Lithofacies Architecture and Construction of Volcanoes Erupted in Englacial Lakes: Icefall Nunatak, Mount Murphy, Eastern Marie Byrd Land, Antarctica
Mount Murphy is a large Miocene shield volcano flanked by several small basaltic satellite centres that were erupted beneath a thick (> 200 m) ice sheet. Three empirical models illustrating the hydraulic evolution of glacio-volcanic systems are deduced from glacier physics, with distinctly different implications in each case for the resultant lithofacies architecture. Glacier hydraulic considerations and facies analysis are used to describe the evolution of one of the satellite centres (Icefall Nunatak). The nunatak was constructed from several vents during three main stages. Each stage demonstrates different aspects of englacial volcano construction, mainly in a flooded vault or lacustrine setting. An initial mainly effusive phase was dominated by lava and cogenetic joint-block breccia, and eruption was probably confined mainly within an englacial vault or lake (stage I). Renewed activity, at a different vent and beneath a re-established ice sheet (stage II), began with coarse sediments flushed away subglacially. A subaqueous tuff cone was then constructed in an englacial lake, from explosively erupted coarse glassy tephra probably produced mainly during sustained eruptions and distributed by high-density turbidity currents. Fine detritus is common only in the basal tuff cone unit, possibly as a result of lower, denser (largely subaqueous?) eruption columns. A spectacular slope failure is represented by numerous large blocks, which were displaced to low elevations on extensively fractured tuff cone flanks, and the failure event may have initiated zones of high pore-water discharge. Stage II culminated with two phases of lava delta progradation, indicating that the volcanic edifice ultimately penetrated the entire ice-sheet thickness and that the vent became emergent. Stage III commenced with lava effusion, probably through a thin re-formed cover of permeable snow and firn. A small cinder cone was also constructed and was partially palagonitized because of its structural position on top of a water-saturated volcanic pile and likely presence of vent intrusions driving hydrothermal circulation.