Geomorphological evolution of Lord Howe Island and carbonate production at the latitudinal limit to reef growth
Lord Howe Island is a volcanic island, rising to over 800 m, draped with Late Quaternary submarine and subaerial carbonate sediments. The island and neighbouring islets lie within a chain of seamounts and is presently at or close to the latitudinal limit to coral reef growth. Lord Howe Island and adjacent Balls Pyramid, composed of the basalts erupted around 6 million years ago, sit near the middle of broad shelves on separate peaks of one major volcanic edifice. The central part of the Lord Howe Island is covered by calcarenite that was deposited primarily as dunes (eolianite), but with isolated beach units. Uranium-series, amino acid racemisation, and thermoluminescence dating indicate that many of these were deposited during marine oxygen isotope stage 5. Eolianite units stratigraphically below the beach deposits are of penultimate interglacial, or in places perhaps older, age. Different suites of erosional landforms are associated with different lithologies. Towering plunging cliffs characterise the resistant Mount Lidgbird Basalt, in some cases fringed with large talus slopes. On less resistant lithologies or where nearshore topography means greater wave force as a result of waves breaking, there are shore platforms. Slumping cliffs abut broad erosional platforms on the poorly lithified calcarenite. A fringing reef on the western side of Lord Howe Island, the southernmost coral reef in the Pacific, is dominated by coral and coralline algae. Carbonate sediments veneering the shelf around the islands contain a more temperate biota. Located at the southern limit of reef-forming seas, but apparently having undergone erosion for much of its history outside of reef seas, Lord Howe Island provides insights into marine planation of volcanic islands close to what has been termed the Darwin Point. It represents the initial stages of fringing reef development on a volcanic island. Middleton and Elizabeth Reefs, north of Lord Howe Island, have the morphology of coral atolls and appear to be gradually subsiding. The Darwinian sequence, fringing reef to atoll, appears particularly compressed in this chain of islands. However, a fossil reef in water depths of around 30 m on the shelf around Lord Howe Island, of unknown age, implies a more complex history.