Sound velocities in calcareous oozes and chalks from sonobuoy data: Ontong Java Plateau, western equatorial Pacific

Abstract

The Ontong Java Plateau, in the western equatorial Pacific Ocean, is blanketed by over 1 km of flatlying, highly stratified calcareous ooze, chalk, and limestone. Wide‐angle reflection data from 17 sonobuoys deployed over the plateau were analyzed for compressional wave velocities in these sediments. No particular velocity horizons stand out as prominent features that can be correlated from one sonobuoy station to the next. When the data from all stations are plotted together, a fairly consistent velocity‐depth relationship is derived: V = 1.56 + 2.97t with the standard error of estimate equal to 0.08, where V is the compressional wave velocity in kilometers per second at depth t and t is the depth below the sea floor in one‐way travel time of sound in seconds. When the velocity data are plotted against depth in kilometers below the sea floor, the velocity gradient is 1.4 s−1, which is considerably greater than that in most marine turbidites, silts, and clays that are buried to 1 km. The linear uniform velocity gradient is consistent with the type of lithology underlying the Ontong Java Plateau as determined by the Deep‐Sea Drilling Project. That is, there are gradual transitions from ooze to chalk to limestone with intervals of the different lithologies interbedded rather than a single abrupt change from ooze to chalk at one level and from chalk to limestone at a deeper level. A slight difference was seen between velocity profiles in calcareous sediments that had been deposited below and those that had been deposited above the lysocline (the depth below which calcareous ooze is markedly corroded). Sublysoclinal sediments have anomalously low acoustic velocities in the upper 200 m of strata but have normal velocities at greater burial depths. These observations suggest that calcareous sediments deposited below the lysocline lithify more slowly after burial than those deposited above the lysocline.