Processes and products of grounding-line fans from the Permian Pagoda Formation, Antarctica: Insight into glacigenic conditions in polar Gondwana

Abstract

Glacimarine grounding-line fans are subaqueous depositional features that form due to the expulsion of sediment-laden subglacial waters at the grounding-lines of temperate tidewater glaciers. Few studies of ancient grounding-line fans exist. This study describes and interprets several ancient glacimarine grounding-line fans and their physical characteristics using the Lower Permian Pagoda Formation along Tillite Glacier in the central Transantarctic Mountains, Antarctica as a deep-time example. Wedge-shaped sandstone bodies are used to identify major depositional processes and products that occurred during fan formation. The depositing processes on these fans operated in tandem; as tractive freshwater discharge from beneath the glacier(s) was progressively replaced down the fans, in the direction of flow, by buoyant upwelling and the development of overflow plumes. Tractive flow deposits are characterized by trough and climbing cross-stratification, while deposits resulting from settling from meltwater plumes consist of horizontal laminations and abundant soft-sediment deformational structures (load, flame, and diapiric structures). Secondary features, such as thrust and normal faults, resulted from advance and retreat dynamics of the ice front; whereas, mass flow deposits (convolute bedding, recumbent and S-shaped folds, slumps and slides) resulted from resedimentation. Because glacimarine grounding-line fans require specific depositional conditions to develop, their identification is useful in constraining environmental conditions in the rock record. For the Pagoda Formation, the occurrence of grounding-line fan deposits is significant, as these strata were originally interpreted as deposits of a large terrestrial ice sheet centered over the location of the present Transantarctic Mountains that extended outward and covered much of Gondwana for up to 90millionyears. However, this study demonstrates that these rocks were deposited in a glacimarine setting. Results, combined with other recent studies, suggest that multiple small temperate ice sheets occurred in South Polar Antarctica during the late Paleozoic ice age rather than a single massive ice sheet.