Thermal alteration of clastic organic particles as an indicator of contact and burial metamorphism in sedimentary rocks

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Abstract
Most sedimentary rocks contain dispersed solid organic grains of fine sand or silt size. These generally amount to a fraction of 1% of the rock mass, but may be concentrated after macerating the rock with acids. Study by transmitted or reflected light microscopy shows that these grains are largely plant fragments deposited like other grains of the rock; therefore they are termed phytoclasts. Phytoclasts are altered chemically and physically when exposed to increased temperature. This alteration is similar for phytoclasts from rocks near volcanic dikes, from laboratory bombs, from deep wells, and from strata formerly deeply buried. The degree of alteration is quantified best by measuring reflectance of selected phytoclasts—which increases with temperature increase and time of exposure. Reflectance‐temperature standards from laboratory bombs apply to dike metamorphosed phytoclasts, and temperature plots based on phytoclast reflectance in contact rocks sampled are like those expected from a simple model of heat flow. Suites of well core samples with measured temperature have phytoclast reflectance‐temperature gradients much higher than those of bomb samples. It is uncertain which reflectance‐temperature scale can be used to measure actual burial temperature at specific sites, but the relative temperatures are useful in stratigraphic and structural problems and indicate approximate rank of burial metamorphism.

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