Sedimentology of rocky shorelines: 4. Coarse gravel lithofacies, molluscan biofacies, and the stratigraphic and eustatic records in the type area of the Pleistocene Hulopoe Gravel, Lanai, Hawaii

Abstract

During the Pleistocene, the south flank of Lanai was the site of repeated erosion and deposition as a consequence of glacio-eustasy superimposed on tectono-isostasy. Throughout this time, terrigenous and bioclastic gravel was generated along mainly rocky shorelines. Gravelly sediment was deposited in topographically low areas of the shoreface and inner shelf that later were uplifted isostatically. The sedimentary lithofacies and biofacies in these deposits are distinctive, but they represent a deposystem and suite of environments that hitherto have not been studied systematically. These unrecognised circumstances have fuelled controversies about the nature and significance of the Hulopoe Gravel, most notably the claim that it represents a mega-tsunami deposit. The Hulopoe Gravel in its type area displays a lithofacies succession that encompasses a complete disconformity-bounded transgressive–regressive cycle on a sand-poor rocky shoreline. This succession incorporates not only a distinctive vertical assemblage of rocky shoreline litho- and biofacies, but also internal disconformities that record episodes of erosion and non-deposition during shoreline migration. Repetition of the lithofacies succession imparts overall stratigraphic order to the Hulopoe Gravel. Four unconformity-bounded transgressive–regressive cycles correspond to eustatic highstands of Oxygen Isotope Stages 5 and 7, superimposed on continuing tectono-isostatic uplift driven by growth of the island of Hawaii. This study of the Hulopoe Gravel demonstrates that the sedimentary record of mixed volcaniclastic-carbonate depositional systems developed on rocky shorelines of tropical and subtropical oceanic island coasts can be preserved. This record may include depositional products of both transgressive and regressive shorelines. Although tsunami and giant submarine landslides are known to occur in the Hawaiian Islands, deposits and features that result from tsunami inundation above the shoreline, such as might be expected from landslide-related mega-tsunami, have not been recognised in the type area of the Hulopoe Gravel. Rocky palaeoshoreline records on oceanic volcanic islands may yield oceanographic and climatic clues that complement data obtained from other palaeoshoreline types.