The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Friday, September 26, 2014

September 26. Palisade disturbance




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
After those sediments were laid down, and after they were injected with molten rocks that became sills and lava flows, the whole thing was tilted. The basins, which were already fault-bounded, broke even more. This probably represents a continuation of the extension that formed the basins in the first place, but now the basin rocks were being faulted as well. The newer faulting, near the end of the Triassic Period, produced asymmetrical grabens because typically one side was faulted more strongly than the other – or in some cases, one side was faulted and the other not at all. This resulted in tilting of the formerly horizontal layers of the Newark Supergroup rocks. 

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

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