The Pagodroma Tillite
Geographic and stratigraphic location
The Pagodroma Tillite has two main areas of exposure within the northern Prince Charles Mountains, the Amery Oasis and Fisher Massif. Within the Amery Oasis area this semi-lithified glacigene deposit is widely distributed west of Beaver Lake where it unconformably overlies the Amery Group, forms a prominent exposure in the walls of Pagodroma Gorge, and comprises subhorizontal sequences which are mostly <30m in thickness. West of Radok Lake, the Pagodroma Tillite forms a more than 100m thick deposit which directly overlies Precambrian metamorphic basement rocks. South of the Amery Oasis, the Pagodroma Tillite is present on both the flanks and crest of Fisher Massif. In addition, it has also been sighted at Mt. Bunt and near Gorman Crags, both of which are located within the Aramis Range at some distance (>90 km) west of Pagodroma Gorge, as well as further south at Mount Collins. Within the southern Prince Charles Mountains, the glacial deposits observed at Mount Menzies may also comprise Pagodroma Tillite.
Where Pagodroma Tillite overlies the Permian fluvial sediments of the Amery Oasis region, between Beaver and Radok Lakes, its basal contact is characterised by fragmentation and disruption of the underlying Amery Group sedimentary sequence. However, where the Pagodroma Tillite directly overlies basement metamorphic rocks, it rests upon a glacially smoothed erosion surface. The exposed and scree-blanketed top of the Pagodroma Tillite represents an undulating ablation surface, the partial erosion of which is considered to reflect a subsequent, but little known, glacial event.
The Pagodroma Tillite is a marine lodgement tillite comprising massive, matrix-rich diamictite. Individual sequences measure up to several metres in thickness and are interbedded with thinner beds (<0.5m thick) of matrix-free pebble- and cobble-conglomerate as well as rare beds (<3m thick) of laminated silty sandstone. The Pagodroma Tillite comprises subangular to rounded clasts of both Amery Group and Precambrian basement lithology's, which are generally less than 1m in diameter and widely dispersed throughout a non-sorted matrix. The preferred orientation of the clasts is often indicative of a pronounced lodgement till depositional fabric. Occasional thin (<1m thick) matrix-rich, fine-grained diamictites contain shelly fragments and may represent gravity-flow deposits. Although no distinct stratification has been recognised within the Pagodroma Tillite, the basal few metres of diamictite overlying the sedimentary sequences of the Amery Oasis region are dominated by Amery Group clasts as well as a dark matrix which contains abundant carbonaceous material derived from the underlying Bainmedart Coal Measures.
Fossil record and age of deposition
The fossil assemblage of the Pagodroma Tillite includes reworked microfossils of both marine and non-marine (?lacustrine) origin. Since the age of this unit must be younger than that of the youngest identified reworked fossils, the presence of marine diatom flora of Upper Miocene (Denticulopsisdimorpha and Denticulopsishustedtii zones) to Middle Pliocene (Cosmiodiscusinsignis and Nitzschiakerguelensis zones) age has been interpreted to suggest a maximum Late Pliocene age of deposition. In addition to planktonic and benthic marine diatoms, other fossils identified within the Pagodroma Tillite include non-marine diatoms, silicoflagellate and radiolarian fragments, sponge spicules and reworked, but in situ, shelly macrofossil fragments.
Environment of deposition
Like the Sirius Group of the Transantarctic Mountains, deposition of the Pagodroma Tillite is considered to reflect a Late Pliocene or Early Pleistocene expansion of the East Antarctic Ice Sheet within a major ice drainage basin. In the case of the Pagodroma Tillite, this drainage basin is the Lambert Graben system. The reworked microfossil assemblages within the diamictite sequences are thought to derive from higher latitude sites located well below sea level within the Lambert Graben. The presence of marine fossils within the sediments at these sites are accounted for by proposed marine incursions into the Lambert Graben in response to previous glacial retreats. Subsequent expansions or advances of the grounded Lambert Glacier would then have resulted in the erosion of these marine sediments and their eventual incorporation into the Pagodroma Tillite. The fact that the base of this unit has been recognised at a range of altitudes, including sea level in the Amery Oasis area, 300m above sea level on the flanks of Fisher Massif and ~1450m above sea level on the crest of Fisher Massif, indicates that phases of deposition were interspersed with periods of considerable uplift and erosion.
Glacial erratics, which comprise basement metamorphic lithology's and measure up to 5m in diameter, as well as terminal and lateral moraines rest upon the upper ablation surfaces of the Pagodroma Tillite in the Amery Oasis area. Although such features support the idea of a younger glacial event, the limited extent of these younger glacial moraines indicates that the ice sheet expansion recorded by the Pagodroma Tillite was of a much greater magnitude than any subsequent glacial event in this region.