A PLIOCENE SHOALING BASALTIC SEAMOUNT - BA VOLCANIC GROUP AT RAKIRAKI, FIJI

At Rakiraki in northeastern Viti Levu, the Pliocene Ba Volcanic Group comprises gently dipping, pyroxene-phyric basaltic lavas, including pi llow lava, and texturally diverse volcanic breccia interbedded with co nglomerate and sandstone. Three main facies associations have been ide ntified: (1) The primary volcanic facies association includes massive basalt (flows and sills), pillow lava and related in-situ breccia (pil low-fragment breccia, autobreccia, in-situ hyaloclastite, peperite). ( 2) The resedimented volcaniclastic facies association consists of bedd ed, monomict volcanic breccia and scoria lapilli-rich breccia. (3) The volcanogenic sedimentary facies association is composed of bedded, po lymict conglomerate and breccia, together with volcanic sandstone and siltstone-mudstone facies. Pillow lava and coarse hyaloclastite brecci a indicate a submarine depositional setting for most of the sequence. Thick, massive to graded beds of polymict breccia and conglomerate are interpreted as volcaniclastic mass-flow deposits emplaced below wave base. Well-rounded clasts in conglomerate were reworked during subaeri al transport and/or temporary storage in shoreline or shallow water en vironments prior to redeposition. Red, oxidised lava and scoria clasts in bedded breccia and conglomerate also imply that the source was par tly subaerial. The facies assemblage is consistent with a setting on t he submerged flanks of a shoaling basaltic seamount. The coarse grade and large volume of conglomerate and breccia reflect the high supply r ate of clasts, and the propensity for collapse and redeposition on ste ep palaeoslopes. The clast supply may have been boosted by vigorous fr agmentation processes accompanying transition of lava from subaerial t o submarine settings. The greater proportion of primary volcanic facie s compared with resedimented volcaniclastic and volcanogenic sedimenta ry facies in central and northwestern exposures (near Rakiraki) indica tes they are more proximal than those in the southeast (towards Viti L evu Bay). The proximal area coincides with one of two zones where NW-S E-trending mafic dykes are especially abundant, and it is close to sev eral, small, dome-like intrusions of intermediate and felsic igneous r ocks. The original surface morphology of the volcano is no longer pres erved, though the partial fan of bedding dip azimuths in the south and east and the wide diameter (exceeding 20 km) are consistent with a br oad shield.