PLIOCENE-PLEISTOCENE CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF IODP HOLE 1396C, ADJACENT TO MONTSERRAT ISLAND IN THE LESSER ANTILLES: OBSERVATION OF EXTREME DIAGENESIS

Integrated Ocean Drilling Program Hole 1396C, adjacent to Montserrat Island in the Lesser Antilles, provides a lower Pliocene to Pleistocene record of calcareous nannofossil assemblages (CN11 to CN15). The nannofossil assemblages are generally common to abundant with poor to moderate preservation in the upper Pleistocene, and very abundant with moderate to good preservation in the lower Pleistocene to the Pliocene. The sequence was zoned via the Gartner (1977) scheme for the Pleistocene and the Okada and Bukry (1980) zonation for the Pliocene using the recent age updates from Backman et al. (2012).

Sedimentation rates inferred by nannofossil biostratigraphy suggest low sedimentation rates in the Pleistocene and high sedimentation rates during the Pliocene. This sedimentation pattern was also observed at Site 1000 from Ocean Drilling Program Leg 165 in the central Caribbean Sea, indicating a regional event caused by the closure of the Central American Seaway.

Scanning Electron Microscopy (SEM) observation of calcareous nannofossil assemblages in selected samples from Hole 1396A shows severe diagenesis (dissolution and/or overgrowth) even near the top of the sequence. Extreme overgrowth features are recorded in each sample, whereas partial dissolution is present in some species and taxa identification may be possible only at the species level. The extreme diagenesis of nannofossils in this relatively shallow basin (e.g., 800 m) resembles the severe diagenesis that may be observed in some older deposits (e.g., Cretaceous). Our hypothesis to be tested is that volcanic activity in the region may be the cause. Further examination of these samples should provide a better understanding of the progression of carbonate diagenesis in this shallow basin.