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