Abstract

Summary. The degree of organic metamorphism of carbonaceous matter is expressed in terms of the reflectance of vitrinite. Vitrinite reflectance is a function of temperature and time. A model relating vitrinite reflectance to temperature and time is proposed from published vitrinite reflectance—temperature—time nomograms. A plot of the logarithm of vitrinite reflectance versus depth gives a linear curve (for vitrinite reflectance greater than 0.3 per cent) in a sedimentary basin which has had a constant geothermal gradient throughout its tectonic history; the geothermal gradient (°C km−1) is equal to the slope of the curve multiplied by 194.8. The model is also applied to sedimentary basins with decreasing geothermal gradient with time (i.e. cooling continental-margin basins). The vitrinite reflectance model is demonstrated to be consistent with observed reflectance data and tectonic information from the Sydney, Bowen-Surat and Gippsland basins in eastern Australia.

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