Text Box: Introduction Ouachita Province | Physical Features | Resources
The Ouachita  (pronounced wah-shi-tah) province is a region of about 225 miles in east-west extent and  approximately 100 miles in the north-south direction in west-central Arkansas and east-central Oklahoma.  The province is bordered on the north by the Ozark Plateaus, on the west by the Central Lowlands, and on the east and south by the Atlantic-Gulf Coastal Plain.  It is the second smallest of the 28 main provinces that comprise the United States; only the Adirondacks province is smaller.  The Ouachita province and the Ozark Plateaus province to the north sometimes are referred to collectively as the Interior Highlands.
The Ouachita province is believed to be closely related to the Ridge and Valley province of the Appalachians, as the rock types and structures of the Ouachita province have been shown to be a westward continuation of Appalachian geology.  Hundreds of thousands of geological wells and oil wells have revealed that the structures of the Ouachita province and the Ridge and Valley region are directly connected; the connecting portion lies under sediment in the intervening Mississippi River Basin.  Part of the mountain building history of the two provinces is also related.  The Ouachita disturbance (sometimes referred to as the Ouachita orogeny), which created the main portion of the present Ouachita Mountains, as well as the Arkansas Valley to the north, occurred simultaneously with the last of the four orogenies–the Allegheny orogeny–that created the Appalachians and produced the linear folding in the Ridge and Valley province.  While Europe and Africa were colliding with the eastern coast of the United States producing the last stage of Appalachian mountain building, the Ouachitas were being produced by the collision of South America, and perhaps northwestern Africa, with what was at that time the southeastern edge of the United States.  The North American plate was subducted beneath the northward-moving South American plate, producing folding and thrust faulting that yielded the Ouachita Mountains, the Arkansas Valley, some uplifting of the Ozark Plateau, as well as the Marathon Mountains in Texas, and perhaps two mountain ranges just west of the Ouachitas in the southern portion of the Central Lowlands province–the Arbuckle Mountains in south-central Oklahoma and the Wichita Mountains in the southwestern portion of that state.  These latter two mountains coincide with an ancient buried rift in southern Oklahoma.  Many consider the Ouachita disturbance to have been a southern extension of the Allegheny orogeny.  One idea has been presented suggesting that the Yucatan Peninsula of present-day Mexico was an independent microplate and was the collisional landmass responsible for raising the Ouachitas, and the microplate became attached to Mexico as a terrane after it pulled away from the United States as the Gulf of Mexico was forming.  Regardless of which section of land in the south was actually involved, it is known with certainty that most of the uplift in the Ouachitas occurred between 310 and 270 million years ago.  Through mechanisms not completely understood, but most likely related to isostatic adjustments, minor slow uplifting continued in the region for about another 200 million years after cessation of the initial Ouachita disturbance, terminating in the early Tertiary.  There was little intrusive igneous activity during the disturbance; the small amount of igneous rocks that occur in a few isolated areas in the Ouachita province intruded while the post-orogenic uplifting was taking place, nearly all during the middle Cretaceous. 
 Introduction Ouachita Province | Physical Features | Resources