Formation Micro-Scanner (FMS) images from carbonate drift sequences in the Maldives Inner Sea, IODP Expedition 359: Electrofacies analysis and astrochronology
DESCRIPTION: This project, using IODP Exp. 359 data and sediments, focused on investigating the style of Neogene carbonate sedimentation in the Maldives in order to unravel the joint contributions of sea level variations and ocean current system changes to the carbonate platform evolution through time. This work used geophysical measurements from downhole logging to: (1) establish a depth-time relationship to link newly collected cores and downhole logs with existing seismic data, and (2) characterize the carbonate sequences and critical seismic stratigraphic horizons in the region.
OUTCOMES: Downhole logging data provide continuous, in situ measurements of physical properties but are limited to the logged interval (typically only deeper than ∼100 meters below seafloor) and by borehole conditions; core data are higher resolution but limited by core recovery and quality. Integration of these two data sets is a powerful tool for interpreting the complete drilled interval collected during Expedition 359. Natural gamma radiation measured both in boreholes and on recovered sediments is the primary measurement used for core-log data integration. Spectral gamma radiation data from downhole logging reveals that the gamma radiation signal is dominated by uranium at all Expedition 359 sites, indicating that its variations and cycles are likely controlled by changes in organic matter. A shipboard age model was established on the basis of all available biostratigraphic and paleomagnetic age control points. In considering which orbital frequencies might influence sedimentary patterns and geochemistry, the Maldivian carbonate atolls should respond to sea level changes affecting production of carbonate as well as the monsoon system, which responds largely to eccentricity forcing. For sea level, the reflectance (L*) data from Site U1467 show evolution from the 40-ky to 100-ky world in the upper ~30 mbsf. The downhole gamma ray record from Site U1467 was compared to Laskar et al.’s (2004) orbital solution, showing clear correlations to the ~2.4 My eccentricity modulation. There is also clear evidence of the 400 and 100 ky eccentricity cycles in these data. Combining these correlations with all available age control points from shipboard biostratigraphy and magnetostratigraphy, a new proposed age model is developed. New correlation points from gamma ray data show good correspondence with shipboard data. Work continues to identify correlation points for additional cycles, in order to further refine the Site U1467 age model. Additional evidence of cyclicity in Site U1467 data will also be explored, for example, a series of alternating bright/dark pairs in downhole Formation Microresistivity (FMS) images.