Planktonic biostratigraphy of ODP Leg 144 holes

Five guyots were drilled on Ocean Drilling Program Leg 144, three of which possess thick caps of pelagic sediment. These guyots (Limalok, Site 871; Lo-En, Site 872; and Wodejebato, Site 873) belong to the Marshall Islands group of seamounts. Pelagic sediments of late Oligocene to Holocene age were recovered from them. In each case, the sediment was found to be unconsolidated on recovery and contain very abundant planktonic foraminifers, particularly in the >150-?m size range. Preservation of tests is generally good, with most showing only minor signs of dissolution or recrystallization, although many samples have a high proportion of fragmented material in the fine fraction.
Planktonic foraminifer faunas are diverse and consist predominantly of warm-water species. A typically western Pacific fauna occurs throughout the Miocene. Biostratigraphic assignment was generally straightforward except for the bottommost interval of the pelagic caps where severe reworking (sediment mixing) is a common feature. A significant hiatus was found at each site between drowning of the carbonate platform and the onset of pelagic sediment accumulation. Thus, the platforms were apparently swept clean of sediments, with the exception of isolated ponds, until subsidence took the guyots sufficiently deep for sediment to accumulate in large quantities. Backtracking of subsidence paths for each guyot suggests that pelagic cap formation began at depths of between 700 and 1000 mbsl. The first phase of pelagic cap formation was accompanied by significant transport and mixing of sediment. Eventually, with further subsidence, more continuous and less disturbed sequences were deposited.
Several hiatuses occur at various levels within the pelagic caps, the durations of which range from more than 5 m.y. down to the limit of biostratigraphic resolution. These probably relate to changes in the local hydrographic regimes because there is little similarity in the timing of hiatuses between the guyots