Deposition and diagenesis of Miocene arc-fringing platform and debris-apron carbonates, southwestern Viti Levu, Fiji

Abstract

Carbonate deposition was widespread in the eastern Melanesian arc during the late Early to early Middle Miocene. In southwestern Viti Levu, Fiji, limestones of this age include a massive to crudely bedded reef or platform facies, and a well-bedded horizon of redeposited limestone rudites and calcarenite turbidites interbedded with hemipelagic lime mudstones. The redeposited limestones are considered to represent a series of carbonate debris aprons deposited in relatively deep water in the early Middle Miocene. The subsurface equivalents of the reef/platform limestones are likely to form the best potential hydrocarbon reservoir rocks in Fiji. Carbonate deposition coincided with an increase in epiclastic sediment yield to the forearc basin, indicating increased emergence and erosion of the arc. It is suggested that by bringing larger submerged areas into the photic zone, uplift of the arc was the main factor allowing widespread shallow-water carbonate deposition at this time. Primary pore space in the reef/platform carbonates was largely occluded in the marine phreatic zone by microcrystalline cements, isopachous fibrous or blocky cement fringes, and micritic internal sediments. Remaining primary pores and secondary biomouldic pores were filled by equant sparry calcite during meteoric phreatic and burial diagenesis. Reef- or platform-derived limestone clasts are abundant in the redeposited limestones, and there is evidence of an Early to Middle Miocene depositional hiatus accompanied by early subaerial exposure within the largest platform carbonate body. It is therefore suggested that the debris-apron carbonates were deposited as the result of emergence and erosion of the nearby shallow-water limestones, which may have been related to sharp but short-lived eustatic sea-level falls at 16 and 15 Ma.