SILICOFLAGELLATE ZONATION

A review of silicoflagellate literature concerning deep-ocean sites (McCartney et al., 1995) shows that only a few papers provide silicoflagellate biostratigraphy of low-latitude (<25°) Paleogene sediments. Only four legs have investigated silicoflagellates from the Oligocene (see Ling, 1980 [for Deep Sea Drilling Project (DSDP) Holes 165A and 166]; Bukry, 1977, 1978d, 1985), and there have been no equatorial studies of the Eocene and Paleocene, except a data report by Bukry (1989). The low-latitude and cosmopolitan zonation provided by Bukry (1981b) uses information from middle and high latitudes for the Eocene intervals. Ciesielski (1991) presented a zonation for the Eocene and Oligocene for middle and high latitudes, which was emended by McCartney and Harwood (1992). Leg 199 provides an excellent opportunity to build on this more recent work to develop a low-latitude Eocene and Oligocene zonation. This zonation complements the low-latitude Neogene biostratigraphy provided by McCartney et al. (1995).

Definition: Interval between the first and last occurrence of Dictyocha hexacantha.

Author: Bukry and Foster, 1974; modified by Bukry, 1977.

Common species: In addition to D. hexacantha, common species include Corbisema bimucronata, Corbisema hastata, and Corbisema regina. Naviculopsis constricta occurs sporadically and is dominant in Sample 199-1219A-21H-4, 70–71 cm.

Remarks: The interval from Sample 199-1219A-21H-6, 70–71 cm, to 21H-2, 70–71 cm, is designated the Dictyocha spinosa Subzone. The subzone also includes C. bimucronata and C. regina, though these taxa are not restricted to this subzone. Bukry (1981b) placed this subzone in the Naviculopsis foliacea Zone, a zone not recognized in this study, which he placed below the D. hexacantha Zone.

Definition: Last occurrence of D. hexacantha to the initial occurrence of N. foliacea.

Author: Ciesielski (1991), emended.

Common species: Corbisema triacantha is present with only rare occurrences of other taxa except N. constricta. Dictyocha byronalis is present and generally dominant in the four samples that constitute the D. byronalis horizon, as defined below.

Remarks: The original description of this zone by Ciesielski (1991) used the last occurrence of Dictyocha precarentis, a taxon not found in this study, as the bottom of the zone. An equivalent of this zone is the C. hastata–Corbisema apiculata Interval Range Zone described by Perch-Nielsen (1975).

The interval placed in this zone is largely barren of silicoflagellates except for three samples at the top of the interval that include N. constricta and a four-sample interval from Sample 199-1219A-15H-5, 70–71 cm, to 6H-1, 70–71 cm, that contains D. byronalis. The latter is designated the D. byronalis Horizon, defined as the interval from the first to last common D. byronalis.

Common species: C. triacantha, with sporadic occurrences of Naviculopsis biapiculata and rare occurrences of Distephanus crux. The interval is left unzoned because of a lack of diversity and the lack of key zonal species.

Remarks: There is little general agreement on how best to zone the upper Eocene and lower Oligocene interval between the middle Eocene Corbisema hexacantha Zone and the upper Oligocene Bachmannocena apiculata Zone. This is well illustrated for Leg 29 sites, where Bukry (1975b) applied the Dictyocha deflandrei Interval Zone, while Perch-Nielson (1975) applied the C. hastata–C. apiculata Interval Zone and other thinner zones. Martini and Müller (1976) used C. bimucronata and Dictyocha quadria as zones to cover this general interval. The interval between Samples 199-1219A-14H-6, 70–71 cm, and 8H-6, 70–71 cm, did not contain D. deflandrei, C. apiculata, or D. quadria. The interval also lacked taxa above the last occurrence of N. constricta that might be used for describing a new zone, and thus this interval is left unzoned in this study. The interval could be applied to the N. foliacea Zone described by Martini and Müller (1976), but N. foliacea is only found in abundance in the bottom sample of the interval.

Definition: Interval from the first to last common occurrence of B. apiculata.

Author: Perch-Nielsen (1975).

Common species: B. apiculata and Distephanus speculum speculum are commonly present. Other species found sporadically include D. crux and N. biapiculata.

Remarks: Bukry (1981b) defines the bottom of this zone as the last occurrence of D. hexacantha.

Definition: Interval between the last common occurrence of B. apiculata and the first occurrence of Naviculopsis lata.

Author: Bukry (1978a).

Common species: Distephanus speculum and D. crux.

Remarks: N. biapiculata was only found in the lowermost two samples of the interval placed in this zone.

Definition: Interval between the first and last common occurrence of N. lata.

Author: Martini (1972).

Common species: N. constricta, N. biapiculata, and D. crux.

Remarks: This zone was applied in only three samples covering an interval of ~3 m (31.7–34.5 meters below seafloor [mbsf]).

Definition: Interval from the first appearance of Naviculopsis ponticula to the last appearance of Naviculopsis ssp.

Author: Bukry (1980a).

Common species: D. crux and N. biapiculata.

Remarks: Bukry (1985, see also Bukry, 1982) noted that N. ponticula is most abundant in the lower part of the zone, whereas N. ponticula spinosa is dominant in the upper part of the zone. A similar pattern was found in this study, where N. ponticula was found to have more prominent apical spines in the uppermost sample from the zone.

Definition: Interval from last occurrence of Naviculopsis species to last occurrence of C. triacantha.

Author: Martini (1971).

Common species: Common species include Dictyocha fibula fibula, D. fibula ausonia, and Distephanus crux crux. Both D. speculum and D. crux are less abundant in this zone than in the underlying N. ponticula Zone.

Remarks: This zone includes the Distephanus stauracanthus horizon described by Martini (1972), but the horizon is only represented by a single specimen in each of the uppermost three samples of the zone. This horizon was also applied by Locker and Martini (1986), and in this study, the horizon is much thinner than the 50-m presence in Hole 844B (McCartney et al., 1995).

Definition: The bottom of this zone is defined as the last occurrence of C. triacantha. Locker and Martini defined the top of this zone as the first consistent occurrence of Dictyocha extensa, which McCartney et al. (1995) modified to the first occurrence of Distephanus speculum tenuis. Neither D. extensa nor D. speculum tenuis were found in this study.

Author: Locker and Martini (1986; see also McCartney et al., 1995).

Common species: D. fibula fibula was the most common species.

Remarks: Only four taxa were found in the samples that were placed in this zone. They were D. fibula fibula, D. speculum, D. crux, and D. varia. Only two specimens of D. varia were identified in this interval.