For the last two and one-half decades, many Oligocene to early Miocene dinoflagellate cyst zonations have been proposed. These include
Detailed correlation of a given individual zone with coeval zone(s) elsewhere is often difficult to accomplish because the zonal concepts may be different or provincialism or environmental setting may limit the occurrence of some key species. This may be caused by more differentiated oceanic regimes and complicated environments in the Oligocene and Miocene compared to pre-Oligocene time. For example, the Oligocene Deflandrea heterophlycta Assemblage Zone and Chiropteridium dispersum Assemblage Zone proposed by Williams (1975) for offshore eastern Canada cannot be correlated satisfactorily with the Oligocene Chiropteridium lobospinosum Zone, Areosphaeridium? actinocoronatum Zone, and Impagidinium spp. Zone proposed by Manum et al. (1989) for the Norwegian Sea. Brinkhuis et al. (1992) efficiently used the youngest acmes of Chiropteridium and Deflandrea when working on a high-resolution dinoflagellate stratigraphy of the Oligocene–Miocene transition interval in the Piedmont Basin of northwest Italy and the Marche Basin of central Italy, but Chiropteridium is very rare and Deflandrea is lacking in the South China Sea section we studied. Regardless of these difficulties, comparison of similarities or differences between our zones and other coeval zones is possible and may reveal some interesting results on the distribution patterns of some taxa relating to provincialism or environmental settings.
The C. diversispinosum Assemblage Zone (Zone A) can be generally correlated with the Oligocene dinoflagellate assemblage(s) or zone(s) from areas where upwelling is not well developed (e.g., from offshore eastern Canada) (Williams and Bujak, 1977), with which it has 21 species in common (Table T3). However, our Zone A has less similarity in species composition when compared with the coeval dinoflagellate cyst assemblages from the Niger Delta (Biffi and Grignani, 1983), which are dominated by peridinioid cysts like Lejeunecysta, with only few gonyaulacoid chorate taxa being present; this is typical for environments with well-developed upwelling of the water mass.
The E. pectiniformis Subzone (Subzone A-1) has 12 species in common with the early Oligocene C. lobospinosum Zone from the Norwegian Sea (Manum et al., 1989). They are C. cantharellum, C. inodes, C. placacanthum, D. ellipticum, E. arcuata, E. pectiniformis, H. campanula, L. machaerophorum, P. laticinctum, Spiniferites pseudofurcatus, T. pelagica, and W. articulata. Subzone A-1 shares 24 species with the early Oligocene four interval zones from central Italy (Brinkhuis and Biffi, 1993), which constitutes more than 40% of all species (subspecies) recorded from Subzone A-1. Except for C. placacanthum, E. arcuata, L. machaerophorum, and W. articulata, the other 8 are common to both the early Oligocene C. lobospinosum Zone of the Norwegian Sea and four interval zones from central Italy. In addition, C. ancyreum, H. plectilum, H. cinctum, R. actinocoronata, and W. gochtii are also among the 24 species. Twelve dinoflagellate assemblages from the early Oligocene Boom Clay Formation of northwest Belgium (Stover and Hardenbol, 1993) have seven species in common with our Subzone A-1 including C. diversispinosum and E. pectiniformis. The coeval dinoflagellate Zone D14 from northwest Germany (Köthe, 1990) shares 18 species with Subzone A-1. Among these 18 species, 8 are also found in the early Oligocene C. lobospinosum Zone from the Norwegian Sea, 11 are also found in central Italy, and 2 are also found in the Boom Clay Formation in northwest Belgium (Table T3).
Subzone A-1 has less similarity to the early Oligocene D. heterophlycta Assemblage Zone from offshore eastern Canada (Williams, 1975) and Zone VI of the Rockall Plateau, North Atlantic (Costa and Downie, 1979) because they have few species, such as E. multicornuta, in common. Deflandrea spp. are common in the latter two zones but are absent from our Subzone A-1.
The C. gracile Subzone (Subzone A-2) can be correlated with the H. plectilum–C. gracile Assemblage from the Pearl River Mouth Basin of the South China Sea (Mao and Lei, 1996) with 28 species in common, which represents 50% of the species (or subspecies) recorded from Subzone A-2. Cleistosphaeridium ancyreum, C. diversispinosum, C. placacanthum, C. gracile, P. zoharyi, and H. plectilum occur throughout both Subzone A-2 and the assemblage; H. plectilum is particularly abundant in both. Subzone A-2 contains 15 species (including H. campanula, H. plectilum, and R. actinocoronata) in common with the Areosphaeridium? (now Reticulatosphaera) actinocoronata Zone and Impagidinium sp. 1 Zone from the lower upper Oligocene–upper Oligocene in the Norwegian Sea (Manum et al., 1989) (Table T3).
Subzone A-2 has 13 species (including C. inodes, Dapsilidinium pseudocolligerum, and E. arcuata) in common with the late Oligocene assemblage from Blake Plateau, North Atlantic (Stover, 1977). A number of species, such as C. cantharellum, H. cinctum, H. rigaudiae, P. laticinctum and S. pseudofurcatus, are common to all four areas: the Norwegian Sea, the Blake Plateau, the Pearl River Mouth Basin, and Subzone A-2. The latest Oligocene dinoflagellate assemblage from the Lemme section of northwest Italy (Powell, 1986a) may be correlated with our Subzone A-2, as it shares 22 species, including C. gracile, C. cantharellum, H. plectilum, H. tenuispinosum, and T. pelagica. However, the coeval T. vancampoae Assemblage Biozone from the same region but near the Langhe Region (Powell, 1986b) has only 3 species (C. cantharellum, T. pelagica, and T. vancampoae) in common with Subzone A-2. The Zone D15 from northwest Germany (Heilmann-Clausen and Costa, 1990; Köthe, 1990) may be correlated as well with our Subzone A-2, having 8 species in common.
The late Oligocene Operculodinium xanthium Zone and Homotryblium floripes Zone from the Lower Rhine Basin of western Germany (Benedek and Müller, 1974) have less similarity with Subzone A-2 because both have only few species, such as O. xanthium, in common.
The P. zoharyi Assemblage Zone (Zone B) has many similarities to the early Miocene P. zoharyi–L. machaerophorum Assemblage from the Pearl River Mouth Basin of the South China Sea (Mao and Lei, 1996). Fossil abundance and species diversity are distinctly low in both Zone B and the assemblage, but they still have 12 species (including H. obscura and P. zoharyi) in common. The early Miocene BB I to lower part of BB II Assemblage Zones from Baffin Bay (Head et al., 1989) have 10 species in common with Zone B, including C. ancyreum, C. tenuitabulatum, H. obscura, M. choanophorum, R. actinocoronata, and T. vancampoae.
Zone B may correlate to some extent with the early Miocene dinoflagellate assemblages from the Rockall Plateau of the eastern North Atlantic (Costa and Downie, 1979) and the Goban Spur of the Atlantic (Brown and Downie, 1985), in sharing 7 species, which include C. placacanthum, H. obscura, Impagidinium aculeatum, and M. choanophorum. Likewise, Zone B may also correlate to some extent with the early Miocene dinoflagellate assemblages from the Gulf of Mexico, offshore Louisiana (LeNoir and Hart, 1986), with which it shares 8 species, including C. tenuitabulatum, H. obscura, P. zoharyi, Tectatodinium pellitum, T. vancampoae, and others (Table T4). However, it has less similarity with the coeval assemblages from near–delta shelf environments of the same area (Duffield and Stein, 1986), which are characterized by the dominance of peridiniacean cysts and scarcity of Impagidinium.
Zone B has little similarity with the early Miocene dinoflagellate assemblages from the western and northern Pacific (Bujak, 1984; Bujak and Matsuoka, 1986) because it lacks the cold-water indicator Spiniferites ellipsoideus and the abundance of protoperidiniacean cysts seen in the north Pacific. Although both have eight species in common, most of these, except for H. obscura, are long-ranging species such as L. machaerophorum and O. centrocarpum.
De Verteuil and Norris (1996) studied Miocene dinoflagellate stratigraphy of the Maryland and Virginia Mid-Atlantic coastal margins and established 10 interval zones with detailed information. Unfortunately, it is difficult to compare their four early Miocene zones with our Zone B in detail, except for the ubiquitous occurrence of C. placacanthum and C. satchelliae in both, which may correlate them in a general way. However, most of their key species, such as D. phosphoritica, S. soucouyantiae, and Cousteaudinium aubryae, were not recognized in Zone B. The earliest Miocene dinoflagellate assemblage from the Lemme section of northwest Italy (Powell, 1986a) may be correlated with our Zone B in that they share 11 species, which include C. placacanthum, H. obscura, M. choanophorum, and P. laticinctum. But the three early Miocene Oppel zones DM1–DM3 from the Marche Region of central Italy (Biffi and Manum, 1988) may be linked with our Zone B by virtue of sharing M.? picena.
It is difficult to compare the four early Miocene interval zones of Ascostomocystis granosa, Evittosphaerula paratabulata, Impagidinium patulum, and Apteodinium spiridoides from the Norwegian Sea (Manum et al., 1989) with our Zone B in any detail because the four eponymic species and other key species are not present in Zone B. However, the interval zones do have eight species in common with our Zone B, including H. obscura, P. laticinctum, T. vancampoae, C. tenuitabulatum, and I. aculeatum.
In summary, certain important species are always present in any of these coeval assemblage(s) and zone(s) and make it possible to correlate them (Tables T3, T4). The following 18 species (or most of them), when present together in an assemblage, provide clues to an Oligocene aspect: C. ancyreum, C. placacanthum, C. cantharellum, C. gracile, C. inodes, C. tenuitabulata, D. pseudocolligerum, E. arcuata, H. campanula, H. plectilum, H. tenuispinosum, H. cinctum, H. rigaudiae, L. machaerophorum, P. laticinctum, S. pseudofurcatus, R. actinocoronata, and T. pelagica. When any of E. pectiniformis, P. amoenum, D. ellipticum, or M. aspinatum occurs in association with any of the previous 18 species, it may be evidence for further differentiation into early or late Oligocene age.
The ranges of C. cantharellum, C. tenuitabulata, H. plectilum, H. rigaudiae, L. machaerophorum, P. laticinctum, and P. zoharyi may extend upward into younger strata. However, when present together with key species such as C. satchelliae, H. obscura, M. choanophorum, M.? picena, O. longispinigerum, O. piaseckii, and T. vancampoae, their co-occurrence indicates a Miocene age.
Presence of species of Deflandrea (mainly D. phosphoritica), Wetzeliella (mainly W. articulata, W. gochtii, and W. symmetrica), and P. zoharyi may have some environmental significance. Deflandrea prefers the medium to cold waters of mid- to high latitudes. Abundant P. zoharyi has often been recovered in assemblages from low-latitude warm waters such as the Pearl River Mouth Basin of the South China Sea (Mao and Lei, 1996), the offshore South China Sea, and Taiwan (Mao et al., 2002). Cleistosphaeridium ancyreum, C. diversispinosum, and C. placacanthum appear to favor offshore settings, whereas Wetzeliella prefers a shallow brackish water environment.