Ouachita Mountains - Geology and Physiography

Geology and Physiography

The Ouachita Mountains are a physiographic section of the larger Ouachita province (which includes both the Ouachita Mountains and the Arkansas Valley), which in turn is part of the larger Interior Highlands physiographic division.

The Ouachita Mountains are fold mountains like the Appalachian Mountains to the east, and were originally part of that range. During the Pennsylvanian part of the Carboniferous period, about 300 million years ago, the coastline of the Gulf of Mexico ran through the central parts of Arkansas. The South American Plate drifted northward, riding up over the denser North American continental crust. Geologists call this collision the Ouachita orogeny. The collision buckled the overriding plate, producing the fold mountains we call the Ouachitas. At one time the Ouachita Mountains were very similar in height to the current elevations of the Rocky Mountains. Because of the Ouachitas' age, the craggy tops have eroded away, leaving the low formations that used to be the heart of the mountains.

Unlike most other mountain ranges in the United States, the Ouachitas run east and west rather than north and south. Also, the Ouachitas are distinctive in that volcanism, metamorphism, and intrusions are notably absent throughout most of the system. The Ouachitas tend to be clustered into distinct sub-ranges separated by relatively broad valleys.

The Ouachitas are noted for quartz crystal deposits around the Mount Ida area and for renowned Arkansas novaculite whetstones. This quartz was formed during the Ouachita orogeny, as folded rocks cracked and allowed fluids from deep in the Earth to fill the cracks.

The Ouachita Mountain area of Arkansas is dominated by Cambrian through Pennsylvanian clastic sediments, with the lower Collier formation in the core of the range and the Atoka formation on the flanks. The Atoka Formation, formed in the Pennsylvanian Period, is a sequence of marine, mostly tan to gray silty sandstones and grayish-black shales. Some rare calcareous beds and siliceous shales are known. The Collier sequence is composed of gray to black, lustrous shale containing occasional thin beds of dense, black, and intensely fractured chert. An interval of bluish-gray, dense to spary, thin-bedded limestone may be present. Near its top, the limestone is conglomeratic and pelletoidal, in part, with pebbles and cobbles of limestone, chert, meta-arkose, and quartz. It was formed during the Late Cambrian.