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lithostratigraphic unit (rock unit, rock-stratigraphic unit) A body of rock forming a discrete and recognizable unit, of reasonable homogeneity, defined solely on the basis of its lithological characteristics (see LITHOLOGY). A lithological unit may be sedimentary, igneous, metamorphic, or a combination of these. As with other stratigraphic units, lithostratigraphic units are defined according to type sections. Their boundaries are placed at surfaces of lithologic change, usually sudden but sometimes gradational. As the physical nature of the units reflects depositional environments rather than time spans, the boundaries of lithological units may be diachronous. Lithostratigraphic units are comparatively local in extent when compared to the world-wide compass of chronostratigraphic units. They are ranked in decreasing order of magnitude in supergroups, groups, formations, members, and beds. A diverse, but distinctive and interrelated body of rock that cannot be subdivided into any other lithostratigraphic unit is termed a ‘complex’. See also BIOSTRATIGRAPHIC UNIT; CHRONOSTRATIGRAPHIC UNIT; and STRATIGRAPHIC UNIT.
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