The Eocene volcaniclastic Sejong Formation, Barton Peninsula, King George Island, Antarctica: Evolving arc volcanism from precursory fire fountaining to vulcanian eruptions

Abstract

The Sejong 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): (2) spatter/cinder-cone association (FA I), (2) volcaniclastic-apron association (FA II), and (3) distal-apron association (FA III). FA I, 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 flows. These field relations suggest fire-fountaining (Hawaiian) to Strombolian eruptions and subsequent emplacement of "ponded" lavas filling the vents of small-scale spatter/cinder cones at the precursory phase of arc volcanism in Barton Peninsula. FA II, unconformably overlying FA I, is represented by very thick, tabular beds of basaltic to andesitic, welded to non-welded, tuff breccias and lapilli tuffs, emplaced by pyroclastic flows (largely block-and-ash flows), with rare intervening andesite lava flows. FA II 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 III is intercalated with FA II as thin lenses and is characterized by fluvial red sandstone/siltstone couplets, locally alternating with channelized mass-flow conglomerates. FA III represents active hydrologic remobilizations during inter-eruptive periods and thereby development of ephemeral streams and floodplains in lowlands on and beyond the distal volcaniclastic aprons. These eruptive and depositional processes indicate a full emergence (subaerial setting) of the King George Island during the Eocene.