13C positive excursion (A. Bartolini, pers. comm., 2001) (Fig. F2) and correlates to peaks in this taxon in the North Sea (Williams and Bralower, 1995) and Southern Alps (Erba and Quadrio, 1987; Tremolada and Erba, 2000).
In the last three decades, calcareous nannofossils have become a very important fossil group for age dating and correlation of Mesozoic pelagic carbonates.
Lower Cretaceous biostratigraphy has achieved considerable stability, and cosmopolitan zonations have been proposed (Thierstein, 1971, 1973, 1976; Sissingh, 1977; Roth, 1978; Perch-Nielsen, 1985). More detailed biozonations were recently proposed for several paleoprovinces, but their reliability is geographically restricted. In this study we applied the zonations of Thierstein (1971, 1973) and Bralower et al. (1995) for the Lower Cretaceous sediments recovered at Site 1149.
In the Lower Cretaceous sedimentary succession studied in Holes 1149B, 1149C, and 1149D, calcareous nannofossils are quite common in the carbonate intervals and their preservation ranges from poor to moderate. Despite the absence or sporadic occurrence of the useful zonal markers, it was possible to identify two nannofossil zones.
The sediments recovered in these holes contain nannofossil assemblages spanning from late Valanginian to late Hauterivian age. In general, the nannofossil assemblage in the three Holes 1149B, 1149C, and 1149D is dominated by Watznaueria barnesae, a species resistant to diagenesis.
CaCO3-rich sediments were recovered from Cores 16R through 29R in Units IV and V.
The nannofossil assemblages in the interval from Cores 23R through 29R indicate a late Valanginian age, belonging to the Calcicalathina oblongata Zone (Thierstein, 1971, 1973) (Zone NK3 in Bralower et al., 1989, 1995). C. oblongata is very rare in the samples investigated, but other useful markers whose combined ranges suggest a late Valanginian age such as Tubodiscus verenae, Tubodiscus jurapelagicus, and Rucinolithus wisei were recognized (Table T1, Pl. P1). In particular, the stratigraphic ranges of T. verenae and T. jurapelagicus are restricted to the middle to upper part of the C. oblongata nannofossil Zone. T. verenae and R. wisei display a very continuous occurrence and were detected from the bottom of the sedimentary section, suggesting a late Valanginian age for the basement. The last occurrences (LO) of T. verenae and R. wisei correspond to Samples 185-1149B-23R-1, 22-26 cm, and 24R-1, 24-28 cm, respectively. Other species commonly recorded in this interval are Cyclagelosphaera margerelii, Parhabdolithus embergeri, Cruciellipsis cuvillieri, Discorhabdus rotatorius, Assipetra infracretacea, Cretarhabdus angustiforatus, Diazomatolithus lehmanii, and Watznaueria spp.
The nannofossil assemblage in Cores 21R through 23R is dominated by W. barnesae but displays an abundant (up to 30%) D. lehmanii, including normal-sized and slightly oversized specimens. This sharp increase corresponds to the base of the 13C positive excursion (A. Bartolini, pers. comm., 2001) (Fig. F2) and correlates to peaks in this taxon in the North Sea (Williams and Bralower, 1995) and Southern Alps (Erba and Quadrio, 1987; Tremolada and Erba, 2000).
The interval from Cores 20R through 16R is assigned to the Lithraphidites bollii Zone (Thierstein, 1971, 1973). We identified the lower boundary of this nannofossil zone by the first occurrence of the zonal marker L. bollii, observed in Sample 185-1149B-20R-1, 139-140 cm. L. bollii is rare in the Pacific Ocean (Bralower, 1987) and in the Tethys as well (e.g., Erba and Quadrio, 1987; Channell and Erba, 1992), but it shows a quite continuous occurrence in the analyzed samples. The first occurrence of Rucinolithus terebrodentarius occurs in Sample 185-1149B-20R-1, 46-47 cm, where geophysical logging records a major change in dip and strike in the sedimentary section (R. Pockalny, pers. comm., 2001) (Fig. F3). The stratigraphic range of R. terebrodentarius spans from late Hauterivian to Turonian; its first occurrence just above the change in dip suggests that the lower-middle Hauterivian corresponds to an unconformity. In addition, the last occurrence of C. cuvillieri is recorded in Sample 185-1149B-20-1, 8-10 cm, indicating a late Hauterivian age (Thierstein, 1971, 1973). In general, the first occurrence of R. terebrodentarius postdates the last occurrence of C. cuvillieri, but our data show the range of C. cuvillieri overlapping with that of R. terebrodentarius. This is in agreement with the results of Erba et al. (1999) and Channell et al. (2000) from the Cismon drill core (Southern Alps, Northeastern Italy). Other species frequently observed in this interval are C. margerelii, Lithraphidites carniolensis, A. infracretacea, Cretarhabdus conicus, C. surirellus, C. angustiforatus, and Watznaueria spp. The abundance of D. lehmanii decreases abruptly in Core 20R. In the upper portion of this interval, overgrown specimens of C. cuvillieri, T. jurapelagicus, and T. verenae were observed in several samples, suggesting a reworking of older strata.
The overlying noncalcareous interval, in Core 16R, is barren of nannofossils, probably reflecting the passage of the site into the high-fertility equatorial zone, where calcareous plankton is overcome by siliceous plankton (Erba, 1992). This transition is also suggested by the change to more cherty lithologies in Unit III (Plank, Ludden, Escutia, et al., 2000).
The sequence of bioevents recognized in Hole 1149B correlates with that of the Tethys section at Capriolo, Northern Italy (Fig. F2). The Capriolo section is a continuous and expanded section spanning from late Berriasian to Barremian with a good bio- (radiolarians and calcareous nannofossils), magneto-, and chemostratigraphic control.
This hole was only spot-cored in two selected intervals, the top and bottom of lithostratigraphic Unit IV, recovering the sediment/basement contact. Recovery rate in Hole 1149C was low (~4%).
The calcareous nannofossil assemblage recorded in this hole is very similar to that recorded in Hole 1149B. Below Core 8R the nannofossil assemblage is assignable to the C. oblongata Zone (Thierstein, 1971, 1973) based on the occurrences of T. verenae, C. cuvillieri, T. jurapelagicus, C. oblongata, and the large abundance of the genus Diazomatolithus (Table T2). The last occurrence of R. wisei was observed in Sample 185-1149C-8R-1, 13-14 cm, whereas the last occurrence of T. verenae was detected in Sample 8R-1, 0-3 cm. The first occurrence of L. bollii, which defines the lower boundary of the L. bollii nannofossil Zone (Thierstein, 1971, 1973), was identified in Sample 185-1149C-6R-1, 35-41 cm, together with the last occurrence of C. cuvillieri. The identification of these two events indicates that Core 6R can be attributable to the middle part of the L. bollii Zone. Core 7W is a wash core spanning a ~60-m-thick interval (from 322 to 388.2 mbsf) that corresponds to the upper part of the C. oblongata Zone and the lower part of the L. bollii Zone. The last occurrence of C. cuvillieri precedes the first occurrence of R. terebrodentarius, which lies in Sample 185-1149C-6R-1, 30-34 cm. The first occurrence of R. terebrodentarius is unreliable because of the scarcity of this species near the beginning of its stratigraphic range. Reworked specimens of T. verenae and T. jurapelagicus were observed frequently in the upper portion of the cored section.
Recovery in this hole was very low (~4%). A few samples were studied to assess the age of the oldest sediments overlying the basement and to compare the results with those from Hole 1149B (Table T3). Preservation is generally poor, and Sample 185-1149D-3R-1, 34-35 cm, is barren of calcareous nannofossils. The two lowermost Samples (185-1149D-5R-1, 13-15 cm, and 4R-1, 66-70 cm) contain a sparse, poorly preserved assemblage, characterized by the rare occurrence of highly resistant species (W. barnesae, Watznaueria ovata, and C. margerelii). The occurrences of marker species T. verenae, R. wisei, and C. oblongata are likely affected by poor preservation and cannot be excluded. Based on the occurrence of T. verenae and R. wisei in Sample 185-1149D-4R-1, 1-5 cm, the base of the hole corresponds to the C. oblongata Zone of Thierstein (1971), but no further biostratigraphic detail is possible. This basal age is similar to that in Hole 1149B. Overlying sediments do not contain L. bollii. Thus, the lower boundary of the L. bollii Zone of Thierstein (1973) has not been cored in this hole, and direct correlation with Holes 1149B and 1149C is only hypothetical (Fig. F4).
