Ross, Pierre-Simon; White, James D. L.; McClintock, Murray
(2008).
Geological evolution of the Coombs-Allan Hills area, Ferrar large igneous province, Antarctica: debris avalanches, mafic pyroclastic density currents, phreatocauldrons
Journal of Volcanology and Geothermal Research
, vol. 172
, nº 1-2.
p. 38-60.
DOI: 10.1016/j.jvolgeores.2005.11.011.
Résumé
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 mixed with detrital material to form the Carapace Sandstone in alluvial
and eventually lacustrine environments.
Eruptions then became largely effusive, producing hundreds of metres of flood lavas that covered the landscape
(‘Kirkpatrick time’). In places, lava flowed into ephemeral lakes to form pillow-palagonite breccias (base of sequence,
Carapace Nunatak) or pillow lavas (top of sequence, Coombs Hills). Several generations of Ferrrar intrusions were
emplaced during the course of these events; at least three can be distinguished based on field relations. New geochemical
data indicates that for the Ferrar province, magma involved in the explosive eruptions had the same major element
composition as that which produced shallow intrusions and lavas. We also note the possibility that flood lavas were fed by
plugs cross-cutting the Mawson Formation at Coombs Hills, rather than by major dikes extending to the surface. Finally, we
infer that eruption plumes were limited to the troposphere and that direct environmental impacts were thus likely restricted
to the southern hemisphere.
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