The Volcanic Architecture of Rigil (Hole U1374A) and Burton Guyots (U1376A), IODP Expedition 330

Downhole wireline logging measurements taken during IODP Expedition 330 to the Louisville Seamount Chain aimed to contribute data towards one of the key scientific objectives; to constrain the paleolatitude history of the Louisville hotspot between 80-50 Ma. To this end, establishing an accurate architecture of the rocks drilled is important and identifying the number of in situ cooling units drilled is essential. In addition, in situ magnetic field data measured with the Göttingen Borehole Magnetometer (GBM) will contribute to this objective. By separating and identifying the magnetic signals of the different flow units and determining the inclination and declination of the natural remanent magnetization, the estimation of the virtual geomagnetic pole position and the paleolatitude of the Louisville hotspot should be possible. Downhole logs are used to determine the physical, chemical, and structural properties of formations penetrated by drilling. The data are rapidly collected, continuous with depth, and measured in situ; they can then be interpreted in terms of stratigraphy, lithology, mineralogy, magnetic characteristics, and geochemical composition of the penetrated formation. Where core recovery is good (as with this Expedition), log and core data complement one another and may be interpreted jointly. During Expedition 330 downhole measurements were taken in Holes U1374A and U1376A. Even though core recovery levels were consistently high, only by correlating the core with the down-hole logs can continuous information regarding the lithologies and mineralogy be obtained for the entire drilled hole, i.e. the logs can circumvent the issue that 12 and 25% of the rock was lost in the two holes. In particular, in Hole U1376A between ~130 and 144 mbsf, core recovery was very poor. However, using Formation MicroScanner (FMS) imagery and other standard log data, we can make a competent attempt at filling the data gap. FMS images are particularly powerful in highlighting contrasts in formation structure and identifying brecciated zones, highly fractured zones, vessiculated units, banded units and revealing unrecovered contacts. Additionally, by 'picking' structures (such as fractures) on FMS images a clearer picture of the stress regime can be established and the nature of observed contacts refined (steep vs. shallow and hence flow vs. intrusion).