Mawson Formation, south Victoria LandEntry ID: Elliot_mawson
Abstract: The research in the Allan and Coombs Hills area of south Victoria Land focused
on the mode of formation of exceptionally thick tuff-breccia deposits that are
part of the Jurassic Ferrar tholeiites, the magmatic rocks associated with the
break-up of Gondwana. Field investigations demonstrated that the tuff breccias
fill part of a major intrusive feature, or collapse ... caldera, at Allan Hills,
rather than having an unconformable relationship with the older Beacon strata
as had previously been reported.
The sequence of events involved magma emplacement as a thick sill, phreatic
interaction with the overlying Beacon strata, withdrawal of magma, and collapse
of unsupported roof strata. Subsequent resurgence of magma led to the in situ
formation of massive, unbedded breccia with varying amounts of primary basaltic
debris and sedimentary particles, followed by direct extrusion of thick, weakly
stratified tuff-breccia units and intrusion of breccia pipes and basalt plugs
and dikes. This example of volcanic activity is unusual for the role of magma
emplacement in the generation of coarse pyroclastic debris and for the scale of
the deposits so formed. Further, it offers an explanation for enigmatic
occurrences of similar thick but poorly exposed rocks in other parts of the
Transantarctic Mountains. Other flood basalt fields in similar tectonic
settings may also have been initiated with large scale phreatomagmatic
Some of the additional findings include:
1. Members B, C and D of the Triassic Lashly Formation are exposed on Coombs
Hills. Member D is thicker than at any other recorded locality, and also
passes up into beds containing silicic ash which is not known elsewhere in
south Victoria Land. The ash-bearing beds may be equivalent to the well
documented silicic tuff sequence of Early Jurassic age in the Beardmore Glacier
2. Phreatic intrusions cut the Lashly Formation on Coombs Hills and also on
Allan Hills. These clearly represent the effects of sill emplacement at depth
and explosive vaporization of groundwater. These features are probably more
widespread in the Beacon strata than currently recognized.
3. The recognition that the contact relationship between Beacon strata and
pyroclastic rocks is intrusive, not stratigraphic, necessitates revision of the
tectonic history of the Transantarctic Mountains in pre-Ferrar time. The
inference of 500 m of paleotopography prior to eruption of the Ferrar
pyroclastic rocks is no longer tenable. Significant uplift and erosion prior
to basaltic magmatism must have been distant from the present Transantarctic
4. Subsurface fragmentation and block rotation of sedimentary strata appear to
be a major processes in the zone of interaction between new basaltic magma and
Beacon country rocks. This appears to be the case also in the Beardmore
Analysis of the rocks and field data are continuing and publications are now
appearing in the literature.
(Click for Interactive Map)
Paleo Temporal Coverage
SOLID EARTH > ROCKS/MINERALS/CRYSTALS > IGNEOUS ROCKS
SOLID EARTH > ROCKS/MINERALS/CRYSTALS
SOLID EARTH > TECTONICS > VOLCANIC ACTIVITY > ERUPTION DYNAMICS > MAGMA COMPOSITION/TEXTURE
SOLID EARTH > TECTONICS > VOLCANIC ACTIVITY > ERUPTION DYNAMICS > PYROCLASTICS COMPOSITION/TEXTURE
SOLID EARTH > TECTONICS > VOLCANIC ACTIVITY > ERUPTION DYNAMICS > ASH/DUST COMPOSITION
ISO Topic Category
Email: elliot.1 at osu.edu
School of Earth Sciences
Province or State: Ohio
Postal Code: 43210
Atkins, C.B., Barrett, P.J., and S.R. Hicock. 2002. Cold glaciers erode and
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Ballance, P.F., Watters, W.A. 1971. The Mawson Diamictite and the Carapace
Sandstone Formations of the Ferrar Group at Allan Hills and Carapace Nunatak,
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... Borns, H.W.Jr., Hall, B.A. 1969. Mawson "Tillite" in Antarctica: Preliminary
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Grapes, R.H., Reid, D.L. and McPherson, J.G. 1974. Shallow dolerite intrusion
and phreatic eruption in the Allan Hills region, Antarctica. N. Z. J. Geol.
Geophys., 17, 563-577.
Elliot, D.H., Hanson, R.E. 2001. Origin of widespread, exceptionally thick
basaltic phreatomagmatic tuff breccia in the Middle Jurassic Prebble and Mawson
Formations, Antarctica. J. Volc. Geotherm. Res., 111, 183-201
Gunn, B.M., Warren, G. 1962. Geology of Victoria Land between the Mawson and
Mulock Glaciers, Antarctica. Bull. Geol. Surv. N.Z., no. 71, 157 pp.
White, J.D.L., McClintock, M.K. 2001. Immense vent complex marks flood basalt
eruption in a wet failed rift: Coombs Hills, Antarctica. Geol., 29, 935-938.
Creation and Review Dates
DIF Creation Date: 2005-08-24
Last DIF Revision Date: 2010-07-07