ABSTRACT: Sedimentology of the Navosa Group (Miocene-Pliocene), SW Viti Levu, Fiji (South Pacific)

James E. Evans

The Navosa Group consists of the Vunamaoli Conglomerate, newly defined Sovi Sandstone, and Tamanua Formation. The first two units are lateral-facies equivalents in a coarse-grained, volcaniclastic, slope apron-submarine fan depositional system. These marine rocks are restricted to a narrow, linear fault-bounded basin in southwest Viti Levu that opened and then deformed in an episode of late Miocene transform tectonics. The marine rocks are unconformably(?) overlain by the nonmarine, early Pliocene Tamanua Formation.

Slope-apron deposits in the Vunamaoli Conglomerate consist of thick, laterally discontinuous, massive boulder diamictites, basement blocks, and slumps along the southern margin of the basin. The diamictites are interpreted as high-density gravelly mass-flow deposits resulting from basin-margin faulting. Along the basin edge these are interspersed with proximal submarine fan deposits. Inner fan regions contain prograding, lenticular, conglomeratic fan lobs composed of cobble-boulder diamictite, normal to inversely graded pebble-cobble conglomerate, pebbly mudstone, and coherent slump blocks with adjacent levee deposits and interdistributary channel thin-bedded turbidites. Mid-fan regions are composed of sheet-like to broadly lenticular high- and low-density sandy turbidites. Outer fan egions consist of poorly exposed, sheet-like, low-density sandy turbidites, and pelagic deposits.

The Tamanua Formation represents deposits from north-flowing, gravel-bed load streams and colluvium. Recognizable features include channels, cutbanks, scours, cluster bed forms, cross-bedded gravels, intraclasts, overbank deposits, and paleosols. Paleocurrent and provenance data from the Tamanua Formation suggest there was little topographic relief across the faults that influenced earlier marine deposition.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990