The major events used for identifying the Bolboforma zonal boundaries and zones are the same as those used in Spiegler and von Daniels (1991) and updated in Spiegler (in press b). They are reported from oldest to youngest.
The stratigraphic resolution of Bolboforma is high in Pliocene and Miocene sediments at the Hatton-Rockall Basin. The results from Hole 982A are in accordance with the observations made for Hole 982B. Therefore, they are not reported here in detail. The main observations concerning Bolboforma distribution were made at Hole 982B, where one sample per section and the CC samples were analyzed. For Hole 982B, the uppermost 33 samples of Holocene and Pliocene sediments investigated (Samples 162-982B-2H-1, 114-116 cm, through 7H-5, 18-20 cm, at 6.64 to 59.18 mbsf) are barren of Bolboforma. The LO of the genus Bolboforma is observed in Sample 162-982B-7H-5, 31 cm, at 59.31 mbsf. The Pliocene sequence down to 63.92 mbsf and again between 140.24 and 170.25 mbsf only contains sporadic Bolboforma specimens. In between this interval, Bolboforma specimens are constantly present in increasing quantities, and they are common to abundant in the upper and middle Miocene sediments down to Sample 162-982B-56X-CC at 527.90 mbsf. The sediments of the lower middle Miocene (from Core 162-982B-57X at 528 mbsf) and lower Miocene (through Core 60X at 561 mbsf) are generally barren of Bolboforma. The samples of Cores 162-982B-61X, 62X, and 65X contain sparse, poorly preserved lower Miocene Bolboforma assemblages.
At Hole 982B, the Pliocene assemblages consist of 10 taxa (Table 1). All Pliocene Bolboforma specimens display a slightly elongated oviform test, covered by elongated ribs and wings or spines of different length, but they show no reticulations. The rarely observed smooth tests are interpreted as cysts. The B. costairregularis Zone, spanning parts of the upper and lower Pliocene, can be subdivided into an upper and lower part by the TR of B. costairregularis costairregularis and B. costairregularis variabilis. The co-occurrence of B. costairregularis costairregularis and B. elegans characterizes upper Pliocene sediments. The upper/lower Pliocene boundary lies between Samples 162-982B-9H-1, 84-86 cm (72.84 mbsf), and 9H-3, 84-86 cm (75.84 mbsf). The successive LOs and FOs of B. hispida, B. alata, and B. stella as well as B. carina, B. carinaspinosa, B. tenuis, and B. sphaerica observed downhole contain stratigraphic potential for further subdividing the lower Pliocene sequence. The Pliocene/Miocene boundary lies between the B. costairregularis Zone and the B. intermedia Zone and is documented between Samples 162-982B-18H-7, 24-26 cm, at 166.74 mbsf and 19H-4, 24-26 cm, at 171.74 mbsf. Three samples in this interval were barren of Bolboforma.
The upper Miocene Bolboforma intermedia, B. metzmacheri, and B. capsula Zones are also recognized. The B. intermedia Zone is recorded down to 262.09 mbsf. The TR of B. lockeri between 192.19 and 201.69 mbsf marks an interval in the upper B. intermedia Zone, and B. praeintermedia is dominant in the lowermost part of this zone (Table 2). Single specimens of B. metzmacheri metzmacheri and B. metzmacheri ornata are present in Sample 162-982B-29H-3, 19-21 cm, indicating the transition to the underlying B. metzmacheri Zone, which extends to 331 mbsf. B. polygonalis marks a distinct horizon (from 287.57 to 292.49 mbsf), and B. metzmacheri solida, a thick-walled subspecies, characterizes the base of the B. metzmacheri Zone (Table 3). The B. capsula Zone is identified from 332.49 to 368.09 mbsf.
Single Bolboforma specimens of middle Miocene age, such as B. badenensis, B. platyreticulata, B. danielsi, and B. subfragoris, testify to the continuous redeposition of middle Miocene sediments from the Rockall Bank into the Hatton-Rockall Basin during the late Miocene and latest middle Miocene (from 195.19 to 390.29 mbsf; Table 2 and Table 3). Sample 162-982B-29X-CC at 264.84 mbsf contains a mixed assemblage of few B. metzmacheri and common B. subfragoris, which were heavily encrusted by crystal overgrowths of calcite (Pl. 3, fig. 1). This fact indicates extensive redeposition of sediments containing the B. subfragoris Zone into sediments spanning the B. metzmacheri Zone. Logging data show a turbidite (Jansen, Raymo, Blum, et al., 1996) for the sedimentary sequence of Core 162-982B-30X, where only a single fragment of silica-cemented sand was recovered.
The assemblages in the middle Miocene contain abundant Bolboforma and therefore allow a high stratigraphic resolution (Table 4). All the middle Miocene Bolboforma zones defined by Spiegler and von Daniels (1991) are recognized in Hole 982B. The B. subfragoris Zone is present from 369.59 to 406.59 mbsf; the B. compressispinosa Zone, from 408.09 to 414.69 mbsf; and the B. badenensis Zone, from 416.19 to 435.39 mbsf. In the interval from 432.39 to 433.89 mbsf, the presence of B. atlantica marks the lowermost B. badenensis Zone. The TR of B. danielsi spans the interval from 436.89 to 438.20 mbsf.
The underlying B. reticulata Zone is present from 441.99 to 534.29 mbsf. The TRs of B. platyreticulata and B. reticulata allow a subdivision of the B. reticulata Zone into an upper subzone (down to 488.30 mbsf) and a lower subzone.
The boundary between the middle and lower Miocene at 534.29 mbsf is tentative because of the sparsity of small-sized round Bolboforma specimens of B. rotunda, B. spiralis, B. spinosa, and B. vulgaris, which characterize the lower Miocene sediments in Hole 982B.
DSDP Site 116 is located 6.5 km east of ODP Site 982. At Site 116, two holes (Holes 116 and 116A) were rotary drilled. Hole 116 was only spot cored. In this hole, the first core was taken at 70 mbsf. Until 649 mbsf, cores were cut every 50 m; continuous coring then stopped at 854 mbsf in upper Eocene sediments. Hole 116A was finished at 99 mbsf in lower Pliocene sediments (Laughton, Berggren, et al., 1972).
Hole 116 was reinvestigated for Bolboforma studies. A determination of calcareous nannoplankton zones was given by Perch-Nielsen (1972). Spiegler and Müller (1992) further defined the results more precisely and correlated the nannoplankton zones with the Bolboforma zones. An updated version of the Bolboforma zonation of Hole 116 is given in Table 5.
In the Neogene sequence of Hole 116, 10 Bolboforma zones have been identified. The occurrence of B. costairregularis variabilis and B. tenuis in the samples of Core 12-116-2, between 109.19 and 118.10 mbsf, constrains the lower B. costairregularis Subzone (early Pliocene). Since B. platyreticulata and B. reticulata are here considered as separate species and B. platyreticulata is identified in Sample 12-116-9-2, 74-80 cm, it is possible to recognize the upper B. reticulata Zone in Core 12-116-9 (461.17 to 462.68 mbsf) and the lower B. reticulata Zone in Core 10 (511.99 to 517.70 mbsf). The early Miocene Zones B. rotunda, B. spinosa, and B. spiralis are well documented, whereas Bolboforma are sporadic in the sediments of the Oligocene and Eocene.