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The Mount Gran area, encompassing Mount Gran and the ice-free Gran Valley (unofficial name) adjoining on the north, is located 77°S and 161°E in the rugged, glaciated mountains of South Victoria Land, Antarctica.
Most of the area is underlain by sedimentary rocks, intruded by, and sandwiched between, thick diabase sills. However, a basement complex of metamorphic and igneous rocks crops out in a small, isolated exposure at the northwest corner of Gran Valley. This complex consists of granitic rocks and gneisses which are cut by acid and basic dikes. The gneisses show foliation striking northwest with a nearly vertical dip. The age of the gneisses may be Precambrian or possibly Early Paleozoic.
The sedimentary rocks, all referred to the Beacon Sandstone Group, which ranges from Silurian to Jurassic in age, crop out on the southeast face of Mount Gran and in a thick belt rimming most of the 8-mile long, northeasterly trending Gran Valley. Dips are 3°-8° northwestward. In Gran Valley, the dominant lithologic type consists of light gray to white, well sorted, fine- to medium-grained, cross-bedded, quartzose sandstone between 1,000 and 1,300 feet in thickness. The lower half of this section includes ferruginous concretions as much as one foot in diameter. A few thin beds of green silicified siltstone occur in the upper half of the section.
At Mount Gran 130 feet of the quartzose sandstone is overlain by 470 feet of a nearly cyclic sequence of carbonaceous shale, coal, conglomerate, arkosic sandstone with interbedded shaly siltstone, and sandstone. Correlation between the quartzose sandstone at Mount Gran and Gran Valley is not clear because of lack of diagnostic fossils and discontinuity of exposures, but it is believed that the coal-bearing section stratigraphically overlies the Gran Valley section.
The youngest formation, the Ferrar Dolerite, probably of Jurassic or Cretaceous age, occupies the largest part of the Mount Gran area. Two large sills are present in Gran Valley. One, which probably overlies the basement complex and generally underlies the Beacon Group, forms the floor of the valley. The second overlies the Beacon Group in Gran Valley, forming the escarpment and skyline of the Mount Gran area. To the south, at Mount Gran, the two sills join, leaving only a remnant of Beacon sandstone which is dissected by diabase sills and dikes.
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