Abstract

Between 1940 and 1960, 17 field and laboratory classifications of sandstones were proposed in the North American geological literature. These classifications were based on combinations of several criteria including (1) mineral composition (provenance), (2) diastrophism, (3) mineralogical maturity, (4) textural maturity (index of sorting), (5) fluidity factor (measure of density and viscosity of the transport agent), and (6) primary structures.

Field and laboratory studies confirmed that provenance and mineralogical maturity are the only factors controlling the mineral composition of sandstones. Diastrophism does not control sandstone composition. Classifications which proposed terms denoting a combination of texture and composition were found to ignore the basic sedimentological observation that texture and mineral composition are independently behaving factors in sand deposition. The fluidity factor (per cent matrix indicating depositional fluid's viscosity and density) ignores that increasing or decreasing clay matrix can also be controlled by changes in fluid viscosity, granular abrasion, diagenesis, size sorting of original material in source area, and organic reworking. Primary structures cannot be used to define sandstone classes because the criteria selected for separating deep-marine from shallow-marine sandstones are known to overlap both depositional conditions.

The writer recommends that the approach to sandstone classification outlined by Folk (1954) and Van Andel (1958) be followed because separate terms are proposed for grain size, textural maturity, and mineral composition. Such an approach does not vitiate in any way the classical understanding of sandstone classes. It is logically consistent with a variety of data from both Recent sands and ancient sandstones where the independence of texture and composition are known.

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