PACIFIC ATOLL ENIGMATIC 'CONGLOMERATE PLATFORM' DEPOSITED BY TSUNAMI DURING HOLOCENE HIGHER SEA LEVEL: SUPPORTIVE DATA FROM FAKARAVA, FRENCH POLYNESIA

On Pacific atoll rims, sandy islets perch on a Holocene-coral-rubble 'conglomerate platform' (CP) ~1.5 metres (m) thick. The CP often protrudes oceanward as a bare rock ledge, its top emergent at high tide, the distal scarp eroding by storm waves, and its surfaces darkened by algal boring. Below and oceanward of the CP is the reef-flat. My 2018-19 observations on Majuro and Tarawa atolls necessitated reinterpreting CP coral debris as ejected outward, from the lagoon (not inward from the forereef), by bilateral vortices induced by a tsunami passing overhead when no islets existed and sea level (SL) was ~2m higher than now, during Fairbridge's (1961) Abrolhos highstand (Higgs 2022 GSA abstract 10.1130/abs/2022am-377547). This model is reinforced by my 2023 studies of Fakarava and nearby atolls, where aerial images again show many CP bodies with a "tulip"-shape (Higgs 2022), a long (100s m) "stem" terminating in an oceanward-flaring "flower" (e.g. Google Earth at 16°23'14"S 145°40'37"W). Stems can again show "nested crescents" (e.g. SW Fakarava, 16°26'24"S 145°37'23"W); visiting one example I found oblique, near-symmetrical, round-crested bedforms (spacing ~1m, height at least 20cm; antidunes?) of coral-pebble conglomerate, poorly exposed (crests only) between remnants of bouldery CP cover up to ~1m thick. Other CPs on aerial images are simple fans, splaying from their apices at the lagoon edge and ending 10s m short of the ocean. These fans can merge laterally, forming a composite CP, straddled by an islet much longer (km-10s km) than wide (200-500m), like NE Fakarava, where the CP protrudes up to ~80m oceanward, its scalloped front (Google Earth) reflecting fan mergence. Examined at Teariki, this CP is bouldery rubble, patchily removed by erosion except for a distal fringe up to 15m wide and ~1m thick (prolonged sea-water contact slows cement dissolution by rain?). This fringe is a set of oceanward-dipping foresets (cf. Ebon, Majuro, Tarawa atolls; Curray et al. 1970; Newell & Bloom 1970; Higgs 2022) with shore-parallel strike (e.g. 16°07'59"S/145°35'55"W). Inboard of the fringe, erosional windows in the CP expose benthic-foram-rich grainstone of the reef-flat; usually <10cm are visible, with up to 40cm more in trough-like 'sags' ~3m wide (concave-up base), >50m long, and kinked. Sags suggest subsurface karstic-void growth, requiring a nearby rain-catchment (islet).