Glacimarine processes at grounding-line fans and their growth to ice-contact deltas

Abstract

Grounding-line fans originate from subglacial and basal stream tunnels at grounding lines of glaciers terminating in a marine environment. During melt seasons discharge forms a turbulent jet beyond the efflux. Over the initial zone of flow establishment in the jet, it may remain in contact with the sea floor for up to 13.4 D ( D -tunnel diameter) along a runout distance, if velocity is constant. Plug flow in this zone deposits marine outwash sediment commonly having chaotic texture and graded and welded contacts. Texture may vary rapidly because of discharge pulses, and the jet may move laterally to produce sheet or cut-and-fill geometries. Imbricate gravel is deposited near the efflux, then farther out are sheet or weakly channelized finer-grained gravels and sands. In some instances a migrating barchanoid bar forms at the detachment zone and forms large-scale trough cross-beds. Beyond the detachment zone the turbulent jet becomes vertical and sediment cascades from it in a ‘veil’. This sediment is added to by flows from continuous failures of bed load sediment in the detachment zone. Resulting deposits are interstratified sediment gravity flows and thick, coarse end-members of cyclopsams. The jet changes to a plume when inertial forces become less than buoyancy forces. Turbulent vorticies within the jet can re-entrain particles into a buoyant plume. The plume remains vertical, although continuously spreading, until reaching neutral buoyancy. Fan depocentres have been observed to accumulate at over 10 ⁶ m ³ a ⁻¹ in temperate glacial areas. Eventually, at quasi-stable grounding lines, fans may aggrade to sea level to form ice-contact deltas. At incipient deltas when the delta plain is intertidal, coarse sediment is redistributed to the prodelta with each tidal cycle. Intense prodelta rhythmites mark the transition of a fan into a delta.