Southern Oklahoma Aulacogen

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Southern Oklahoma Aulacogen
Stratigraphic range: Early to mid Cambrian
An outline of the Aulacogen
AreaApproximately 42,500 sq mi (110,000 km2)
Coordinates 35°00′N99°18′W / 35°N 99.3°W / 35; -99.3 .
CountryUnited States

The Southern Oklahoma Aulacogen Loudspeaker.svg (ah-lah-coh-jin)   is a failed rift, or failed rift arm (aulacogen), of the triple junction that became the Iapetus Ocean spreading ridges. It is a significant geological feature in the Western and Southern United States. It formed sometime in the early to mid Cambrian Period and spans the Wichita Mountains, Taovayan Valley, Anadarko Basin, and Hardeman Basin in Southwestern Oklahoma. [1] The Southern Oklahoma Aulacogen is primarily composed of basaltic dikes, gabbros, and units of granitic rock. [2]

Rift A linear zone where the Earths crust is being pulled apart, and is an example of extensional tectonics

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.

An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Specifically, aulacogens are a rift zone, where new crust is formed, that is no longer active.


A rough outline of the inferred bounds of the Southern Oklahoma Aulacogen Soamap.png
A rough outline of the inferred bounds of the Southern Oklahoma Aulacogen


The Southern Oklahoma Aulacogen extends roughly 500 miles long (805 km) by ~80–90 miles wide (129–145 km). The two remaining continental plate boundary arms of the triple junction from which the Southern Oklahoma aulacogen formed became spreading zones for the spreading of the Iapetus Ocean during the breakup of the supercontinent, Rodinia, estimated to have occurred in the Cryogenian Period, approximately 750 million years ago. [3] These arms closed in the Pennsylvanian Period (~323.2298.9 Ma) and formed part of the Ouachita orogenic belt. The Southern Oklahoma Aulacogen is estimated to contain over 250,000 km3 of igneous rock. [4] The aulacogen is inverted: rather than extending across the surface it penetrates into the North American craton, [3] and is aligned with the northern edge of a deeply buried Proterozoic basin of uncertain origin which may have formed through igneous layering or deposition. [5] The aulacogen terminates on contact with the Ouachita orogenic belt. The Southern Oklahoma Aulacogen is associated with a widespread anomalous area in which seismic waves travel more slowly. [6] A common comparison is drawn from this aulacogen to the Dniepr-Donets Aulacogen in Baltica because both are significant intracratonic rifts. [7]

Iapetus Ocean An ocean that existed in the late Neoproterozoic and early Paleozoic eras

The Iapetus Ocean was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale. The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia. The ocean disappeared with the Acadian, Caledonian and Taconic orogenies, when these three continents joined to form one big landmass called Euramerica. The "southern" Iapetus Ocean has been proposed to have closed with the Famatinian and Taconic orogenies, meaning a collision between Western Gondwana and Laurentia.

Rodinia Hypothetical neoproterozoic supercontinent from between about a billion to about three quarters of a billion years ago

Rodinia is a Neoproterozoic supercontinent that was assembled 1.1–0.9 billion years ago and broken up 750–633 million years ago. Valentine & Moores 1970 were probably the first to recognise a Precambrian supercontinent, which they named 'Pangaea I'. It was renamed 'Rodinia' by McMenamin & McMenamin 1990 who also were the first to produce a reconstruction and propose a temporal framework for the supercontinent.

The Cryogenian is a geologic period that lasted from 720 to 635 million years ago. It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran.

The Southern Oklahoma Aulacogen contains numerous igneous rocks. Among these rocks are a multitude of gabbros, including anorthosite, titanium-rich, iron-rich, phosphorus-rich, and biotite gabbros. [2] Also included are rhyolites and granites. This assemblage is very similar to the mid-Proterozoic age anorthosite-mangerite-charnockite-granite (AMCG) complexes of North America, but for the lack of coarse massif anorthosites. This is significant in that AMCG complexes tend to form at huge depths in the Earth's crust and thus cool more slowly, allowing the massif anorthosites to form coarse-grained. The similar igneous assemblage suggests that the magmas that formed the igneous rocks of the Southern Oklahoma Aulacogen quickly cooled to at or near their crystallization point, much more quickly than the magmas of AMCG complexes, thus resulting in finer-grained anorthosites. [2]

Igneous rock Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.

Anorthosite A mafic intrusive igneous rock composed predominantly of plagioclase

Anorthosite is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Titanium Chemical element with atomic number 22

Titanium is a chemical element with the symbol Ti and atomic number 22. It is a lustrous transition metal with a silver color, low density, and high strength. Titanium is resistant to corrosion in sea water, aqua regia, and chlorine.

More recently, different interpretations of seismic and outcrop data, as well as stratigraphy in the area have led some studies to postulate that this formation may not be an aulacogen after all, but a system of transform faults. [8]

Outcrop visible exposure of bedrock or ancient superficial deposits on the surface of the Earth

An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth.

Stratigraphy The study of rock layers and their formation

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has two related subfields: lithostratigraphy and biostratigraphy.

Tectonic evolution

An illustration of the likely formation history of the Southern Oklahoma Aulacogen. In A, volcanoes form along a continental plate boundary. B shows the triple junction's basic appearance following its formation. C shows that one of the three "arms" has not split while the other two do, continuing into D, in which the Iapetus Ocean forms. Aulacogendiagram.png
An illustration of the likely formation history of the Southern Oklahoma Aulacogen. In A, volcanoes form along a continental plate boundary. B shows the triple junction's basic appearance following its formation. C shows that one of the three "arms" has not split while the other two do, continuing into D, in which the Iapetus Ocean forms.

The Southern Oklahoma Aulacogen formed sometime in the Late Proterozoic Eon, between 525 and 550 million years ago, [9] during the rifting of the Laurentia supercontinent or North American Craton, the geological core of North America. Its formation and bimodal igneous activity occurred simultaneously, with two definite episodes of magma activity, mafic and felsic, the former of which being chiefly composed of gabbro-heavy magma and the latter phase being primarily composed of rhyolitic magma. It is hypothesized that between the mafic and felsic stages of magmatic activity substantial uplift occurred, which correlates to the lack of coarse-grained massif anorthosites presented previously. [2] The remaining two arms of its original triple-junction became spreading zones for the nascent Iapetus Ocean. The aulacogen penetrated the craton, causing normal faults to form in what became the Anadarko Basin. [5]

The Proterozoic is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life on the Earth. The name Proterozoic combines the two forms of ultimately Greek origin: protero- meaning "former, earlier", and -zoic, a suffix related to zoe "life". The Proterozoic Eon extended from 2500 mya to 541 mya, and is the most recent part of the Precambrian "supereon." The Proterozoic is the longest eon of the Earth's geologic time scale and it is subdivided into three geologic eras : the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.

Laurentia A large continental craton that forms the ancient geological core of the North American continent

Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of the North American continent. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although originally it also included the cratonic areas of Greenland and also the northwestern part of Scotland, known as the Hebridean Terrane. During other times in its past, Laurentia has been part of larger continents and supercontinents and itself consists of many smaller terranes assembled on a network of Early Proterozoic orogenic belts. Small microcontinents and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian craton seen today.

Mafic Silicate mineral or igneous rock that is rich in magnesium and iron

Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.

The aulacogen underwent crustal shortening and inversion sometime in the Mississippian Period to the Early Permian Period, roughly 330280 million years ago. This coincides with the closing of the Iapetus spreading zones and the overthrusting of the Ouachita uplift over the Anadarko Basin, forming the Wichita Mountains. [5] This also resulted in the reactivation of Cambrian rift faults, often becoming reverse or listric thrust faults. [3] Anticlines formed in the sedimentary rock layers of the basin, contributing to the formation of the deep hydrocarbon reservoir of the Anadarko Basin. The formation of these listric faults and anticlines indicates that the crustal shortening was significant, up to 10–15 km or more. [5]

The Mississippian is a subperiod in the geologic timescale or a subsystem of the geologic record. It is the earliest/lowermost of two subperiods of the Carboniferous period lasting from roughly 358.9 to 323.2 million years ago. As with most other geochronologic units, the rock beds that define the Mississippian are well identified, but the exact start and end dates are uncertain by a few million years. The Mississippian is so named because rocks with this age are exposed in the Mississippi River valley.

Ouachita orogeny

The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains. The more extensive Ouachita system extends from the current range in Arkansas and Oklahoma southeast to the Black Warrior Basin in Alabama and to the southwest through the Llano, Marathon, and Solitario uplifts in Texas on into Coahuila and Chihuahua in Mexico.

Wichita Mountains Mountains in the US state Oklahoma

The Wichita Mountains are located in the southwestern portion of the U.S. state of Oklahoma. It is the principal relief system in the Southern Oklahoma Aulacogen, being the result of a failed continental rift. The mountains are a northwest-southeast trending series of rocky promontories, many capped by 500 million-year old granite. These were exposed and rounded by weathering during the Pennsylvanian & Permian Periods. The eastern end of the mountains offers 1,000 feet (305 m) of topographic relief in a region otherwise dominated by gently rolling grasslands.

The igneous rocks found in the aulacogen were also uplifted during the Ouachita uplift and subsequently reburied by both local and transported sediments. As there was no major deformation of the midsection of North America during the Mesozoic and Cenozoic eras, the aulacogen's structure and rift assembly were mostly preserved. Erosion in recent eras has eroded sediments overlying a section of plutonic and volcanic rocks that once formed the bedrock of the aulacogen, and as a result this aulacogen is "one of the best preserved and best exposed" selections of the igneous results of ancient rift activity. [2] Consequently, the Southern Oklahoma Aulacogen is the designated type in the United States. [5]

Mafic rocks

The mafic rocks of the Southern Oklahoma aulacogen can be separated into two primary groups, the Raggedy Mountain Gabbros and the Late Diabase Dikes. The Raggedy Mountain Gabbros can be further separated into two subgroups due to petrographic analysis and field mapping. These subgroups are the Glen Mountains Layered Complex and the Roosevelt Gabbros. [2]

Felsic rocks

The felsic rocks of the Southern Oklahoma aulacogen are broken into two main units, the Carlton Rhyolite Group and the Wichita Granite Group. There is a distinct change in texture in the layers of granite, with earlier deposited layers being very fine-grained in comparison to later, coarse-grained granite layers. [10]

Significance to petroleum exploration

Due to its unique structure and faulting, the area within and around the aulacogen developed very deep basins (such as the Anadarko Basin), forming excellent petroleum sources. Igneous rock deposits often form the hanging walls of anticline reverse faults in this area, leading to an unusual number of petroleum wells drilled into them in order to access the petroleum-bearing rock layers below. [12] A graph of the geothermal history of the Southern Oklahoma Aulacogen suggests that sections of rock in the area may have been, at one point, in the temperature range of the "liquid window," the range of temperatures that are ideal for oil formation. The isotherms of this window range from 65 °C to 150 °C. This further suggests that the area may have served as an oil formation bed before a late Ordovician fluid migration pulse. [13]

Related Research Articles

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

Granite A common type of intrusive, felsic, igneous rock with granular structure

Granite is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar.

Gabbro A coarse-grained mafic intrusive rock

Gabbro is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".

Rhyolite An igneous, volcanic rock, of felsic (silica-rich) composition

Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2 – see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 – see the QAPF diagram). Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.

Granophyre A subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths

Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image.

Layered intrusion large sill-like body of igneous rock

A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions are Archean to Proterozoic in age, they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.

Cumulate rock

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.

Musgrave Block orogen/fold belt tectonic superprovince in Australia

The Musgrave Block is an east-west trending belt of Proterozoic granulite-gneiss basement rocks approximately 500 kilometres (310 mi) long. The Musgrave Block extends from western South Australia into Western Australia.


Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly peraluminous (Al2O3/ (CaO + Na2O + K2O) = 0.98–1.11), and contain ilmenite, sphene, apatite and zircon as accessory minerals. Although the compositional range of the monzogranites is small, it defines a differentiation trend that is essentially controlled by biotite and plagioclase fractionation. (Fagiono, 2002). Monzogranites can be divided into two groups (magnesio-potassic monzogranite and ferro-potassic monzogranite) and are further categorized into rock types based on their macroscopic characteristics, melt characteristics, specific features, available isotopic data, and the locality in which they are found.

Quartz Mountain mountain in United States of America

Quartz Mountain is located in Greer County in southwest Oklahoma. It is the namesake of Quartz Mountain Nature Park and its eastern flank is enclosed by the park boundaries. It is near the cities of Mangum, Oklahoma and Altus, Oklahoma. The park is open to the public year round for rock climbing, hiking, boating, camping, nature observation and photography, and environmental education and interpretation. The mountain overlooks scenic Lake Altus-Lugert.

Stillwater igneous complex PGE (Pt-Pd) deposit

The Stillwater igneous complex is a large layered mafic intrusion (LMI) located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of being mined for chromium. More recent mining activity has produced palladium and other platinum group elements.

Mackenzie Large Igneous Province

The Mackenzie Large Igneous Province (MLIP) is a major Mesoproterozoic large igneous province of the southwestern, western and northwestern Canadian Shield in Canada. It consists of a group of related igneous rocks that were formed during a massive igneous event starting about 1,270 million years ago. The large igneous province extends from the Arctic in Nunavut to near the Great Lakes in Northwestern Ontario where it meets with the smaller Matachewan dike swarm. Included in the Mackenzie Large Igneous Province are the large Muskox layered intrusion, the Coppermine River flood basalt sequence and the massive northwesterly trending Mackenzie dike swarm.

The geology of Virginia began to form 1.8 billion years ago and potentially even earlier. The oldest rocks in the state were metamorphosed during the Grenville orogeny, a mountain building event beginning 1.2 billion years ago in the Proterozoic, which obscured older rocks. Throughout the Proterozoic and Paleozoic, Virginia experienced igneous intrusions, carbonate and sandstone deposition, and a series of other mountain building events which defined the terrain of the inland parts of the state. The closing of the Iapetus Ocean, to form the supercontinent Pangaea added additional small landmasses, some of which are now hidden beneath thick Atlantic Coastal Plain sediments. The region subsequently experienced the rifting open of the Atlantic Ocean in the Mesozoic, the development of the Coastal Plain, isolated volcanism and a series of marine transgressions that flooded much of the area. Virginia has extensive coal, deposits of oil and natural gas, as well as deposits of other minerals and metals, including vermiculite, kyanite and uranium.

The geology of North Carolina includes ancient Proterozoic rocks belonging to the Grenville Province in the Blue Ridge. The region experienced igneous activity and the addition of new terranes and orogeny mountain building events throughout the Paleozoic, followed by the rifting of the Atlantic Ocean and the deposition of thick sediments in the Coastal Plain and offshore waters.


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