Geology of the central nimrod glacier area, Antarctica

Abstract

The area studied includes the Cobham, Holyoake and parts of the Swithinbank, Surveyors and Queen Elizabeth Ranges and forms part of the Trans-Antarctic Mountains. The sedimentary rocks can be divided into three major groups separated from each other by an unconformity. The oldest sequence, the unfossiliferous Beardmore Group, is probably late Precambrian. It consists of isoclinally folded sediments and metasediments and comprises the Cobham Formation of regularly bedded marble, quartzite and schist and the younger Goldie Formation of argillite and graded sandstone with rare schist and marble beds. Overlying this group with angular unconformity is the predominantly calcareous Byrd Group comprising the Shackleton Limestone and Starshot Formation. It is characterized by north-west- to north-trending open folds of large amplitude with near vertical axial planes. The Shackleton Limestone consists of limestone and marble with breccia and minor quartzite, conglomerate, sandstone and shale. Archaeocyatha found throughout the 8,300 m sequence date the formation as lower to middle Cambrian. Facies changes suggest biohermal development on a tectonically unstable shelf with a shoreline to the west. The Starshot Formation consists largely of graded fine conglomerate and sandstone locally interbedded with thin rhyolite and trachyte flows. Although the two formations comprising this group were not seen in contact they may be in part contemporaneous. The Beacon Supergroup was deposited on the Kukri Peneplain which cuts across both the Beardmore and Byrd Groups, and five formations are recognised within it. The newly erected Castle Crags Formation comprises siltstone, carbonaceous shale and cross-bedded sandstone laid down in local depressions in the Kukri Peneplain. As subsidence continued, a coarse quartzose cross-bedded sandstone—the Alexandra Formation—was deposited. This is overlain disconformably by the Pagoda Tillite, which includes poorly sorted ice contact deposits (tillite) and fluvioglacial sandstone. The overlying Mackellar Formation, of dark carbonaaceous shale and sandstone, was deposited mainly in proglacial lakes and streams after the ice front retreated. The youngest formation, the Buckley Coal Measures, consists of cross-bedded sandstone overlain by coal measures thought to have been deposited on a deltaic plain. Both the Beardmore and Byrd Groups have been intruded by post-tectonic microclinebiotite granite and tonalite of the Granite Harbour Intrusives. The Beacon Supergroup has been intruded by tholeiitic dolerite sills of the Ferrar Group. The Holyoake Gabbro, a quartz gabbro, intrudes the. Byrd Group and is thought to. be a deeper equivalent of the Ferrar Group. Intrusions of finely dlssemmated hematite and veins carrying arsenic sulphide are also found in several localities within the Byrd Group. © 1971 Taylor & Francis Group, LLC.