COMPACTION AND INCLINATION SHALLOWING IN DEEP-SEA SEDIMENTS FROM THE PACIFIC-OCEAN

Progressive downcore shallowing of the remanent inclination has been observed in a 120-m section of marine sediments at Deep Sea Drilling Project site 578 in the northwest Pacific, representing the past 6.5 m.y. Near the top of the section the average inclination corresponds to the expected geocentric axial dipole value of 53° but shallows down section by about 6° to 8°. Northward translation of the Pacific plate accounts for only about a quarter of the inclination shallowing. Moreover, no inclination shallowing was observed at neighboring site 576, which has considerably lower sedimentation rates, and the last 5 m.y. are represented by a 26-m sedimentary section. The inclination shallowing at site 578 was correlated with an average decrease in porosity of 3-4%. The porosities in the top 26 m at site 576 are slightly higher than at site 578 and show no definite trend downhole. These results suggest that both the downhole inclination shallowing and decrease of porosity in site 578 are caused by sediment compaction. Compaction does not play a significant role for the section at site 576 due to its much shorter length. -after Authors