The Ouachita System


Contents

  1. Description and History
  2. Rocks of the Ouachita System
  3. References

Description and History

The Ouachita Mountains (Pron. "Washita") are a region of east-west trending ridges of hard rocks with soft-rock valleys in southeastern Oklahoma and west central Arkansas, extending from Little Rock, AR (LR on the map) to Atoka, OK (A on the map), about 220 miles long and 50 miles wide, covering about 12,000 sq. miles. The climate is mild and rainy, and the soils are poor. There are no mineral resources. The region is forested with yellow and shortleaf pine, and some oak. Where logging has occurred, the hillsides are scarred, deeply eroded and the soil rendered even more poor. Most large wildlife was exterminated long ago. Aside from this, the area is beautiful and thinly populated. The western area is drained by the Kiamichi River, which flows westward through the middle, then south and southeast to the Red River. The Little River, and its tributary the Mountain Fork, do the same a little farther east. The eastern half is drained by the Fourche La Fave River, which flows eastward into the Arkansas, and the Ouachita River and its tributaries, which flow southeastward into Louisiana, where they meet the Red River not far from its mouth into the Mississippi.

In Oklahoma, the county names of Choctaw and Pushmataha recall that this was the territory assigned to the Choctaw in place of their rich lands in Mississippi and Alabama. Pushmataha was the Choctaw leader who supported the United States against Tecumseh in the War of 1812, and was first to feel the cruelty of the Removal Act of 1830. Of the 13,000 Choctaw that emigrated west, 4000 died on the trail. Later support for the Confederacy against their oppressors gave a good reason for seizure of the poor lands they still possessed.

Similar-sounding names in this area, such as Washita (very close to the French pronunciation of Ouachita), Wichita and so on are related to Ouachita, the spelling of which is French, representing a native name. The Ouachita Mountains are not an extension of the Ozark Plateau, and are completely distinct in rocks and structure. Murfreesboro, Pike County, AR, near where diamonds from a lamproite pipe were discovered in 1896, is on the southeastern edge of the Ouachita Mountains.

The Ouachita Mountains are a small part of the extensive Ouachita System, which extends from its junction with the Appalachian System near Black Warrior Basin in Alabama (BW on the map), westward through Mississippi and Arkansas into the southwestern corner of Oklahoma, then south around the Llano Uplift (LU on the map) in Texas, northwestward roughly along the Rio Grande (Rio Bravo) and bending southward west of the Big Bend region in Texas, and passing into Coahuila and Chihuahua in Mexico, where it can no longer be traced. Of this thousand-mile trend, only the 200 miles of the Ouachita Mountains on its northern flank, the Marathon Uplift (MU on the map) east of Alpine, Texas, and the small window of the Solitario near Presidio are exposed at the surface, giving little hint of the size of the system. The rest is covered with thick Cretaceous and Tertiary sediments of the Gulf Coastal Plain, with the rocks of the Ouachita System deep below them.

The Ouachita System would still be a mystery were it not for the many wells drilled for petroleum (and some for water) that sampled its rocks, whose cuttings could be examined, and which could be cored to bring samples to the surface. The information supplied by these wells was carefully studied, and the report in the References was the result, giving as good a picture as could be obtained of the system as a whole.

The report came out before plate tectonics was accepted in the United States, so all its explanations are in terms of geosynclines. However, the authors seem doubtful of these traditional views. Much evidence is presented against them, but the possibility of continental drift is never even hinted at. Had the authors done so at this time, they would have been assaulted with contempt and ridicule for supporting absurd "foreign" ideas. "Why does a duck have flat feet?" goes the old grade-school joke. "To stomp out forest fires," is the answer. "Why do elephants have flat feet?" The response is "to stomp out burning ducks." The Ouachitas (and many other examples) were the singed, flat ducks. Most geologists, asked to explain these strange objects, would conclude that the ducks grew to a large size, then tended to catch fire as a result. Having caught fire, they would then flatten themselves in a "paroxysm of orogeny." (This phrase is actually used in the report.) The possibility of elephants would not be broached, however many flat ducks were encountered.

However, all that is contained in the report is consistent with the creation of a destructive plate boundary along the Ouachita trend in Silurian time, the creation of a volcanic arc in the Mississippian, and collision with continental crust in Pennsylvanian time. This is an event, as we now know, in the assembly of Pangaea by the collision of Gondwanaland and Laurasia. At the same time that Africa collided with what is now the eastern coast of North America, forming the Appalachian System, South America collided with what is now the Gulf Coast, forming the Ouachita System. South America was moving to the northwest (relatively), and the mélange in the middle wrapped itself around the rigid Llano uplift, while the sediments now forming the Ouachitas were squeezed up against the hard continental craton. It was similar to the creation of the Alps, but on a much more modest scale. Thick layers of flysch that had accumulated in the subduction trench were contorted against the craton, and slid forward to form the "thrust faults" seen at the northern edge of the Ouachitas. The Ouachitas then emerged above the shallow seas, where they have been ever since. In the Cretaceous, a constructive plate boundary was formed around what is now the Gulf of Mexico, and South America began pulling away, as did Europe and Africa from the Americas somewhat earlier as the ancestral Atlantic Ocean began to separate. The magnificent Louann Salt was torn into three parts, one remaining with North America, one going with South America, and the third part carried away by Africa. Some of Mexico was left behind as well, the nucleus of that area that was later built up from terranes riding in from the west. The history of the Caribbean is complex, including the formation of one or two new crustal plates. Not until late in the Tertiary were North and South America connected by land; previously there was free communication between the shrinking Pacific and the growing Atlantic.

Rocks of the Ouachita System

The rocks exposed in the Ouachita Mountains are all that are available for detailed study. Although this is a peripheral area, the rocks are generally representative of Ouachita System rocks, if the tendencies exhibited in the more southern examples are extrapolated. The cratonic rocks are called the foreland, while further south is the interior region. The foreland is covered with a thin layer of Paleaeozoic sediments, with Pennsylvanian limestones, sandstones and shales on the surface. Mesozoic rocks were probably deposited here, but they have now been completely eroded away. In the interior region, thick layers of calcareous Cretaceous rocks overlap earlier Mesozoic rocks, and come right up to the southern boundary of the Ouachitas. Elsewhere, they completely cover the Ouachita rocks except for the two exceptions mentioned above. To the east, muddy Tertiary rocks of the Mississippi embayment cover the Cretaceous. The Mississippi embayment itself may be a failed arm of the spreading margin to the south. The Ouachita Mountains have squeezed out northward just east of the rigid Precambrian buttress of the Arbuckle Mountain uplift, west of Atoka.

The Ouachita facies rocks are all Palaeozoic, from 10,000 to 46,000 feet of shales and sandstones, contaning a great deal of silica. There are few limestones, and fossils of marine invertebrates are rare. The epicontinental sea seemed to be euxinic, that is, poor in oxygen as the Black Sea is today, and high-silica to boot. Black, carbonaceous shales are the typical sediment. Much silica came from acidic volcanism, and the abundance of silica encouraged siliceous sponges and radiolarians. (Diatoms did not yet have siliceous frustules, and so are not represented as fossils). Possibly Cambrian rocks are the lowest found, dark cherts and slates, or quartzite-like sandstones. The lower Palaeozoic sediments are all weakly metamorphosed so that the shales often become slates, with slaty cleavage instead of bedding cleavage, the clays in the sandstone become chlorite, and the quartz grains become corroded and tightly cemented with quartz.

The Ordovician is an alternation of slaty shales, quarzose sandstones, and black chert, all well-dated by abundant graptolites, that first appear in the lower Ordovician Mazarn formation. In the mid-Ordovician Womble formation, radiolarians and sponges are prominent, as much silica is introduced by volcanic activity. On top of the Womble is the Bigfork black chert, the first really significant bedded chert. The black shales of the Polk Creek formation finish off the Ordovician, which is topped with an unconformity. The Silurian is represented by the Blaylock sandstone and the Missouri Mountain slate and chert. These are not found generally, so the surface was above and below water at intervals. Heavy minerals begin to appear in the sediments, some typical of "basement complex" rocks. The Devonian is represented by the remarkable Arkansas Novaculite, from 230 to 950 feet thick. The lowest half is a white or green spiculitic (made of sponge spicules) chert, with green laminated siliceous slate. The next quarter is grey or black novaculite, the famous whetstone, with thin paper shale partings. The rest is reddish or greenish radiolarian chert and slates. A novaculite is a light-colored, very dense, even-textured cryptocrystalline silica rock, first described in the Ouachitas. It made a superior whetstone for sharpening edges, and was famous for this.

Above an unconformity is usually found the thick Stanley Shale of Mississippian age, which crops out in the southern part of the mountains where the hard Jackfork sandstone, upper Mississippian, caps it, holding up the ridges. The greenish black Stanley is a flysch facies, with dark shales and graywacke interbedded, lightly metamorphosed. It contains a great deal of garnet derived from its source beds, and lenses of Hatton tuff, the consequence of major acidic eruptions. The acidic nature of the volcanic activity is typical of subduction zones. At the top of the Jackfork is the Johns Valley shale, on the Mississippian-Pennsylvanian boundary, another flysch facies, with erratic boulders that cascaded down in mud flows under the sea. This represents the climax of the plate collision, but not the end of the deformation.

Above the Johns Valley in the west, and the Jackfork in the east, the quite variable Pennsylvanian Atoka formation appears. There is some conglomerate here, giving evidence of rapid erosion at the crest of the range. Metamorphism is slight. The Atoka crops out at the northern side of the mountains, slumped in four or five successive lobes represented by thrust faults.

All of these rocks are very different from those of the craton to the north, and could not have been derived from them by erosion. All the principal beds thicken to the south, and when they sink from view beneath the Cretaceous are still thickening. A hypothetical source for these sediments was a land named Llanoria lying to the southeast, well provided with volcanoes and other interesting features. It was even suggested that the sedimentary cover of Llanoria was stripped by late Ordovician, exposing the "basement" rocks to erosion and explaining the change in the type of sediments at that time. This imaginary land was no more real than Lyonesse, and dissolved into the mist in the light of plate tectonics.

Gravity measurements show the Ouachita trend to exhibit negative Bouguer anomalies, meaning that the root of light rocks is very deep. A subduction zone explains this very readily.

References

P. T. Flawn, A. Goldstein, Jr., P. B. King and C. E. Weaver, The Ouachita System (Austin, TX: The University of Texas, 1961).


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Composed by J. B. Calvert
Created 3 March 2003
Last revised 14 February 2007