Attempts to model the Lower Cretaceous pelagic sedimentary cover of Shatsky Rise (NW Pacific Ocean) from downhole geophysical records

Ocean Drilling Program (ODP) Leg 198 to Shatsky Rise cored Lower Cretaceous sediments at three sites (Sites 1207, 1213, and 1214). Unfortunately, core recovery was poor at all sites, most likely due to the contrast in physical properties between poorly cemented, very soft carbonate ooze/chalk and well-cemented, very hard biosiliceous chert. Downhole geophysical logs from two sites (1207 and 1213) provide continuous, in situ records of physical and chemical properties that can be used to elucidate the sedimentary history of Shatsky Rise in the early Cretaceous. The downhole logs show that at both sites two stratigraphic intervals (dated as late Barremian–Aptian and early Albian) are characterised by low porosity, high resistivity sediments. The timing of these intervals is synchronous at both sites and it has been previously proposed that the ‘resistivity highs’ are the result of increased biosiliceous sedimentation and subsequent silica cementation. In order to test this hypothesis, we use matrix algebra to invert the logging data from Site 1207 to produce two compositional models — the first reconstructs lithology, the second reconstructs mineralogy. The results of the lithology model suggest that the abundance of chert is highest during the resistivity highs. The results of the mineralogical model are more complex because of the probable presence of additional minerals such as barite.