We made biostratigraphic assignments to core catcher and other selected samples using calcareous nannofossils. Figure F5 presents summaries of biozonal assignments. Abundance and distribution of nannofossils in each sample studied were recorded in the paleontology (PAL) database and are shown in Tables T3 and T4.
Hole 1204A was drilled in a water depth of 2371 m. The hole was drilled ahead to 761.9 mbsf with the rotary core bit, where coring commenced. Basalt was reached at Section 197-1204A-6R-5, 10 cm. Samples from Cores 197-1204A-1R to 6R contain nannofossils ranging from few to common in abundance. Preservation ranges from poor to moderate.
We assign Samples 197-1204A-1R-1, 50-51 cm, to 1R-CC, to Zone NP15 (middle Eocene), based on the presence of Nannotetrina fulgens and the absence of Discoaster lodoensis and Reticulofenestra umbilicus. Samples 197-1204A-2R-1, 50-51 cm, to 2R-3, 50-51 cm, are in Zone NP14 (which spans the early Eocene/middle Eocene boundary), as indicated by the presence of D. lodoensis and Discoaster sublodoensis. Samples 197-1204A-2R-CC to 3R-CC lack D. sublodoensis. A few specimens of overgrown Tribrachiatus orthostylus are present in these samples, but we believe these specimens to be reworked from Zone NP12 below, where T. orthostylus is a common component of the assemblage. We therefore discount the few specimens of T. orthostylus in these cores and assign Samples 197-1204A-2R-CC to 3R-CC to Zone NP13 (early Eocene). Samples 197-1204A-4R-1, 50-51 cm, to 5R-CC, contain D. lodoensis and T. orthostylus, indicating Zone NP12 (early Eocene).
Sample 197-1204A-6R-1, 25-26 cm, contains Heliolithus riedelii, Heliolithus kleinpellii, and Fasciculithus tympaniformis but lacks Discoaster multiradiatus, indicating Zone NP8 (late Paleocene). Therefore, a hiatus exists between Samples 197-1204A-5R-CC and 6R-1, 25-26 cm, representing at least 3.4 m.y. (duration of the NP9-NP11 zonal interval). Zone NP8 continues downward to Sample 197-1204A-6R-1, 58-59 cm. Samples 197-1204A-6R-1, 79-80 cm, to 6R-1, 107-108 cm, are in Zone NP7, based on the presence of Discoaster mohleri and the absence of H. riedelii.
The next nannofossil-bearing sample is 197-1204A-6R-2, 86-87 cm, which contains Quadrum trifidum, indicating a Campanian age (Zone CC22 or CC23). Although the CC22/CC23 zonal boundary marker (Eiffellithus eximius) is not present, Aspidolithus parcus is present in the sample. The extinction of A. parcus is most often given in the literature as occurring low in Zone CC23 at 74.5 Ma (e.g., Erba et al., 1995) (see also Table T3). However, a recent compilation of Late Cretaceous zonations and datums by Burnett (1998) places the extinction of A. parcus higher in Zone CC23, at a level just below the Campanian/Maastrichtian boundary, which is dated at 71.3 Ma. Q. trifidum is also present in Sample 197-1204A-6R-3, 80-81 cm, again indicating Zone CC22 or CC23.
Closely spaced "toothpick" samples taken from the 128 cm of altered volcanic ash layers separating the lowermost Paleocene sample (197-1204A-6R-1, 107-108 cm) and the uppermost Campanian sample (197-1204A-6R-2, 86-87 cm) contain no nannofossils.
Hole 1204B was drilled in a water depth of 2370 m. The hole was drilled ahead to 810.7 mbsf with the rotary core bit. The first core taken contains the sediment/basalt contact in Section 197-1204B-1R-3, 33 cm. Samples 197-1204B-1R-1, 5-6 cm, to 1R-1, 31-32 cm, contain common to abundant, mostly poorly preserved nannofossils. The samples are dominated by species of Micula, including M. decussata, M. swastica, M. concava, and M. murus. The presence of M. murus indicates these samples are in upper Zone CC25 or CC26 (Maastrichtian). Samples between 197-1204B-1R-1, 125-126 cm, and 1R-2, 35-36 cm, are barren. Samples 197-1204B-1R-2, 89-90 cm, to 1R-3, 19-20 cm, contain few moderately preserved nannofossils, including Q. trifidum and A. parcus. These species indicate Zone CC22 or CC23 (Campanian), the same zonal assignment made in Hole 1204A (Fig. F5). A sample in the last core taken, Sample 197-1204B-17R-3, 80-81 cm, also contains Q. trifidum, indicating the same zonal interval is present to the bottom of the hole.