The Seismic Architecture and Geometry of Glacial Grounding-zone Wedges from Greenland and North Africa

Carole Decalf

Grounding-zone wedges (GZW) form where still-stands during ice-sheet retreat across polar shelves allow sediment build-up at the grounding-zone through delivery of deforming basal debris from fast-flowing ice streams. They are asymmetric in the ice-flow direction with steeper ice-distal (lee) sides. GZW dimensions are controlled by sediment flux, still-stand duration, cavity shape, and ice-stream width. Ice shelves beyond the grounding-zone appear to restrict accommodation space, preventing formation of high-amplitude ridges. GZW are mainly transparent or chaotic seismically, resulting from delivery of diamictic debris and reworking of sediments during minor ice oscillations. Off-lapping reflectors represent progradation into an ice-roofed cavity. Reflector truncations at the base indicate erosion during initiation. Channels are sometimes observed; meltwater flow is under high pressure. V-shaped incisions suggest high-energy flow. The observed architectural complexity suggests subglacially derived diamictic debris, sorted sediment in internal channels and turbidite deposits in a more ice-distal position. Recent study of NE and NW Greenland shelves reported and characterized several GZW and their dimensions; for example, sediment thickness, length in the direction of former ice flow, and ice-marginal width. GZW internal structure and geometry are examined and then compared with the geometry and sedimentology of examples from Late Ordovician glacial deposits onshore North Africa. GZW are indicators of periods of ice-sheet marginal stability, although retreat across a shelf between the locations of GZW may be rapid. The observed series of Algerian GZW thus records the episodic retreat of ice during deglaciation across a Late Ordovician continental shelf.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013