Stratigraphy and Petrography, Mount Gran Area, Southern Victoria Land, Antarctica1
- Jarvis B. Hadley
Published Online: 3 APR 2013
Copyright © 1965 by the American Geophysical Union.
Geology and Paleontology of the Antarctic
How to Cite
Mirsky, A., Treves, S. B. and Calkin, P. E. (1965) Stratigraphy and Petrography, Mount Gran Area, Southern Victoria Land, Antarctica1, in Geology and Paleontology of the Antarctic (ed J. B. Hadley), American Geophysical Union, Washington, D. C.. doi: 10.1029/AR006p0145
- Published Online: 3 APR 2013
- Published Print: 1 JAN 1965
Print ISBN: 9781118655733
Online ISBN: 9781118668528
- Diabase sills and dikes;
- Mount gran area;
- Mount razorback section;
- Sedimentary rocks;
- Southern Victoria Land;
- Stratigraphy and petrography
The Mount Gran area of southern Victoria Land, Antarctica, includes Mount Gran (77°S latitude and 161°E longitude) and the 8-mile-long ice-free Alatna Valley adjoining on the north.
A basement complex consists of igneous and metamorphic rocks, which show near-vertical foliation striking northwest and which may be Precambrian or possibly Early Paleozoic in age. The complex is overlain by sedimentary rocks that crop out along the lower valley walls and dip 3°–8° northwestward. They are divisible into two mappable units: the Razorback Formation and the overlying Mount Bastion Formation.
The Razorback Formation, probably Devonian in age, is about 1300 feet thick and is almost wholly crossbedded quartz sandstone with rounded to subrounded, well-sorted, fine to medium grains. The major heavy minerals are opaques and rounded zircon, but very well-rounded tourmaline is important in the upper part of the Razorback, indicating a change in the source. The significant minor minerals are amphibole, orthopyroxene, and, especially, clinopyroxene. The depositional area was a broad alluvial plain over which a braided stream system flowed in a semiarid climate, where eolian sand was contributed to the water-laid deposits. The most suitable source terrain is a relatively close area of older elastic sedimentary rocks intruded by mafic dikes and sills.
The Mount Bastion Formation, about 500 feet thick, disconformably overlies the Razorback. It is dominantly arkosic but includes rocks ranging from conglomerate to shale. Quartz and feldspar are present as subrounded to angular, moderately sorted grains in crossbedded or thinly parallel-bedded strata. Most beds are micaceous and carbonaceous and several thin coal beds occur in the section. The major heavy minerals are angular garnet and tourmaline, and opaques. Metaquartzite pebbles are included in the conglomeratic units. Sediments derived from a metamorphic terrain were deposited in a swampy area with a warm or temperate climate favorable to growth of wooden plants. Characteristic elements of the Glossopteris flora indicate a Permian age for this formation.
The youngest formation, the Ferrar Dolerite, which is probably of Jurassic or Cretaceous age, consists of two large sills. One sill, which forms the floor of Alatna Valley, probably overlies the basement complex and generally underlies the Razorback Formation. The other overlies the sedimentary rocks along the upper valley walls and occupies the largest part of the Mount Gran area, forming the main escarpment and skyline of this area. It is associated with thin dikes and sills that intrude the Razorback and Mount Bastion formations. To the south, at Mount Gran, the two sills cannot be distinguished; a single large body includes only a remnant of Beacon rocks, which is dissected by diabase sills and dikes.