11. Ferrar Group: Dolerite Sills

Ferrar Group Dolerite Sills and the Dufek Intrusion... 

The sedimentary rocks of the Beacon Supergroup north and south of the ice-free valleys of southern Victoria Land contain an abundance of intrusive dolerite sills of the Ferrar Group. The sills are not popular with stratigraphers and paleontologists because they interrupted the continuity of the exposures of the Beacon rocks and caused contact metamorphism of reactive sedimentary rocks that contain diagnostic fossils. 

Fig. 4.19 The Bowers Terrane occupies the central area of northern Victoria Land flanked by the Wilson Terrane in the west and the Robertson Bay Terrane in the east The major rock types of this segment of the Transantarctic Mountains are identified by capital letters in alphabetical order A = Admiralty Intmsives, B = Beacon Supergroup, BT = Bowers Terrane, G = Granite Harbor Intrusives, P = Galhpoli Porphyries, R = Robertson Bay Group, W = Wilson Group. The Kirkpatrick Basalt has been onritted from this map for the sake of clarity and the Ferrar Dolerite sills are included with the Beacon Supergroup. The Lanterman and Salamander ranges constitute the eastern province of the Wilson Terrane (Adapted from Gair et al. 1969)...

The Exposure Hill Formation of northern Victoria Land occupies the same stratigraphic position as the Mawson Formation in the Allan and Coombs hills of southern Victoria Land. Both formations are composed of volcanic breccias and related pyroclastic rocks that formed at the start of the eruption of the Kirkpatrick Basalt and the intrusion of sills of the Ferrar Dolerite of the Middle Jurassic Ferrar Group which is the subject of Chapter 12. 

The Mawson Formation in the Coombs HiUs is locally overlain by the Kirkpatrick Basalt and was intruded by dikes and sills of the Ferrar Dolerite. This evidence could mean that the Mawson Formation predates the Ferrar Group. However, the presence of basalt and dolerite clasts in the Mawson Diamictite indicates that basalt was already erupting and that sills of dolerite had already intruded the underlying Beacon rocks when the phreatomagmatic explosions occurred. Therefore, the Mawson Formation is part of the Ferrar... 

Brief statements about the occurrence of basalt flows and dolerite sills of the Ferrar Group were included in the text that accompanied the geologic maps of the Transantarctic Mountains edited by Craddock (1969). For example, Grindley and Laird (1969), who mapped the geology of the Shackleton Coast of the central Transantarctic Mountains, stated ... 

The numerous basalt dikes which intruded the flows of southern Vestfjella are unaltered in contrast to the flows (Fumes and Mitchell 1978). Nevertheless, Fumes et al. (1982) reported that the initial Sr/ Sr ratios of ten dikes range from 0.70347 to 0.70687 and average 0.70460 0.00061 (2a). This result reinforces the evidence that the initial Sr/ Sr ratios of basalt flows and dikes of the Kirwan Volcanics in Queen Maud Land are not constant and that average values of this ratio are lower than the initial Sr/ Sr ratios of the basalt flows and dolerite sills of the Ferrar Group in the Transantarctic Mountains. 

Fig. 15.12 According to a proposal by Schmidt and Rowley (1986), the Antarctic plate was split by a rift that developed during the Jurassic Period. This rift caused the eruption of basalt flows and the intrusion of dolerite sills of the Ferrar Group in the Transantarctic Mountains along the raised margin of the rift valley. Marie Byrd Land, which was originally located adjacent to northern Victoria Land, moved to its present position in West Antttfctica by right-lateral motion along the Transantarctic rift system tmd the EUsworth Mountain block was rotated clockwise out of alignment with the Transantarctic Mountains at about the same time. These displacements of blocks of continental crust were caused by transform faults that developed in the spreading ridge that separated Africa from Queen Maud Land of East Antarctica (Adapted from Fig. 1 of Schmidt and Rowley (1986))...

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