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4 - Western Canada and United States
- 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 29-92
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Summary
INTRODUCTION
Prior to the general acceptance of the theory of plate tectonics, western North America was both a classic model and an enigma for structural geologists captivated by the eugeosyncline–miogeosyncline paradigm. Soon after the theory of plate tectonics settled in, accreted terranes followed, pioneered by one of the leaders of the plate-tectonic movement (Wilson 1968). Western North America, unique among the various boundary regions of the Pacific Ocean in being dominated by thin slivers of accreted terranes separated by transcurrent faults (Howell and Jones 1989), has played a leading role in the development of the concepts of terrane accretion. Insofar as the Jurassic was a primary period of accretion, the Jurassic rocks in western North American have played a major role in these developments.
The North American continent that split away from Europe during the Jurassic is estimated to have been 20–30% smaller than at present and to have grown by 300–500 km along its Pacific Coast by the accretion of about 100 terranes between 200 and 50 m.y. ago (Figure 4.1 A). The western margin of North America prior to about Middle Jurassic time was a passive margin (miogeocline), with mainly cratonic sediment sources, persisting 450 m.y. since a major Late Proterozoic rifting event (Sloss 1982; Stott and Aitken 1982).
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
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- Chapter
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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.