sand and sandstone

sand and sandstone From a geologist's point of view, the word ‘sand’ refers to a sediment containing 50 per cent or more of grains with a diameter between 0.063 and 2.00 mm, regardless of their composition; for example, quartz, skeletal debris, volcanic rock fragments, etc. However, it is also commonly understood, unless there is a qualifying adjective, that ‘sand’ refers to deposits formed of silicate minerals, of which quartz is the most abundant. Sand is part of a large group of sediments derived from the breakdown of silicate rocks which are termed detrital, clastic, terrigenous, or, more usually today, siliciclastic deposits. Sandstone is indurated (hardened) sand, composed of silicilastic grains, 50 per cent or more of which are of sand size, and are bound together by chemically precipitated cement or a recrystallized matrix of finer sediment.

Sand and sandstone are second to muds and mudstones in abundance, and form approximately 10–15 per cent of the total sediments of the Earth's crust. The size and degree of homogeneity of the population of grains is usually used to qualify the terms sand and sandstone; for example, very coarse, coarse, medium, fine, and very fine. A measure of the central tendency of the grain-size distribution is used in general description and statistical analysis; for example mean, median, mode, or modal size. The spread of the population of grain sizes is described by the term sorting. A well-sorted sediment has a small range of grain sizes, and a poorly sorted sediment the reverse. Sorting is the opposite to the term graded, with which it is sometimes confused. A well-graded sediment is the reverse of a well-sorted sediment and has a wide variety of grain sizes. Statistical terms such as skewness and kurtosis are also employed to describe the asymmetry of the grain population and the relationship of the central and peripheral parts of the grain-size distribution. Although these grain-size properties are initially dependent upon the source, they are mainly a reflection of the hydrodynamics of the environment in which the sand is deposited, and hence they can be used to determine conditions in the environment of deposition. Coarse-grained sands are characteristic of deposition in high-velocity regimes, whereas fine-grained sands are deposited in low-velocity regimes; the mean grain size of the sediment indicates the flow strength. Sands deposited in high-energy environments, such as beaches or dunes, or environments of stirring by persistent current or wave action, are usually better sorted than sands deposited by a flash flood on, say, an alluvial fan. Winds blowing over a beach remove only the finer-grained sands and leave a deposit with an increased coarse fraction; that is, a negatively skewed fraction, whereas aeolian sand lacks coarser material but may have a tail of fines; that is, it is positively skewed.

The geometry or shape of individual grains appears to be a property inherited from the original crystals of the silicate minerals in the source rock. In contrast, the sharpness of the edges of the individual grains or their angularity (i.e. roundness) records the history of the grain during transport and, to a lesser extent, during diagenesis. Constant collision of grains during transport smooths the sharp edges of the crystals and grains that were released during weathering. Wind is particularly effective in rounding grains, because of the greater force of impact in lower-density air compared to higher-density water. Nevertheless, the rounding process is very slow, and the famous Dutch geologist Ph. H. Kuenen claimed, on the basis of experimental work, that a cube of quartz would have to be transported over a distance equivalent to travelling several times around the Equator to produce a well-rounded grain, and, furthermore, that wind transport would need to be involved at some time during this process. The surface texture of an individual grain records the history of transport and deposition, although this can be modified or altered during diagenesis. Weathering, transportation by wind, and ice impart characteristic patterns of grooves, pits, and depressions which are revealed when viewed using an electron microscope.

The main components of siliciclastic sands and sandstones are quartz, feldspar, fragments of fine-grained rocks (lithic grains), mica, and accessory minerals. The last-named are often called ‘heavy minerals’, since they are usually denser than the more important constituents. They are usually present only in small amounts but may be concentrated by hydrodynamic processes to form accumulations which are sometimes of great economic importance—placer deposits. In addition, they can be very useful as tracers of the source of a sediment.

Sedimentologists have for many years been attracted by the possibility of using the composition of sand grains to determine the source rock and hence, in some situations, the tectonic evolution of the source area, a subject we can call palaeogeology. Initially, the more varied heavy minerals were used and a vast literature on this subject accumulated in the early part of the twentieth century. Little work was done on the apparently less promising major components, except by a few isolated workers. However, pioneer studies were initiated by Krynine in the United States to relate the relative abundance of the more common minerals of sands and sandstones to the geological character and evolution of some areas. This interest has developed enormously during the past few decades, largely because of the advent of plate-tectonic theory. It has become apparent that not only have sources of sediments, at various times in geological history, become buried or removed by erosion, but also that parts of the Earth's crust have been displaced, during plate-tectonic movements, vast distances from their previous positions. The record of their former presence is reflected in the composition of the sands they shed during erosion into adjacent basins; and the record left in sandstones is one of the few permanent records of the proximity of a particular source-land.

Studies of sands from the Mississippi River by Rittenhouse in the 1930s showed that sediments retained their source characteristics with only minor changes in mineralogy, even when transported thousands of miles. Later work by Potter and his students in the United States and South America has confirmed that this appears to be generally true for most large rivers, which are, in temperate regions, the most important suppliers of sand to sedimentary basins. These geologists also demonstrated that in tropical regions sands show considerable changes in composition during transport. Furthermore, sorting during transport and deposition according to size and density also fractionates the initial population into several populations of varying composition, and this has to be taken into account if reconstructions of source-lands are to be attempted. Diagenetic changes can alter the composition of the sands after deposition; for example, unstable components, such as feldspars and volcanic fragments, may be altered. Petrographic work will sometimes reveal the composition of the original grain, although this is not always possible.

The detailed understanding of the relationship between the composition of a sand or sandstone and the nature of the source rock, the weathering it undergoes, and the effects of transport, deposition, and diagenetic changes is still in its infancy. It has, however, been claimed by Dickinson and his co-workers in the United States that the influence of the source is, in most sands and sandstones, strong enough to persist through the vicissitudes of a grain's history. The relative percentages of sand grains composed of quartz, feldspar, volcanic fragments, etc. can be used to distinguish source-lands such as the stable cratons of continental interiors; continental and island arcs; and collisional mountain belts (Fig. 1). The study of the accessory minerals, which had gone out of fashion except in a few countries, notably the Netherlands, has received renewed attention. The introduction of the electron probe and the ability to date individual grains has increased the chances of identifying source rocks of sediments.

Sands undergo relatively small physical changes when buried except for an increase in bulk density that results from tighter packing of grains under overburden pressure. Sands buried to over 3000 m in the Gulf coast of the United States still retain porosities close to those of recently deposited sand. However, those containing large amounts of unstable lithic fragments or matrix show greater porosity reduction on loading owing to deformation of the grains and water loss from grains or matrix.

The most important diagenetic change in the conversion of sand to sandstone is the precipitation of mineral cement between the grains. Silica, calcium carbonate, and iron oxides are the most important, but in many deposits precipitated clay cements are common. Several generations of cement are present in some sandstones, sometimes replacing one another. During deep burial, overburden pressure causes solution at points of contact between grains, a process known as pressure solution, to produce sutured, interdigitating contacts between grains or their overgrowths. Although a considerable amount of the cement may be produced locally by solution of carbonate skeletal debris, siliceous organisms, volcanic material, and pressure solution, most must come from solutions driven into the sand from adjacent fine-grained mudstones during compaction and from metoric waters draining down into a sedimentary basin. The fine-grained matrix usually recrystallizes during diagenesis. Occasionally, however, aggressive waters from adjacent organic-rich sediments dissolve earlier cements and produce horizons of enhanced porosity and permeability in the subsurface. Although there are some striking examples of solution of some minerals by incoming fluids (intrastral solution), this is not a process of major significance.

Sandstones are classified into two main groups: arenites and wackes (Fig. 2). Arenites are siliciclastic sediments of sand size with less than 15 per cent of fine-grained material between the framework of sand grains (Fig. 2a). They are usually infilled to a varying degree by a precipitated mineral cement. Wackes are sand-sized sediments which contain more than 15 per cent matrix. The matrix has generally recrystallized to bind the sand grain together, to form a indurated rock (Fig. 3). Interpreting the origin of the matrix of such rocks has been a subject of great controversy because they may be partly primary, but in some sandstones may have been introduced later by infiltration of fine-grained materials or by the breakdown, compaction, and recrystallization of unstable sand grains. Greywacke (or graywacke) is an old term still sometimes used for a particular type of wacke especially in grey-coloured turbidite sequences. The term arkose or the prefix ‘arkosic’ are used for feldspar-rich sands or sandstones. The concept of maturity is often applied to sandstone description: those sandstones that are composed of well-sorted and well-rounded grains are said to be texturally mature, implying considerable transport; a mineralogically mature sediment is composed only of stable quartz with no unstable components, implying that intensive weathering has occurred in the source area or that the deposit has undergone many cycles of sedimentation. Some deposits may be both texturally and mineralogically mature.

Sand is transported by traction currents; it slides, rolls, and saltates along the surface. Fine and very fine sand are also transported in suspension. Sand can be fashioned into a wide variety of bedforms by the transporting fluid, to produce cross-stratified sediments which are invaluable in attempts to ascertain the location of a source area via this palaeocurrent indicator. Such phenomena can also be used to reconstruct past environments of deposition and palaeogeographies.

Sand is deposited in a wide variety of environments, extending from terrestrial settings such as fluvial and desert environments, through coastal beaches and dunes to shelf seas and the turbiditic deposits of the deepest parts of the oceans.

Sands and sandstones are important as raw materials for the construction industry (building stones, building sand, filter-bed sand), as well as for moulding sands and glass sands. In addition, places deposits in sands are often worked for industrial minerals (cassiterite, titanium ores, iron ores), as well as for semi-precious gemstones and precious minerals such as gold, diamonds, and emeralds. Because of their porosity and permeability, sandstones act as hosts for water, and they are important aquifers and reservoir rocks for oil and gas (e.g. the Permian sands of the North Sea). Valuable but small deposits of uranium and copper were precipitated in some geologically ancient sandstones.

G. Evans

Bibliography

Dickinson, W. R. and and Suczek, C. A. (1978) Plate tectonics and sandstone composition. American Association of Petroleum Geologists Bulletin, pp. 2164–82.
Pettijohn, F. J.,, Potter, D. E.,, and and Siever, R. (1973) Sand and sandstone. Springer-Verlag, Berlin.
Siever, R. (1988) Sand. Scientific American Library, New York.