Microfossil assemblages on Tuscaloosa Seamount and their constraints on the age of the Nu´uanu landslide, north of Oahu, HI

The Núuanu submarine landslide, north of the Island of Oahu, is the largest of many debris avalanches along the flanks of the Hawaiian Islands. Several attempts have been made to date the Núuanu landslide and to assess its relationship to the shield-building stage of Kóolau Volcano as an indication of hazards that might be anticipated during the evolution of oceanic volcanoes. Previous age determinations based on magnetostratigraphy and the temporal distribution of turbidites have yielded a range of possible ages. We describe a distinctive fine-grained silty mudstone representing mantling sediment on the largest landslide block within the debris field, Tuscaloosa Seamount. This mudstone contains foraminifera and nannofossil assemblages that yield a micropaleontological age and paleoenvironmental determination for the interval immediately following landslide emplacement. The microfossils constrain the mantling sediment to have been deposited between 2.5 and 3.3 Ma, in offshore, mid- to lower-bathyal (∼1500-3000 m) water depth, consistent with a position on the upper flanks of Tuscaloosa Seamount. These findings indicate that landsliding occurred prior to or near the end of shield building at Kóolau Volcano (2.9 Ma), and may have precipitated dramatic changes in volcano growth and deformation behavior, as well as changes in basalt geochemistry.