GEOPHYSICAL RESPONSE OF FLOOD BASALTS FROM ANALYSIS OF WIRE LINE LOGS- OCEAN DRILLING PROGRAM SITE-642, VORING VOLCANIC MARGIN

Analysis of wire line logs at Ocean Drilling Program Site 642, the dee pest scientific hole yet drilled in an offshore large igneous province (LIP), reveals the main features of the extrusive complex constructed during formation of a volcanic margin. A characteristic cyclic log re sponse images tholeiitic lavas and thin tuffs corresponding to the sea ward dipping reflectors in the seismic record. Flows are 0.6-18.5 m th ick, while most sediments are < 1 m thick. Porosity dependent sonic, d ensity, neutron porosity and resistivity logs respond to gently decrea sing porosity in upper 5-7 m of the flow and rapid porosity increase n ear the flow base. Velocity and density top and bottom gradients are l argely independent of flow thickness, with velocity gradients of 400 t o 600 s-1 and -1000 to -2000 s-1, respectively. A positive correlation between flow thickness and mean velocity simply reflects increased pr oportion of massive basalt in thicker flows. There is also an overall increase in mean fine-grained basalt velocity with depth with a gradie nt of 0.8 +/-0.3 s-1. Flow velocity-density relations show a downward velocity strengthening related to a decrease in high aspect ratio void s. High velocities near the flow base correlate with high, > 2000 OMEG A-m, focused resistivity kicks. The large flow-scale variations in phy sical properties show that representative core measurements must be de nsely sampled. Both on flow and sequence scales Site 642 log responses appears typical for the North Atlantic Volcanic Province and other LI Ps but differs from logs in deep ocean crustal holes. Principal differ ences in log responses reflect mode of crustal emplacement and alterat ion. Hence Site 642 forms a reference hole for extrusive constructions on oceanic LIPs.