The Sirius Group

Outcrops of glacigenic sedimentary rocks of the Cenozoic Sirius Group occur as erosional remnants scattered throughout the Transantarctic Mountains as far south as 86šS. The deposits are exposed in two typical settings: (1) as thin erosional remnants at high elevation in palaeovalleys or on flat mountain summits, or (2) as sequences up to around 100 m thick along the walls of broad trunk valleys occupied by large outlet glaciers draining the modern East Antarctic Ice Sheet.

A debate was ignited when Webb et al. (1984), based on recycled marine diatoms, linked the Sirius Group to glacial expansion after early Pliocene deglaciation and marine incursion of East Antarctica. Their model envisioned expansion and reduction of the Antarctic ice sheet throughout the Cenozoic. In contrast, Denton et al. (1984) attributed the Sirius Group to mid-Miocene overriding of the Transantarctic Mountains based on geomorphological evidence from the Dry Valleys. A dynamic ice-sheet hypothesis and a stable ice-sheet hypothesis developed, which represented contrasting views of Neogene Antarctic climate and glacial dynamics. Since the 1980s a substantial body of evidence has been published in favor of either of the two contrasting views. In Passchier (2001, 2004) I proposed a new hypothesis based on mineralogical and geochemical provenance of the Sirius Group to show that the glacigenic sequence is the result of multiple glaciations and link the sedimentary rocks to the glacial denudation of the Transantarctic Mountains. The map to the left shows the locations of samples incorporated in this study.

The illustration to the right shows a schematic cross-section of the central Transantarctic Mountains with large outlet glaciers currently eroding sequences of sedimentary rocks and dolerite sills, as well as crystalline basement. Older Sirius Group deposits crop out at higher elevation than the present surfaces of the outlet glaciers. These deposits were laid down during several stages of glacial denudation of the Transantarctic Mountains.

The sequences have significantly different mineralogical and chemical composition, as expressed in e.g. heavy mineral composition and the chemical index of alteration (CIA). Higher CIAs indicate more advanced conditions of chemical weathering prior to glaciation, and these Sirius Group deposits occur at the highest levels in the Transantarctic Mountains. The conclusion is then that the Sirius Group represents multiple phases of erosion and deposition, spanning a much longer age range than previously anticipated. Further field studies should therefore, in my opinion, look into the link between neotectonics and Cenozoic glaciation.

Research publications:

Passchier, S., 2004. Variability in geochemical provenance and weathering history of Sirius Group strata, Transantarctic Mountains: implications for Antarctic glacial history. Journal of Sedimentary Research, 74(5), 607-619.

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