Site U1465: Expedition 359

Abstract

The main goal of International Ocean Discovery Program (IODP) Expedition 359 was to retrieve cores from platform and drift sequences that carry the record of Neogene sea level changes and the onset of monsoon-related currents in the Inner Sea of the Maldives. In addition, the cause of platform drowning related to these environmental changes was a focus of the expedition. The drilling strategy to reach this goal was to drill two transects that penetrated the prograding portions of the drowned platforms and the overlying current deposits and the thick drift sequences adjacent to the drowned platform. Site U1465 is situated above the margin of a drowned platform overlain by 70 m of sediment. Within the platform, 11 sequences were identified and interpreted as related to sea level changes by recognizing lowstand and highstand units in the lowermost six sequences. Site U1465 penetrates the topset of the youngest sequence (Platform Sequence [PS] PS11) and lies below the foresets of the underlying sequence (PS10). The sequence boundaries are onlap unconformities at and below the platform edges and are high-amplitude reflections in the foresets and bottomsets. By dating the same seismic sequence boundaries at the other sites along the transect (Sites U1466 and U1468), a robust age model for sea level changes can be established. The specific objectives of Site U1465 were as follows: (1) provide detailed reconstruction of the predrowning, drowning, and postdrowning evolution of the carbonate bank by linking the seismic stratigraphic record to the sedimentary record; (2) constrain the timing of this evolution, thus allowing age assignments of unconformities, sedimentary interruptions, sedimentary turnovers, and onset of drift deposition; and (3) reconstruct and date the bank to drift turnover.