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On the northeast edge of the Wichita Mountain uplift, northwest of Loco, Oklahoma, is a depositional syncline with Pennsylvanian sediments from Atokan through Virgilian. This depositional syncline was named the Harrisburg trough by Harlton. Sedimentary onlap relations within the Harrisburg trough suggest that it formed as a topographic valley that was at times partly above and below sea level. Seismic evidence indicates that this buried valley (canyon?) has paleotopographic relief of more than 10,000 ft (3,048 m). Atokan drainage in the Harrisburg trough was from the southeast to the northwest. Continued erosion of the highlands of Mississippian through Ordovician rocks, and located mainly to the south but also present to the north, provided the clastics deposited in the rough.
The compression that probably caused the uplifted source areas continued through Virgilian time and folded the northeast flank of the Harrisburg trough into the Northwest Velma and West Velma anticlinal hydrocarbon traps. Future hydrocarbon discoveries will probably find production in stratigraphic traps of Atokan sands that pinch out or onlap the flanks of the trough.
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