APTIAN-ALBIAN SEA-LEVEL HISTORY FROM GUYOTS IN THE WESTERN PACIFIC

Relative sea level fluctuations are an important control on patterns o f sedimentation on continental margins and provide a valuable tool for regional correlations. One of the main objectives of combined Ocean D rilling Program Legs 143 and 144 was drilling the thick carbonate caps of a suite of seamounts, called guyots, scattered over the northweste rn Pacific. The array of drowned Cretaceous banks includes four carbon ate banks of Aptian-Albian age. These particular carbonate banks displ ay emergent surfaces if regional sea level falls faster than the rate of guyot subsidence, or intervals of condensed parasequences and well- cemented peritidal crypto-algal flats if the rate of sea level fall is slightly less than guyot subsidence. Rapid rises of sea level followi ng these sequence boundaries are recorded as drowning of the emergent horizons or as pronounced deepening of facies. The cored lithologies a nd downhole geophysical and geochemical logs were used to identify dep ositional sequences and surfaces of exceptional shallowing or deepenin g. A combination of biostratigraphic datums, carbon and strontium isot ope curves, relative magnitude of surfaces of emergence, relative thic knesses of depositional sequences, sea level events, and counts of upw ard shallowing cycles or parasequences were used to correlate sequence s among the four sites. After compensating for thermal subsidence rate s at each guyot, an identical pattern of major Aptian-Albian eustatic sea level events is evident throughout this large portion of the Pacif ic Ocean. There are approximately 12 Aptian and 12 Albian significant sequence boundaries, of which a third were associated with major episo des of emergence. When these events are compared with Aptian-Albian re lative sea level changes observed in European shelf successions, the m ajor sequence boundaries and transgressive surges can be easily correl ated, and it appears that both regions also display the same number of minor events. Therefore we can apply the relative timing of these eve nts from the thermal subsidence rates and parasequence counts of the P acific banks to construct an improved scaling of the associated ammoni te zones and biostratigraphic datums in the Aptian-Albian interval.