Landslide and debris flow deposits at the margin of a large vent complex, Mawson Formation, Allan Hills, southern Victoria Land, Antarctica

Lockett, Gillian M. (Gillian Mary)

In the Allan Hills, Victoria Land, Antarctica, Beacon Supergroup sedimentary rocks are juxtaposed with volcaniclastic rocks of the Ferrar-age Mawson Formation. The Mawson Formation and its correlatives along the Transantarctic Mountains contain an abundance of rather chaotic rocks consisting of once-glassy, juvenile basaltic grains mixed with broken country rock from the Beacon. Many of these rocks, including those at Allan Hills, have been interpreted in terms of a large vent complex that formed in advance of Kirkpatrick flood-basalt eruptions of the Ferrar large igneous province. New Watters peak, the Mawson Formation at Allan Hills can be divided into two units. One unit ("Mawson A") contains little or no juvenile basaltic material. Mawson B is more typical Mawson Formation tuff breccia, containing a variably high concentration of glassy juvenile material; it forms most of the Allan Hills and much of the nearby Coombs Hills. Mawson B is interpreted to be a intra-vent deposit, within a large shallow vent complex. Mawson A has a sharp to gradational contacts with unit B, and a gradational contact with Beacon Supergroup country rock. There are two main facies within Mawson A. The lower facies (Mawson Al) consists entirely of blocks of medium grained sandstone. Nearest the contact, blocks are of decimeter scale, rotated only slightly or not all, and beds within blocks can be traced from block to block for metres. Thin seams of sand matrix separate the blocks, and the sand consists of the same grains as the sand constituting the sandstone in the blocks. Further from the contact, blocks are commonly rotated and squeezed back together with little or no matrix between them. This unit is interpreted to have formed at the vent margin due to marginal subsidence caused by the removal of fragmented rock from the site of phreatomagmatic eruptions. A second facies (Mawson A2) lies further from the contact with the Beacon Supergroup, is more chaotic in appearance, and comprises predominately medium grained sandstone, but also contains clasts of coarse-medium sand pebbly congmplerate, fine grained sandstone, coal and glassy juvenile material. Clasts are randomly orientated and no internal structures are visible. This unit is interpreted to a debris flow Analysis on coal within this unit suggests that it was sourced from high in the Lashly Formation. The flow travelled into the vent complex implying it had negative relief. The flow was most likely the result of continued volcanic/tectonic seismicity causing failure along incipient weaknesses in the country rock surrounding the vent. After the flow volcanic activity continued and new intra-vent Mawson B was formed which incorporated some of the debris of the flow.

viii, 105 leaves : ill. (some col.), maps (some col., 1 folded) ; 30 cm.

2003Lockett

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