Eocene to Miocene geology of southwest Viti Levu, Fiji

The oldest rocks in Fiji belong to the Yavuna Group of southwest Viti Levu. These represent part of an Upper Eocene to Lower Oligocene volcanic arc and were probably rifted to their present position during the formation of the South Fiji Basin in the mid-Oligocene. They are overlain with angular unconformity by the Upper Oligocene to Middle Miocene Wainimala Group which includes volcaniclastic turbidites, hemipelagic carbonates and vitric tuffs deposited in the Sigatoka sedimentary basin, and further south volcaniclastic rudites, intrusive/extrusive dacites and andesites, and minor reef/platform limestones representing the Wainimala volcanic arc. The Sigatoka basin is floored by Wainimala pillow basalts and Yavuna Group rocks and is interpreted as a intra/forearc half-graben that may have originated during mid-Oligocene arc rifting. During the Late Oligocene to Middle Miocene Fiji was on line with similar arc-forearc systems in the Lau Ridge-Tonga Platform and Western and Eastern Belts of Vanuatu as part of a continuous Melanesian Arc. Increased terrigenous input to the Sigatoka basin and the deposition of reef/platform limestones, including a major redeposited carbonate horizon, along the southern basin margin indicate increased emergence of the arc during the later Early and early Middle Miocene. This trend culminates in a regional mid-Miocene hiatus during which trondhjemite/gabbro stocks of the Colo Plutonic Suite were emplaced along the axis of the Wainimala arc. The sedimentary record in the Sigatoka basin recommences in the Late Miocene with the development of a sand-rich turbidite system (Tuva Group) fed from the uplifted post-Colo arc massif. These rocks rest on the Wainimala Group without angular unconformity. An intra CN9 nannozone angular unconformity separating the Tuva from the overlying Navosa Group marks the onset of sinistral strike slip tectonism as the Melanesian Arc began to break up with the opening of the North Fiji Basin in the Late Miocene