Nannofossil bioevents and biozones of ODP Holes 197-1203A and 197-1204A on Detroit Seamount

Calcareous nannofossils were studied in sedimentary successions recovered from two holes on the Detroit Seamount in the northwestern Pacific Ocean. Preservation of calcareous nannoflora assemblages varies from poor to good throughout the sediments recovered from both Holes 1203A and 1204A. Biostratigraphic investigation allowed the identification of 19 nannofossil zones in Hole 1203A and 7 in Hole 1204A. The sedimentary cover in Hole 1203A ranges from lower Eocene (Zone NP12) to upper Miocene (Zone NN9). The sedimentary interval investigated directly overlying the basalt recovered at Hole 1204A is late Campanian in age (Zones CC22-CC23), and the top of the section is middle Eocene (Zone NP15) in age. Major unconformities were observed in Hole 1204A between upper Campanian (Zones CC22-CC23) and lower Thanetian (Zone NP7) sediments and between upper Thanetian (Zone NP8) and upper Ypresian (Zone NP12) sediments.

Paleomagnetic of basalts from ODP Site 145-883 on Detroit Seamount

Paleomagnetic data were measured from basaltic flows cored by the Ocean Drilling Program (ODP) at Site 883 on the summit of Detroit Seamount, located in the northernmost Emperor seamounts. These data are important because they reflect the paleolatitude of Hawaiian volcanism for the Late Cretaceous and bear upon geodynamic models of hotspot drift. A total of 143 samples were measured, from cores acquired at two ~20-30 m apart. Most samples gave apparently reliable magnetic directions that were analyzed in a tiered fashion to compute a composite inclination vs. depth curve. One hole gave 13 distinct inclination groups, the other 10, and the two were combined into nine groups thought to represent independent measurements of paleofield direction. These data indicate normal magnetic polarity and give a mean inclination of 61.5+10.6°/-6.4° and paleolatitude of 42.8+13.2°/-7.6° (95% confidence limits). This paleolatitude is 6.2° higher than results from another ODP site (884) drilled on the lower flank of the same seamount. The difference is thought to result partly from an age difference (1-3 Myr) and partly from incomplete averaging of paleosecular variation at both drill sites. Together, the data from the two sites reinforce the conclusion that the northern Emperor seamounts were formed far north of the present-day hotspot latitude (~19.5°N) and suggest prior estimates of the amount and rate of southward drift may have been low. This analysis also illustrates uncertainties in determining paleolatitude from a small number of lava flow units from a single drill site.