Hole 1203A was drilled in a water depth of 2594 m. The hole was drilled ahead to 300 mbsf with the rotary core bit, where coring commenced. Seventeen sediment cores were recovered above basalt, which was encountered at 462 mbsf. We made biostratigraphic assignments to core catcher and other selected samples using calcareous nannofossils. Table T3 and Figure F4 show a summary of biozonal assignments.
Samples from Cores 197-1203A-1R to 17R contain nannofossils ranging from few to dominant (mostly common) in abundance. Preservation is generally poor, with a few samples displaying moderate preservation. Nannofossil preservation in Cores 197-1203A-16R and 17R is generally better than in the overlying cores.
Samples 197-1203A-1R-1, 68-69 cm, to 3R-1, 50-51 cm, are in the NN8-NN10 zonal interval (late Miocene), based on the presence of Discoaster bollii, Discoaster brouweri, Discoaster calcaris, and Discoaster cf. Discoaster hamatus. We could not zone sediments between Sample 197-1203A-3R-1, 50-51 cm, and 8R-5, 50-51 cm, owing to poor preservation and lack of diagnostic marker species. Samples 197-1203A-8R-5, 50-51 cm, and 9R-CC contain Discoaster druggii and are within the zonal interval of that species, NN2-NN5 (early to middle Miocene). Cyclicargolithus abisectus is present in Sample 197-1203A-10R-2, 82-83 cm, suggesting an assignment to Zone NP24-NN1 (late Oligocene to early Miocene); C. abisectus together with Reticulofenestra bisecta in Sample 197-1203A-10R-CC indicates a zonal interval of NP24-NP25 (late Oligocene). The species normally so helpful for Oligocene nannofossil zonation (Sphenolithus ciperoensis and Sphenolithus distentus) are not present in these cores, probably owing to poor preservation.
Sample 197-1203A-11R-2, 83-84 cm, contains both Chiasmolithus oamaruensis and Reticulofenestra umbilicus, suggesting a zonal range of NP18-NP22. However, the assignment of Zone NP21 below this level (in Sample 197-1203A-14R-CC) indicates that the interval from Sample 197-1203A-11R-2, 83-84 cm, to 13R-1, 50-51 cm, can be assigned to Zones NP21-NP22. Isthmolithus recurvus together with common Ericsonia subdisticha (i.e., the acme of E. subdisticha) in Sample 197-1203A-14R-CC indicates Zone NP21 (Eocene/Oligocene boundary). Sample 197-1203A-15R-CC contains Discoaster barbadiensis and C. oamaruensis, indicating a zonal interval of NP18-NP20 (late Eocene). The absence of Chiasmolithus solitus and C. oamaruensis and the presence of Chiasmolithus grandis in Samples 197-1203A-16R-1, 15-16 cm, and 16R-1, 29-30 cm, suggest the assignment of Zone NP17. Samples 197-1203A-16R-1, 70-71 cm, to 16R-1, 131-132 cm, are in Zones NP15-NP16 (middle Eocene), based on the co-occurrence of R. umbilicus and C. solitus. Sample 197-1203A-16R-2, 20-21 cm, is in Zone NP14 (early Eocene/middle Eocene boundary) on the basis of the co-occurrence of Discoaster lodoensis and Nannotetrina sp. Sample 197-1203A-16R-CC contains D. lodoensis, Tribrachiatus orthostylus, and Coccolithus crassus, indicating Zone NP12 (early Eocene). Overlying Samples 197-1203A-16R-2, 61-62 cm, to 16R-3, 81-82 cm, are therefore in the zonal interval NP12-NP14. Zone NP12 continues downward to the last soft sediment sample (197-1203A-17R-3, 47-48 cm) immediately overlying basalt. The lower and upper boundary ages of Zone NP12 are estimated to be 52.8 and 50.6 Ma, respectively (see "Biostratigraphy" in the "Explanatory Notes" chapter). However, C. crassus occurs in all the Zone NP12 samples, narrowing the age assignment to 51.5-50.6 Ma.
Some volcaniclastic sediment intercalated in basalt in this hole contains a sparse but moderately to well-preserved assemblage of nannofossils. Volcaniclastic beds in Samples 197-1203A-20R-6, 36-40 cm; 29R-2, 116-117 cm; 29R-3, 19-20 cm; and 33R-CC all contain the same Campanian species: Ceratolithoides aculeus, Quadrum gothicum, Quandrum sissinghii, Quandrum trifidum, Micula decussata, and Micula concava. We assign these samples to the CC22-CC23 zonal interval. Q. trifidum is the zonal indicator species. It ranges from the base of Zone CC22 (76.0 Ma) to just above the middle of Zone CC23 (71.3 Ma). Samples 197-1203A-39R-3, 61-62 cm, and 39R-4, 18-19 cm, contain the same nannofossil assemblage, together with Eiffellithus eximius and Aspidolithus parcus. These samples are therefore in Zone CC22 (75.0-76.0 Ma). Zone CC22 is also assigned, based on this assemblage, to volcaniclastic Samples 197-1203A-63R-5, 55-56 cm; 63R-5, 74-75 cm; 63R-5, 118-119 cm; 63R-6, 10-11 cm; and 64R-1, 112-113 cm.