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Gillian Lockett

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

(MSc thesis completed February 2003, supervisor: Assoc. Prof. James White)

Abstract

In the Allan Hills, Victoria Land, Antarctica, Beacon Supergroup sedimentary rocks are juxtaposed with volcaniclastic rocks of the Jurassic, 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.

Near 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 an intra-vent deposit, within a large shallow vent complex.

Mawson A has a sharp to gradational contact with unit B, and a gradational contact with Beacon Supergroup country rock. There are two main facies within Mawson A. The lower facies (Mawson A1) consists entirely of blocks of medium grained sandstone. Nearest the contact, blocks are of decimeter scale, rotated only slightly or not at 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 predominantly medium grained sandstone, but also contains clasts of coarse-medium sand pebbly conglomerate, fine grained sandstone, coal and glassy juvenile material. Clasts are randomly orientated and no internal structures are visible. This unit is interpreted to be the deposit of a debris flow. Analysis of coal within this unit suggests that it was sourced from high in the Lashly Formation. The flow travelled into the vent complex, implying the vent 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 deposit.

Field Area

Abstract for Geological Society of New Zealand conference, 2002

Landslide Deposits At The Margin Of A Large Basaltic Vent Complex, Mawson Formation, Ferrar Supergroup, Allan Hills, Antarctica

G.M. Lockett, Geology Department, University of Otago, PO Box 56, Dunedin, locgi011@student.otago.ac.nz
J.D.L White, Geology Department, University of Otago, PO Box 56, Dunedin, james.white@stonebow.otago.ac.nz

 

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 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 previously interpreted in terms of a regional lahar field that formed in advance of Kirkpatrick flood basalt eruptions of the Ferrar large igneous province.

Near Watters Peak, the Mawson Formation at Allan Hills can be divided into two units. One unit ("Mawson A") contains fragmented Beacon strata, with little or no juvenile basaltic material, and is the focus of this presentation. "Mawson B" is more typical Mawson Formation tuff breccia, containing a variably high concentration of juvenile material; it forms mist of the Allan Hills and much of nearby Coombs Hills.

Mawson A is further subdivided into two facies. The lower facies (MA1) consists entirely of blocks of medium-coarse sandstone from the immediately underlying Beacon strata. Nearest the contact blocks are of decimeter scale, with block size decreasing exponentially to less than 5cm towards Mawson B. Thin seams of sand matrix separate the blocks; the sand consists of the same grains as the sand comprising the blocks. The blocks dip to the south, towards the vent system. The upper facies (MA2) is more chaotic in appearance, and comprises predominately medium grained sandstone along with blocks of siltstone, coarse grained pebbly sandstone conglomerate, coal and rare juvenile material. Clasts are randomly orientated and no internal structures are visible.

MA1 is interpreted to have formed on a peripheral ring fault surrounding a maar-diatreme system. As magmatic support within the vent system was withdrawn and downsag features formed causing the beds to dip into the vent and fragment. Fragmentation of the blocks due to eruptions within a growing vent would fragment the rock further causing the stratification seen. MA2 is the result of a debris avalanche, created by continued volcanic activity causing instability of country rocks and failures.