What Is Arkose Sandstone? How Does it Form and Where?

Arkose is a detrital sedimentary sandstone with over 25% feldspar. It comprises mainly quartz and feldspar clasts with minor amounts of mica and some heavy minerals. These minerals are cemented in calcite, iron oxide, or silica.

This rock resembles granite due to the abundance of pinkish or reddish K-feldspar and forms from erosion of grainy feldspar and quartz granite or felsic gneiss rock and subsequent lithification.

Some of the uses of arkose include as a building material, paving, landscaping, and asphalt concrete. You can essentially use it like any other sandstone.

Learn more about arkose rock, including its chemical and mineral composition. I will also tell you how it forms, depositional environments or where it occurs, and a lot more.

Quick facts and properties

• Name: arkose or arkosic sandstone
• Pronunciation: /ˈɑːrkoʊs/
• Rock type: Sedimentary
• Origin: Clastic or detrital
• Texture: Medium-grained clastic
• Grain size: 0.06 – 2 mm or (0.0024–0.08 inch)
• Sorting: Poorly to moderately well sorted
• Clast shape: angular to subangular
• Composition: Mainly quartz and feldspar and a small amount of mica and other minerals cemented with calcite
• Color: Pink, red, or grayish, usually lighter than graywacke
• Feel: Sandy
• Depositional environment: Mainly alluvial fans, braided stream, and point bars
• Resembles: Granite when not banded
• Acid test: Slight effervesce in dilute hydrochloric acid.
• Mohs hardness scale: 5.5 – 6.5

What is arkose?

Arkose is a reddish, pink, or sometimes grayish sandstone with over 25% feldspar. It is a medium-grained (with 0.0625-2 mm grains) clastic textured sedimentary rock with angular to subangular moderately to poorly sorted clasts.

These angular and subangular poorly sorted detrital material indicates minimal transportation, i.e., deposition happened quickly and close to the source of sediments. Thus, arkose is a texturally and compositionally immature detrital sedimentary rock.

However, although desert arkose may be compositionally immature, they are texturally mature with well-sorted and rounded clasts. This happens because the dry environment allows for erosion with little chemical weathering. Also, the wind and rugged landscape favored transportation, rounding, and sorting detrital materials.

Sometimes, the term feldspathic sandstone or feldspathic arenites may refer to arkoses. However, the term feldspathic only implies it is rich in feldspars. It doesn’t give specific amounts.

On the other hand, an arenite is any sedimentary rock with sand-sized grains, with the matrix accounting for less than 15% of the stone. Other arenites include greensand, greywackes, and sandstones.

Some arkose rocks with feldspar between 5% and 25% are as subfeldspathic arenites or subarkose. It lies between quartz arenite and arkose.

Lastly, arkose forms with minimal chemical weathering, possibly in 1) dry/arctic conditions, 2) rapid erosion, deposition, and burial of sediments favored by an uplift or high relief area or steep terrain, and 3) deposition close to the original rocks.  

Arkosic sand

Arkosic sand is nothing other than a feldspar-rich sand that is a possible precursor of this rock.

Arkose composition

Most arkose sandstones have mineral and chemical compositions like granite, granodiorite, gneiss, or other mostly acidic or silicic rocks. This mineral and chemical composition confirms this origin.

Here is more on the chemical and mineral composition of arkosic sandstone.

i. Chemical composition

The chemical composition of arkose is about 60-80% silica (SiO₂) and a relatively high amount of aluminum oxide (Al₂O₃). Also, this rock has considerable potassium and sodium oxides.

Usually, arkosic sandstones have 2-4% potassium oxide, which is more than the amount of sodium oxide if the given sample has more K-felspar than plagioclase.

ii. Mineral composition

Mineralogically, arkose has predominantly quartz and at least 25% feldspar. The feldspar is often potassium (K-feldspar), especially microcline, but plagioclase may be present. Microcline is often pinkish, giving this rock a hue, but it may appear reddish due to a hematite surface coat.

Note: The usage of the term arkose is widespread among geologists. It generally means a sandstone with a conspicuous amount of feldspar. Some put it at 20%, others at 25% or 30%. A more widely accepted is sandstone or feldspathic sandstone with at least 25% feldspar.

Besides quartz and feldspar, this sedimentary rock may have minor amounts of mica (biotite and muscovite) and igneous or metamorphic rock clasts.

Usually, muscovite is unaltered but may be stained with limonite or hematite, while biotite will be altered to chlorite or hematite.

Other heavy minerals make up a small percentage of the framework grains. These include metastable types like amphibole, kyanite, garnet, epidote, magnetite, pyroxene, and olivine, or ultra-stable ones like tourmaline and rutile.  

The clasts cementing matrix material is usually less than 15%, mainly calcite, rarely dolomite. However, it may sometimes be silica, hematite, clay minerals, barite, and pyrite. Also, some kaolinite or illite from weathered feldspar may be present in the cementing matrix.  

Lastly, an arkose thin section will show mainly angular quartz grains and grid twinning microcline feldspar under cross-polarized or XP light. Muscovite, on the other hand, will appear as highly birefringent flakes in a calcite matrix.

How does arkose form?

Arkose rocks form from the weathering of crystalline feldspar and quartz-rich igneous or metamorphic rocks. These rocks include granite, granodiorite, gneiss, and similar acidic or silicic rocks.

Usually, these rocks form on stable continental shelves or cratons and are associated or interbed with conglomerates, shale, limestones, evaporates, and shallow-water lithic or quartz arenites.

However, they may occur in unstable basins, mobile belts, or deep-water settings but are less common in these settings.

As you may know, feldspar undergoes characteristic cleavage along the lines of its weakness into smaller fragments. Also, it will undergo chemical weathering to form clay minerals, especially kaolinite.

Therefore, there must be little to no chemical weathering for arkosic sandstones to form. This can only happen under two conditions:

1. When erosion or mechanical weathering, transportation, deposition, and burial occur faster than chemical weathering and
2. The climatic condition doesn’t favor chemical weathering.

Thus, for felspars to survive chemical weathering, arkosic sandstones may form in the following environments or places with granitic, gneiss outcrops or other parent rock outcrops:

1. Mountainous area with nearby low-relief basins

Arkosic sandstones may form in mountainous areas or places with local uplifts exposing granitic crystalline rocks with a nearby low-relief basin or fault trough. Such places include fault-bounded or rift-related grabens or half grabens with a steep-sided basement rock.

The higher relief will promote quicker erosion, transportation, and burial of sediments giving little time for chemical weathering.

Therefore, such a place will allow the formation of arkose irrespective of the climate. However, these rocks must form close to the origin.

2. Cold or artic conditions

Cold or arctic climatic conditions inhibit or limit chemical weathering but allow mechanical weathering. This will prevent feldspars from weathering chemically and favor the formation of these rocks.

3. Arid and semi-arid areas

In arid or semi-arid areas with crystalline granitic/gneiss outcrop, rugged terrain, and wind will promote faster erosion and deposition before the clasts undergo chemical weathering.

Also, it will favor a mountainous topography with arid or semi-arid conditions with these crystalline parent rocks.

Arkose depositional environments

Arkosic sandstone deposits occur mainly on non-marine wedge-shaped alluvial fans, point bars, braided channels, or inside river channels near the origin source.

However, they can occur in other lacustrine and transitional marine deposits, including on seas advancing granitic bedrock, but rarely. Those that go to the marine will have lesser feldspar and will mix with clasts from other sources.

Some deposits may occur in situ or short-distance downslope. Such will be poorly sorted, with more angular clasts and higher detrital matrix content than those transported further. Also, thin layers of arkose resting on a granitic rock in a sedimentary sequence formed from clasts of the underlying rock are known.

Usually, arkosic rocks may show depositional features like crossbreds, cut-fill structures, current ripple marks, horizontal lamination, and deposits thin basin-ward and will have finer sediments.

However, most arkosic sandstones will rarely have fossils due to the depositional process involved, except for a few terrestrial or shallow marine fossils, including plants.

Lastly, deposition may also happen adjacent to dissected or active magmatic arcs.

Where is arkose found?

Arkose sandstones are not characteristic to a certain age as they are present from Precambrian to Holocene geological formations. Their occurrence is local, and it depends on tectonic activities that may create an uplift of silicic crystalline rocks and sometimes weather conditions.

Usually, arkose forms thick (2000 meters or more), wedge-shaped deposits adjacent to uplifts geographically limited by fault troughs or quickly subsiding basins.

However, individual arkose beds may be a few centimeters to several meters thick. These beds are poorly sorted, irregularly banded, and laterally discontinuous.

Also, they are associated with conglomerates whose source is granitic rock terrains and may occur above unconformities near granite terrain.

Some notable formations with arkose sedimentary rocks include the following:

• Uluru inselberg (Ayers Rock) in Australia
• Pennsylvanian Minturn and Fountain Formations in Colorado, USA
• Triassic Newark Group of Connecticut
• Rocky Mountains front ranges
• Precambrian Torridon sandstones of Scotland,
• Silurian Clinton Formation in Pennsylvania, USA
• Devonian Old Red Sandstone of England
• Paleocene Swauk Formation in Washington, USA

References

• Boggs, S. (2012). Petrology of sedimentary rocks (2nd ed.). Cambridge University Press.
• Prothero, D. R., & Schwab, F. (2014). Sedimentary geology: An introduction to sedimentary rocks and Stratigraphy (3rd ed.). W. H. Freeman and Company.
• Klein, C., & Philpotts, A. R. (2017). Earth materials: Introduction to mineralogy and petrology (2nd ed.). Cambridge University Press.
• Martin, R. (2018). Earth’s evolving systems: The history of planet earth (1st ed.). Jones & Bartlett Learning.
• Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill/Education, Inc.
• Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.