PHYSICAL-PROPERTIES OF A PORCELLANITE LAYER (SOUTHWEST INDIAN RIDGE) CONSTRAINED BY GEOPHYSICAL LOGGING

A distinct porcellanite layer from the Southwest Indian Ridge intercal ated in Pleistocene diatom ooze was studied using nondestructive physi cal property measurements and sedimentological data. This bed was samp led by two piston cores at a water depth of 2615 m. The 3-5 cm thick p orcellanite layer appears in the cores at a depth of 6.03 m (Core PS20 89-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its charac teristic physical properties the porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped wit h echosounding. The physical index properties, wet bulk density and el ectrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both core s and to distinguish whether resistivity anomalies are caused by a hig her amount of terrigenous components or by the presence of porcellanit e. The porcellanite has the special characteristic to affect a positiv e anomaly ir, resistivity but not in susceptibility, Most marine sedim ents, in contrast, show a positive correlation of magnetic susceptibil ity versus electrical resistivity; therefore a combination of electric al resistivity and magnetic susceptibility logs yields a definite dete ction of the porcellanite bed. Images from the X-ray CT survey indicat e that the porcellanite is lithified and brittle and fragmented when t he piston corer penetrated the bed. (C) 1997 Elsevier Science B.V.