General Geology

Rocks are generally placed into 1 of 3 major categories: igneous, metamorphic, or sedimentary. Igneous rocks have solidified from molten or partly molten mineral matter. Metamorphic rocks have been altered in the solid state from some pre-existing condition in response to significant changes in temperature, pressure, or chemical environment. Sedimentary rocks are composed of particles of sediment, which are derived by the weathering and/or the erosion of pre-existing rock. Most of the rocks exposed at the surface in Arkansas are sedimentary, but there are some igneous rocks (with adjacent contact metamorphic rocks) and very low grade regional metamorphic rocks in Arkansas also.

A sedimentary rock consists of two components: the particles and the cement that holds them together. Sedimentary rocks are classified as clastic (rocks made up of grains of sand, silt, and clay) or chemical (rocks made up of shell fragments, saline water deposits, and other materials that are deposited from solution). The most common clastic sedimentary rocks are shales, siltstones, and sandstones. The most common chemical sedimentary rocks are limestone and dolostone.

To understand how sedimentary rocks form, we must account for the processes that create the original particles of sediment, the mechanisms of sediment transport, the processes of deposition or precipitation of a given sediment, and what has happened to the sediment over time. By studying rocks and depositional systems (the processes by which sediments are deposited), geologists recognize that most of the sedimentary rocks in the Paleozoic Highlands of Arkansas are marine. In the southern and eastern parts of the state, the sedimentary deposits are predominantly fluvial (fresh-water processes).

The exposures of igneous rocks in Arkansas are less than 0.1 percent of the entire area of the state. Most are exposed over 15 square miles, principally in Pulaski, Saline, Hot Spring, Garland, and Pike Counties. A few small igneous dikes and sills are present outside the Ouachita region, mostly in the Arkansas Valley, and in at least one case, in the Boston Mountains. Except for some localized contact metamorphism adjacent to the larger igneous intrusions, only very low grade metamorphic rocks are present in the state.

Arkansas is divided into a highland area in the northwest and a lowland region in the south and east. The rocks in the highland area are dominated by well-lithified sandstones, shales, limestones, and dolostones of Paleozoic age. A thin drape of younger unconsolidated clays, sands, and gravel, termed alluvium, is often found in valley floors and associated with the streams and rivers. The sedimentary deposits of the lowlands are mainly unconsolidated clay, sand, and gravel of Quaternary age, poorly consolidated deposits of clay, sand, silt, limestone, and lignite of Tertiary age, and consolidated (to a limited extent) deposits of Cretaceous marl, chalk, limestone, sand, and gravel (see Geologic Map of Arkansas below). Download Arkansas Physiographic Province Map (subdivisions) and Arkansas Physiographic Province Map (county base).

When most of the sediments that compose the rocks in the highland region of Arkansas were being deposited, north Arkansas was a shallow south-sloping sea floor (continental shelf), the Arkansas River Valley was near the edge of the shelf, and the Ouachita area was a deep abyssal plain (see Geologic History). An abyssal plain is the relatively smooth and deep (more than 3,000 feet below sea level) parts of the ocean floor where accumulating sediments have buried the pre-existing topography. In the late PreCambrian Era, a broad uplift domed the Ozark strata with little structural disruption. Starting in the middle to late Paleozoic, a collision of two of the earth's mobile continental plates compressed the sediments of the abyssal plain into the Ouachita Mountains. This multimillion-year-long process folded and faulted the Ouachita strata into a structurally complex mountain chain. The Arkansas River Valley area is the transition zone between the structurally simple Ozarks and the structurally complex Ouachitas with subdued characteristics in each region.

Today, the rocks of the Ozarks tilt slightly to the south and have a dendritic drainage pattern. Since shales and siltstones erode faster than sandstones and limestones, the basic topography is flat-topped mountains with stepped flanks. By contrast, the topographic expression of the Ouachitas is controlled not only by the erosional resistance of the rocks, but also by their internal structure. The strata are complexly folded and frequently faulted. The mountains are mostly east-west-trending ridges supported by erosionally resistant rocks and separated by less resistant rocks. The Arkansas River Valley is characterized by much less intensely folded and faulted strata than the Ouachita region. Erosional processes left the synclines as mountains and the anticlines as valleys.

Facts About Arkansas

Arkansas Bauxite
Bauxite - State Rock

This rock was formed by the weathering of nepheline syenite under tropical conditions, a process called laterization.  It is a type of lithified soil which is relatively low in silica and high in aluminum.  Bauxite was discovered in Arkansas before 1900 and was a major source of ore for aluminum metal for some 90 years in Saline and Pulaski Counties.  There are many uses for processed bauxite, aside from metal, including abrasives, cement, refractories, and chemicals.  Aluminum compounds are essential ingredients in many common household items, including deodorants, antacids, and paper.

History of Bauxite in Arkansas Pamphlet PDF (687 KB)

Arkansas Quartz
Quartz - State Mineral

Quartz is composed of oxygen and silica combined in a ratio of 2: 1. This mineral is hard, durable, weather-resistant, and relatively common.  Quartz crystals formed as hot waters percolated through fractured rock in the Ouachita Mountains some 245 million years ago.  Chemically pure sources of quartz are in much demand by industry as a source of the raw chemical feedstock for the manufacture of quartz wafers, silicon metal, glass, fused quartz, and optical fiber.  Arkansas has the most significant economically valuable deposits of high-quality quartz in the United States.

Arkansas Quartz Crystals Pamphlet PDF (526 KB)

Arkansas Diamond
Diamond - State Gem

Diamonds were first discovered in Arkansas in 1906. Since that time over 100,000 diamonds have been recovered from the site now known as Crater of Diamonds State Park. The average size recovered is about .21 carat. Colors of the diamonds range from white to yellow and brown and the natural crystals are usually rounded. The largest diamond found in Arkansas is The Uncle Sam, found in 1924. This diamond weighed 40.24 carats.

Finding Diamonds in Arkansas Pamphlet PDF (359 KB)

Geology And Geography
  • Oldest known mapped geologic formation (Collier Shale) about 520,000,000 years old
  • Oldest known surface rock (an altered igneous body, Saline County) about 1,025,000,000 years old
  • Most abundant sediments – sand, clay, silt, gravel, and marl
  • Most abundant sedimentary rocks – shale, sandstone, dolostone, limestone, and chert
  • Most abundant igneous rock– syenite (resembles granite, but rarely contains quartz)
  • In recent years, an average of 44 earthquakes per year are detected in Arkansas. However, from Jan. 12, 1982 to Jan. 12, 1983, a swarm of nearly 20,000 small earthquakes occurred in Faulkner County.
  • Fourteen meteorites have been discovered in Arkansas.
  • Area – 53,182 square miles, making Arkansas larger than half of the world’s countries
  • Highest point – 2,753 feet above sea level (Magazine Mountain)
  • Lowest elevation – 54 feet above sea level (Ouachita River at Arkansas/Louisiana state line)
  • Eighteen percent of the state (by area) is irrigated land.
  • Navigable length of major rivers: Arkansas River – 308 miles, Ouachita River – 128 miles, Mississippi River – 321 miles, and White River – 255 miles

Minerals And Fossil Fuels
  • Arkansas had the 1st diamond mine in the United States and led the nation in the recovery of diamonds for over 50 years. It is the only place where you can find (and keep) a diamond.
  • The two largest diamonds discovered in the United States came from Arkansas.
  • Arkansas led the nation in the production of barite for over 30 years.
  • Arkansas’ annual value of mineral and fossil-fuel production is more than $1,000,000,000.
  • Arkansas’ most valuable non-fuel mineral resources, based on annual production (2007 data), are: bromine, crushed stone, cement, and construction sand & gravel, amounting to 92% of hard minerals value.
  • Three fossil fuels – natural gas, oil, and coal – are produced in Arkansas today and vast reserves of lignite are essentially untouched.
  • In 2012, there were about 207 oil and 150 gas fields in production in Arkansas.
  • In 2012, about 6,594,951 barrels of oil were produced in Arkansas.
  • As of 2012, about a total of 1,863,343,830 barrels of oil were produced in Arkansas.
  • As of 2012, about a total of 10,766,937,122 mcf of gas were produced in northern Arkansas.
  • Deepest well ever drilled in Arkansas – 20,661 feet (a test well for gas/oil in Yell County)
  • Rocks and minerals currently produced or recovered in Arkansas: Bauxite, Dolostone, Gypsum, Quartz, Tripoli, Cement rock, Gemstones, Limestone, Sandstone, Sulfur, Tuff, Clays, Glass/Industrial Sand, Novaculite, Slate, Syenite
  • In 2013, 28% of the global bromine was produced in Arkansas, with Arkansas being the only U.S. producer.
  • More bauxite and vanadium ore has been mined in Arkansas than in all other states combined.

Among the states, Arkansas Ranked (as of 2007):
  • 1st in the production of bromine
  • 1st in the production of quartz crystal and lasca
  • 1st in the production of novaculite and silica stone
  • 1st in the recovery of diamonds
  • 3rd in the production of tripoli
  • 4th in the production of kaolin
  • 5th in the production of crude gypsum
  • 6th in the production of common clays

Most current data for 2007, Arkansas Geological Survey, 2010