Mawson Formation, south Victoria Land

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 activity. 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 region. 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 Mountains. 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 Glacier region. Analysis of the rocks and field data are continuing and publications are now appearing in the literature.