Dr. Pierre-Simon Ross
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email: p_s_ross@hotmail.com
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PhD Thesis
Volcanology of the Mawson Formation at Coombs and Allan Hills, south Victoria Land, Antarctica.
{ complete PDF of thesis available upon request }
PhD Supervisor: Assoc. Prof. James D.L. White
Modified abstract
The Jurassic Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, underlying the Kirkpatrick flood basalts. In South Victoria Land, the mafic volcaniclastics are referred to as the Mawson Formation. At Coombs Hills (Fig. 1), the Mawson is interpreted as filling a large vent complex (Fig. 2), which was re-examined in detail to better understand vent-forming processes. Two contrasting types of cross-cutting volcaniclastic bodies were found in the complex, both of which are interpreted to have been forcefully emplaced from below into existing, non-consolidated debris. The first type consists of country rock-rich lapilli-tuff pipes. These are interpreted as traces of the passage of subterranean debris jets which originated when phreatomagmatic explosions occurred near the walls or floor of the vent complex, causing fragmentation of both magma and country rock. The second type of cross-cutting body consists of basalt-rich tuff-breccias and lapilli-tuffs (Fig. 3), some of which could have been generated by explosions taking place within pre-existing basalt-bearing debris, well away from the vent walls. Other basalt-rich zones, accompanied by domains of in situ peperite and coherent basalt, are inferred to have originated by less violent processes.
Fig. 3. Sub-vertical contact between juvenile-rich tuff breccia (right) and host lapilli tuff (left).At nearby Allan Hills, the Mawson can be divided into two informal members, m1 and m2. Member m1 is exposed only at central Allan Hills, consists essentially of sedimentary material from the underlying Beacon Supergroup, and is interpreted as a 180 m-thick debris avalanche deposit (Fig. 4). Most megablocks in m1 were derived from the late Triassic Lashly Formation, parts of which were probably only weakly consolidated in the Jurassic. Sandstone breccias dominate volumetrically over megablocks within the deposits. This indicates pervasive and relatively uniform fragmentation of the moving mass, and probably reflects the weak and relatively homogeneous nature of the material involved. The avalanche flowed into a pre-existing topographic depression carved into the Beacon sequence, and flow indicators reveal a northeastward movement. Sparse globular basaltic megablocks (Fig. 5) suggest that Ferrar intrusions played a role in triggering the avalanche.
Member m2, which is exposed at both central and southern Allan Hills, consists predominantly of metre-thick basaltic volcaniclastic layers that fall into three broad categories: (1) poorly sorted, coarse lapilli-tuff and tuff-breccia; (2) block-rich layers (Fig. 6); (3) tuff and fine lapilli-tuff. The former type is interpreted as the deposits of high-concentration pyroclastic density currents (PDCs), probably formed during the collapse of phreatomagmatic eruption plumes. Occasional block-rich layers probably were formed by both ballistic fall from local vents and pyroclastic flows, and the finer-grained layers were probably deposited by dilute PDCs. Dilute, moist turbulent currents were also likely responsible for the generation and deposition of large (≤4.5 cm) rim-type accretionary lapilli (Fig. 7). The thick layers are locally underlain by or interbedded with thin tuff ring-style volcaniclastic layers, and all the layers are underlain and invaded by basalt-rich tuff-breccias and lapilli-tuffs.
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Fig. 6: Graded block-rich layer in member m2 of the Mawson Formation at Allan Hills. |
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Fig. 7: Large rim-type accretionary lapilli in member m2, southern Allan Hills (scale bar in cm). |
