Coastal Processes and Landforms of Fiji: Their Bearing on Holocene Sea-Level Changes in the South and West Pacific

Abstract

In Fiji, the legacy of past sea-level changes, notably those of Holocene age, cannot be deciphered on modern coastlines without reference to the contemporary local tectonic regime. The structure and the dominant tectonic tendency for each constituent element of the Fiji islands during the late Quaternary include: (a) areas of predominant uplift and perhaps vertical creep, such as Cikobia and Taveuni; (b) areas where subsidence dominated between intermittent bursts of uplift, such as the Cakaudrove coast of Vanua Levu and parts of Viti Levu's south coast; areas where subsidence was dominant, such as the Yasawa and Mamanuca island groups and the Yasayasa Moala; and tdl areas which were (effectively) stable, such as the islands of the Lau Ridge and Lomaiviti. Evidence for low-level/Holocene shoreline displacement from Fiji's coasts can be interpreted in the context of local tectonics. Average shoreline displacement increases from areas where subsidence has been dominant through those which have been (effectively) stable to those where uplift has been dominant. The likeliest explanation for this pattern of shoreline displacement is one involving a uniform Holocene emergence, probably a sea-level fall and not a regional hydro-isostatic effect, being imposed on local tectonics. Dates from emerged Holocene shorelines in Fiji suggest that the Holocene transgressive maximum reached 1-2 m above present mean sea level some 3,000-2,000 years ago. Selected aspects of Holocene coastline development are described in the context of the sea level history established earlier. These include river-mouth and coastal progradation, which seems to have been most marked on stable or slowly subsiding coasts; coastal dune accumulation, which may have been more closely linked to Holocene eustatic and tectonic changes than hitherto suspected; beach rock and related deposits; lagoon infilling and coral-reef emergence; and the effects of catastrophic events, particularly storm surges, on Fiji's coasts. An account is also given of the explanations which the deduced Holocene sea-Ievel history provides for aspects of contemporary settlement history which have long puzzled prehistorians. Some discussion of shoreline movements in the last fifty years and in the future in Fiji concludes the paper.