PRE-CRETACEOUS COALIFICATION IN SOUTHERN ISRAEL - CONTRIBUTION TO TIME-TEMPERATURE COALIFICATION MODELING

The possibility that coal rank can remain "frozen" or static over a long period of time has important implications for time-temperature coalification modelling. In the study area in southern Israel, the relationship between the 0.5-0.8% Ro vitrinite iso-reflectance contours and the strata (concordant in the Permian-Jurassic sequence, discordant to the disconformably overlying Cretaceous) indicates that coalification within the Permo-Triassic succession predated the development of the basal-Cretaceous unconformity. The fact that coalification has remained static since at least Late Jurassic time, i.e. for more than 140 million yrs, suggests that the given coal rank was stable under the temperatures that subsequently prevailed. Examples of preserved pre-deformation coalification are widespread elsewhere. Preservation of coal rank, static over long intervals due either to prolonged heating at constant temperature or to cooling, requires the existence of a state of coalification-temperature pseudo-equilibrium. Among the major implications is that the minimum temperature at which a certain coal rank can be attained is the pseudo-equilibrium temperature for this rank. Coalification would not proceed beyond this rank, regardless of the duration of heating, unless the temperature exceeded its pseudo-equilibrium level. Hence, since coalification reactions are irreversible, a measured coal rank is actually a "minimum thermometer" for the maximum temperature experienced by the rock. This further implies that time-temperature coalification modelling should be designed to allow for a discontinuous process and for the slowing-down of coalification rate with time.