During the late Proterozoic and Paleozoic, the southern margin of North America underwent a complete cycle of continental rifting, ocean opening and closing, and collision that created the Ouachita orogenic system. Initial rifting was along a network of transforms and spreading zones from which failed rift basins, called aulacogens, extended inland. From late Cambrian through Devonian time, the continental margin was a passive region of subsidence, where shelf sediments accumulated near land and a deep ocean basin developed farther offshore.
The Ouachita system displays the "starved basin" phase of development from late Ordovician through Devonian time. Representative formations include the Big Fork Chert, Arkansas Novaculite, and Caballos Novaculite. These chert and shale formations were deposited slowly in deep water of a subsiding ocean basin. The starved-basin phase represents the maturing ocean basin following earlier continental rifting and prior to subsequent collision.
Beginning in early Mississippian time, a dramatic change in sedimentation took place, with rapid accumulation of exceptionally thick flysch (turbidites) and wildflysch (submarine landslides). Northward thrusting of the continental margin culminated in uplift of mountains by Pennsylvanian time and draining of shallow inland seas during the Permian. Crustal stress was transmitted into the continental interior and resulted in local uplifts, such as the Arbuckle Mountains, normal to the Ouachita trend. The region was once again subjected to continental rifting during Jurassic and Cretaceous time, as evidenced by the Gulf Coastal Plain sedimentary sequence and by Cretaceous intrusive rocks.
The Ouachita orogeny is distinctive in that volcanism, metamorphism, and intrusion are notably absent throughout most of the system. The obvious interpretation is that a subduction zone dipped southward beneath the converging plate. By early Mississippian time, the Ouachita basin had become a narrow trough into which a vast amount of clastic and some (minor) volcanic sediment was rapidly deposited from the south. Thrust uplift of this material was the result of a collision with a continental terrane that had been rifted from North America earlier. This terrane underlies the Gulf Coastal Plain, over which a great thickness of Cretaceous and Tertiary sediment has accumulated on a slowly subsiding, passive continental margin.
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View from the top of Magazine Mountain, the highest point in Arkansas. The mountain crest is supported by thick sandstone preserved in the trough of a gently folded syncline. Road section through vertical strata in a ridge of the folded Ouachitas, west-central Arkansas. Ordovician Big Fork Chert, Hot Springs, Arkansas. Interbedded chert and shale are strongly deformed in an irregular style. Closeup view of contorted folds in the Big Fork Chert, Hot Springs, Arkansas. Arkansas Novaculite outcropping on ridge crest, Hot Springs, Arkansas. Devonian formation of nearly pure silica (chert) represents a deep-sea deposit. This rock is hard, brittle, and extremely resistant to chemical weathering. Caddo Gap, a water gap across vertical beds of the Arkansas Novaculite, west-central Arkansas. Thick sequence of tilted flysch strata (Carboniferous) exposed on the shore of DeGray Lake, Arkansas. Tilted and deformed flysch strata (Carboniferous), eastern Oklahoma. Native sandstone makes up the foundation in the Fayetteville County Courthouse, northwestern Arkansas. Students examine an exposure of carbonatite near Magnet Cove, Arkansas. Carbonatite, syenite, and other unusual igneous rocks mark rifting of continental crust in the Mississippi Embayment during the Cretaceous.
Arbuckle Mountains, Oklahoma
The Arbuckle Mountains are a narrow range in southern Oklahoma that trends WNW-ESE, approximately perpendicular to the Ouachita structures. The Arbuckles are interpreted as an aulacogen during late Proterozoic continental rifting (Arbenz, Geology of North America, vol. A, 1989), and the basin was infilled with marine sedimentary strata during the Paleozoic. Vertical uplift and deformation took place in the Pennsylvanian simultaneous with the main phase of thrust deformation in the Ouachitas. The overall structure of the Arbuckles is an anticline in which Proterozoic crystalline rocks are exposed at the core with successively younger Paleozoic sedimentary strata on the flanks. No metamorphism, volcanism or intrusions are associated with deformation of the Arbuckle Mountains. Post-orogenic sediment is represented by Pennsylvanian conglomerates that fill fault basins.
Geologic highway section across the Arbuckle Mountains along US 77 (I-35), south-central Oklahoma. Note the arrangement of Paleozoic formations, either side of the Proterozoic core (porphyry), and the position of Permian conglomerate (red). Turner Falls, a touristic landmark in the Arbuckle Mountains, Oklahoma. Roadcut section in Mississippian-age Sycamore Limestone, on U.S. highway 77. Strata are tilted into near-vertical position, northern margin of the Arbuckle Mountains. Roadcut section in Late Ordovician Viola Group limestone, on U.S. highway 77. Strata are tilted into near-vertical position, northern margin of the Arbuckle Mountains. Outcrop of Arkbuckle Group in the northern margin of the Arbuckle Mountains. The Arbuckle Group comprises >2000 m of late Cambrian to early Ordovician limestone and dolostone (Fay 1988). Tombstone outcrop of the Arbuckle Group in the Arbuckle Mountains, Oklahoma. Erosion of near-vertical dolostone beds creates the tombstone appearance of this exposure. Billy Jacobson demonstrates fault breccia in the Arbuckle Group. This fault bounds a graben filled with Pennsylvanian conglomerate (see next picture). Pennsylvanian conglomerateóred beds of the Collings Ranch Conglomerate. Conglomerate is preserved in fault basins, and sediment fill was derived from primarily the Arbuckle Group and other sources in adjacent uplifts (Donovan and Heimlen 1988). Exposure along I-35, Arbuckle Mountains, Oklahoma. Ira Smith granite quarry, Oklahoma. Proterozoic crystalline basement rock exposed by tectonic uplift in the Arbuckle region. Tishomingo Granite, near Ardmore, Oklahoma. Proterozoic crystalline basement rock exposed by tectonic uplift in the Arbuckle region. Carlton Rhyolite, Oklahoma. Proterozoic crystalline basement rock exposed by tectonic uplift in the Arbuckle region. Intruded gabbro, Oklahoma. Proterozoic crystalline basement rock exposed by tectonic uplift in the Arbuckle region. Exposure of Proterozoic crystalline basement rock in a stream valley, near Tishomingo, Oklahoma. Mafic intrusive dike exposed in Proterozoic basement, near Tishomingo, Oklahoma. Contact between granite (upper left corner) and rhyolite, near Tishomingo, Oklahoma.
Marathon Basin, West Texas
Typical rocks and structures of the Ouachita system are exposed in the Marathon Basin of West Texas, where overlying Cretaceous strata have been eroded away. Although formation names differ, the overall sequence and history of deformation are quite similar to the Ouachitas of Arkansas and Oklahoma. The rocks are well exposed in the desert climate.
Geological marker for the "denuded Ouachita rock belt" in the Marathon Basin, West Texas. View over the Marathon Basin, with overlying Cretaceous strata forming the rim rock in the left background and Mississippian Tesnus Formation (flysch) in the foreground. Ridge of Caballos Novaculite, Marathon Basin, West Texas. The Caballos Novaculite is quite similar to the Arkansas Novaculite in age (Devonian) and origin (deep sea). Closeup view of the Caballos Novaculite, Marathon Basin, West Texas. Note the distinctive fractures in this brittle rock. Pennsylvanian Haymond Formation (turbidites) tilted into near-vertical position. Marathon Basin, West Texas. Breccia and exotic boulders (wildflysch) in the Haymond Formation. Marathon Basin, West Texas. Pennsylvanian Dimple Limestone, carbonate turbidites interbedded with chert (dark). Marathon Basin, West Texas.
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