DISTAL GLACIMARINE FACIES OF THE UPPER MIOCENE BATTYE GLACIER FORMATION, AMERY OASIS, NORTHERN PRINCE CHARLES MOUNTAINS, ANTARCTICA

Glacimarine strata of the upper Miocene Battye Glacier Formation (~130 m-thick), Pagodroma Group, exposed in the Amery Oasis of East Antarctica, provide an inland record of paleoenvironmental conditions in the Lambert paleo-fjord, during warmer-than-present climate in Antarctica. The formation overlies the Amery Erosion Surface (~300 m to ~270 m above sea-level) that formed during an advance of the Lambert Glacier. Ice-distal, glacimarine, diatom-bearing mud (up to 7-12% biogenic silica) and in situ articulated molluscs occur in the lower member. Three stratigraphic intervals of diatom-bearing mud are recognized from (1) in situ strata referred to as the McLeod Beds; (2) informal Bed A; and (3) and from glacially reworked clasts of diatomaceous mud near the base of the section. The diatom-bearing mud also contains sponge spicules and minor silicoflagellates and ebridians. Marine diatom biostratigraphy constrains the age of the beds between 10.7 and 9.0 Ma (late Miocene). This event is likely correlative with an open marine event in the Fisher Bench Formation on Fisher Massif, 50 km further south in the Lambert Embayment. Abundant benthic diatoms suggest deposition within shallow euphotic depths. The high abundance of intercalary valves of Eucampia antarctica from an interval of the McLeod Beds suggests there was less winter sea-ice than in Prydz Bay today. It is unlikely that sea-ice was perennial because the presence of Thalassionema spp. and Stellarima stellaris suggests that summer sea surface temperatures were too warm (>0 degrees C and >3 degrees C, respectively). Paleoclimate appears analogous to that in modern fjords of East Greenland, which is consistent with the depositional model proposed for the Pagodroma Group, and distinct from Antarctic conditions today. The diatom-bearing mud intervals were deposited on the Amery Oasis, ~250 km inland of the current Amery Ice Shelf edge, when the East Antarctic Ice Sheet was reduced in size. This deposit documents a significant deglacial event, and the persistence of warm, polythermal ice in Antarctica several million years after the mid-Miocene oxygen isotope shift that was inferred to reflect the onset of the present cold, polar ice sheet in East Antarctica.