Age and uplift rates of Sirius Group sediments in the Dominion Range, Antarctica, from surface exposure dating and geomorphology

Abstract

The recent tectonic history of the Dominion Range is a key component in the debate over the age and significance of the Sirius Group glacial sediments. Outcrops at Oliver Bluffs along the Beardmore Glacier contain fossil wood ( Nothofagus), mollusks, fish and insects that have been biostratigraphically dated by close association with late Pliocene (<3.8 Ma) marine diatoms, implying a significantly warmer climate at that time. The sediments are inferred to have been deposited adjacent to a tidewater glacier at the head of an ancestral Beardmore Fjord and subsequently uplifted to their present elevation of ∼1800 m. Evidence for post depositional uplift at the Dominion Range includes the presence of glacial-marine sediments and normal faults which displace Sirius Group deposits up to 300 m. New surficial mapping of lateral moraines and fault scarps, combined with 3He surface exposure dating, place limits on the age and uplift rates of these key Sirius Group sediments. A series of Beardmore lateral moraines with exposure ages that range from <20 ka to >2 Ma are separated by only 130 m elevation, indicating little change in elevation during the interval and that Plio-Pleistocene tectonic activity in the Dominion Range has been minimal. Surface exposure ages indicate the Koski fault scarp formed prior to 2 Ma and that erosion rates of the semi-lithified Sirius tills exposed in the scarp are only ca. 2.5 m/Ma, consistent with pervasive cold, arid climatic conditions. Surface exposure ages of moraine boulders overlying Sirius till, and offset by the faults, are between 1.9 and 5.1 Ma, assuming no erosion and constant elevation. Models of cosmogenic 3He accumulation in the moraine boulders, that place the Sirius Group near sea level 3.8 Ma, show that the biostratigraphic age and the cosmogenic nuclide concentrations can be reconciled only with long (>4 Myr) exposure prior to deposition of the Nothofagus bearing units. This scenario is incompatible with the observed stratigraphy and climatic conditions necessary to support Nothofagus, and indicates these key Sirius group sediments are much older than 3.8 Ma. The conspicuous faults and grabens in the Dominion Range are interpreted as sackungen features attributed to gravitational failure resulting from oversteepening of the range flanks by glacial erosion by the Beardmore and Mill outlet glaciers rather than the result of tectonic uplift.