Gerald M. Friedman writes: the paper by Marshall et al. 1994 uses vitrinite reflectivities from Lower Old Red Sandstone (ORS) sediments of the Midland Valley of Scotland to interpret burial and uplift history. Vitrinite reflectivies give values of 1.3% for the Stonehaven Group (Kincardineshire) and 1.3% for the Strathmore Group (Perthshire). They conclude that 'the Stonehaven Group reflectivity values are considerably lower than expected under the 9 km of sediments aggregated for the Lower ORS of Kincardineshire and demonstrate a maximum burial of 3 to 5 km’.

The ORS correlates with the Devonian red beds of the Appalachian Basin. Here carbonized plant debris in uppermost Middle Devonian strata within the eastern Catskill Mountains of New York have been converted to anthracite (Friedman & Sanders 1982). The anthracite has a vitrinite reflectance of 2.5%. This implies a level of organic metamorphism (LOM) of 16. A similar degree of thermal activity is implied by the black colour (Staplin kerogen alteration index of 4) of the associated (possibly recycled) carbonized kerogen; a conodont alteration index of 4; and authigenic chlorite and local sericite fillings of the interparticle pores of interbedded sandstones. The specimens are about 350 Ma old. If 200 Ma is taken as the duration of exposure to the maximum geothermal temperature, then the LOM of 16 and other thermal indicators imply a maximum temperature of 190 °C. Using a geothermal gradient of 26°C km−1, a former depth of burial of 6.5 km is implied. Such a former depth is consistent with

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