Mid-Atlantic Region in the Late Silurian

The Mid-Atlantic Region in the Late Silurian
The late Silurian in the Mid-Atlantic region was tectonically quiet, very hot, and very dry - a tropical desert. To the east lay the old Taconic terrane, now eroded down to a peneplain. Occasionally small amounts of Quartz sand or Shale erode from it, but otherwise it is low and stable. To the west, in central Ohio and Kentucky is the Cincinnati arch, a shallower region, occasionally exposed when sea level drops. And in between is the Central Appalachian Basin (CAB), an area of mild subsidence trending NE to SW, lying more or less over the old foreland basins of the Taconic orogeny. The CAB opens to rest of the Tippecanoe epicontinental sea in the southwest, but in the Mid-Atlantic it is mostly a closed inlet.

Mostly Carbonate sediments accumulate in the CAB during the late Silurian, although at the peak of the heat and dryness hundreds of feet of evaporites, Halite and Gypsum in the Salina Formation, also accumulate because of the high evaporation rates. Around the edges of the CAB are Carbonate tidal flats, the Tonoloway Formation. These coastal deposits lap up onto the edge of the eroded Taconic terrane in the east, and the Cincinnati arch in the west, and swing around in a horseshoe shaped arc into Pennsylvania. 
Transgression of the seas began at this time and continued until the coastal regions end up in upstate New York, and the rest of the CAB is under deeper water. It is likely that even the Cincinnati arch was underwater at the peak of this transgression. The late Silurian brought about a short interval in a longer period of tectonic calm between the Taconic and Acadian orogenies. At the bottom of the stratigraphic chart we can see the Oswego and Juniata Formations that are the last of the Taconic formations, and at the top of the chart the Needmore, that represents the beginning of the Acadian orogeny. 

In this overall inter-orogenic interval of tectonic calm the late Silurian acted as a transition period. The Clinton and Cayuga groups below it are mostly clastic and above it, the Helderberg group is abundantly fossiliferous Carbonates. These Formations are not thick (often only 10's of feet) and their geographic distribution tends to be patchy and limited. This is typical of non-orogenic intervals. Formations deposited during an orogeny tend to be thick (hundreds to thousands of feet), and geographically widespread. 
Contributed by Lynn Fichter  
Monday, August 04, 2014
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