The overlap in the ranges of S. bigotii and the two subspecies of C. mexicana is clearly demonstrated in our two drill sites in the IAP, thereby confirming the co-occurrence of these taxa as suggested by Wind (1978) for the western Atlantic and in the Iberian region by de Kaenel and Bergen (1996) (see also Bown and Cooper, 1998, fig. 4.2). Such an overlap reliably indicates the presence of Tithonian-age sediments in our study area. Nonetheless, Bown and Cooper (1998, p. 47) note that "S. bigotii bigotii appears to have a diachronous LO," and one must be wary of the fact that the Galicia interior basin was actively subsiding at this time as a result of minor rifting or stretching of the lithosphere, thus the possibility of reworking of nannofossils from older strata was a distinct possibility. The fact that the central structure of this taxon, however, is rather delicate, that the specimens are well preserved (some with exceptionally long central spines), and that they are relatively uncommon to begin with in most Upper Jurassic sequences leads us to consider reworking in this instance unlikely (as did de Kaenel and Bergen, 1996).
C. mexicana minor in the presence of very small (4 to 5 µm) C. mexicana mexicana in Cores 173-1065A-10R to 13R suggests that the interval is early to mid Tithonian in age. Bralower et al. (1989) found that a number of Jurassic evolutionary lineages begin with very small forms (to which they give the subspecies designation of minor) that eventually develop to a larger size. They further suggest that this size progression may have biostratigraphic utility. Given the uniformly small size of the Hole 1065A conusphaerid assemblage, an assignment to the lower to mid Tithonian seems appropriate (see, however, further discussion of this assemblage in relation to its restricted environment below). This section, therefore, is older than that sampled at Site 1069, where full- length (~8 µm) specimens of C. mexicana mexicana are present.
Some of the specimens of C. mexicana in Hole 1065A, however, exhibit a peculiar diverging, if not crossing, pattern reminiscent of the Cretaceous C. rothii (e.g., Pl. P2, Fig. 3). The Hole 1065A specimens, however, are considerably smaller than typical C. rothii, measuring only 4 to 5 µm or less as mentioned above. Thus, these are not assigned to that species here. Whether the C. rothii diagonal cross-hatched pattern actually originated in the Tithonian is an interesting question for further research.
As noted previously, the predominantly fine-grained Jurassic clastic sediments at Site 1065 were deposited below wave base. Basin restriction at that site is indicated by the paucity of calcareous nannofossils, the general lack of bioturbation, and relatively high organic contents (up to 0.9%) (Shipboard Scientific Party, 1998b: table 7). Thin streaks of nannofossil ooze only a millimeter or two thick at Sites 901 and 1069 are the only indications of brief communications of this basin with the open ocean. These thin laminae of nannofossil ooze, which indicate enhanced nannoplankton productivity in the surface waters, were preserved because of the general absence of bioturbation at the sediment/water interface. They therefore indicate intervals when well-oxygenated surface waters existed above dysaerobic bottom waters.
One other characteristic of dysaerobic or anoxic bottom-water conditions is the presence of common nannofossil coccospheres, such as those illustrated in Plate P3, Figures 6 and 7. An occurrence analogous to this has been described by Gallois and Medd (1979: pl. 1, fig. d), who noted high abundances of Ellipsogellosphaera coccospheres in thin chalk laminations (bands) of the Kimmeridgian Clay "black shales" of England.
The restricted nature of the basin in our study area raises the question as to whether the small size of the Hole 1065A conusphaerids discussed above might actually be due to environmental factors rather than to evolutionary development. Erba et al. (1995) described dwarf nannofossil assemblages from Campanian lagoonal deposits of Wodejebato Guyot in the central Pacific Ocean (ODP Sites 873, 874, and 877), which they attributed to restricted environmental conditions. They presented biometric measurements for seven members of their assemblages, noting that the average sizes were about half that of normal oceanic assemblages of the same age at nearby Site 869. Among the Upper Cretaceous taxa they measured is Watznauria barnesae, a long-ranging form that also occurs in our Jurassic assemblages. We therefore made similar biometric measurements of that taxon in our Site 1065A material (Sample 173-1065A-11-CC) and compared our results with those of Erba et al. (1995: table 8) in our Table T3. We find that our Hole 1065A W. barnesae specimens match in size those of Erba et al.'s (1995) normal oceanic assemblage specimens, rather than those of their dwarf (lagoonal) floras, which suggests that our small Jurassic conusphaerids are indeed the products of early evolutionary development rather than environmental restriction.
Although we interpret the age of the nannofossil assemblages discussed here as Tithonian, as noted previously those in the thin lamina sampled at Site 1069 do contain a single specimen of an Early Cretaceous form, Diadorhombus rectus, which we consider to be a downhole contaminant. If this specimen is in place, however, then basin restriction in this area would have clearly continued into the earliest Cretaceous, which it may have done anyway. Lower to middle Berriasian sediments have not been identified at any site in the study region; thus, this part of the record is missing, probably as a result of erosion associated with tectonic activity. Major rifting that ventilated the basin was underway at least by the late Berriasian, as recorded by the slumped chalk unit immediately above the Tithonian at Site 1069 (Fig. F3) (see Whitmarsh, Beslier, Wallace, et al., 1998; Wise et al., 1999; Wilson et al., in press).
Although drilling was terminated before the basement was reached, 323 m of Upper Jurassic sediments was penetrated at Site 1065, indicating that the basin was undergoing significant subsidence. This, plus the other features discussed above, would fit the model of an interior basin formed during an early phase of rifting, as has been postulated for the Galicia Basin (Fig. F4) (see discussion by Pinheiro et al., 1996, and Shipboard Scientific Party, 1998a, p.8). Although Pinheiro et al. (1996: fig. 5) illustrated the Jurassic clastic sediments of Leg 149 Site 901 as an offshoot of that basin, it is difficult to envision this basin extending as far west as Leg 173 Sites 1065 and 1069.
ODP Leg 173 was designed to test a variety of models that had been proposed since Leg 149 for lithospheric rifting and the formation of the OCT on the west Iberia margin. Of those discussed by the Shipboard Scientific Party (1998a, pp. 15-19), we believe that the simplest model to explain the dispersal of Jurassic sediments from a restricted interior basin originally aligned with or lying within the Galicia interior basin is the model of detachment faulting proposed by Krawczyk et al. (1996). Although a simplified model, we find it readily adaptable to explain the dispersion of remnants of our postulated Jurassic interior basin from east to west over the OCT (Fig. F5). A more sophisticated detachment-fault model for the IAP has recently been proposed by Manatschal and Froitzheim (1999).