Middle Miocene to Pleistocene magneto-biostratigraphy of ODP Sites 1150 and 1151, northwest Pacific: Sedimentation rate and updated regional geological timescale

Abstract Shipboard and shore‐based investigation on siliceous and calcareous microfossil biostratigraphy, magneto‐stratigraphy and tephrostratigraphy identified numerous datum events from the sedimentary sequences of Sites 1150 and 1151 drilled on the forearc basin of northern Japan by the Ocean Drilling Program Leg 186. Some 83 datum events were selected to construct new age–depth models for the sites. Based on the reliable magneto‐stratigraphy from the Pleistocene to the Upper Miocene, which were correlated to the standard geomagnetic polarity timescale, and on excellent records of diatom and radiolarian biostratigraphy throughout the sequences, the shipboard age model was revised. Major revisions referred to stratigraphic position of the Miocene–Pliocene boundary that has been shifted more than 200 m downward in each sequence. The age–depth relations of the forearc sites represent drastic changes in the sedimentation rate—extremely high (40 cm/k.y. on average) in the Early Pliocene and low (less than 2 cm/k.y. on average) in the Middle Miocene—and several hiatuses exist throughout the sequence. The drastic changes can be related mostly to changes in diatom sedimentation and the tectonics of the Japanese Island Arc. Local ages for some foraminiferal, calcareous nannofossil and radiolarian bioevents are estimated from the age–depth models at each site. These newly calibrated bioevents and biozones as well as established diatom biostratigraphy are incorporated into the updated magneto‐biochronologic timescale, which will contribute to an improvement in biochronologic accuracy of Neogene sediments in northern Japan and adjacent areas.     

Shear wave velocity measurement during a standard penetration test

An experiment was carried out to develop a technique to measure shear wave velocity simultaneously with the standard penetration test popular in soil engineering. In the standard penetration test an impact at the bottom of a borehole is produced by weight dropping and may be expected to generate seismic waves. A three-component geophone was set on the ground surface near the borehole and the waves generated were recorded with a magnetic recorder at successive depths of the penetration test. The predominance of the SV wave obtained with this simple method was assured by measurement of the particle orbit. Signal amplitudes decrease with depth and become less than the noise level at a certain depth. Therefore records from deeper sources must be processed to disclose the shear waves. Since waveforms of SV events generated by blows of the penetration test at a given depth are very similar, the signal to noise ratio would be expected to be improved by a stack of wave trains. A paste-up of the radial component after stacking was compared with that before stacking and a refinement was clearly recognized. A vertical distribution of shear wave velocity was obtained by reading the onset time at each depth. Shear wave velocities thus obtained were compared with N values from the standard penetration test and specific resistivities from electrical logging in the same borehole. The data were mutually consistent. This experiment showed that a convenient, precise shear wave velocity measurement can be conducted during the routine work of a standard penetration test.     

Measuring Flume Surfaces for Hydraulics Research Using a Kodak DCS460

A critical problem in hydraulics research is accurate measurement of fluvially worked sediments, both in the field and in scaled representations of field situations in laboratory flumes. Such measurement must provide information on individual grain characteristics, and their organisation into structures referred to as bedforms. Existing measurement approaches are based upon mechanical or laser profiling devices, which are both expensive and take considerable time to acquire data, particularly where information is required at very high densities. This paper demonstrates how conventional automated terrain model extraction software, combined with image acquisition using a Kodak DCS460 digital camera, has been effective in generating digital elevation models of complex bed morphology. This has reduced time spent collecting data in the flume and has allowed data collection at much higher spatial and temporal densities. Application of the method is illustrated by research carried out at Hydraulics Research Wallingford. Issues discussed include configuration of photographs and control coordinates; appropriate camera calibration methods; stability of inner orientation of the Kodak DCS460; and accuracies obtained. Comparisons with independent check data reveal that accuracies of ±2.5mm have been achieved using a camera-to-object distance of 4.2 m.