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People looking at folds. Faces obscured to protect identities. -
¡Mas folding!. -
Action shot of man pointing at a fold -
Normal fault in sandstones and shales
Recently, I got to join a field trip to the Ouachita Mountains to discuss a spectacular structural event that took place in the Late Paleozoic. The strata of the Ouachita Mountains are dominantly sandstone and shale beds from the Cambrian to the Pennsylvanian that were deposited in an offshore, deep water environment somewhat similar to the deep water regime of the Gulf of Mexico. Beginning in the Mississippian Subperiod, a compressional event initiated as proto-North America collided with a small land mass (volcanic arc) to the south creating the Ouachita Mountains, and concluded during the Pennsylvanian Subperiod.
Red lines are thrust faults in the Ouachitas; black lines are normal faults.
From the Geologic Map of Arkansas (1993).
From the Geologic Map of Arkansas (1993).
Weirdly, there are two large silica-rich formations in the Ouachitas, one is called the Bigfork Chert, and the other, the Arkansas Novaculite. Of these two, the Novaculite is the largest (up to 900 feet thick) and densest, and is a bit of an anomaly. It is not well understood how so much silica was deposited here. Was it biogenic? Was it siliceous ooze? Was it ash fall from the volcanic arc? Although it’s origin is debatable, there has been some suggestion from U of A grad student research that it may be the latter.
Vertical beds of Arkansas Novaculite at Caddo Gap
(picture taken on a previous trip)
From a structural geology perspective, lots of fun can be had in the Ouachitas, particularly in the deep subsurface, where much has yet to be explored.
Sand intrusion (or sand blow) in shale.