A MARINE AND TERRESTRIAL SIRIUS GROUP SUCCESSION, MIDDLE BEARDMORE GLACIER QUEEN-ALEXANDRA RANGE, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

A succession of Sirius Group glacigene sediments which crop out along the western margins of the Beardmore valley between Cherry Icefall and Hewson Glacier, below The Cloudmaker, is designated as the stratotype of the Cloudmaker Formation. This new formation overlies a multiply g laciated pavement (Dominion Erosion Surface) cut into the Precambrian Goldie Formation, and is disconformably overlain by the Meyer Desert F ormation (Sirius Group). The Cloudmaker Formation comprises bedded aci d massive diamictons, bedded sands and silts, and laminated clays. Ass emblages of foraminifera occur throughout the Cloudmaker Formation and indicate that these basal Sirius Group sediments were deposited in br ackish glacial marine environments. The general absence of diatoms sug gest these marine waters were ice-covered, Similar marine assemblages are also present in basal Sirius Group sediments at Oliver Bluffs, Dom inion Range. Recycled marine diatom assemblages in the Sirius Group at the latter locality indicate that the host sediments have an age of < 3.8 Ma (Pliocene). The Cloudmaker Formation is placed in the Pliocene , although a latest Miocene age for the basal sediments cannot be rule d out. Stratigraphic, sedimentologic, and paleontologic evidence sugge sts that Beardmore valley was occupied by a fjord and tidewater glacie r system that extended at least 165 km through the Transantarctic Moun tains from the southwestern Ross Sea. The stratigraphy of The Cloudmak er Formation consists of a succession of members separated by disconfo rmities. It is hypothesised that these strata were deposited by a dyna mic valley glacier system that underwent a history of glacier advance and grounding alternating with glacier retreat and flotation over a ma rine water column. A combination of fjord basin sediment filling and s ea-level oscillations may also have influenced the pattern of glacier ice advance and retreat within Beardmore Paleofjord. The marine Cloudm aker Formation is overlain by the terrestrial diamicton dominated Meye r Desert Formation. At Oliver Bluffs, the Meyer Desert Formation diami ctons are interbedded with fluvial, and lacustrine sediments; successi ons that contain in situ vascular plant fossils: (principally the Sout hern Beech Nothofagus), mosses, and beetle remains, A Magellanic-type flora and fauna occupied the coastal margins of the Beardmore Paleofjo rd. The vertical transition from the basal marine Cloudmaker Formation to terrestrial Meyer Desert Formation provides a sea-level datum that can be used to assess the extent of post-Sirius Group tectonic uplift . Uplift rates at the Cloudmaker section, 90 km inland from the Transa ntarctic Mountain front or rift shoulder margin in the Queen Alexandra Mountain block are determined to be similar to 429 or similar to 350 m/Myr. This assumes a total uplift of 1331 m for the uppermost marine sediments of the Cloudmaker Formation, and maximum diatom-based ages f or the Sirius Group of <3.1 Ma or <3.8 Ma. Gross similarities in strat igraphy and interpreted paleoenvironments are apparent between The Clo udmaker succession (Beardmore Paleofjord) and the upper Miocene-Plioce ne successions at the mouth of Taylor Paleofjord, 800 km to the north. Contrasting present day elevational settings for these two widely sep arated marine successions indicates the post-Sirius rate of tectonic u plift for the Transantarctic Mountains has been significantly greater in the Queen Alexandra block.