Morphology and Stratal Geometry of the Antarctic Continental Shelf: Insights from Models

Abstract

Reconstruction of past ice-sheet fluctuations from the stratigraphy of glaciated continental shelves requires understanding of the relationships among the stratal geometry, glacial and marine sedimentary processes, and ice dynamics. We investigate the formation of the morphology and the broad stratal geometry of topsets on the Antarctic continental shelf with numerical models. Our models assume that the stratal geometry and morphology are principally the results of time-integrated effects of glacial erosion and sedimentation related to the location of the seaward edge of the grounded ice. The location of the grounding line varies with time almost randomly across the shelf. With these simple assumptions, the models can successfully mimic salient features of the morphology and the stratal geometry. The models suggest that the current shelf has gradually evolved to its present geometry by many glacial advances and retreats of the grounding line to different locations across the shelf. The locations of the grounding line do not appear to be linearly correlated with either fluctuations in the 6180 record (which presumably represents changes in the global ice volume) or with the global sea-level curve, suggesting that either a more complex relationship exists or local effects dominate. The models suggest that erosion of preglacial sediments is confined to the inner shelf, and erosion decreases and deposition increases toward the shelf edge. Some of the deposited glacial sediments must be derived from continental erosion. The sediments probably undergo extensive transport and reworking obliterating much of the evidence for their original depositional environment. The flexural rigidity and the tectonic subsidence of the underlying lithosphere modify the bathyrnetry of the shelf, but probably have little effect on the stratal geometry. Our models provide several guidelines for the interpretation of unconformities, the nature of preserved topset deposits, and the significance of progradation versus aggradation of shelf sediments.

Ross Sea West Antarctic Ice Sheet History

“…At 300 mbsf in Core AND-2A, a 300 ky disconformity equivalent to RSU4 (middle Miocene) suggests a shelf-wide advance of grounded marine-based ice originating from the EAIS during the MMCT (Figures F6, F7, F8) (De Santis et al, 1999;Passchier et al, 2011;Levy et al, 2016). This unconformity is the first unequivocal seismic evidence for a glacially carved trough in the central Ross Sea and is a key target of Site U1521 (Figures F7, F8) Ten Brink et al, 1995;Anderson, 1999;Anderson et al, 2018;Colleoni et al, 2018a). At DSDP Site 272, a ~400 m thick early-middle Miocene sequence of glaciomarine mudstone was recovered (Shipboard Scientific Party, 1975b) ( Figure F6).…”

Glacial Intensification During the Neogene: A Review of Seismic Stratigraphic Evidence from the Ross Sea, Antarctica, Continental Shelf

“…During major glacial periods, the extent of grounded ice expands toward the outer shelf ( Figure 4a). The ice-covered inner continental shelf becomes a zone of net erosion (ten Brink et al, 1995;Anderson, 1999). The trough and bank topography on the Ross Sea outer shelf provides strong evidence that six paleo-ice streams occupied these basins during the last glacial maximum (Hughes et al, 1981).…”

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“…The first unequivocal seismic evidence of a glacially carved trough in the central Ross Sea (key target of Site U1521) occurs at Unconformity RSU4 (mid-Miocene) and is interpreted as an expansion of a grounded marine-based ice stream originating from the EAIS to the west (Figures F7, F8) (De Santis et al, 1995;Ten Brink et al, 1995;Anderson, 1999). At DSDP Site 272, a ~400 m thick middle-upper Miocene sequence of glaciomarine mudstone was recovered (Shipboard Scientific Party, 1975b) (Figure F6).…”