The ancient shorelines of Lanai, Hawaii, revisited

Abstract

Abstract Three ancient shorelines (the Mahana, Kaluakapo and Manele) have been described at elevations of 365, 190 and 170 m in the Kaluakapo Crater, Lanai, Hawaii [Bernice P. Bishop Mus. Bull. 237 (1978) 1]. Stearns' original observation of a fossil-bearing outcrop at 326 m was interpreted as an ancient shoreline. Subsequently, Moore and Moore (1984, 1988) [Science 226 (1984) 1312; Geol. Soc. Am., Spec. Pap. 229 (1988) 101] argued that Stearns' fossil evidence represented the highest inundation of tsunami waves associated with the collapse of the flanks of the Hawaiian Islands chain (the Giant Wave Hypothesis, GWH). Subsequently, Stearns' fossil-bearing swale site has never been observed. Geological field studies of Kaluakapo Crater were conducted to clarify the nature of the highest coral-bearing outcrops preserved on Lanai. These field studies found no support for a shoreline at 326 m. However, the field studies did document a shingle terrace with numerous coral clasts at 190-m elevation. We find in situ fossil-bearing marine deposits at 170-m elevation within Kaluakapo Crater. Furthermore, we observe: the undisturbed nature of marine deposits at 190 m, the preservation of fine-grained materials at 170 m, the absence of in situ deposits with a biological component between 200 and 365 m, the dearth of rounded boulders (lacking weathering rinds) between 200 and 365 m, and the lack of post-depositional burial of the terrace deposit (despite loose debris being present on the high ground surrounding the 190-m terrace). These observations lead us to believe that erosion by the hypothesized giant waves did not take place. Our observations are inconsistent with the “Giant Wave” tsunami inundation of the Kaluakapo Crater area. Thus, we concur with Stearns (1938, 1978) [Geol. Soc. Am. Bull. 49 (1938) 615; Bernice P. Bishop Mus. Bull. 237 (1978) 1], Jones (1993) [Jones, A.T., 1993. Elevated fossil coral deposits in the Hawaiian Islands: a measure of island uplift in the Quaternary. PhD Dissertation, Department of Oceanography, University of Hawaii, 1–274] and Grigg and Jones (1997) [Mar. Geol. 141 (1997) 11.] that the coral-bearing deposits on Lanai represent ancient shorelines and reflect a history of uplift of the island associated with lithospheric deformation of the sea floor around the Hawaiian hot spot.