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16 - Bivalve zones and assemblages of the circum-Pacific region
- Edited by Gerd E. G. Westermann, McMaster University, Ontario
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- Book:
- The Jurassic of the Circum-Pacific
- Published online:
- 04 August 2010
- Print publication:
- 26 March 1993, pp 300-307
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Summary
The use of Late Jurassic bivalves (buchiids) in biostratigraphy has been successful in the northern circum-Pacific for a long time. In recent years the increase of detailed studies on other bivalve faunas has extended the potential biostratigraphic usefulness of bivalves to cover also the Early and Middle Jurassic of other circum-Pacific regions. Only some bivalve groups have proved useful, notably monotaceans and pectinaceans for the Lower Jurassic, inoceramids (s.l.) for the Middle Jurassic, and buchiids for the Upper Jurassic. As knowledge of systematics and vertical ranges of bivalve species is still very uneven, many more studies are needed to obtain a comprehensive picture. A correlation chart for several circum-Pacific regions has been compiled showing approximate equivalences (Table 16.1). This chart is mainly intended to stimulate further discussions on this subject.
As seen in Table 16.1, many of the biostratigraphic units recognized thus far are based on and named after local species belonging to mainly circum-Pacific genera, such as Otapiria, Radulonectites, Parainoceramus, Retroceramus (or Mytiloceramus), and Buchia. Although this is only a consequence of the fact that correlation tables are commonly based on offshore species, it makes comparison between circum-Pacific regions easier. This analysis should not overlook the progressive displacement of local vertical ranges of some of these bivalve taxa along their geographic ranges.
11 - Eastern Russia
- 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 225-246
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- Chapter
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Summary
INTRODUCTION
Diastrophism
The Jurassic of eastern Russia is part of the Mesozoic tectonosedimentary megacycle, from Late Triassic to Neocomian, which formed the Mesozoids of Northeast Asia and laid the foundations of its present structural plan. The Jurassic changed the geodynamic setting from active geoclinal to continentalization, except for the Pacific margin, where new geoclines were initiated the close of the Jurassic.
Lower and Middle Jurassic deposition occurred during the major cycle of geoclinal evolution of the Mesozoids (North-East Russia: Yukagir cycle), and it comprises a single structural complex together with the Upper Triassic (commonly Norian) deposits. The lower boundary of this complex in the North-East is characterized by hiatuses of differing lengths and minor structural changes (Figure 11.1). In areas of stable down-warping (Oldzhoi and Inyali–Debin Troughs), a weak compressional phase at the beginning of the Yukagir cycle resulted in small thrusts and olistostromes (Parfenov 1984). In Far East Russia, the emplacement this complex was preceded by a major restructuring accompanied by folding (Figure 11.1).
The upper boundary of the Yukagir cycle is almost ubiquitously marked by angular and stratigraphic uncomformities. This regressive phase was of long duration. In the North-East, uncomformities are most distinct in the Bathonian, and less pronounced in the Oxfordian. In the Far East, the first folding occurred in the middle Bajocian and the main phase in the Callovian; the latter is associated with a total inversion of most of the Amur-Okhotsk Geocline.