TGIF Seminar Series

December 5, 1998.

A Submerged Stage 7 Terrace and Reevaluation of the Late Pleistocene Stratigraphy of Oahu, Hawaii

Clark Sherman
Department of Geology and Geophysics
University of Hawai'i

Abstract:
    Results of sedimentologic and geochronologic investigations of a nearshore terrace require a reevaluation of the Late Pleistocene stratigraphy and relative sea-level/tectonic history of Oahu. The shallow submerged slope of Oahu consists of a shallow-dipping shelf extending from the shoreline out to the ~-20 m contour where there is a sharp break (wall) down to ~-30 m forming the start of a deeper terrace. The composition as well as shoreward zonation of facies suggests that the nearshore terrace represents an in situ fossil reef complex. Th-U ages of in situ corals indicate that the terrace complex is entirely Pleistocene in age and suggest that the bulk of the feature is composed of a fossil reef complex formed during marine oxygen isotope stage 7.
    The emerged carbonate record on Oahu contains two well-identified Late Pleistocene formations. The Kaena Formation, which reaches elevations as high as +30 m, has been correlated with marine oxygen isotope stage 13 or 15. The Waimanalo Formation, which reaches elevations as high as +12.5 m, has been correlated with marine oxygen isotope stage 5e. The age and position of these formations has been used to determine the long-term uplift rate for Oahu. In coastal sections, the Waimanalo Formation either rests unconformably on the Kaena Formation or is separated from it by the Bellows Field Eolianite of undetermined age.
    The data presented here suggest relative sea levels on Oahu during stage 7 were much lower than sea levels during stage 5e or stage 13/15. In addition, hiatuses in the emerged record of Oahu may be accounted for in the nearshore submarine terraces.

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