Quartz arenites: review and interpretation

Quartz arenites (orthoquartzites) are exceptional sands (Pettijohn et al., 1972) that, in the past, have formed extensive and commonly thick formations. They are somewhat enigmatic in that they do not appear to be forming at the present time. This paper examines the origin of quartz-rich sands, documents the range of environments in which they are found and suggests a model for their genesis. Quartz sand is most commonly formed by tropical weathering of coarse-grained quartz-bearing rocks such as granite. It may be deposited as texturally immature fluvial sandstones, generally under a humid climate. With drying of the climate, quartz-rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts. Quartz sand, when transported into shallow marine environments, becomes very well rounded through reworking by tidal currents. Such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings. Quartz-rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones. Precambrian orthoquartzites are commonly thick and widespread when compared to younger examples. The thinness of post-Silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles. There is an abundance of shallow marine and fluvial quartz arenites in the Early Paleozoic and the Cretaceous; this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents. The shallow marine examples are commonly associated with bituminous, sometimes phosphatic shale, glauconite and possibly chamositic ironstone. There is an abundance of eolian quartz arenites in the Permo-Triassic. This is related to a widespread dry continental climate during assembly of the Pangean supercontinent.