Holocene lagoonal sedimentation at the latitudinal limits of reef growth, Lord Howe Island, Tasman Sea

Abstract

The southernmost limit to coral reef growth occurs on Lord Howe Island (33°30′S, 159°05′E) where a discontinuous fringing reef flanks the western side of the island. Coring and radiocarbon dating indicate that carbonate sediments were first deposited within the lagoon around 6500 radiocarbon years BP coincident with sea level reaching close to its modern level. High-energy conditions dominated the reef with robust branching corals dominating the developing reef. Growth of the reef crest between 6000 and 5000 yr BP is inferred to have caused a reduction in the energy conditions within the lagoon resulting in mud deposition. During this period, sedimentation occurred at average rates of around 5 mm/yr, though locally up to 10 mm/yr, which infilled almost all the available accommodation space in the lagoon. By 4000 yr BP most of the lagoon had accreted to a level very close to the modern lagoon floor having accumulated over 11 m of sediment in the northern part of the lagoon and possibly up to 30 m in the south. Sediments younger than 3000 yr BP form a thin veneer over these older units with the main deposition being confined to embayments, the coastal plain and the infilling of blue-hole depressions. The reef and lagoon fringing Lord Howe Island appear to have developed their present form primarily during a phase of luxuriant growth in the mid-Holocene.

Introduction

Lagoons are important depositional systems within coral reefs, often occupying a large proportion of the total reef area. The general morphology of a lagoon can be attributed to a combination of morphological inheritance from the underlying pre-Holocene topography and a concentration of reef growth on the periphery of a pre-existing platform (Hopley, 1982). The evolutionary stage of the surrounding reef in relation to sea level may also have an important role in relation to the shape and sedimentology of the lagoon (Davies and Montaggioni, 1985, Neumann and Macintyre, 1985).

Investigations into the sedimentological history of Holocene reefal lagoons have indicated that infill occurred primarily during three depositional periods: (i) an initial high-energy initiation stage when the antecedent surface was flooded by rising sea level and coral growth was established across this surface; (ii) a low-energy stage when the reef crest became established as a result of keep-up or catch-up growth in relation to sea level; and (iii) a subsequent moderate-energy stage when the lagoon surface infills to wave base (Tudhope, 1989). Coral gravel and sand tend to dominate the earliest periods of infill, with mud only being deposited during low-energy periods. The majority of lagoonal infill, however, occurs under storm-induced high-energy conditions when coarse sand and gravel are reworked off the reef crest, although in some cases such as Mataiva and Takapoto atolls, in the Central Pacific, autochthonous sediments may still dominate (Adjas et al., 1990). The change to allochthonous sedimentation occurs as the reef crest approaches the sea surface and sediments are reworked laterally into the lagoon. The resulting sediments are shallowing upwards, characterised by a fining-upwards base and coarsening-upwards top (Scoffin and Tudhope, 1988, Tudhope, 1989). Reefs that appear to have at least partially infilled in this manner include Davies Reef (Tudhope, 1989), Heron Island (Smith et al., 1998), and One Tree Island (Davies and Kinsey, 1977, Marshall and Davies, 1982) in the Great Barrier Reef and the Cocos (Keeling) Islands in the Indian Ocean (Smithers et al., 1992, Kench, 1998).

The earliest stages of infill on these reefs have been sampled rarely as they lie at the base of thick sequences of Holocene sediment. Investigations of smaller lagoons, such as those associated with a reef crest close to shore, can provide the opportunity to investigate the entire sequence of sediments from initial deposition. A lagoonal depth of 10 m is often used to differentiate between fringing and barrier reefs (Milliman, 1974, Guilcher, 1988). Many fringing reef lagoons contain only a few metres of sediment such as those around the Grand Cayman Island in the Caribbean (Li et al., 1997, Kalbfleisch and Jones, 1998, Li et al., 1998) and therefore do not develop thick shallowing-upwards sequences typical of platform reef lagoons. Lagoons that are bounded by a subaerial landmass on one side have additional terrestrial influences associated with freshwater runoff and non-reefal sediment, which can adversely affect carbonate sedimentation both within the lagoon and on the reef.

This paper investigates Holocene lagoonal sedimentation on Lord Howe Island in order to determine whether lagoonal infill close to the environmental limits of reef growth conforms to the patterns established at lower latitudes. The lagoon and reef are small compared with the larger platform reefs that have been investigated on the Great Barrier Reef. The Lord Howe reef is shore-attached at its northern and southernmost ends enclosing a shallow lagoon (Slater and Phipps, 1977). Guilcher (1988) has classed it as a fringing reef with a wide boat channel.