AGE AND BIOSTRATIGRAPHY

The age and duration of OJP magmatism has not yet been established with certainty. OJP basalts are difficult to date by the widely used ⁴⁰Ar/³⁹Ar method because of their very low potassium contents. Published ⁴⁰Ar/³⁹Ar data (Mahoney et al., 1993; Tejada et al., 1996, 2002) suggest a major episode of OJP volcanism at ~122 Ma and a minor episode at ~90 Ma. ⁴⁰Ar/³⁹Ar analysis (Chambers et al., 2002; L.M. Chambers, unpubl. data) of samples from ODP Leg 192 Sites 1185, 1186, and 1187 (Fig. F1) gives ages ranging from 105 to 122 Ma. Chambers et al. (2002) suggest that their younger apparent ages (and, by implication, the data on which the 90-Ma episode is based) are the result of argon recoil and therefore represent minimum ages. Biostratigraphic dating based on foraminifers and nannofossils (Sikora and Bergen, 2004; Bergen, 2004) contained in sediment intercalated with lava flows at ODP Sites 1183, 1185, 1186, and 1187 suggests that magmatism on the high plateau extended from latest early Aptian on the plateau crest to late Aptian on the eastern edge. This corresponds to age ranges of 122–112 Ma (Harland et al., 1990) or 118–112 Ma (Gradstein et al., 1995). However, Re-Os isotopic data on basalt samples from these same four drill sites define a single isochron with an age of 121.5 ± 1.7 Ma (Parkinson et al., 2002).

The oldest sediment overlying basement on the crest of the OJP occurs within the upper part of the Leupoldina cabri planktonic foraminiferal zone and corresponds to a prominent ¹³C maximum (Sikora and Bergen, 2004). This result shows that eruption of basaltic lava flows continued through much of Oceanic Anoxic Event 1a, of which the formation of the plateau is a postulated cause (e.g., Larson and Erba, 1999). Nannofossil studies (Bergen, 2004) reveal six unconformities in the lower Aptian to Miocene pelagic cover sequence recovered during Leg 192. The biostratigraphic data are in good agreement with the mid- to Late Cretaceous strontium isotope stratigraphy determined on samples from Sites 1183 and 1186 by Bralower et al. (this volume). Oxygen and carbon isotope analysis of bulk carbonate samples from the OJP and Manihiki Plateau (MacLeod and Bergen, this volume) provide evidence for cooling in the region during the Maastrichtian. These data extend existing evidence for Maastrichtian cooling into the southwestern tropical and subtropical Pacific. Sano et al. (this volume) determined boron contents in the sedimentary rocks recovered at Sites 1183 and 1186. They show a correlation between boron and hydrogen contents, suggesting that boron is expelled along with water during compaction of the sediments.