COMPLEX ORIGIN AND STRUCTURE OF THE OAHU CARBONATE SHELF: HAWAIIAN ISLANDS

Few studies of island carbonate shelves integrate emerged and submerged records of accretion over successive sea-level stands of the late Quaternary. Here we present an accretion model of the coastal plain and shallow submarine terrace of Oahu, a slowly uplifting, high volcanic (pre-atoll) island in the Hawaiian Archipelago. Reef framework accretion during MIS 7 (Waianae Reef) forms the massive principal structural component of the island margin from ~-5 to -20 m elevation. Subsequent framework accretion during MIS 5e ranges from ~+3 to -5 m and rests unconformably on the underlying Waianae Reef. New dates of late stage 5 corals and carbonate dunes establish the end of the last interglacial (MIS 5a-c) as a period of combined framework accretion, sediment production and eolianite accumulation leading to coastal plain development and reef extension. Holocene reef accretion is limited to areas of low wave energy where the antecedent foundation is absent or at greater depths, or on the seaward front of the shelf often in areas where subaerial weathering processes have opened deeper substrate on the antecedent surface. Early to middle Holocene framework accretion was rapid in these areas but terminated by late middle Holocene time except for accretion marked by sand and gravel accumulation and cementation forming “pile-up” reefs with debris cores. Modern coral growth occurs in the narrow window between wave base and the antecedent surface by utilizing a range of ecotypic growth forms (species plasticity). Most of the modern Oahu shelf is devoid of significant Holocene accretion and modern reefs experience significant natural limitations. Carbonate sand age reflects relatively low rates of sediment production. Recent carbonate sand deposition peaked under the late middle Holocene geoidal highstand (~ +1 to 2 m, ca. 2000 to 3000 yr BP). The fall of sea level over the period 2000 yr BP to present led to sandy accretion strand plains that further extended Oahu’s coastal plains. We infer modern reef accretion and sediment productivity are presently limited by this geologic framework.