User:Tanordstrom/Paradox Basin

Paradox Basin
Map of United States, red box indicates location of Paradox Basin.
Map of Paradox Basin
Coordinates	36°30′N 109°00′W
Etymology	Paradox Valley
Location	Central Western North America
Country	United States
State(s)	Utah, Colorado, Arizona & New Mexico
Cities	Moab, Utah
Characteristics
Boundaries	Uncompahgre Plateau (NE) San Rafael Swell (NW)
Area	>86,000 sq mi (220,000 km2)
Geology
Basin type	Intraflexural Foreland Basin
Orogeny	Uncompahgre Uplift
Age	Mid-Pennsylvanian - Late-Permian
Stratigraphy	Stratigraphy
Field(s)	Cane Creek

Paradox Basin

The Paradox Basin is an asymmetric foreland basin located mostly in southeast Utah and southwest Colorado, but extends into northeast Arizona and northwest New Mexico. The basin is a large, elongate northwest to southeast oriented depression formed during the late Paleozoic Era, making it a little older than 300 million years. The basin is bordered on the east by the tectonically uplifted Uncompahgre Plateau, on the northwest by the San Rafael Swell, and on the west by the Circle Cliffs Uplift.

Its areal size is around 86,000 square miles (220,000 km²). The combined sedimentary strata of the Paradox Basin are more than 15,000 feet (4,600 m) thick in some places.

Unlike most Rocky Mountain basins, the Paradox Basin is an evaporite basin containing sediments from alternating cycles of deep marine and very shallow water. As a result of the thick salt sequences and the ductile nature of salt at relatively low temperatures and pressures, salt tectonics play a major role in the post-Pennsylvanian structural deformation within the basin.

Regional Tectonic Setting

The Paradox Basin is located in the interior of the North American Plate and began forming during the mid-Pennsylvanian as a piece of the Ancestral Rockies Orogeny. During this time, at least 20 different uplifts involving crystalline basement exhumed in the western interior of the United States, creating a swatch of basins across this area. The Paradox Basin is one of the largest, and best exposed basins from the Ancestral Rocky Mountain orogeny.

It had been suggested that the initial development of the Paradox Basin was effected by the Ouachita orogeny and created a pull-apart basin. At the same time of the Ancestral Rockies Orogeny, this belt was suturing the southern portion of North America to the northern portion of Gondwana. Potentially, this could've created a scenario to describe the Paradox Basin as a pull-apart basin, but this has since been disproved. The width, length, and facies architecture of the Paradox basin are very characteristic of other well-known flexural foreland basins.

Basin Formation

Cross-section of the Paradox Basin. Image shows transition from proximal evaporite basin to the distal carbonate shelf.

The Paradox Basin is classified as a foreland basin that formed along the side of the Uncompahgre Uplift, a northwest-southeast trending basement arch, which borders the northeastern edge of the basin. This uplifted arch is composed of Precambrian crystalline rock. The area between the basin and the Uncompahgre uplift is marked by a northeast-dipping fault zone extending over 200 km. As this basement uplifted, it displayed roughly 10 km of southwestern heave, placing the basement rock over the most proximal syntectonic sediments of the basin. These events caused substantial enough loading on the crust to create the flexure and accommodation space required to eventually form a foreland basin. This is supported by the general aspect ratio of the dimensions of the basin, and the asymmetric profile of the basin seen in [[cross-sections.

Stratigraphy and Sedimentology

Paradox Basin Stratigraphy. Image of synorogenic basin fill within Paradox Basin.

There are three synorogenic lithostatic facies composing the majority of this basin fill and they’re distinguished by containing carbonates and evaporite (Paradox Formation), a mixture of siliciclastic and carbonate (Honaker Trail Formation), or containing only siliciclastic material (Cutler Formation). During the Desmoinseian, the Uncompahgre uplift created accommodation space of several km in the proximal basin, and only 10% of that space in the distal portions. The rate of subsidence was also very high and because of that, the majority of the basin is filled with sediments of the same age as the basin’s formation.

In the proximal portions of the basin containing the thickest layers, the sediments were reworked and eroded directly from the basement uplift. They are coarse-grained sediments ranging from boulders to pebbles, overwhelmingly granitic and arkosic sandstone in composition, and make up the Cutler Formation. The deposition is thought to be largely driven by alluvial fans given the presence of the gravelly sandstones and clast-supported cobble conglomerates. The portions of sediment found closest to the uplift can be up to 5 km in thickness. Although this formation is characterized by the thick siliciclastic sediment, as it extends into the medial basin it does become gradually finer-grained and more interbedded with carbonates and evaporites from other strata.

The Paradox Formation extends through the medial and distal portions of the basin but reaches a maximum thickness, about 3 km, in the medial basin. It contains alternating sequences of dolomites and evaporites, whose deposition has been found to correlate to glacio-eustatic cycles and restricted marine systems. Despite rarely being visible in outcrops, these units are very recognizable in well logs across the basin. Quickly after deposition, the loading of sediment from prograding fans over the evaporate deposits caused halokinesis and salt-cored anticlines formed. The movement of these salt bodies has made it difficult to find the autochthonous sections of evaporite deposits, but some have been found with thicknesses of 2.2 km.

The Honaker Trail Formation was deposited over the Paradox Formation and in the most distal portion of the basin. Like the Paradox Formation, it contains carbonates, which dominate on the edge of the basin, but it also has significantly more siliciclastic sediments. The abundance of shallow marine fauna (crinoids, brachiopods, fusulinids and bryozoans) and certain sedimentary structures (lenticular beds, cross-beds, disaggregated bioclasts) indicate an interaction between an environment of carbonate shoals and coastal channels near a terrestrial fan. This supports the theories of a transition at this time between marine and restricted-marine systems within the basin.

The Cutler Group, which deposited in the Permian, contains mostly fluvial and eolian deposits. It’s about 530 m thick and is comprised of arkosic and quartzose sandstones, and siltstones. Although the proximal basin had been filled in by the Cutler Formation sediments, the Uncompahgre uplift was still a considerable height and provided a continuous and healthy supply of sediment that was able to stretch farther distances throughout the basin. The Cutler Group represents a basin-wide depositional period of sediments eroded from the uplift.

Hydrocarbon Potential

There are significant estimates of undiscovered hydrocarbons in the Paradox Basin (560 million barrels of undiscovered oil, 12,701 billion cubic feet of undiscovered natural gas, and 490 million barrels of undiscovered natural gas liquids) and although there are more than 100 small oil fields, there are challenges in recovering these hydrocarbons. Each of these oil fields is capable of producing anywhere from 2 to 10 million barrels of oil, but fields within the Paradox Basin are often characterized by early abandonment. Using conventional methods, the wells tend to produce in the first year or two, but after the point production decreases and it's no longer profitable to operate, the field is abandoned for more lucrative prospects.

The Paradox Formation contains hydrocarbons that can be sourced from layers of black shale and this accounts for the majority of the discovered hydrocarbons. This layer is also interbedded with ductile salt which creates traps from fractures as it moves upwards. These salt layers are also what pose the immense problem of hydrocarbon recovery because they’re thick and laterally persistent throughout the basin. Even while utilizing unconventional methods, like directional drilling and hydraulic fracturing, the ductile nature of the salt deposits causes it to move around between layers of strata as hydrocarbons are recovered.

Natural resources

Natural resources extracted from the basin include petroleum, uranium, copper, and potash.

Much of the petroleum production in the basin has come from porous carbonate deposits, such as algal mounds, of Pennsylvanian age. Additional reservoir types include uplifted fault blocks and discontinuous clastic beds with both stratigraphic and structural traps. The principal productive horizons in the basin include the Mississippian age Leadville Limestone, the Pennsylvanian Age Hermosa Group (Honaker Trail, Paradox, and Pinkerton Trail formations) and the Permian age Cutler Formation.

Discoveries in the cane creek formation have resulted in new oil production in the cane creek unit, near Moab, Utah.