Doctor of Philosophy
School of Geosciences
Brooke, Brendan, Quaternary stratisgraphy and evolution of aeolianite on Lord Howe Island, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 1999. http://ro.uow.edu.au/theses/1978
Lord Howe Island is a remote subtropical island in the Tasman Sea, situated 700 k m eastnortheast of Sydney. It is composed of Miocene basalt and outcrops of aeolianite, a bioclastic limestone deposited as dune units with subordinate beach, palaeosols and reefal units. This thesis investigates the chronostratigraphy of the aeolianite, identifying a far longer Quaternary record of dune emplacement on the island than has been reported in previous studies. This study also reveals that the major periods of dune emplacement on the island are linked to glacio-eustatically controlled sea-level highstands. Past studies of the island have concluded that the dunes which comprise the aeolianite formed during periods of low sea level around the time of the Last Glacial Maximum, when the broad shelf surrounding the island was exposed.
The aeolianite comprises two lithostratigraphic formations, the Searles Point Formation the Neds Beach Formation. This classification revises the previous proposed lithostratigraphy of the aeolianite which recognised only a single formation. The diagenetically-mature lower unit, the Searles Point Formation, consists of well-cemented dune units comprising red algal, molluscan, micritic and recrystallised grains with minor proportions of foraminiferal and volcanic grains. Major units within the formation are capped by well-developed, clay-rich palaeosols. The upper formation, the Neds Beach Formation, comprises mostly very pale-brown, lightly- to moderately-cemented dune and beach units which exhibit only minor diagenetic alteration. Most units of this formation exhibit a mix of skeletal grains similar to that observed in the Searles Point Formation. The Neds Beach Formation includes several outcrops of beach deposits that record open coastal environments. The beach units are considerably more extensive than has previously been recognised. This study shows that the major aeolianite ridge on the island is predominantly composed of dune units of the Neds Beach Formation. Both the bedding structures within and the shore-aligned orientation of the ridge suggest much of the deposit was emplaced as a source-bordering transverse dune.
The Neds Beach Formation includes two members, the Middle Beach and Cobbys Corner Members. The Middle Beach Member is characterised by white dune units with a major proportion of miliolid foraminiferal grains and rare volcanic grains. This is distinct from the Cobbys Corner Member which comprises dune units with a major proportion of volcanic grains and brown micritic cement. The two members represent distinctly different shoreline environments on the east and west coasts that appear to have existed during the Last Interglacial.
Geochronological determinations using the uranium-series (U/Th), radiocarbon ( AMS 14C), thermoluminescence (TL) and amino acid racemisation (AAR, mollusc and "wholerock") techniques provide reliable chronologies for the lithostratigraphic units and help correlate disjunct outcrops. Speleothems from the Searles Point Formation range in age from approximately > 350 ka to 95 ka and dune and palaeosol units range from approximately 274 ka to 201 ka. Speleothems, corals, aeolianite and fossil land snails from the Neds Beach Formation range in age from -125 ka to > 40 ka.
Allostratigraphic mapping of the aeolianite indicates up to nine major phases of dune building in contrast to previous studies which recognised two phases. During each phase the emplacement of dunes was followed by a relatively long period of pedogenesis. These cycles of sedimentation have occurred since the Middle Pleistocene or possibly earlier. The Middle Beach Alloformation outcrops at all the main sites except North Bay and represents the final phase of Pleistocene dune sedimentation on the island, probably occurring at the end of oxygen isotope stage 5.
As on the islands of Bermuda and the Bahamas, most dune units on Lord Howe Island appear to have been emplaced during periods of high sea level in the Middle and Late Pleistocene. Dune emplacement occurs predominantly after the sea has flooded the platforms surrounding the islands during interglacials. Thus, carbonate sedimentation is linked to global eustatic changes in sea-level. In contrast to Bermuda and the Bahamas, during the last glacial cycle on Lord H o w e Island, aeolianite appears to have been more a product of sea-level highstands during oxygen isotope substages 5c and 5a rather than substage 5e, when sea level was approximately 20 m below present. During these periods, the island was surrounded by a shallow sea, 10 - 30 m deep, that formed a highly productive Heterozoan subtropical to temperate carbonate province.
The stratigraphic and geochronological findings presented in this thesis show Lord Howe Island possesses a long record of Quaternary coastal environments. This mid-ocean record provides important new palaeoenvironmental information for the southwest Pacific.