2 results
12 - Ammonite zones of the circum-Pacific region
- Edited by Gerd E. G. Westermann, McMaster University, Ontario
-
- Book:
- The Jurassic of the Circum-Pacific
- Published online:
- 04 August 2010
- Print publication:
- 26 March 1993, pp 247-272
-
- Chapter
- Export citation
-
Summary
LOWER JURASSIC
Introduction
In recent years, many publications on Lower Jurassic ammonites from the western side of the Pacific Ocean have been published, and together with earlier publications they provide a much more complete understanding of regional biostratigraphy (Table 12.1). Various Russian authors have described Lower Jurassic ammonites from eastern and northeastern Asia, and a detailed biozonation has been developed based, for the most part, on ammonites; for summaries, see Repin (1984) and Krymholts, Mesezhnikov, and Westermann (1988). Lower Jurassic ammonites are rare in most areas in Japan, and only a few papers have been published since the synthesis by Arkell (1956). A summary, regional zonation for part of the Lower Jurassic, along with range charts, has been provided by Hirano (1973, 1985) and Sato and Westermann (1985, 1991). Lower Jurassic ammonites from the Pacific border of southern China are mainly known from the Guangdong province, but only three Sinemurian assemblages have been established (Wang and Smith 1986). In Indochina, Lower Jurassic ammonites are mainly represented in Vietnam. All stages have been proved, but a detailed biozonation is not yet possible (Vu Khuc 1984). From Thailand (Mae Sot region), only late Toarcian ammonites are known (Braun and Jordan 1976). No Lower Jurassic ammonites have been described from the Malay Peninsula, the Philippines, or Sumatra. Some ammonites of Sinemurian and Toarcian age were found in Borneo (Krause 1896, 1911; Hirano et al. 1981).
24 - Ammonites of the circum-Pacific region
- Edited by Gerd E. G. Westermann, McMaster University, Ontario
-
- Book:
- The Jurassic of the Circum-Pacific
- Published online:
- 04 August 2010
- Print publication:
- 26 March 1993, pp 342-359
-
- Chapter
- Export citation
-
Summary
LOWER JURASSIC1 (Figure 24.1)
Hettangian
Hettangian ammonites are found on both sides of the Pacific, from northeastern Asia to New Zealand and from the Arctic to central Chile. Localities, especially those with diverse assemblages, are normally less frequent than for the rest of the Lower Jurassic. Assemblages with Badouxia, Paracaloceras, Pseudaetomoceras, and some species of Vermiceras or Metophioceras are included that in part may be latest Hettangian (Bloos 1983, 1988) or even earliest Sinemurian (Guex and Taylor 1976; Guex 1987).
The systematics of Hettangian ammonites include genera whose synonymity remains ambiguous. Some Hettangian ammonites found in the circum-Pacific area are very similar in test morphology to genera described from the northeastern Alps, but the circum-Pacific species frequently show much simpler septal sutures. The same difference exists between northwestern Europe and the northeastern Alps. The significance of this phenomenon is controversial (G. Bloos personal communication; Guex 1982), but a possible reason could be the repeated immigration of genera and species from the open sea to shelf environments.
Faunal differentiation in the Hettangian was still weak. The richest assemblages are known from North America (mainly Nevada), Peru, and northern Chile. They all have strong Tethyan affinities. A proper Boreal Realm did not then exist, because those genera found in the northernmost part of the Pacific also occur in the Tethyan Realm and must be designated as pandemic. Differentiation between the Tethyan and Boreal Realms is achieved mainly because of richer assemblages in genera and species in the first region.