I hope you remember the grabens, those fault-bounded down-dropped basins that were forming in what is now eastern United States toward the end of the Triassic. They were the response to the extension, the rifting that was beginning to pull Africa away from North America to form the modern Atlantic Ocean. The sediments and igneous rocks that filled those basins are called the Newark Supergroup, which we talked about on September 19.
|Image source: National Park Service - Newark Basin|
These basins were set into the much older, highly deformed and metamorphosed rocks that were the result of the earlier collisions dating all the way back to the Ordovician – the Taconic, the Caledonian, and the Alleghenian-Appalachian Orogenies. So the whole works is tilted, but it’s most evident in the relatively flat layers of the earlier Triassic sedimentary rocks that filled the earlier basins.
It’s possible that the Palisades Sill, an igneous intrusion in the Newark beds, was formed at about the same time as the tilting of the rocks. It creates the Palisades of the Hudson River in New Jersey and New York, and its age is typically given as about 200 million years, which would put it just about at the end of the Triassic. But there are some more recent age dates that put it a little later, about 190 million years, the early Jurassic, so we’ll talk more about it next month. This tectonic activity, normal faulting and gentle tilting of the older Triassic rocks, is called the Palisade Disturbance. Not really an orogeny, but not a passive time, either.
—Richard I. Gibson
Image source: National Park Service - Newark Basin