Calcareous Nannofossils

Hole 1205A was drilled in a water depth of 1309 m using a rotary core bit. Five sediment cores were recovered above basalt, which was encountered in Section 197-1205A-5R-2. Core 197-1205A-1R was barren of nannofossils. Samples from Cores 197-1205A-2R to 5R contain nannofossils ranging from scarce to high in abundance. Nannofossil preservation is poor to moderate in samples from Cores 197-1205A-2R to 3R and poor to good (mostly moderate) in samples from Cores 197-1205A-4R to 5R.

We made biostratigraphic assignments to core catcher and other selected samples using calcareous nannofossils. Figure F6 shows the biozonal assignment. Abundance and distribution of nannofossils in each sample studied were recorded in the paleontology (PAL) database and are shown in Table T3.


We assign Samples 197-1205A-2R-1, 10-11 cm, to 5R-1, 32-33 cm, to nannofossil Zone NP10. The nannofossil assemblage is dominated by species typical of this zone (e.g., Campylosphaera dela, Chiasmolithus bidens, Neochiastozygus chiastus, Neococcolithes protenus, Rhabdosphaera pinguis, Toweius eminens, Toweius gammation, and Toweius pertusus). Rare specimens of Discoaster binodosus, Discoaster multiradiatus, Zygodiscus adamas/bramlettei, and Tribrachiatus contortus are also present. Zone NP10 straddles the upper Paleocene/lower Eocene boundary. However, the evolutionary first appearance of T. contortus occurs just above the lower NP10 zonal boundary. The first appearance datum of T. contortus and the Paleocene/Eocene boundary are both estimated to be at 54.7 Ma (see "Biostratigraphy" in the "Explanatory Notes" chapter). The presence of T. contortus therefore indicates these samples are in the lower Eocene part of Zone NP10 (estimated age range of 53.6-54.7 Ma).

All samples examined below Sample 197-1205A-5R-1, 32-33 cm, were barren.


Significant numbers of specimens belonging to the Family Braarudosphaeracea (viz., Braarudosphaera bigelowii, Braarudosphaera discula, Micrantholithus cf. Microantholithus entaster, and Micrantholithus spp.) are present in the samples. Members of this family are normally very rare in open-ocean sediments and suggest nearshore conditions at this site during Zone NP10 (see Siesser et al., 1992, for a review of the environmental preferences of the Braarudosphaeracea).


During DSDP Leg 55, Holes 432 and 432A were drilled very close to Hole 1205A (Hole 432A is <100 m from Hole 1205A). A thin sedimentary section consisting of 5.5 m of Quaternary foraminiferal ooze overlying Paleogene volcanic sand was recovered from Hole 432. In Hole 432A, the upper 36 m of sediments was washed, after which ~3.5 m of volcanic sandstone, calcareous conglomerate, basalt, and red lateritic clay were recovered in Core 1 and Core 2. Basalt flows were reached in Core 3 (Shipboard Scientific Party, 1980).

In Hole 432, only Quaternary nannofossils were found; no nannofossils were found in Hole 432A. Planktonic foraminifers were found, however, in Holes 432 and 432A and suggested an age range of late Paleocene to early Eocene for the Paleogene sediments overlying basalt in both holes (Shipboard Scientific Party, 1980). As described earlier, nannofossils found in Hole 1205A show the sedimentary sequence here can be restricted to early Eocene.

Benthic foraminifers, ostracodes, and benthic macrofossils indicated a shallow-water reef environment for the Paleogene sediments in Holes 432 and 432A. However, the Leg 55 Shipboard Scientific Party (1980) believed the fossils had probably been reworked from a nearby reef complex and redeposited on the flanks of the seamount, rather than representing an in situ shallow-water reef environment at the drill site. The Braarudosphaeracea we found in Hole 1205A suggest simply "nearshore" conditions.