Volcanology and sedimentology of the phreatomagmatic eruptive system of the Mawson Formation in Coombs Hills, Victoria Land, Antarctica

The volcanology and sedimentology of a phreatomagmatic eruptive system in the Coombs Hills, Victoria Land was investigated to determine how magmatism is initiated and evolves during continental rifting.
Basic field mapping and thin section analysis were used on the Mawson formation at Coombs Hills to distinguish between and map the realtionships among
a) non-explosive intrusive rocks,
b) deposits of explosive eruptions preserved in volcanic craters and
c) deposits of explosive eruptions preserved in areas surrounding volcanic craters.
The position and morphology of local vents within the Mawson formation were identified and used to establish beyond-vent and within vent deposition. These observations, combined with stratigraphic columns and appropriate lithofacies maps give a picture of the scale and evolution of the Coombs Hills volcanic system during the Middle Jurassic. Mapped units are defined by the depositional features (layering, grading, cross-stratification, etc, or their absence), by clast composition subdivided where possible by major changes in the abundance of different clast groups (based on field observations, thin sections, clast counts, etc), by the apparent rheology of non-volcanic clasts at the time of their incorporation and deposition (e.g. rigid or brittly deformed clasts vs. plastically deformed or partially disaggregated clasts), by the nature of volcanic clasts and by the nature of juvenile clast-host contacts. Combination of this information from this area and others will address the question of whether such phreatomagmatic activity is characteristic of specific environments of continental rifting.