Abstract:
The Shatsky Rise is a large Mesozoic igneous province in the northwestern Pacific that was situated near the equator (0-20 degrees ) during the Cretaceous. At Ocean Drilling Program Site 1213 on the southern high of the Shatsky Rise, a thick ( approximately 400 m) Cretaceous section was drilled consisting of nannofossil ooze and chalk, and radiolarian-bearing chert and porcellanite. Although core recovery through this interval was relatively poor and biased towards the harder silicified lithologies, shipboard biostratigraphic studies indicate a fairly complete Lower Cretaceous section and provide microfossil datums for age-depth plots. The high carbonate content of the non-chert intervals indicates deposition above the CCD. It is uncertain whether the radiolarian content in the section is a function of higher productivity or enhanced preservation. Various lines of evidence favor early silicification as a result of diagenetic mobilization of biogenic silica (radiolaria tests) in the calcareous deposits and concentration within chert/porcellanite horizons. The Cretaceous chert at Site 1213 is highly variegated. There are stratigraphic changes in chert color from yellow, orange, red and brown hues to gray hues. These color changes correlate to variations in rates of sediment accumulation based on age-depth plots. Intervals with warm colored chert correspond to periods with lower sediment accumulation rates, whereas the intervals with gray chert correspond to periods when sediment accumulated more rapidly. Chert color reflects the color of the calcareous host sediment, which in turn can be linked to Redox conditions in the sediment. Lower sediment accumulation rates produced oxic conditions in the sediment, whereas rapid rates led to reducing conditions. At Site 1207 on the northern high of the Shatsky Rise, sediment accumulation rates were less variable and stratigraphic trends in chert color are less distinct. This paper was co-authored by the Leg 198 Shipboard Scientific Party.