic, dinosaurs were clearly expanding both in terms of diversity and geographically. Coelophysis was a 9-foot, 100-pound carnivore that lived in southwestern North America by about 225 million years ago. Fossils similar to Coelophysis have since been found in many parts of the world, indicating it was a successful and adaptable animal that took advantage of the more-or-less connected nature of the supercontinent of Pangaea.  

Coelophysis was a theropod, meaning “animal foot,” a branch of the dinosaurs that were bipedal, with an upright stance. This group also includes birds, which descended from a dinosaur lineage.

Herbivorous dinosaurs had also evolved by late Triassic time. Plateosaurus was an exclusively bipedal animal up to 30 or so feet long, much of that in its neck. This presumably made it able to reach higher into trees and other leafy vegetation.

Plateosaurus skeleton photo by FunkMonk, under creative commons license


Dinosaurs are defined taxonomically as archosaurs with distinctive bone structures that allowed them to carry their bodies more directly above their legs. This applies to both two-legged stances and four-legged stances. The non-dinosaur crocodilians continued a more sprawling stance, with the legs extended away from the body rather than beneath it. The upright stance allowed for a speedier gait. It’s possible that the earliest dinosaurs were bipedal, and that the later 4-legged varieties actually reverted to that anatomy somewhat later.

Dinosaurs competed with other varieties of reptile and with therapsids like cynodonts. Two extinction events, one at about 215 million years ago during the late Triassic and the other at the end of the Triassic affected the primitive reptiles and therapsids more drastically than the newly evolved dinosaurs, and this may have given them more ecological niches to expand into. We’ll talk more about the extinction events at the end of the month. But that end-Triassic extinction seems to have ultimately proved beneficial to the dinosaurs, because they really began to take off after it happened.
—Richard I. Gibson

Plateosaurus skeleton photo by FunkMonk, under creative commons license.