Hole 1065A was drilled in 5412 m of water with the intention of reaching a tilted fault block to determine whether the block consisted of continental crust or not. An ancillary objective was to date any prerift or synrift sediment overlying basement.
More than 322 m of Jurassic clays, claystones, and dolomitic claystones was cored, although basement was not reached. The sequence was subdivided into two subunits. Dark greenish gray to medium dark gray soft clay dominates lithologic Subunit VA (308.8 to 501.5 meters below seafloor [mbsf]), whereas Subunit VB (501.5-631.4 mbsf) consists of claystone, some of which is dolomitic. Turbidites, some consisting of sandstones and conglomerates, are minor constituents. These lithologies represent primarily offshore, sub-wave base suspension deposits (Shipboard Scientific Party, 1998a).
Core recovery was poor (<5%) in this portion of the hole, and no nannofossil ooze was encountered. Assemblages, therefore, were highly diluted by the clastic sediments. The presence of much fine silt made it difficult to concentrate the nannofossils by gravitational settling techniques.
The stratigraphic distribution of nannofossils in Cores 173-1065A-8R to 35R is given in Table T1; the fossils are illustrated by light micrographs in Plates P1, P2, and P3. Their overall abundance ranges from few to very rare and their preservation from poor to good, with the best preservation in Cores 1065A-8R to 20R. No overgrowths were noted; only dissolution affected the preservation of the specimens.
S. bigotii is present down to Core 173-1065A-20R, and Conusphaera mexicana minor along with small specimens of C. mexicana mexicana (4-5 µm) were traced down to Core 173-1065A-13R. Among the other taxa present are Zeugrhabdotus erectus, Zeugrhabdotus embergeri, Miravetesina favula, Diazomatolithus lehmanii, and Diazomatolithus galicianus. The latter was described as new from Hole 901A by de Kaenel and Bergen (1996), but it has not been recorded outside of this region. The only taxa noted as common in Table T1 belong to the Watznaueriaceae. These include members of the Ellipsagelosphaera/Watznaueria plexus, in which the Watznaueria are dominant. Cores 173-1065A-21R to 31R are barren of nannofossils.
There are rare to very rare occurrences of nannofossils from Section 173-1065A-32R-CC to the bottom of the hole consisting only of dissolution-resistant members of the Watznaueriaceae in which Watznaueria remains dominant. Unlike the well-preserved members of this plexus noted higher in the hole, however, these specimens exhibit mostly broken or fragmented shields.
Cores 173-1065A-8R to 20R are clearly no younger than Tithonian in age, based on the presence of S. bigotii and Discorhabdus patulus in the absence of lower Berriasian index taxa such as Nannoconnus steinmannii, Retacapsa angustiforata, and Assipetra infracretacea (see Bergen, 1994). An age no older than Tithonian for the upper portion of the section is indicated by the presence of C. mexicana mexicana and C. mexicana minor in Cores 173-1065A-10R to 13R (Bown and Cooper, 1998: fig. 4.2). C. mexicana minor is confined to the lower Tithonian in the distribution charts of Bralower et al. (1989) but ranges into the upper Tithonian in that of de Kaenel and Bergen (1996) (Table T2) for Hole 901A. The presence of conusphaerids of only five µm or less in length would suggest that top of the Jurassic section in Hole 1065A (Cores 173-1065A-10 to 13) belongs to the lower to mid-Tithonian (see "Discussion").
Below the barren interval from Cores 173-1065A-21R to 31R, watznauerids continue to dominate the ellipsagelospherids, which could indicate a Tithonian age (J. Bergen, pers. comm, 1997). The paucity of nannoliths in this interval, however, makes any such determination highly speculative.
Hole 1069A was drilled in 5075 m of water within the ocean-continent transition (OCT) to determine the nature of a relatively flat-topped, north-south basement ridge buried under postrift sediments (Figs. F1, F2). Jurassic clastic sediments were encountered at 867.83 mbsf where a 10-cm-thick medium dark clay (lithologic Subunit VA) was recovered at the base of Core 173-1069A-16R (Fig. F3). Beneath that level core recovery was poor (<3%), with only a series of limestone and low-grade metamorphic cobbles retrieved without any fine-grained matrix (Subunit VB, >85.65 m thick). Nonetheless, these rocks indicated that the basement at this locality was most certainly continental (Shipboard Scientific Party, 1998b).
Although the cores from Subunit VB were barren of nannofossils, the dark clay in Subunit VA did contain a thin (2 cm thick) light gray nannofossil ooze, the only such concentration of Jurassic coccoliths recovered on this cruise. These provided the opportunity to obtain scanning electron micrsocope (SEM) images of the assemblage (Pls. P2, P3) along with light micrographs (Pls. P4, P5).
The thin streak of nannofossil ooze was recorded in Sample 173-1069A-16R-3, 129 cm. It contains an assemblage generally similar to that in Cores 8R to 20R at Site 1065 except that more delicate, less dissolution-resistant forms such as Stradnerlithus sexiramatus are present and C. mexicana mexicana is much larger in size (Table T2). In the SEM, some specimens exhibit some dissolution, whereas others show minor overgrowth by secondary calcite. S. bigotiiis also present along with D. galicianus, C. cuvillieri, Hexapodorhabdus cuvillieri, Paleopontosphaera erismata, D. patulus (= Podorhabdus grassei of authors), and Tubirhabdus patulus. Complete coccospheres belonging to various taxa are common (Pl. P2, figs. 6, 7).
Of these forms, H. cuvillieri, S. bigotii, D. patulus, and T. patulus indicate an age no younger than Tithonian, whereas the presence of C. mexicana mexicana requires an age assignment no older than Tithonian. As discussed for Site 1065 above, the range charts of most authors indicate an early Tithonian age for this assemblage; however, all of these taxa range into the upper Tithonian at Site 901 as delimited by de Kaenel and Bergen (1996, table 2; fig. 2). The presence of C. cuvillieri and Umbria granulosa ssp. minor indicates a placement very close to the top of the Tithonian according to the biostratigraphic summary tables of Bralower et al. (1989, fig. 14) and Bown and Cooper (1998, fig. 4.2). As noted previously, there are varying taxonomic concepts in the literature for C. cuvillieri, and there does not yet seem to be a consensus as to the correlation of its first occurrence with respect to the Jurassic/Cretaceous boundary.
A single specimen of Diadorhombus rectus, whose first occurrence (FO) is considered to be in the Berriasian (Lower Cretaceous) (e.g., see Bralower et al, 1989, fig. 14), was noted by one of us but not photographed. We consider this to be a downhole contaminant. Because the Tithonian ooze lamina is very thin (2 mm) and only lies a few centimeters below the subjacent upper Berriasian chalk (Fig. F3), downhole contamination is possible.