Litho-bound basin scale thin sill emplacement, Raton Basin, USA. 

The Raton Basin of Colorado and New Mexico host one of the world’s most astonishing high-level “basin-scale” sill network. Within the late Cretaceous / early Paleogene basin fill, coal seams are intruded by thin, discrete (0.2 to 4.0 m thick) and laterally continuous Oligocene/Miocene sills. Individual coal-sill packages are readily traceable in the subsurface over areas >250 sq.km. Outcrop, core, and petrographic data demonstrate that sill bodies actively fluidized bituminous coals during emplacement, forming chaotic complex brittle and ductile melt – host-rock coal slurry intercalations, with many features recording liquid-liquid immiscibility. This process resulted in the development of “Thuidite”, a genetic term introduced to describe rock types resulting from the thermal fluidization of previously consolidated “lithified” rock. Coal rank, reactive macerals, preferential geomechanical contrast, and coal volatilization likely all combined to effectively fluidize the host rock during magma emplacement, while a coked carapace effectively ensured heat retention, maintaining coal/magma plasticity over a broad time-space temperature spectrum resulting in a wide range of solidification effects. The author will document the complex coal/sill relationships from outcrop through the subsurface, outlining key magma emplacement mechanisms and regional 3D “plumbing” style. It being readily evident in the Raton Basin that by far the most important control on sill distribution is host litho-type. It being likely that in most all mature coal bearing basins, sills when present, will preferentially locate and propagate within individual seams often over many tens of kilometers.