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GSA Connects 2022 meeting in Denver, Colorado
Paper No. 122-4
Presentation Time: 2:20 PM
PACIFIC ATOLLS: SEDIMENTOLOGICAL DISCOVERIES IN ENIGMATIC
'CONGLOMERATE PLATFORM' SUPPORT HOLOCENE RAPID METER-SCALE
SEA-LEVEL OSCILLATION AND TSUNAMI DEPOSITION
HIGGS, Roger, DPhil, Geoclastica Ltd, Coventry, CV1 2NT, United Kingdom
Pacific atoll rims comprise a 'reef flat' (separating lagoon and ocean), partly covered by tabular bodies of coral rubble ~1.5m thick
jointly called 'conglomerate platform' (CP; e.g. Montaggioni et al. 2021), overlapped by sandy islets. The reef flat is dry at low spring
tide (tide range 0.5-2m). Exposed CP surfaces are dark (boring algae). Each CP body is bounded by an eroding scarp. Coral clasts in
the scarp are mostly 6.5-2.5kyBP.
Most workers assume the CP rubble source is the reef front. Instead proving a lagoon source: 1) well-exposed CP bodies on Google
Earth images (e.g. Majuro, Tarawa, Caroline, SW Fakarava) have an ocean-facing 'tulip' shape, i.e. a thin (<25m) 'stem' extends up to
600m from the lagoon edge and widens abruptly into a 'flower' 150-250m tall, never reaching the ocean; and 2) flowers protruding in
front of islets on both N and S rims of Majuro and Tarawa show cross-bed foresets (previously unreported in CPs) dipping oceanward
(my 2018-19 fieldwork). Stems also show likely cross-beds, expressed as nested crescents facing in- or outward on Google Earth
images.
Most authors interpret CP as inter- or subtidal storm beds. A new model follows. Around 3kyBP, the reef flat was exposed for centuries
('Pelham Bay Emergence', Fairbridge 1961). About 2.7kyBP, world sea level (SL) quickly rose ~5m in ~100y ('Abrolhos
Submergence'), to ~2m above modern SL, re-flooding the reef flat, removing any sand cover (wave attack) and outpacing coral
growth. After a threshold water depth was reached, the first tsunami to cross the newly submerged atoll rim induced contra-rotating
vortices behind the rim's tsunami-facing sector (now an 'underwater barrier' sensu Boshenyatov & Zhiltsov 2019). The vortices ejected
patch-reef talus bilaterally 'sideways' out of the lagoon.
SL later fell ~5m by ~1.7kyBP ('Florida Emergence'). Foram-rich sand exposed on the reef flat and lagoon fringe became heaped into
eolian dunefields (NB ~2kyBP forams in islet sand, Kayanne et al. 2011). An ensuing SL rise of ~3m by 1.0kyBP ('Rottnest
Submergence') re-flooded the reef flat, shrinking the dunefields (wave attack) to form today's islets. Where sand supply is high, islets
now prograde into the lagoon.
The oscillating SL model contradicts Kiritimati's 5-0kyBP 'stable' SL curve, whose clustered samples and wide error bars permit multi-
century data gaps.
Session No. 122
T145. Advances and New Voices in Marine and Coastal Geoscience
Monday, 10 October 2022: 1:30 PM-5:30 PM
407 (Colorado Convention Center)
Geological Society of America Abstracts with Programs. Vol 54, No. 5
doi: 10.1130/abs/2022AM-377547
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