Ice-marginal environment and ecosystem prior to initial advance of the late Palaeozoic ice sheet in the Mount Butters area of the central Transantarctic Mountains, Antarctica

Basal rocks of the Upper Carboniferous to Lower Permian Pagoda Formation at Mount Butters provide an unusual view of periglacial conditions in the cen tral Transantarctic Mountains region prior to the initial advance of the Go ndwanide ice sheet. These rocks were deposited on a high relief unconformit y that developed on granite. Deposition within relief on the unconformity, possibly in the lee of a granite buttress, protected the rocks from erosion during subsequent overriding by the ice sheet. The succession reflects dep osition in a glacial-fed to ice-contact lake that contained a freshwater cr ustacean fauna. Centimetre- to decimetre-scale basal layers include breccia and coarse-grained sandstone. The occurrence of breccia resting on weather ed granite suggests sedimentation as scree and as mass flow deposits. Overl ying decimetre-to metre-scale stratified diamictites interbedded with metre -scale, coarsening-upward successions of siltstone to cross-laminated sands tone suggest lacustrine deposition by suspension settling, rain out of ice- rafted debris, and deltaic progradation. Thin zones with abundant conchostr acans and/or with prolific trace fossils, in addition to less common remain s of other crustaceans, attest to the presence of a low diversity benthic f auna. Conchostracans are concentrated in a series of thin beds that reflect moderately lengthy, perhaps seasonal, periods of free-flowing water. Patch y vertical and lateral distribution of intense bioturbation and profuse tra ce fossils probably reflect repeated colonization events during times of fa vourable environmental conditions. Massive diamictite overlies the basal ro cks and indicates that the ice-marginal lake was subsequently overridden by the late Palaeozoic ice sheet. Occurrences of lodgement till, glacitectoni te and deformation till suggest deposition from temperate or warm-based ice , whereas underlying lacustrine and deltaic deposits, along with a crustace an and trace fossil fauna, suggest temperate periglacial conditions. Previo us studies have stressed that upper Palaeozoic glacigenic deposits in Antar ctica, and in Gondwanaland, record deglaciation events. In contrast, rocks at Mt. Butters provide an unusual glimpse into an ice-margin lake and its f auna just prior to ice sheet advance.