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 13.4D (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 reachin~ neutral buoyancy. Fan depocentres have been observed to accumulate at over 106 m 3 a-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.

The Effect of Meltwater Plumes on the Melting of a Vertical Glacier Face

“…Oceanographic control of sediment transport from subglacial discharge has been the object of many studies (e.g., Syvitski 1989;Powell 1990;Mugford and Dowdeswell 2011). The concentration of sediments suspended in the glacial meltwater is not high enough to overcome its buoyancy, and the sediment-rich water rises as a meltwater plume (Mulder and Syvitski 1995).…”

“…The discharged water often contains high concentrations of glacially eroded basal sediments and iron from the mechanical and chemical weathering beneath glaciers (Drewry 1986). The discharge of sediments and iron play an important role in shaping the landscape (Powell 1990) and marine ecosystem (Hartley and Dunbar 1938;Apollonio 1973;Arimitsu et al 2012) in the fjords.…”

“…Earlier studies speculated that the trajectory of meltwater plumes is determined by the relative strength of buoyancy and inertia of the discharged water (Syvitski 1989;Powell 1990). Under sufficiently large discharge, Syvitski (1989) argued that horizontal inertia could push the trajectory of a meltwater plume away from the glacier, and the plume would surface some distance away from the terminus.…”

“…Under sufficiently large discharge, Syvitski (1989) argued that horizontal inertia could push the trajectory of a meltwater plume away from the glacier, and the plume would surface some distance away from the terminus. In the case of lower discharge, the buoyancy force was thought to dominate the inertia, and the meltwater plume is expected to rise along the ice face (Powell 1990;Cowan and Powell 1990).…”

Late Wisconsinan Sedimentation in the Québec City Region: Evidence for Energetic Subaqueous Fan Deposition During Initial Deglaciation

“…5), suggesting that meltwater discharge increased just prior to gravel deposition. Finally, the flat base of the sheet, sheet-like geometry of the gravel beds and locally convex up nature of the top contact are not compatible with deposition in front of a small efflux, where channelization would be expected (Powell, 1990). In concert, these data are suggestive of sheetflooding from a nearby ice front during final stages of subaqueous fan deposition.…”