Triassic Basin


 
 
Triassic Basin 
 
The lower Mesozoic sediments deposited in fault basins are usually referred to as Triassic Basin deposits, although they are now known to also contain lower Jurassic rocks. These basins formed when Africa separated from North America to create the Atlantic ocean.

The Culpepper Basin in the western Piedmont near the Blue Ridge province is the largest, but numerous smaller basins including the Richmond, Farmville, and Danville are scattered throughout the Piedmont. Structurally these are half-graben features with a main fault only on the western side. Sometimes you know you have crossed into one of these basins because the rocks turn red, or there are  coal beds, however, often times the evidence is more subtle. 
 
The sediments filling the basins are ancient alluvial fan conglomerates along the western borders, but the deep red sediments to the east, often with fish fossils or crisscrossed with dinosaur tracks, indicate the tropical lakes and mudflats which existed at the time, such as are found in this quarry a few miles east of Culpepper along Rt 3.
 
Cutting into the Triassic-Jurassic sediments are numerous igneous dikes, stocks (small bodies of igneous rock), and lava flows that accompanied the volcanic activity accompanying the opening of the Atlantic ocean. 
 
All in all, the Piedmont contains evidence of a complex history of many intense geologic forces.
 
Contributed by Lynn Fitcher 
Tuesday, July 29, 2014
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