FORMATION MICROSCANNER (FMS) DATA AND ORBITAL CYCLE RECORDS: PRELIMINARY INTERPRETATION FROM THE ASIAN MONSOON IODP EXPEDITION 346

Recently, IODP Expedition 346 (29 July–27 September 2013) drilled seven sites in the marginal sea bordered by the Eurasian continent, the Korean Peninsula, and the Japanese Islands and two closely spaced nearby sites in the East China Sea. Expedition 346 was the first scientific drilling expedition to focus exclusively on the climate system in this region.

During the expedition, the Formation MicroScanner (FMS) downhole logging tool was deployed, providing high-resolution electrical resistivity images of borehole wells. Features such as bedding, slump folding and bioturbation can be resolved. Here we analyze FMS images resistivity data collected recorded at Site U1425 that extending back to the Mid-Miocene. FMS facies analysis from FMS images allowed the recognition of several intervals, with vertical extension ranging from several decimeters to few meters, and with an apparent cyclicity. These FMS intervals are well correlated with other downhole logs (Gamma Ray, density, porosity), as well as conductive intervals generally correlate with low gamma ray and low density log values.

An apparent cyclic nature, with several orders of cycles on the FMS images, is locally clearly observed. The low frequency cyclic pattern consists of ~1–3 m thick resistive intervals alternating with conductive intervals, and containing higher frequency, smaller scale apparent cycles. At low frequency, the FMS images have been interpreted in term of facies (conductive = FaciesB ; resistive = facies A ) and their thickness have been measured along the borehole. Vertical changes in thickness are plotted versus depth, and we propose an attempt of interpretation in term of orbital forcing based on the Site U1425 shipboard age model. Preliminary interpretation over a 100-m long interval in the Miocene suggests that the 41-kyr obliquity is well expressed on the FMS images, while both 41-kyr obliquity and 174-kyr obliquity modulation are observed in the gamma ray logs. We now plan to use continuous wavelet transforms (CWT) in order to test the existence of these sedimentary cycles,by providing the amplitudes of the various frequencies, and their time or depth dependence.