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Definition of orthoquartzite

A clastic sedimentary rock that is made up almost exclusively of quartz sand (with or without chert), and relatively free of or lacks a fine-grained matrix, derived by secondary silicification; a quartzite of sedimentary origin, or a pure quartz sandstone. The term generally signifies a sandstone with more than 90% to 95% quartz and detrital chert grains that are well-sorted, well-rounded, and cemented primarily with secondary silica (sometimes with carbonate) in optical and crystallographic continuity with the grains. The rock is characterized by stable but scarce heavy minerals (zircon, tourmaline, and magnetite), by lack of fossils, and by prominence of cross-beds and ripple marks. It commonly occurs as thin but extensive blanket deposits associated with widespread unconformities (e.g., an epicontinental deposit developed by an encroaching sea), and it represents intense chemical weathering of original minerals other than quartz, considerable transport and washing action before final accumulation (the sand may experience more than one cycle of sedimentation), and stable conditions of deposition (such as the peneplanation stage of diastrophism); e.g., St. Peter Sandstone (Middle Ordovician) of midwestern United States.

See Also: quartzite

Ref: AGI


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