BASIN SCALE PEPERITES IN CRETACEOUS COALS; HOST ROCK FLUIDIZATION CA. 30 MILLION YEARS AFTER LITHIFICATION

High level late Oligocene/Miocene sills intruded into late Cretaceous/early Tertiary coals within the Raton Basin of Colorado, demonstrate that hot fluidized magma/sediment “peperite” mixtures can develop several tens of million years after deposition and lithification. Outcrop, subsurface core, petrophysical and petrographic data, indicate that sill bodies actively liquefied coals during the emplacement process, forming complex and often chaotic brittle and ductile melt peperite intercalations tracable over 10's of sq. kilometers. It is believed that preferential geomechanical contrast, coal plasticity (under high temperature) and coal volatilization all combined to fluidize the host rock, while stable insulating vapor-film development coupled with a coked carapace reduced the effects of phreatic and pyroclastic disruption, allowing insulation of the thin (1cm - 1m) intruding bodies. New observations suggest that not all peperite deposits require a generic link to contemporaneous intrusion within unconsolidated or poorly lithified wet sediments, while melt emplacement mechanisms and intrusion distribution may in certain circumstances be strongly controlled by host rock lithology.