ANCIENT STRANDLINES ON LANAI AND EVIDENCE FOR ISLAND UPLIFT

The highest elevated carbonate unit reported in Hawaii occurs on Lanai at 326 m where Harold Stearns reported collecting fragments of marine shells in fractures in volcanic rock. Using a copy of Stearns field map, we revisited the site but found no in-situ fossils-bearing material - only caliche filled fractures within the basalt. Thus we conclude that this should not be used as a valid stratigraphic horizon.

Lower down the slopes, two terraces at the 190 m and 170 m elevation are present in the interior of the Kalaukapo Crater. A coral-bearing boulder beach is present on 190 m terrace with sea stacks preserved on its SW margin. These boulders are brown-black, well-rounded, and lack weathering rinds. Oxisols are developed above this boulder-beach and Vertisols below. Stearns (1978) referred to this terrace as the Kalaukapo Shoreline and named the coral-bearing conglomerate at 170 m the Manele Bay Shoreline.

Further west our geologic studies reveal major carbonate units at several elevations between 100-200 m elevation the most prominent being at roughly 100 m along the Annapuka drainage. Beachrock and a black-brown well- rounded, well-sorted, boulder beach deposit (like that at 190 m in Kalaukapo Crater) is also present.

Between 100-200 m elevation in the Poopoo gullies a number of horizontal layers of coral bearing conglomerates, having the appearance of fossil "bathtub" rings, were found, generally restricted to the eastern sides of the gullies. The fossil assemblage of these upper units include species no longer present in Hawaii. Fossils are absent on interfluves between the identified horizons.

We interpret the outcrops of horizontal coral-bearing deposits, distributed in bathtub ring fashion as traces of ancient strandlines. Ages reported in the literature for coral include 350 ka at an elevation of 171 m, 250 ka at roughly 58 m, 211-230 ka at 28-35 m, and an age of roughly 120 ka at 23 m elevation. These horizons correspond to high-stands in the sea-level record derived from the Marine Isotope Stage record (specifically, 9.3, 7.5, 7.1, and 5.5). These correlations are consistent with an island uplift of 0.5 mm/y between 150-350 ka and 0.2 mm/y between 0-150 ka. The island uplift is likely to be associated with the plate motion carrying the island of Lanai across the lithospheric arch associated with the Hawaiian hot spot.