Characteristics of the turbidite units derived from the Alika debris avalanches on the submarine flanks of the island of Hawaii

Many giantic submarine landslides have been recognized around the Hawaiian Islands, and most of them are considered to be accompanied with giant tsunami. Core samples obtained from deep-sea floor are a useful record for estimating the frequency of giant submarine landslide. Hawever, correlation between turbidite deposits in the core sample and the giant submarine landslides are uncertain. In order to clarify the correlation, we have investigated phisical and chemical properties of the turbidites in the core samples obtained from the western Hawaiian deep. The turbidite were derived from the South Kona slide complex, which is well studied using facilities of submarine investigation (Yokose et al., 2004; Yokose and Kanamatsu, 2007), and are prevailing along the western Hawaiian deep. The three core samples were obtained during KR01-12 cruise in 2001: PC13, PC-14, and PC-15. The distance from the source region of each core, PC-13, PC-14, and PC-15, is approximately 120 km, 330 km, and 440km, respectively. The physical and chemical properties of the sediment in the core samples were analyzed (e.g., bulk composition, magnetic properties, grain size analysis, X-ray CT scan). We identify the Brunhes-Matsuyama boundary in the core samples of PC-14 and PC-15. Some dark brown thin layers are observed in the core samples by naked eyes. Some of them are corresponded to relatively coarser grained layers and higher magnetic susceptibility positions. However, many coarse-grains layers are rich in microfossils but sand grains. Some higher magnetic susceptibility positions are observed in the totally pelagic sediments. These factors are unlikely to indicate the turbidity layers. On the other hand, the bulk chemical composition of the sediments in the core samples are useful discriminator for a turbidits units. The sedimentary sequence bounded by the chemical composition is resembles to the Bouma sequence, especially those of Tb, Tc, Td, and Te (PC-13 has a hole sequence). As grain size distribution pattern and grain shape are different, pelagic sediments and the pelitic division of the turbidities unit are identifiable. Based on the above analysis, changes in the stratigraphic phase may corresponded to the distance from their source. The sedimentary units of Alika 1 debris avalanche deposit in PC-14 and Alika 2 debris avalanche units in PC14 and PC 15 are similar to sandy turbidite and muddy turbidite in a distal part of the landslide deposit, respectively. Based on the above investigation, we cannot define a landslide deposit by a thin sandy layer in the core sample. If a turbidite derived from a huge landslide, it should be composed of several layers as a units. Therefore, there are only two huge landslide events after Brunhes-Matsuyama boundary in the western part of the Hawaiian Islands.