Arkose is a specific type of sedimentary rock, a sandstone distinguished by its high content of feldspar minerals. While most sandstones are dominated by quartz, arkose must contain at least 25% feldspar grains to be classified as such. The presence of this relatively unstable mineral indicates that the rock formed under restrictive geological conditions. These conditions involve a rapid interplay between the source, climate, and transportation, preventing the complete chemical breakdown of feldspar before deposition.
Required Source Rock and Weathering Conditions
The formation of arkose begins with a source rock rich in both quartz and feldspar, typically a granitic or felsic metamorphic rock like gneiss. These basement complexes provide the necessary raw material, often called “granite wash,” which is then subjected to erosion. The resulting sediment, or grus, is a mixture of sand-sized quartz and feldspar grains.
The overriding condition for arkose formation is the suppression of chemical weathering. Feldspar is chemically unstable when exposed to water and atmospheric gases, quickly breaking down into clay minerals. Therefore, the environment must favor rapid physical erosion and transport over slow chemical decomposition. This often occurs in cold (glacial) or arid (dry) climates, where low moisture or temperatures significantly slow the chemical reactions that destroy the feldspar.
Alternatively, even in humid climates, the rate of uplift and erosion must be high enough that feldspar grains are quickly stripped from the parent rock and buried. This scenario, where mechanical weathering far outpaces chemical weathering, results in the coarse, angular, and poorly-sorted texture characteristic of arkoses.
Proximal Depositional Settings
Arkose almost exclusively forms in proximal depositional environments, close to the source mountains, due to the requirement for minimal transport distance. These settings are typically associated with high-relief, tectonically active areas where the granitic basement is exposed and undergoing rapid erosion. The sediment must be dumped quickly to prevent the mechanical abrasion and chemical alteration that occur during long-distance travel.
Alluvial Fans and Braided Streams
One of the most common settings is the alluvial fan, a wedge-shaped deposit formed where a steep mountain stream emerges onto a flatter plain. The rapid decrease in stream velocity causes an abrupt deposition of coarse, poorly-sorted sediment, preserving the feldspar content. High-energy braided stream channels, river systems near mountain fronts, are similarly effective at transporting and depositing the feldspar-rich material over short distances.
Tectonic Basins
Tectonically controlled depressions, such as continental rift basins or graben structures, also provide ideal environments for arkose accumulation. As the Earth’s crust pulls apart, the exposed basement rock is eroded, and the resulting sediment is rapidly shed into the subsiding basin floor. This fast burial rate, combined with the short travel distance, ensures the survival of the feldspar grains.
Cementation and Lithification
Once the feldspar-rich sand is deposited, it must undergo lithification, the process that turns loose sediment into solid rock. This transformation involves compaction, where the weight of overlying sediment squeezes the grains together and reduces pore space. Following compaction, cementation occurs as dissolved minerals precipitate in the remaining pore spaces, binding the grains together.
The cementing agents in arkose are varied but commonly include silica (quartz), calcite (calcium carbonate), or iron oxides. The precipitation of iron oxides, such as hematite, is frequently responsible for the characteristic pink or reddish coloration seen in arkose formations. This coloring is often a visual cue to the presence of iron-bearing minerals in the source rock.
The resulting rock is classified as a feldspathic arenite, an immature sedimentary rock due to the angularity and poor sorting of its grains. The cementation process locks the mineral assemblage in place, preserving the high concentration of feldspar that survived the journey from the source terrain.