The seismic architecture and geometry of grounding-zone wedges formed at the marine margins of past ice sheets

Abstract

Grounding-zone wedges form where halts of decades to centuries during ice-sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast-flowing ice streams. Thirty grounding-zone wedges were identified on two-dimensional seismic-reflection profiles across the continental shelves of NE and NW Greenland. Grounding-zone wedges close to the seafloor are probably of late Weichselian age; others are on paleoshelves, buried beneath thick prograding glacial sediments. Several Greenland-shelf grounding-zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water. Grounding-zone wedges are asymmetric in the ice-flow direction with steeper ice-distal sides. Typical grounding-zone wedges are approximately 5–20 km long and 50–100 m thick, with a lateral width of several tens of kilometers. Forty grounding-zone wedges from the Greenland, Norwegian, and Antarctica margins show considerable variability about these values. Grounding-zone wedge dimensions are controlled by sediment flux, the duration of halts in ice retreat, subice cavity shape, and ice-stream width. Low-angle ice-shelf cavity roofs immediately beyond the grounding-zone probably restrict vertical accommodation space, preventing formation of high-amplitude ridges. Grounding-zone wedges are mainly transparent or chaotic on seismic profiles, probably resulting from delivery of diamictic debris. Offlapping reflectors represent progradation into an ice-roofed cavity. Reflector truncations of grounding-zone wedge bases indicate erosion during initiation. Channels are present within some grounding-zone wedges; meltwater flow is under high pressure, and V-shaped incisions suggest high-energy flow. Channels are not ubiquitous within Greenland grounding-zone wedges and may represent nonsteady meltwater release, perhaps through lake drainage. Where ice-sheet mass loss is dominated by meltwater runoff, grounding-zone wedges probably contain more sorted sediment than those from Greenland. Simple models of grounding-zone wedge architecture are presented. Grounding-zone wedges indicate episodic rather than catastrophic ice-sheet retreat and are a mechanism for ice-shelf stabilization because wedge growth counteracts collapse induced by ice-sheet thinning and sea-level rise.