Geology of Arkansas

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The geology of Arkansas includes deep 1.4 billion year old igneous crystalline basement rock from the Proterozoic known only from boreholes, overlain by extensive sedimentary rocks and some volcanic rocks. The region was a shallow marine, riverine and coastal environment for much of the early Paleozoic as multi-cellular life became commonplace. At the end of the Paleozoic in the Permian the region experienced coal formation and extensive faulting and uplift related to the Ouachita orogeny mountain building event. Extensive erosion of new highlands created a mixture of continental and marine sediments and much of the state remained flooded even into the last 66 million years of the Cenozoic. In recent Pleistocene and Holocene time, glacial sediments poured into the region from the north, down major rivers, forming dunes and sedimentary ridges. Today, Arkansas has an active oil and gas industry, although hydraulic fracturing related earthquake swarms have limited extraction. Mining industries in the state also produce brines, sand, gravel and other industrial minerals.

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Stratigraphy & Geologic History

The oldest rocks in Arkansas are igneous granites encountered in deep wells in the Ozarks and the northern part of the Mississippi Embayment, dated to be 1.4 billion years old. These felsic rocks likely formed sometime soon after the breakup of the supercontinent Columbia, into its component continents, including Laurentia, which today forms the stable basement rocks of most of North America.

The Precambrian, including the Archean and Proterozoic eons is poorly understood in Arkansas, but the entire state is believed to be underlain by deep, igneous and metamorphic crystalline basement rock. In the Ouachita region, geologists have found igneous and metamorphic Precambrian erratic boulders as well as metamorphosed igneous rock intrusions. [1]

Cambrian (542-488 million years ago)

At the time of the Cambrian explosion, as multi-cellular became commonplace, Arkansas was primarily flooded by rivers and a shallow marine environment. In the Ozark region, calcareous, quartzose sand and clay deposited, while the Ouachita area witnessed the formation of alternating layers of sand, clay, silt and small amounts of lime mud. [2]

Ordovician-Devonian (488-359 million years ago)

Water levels may have deepened in Arkansas with sand and clay depositing in a deep marine offshore trough environment in the Ouachita region, which shifted to siliceous ooze and clay by the Devonian. A shallow, near shore environment prevailed in the Ozarks, where thin layers of carbonates were interrupted by periodic erosion during the Ordovician. During the Devonian, carbonate deposition continued, but with added emplacement of carbonaceous clay, siliceous ooze and some shallow marine sand deposits. [2]

Carboniferous-Permian (318-299 million years ago)

While a marine environment continued to prevail into the Mississippian, the period marked rapid clastic sedimentation in the Late Mississippian in the Ouachita region. The depositional environment in the Ozarks area remained much the same as in the Devonian for the Mississippian, but the region experienced rapid clastic sedimentation at the beginning of the Pennsylvanian. The Ozarks lack Early Pennsylvanian rocks.

Arkansas experienced widespread faulting in the Pennsylvanian, associated with the Ouachita orogeny mountain building event. As a result, the quickly accumulated sediments in the Ouachita deep marine trough were faulted, folded, experienced low-grade metamorphism, followed by uplift and the formation of numerous quartz veins. [2]

A shallow environment of nearshore swamps formed in the Arkansas Valley region, bounded by newly formed faults, leading to subsequent coal formation from plant debris. The region experienced erosion during the Permian and widespread uplift of highlands as well as the continued formation of milky quartz veins. [2]

Mesozoic (251-66 million years ago)

At the start of the Mesozoic, in the Triassic period, the highlands experienced continued erosion. Red clay, silt, sand, gravel and other sediments and small anhydrite deposits formed on land.

A marine transgression in the Jurassic brought a renewed marine environment. Red clay, carbonate sand and silt accumulated in a shallow sea. Large deposits of anhydrite from this period point to very high salinity in the water. These anhydrite deposits are emplaced in the subsurface of southern Arkansas.

In the Early Cretaceous, carbonates and clastic sediments accumulated in the shallow seas, along with gypsum and additional anhydrite. Approximately 100 million years, the downwarping of the Mississippi Embayment expanded the marine transgression, leading to the accumulation of marl, sand and chalk as well as volcanic debris cast off by volcanic activity in the region. [2]

Cenozoic (66 million years ago-present)

In the Paleogene and Neogene periods of the Cenozoic (also traditionally known as the Tertiary), Arkansas experienced similar geologic events as in the Mesozoic.

In the Paleocene, nearshore reefs and dark marine clays formed in the flooded Mississippi Embayment. Bauxite developed on islands, formed from exposed Cretaceous igneous rock. During the Eocene, some bauxite was transported and a return to swampy conditions favored the formation of lignite low-grade coal. Throughout the Pliocene and Oligocene, the area experienced widespread erosion.

Much of the soil and sediments near the surface in Arkansas formed during the last 2.5 million years of the Quaternary. In the Pleistocene, zones of alpine snow pack, akin to small glaciers formed in the Ouachita Mountains and Boston Mountains. Periodic extensive glacial outwash from the Ohio River and Mississippi River produced Crowley's Ridge and both the Eastern Lowlands and Western Lowlands. Silt and sand formed loess and dunes. During the last 11,000 years of the Holocene, Arkansas has experienced additional deposition of alluvium due to Mississippi River floods. Sand dikes formed due to significant seismic activity in northeast Arkansas in the vicinity of the New Madrid Seismic Zone. [2]

Natural resource geology

Bauxite and zinc mining have both played a role in Arkansas' economy in the past, along with oil and natural gas, which are increasingly extracted from unconventional resources like the Fayetteville Shale.

Minerals & metals

Antimony was extracted from stibnite, hosted in quartz veins in Sevier County, Arkansas between 1873 and 1947, peaking in World War I and relying on a combination of shallow trenches and tunnels. In the Ouachita Mountains, manganese deposits also have as much as 1.2 percent cobalt, copper, nickel and lithium.

Arkansas has 19 minerals containing copper, including chalcopyrite, malachite and native copper, often situated in the Ozarks and Ouachitas. None form economically viable deposits. In the Ozarks, sphalerite is often found near faults in Paleozoic limestone and dolomite.

In addition to copper, there are numerous iron minerals, but few with economic deposits. Two small pig iron furnaces operated in the 1800s and a tiny open pit mine in the 1960s, near Rosston, Arkansas extracting only a few hundred tons. The largest production overall was 120,000 tons produced from the Wilcox Group where weathering of bedded siderite produced limonite concretions. In 2006, industrial minerals were valued at $913 million with production of bromine, Portland cement, crushed stone, lime and sand and gravel for construction.

Bauxite

In Arkansas, the aluminum hydroxides in bauxite form small oolites and pea-sized pisolites and outcrop or are located very close to the surface under thin sediments. The resource was first mined in 1898, 11 years after the State Geologist, John Branner, identified it in a sample from Pulaski County, Arkansas. In addition to the deposit in Pulaski County, south and east of Little Rock, there is a second deposit in Saline County, Arkansas close to Benton. During the Cretaceous and early Cenozoic, islands of syenite in the flooded Mississippi Embayment experienced tropical monsoon conditions and weathered into laterite, stripping away many elements and leaving sediment enriched in aluminum. Initially, Arkansas bauxite met 90% of US aluminum demand. Underground mining before and during World War II gave way to open pit mining in the 1960s. During the war, up to six million tons were mined in 1943. Arkansas bauxite mines were often passed over in favor of higher quality bauxite reserves in the Caribbean and mining ceased in 1982. The state has small amounts of gallium associated with bauxite deposits. [3]

Oil & natural gas

Between 1920 and 2003, 1.8 billion barrels of oil were produced in Arkansas. Ten counties, spread across the southern part of the state, produce oil. They include Ashley, Bradley, Calhoun, Columbia, Hempstead, Lafayette, Miller, Nevada, Ouachita and Union, although Union, Lafayette, Columbia and Ouachita counties have historically accounted to 85% of total production.

El Dorado, Arkansas was the site of early prospecting. Fifty years before oil was discovered, William Brown's Union Coal Company built an oil refinery in 1860 to distill oil from lignite. Exploratory wells were drilled in the 1880s and other companies in Camden County tried to refine oil from lignite throughout the 1860s.

Oil exploration began in earnest in 1916 with an unsuccessful well east of El Dorado, in Union County (which at the time was known as Urbana County). A week after Samuel S. Hunter found modest amounts of oil in the Hunter No. 1 well near Stephens, Arkansas, the Constantin Oil and Refining Company found large amounts of oil and natural gas. Soon after, in 1921, a gusher in El Dorado spurted up to 10,000 barrels as far as a mile away. Oil production dramatically expanded throughout the 1920s, prompting railroads to run up to 22 trains a day between El Dorado, Shreveport and Little Rock.

At the boom's peak in 1925, 3,483 wells produced up to 73 million barrels of oil. The huge supply overwhelmed local railroads and much of the oil was stored in open pits, where it leached into groundwater or became useless from rainwater contamination. The Arkansas Conservation Commission launched lawsuits related to the spills in the 1920s and the state launched the Arkansas Oil and Gas Commission in 1939, which set out to regulate the industry and limit oil and gas waste, as well as spills of bromine-laden salt brine brought up from the wells.

Oil production dropped precipitously from 58 million barrels in 1926 to 12 million by 1932. However, the industry surged during World War II until the early 1960s, producing 30 million barrels annually. The Shuler and Magnolia pools in Union County, discovered in 1937, produced 110 million barrels over their lifespan. Only one new oil play was discovered in 1971 and by the early 2000s, production hovered slightly above seven million barrels. With changes toward unconventional resources in the 2000s, hydraulic fracturing began on the Fayetteville Shale. However, the fracking related Guy-Greenbrier earthquake swarm in August, 2010 prompted a slower approach to development. [4]

Related Research Articles

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Geology of the Appalachians

The geology of the Appalachians dates back to more than 480 million years ago. A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor – strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians and neighboring Little Atlas near the center. These mountain ranges likely once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.

Permian Basin (North America) Large sedimentary basin in the US

The Permian Basin is a large sedimentary basin in the southwestern part of the United States. The basin contains the Mid-Continent Oil Field province. This sedimentary basin is located in western Texas and southeastern New Mexico. It reaches from just south of Lubbock, past Midland and Odessa, south nearly to the Rio Grande River in southern West Central Texas, and extending westward into the southeastern part of New Mexico. It is so named because it has one of the world's thickest deposits of rocks from the Permian geologic period. The greater Permian Basin comprises several component basins; of these, the Midland Basin is the largest, Delaware Basin is the second largest, and Marfa Basin is the smallest. The Permian Basin covers more than 86,000 square miles (220,000 km2), and extends across an area approximately 250 miles (400 km) wide and 300 miles (480 km) long.

Mississippi embayment Low-lying basin filled with Cretaceous to recent sediments

The Mississippi Embayment is a physiographic feature in the south-central United States, part of the Mississippi Alluvial Plain. It is essentially a northward continuation of the fluvial sediments of the Mississippi River Delta to its confluence with the Ohio River at Cairo, Illinois. The current sedimentary area was formed in the Cretaceous and early Cenozoic by the filling with sediment of a pre-existing basin. An explanation for the embayment's formation was put forward by Van Arsdale and Cox in 2007: movement of the earth's crust brought this region over a volcanic "hotspot" in the Earth's mantle causing an upthrust of magma which formed the Appalachian-Ouachita range. Subsequent erosion caused a deep trough that was flooded by the Gulf of Mexico and eventually filled with sediment from the Mississippi River.

Mudrock Class of fine grained siliciclastic sedimentary rocks

Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 1⁄16 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.

Clastic rock Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Geology of Texas Overview of the geology of the U.S. state of Texas

Texas contains a wide variety of geologic settings. The state's stratigraphy has been largely influenced by marine transgressive-regressive cycles during the Phanerozoic, with a lesser but still significant contribution from late Cenozoic tectonic activity, as well as the remnants of a Paleozoic mountain range.

Geology of Kansas

The Geology of Kansas encompasses the geologic history of the US state of Kansas and the present-day rock and soil that is exposed there. Rock that crops out in Kansas was formed during the Phanerozoic eon, which consists of three geologic eras: the Paleozoic, Mesozoic and Cenozoic. Paleozoic rocks at the surface in Kansas are primarily from the Mississippian, Pennsylvanian and Permian periods.

Shallow water marine environment

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

The geology of Guinea-Bissau is oldest in the east and becomes younger toward the west, with sediments from the past 66 million years nearer the coast. Some rock units in the northeast are as much as 680 million years old and throughout the geologic past Guinea-Bissau was influenced by the Mauritanide Belt orogeny and was submerged or partially submerged as a marine shelf or river delta for most of its existence.

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 Ohio formed beginning more than one billion years ago in the Proterozoic eon of the Precambrian. The igneous and metamorphic crystalline basement rock is poorly understood except through deep boreholes and does not outcrop at the surface. The basement rock is divided between the Grenville Province and Superior Province. When the Grenville Province crust collided with Proto-North America, it launched the Grenville orogeny, a major mountain building event. The Grenville mountains eroded, filling in rift basins and Ohio was flooded and periodically exposed as dry land throughout the Paleozoic. In addition to marine carbonates such as limestone and dolomite, large deposits of shale and sandstone formed as subsequent mountain building events such as the Taconic orogeny and Acadian orogeny led to additional sediment deposition. Ohio transitioned to dryland conditions in the Pennsylvanian, forming large coal swamps and the region has been dryland ever since. Until the Pleistocene glaciations erased these features, the landscape was cut with deep stream valleys, which scoured away hundreds of meters of rock leaving little trace of geologic history in the Mesozoic and Cenozoic.

The geology of South Dakota began to form more than 2.5 billion years ago in the Archean eon of the Precambrian. Igneous crystalline basement rock continued to emplace through the Proterozoic, interspersed with sediments and volcanic materials. Large limestone and shale deposits formed during the Paleozoic, during prevalent shallow marine conditions, followed by red beds during terrestrial conditions in the Triassic. The Western Interior Seaway flooded the region, creating vast shale, chalk and coal beds in the Cretaceous as the Laramide orogeny began to form the Rocky Mountains. The Black Hills were uplifted in the early Cenozoic, followed by long-running periods of erosion, sediment deposition and volcanic ash fall, forming the Badlands and storing marine and mammal fossils. Much of the state's landscape was reworked during several phases of glaciation in the Pleistocene. South Dakota has extensive mineral resources in the Black Hills and some oil and gas extraction in the Williston Basin. The Homestake Mine, active until 2002, was a major gold mine that reached up to 8000 feet underground and is now used for dark matter and neutrino research.

The geology of Kentucky formed beginning more than one billion years ago, in the Proterozoic eon of the Precambrian. The oldest igneous and metamorphic crystalline basement rock is part of the Grenville Province, a small continent that collided with the early North American continent. The beginning of the Paleozoic is poorly attested and the oldest rocks in Kentucky, outcropping at the surface, are from the Ordovician. Throughout the Paleozoic, shallow seas covered the area, depositing marine sedimentary rocks such as limestone, dolomite and shale, as well as large numbers of fossils. By the Mississippian and the Pennsylvanian, massive coal swamps formed and generated the two large coal fields and the oil and gas which have played an important role in the state's economy. With interludes of terrestrial conditions, shallow marine conditions persisted throughout the Mesozoic and well into the Cenozoic. Unlike neighboring states, Kentucky was not significantly impacted by the Pleistocene glaciations. The state has extensive natural resources, including coal, oil and gas, sand, clay, fluorspar, limestone, dolomite and gravel. Kentucky is unique as the first state to be fully geologically mapped.

The geology of Austria consists of Precambrian rocks and minerals together with younger marine sedimentary rocks uplifted by the Alpine orogeny.

The geology of Mississippi includes some deep igneous and metamorphic crystalline basement rocks from the Precambrian known only from boreholes in the north, as well as sedimentary sequences from the Paleozoic. The region long experienced shallow marine conditions during the tectonic evolutions of the Mesozoic and Cenozoic, as coastal plain sediments accumulated up to 45,000 feet thick, including limestone, dolomite, marl, anhydrite and sandstone layers, with some oil and gas occurrences and the remnants of Cretaceous volcanic activity in some locations.

The geology of Missouri includes deep Precambrian basement rocks formed within the last two billion years and overlain by thick sequences of marine sedimentary rocks, interspersed with igneous rocks by periods of volcanic activity. Missouri is a leading producer of lead from minerals formed in Paleozoic dolomite.

The geology of Montana includes thick sequences of Paleozoic, Mesozoic and Cenozoic sedimentary rocks overlying ancient Archean and Proterozoic crystalline basement rock. Eastern Montana has considerable oil and gas resources, while the uplifted Rocky Mountains in the west, which resulted from the Laramide orogeny and other tectonic events have locations with metal ore.

The geology of Denmark includes 12 kilometers of unmetamorphosed sediments lie atop the Precambrian Fennoscandian Shield, the Norwegian-Scottish Caledonides and buried North German-Polish Caledonides. The stable Fennoscandian Shield formed from 1.45 billion years ago to 850 million years ago in the Proterozoic. The Fennoscandian Border Zone is a large fault, bounding the deep basement rock of the Danish Basin—a trough between the Border Zone and the Ringkobing-Fyn High. The Sorgenfrei-Tornquist Zone is a fault-bounded area displaying Cretaceous-Cenozoic inversion.

The geology of the United Arab Emirates includes very thick Paleozoic, Mesozoic and Cenozoic marine and continental sedimentary rocks overlying deeply buried Precambrian. The region has extensive oil and gas resources and was deformed during the last several million years by more distant tectonic events.

References

  1. ↑ Walker & Geissman (2009). "Geologic History of Arkansas". Arkansas Geological Survey.
  2. 1 2 3 4 5 6 Walker & Geissman 2009.
  3. ↑ "Mineral Resources of Arkansas". Arkansas Geological Survey. 2015.
  4. ↑ Bridges, Kenneth (2017). "Oil Industry". The Encyclopedia of Arkansas History & Culture.
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