The main objective of Ocean Drilling Program (ODP) Leg 198 was to examine the sedimentary record of several ancient "greenhouse" climate events (Bralower, Premoli Silva, Malone, et al., 2002). These events include the early Aptian Oceanic Anoxic Event (OAE1a) or "Selli" interval that was deposited over a period of ~0.6 m.y. (Bralower et al., 1993). This Cretaceous black shale has been found in deep marine sections around the globe, from Italy to the Pacific Ocean Basin (e.g., Sliter, 1989; Coccioni et al., 1992; Bralower et al., 1993, 1994). Oceanic anoxia in the OAE1a interval is manifested by enhanced preservation of organic carbon and significant biotic changes (e.g., Premoli Silva et al., 1999; Leckie et al., 2002). It is linked to high fertility characterized by oligotrophic conditions and poorly oxygenated bottom waters (Coccioni et al., 1992). Some workers link the enhancement of preservation to ocean bottom water stagnation and anoxia (e.g., Bralower and Thierstein, 1984), whereas others attribute the preservation to enhanced productivity (e.g., Pedersen and Calvert, 1990; Hochuli et al., 1999). Bralower et al. (1993) suggest that OAE1a represents a condensed section formed during a period of rapidly rising sea level linked to increased rates of seafloor spreading and anomalous midplate volcanism (e.g., Larson, 1991a, 1991b; Tarduno et al., 1991; Arthur et al., 1991). It is therefore not surprising that cores and outcrops that contain the OAE1a horizon often contain altered volcanic tuffs (e.g., Sliter, 1989, 1999). Data and interpretations about this interval are summarized in several publications (e.g., Bralower et al., 1994; Larson and Erba, 1999; Leckie et al., 2002).
Prior drilling on the Shatsky Rise encountered the OAE1a horizon, but recovery was poor (Site 305; Larson, Moberly, et al., 1975). Recovery of this horizon was better during Leg 198 (Fig. F1), particularly near the apex of the Northern High in Cores 198-1207B-43R and 44R (Bralower, Premoli Silva, Malone, et al., 2002). The OAE1a interval was also partly recovered in Cores 198-1213B-8R and 198-1214A-23R on the southern flanks of the Southern High. Gamma and natural uranium logs at Sites 1207 and 1213 suggest that 50% to 30%, respectively, of the OAE1a interval was recovered at these sites (Bralower, Premoli Silva, Malone, et al., 2002). Biostratigraphic and organic carbon analyses suggests that the apex of the OAE1a was at least partly recovered in the sedimentary sections at Sites 1207 and 1213 but was likely missed at Site 1214 (Bralower, Premoli Silva, Malone, et al., 2002). For these reasons, the Site 1207 and 1213 records are emphasized in this discussion.
Given shipboard time constraints, only cursory descriptions of the OAE1a intervals were possible during Leg 198. The goal of this study was to characterize the sedimentology and sedimentary petrology of the OAE1a interval, particularly the organic-rich and volcaniclastic facies. It is important to relate this section to other circum-Pacific occurrences. Deep Sea Drilling Project (DSDP) Site 463 (Fig. F2) was chosen as a comparator because a nearly complete section of the OAE1a interval that is both organic rich and tuffaceous was recovered at that site (Shipboard Scientific Party, 1981). Although there have been many geochemical studies of black shales, their petrographic study has been limited (e.g., Schieber, 2003). Results presented here are meant to complement other postcruise geochemical and biostratigraphic (e.g., Bown, this volume) studies with the ultimate goal of refining the model for OAE1a formation.