Measurements of Porosity in Sediments of the Lower Continental Margin, Deep-Sea Fans, the Aleutian Trench, and the Alaskan Abyssal Plain

Abstract

One objective of Leg 18 was to establish rates of sedimentation in the thick turbidite sequence along North Pacific continental margins. Rates of sedimentation based on the depth to time horizons may be significantly inaccurate if they are not corrected for compaction. Below about 400 meters, for example, compaction has commonly compressed sediment layers to as much as 60 percent of their original thickness. Since the amount of compaction is a function of porosity, the Leg 18 staff took special interest in the laboratory measurements of porosity. However, as the porosity data were analyzed during the cruise, it became apparent thai the routine laboratory procedures gave poor precision. This was caused principally by the sample disturbance during coring, which is the reason most frequently given for questioning the usefulness of the Challenger porosity data. During Leg 18, the scientific staff improved the precision of porosity measurements and by the last week at sea, the results of more careful procedures and selectivity increased the precision by almost an order of magnitude over the routine procedures. (Our procedures are described in the Appendix to this study). Since the end of Leg 18, we have reevaluated and selected the more precise values of porosity, and this paper presents these data and a generalized graph of porosity as a function of depth in North Pacific environments for use in compaction corrections.