Origin and applications of whole-core magnetic susceptibility of sediments and volcanic rocks from Leg 152

Abstract

Nearly continuous records of whole-core magnetic susceptibility obtained during Leg 152 reveal their potential as a lithostratigraphic tool for both sediments and volcanic rocks. For sediments, thermomagnetic and hysteresis measurements show that the whole-core magnetic susceptibility is primarily controlled by the concentration of magnetite associated with detrital minerals. The continuous susceptibility record for cores from Site 918 documents that the detrital influx gradually increased from about 11 Ma and that already at about 9.5 Ma the susceptibility reaches a high level about 50 m below the bottom of the oldest verified glaciomarine sediment (about 7 Ma). The two overlapping cores from Site 919 provide an opportunity for correlation and recovery of a complete composite sequence. In addition, the presence and type of dropstones can be detected by whole-core susceptibility if combined with the gamma-ray attenuation porosity evaluator (GRAPE) density data. For volcanic rocks, the whole-core susceptibility depends on the rock chemistry and the degree of oxidation. The susceptibility of the massive inner part of lava flows is lowest in the least evolved picrites and highest in the evolved olivine-free basalts in the Middle Series. Scoriaceous breccia and fractured rocks are characterized by highs of magnetic susceptibility. Although some of the lava flow tops are marked by susceptibility enhancement, very intense oxidation at other tops reduced the susceptibility by oxidizing magnetite into hematite. In situ susceptibility logging of subaerial volcanic sequences would be useful in taking a continuous profile of successions of lava flows. When combined with shore-based rock magnetic measurements, new applications of whole-core magnetic susceptibility data can be exploited to give new insights to lithostratigraphic variations.

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt

“…Using this information, Fukuma was able to differentiate picritic basalts (f 2000 ASI) from more evolved basalts and dacites (f 4000 ASI). Fukuma's (1998) measurements reveal some interesting characteristics of magnetic susceptibility in basalts. Magnetic susceptibility in very fine-grained, quickly cooled scoriaceous flow tops was generally (Scott et al, 1979).…”

“…Nelson (1993) reviewed the application of magnetic susceptibility logging in sedimentary rocks and tuffaceous volcanoclastic sequences, noting that in some cases, magnetic susceptibility logs were effective in resolving stratigraphic correlations over ones of kilometers. Fukuma (1998) was able to correlate magnetic susceptibility peaks in oceanic sediments between ODP holes over 50 km apart.…”

“…Secondary stratigraphic features, such as sedimentary interbeds between flows, will typically show up on magnetic susceptibility logs as zones of lowered magnetic susceptibility. Fukuma (1998) analyzed magnetic susceptibility in oceanic sediments and submerged, originally subaerial basalts as part of ODP Leg 152, to study the nature of the rifted margin off of southeast Greenland. While there are many ODP and other studies on magnetic susceptibility, Fukuma's (1998) study is a good exam- Fig.…”

“…Some of the fine-grained scoriaceous basalt did not have elevated v values (< 1 mSI) where it was oxidized. Fukuma (1998) specifically noted that the baseline v of flow interiors from different flow series [sic] was controlled by the original composition of unaltered magnetites. Using this information, Fukuma was able to differentiate picritic basalts (f 2000 ASI) from more evolved basalts and dacites (f 4000 ASI).…”

Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone

“…We plot the physical-property data in some of the figures with variable vertical scales to align the turbidites horizontally for visual inspection (Thompson and others, 1975). Physical-property correlations of this type also are common practice with academic and ODP/IODP cores (for example, Fukuma, 1998) and have recently come into use for paleoseismology (Karlin and others, 2004;Abdeldayem and others, 2004;St-Onge and others, 2004;Hagstrum and others, 2004;Iwaki and others, 2004;Schnellmann and others, 2002). Individual event signatures at a given site are established by multiple cores at each site: a minimum of four cores and as many as seven in some cases.…”