Sandstone
The journey of sandstone begins with the physical and chemical weathering of bedrock. Silicate sand grains form as rocks break down under the influence of wind, water, and ice. These processes occur most rapidly in areas of high relief such as volcanic arcs or continental rifting zones. Eroded sand travels via rivers or wind to depositional environments where tectonics creates space for accumulation. Forearc basins often accumulate sand rich in lithic grains and plagioclase. Intracontinental basins and grabens along continental margins serve as common sites for deposition.
As sediments continue to pile up, older layers bury younger ones and undergo diagenesis. This process involves compaction and lithification that turn loose sand into solid rock. Early stages known as eogenesis happen at shallow depths of just a few tens of meters. During this phase bioturbation occurs alongside mineralogical changes with only slight compaction. The red hematite found in red bed sandstones likely forms during eogenesis. Deeper burial brings mesogenesis where most compaction and lithification take place.
Compaction forces sand grains into more compact arrangements while ductile minerals like mica deform. Pore space reduces significantly as pressure increases from overlying sediments. Chemical compaction also takes place through pressure solution where strained contact points dissolve away. Points of contact between grains bear the greatest strain making them more soluble than the rest of the grain. As a result these contact points dissolve allowing grains to come closer together. Lithification follows closely on compaction as increased temperatures hasten cement deposition. Pressure solution contributes to cementing by redepositing dissolved minerals into unstrained pore spaces. Mechanical compaction primarily happens at depths less than one kilometer while chemical compaction continues deeper.
Framework grains make up the bulk of sandstone and measure between 0.0625 and 2 millimeters in diameter. Most framework grains consist of quartz or feldspar because they resist weathering better than other minerals. Quartz grains evolve from plutonic rock which is felsic in origin or from older recycled sandstones. These physical properties allow quartz to survive multiple recycling events while displaying some degree of rounding. Feldspathic framework grains commonly rank as the second most abundant mineral in sandstones.
Feldspar divides into alkali feldspars and plagioclase feldspars distinguishable under a petrographic microscope. Alkali feldspar ranges chemically from KAlSi3O8 to NaAlSi3O8. Plagioclase feldspar ranges from NaAlSi3O8 to CaAl2Si2O8. Lithic framework grains represent pieces of ancient source rock that have not yet weathered away completely. These fragments can be fine-grained or coarse-grained igneous metamorphic or sedimentary rock though volcanic clasts are common. Accessory minerals comprise all other grain types making up just a small percentage of total sandstone content.
Common accessory minerals include micas such as muscovite and biotite along with olivine pyroxene and corundum. Many heavy minerals like zircon tourmaline rutile garnet magnetite resist weathering and serve as indicators of maturity through the ZTR index. Matrix consists of very fine material present within interstitial pore space between framework grains. The nature of matrix results in a twofold classification where arenites remain texturally clean while wackes contain significant amounts of clay.
Geologists typically classify sandstones by point-counting thin sections using methods like the Gazzi-Dickinson Method. This yields relative percentages of quartz feldspar lithic grains and clay matrix amount. Composition provides important information on sediment genesis when used with triangular quartz feldspar lithic charts known as QFL diagrams. Visual aids allow geologists to interpret different characteristics including provenance models showing likely tectonic origins. A stage of textural maturity chart illustrates differences between immature submature mature and supermature sandstones.
Dott's 1964 classification scheme incorporates dual textural and compositional maturity concepts into one system. Dott set the boundary between arenite and wacke at 15 percent matrix content. Arenites types have less than 15 percent clay matrix between framework grains. Quartz arenites contain more than 90 percent siliceous grains including quartz or chert rock fragments. These pure quartz sands result from extensive weathering that removed everything except stable quartz minerals. They commonly affiliate with rocks deposited in stable cratonic environments such as aeolian beaches or shelf environments.
Feldspathic arenites contain less than 90 percent quartz and more feldspar than unstable lithic fragments. Feldspathic sandstones occur in association with cratonic or stable shelf settings derived from granitic-type primary crystalline rocks. Lithic arenites characterize generally high content of unstable lithic fragments like volcanic and metamorphic clasts. Wackes are sandstones containing more than 15 percent clay matrix between framework grains. Arkose sandstones exceed 25 percent feldspar while greywacke sandstones form heterogeneous mixtures of lithic fragments.
When sandstone faces great heat and pressure associated with regional metamorphism individual quartz grains recrystallize. Former cementing material also transforms to create the metamorphic rock called quartzite. Most original texture and sedimentary structures erase themselves during this intense geological process. Grains become so tightly interlocked that breaking the rock fractures through grains forming irregular conchoidal fracture patterns. Geologists recognized by 1941 that some rocks show macroscopic characteristics of quartzite without undergoing high-pressure metamorphism.
These rocks experienced only lower temperatures and pressures linked to diagenesis yet cemented thoroughly enough for microscopic distinction. The term orthoquartzite distinguishes such sedimentary rock from metaquartzite produced by actual metamorphism. Orthoquartzite in narrow sense often contains 99 percent SiO2 with minor iron oxide traces. Although few fossils normally exist original texture and sedimentary structures remain preserved within these formations. Typical distinction between true orthoquartzite and ordinary quartz sandstone lies in how highly cemented the former is.
An orthoquartzite fractures across grains rather than around them allowing field recognition. Distinction between orthoquartzite and metaquartzite marks onset of recrystallization where strained quartz grains replace new unstrained small quartz grains. This produces mortar texture identifiable in thin sections under polarizing microscope. With increasing grade further recrystallization creates foam texture characterized by polygonal grains meeting at triple junctions. Porphyroblastic texture follows featuring coarse irregular grains including larger porphyroblasts.
Pore space includes open spaces within a rock or soil that directly influence porosity and permeability rates. Porosity measures percentage of bulk volume inhabited by interstices within given rock samples. Packing even-sized spherical grains rearranged from loosely packed to tightest packed states affects porosity significantly. Permeability defines rate at which water or other fluids flow through rock materials. Groundwater work permeability may measure gallons per day through one square foot cross section under unit hydraulic gradient.
Sandstone beds allow percolation of water and other fluids making them porous enough to store large quantities. These properties make sandstones valuable aquifers and petroleum reservoirs for human use. The relationship between pore space and fluid storage capacity depends on how sand grains pack together during formation. Compaction reduces pore space while cementation fills remaining voids with secondary minerals like calcite or silica. Chemical compaction via pressure solution dissolves strained contact points allowing closer grain arrangement. This process influences final porosity levels determining suitability for groundwater extraction or oil recovery operations.
Sandstone has been used since prehistoric times for construction decorative art works and tools. A 17,000 year old sandstone oil lamp discovered at Lascaux caves in France demonstrates early human utilization. It has been widely employed around the world constructing temples churches homes and civil engineering projects. Although resistance to weathering varies sandstone remains easy to work making it common building material including asphalt concrete.
Some types used historically like Collyhurst sandstone in North West England suffered poor long-term weather resistance requiring repair. Uniformity of grain size and friability structure make some types excellent materials for grindstones sharpening blades implements. Non-friable sandstone creates gritstone grindstones for grinding grain effectively. Pure quartz orthoquartzite containing 90 to 95 percent quartz proposed nomination to Global Heritage Stone Resource status.
In Argentina orthoquartzite-stoned facades form main features of Mar del Plata style bungalows. The Main Quadrangle of University of Sydney exemplifies so-called sandstone university architecture. Sandstone statues like Maria Immaculata by Fidelis Sporer from 1770 demonstrate decorative applications. Watchtowers of Fort Saint Elmo in Malta show multiple stages of weathering and restoration efforts over centuries.
Common questions
What is sandstone and how does it form from bedrock?
Sandstone is a sedimentary rock that forms through the physical and chemical weathering of bedrock into silicate sand grains. These grains travel via rivers or wind to depositional environments where tectonics creates space for accumulation before undergoing diagenesis.
When did geologists recognize rocks with quartzite characteristics without high-pressure metamorphism?
Geologists recognized by 1941 that some rocks show macroscopic characteristics of quartzite without undergoing high-pressure metamorphism. These rocks experienced only lower temperatures and pressures linked to diagenesis yet cemented thoroughly enough for microscopic distinction.
Where are forearc basins located and what type of sand do they accumulate?
Forearc basins often accumulate sand rich in lithic grains and plagioclase within areas of high relief such as volcanic arcs or continental rifting zones. Intracontinental basins and grabens along continental margins serve as common sites for deposition alongside these forearc settings.
Why is orthoquartzite distinct from metaquartzite in terms of formation history?
Orthoquartzite distinguishes itself from metaquartzite because it formed through diagenesis rather than actual regional metamorphism involving great heat and pressure. This sedimentary rock often contains 99 percent SiO2 with minor iron oxide traces while preserving original texture and sedimentary structures.
How does the Dott's 1964 classification scheme define the boundary between arenite and wacke?
Dott set the boundary between arenite and wacke at 15 percent matrix content where arenites types have less than 15 percent clay matrix between framework grains. Wackes are sandstones containing more than 15 percent clay matrix between framework grains which affects their textural cleanliness.