As opposed to the Paleogene, silicoflagellate assemblages show a marked latitudinal differentiation starting in the Miocene. This has led to the establishment of zonal schemes for high-latitude areas that are more adequate for stratigraphic purposes than the application of low-latitude zonations. Locker and Martini (1989) recognized this aspect and modified previous zonal schemes to improve the stratigraphic value of silicoflagellates in high-latitude realms such as the Norwegian-Greenland Sea. The zonation for the Norwegian Sea's Leg 104 sites that was presented by these authors was basically a refined version of that used for the Norwegian Sea's Leg 38 sites (Martini and Müller, 1976).
Unfortunately, the authors continued to use the Corbisema triacantha Zone without modifying it (other than subdividing it), which had been defined for low-latitude realms (Martini, 1971). This choice is not entirely convenient, since the top zonal boundary is given by the last occurrence of C. triacantha. This taxon is scattered and rare throughout its range at high latitudes, making it hard to clearly distinguish its last appearance. To circumvent this, Locker and Martini (1989) used the peak occurrence of Distephanus stauracanthus together with the last appearance of Caryocha depressa (listed by them as Cannopilus depressus) to identify the top of the zone. These criteria are questionable since D. stauracanthus is another rare taxon at high latitudes (Locker and Martini, 1989, table 1). The last appearance of C. depressa, therefore, might not be as clear-cut as proposed by these authors (see Ciesielski et al., 1989). For high latitudes, the first appearance of Bachmannocena circulus apiculata is a much better choice and is used here to replace the last appearance of C. triacantha as the top zonal boundary of the upper C. triacantha Zone.
This in turn changes the basal zonal boundary of the overlying Bachmannocena circulus apiculata Zone (Locker and Martini, 1989). This marks the extent of the modification of the basic zonal scheme since its top zonal boundary remains unchanged (last occurrence of B. c. apiculata). In this way, the zone now represents this taxon's entire range. The high-resolution sampling interval of Miocene sediments with silicoflagellates from the two high-latitude sites (907 and 982) from Leg 162 considered in this study, however, has allowed the identification of two important bioevents that permit the establishment of three subzones within the B. c. apiculata Zone (see below). Because no biosilica-bearing lower Miocene sections were recovered by Leg 162, early Miocene zonal schemes are not treated here. Miocene biosilica-bearing sections younger than the last occurrence of B. c. apiculata were analyzed but did not provide information useful to refine the pre-existing zones for that interval. Samples for these intervals are assigned to the Bachmannocena diodon Zone (Ciesielski, 1975) and the Distephanus boliviensis Zone (Ciesielski et al., 1989).
In this way, the detailed part of the zonal scheme is improved while maintaining the basic zones known by high-latitude biostratigraphers. Since most zones were found in both Holes 982B and 907B, any differences in the taxa present between the holes are indicated in the description of the assemblage, as well as any disparities between the abundance of the taxa. Given that the sites considered in this chapter are separated by ~10 degrees of latitude, there might be a slight factor of diachroneity in the datums used here to establish the zonal/subzonal boundaries. The taxa selected, however, are not only common constituents of the assemblages, but they also maintain a certain coherence amongst themselves; the datums appear in the same sequence regardless of their climatic sensitivity. Less reliable taxa were discarded for biostratigraphic purposes.
The details of the zonal scheme used herein are as follows:
Definition: Interval from the last occurrence of Naviculopsis quadrata to the first occurrence of Bachmannocena diodon diodon. Early to middle middle Miocene.
Reference: Locker and Martini (1989) subdivided Martini's (1971) Corbisema triacantha Zone into an upper zone and a lower zone based on the first occurrence of Bachmannocena diodon diodon. This zone is used herein without modification, other than pointing out that B. diodon has several subspecies. It is the above mentioned subspecies that is used to define the top of this zone. Locker and Martini (1989) do not distinguish between the taxon's subspecies. Their use by other authors and within this chapter has been shown to be of stratigraphic value, however, and should be continued given the clear morphological distinction between them.
Occurrence interval: Found in Hole 982B, from Sample 162-982B-56X-5, 99-100 cm, to the bottom of the biosilica-bearing interval.
Assemblage: Bachmannocena apiculata curvata, Corbisema triacantha (rare), Dictyocha arcuata, Dictyocha fibula, Distephanus crux ssp. (abundant), Distephanus schulzii, and Distephanus speculum ssp.
Remarks: Only the upper part of this zone is present in the biosilica-bearing interval obtained from Hole 982B during Leg 162. It is placed within the CN4/CN3 nannoplankton zones of Okada and Bukry (1980) by the Shipboard Scientific Party of Leg 162.
Definition: Interval from the first occurrence of Bachmannocena diodon diodon to the first occurrence of Bachmannocena circulus apiculata. Middle middle Miocene.
Reference: As discussed above, this zone was introduced by Locker and Martini (1989) and composed the upper part of Martini's (1971) Corbisema triacantha Zone. The last occurrence of Corbisema triacantha marked the zone's upper boundary. As explained earlier, this datum is considered unreliable at high latitudes. It is here replaced by the first occurrence of Bachmannocena circulus apiculata, a common and conspicuous taxon throughout its range in high latitudes such as those examined herein and in Legs 38 and 104. Since the first appearance of B. c. apiculata is older than the last appearance of C. triacantha, the upper C. triacantha Zone as modified spans a shorter interval of time than it originally did.
Occurrence interval: Found in Hole 982B, from Samples 162-982B-50X-3, 99-100 cm, to 162-982B-56X-1, 99-100 cm; and in Hole 907B, from Sample 162-907B-22H-2, 109-110 cm, to the bottom of the biosilica-bearing interval.
Assemblage: Bachmannocena apiculata apiculata, Bachmannocena apiculata curvata (common), Bachmannocena diodon diodon (common in Hole 982B), Bachmannocena elliptica elliptica (in Hole 982B), Bachmannocena elliptica miniformis, Caryocha depressa (common in Hole 982B), Corbisema triacantha (rare), Dictyocha arcuata (common in Hole 982B), Dictyocha fibula, Distephanus crux ssp. (abundant), Distephanus quinarius (Hole 982B), Distephanus schulzii (Hole 982B), and Distephanus speculum ssp. (Hole 982B).
Remarks: It is placed in Hole 982B within the CN4 nannoplankton zone of Okada and Bukry (1980) by the Shipboard Scientific Party of Leg 162.
Definition: Interval from the first to the last occurrence of Bachmannocena circulus apiculata. Late middle Miocene to middle late Miocene.
Reference: Locker and Martini (1989) introduced the Bachmannocena circulus apiculata Zone as the interval from the last occurrence of Corbisema triacantha to the last occurrence of Bachmannocena circulus apiculata (= Paramesocena circulus apiculata of Locker and Martini, 1989). As discussed above, the last occurrence of Corbisema triacantha as the top of the upper Corbisema triacantha Zone is changed herein to the first occurrence of Bachmannocena circulus apiculata, which in turn changes the base of this zone. In this way, their zone would represent the total range of Bachmannocena circulus apiculata. Two important bioevents allow the recognition of the following three subzones:
Definition: Interval from the first occurrence of Bachmannocena circulus apiculata to the first occurrence of Bachmannocena diodon nodosa. Late middle Miocene.
Reference: This subzone spans the interval from the modified base of Locker and Martini's (1989) Paramesocena circulus apiculata Zone to the first of the important bioevents, the first occurrence of Bachmannocena diodon nodosa, comprising the lower part of their zone and the uppermost part of their upper C. triacantha Zone.
Occurrence interval: Found in Hole 982B, from Samples 162-982B-45X-1, 99-100 cm, to 162-982B-49X-2, 99-100 cm; and in Hole 907B, from samples 162-907B-17H-3, 109-110 cm, to 162-907B-22H-1, 109-110 cm.
Assemblage: Bachmannocena apiculata apiculata, Bachmannocena apiculata curvata, Bachmannocena circulus apiculata (common), Bachmannocena diodon diodon (common in Hole 982B), Bachmannocena elliptica elliptica (in Hole 982B), Bachmannocena elliptica miniformis, Caryocha depressa (common), Caryocha ernestinae (common in Hole 907B), Corbisema triacantha (rare), Dictyocha fibula (common in Hole 907B), Dictyocha subclinata, Dictyocha varia (common), Dictyocha sp. 1 (Hole 982B), Distephanus crux ssp. (common), Distephanus speculum ssp. (Hole 982B), and Distephanus stauracanthus (Hole 982B, first and last occurrence within this zone).
Remarks: The importance of using the first occurrence of Bachmannocena circulus apiculata was recognized by Ciesielski et al. (1989), who defined a similar interval in this new subzone as the Bachmannocena circulus var. apiculata/Caryocha Zone using this bioevent. Their choice of the last occurrence of Caryocha species as the top of their zone, however, is less defined than the first occurrence of Bachmannocena diodon nodosa (used herein) because caryochids become rare and scattered toward the top of their range. In Hole 982B, it spans the middle to lower CN5 nannoplankton zone of Okada and Bukry (1980), as indicated by the Leg 162 Shipboard Scientific Party (1996).
Definition: Interval from the first occurrence of Bachmannocena diodon nodosa to the last occurrence of Distephanus crux stradneri. Latest middle Miocene to earliest late Miocene.
Reference: As discussed above, under Caryocha spp. Subzone, this new subzone comprises the middle part of Locker and Martini's (1989) Paramesocena circulus apiculata Zone. Its top marks the second important bioevent during the range of Bachmannocena circulus apiculata, the time at which the Distephanus crux subspecies, which are common constituents of assemblages until then, disappear. Various of these subspecies disappear close to or at this time (see Table 1, Table 2); Distephanus crux stradneri was chosen only because it is the last one to become extinct.
Occurrence interval: Found in Hole 982B, from Samples 162-982B-32X-3, 99-100 cm, to 162-982B-44X-3, 99-100 cm; and in Hole 907B, from Samples 162-907B-14H-1, 109-110 cm, to 162-907B-17H-2, 99-100 cm.
Assemblage: Bachmannocena circulus apiculata (abundant), Bachmannocena circulus circulus (Hole 907B), Bachmannocena diodon diodon, Bachmannocena diodon nodosa (common), Bachmannocena diodon triodon (first and last occurrence within this zone), Bachmannocena elliptica elliptica, Caryocha depressa (Hole 907B), Dictyocha subclinata, Dictyocha varia, Dictyocha sp. 2 (Hole 907B), Dictyocha sp. 3 (Hole 907B), Distephanus crux ssp. (common), Distephanus polyactis (Hole 982B, first and last occurrence within this zone), Distephanus quinarius (acme in Hole 907B), Distephanus speculum ssp. (common), and Distephanus xenus (Hole 982B).
Remarks: Ciesielski et al. (1989) defined the Distephanus crux scutulatus Zone for an interval similar to this new subzone presented in this chapter. However, in addition to using a different base (see discussion above on their use of the last occurrence of Caryocha species), a different subspecies of Distephanus crux is here used as the top (Distephanus crux stradneri vs. Distephanus crux scutulatus). This is because the subspecies used by them has a less defined last occurrence, based on its low abundance toward the upper part of its range (see Table 1, Table 2). In Hole 982B, it spans the upper CN5 nannoplankton zone to the lower part of the CN6/CN9 interval of Okada and Bukry (1980) as given by Shipboard Scientific Party (1996).
Definition: Interval from the last occurrence of Distephanus crux stradneri to the last occurrence of Bachmannocena circulus apiculata. Middle late Miocene.
Reference: This new subzone comprises the upper part of Locker and Martini's (1989) Paramesocena circulus apiculata Zone.
Occurrence interval: Found in Hole 982B, from Samples 162-982B-24H-5, 99-100 cm, to 162-982B-31X-4, 99-100 cm; and in Hole 907B, from Samples 162-907B-12H-2, 109-110 cm, to 162-907B-13H-5, 120-121 cm.
Assemblage: Bachmannocena circulus apiculata (common), Bachmannocena diodon diodon (common), Bachmannocena diodon nodosa (common), Dictyocha brevispina ausonia (common in Hole 982B), and Distephanus speculum ssp. (abundant).
Remarks: The top part of this subzone is most likely missing in Hole 907B. In Hole 982B, it spans the middle part of the CN6/CN9 nannoplankton zones (Okada and Bukry, 1980) interval given by Shipboard Scientific Party (1996).
Definition: Interval from the last occurrence of Bachmannocena circulus apiculata to the last occurrence of Bachmannocena diodon ssp. Late Miocene.
Reference: Introduced by Perch-Nielsen (1975) as the Mesocena diodon Subzone of her Paradictyocha dumitricae Zone, with the last occurrence of Bachmannocena diodon as the upper boundary, modified by Ciesielski (1975) using the last occurrence of Bachmannocena circulus apiculata as the lower boundary.
Occurrence interval: The lower part of this zone is present in Hole 982B, from Sample 162-982B-23H-3, 99-100 cm, to the top of the biosilica-bearing interval considered in this study. In Hole 907B, Samples 162-907B-11H-4, 109-110 cm, to 162-907B-12H-1, 109-110 cm, are tentatively assigned to this zone based on the absence of B. c. apiculata throughout this interval and the presence of B. d. nodosa in Sample 162-907B-11H-4, 109-110 cm. The preservation of silicoflagellates within this interval, however, is poor, and reworked specimens were also noted (see Table 2). A clearer last occurrence of B. diodon is needed to assign this interval without doubts to the B. diodon Zone in this hole. For these reasons, the top of the zone is represented by a dotted line in Table 2.
Assemblage: Bachmannocena diodon borderlandensis (Hole 982B), Bachmannocena diodon diodon (Hole 982B), Bachmannocena diodon nodosa (common in Hole 982B, rare in Hole 907B), Bachmannocena quadrata (rare), Dictyocha brevispina ausonia (Hole 982B), Dictyocha messanensis stapedia f. stapedia (Hole 982B), and Distephanus speculum ssp. (abundant).
Remarks: The upper part of this zone is missing in Hole 982B, but the good preservation in the lower part provides information that is otherwise missing in Hole 907B. Preservation during this section is extremely poor, and at least some part of the zone toward its base is missing. In Hole 982B, the section studied herein spans the upper part of the CN6/CN9 nannoplankton zone interval (Okada and Bukry, 1980) as given by Shipboard Scientific Party (1996).
Definition: Interval from the last occurrence of Bachmannocena diodon ssp. to the first occurrence of Distephanus jimlingii. Latest Miocene to earliest Pliocene.
Reference: Introduced by Ciesielski et al. (1989).
Occurrence interval: Samples 162-907B-11H-3, 109-110 cm, to the top of the biosilica-bearing interval in Hole 907B are tentatively assigned to this zone, based on the last occurrence of B. d. nodosa in Sample 162-907B-11H-4, 109-110 cm. As discussed above under B. diodon Zone, this datum is not entirely reliable.
Assemblage: Distephanus boliviensis and Distephanus speculum ssp. (abundant).
Remarks: Given that Distephanus jimlingii was not encountered in the well-preserved samples of this interval (Samples 162-907B-10H-2, 70-71 cm, and 162-907B-10H-5, 117-118 cm), it is assumed that its first occurrence is in sediments younger than those examined in this study. Therefore, only the lower part of the zone is present in the section considered herein (Hole 907B). This interval would fit mostly within Subchron 3r according to Shipboard Scientific Party (1996).