Geological evolution of the Coombs-Allan Hills area, Ferrar large igneous province, Antarctica: debris avalanches, mafic pyroclastic density currents, phreatocauldrons

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Ross, Pierre-Simon, White, James D. L., McClintock, Murray
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2008
• Subjects: Allan Hills; Antarctique; Coombs Hills; Ferrar; basalte de Kirkpatrick; grande province ignée; formation de Mawson; Carapace Nunatak; Victoria Land; Antarc*; Antarctica; Antarctique*
• Online Access: https://espace.inrs.ca/id/eprint/1900/; https://espace.inrs.ca/id/eprint/1900/1/Ross-et-al%28geological-evolution%292008_web%5B1%5D.pdf; https://doi.org/10.1016/j.jvolgeores.2005.11.011

The Jurassic Ferrar large igneous province of Antarctica comprises igneous intrusions, flood lavas, and mafic volcaniclastic deposits (now lithified). The latter rocks are particularly diverse and well-exposed in the Coombs-Allan Hills area of South Victoria Land, where they are assigned to the Mawson Formation. In this paper we use these rocks in conjunction with the pre-Ferrar sedimentary rocks (Beacon Supergroup) and the lavas themselves (Kirkpatrick Basalt) to reconstruct the geomorphological and geological evolution of the landscape. In the Early Jurassic, the surface of the region was an alluvial plain, with perhaps 1 km of mostly continental siliciclastic sediments underlying it. After the fall of silicic ash from an unknown but probably distal source, mafic magmatism of the Ferrar province began. The oldest record of this event at Allan Hills is a ≤ 180 m-thick debris avalanche deposit (member m₁ of the Mawson Formation) which contains globular domains of mafic igneous rock. These domains are inferred to represent dismembered Ferrar intrusions emplaced in the source area of the debris avalanche; shallow emplacement of Ferrar magmas caused a slope failure that mobilized the uppermost Beacon Supergroup, and the silicic ash deposits, into a pre-existing valley or basin. The period which followed (‘Mawson time’) was the main stage for explosive eruptions in the Ferrar province, and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years. Phreatomagmatic explosions were the dominant fragmentation mechanism, with magma-water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris. At Coombs Hills, a vent complex or ‘phreatocauldron’ was formed by coalescence of diatreme-like structures; at Allan Hills, member m₂ of the Mawson Formation consists mostly of thick, coarse-grained, poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents. Meanwhile at Carapace Nunatak, mafic clasts were ...