Geophysical response of flood basalts from analysis of wire line logs: Ocean Drilling Program Site 642, Vøring volcanic margin

Abstract

Analysis of wire line logs at Ocean Drilling Program Site 642, the deepest 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 response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record. Flows are 0.6–18.5 m thick, while most sediments are <l m thick. Porosity dependent sonic, density, neutron porosity and resistivity logs respond to gently decreasing porosity in upper 5–7 m of the flow and rapid porosity increase near the flow base. Velocity and density top and bottom gradients are largely independent of flow thickness, with velocity gradients of 400 to 600 s−1 and −1000 to −2000 s−1, respectively. A positive correlation between flow thickness and mean velocity simply reflects increased proportion of massive basalt in thicker flows. There is also an overall increase in mean fine‐grained basalt velocity with depth with a gradient of 0.8±0.3 s−1. Flow velocity‐density relations show a downward velocity strengthening related to a decrease in high aspect ratio voids. High velocities near the flow base correlate with high, >2000 Ω‐m, focused resistivity kicks. The large flow‐scale variations in physical properties show that representative core measurements must be densely sampled. Both on flow and sequence scales Site 642 log responses appears typical for the North Atlantic Volcanic Province and other LIPs but differs from logs in deep ocean crustal holes. Principal differences in log responses reflect mode of crustal emplacement and alteration. Hence Site 642 forms a reference hole for extrusive constructions on oceanic LIPs.