The Eocene Volcaniclastic Sejong Formation, Barton Peninsula, King George Island, Antarctica: Evolving Precursory Fire Fountaining to Vulcanian Eruptions

Abstract

The 5리。ng Formation (100-200 m thick) represents a newly recognized Eocene volcaniclastic unit in Barton Peninsula, King George Island, West Antarctica. Detailed field mapping and lithofacies analysis indicate that the formation can be subdivided into three distinct facies associations (FA): (1) spatter/cinder-cone association (FA 1), (2) volcaniclastic-apron association (FA 11), and (3) distal-apron association (FA 111). FA 1, occurring at the base of the formation, comprises massive and jointed basalt lavas, which pass laterally into basaltic agglomerates and agglutinates through a transitional zone of fractured basalt lava f1ows. These field relations suggest fire-fountaining (Hawaiian) to Strombolian eruptions and subsequent emplacement of' ponded\"lavas filling the vents of smallscale spatter/cinder cones at the precursory phase of arc volcanism in Barton Peninsula. FA 11, unconformably overlying FA 1, is represented by verythick, tabular beds of basaltic to andesitic, welded t。 non-welded, tuff breccias and lapilli tuffs, emplaced by pyroclastic flows (Iargely block-and-ash f1ows), with rare intervening andesite lava flows. FA 11 indicates onset of the main-phase of explosive and effusive eruptions (Vulcanian), probably associated with repetitive extrusions and collapses of lava domes at the summit crater of a stratovolcano,and thereby formation of large volcaniclastic aprons. The changes in eruption styles probably resulted from generation of more evolved (intermediate) magma, possibly due to compositional differentiation of the parental magma, and interaction of the magma with groundwater.FA lll 's intercalated with FA 11 as thin lenses and is characterized by fluvial red sandstone/siltstone couplets,locally alternating with channelized mass-flow conglomerates.FA lll represents active hydrologic remobilizations during inter-eruptive periods and thereby development of ephemeral streams and floodplains in lowlands on and beyond