Coal-fragment rank and contact relationships of debris avalanche and primary pyroclastic deposits in the Mawson Formation, Ferrar LIP, Allan Hills, Antarctica

Abstract

Mafic volcaniclastic deposits are known from many large igneous provinces, and are generally ascribed to explosive phreatomagmatic activity that modifies the climatic effects of flood-basalt volcanism. The volcaniclastic deposits occur both as vent-complex infills and as more widely dispersed deposits that may be strongly localized in pre-LIP topography. At Allan Hills, in the Ferrar Province of Antarctica, mafic volcaniclastic deposits are separated from country rock of the Beacon Supergroup by country rock-derived sandstone breccias. These are coarse, monomict and matrix-poor near the contact, grade upward to polymict breccias derived from several country rock units, and are overlain by mafic volcaniclastic deposits. Fragments of Beacon coal show progressively higher rank away from the country rock contact, interpreted to indicate heating by basaltic fragments in the volcaniclastic deposits. The sequence overall is inferred to represent volcaniclastic deposits overlying a debris–avalanche deposit, with damaged in situ country rock at the base of the avalanche. Rocks in this area lie along one margin of volcaniclastic deposits extending over tens of kms 2. Relationships among them are a reminder that analysis of contact zones is critical to establishing the origin and context of LIP volcaniclastic deposits.