Specimens of coalified plant debris in Tully-correlative strata of the Gilboa Formation (uppermost Middle Devonian) within the eastern Cat-skill Mountains of New York State have been converted to anthracite having 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 color (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 former interparticle pores of interbedded sandstones.
The specimens are about 350 m.y. old; if 200 m.y. is taken as the duration of the time 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 (17 °F · 1,000 ft−1), a former depth of burial of 6.5 km is implied. Such former deep burial is not usually inferred for the Catskills, but it is consistent with the idea that the thick (about 6.4 km or 21,000 ft) Carboniferous strata of northeastern Pennsylvania formerly extended northeast far enough to bury the Catskills.
The lack of metamorphism of the Paleozoic strata lying about 4.5 km beneath the Tully-correlative rocks and exposed in the adjacent Hudson Valley places low limits on the former geothermal gradient; this supports the concept of great depth of former burial of the Catskills. For example, 6.5 km of former burial and a geothermal gradient of 26 °C · km−1 imply a temperature of 307 °C for the base of the Paleozoic. By contrast, only 1 km of former burial requires a geothermal gradient of 170 °C · km−1, which would have subjected the base of the Paleozoic to a temperature of 955 °C, which is far higher than the 600 to 650 °C recently inferred for the Acadian-age metamorphism of the Taconic allochthon in southwestern Massachusetts and adjoining areas.