Ecuaciones como objeto de estudio en sí mismo

Two types of biostratigraphic schemes have been recognised by the Japanese palaeontologists. Firstly, assemblage-zones were established by (Yao, 1982; Yao et a l, 1982; Kishida and Sugano, 1982; Nishizono and Murata, 1983; Igo and Nishimura, 1984; Kishida 1986). These have been useful for general correlation and age assignment (Yao, 1991). More recently, increased precision has been possible with radiolarian zonations based on biohorizons (see Matsuoka and Yao, 1986, Yoshida, 1986) forming range and interval zones.

Nakaseko and Nishimura (1979) recognised three radiolarian assemblages from the Late Triassic of southwest Japan: the Capnuchosphaera theloides, Tripocyclia cf. acythus and Emiluvia (?) cochleata assemblages. The first contained species of Capnuchosphaera, Capnodoce, Syringocapsa and Poulpous, and is similar to assemblages recovered from Timor during this study. Yao (1982) distinguished four radiolarian assemblages from the Mino Terrane: Triassocampe deweveri, Triassocampe nova, Canoptum triassicum, and Parahsuum simplum covering the Middle and Upper Triassic and Lower Jurassic. These contain species which are routinely recovered from the sediments studied during this study.

Chichibu Belt, Japan. The oldest was the Emiluvial cochleata (Anisian-Ladinian boundary) followed by Eptingium manfredi (Ladinian-Carnian boundary), Capnodoce anapetes (Camian-Norian boundary) Spongosaturnalis multidentatus (later changed to Palaeosatumalis multidentatus) for the upper Norian and Rhaetian.

Kishida and Hisada (1985) carried out an extensive study of the Ueno-mura area in the Kanto Mountains of central Japan. Two subassemblages were recognised within the Norian-Rhaetian Palaeosatumalis multidentatus Assemblage zone. These were the Canoptum aff. triassicum subassemblage consisting of Palaeosatumalis multidentatus, P. cf. triassicum , P. cf. quinquespinosa, Pseudoheliodiscus finchi, and Gorgansium gongyloideum. The younger Canoptum lubricum subassemblage zone is characterised by Canoptum lubricum, Palaeosatumalis multidentatus,

Pseudoheliodiscus finchi, Gorgansium gongyloideum, G. crassum, and

Orbiculiforma multifora.

Yoshida (1986) defined seven radiolarian zones (Textfigs. 6.3, 6.6, 6.9.4) covering

the Upper Triassic to Lower Jurassic of Gifu Prefecture, Central Japan. The faunal assemblages of Yoshida (1986) are recovered from continuous sequences of chert which enables an apparently unbroken time interval to be studied. Only radiolarians were recovered from the samples and therefore age determination was based on a comparison with other, integrated, radiolarian faunal sequences in the nearby Inuyama area. This lack of index fossils from other groups for calibration is unsatisfactory, however, detailed relative abundance data proved to be a useful tool. Yoshida (1986) reported 43 genera and 150 species and used the first and last appearances of characteristic forms (genera and species) to construct the zones {Capnuchosphaera, Capnodoce, Acanthocircus-Pseudoheliodiscus, Betraccium deweveri, Livarella-Canoptum, Justium cf. J. novum and Parshsuum Zones).

1) Capnuchosphaera Zone

This is the oldest zone and appears to be Carnian to late C ami an in age. The zone is characterised by the occurrence of members of the genus Capnuchosphaera, in particular C. sockensis, C. theloides, C. tricomis, C. cf. C. schenki, C. sp. A and C. sp. B (of Yoshida, 1986) and Kahlerosphaera cf. K. parvispinosa. The base is defined by the first occurrence of Capnuchosphaera with Kahlerosphaera.

2) Capnodoce Zone

The first occurrences of Capnodoce anapetes and C. antiqua, with the Canesium species group first found just above the base of the zone is characteristic. The top of the zone is

significant for the last occurrence of Capnodoce (in particular C. traversi, C. miniscula, C. cf. C. malaca, C. antiqua, C. fragilis, C. cf. C. kochi, C. beaulieui and C. anapetes) and Triassocampe, Napora and Canesium. Nakaseko and Nishimura (1979) reported that Capnodoce anapetes and C. sarisa occurred within the Capnuchosphaera theloides Zone (early Norian). Pessagno et al (1979) indicated that the Capnodoce species group of the Capnodoce Zone was early Norian in age (based on ammonites and Halobia in N. America). Yoshida (1986) suggested that the correlations indicate that this zone may, in fact, be as old as late Carnian.

3) Acanthocircus-Pseudoheliodiscus Zone

The final occurrence of the Capnodoce species group defines the base of this zone. The characteristic species of this zone include: Acanthocircus heisseli, A. convenus, A. zapfei, A. cf. A. elegans, and Pseudoheliodiscus quadriradiatus. Yao (1982) reported that the Acanthocircus species group and Pseudoheliodiscus species group were included in the Triassocampe nova and Canoptum triassicum assemblages respectively (mid to late Norian). Yoshida (1986) commented that the saturnalid species of Kishida and Sugano (1982) were very similar to the species from Europe reported by Kozur and Mostler (1972) and are in turn similar to some forms recovered during this study. The Acanthocircus species group, as reported from North America by Blome (1984), is included in the Capnodoce Zone (late Camian-mid Norian). Yoshida (1986) inferred that the Acanthocicus-Pseudoheliodiscus Zone in the Kagamigahara section (Japan) is as old as middle Norian. This zone is split into two subzones by Yoshida (1986) the Acanthocircus Subzone (oldest) and the Pseudoheliodiscus Subzone (youngest).

4) Betraccium deweveri Zone

The base is defined by the first occurrence of Betraccium deweveri, which also defines the top of the zone by its last occurrence, forming a total range zone. Another characteristic species is Xenorum cf. X. largum. Blome (1984) defined a subzone within the range zone of Betraccium deweveri by the co-occurrence of B. deweveri, B. inomatum, B. macleami and B. yakounense from the late Norian of Northwest America and so is considered to be that age.

5) Livarella-Canoptum Zone,

The base of this zone is defined by the final occurrence of Betraccium deweveri and the first occurrence of the Uvarella species group. Yoshida (1986) reported that several species of Dreyericyrtium (?) occur near the base of this zone.The section of Yoshida (1986) has its base defined by the first occurrence of Livarella validus, L longus, Dreyericyrtium (?) sp. A and D. (?) sp. B. The top of this zone is is marked by the first occurrence of Justium cf. J. novum firom the zone above. The age is ?early to mid Rhaetian. Characteristic species of this zone include: Livarella densiporata, L. validus, L. longus, L gifuensis, L. sp. A, Canoptum laxum, C. farawayanse, C. triassicum, Dreyericyrtium (?) sp. A, D. (?) sp. B,

and Ferresium sp. A. The species assemblage of the Livarella-Canoptum Zone is almost identical to that of Sample SB 220 (this study), however, Betraccium deweveri is present with forms of Livarella, Canoptum and the distinctive Nassellarian gen, and sp. indet C. The zone as defined by Yoshida (1986) is divided into two subzones, the older Livarella Subzone and thCanoptum Subzone.

6) Justium cf. J, novum Zone

The base of this zone (occurs in the ?late Norian to Rhaetian) is defined by the first occurrence of Justium cf. J. novum which is found throughout the zone, forming a range zone with the final occurrence of Livarella densiporata and the first appearance of Parahsuum group. Species characterising this zone include: Justium cf. J. novum, Livarella densiporata, Canoptum triassicum, Tripocyclia sp. A, Dictyomitrella sp. A. The Justium novum Subzone, according to Blome (1984) from western North America, is within the

a

Capnodoce Zone making Justium cf. 7. novum Zone of Yoshida (1986) biostratigrphically

different from the Justium novum Subzone of Blome (1984). *

7) Parahsuum Zone

This is the youngest zone defined from the Kagamigahara section. Central Japan. The base of the zone is defined by the first occurrence of the Parahsuum group and final occurrence of Justium cf. 7. novum. The top of this zone is not defined.

The work of Yoshida (1986) is one of the more significant studies of the Upper Triassic of Japan and exhibits clear similarities with other areas around the world (see Textfig. 5.7) and with the samples recovered during this study.

6.2.6 Biostratigraphy of China