Toward the end of the Ordovician, last March, we talked about the beginning of tectonic collisions in what is now eastern North America – the Taconic Orogeny. The rivers that eroded those mountains shed a vast pile of sediment into what is now Pennsylvania, West Virginia, and Ohio and beyond, creating the Queenston Delta. As collisions continued, mountainous terrains continued to be uplifted and erosion continued. In Middle to Late Devonian time, ongoing collision between parts of the Avalonian extension of Europe with northeastern North America created more uplifts. This is sometimes called the Acadian Orogeny, which we mentioned last month during the Silurian. It pretty much reached its peak during the Devonian.
The upland was located in southeastern Pennsylvania and northern New Jersey and points further north and south. The sediment that was eroded off the mountains was dumped in especially thick piles in central Pennsylvania and southern New York. River floodplains and deltas lay along a shallow sea. The whole complex is called the Catskill Delta, because the sediments are now exposed as rocks in the present-day Catskill Mountains, as well as the Pocono Mountains of Pennsylvania.
This was a long-lived environment, spanning probably 20 million years about 380 to 360 million years ago. The rocks are similar to those of the Ordovician Queenston Delta – red beds indicating oxidation of iron, river sandstones, flood plain shales, and ephemeral lake deposits. There’s 3,000 feet and more of these rocks. They were deposited on top of the older Marcellus Shale that we talked about yesterday, so that means there was a pretty noteworthy change in the environment, from the oxygen-poor sea waters of the Marcellus to the above-sea-level terrestrial environment of the Catskill Delta.
|Catskill delta rocks are to the east (right). Cross-section from USGS.|
Because the sea level was fluctuating, there are also some layers of marine rocks interbedded with the deltaic sediments, just as you would find in the Mississippi Delta today.
The sandstones that formed from the distant limits of the Catskill Delta serve as oil reservoirs in western Pennsylvania – the first oil deposits that were exploited in the United States beginning in the 1850s but really taking off in the 1870s.
Because they are Devonian in age, and many of the rocks are red with oxidized iron, American geologists in the 19th century initially thought the rocks of the Catskill Delta were part of the Old Red Sandstone of Europe. But they’re not, really. The Catskill sediments were eroded off a rising mountain uplift, and deposited in a coastal flood plain. The Old Red Sandstone was deposited within intermontane basins, pretty much entirely terrestrial in origin, except in a few places.
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Maurice Ewing was born today, May 12, 1906, in Lockney, Texas. He was a pioneer in the field of oceanography, especially the use of seismic studies to understand the nature of the ocean basins. Much of his work was fundamental knowledge that led to the comprehensive theory of plate tectonics.
—Richard I. Gibson
Appalachian maps and cross sections
Cross-section from USGS