GEOLOGY AND ERUPTIVE HISTORY OF THE RABAUL-CALDERA AREA, PAPUA-NEW-GUINEA

Rabaul Caldera is the most recently active (1937-1943) of four adjoini ng volcanic centres aligned north-south through the northern extremity of eastern New Britain. Geological mapping after the 1983-1985 Rabaul seismic and deformation crisis has partially revealed a long and comp lex eruption history dominated by numerous explosive eruptions, the la rgest accompanied by caldera collapse. The oldest exposed eruptives ar e the basaltic pre-caldera cone Tovanumbatir Lavas K/Ar dated at 0.5 M a. The dacitic Rabaul Quarry Lavas exposed in the caldera wall and K/A r dated at 0.19 Ma, are overlain by a sequence of dacitic and andesiti c pyroclastic flow and fall deposits. Uplifted coral reef limestones, interbedded within the pyroclastic sequence on the northeast coast, su ggest that explosive eruptions in the Rabaul area had commenced prior to the 0.125 Ma last interglacial high sea level stand, The pyroclasti c sequence includes the large Boroi Ignimbrites and Malaguna Pyroclast ics both Ar-40/Ar-39 dated at about 0.1 Ma, and the Barge Tunnel Ignim brite Ar-40/Ar-39 dated at around 0.04 Ma. Few reliable ages exist for the many younger eruptives, These include Holocene ignimbrites of the latest caldera-forming eruptions - the Raluan Pyroclastics variously dated (C-14) at either about 3500 or 7000 yr B.P., and the ca. 1400 yr B.P. Rabaul Pyroclastics. At least eight intracaldera eruptions have occurred since the 1400 yr B.P. collapse, building small pyroclastic a nd lava cones within the caldera. A major erosional episode is evident as a widespread unconformity in the upper pyroclastic stratigraphy at Rabaul. Lacking relevant radiometric ages, this episode is assumed to have occurred during last glaciation low sea levels and is here arbit arily dated at ca. ?20 ka. At least five, possibly nine, significant i gnimbrite eruptions have occurred at Rabaul during the last ?20 ka. Th e new eruptive history differs considerably from that previously publi shed, which considered ignimbrite eruption and caldera collapse to hav e first occurred at 3500 yr B.P. Rabaul volcanism has been dominated b y two main types: (a) basaltic and basaltic andesite cone building eru ptions; and (b) dacitic, and rarely andesitic or rhyolitic, plinian/ig nimbrite eruptions of both high- and low-aspect ratio types. The 1400 yr B.P. Rabaul Ignimbrite is a type example of a low-aspect ratio, hig h-energy, and potentially very damaging eruption. Fine vitric ash depo sits, common in the Rabaul pyroclastic sequence, demonstrate the frequ ent modification of eruptions by external water probably related to ea rly caldera lakes or bays. Interbedding of these fine ashes with plini an pumice lapilli beds suggests that many early eruptions occurred fro m multiple vents, located in both wet and dry areas.